Stennis Space Center – Liquid Rocket Engines (J-2X, RS-25, general)

J-2X Progress: Shaking up the Night

The first rocket engine test that I ever saw in person at the NASA Stennis Space Center in southern Mississippi occurred well over twenty years ago. I’d already been doing test data analysis and power balance analysis in support of the Space Shuttle Main Engine (SSME) Project for some months. I had made several data review presentations in front of then chief engineer (and rocket engine legend) Otto Goetz. I could quote engine facts, statistics, and tell you all about how the SSME worked. I’d even seen some videos of engine tests. But it was not until I saw a test in person that I achieved the state that can only be described as awestruck.

It was late in the evening and a little chilly. Though we’d arrived at the control center before dusk, test preparations had dragged on so that now darkness had enveloped the center. The test stand stood out against the blackened sky like a battleship docked in the distance. Brian, my team lead at the time with whom I’d driven down from Huntsville, and I were standing outside in the control room in the parking lot. The radiation from the hydrogen flair stack off to our left warmed one side of our face as the breeze cooled the opposite cheek. The wailing of the final warning sirens drifted off and all that could be heard was the burning torch of the flair stack and, in the distance, the low surging and gushing of water being pumped into the flame bucket. We were a just a couple of hundred yards from the stand.

Then, the engine started.

First, there is the flash and then, quickly, the wave of noise swallows you where you stand. Unless you are there, you cannot appreciate the volume of the sound. It is not mechanical exactly. It is certainly not musical. It is not a howl or a screech. It is, rather, a rumble through your chest and a shattering roar and rattle through your head. You think instinctively to yourself that something this primal, this terrible must be tearing the night asunder; it surely must be destructive, like a savage crack of thunder that continues on and on without yielding. You are deafened to everything else, deprived of hearing because of all that you hear. Yet before your eyes there is the small yet piercing brightness of the engine nozzle exit that can just be seen on what you know to be deck 5 of the stand and, to the right, there are flashes of orange flame stabbing into the billowing exhaust clouds mounting to ten stories high, tinged rusty in the fluctuating shadows. It is like a bomb exploding continuously for eight minutes and yet the amazing thing, in incongruent fact so difficult to grasp as you are trying to absorb and appreciate the sensation is that the whole event is controlled and contained. You cannot believe that so much raw power can be expressed by what is only a distant dot within your field of vision.

This is an experience that I wish everyone could have. There are so many extraordinary feats of engineering all around us that we can appreciate and admire, but nothing for me has ever been as visceral as seeing an engine test, especially at night, with the nozzle open to the night air (and not buried in a diffuser). No engine schematic or listing of characteristics or series of still pictures is an adequate substitute for the majesty of that controlled power.

Since that first test, I’ve seen any number of other engine tests including SSME (what we now call RS-25), a couple of other, smaller engines, and, of course, J-2X. But it was not until the end of June of this year that I again had the opportunity to see a nighttime test on NASA SSC test stand A1. This was J-2X E10002. Below is the video, and it’s really cool, but I wish that you could have been there, standing beside me in the parking lot. Listen carefully to the end of the recording and you’ll hear people cheering. I was amongst the appreciative, awestruck chorus.

LEO Progress: J-2X to Test Stand A1

“Nothing behind me, everything ahead of me, as is ever so on the road.”
– Jack Kerouac, On the Road

Recently, J-2X development engine 10002 was on the road. If you remember, E10002 went through a six-test series on test stand A2 that began in February and finished up in April. The next planned phase of E10002 testing is on test stand A1. In between these series, the engine was back in the assembly area of NASA Stennis Space Center Building 9101.

This respite between test series allowed for a complete series of inspections of the engine hardware. This is vital piece of the learning process for engine development. The basic truth is that a rocket engine is just darn tough on itself when it fires. The reason that we test and test and test is to make sure that our design can stand up to the recurring brutal conditions. The chance to look for the effects of that testing through detailed inspections away from the test stand is an opportunity to collect a great deal of useful information.

Also, even before the engine arrived at the assembly area, the stub nozzle extension was removed. This was done while the engine was still installed in the test stand. Remember, the testing on test stand A2 was performed with a passive diffuser and so we were able to use an instrumented stub nozzle extension to examine the nozzle thermal environments. On A1, there will be no diffuser. We’ll be firing straight into the ambient Mississippi afternoon and so we’ll not have any nozzle extension attached. The other change made to the engine — this one made while in the assembly area — was that we swapped out the flexible inlet ducts so that we can use our specially instrumented ducts for the gimbal testing on A1. These ducts will provide a great deal of unique data when we gimbal the engine and force the inlet ducts to twist and bend and they are designed to do.

Below is my favorite picture from the recent assembly activities for E10002 back in Building 9101.

“FOE” stands for “foreign object elimination.” I love this picture because it is a demonstration of how dedicated and meticulous are our assembly techs and how much basic integrity they bring to their job every single day. In the rocket engine business we tend to be fanatics about foreign objects (i.e., random debris of unknown origin) hanging around. The reason for this is that if you spend enough time in the business, you will eventually have a story of what happens when trash gets into the engine. The rocket in question might be an amazingly powerful beast pumping out five hundred pounds of propellant per second generating 300,000 pounds of thrust, but all it takes is one bit of junk in the wrong place to destroy the whole thing in fractions of a second.

In the picture above is a small nut that was found in the periphery of the assembly area. Shoot, if this was my garage you’d be lucky to find a clean patch not strewn with various bits of stuff like nuts, bolts, wads of duct tape, old hunks of sandpaper, that lost pair of pliers, a “Huey Lewis and the News” cassette, or, well, who knows what. But the rocket engine assembly area is NOT my garage (thank goodness). When something is found like a stray nut in the picture above, that sets off an investigation. Where did it come from? How did it get loose? What procedure allowed this nut to escape control? This is serious stuff. Yet, just think about how easy it would have been for a tech to see that stray nut, pick it up, and stick it in his pocket. They could have avoided the whole minor investigation thing entirely. But that’s not what they did and that is not what they do. Because they know that if they do not show the necessary integrity and methodical approach to continue to learn and perfect our procedures, then the next stray nut could be lodged where it could do terrible damage.

Here are the techs moving E10002 out of its assembly bay and unpacking it for transport.

And a couple more pictures of the process in Building 9101.

Here’s the engine ready for the road, then being lifted up the side of the test stand, and then sitting in the porch area while sitting on the engine vertical installer. I really enjoy the pictures of the engine trucking about sunny NASA SSC. That picture was the inspiration for including the Jack Kerouac quote at the beginning of the article. It’s bright and shiny and full of so much thrill and promise.

All of this should look reasonably familiar. It is the process that we follow, more or less, whenever we take an engine out to the stand. Getting the engine into and out of A1 is a bit easier since you don’t have to deal with moving the diffuser out of the way, but they’re really quite similar. The slightly different flavor that we’ve got for this testing is the addition of the thrust vector control system. In the picture below you can see where these hang. The engine mounts up with the gimbal bearing to the stout, yellow thrust take-out structure.

The two hydraulic actuators are also attached to the thrust take-out structure but slightly outboard and at 90 degrees apart. These actuators are what will swing the engine around as if we were steering a vehicle. Here, below, is another, closer view of the thrust take-out structure and the actuators.

In the picture below, E10002 is mounted up to the thrust take-out structure. The gimbal bearing is the shiny object towards which the arrow is pointing. If you look over to the right side of the picture, you can see one of the “scissors ducts,” i.e., the flexible propellant inlet ducts.

The next picture shows one of the hydraulic actuators hooked up to the engine. As you can see the tolerances are awfully tight. That’s an important vehicle integration consideration. If this was a vehicle stage rather than a test stand to which we were attaching the engine, the thrust structure and the actuators would be the responsibility of the stage manufacturer. Making sure that the stage and the engine can work together in such close quarters takes a great deal of vigilance between the two teams.

So, you’re probably asking yourself, to what do these actuators connect on the engine? That’s a very good question. It certainly isn’t obvious from the assembly pictures. The actuators connect to the forward manifold of the main combustion chamber (MCC). Below is a computer model of the MCC with the two actuator attachment points shown.

The MCC is really the heart of the whole engine, the sturdy framework stuck right in the middle, so it makes sense that when you want to push the engine around, this is what you’d have to push. This final picture below of the engine completely mounted into the stand. Again, it’s amazing to think about that whole thing being able to move about and not having one component run into another component or the actuators or the stand. It is quite the integration miracle.

Testing for E10002 on test stand A1 will commence in June. So, if all goes well, for the next J-2X update, I’ll be able to link in a video of the engine firing and twisting about.

BTW, NASA is in the process of swapping software used for posting blog articles and comments and such. As part of this process, they have to shut down the capability to accept input comments for a short time, specifically the first two weeks of July 2013. Sorry about that. But after that, it ought to be up and running as normal.

Liquid Engines Extra — Introducing LEO

The following picture is a test. Find me amidst the mess…

That’s right. I’m the handsome chap with the snazzy specs. See, I’m just oozing with exactly the vitality that you’d expect from your typical government-trained civil servant! Well, okay, so maybe I’m not exactly Milton Waddams. All personal resemblance and charm aside, I don’t actually have quite that much paper clutter in my office. No. Instead, I just have electronic clutter. Indeed, if someone spent the time to actually print out all the stuff on my computer here at work, then we wouldn’t need a rocket to get beyond the moon. We could just build a staircase from the resulting gargantuan pile of paper.

Why is this the case? It could be that I’m just a data hoarder. Some people hoard clothes (that’s not me). Some people hoard automobiles (that’s not me). Some people hoard books (okay, that is me). And some people hoard data. All data. All of the way back to class notes from Aerodynamics I (AERSP 311, junior year, professor Bill Holl — great teacher, great guy). Yes, okay, so maybe I’m a little guilty of all that. In my defense, however, I will simply say that there’s a lot that goes into making rocket engines and much of it is quite removed from the exciting cutting-metal stuff or the making-smoke-and-fire stuff. And I get to stick my nose into much of it. For this article, I am going to reveal super-duper, deep-and-dark secrets that they won’t even teach you in college. I am going to tell you a little about …

…wait for it…

…project management. Yes, it’s true: I am going to invite you into our little piece of office space and I will do so to tell you about how the office has recently evolved. Also, towards the end as a reward, I’ll give you an update of J-2X testing progress to date.

So, let me introduce you to LEO —

No, none of those LEOs. Instead, let me introduce you to the Space Launch System (SLS) Program Liquid Engines Office (LEO). The LEO is responsible for development and delivery of the liquid rocket engines for the core stage and upper stage of the SLS vehicle. For more information about the SLS Program in general, I highly recommend the following site: https://www.nasa.gov/exploration/systems/sls/

(Oh, and I want to say something briefly about the term “liquid engines.” Probably every single person who would bother to read a blog about engines or rockets or space travel in general knows that this is just a shorthand term. No, we are not talking about engines made of liquid — although that would be really cool. “Liquid engines” is a quick and easy way to denote “liquid-propellant rocket engines.” In case I’ve disappointed anyone, I’m sorry. If ever we are able to make an engine out of liquid, I promise to be the first to report it. Probably the most far-out thing that I once heard was the suggestion to make a hybrid rocket motor using solid hydrogen and liquid oxygen. I cannot even imagine what the infrastructure would be to make the use of solid hydrogen plausible, but you never know…)

For the SLS vehicle, the upper-stage engine is the rocket engine so near and dear to our hearts after several years of design and development and fabrication and assembly and test: J-2X. The core-stage engine is the RS-25. No, the RS-25 is not a brand new engine. Rather, it is the generic name for that workhorse of the last thirty years, the Space Shuttle Main Engine (SSME).

At the end of the Space Shuttle Program, there were fourteen SSMEs that had flown in space on the Shuttle and that still had usable life remaining. I’m not sure that everyone knows this, but rocket engines have limited useful lives. I guess that most things do, but with rocket engines it’s often pretty short. Think of them like cherry blossoms (popular motif in Japanese tattoos): amazingly beautiful and quickly gone too soon. The stresses within an operating rocket engine are tremendous. For example, the J-2X has an official, useful life of only four starts and less than 2,000 seconds of operational run time after the engine has been delivered for use as part of the vehicle. No, the engine doesn’t crumble into dust after that, but based upon our certification strategy and on our analysis of margins, that is the official life for our human-rated launch system. After that point, depending on the proposed usage and risk considerations, and based on the likely reassessment of our margins with the proverbial “sharper pencil,” we can and do routinely talk ourselves into longer active lives for engine hardware. On the test stand, we can test the J-2X upwards of 30 times and for lots of run time, but that is a lower risk situation. Nobody is riding the test stand into space.

Thus, when you come to the end of a program and you have fourteen engines with remaining, usable life, then you’ve got one heck of a residual resource. In addition, there was one SSME assembled and ready to go, but it never made it to the test stand or the vehicle. So it’s brand new. And, on top of that, there were enough leftover pieces and parts lying around of flight-quality hardware to cobble together yet another engine. And, there’s more! (Yes, I feel like the late-night infomercial guy, “and if you call in the next 10 minutes you will get this special gift!”) There are also two development SSMEs. These are not new enough to fly, but they are useful for ground testing and issues resolution. That means that there are a total of sixteen RS-25 flight engines and two RS-25 development engines available to support the SLS Program.

However, before your excitement bubbles over, you have to understand that when you see a sign for “free puppies,” you probably shouldn’t take that whole notion of “free” too literally. As in, well, not at all. Yes, we still have an extraordinary asset in the residual RS-25 engines. No question. But, we have work to do to integrate them into the SLS Program. In a future article, I will discuss the multiple facets of this work. By the way, I cannot claim to be immune from the “free puppies” thing myself. Meet Ruugie –

The Liquid Engines Office (LEO) was formed to manage both the J-2X and the RS-25. This office will also manage other liquid rocket engines used to support the SLS Program as it matures. It was decided from a project management perspective that it would be best to have one office manage both engines. In this way, we can be more efficient by leveraging the expertise across various disciplines and components. For example, do we really need two turbomachinery subsystem managers? No, Gary Genge is our turbomachinery subsystem manager and in that position he can understand and evaluate the relative programmatic and technical risk across all of the various turbomachinery pieces under his purview. If in some utopian future our office responsibilities expands to three or four or eight different engine development or production efforts, we would, in theory, maintain the same structure but provide Gary with the support necessary to effectively manage turbomachinery across so many activities.

So, for LEO we have subsystem managers for Engine Systems (effectively systems engineering and integration), Engine Assembly and Test (also includes asset management, logistics, and operations), Engine System Integration and Hardware, Valves and Actuators, Engine Control Avionics, Turbomachinery, and Combustion Devices. LEO is supported by a Chief Engineer, a Chief Safety and Mission Assurance (S&MA) Officer, Program Planning and Control (i.e., the business office), and Procurement. Plus, of course, we have support from the engineering and S&MA organizations across the many technical disciplines. The structure is really quite similar to how we’ve been managing J-2X for these past several years. We’ve just expanded our responsibilities.

So, that’s LEO and I’ll be talking more about RS-25 and SLS in the future.

Now, while I’ve been off doing my little part to get the foundation of LEO solid, including refreshing and getting into place our prime contracts for both J-2X and RS-25, how has J-2X been doing? Well, in short, J-2X has been just cruising along. E10002 has gone through six tests on NASA Stennis Space Center (SSC) test stand A-2. Below are a series of images showing what an E10002 start looks like if you stood in view of the flame bucket (which I would very strongly advise against, by the way):

First, all you see is the facility water being pumped into the flame bucket. Then you can see the ignition and everything glows orange. Then the whole flame bucket is filled with exhaust. And, finally, the exhaust coming barrelling down the spillway and eventually engulfs the camera. The final step is not shown since there’s nothing to see but solid whitish grayness.

Here are the stats on the six tests:
     • Test:          A2J022          2/15/2013          35 seconds duration
     • Test:          A2J023          2/27/2013          550 seconds duration
     • Test:          A2J024          3/07/2013          560 seconds duration
     • Test:          A2J025          3/19/2013          425 seconds duration
     • Test:          A2J026          4/04/2013          570 seconds duration
     • Test:          A2J027          4/17/2013          16 seconds duration

So the total accumulated time is 2,156 seconds. Tests #22, #25, and #27 all experienced early cuts, but all three were instigated by different flavors of instrumentation or monitoring system issues or oddities. The engine is fine and running well. Some of the key objectives included gathering additional data about the nozzle extension cooling characteristics, additional samples of the turbomachinery design, and main chamber combustion stability trials. Something else that we did for this test series is that we tested a very special fuel turbopump port cover. Here’s a picture of it:

Now, port covers are not something about which one usually says anything at all. What makes this one special is that it was made by using a process known as Selective Laser Melting (SLM). That is a fabrication method that is somewhat analogous to “3-D printing.” A long time ago, I wrote a blog article about a gas generator discharge duct that we made for component-level testing using this technique. This, however, is an engine test and this small, seemingly innocuous, piece of engine hardware may be the humble harbinger of a revolution in rocket engine fabrication. The fact that we systematically stepped through the process of validating this port cover as a piece of hardware for an engine hot fire demonstration paves the way for pursuing other parts in the future, more complex parts, and, hopefully one day, regular production parts as part of a human-rated launch architecture.

E10002 was removed from NASA SSC test stand A-2 on April 30th. It is currently being retrofitted with instrumented inlet ducts and other hardware in preparation for the next phase of testing that will occur on NASA SSC test stand A-1. As you’ll remember, in the past A-1 was used for the PowerPack Assembly testing. Well, the talented and productive folks at NASA SSC remodeled the stand back to the configuration for engine testing. The current plan is to install E10002 into A-1 by mid-May and to perform a series of five to seven tests through probably August. The reason for using A-1 for the next series is because that stand does not have a diffuser. That means that we can gimbal the engine, i.e., twist it around as if we were providing steering for a vehicle. The thrust vector control (TVC) system composed of the hydraulic push-pull actuators that will be performing the gimballing is a component belonging to the stages element of the vehicle. This testing will be providing those folks with data to inform their system design for the SLS Program. See, it’s all win-win when we play nicely together.

And, finally, right on the heels of E10002, the assembly of E10003 will commence in June with scheduled installation into NASA SSC A-2 in September. That’s my report for where things stand. To finish up, I’ll leave you with a purely gratuitous glamor shot of the J-2X. Isn’t she pretty?

J-2X Progress: Current Status, The End of 2012

Once upon a time, not that long ago, people used to communicate by what were known as “letters.” These were written documents. Yes, actual hardcopy, paper items. And they were often transcribed by hand or, sometimes, generated on what was known as a “typewriter,” which was basically a manual, analog printer with no I/O port beyond direct keypad entry. These “letters” were sent to their intended recipients using a small denomination currency with an adhesive backing that is recognized for exchange by only one quasi-governmental agency.

I know that some of you may have doubts that people communicated with each other in primitive ways prior to email and text messages, but witness the cultural clues from the 1961 song illustrated above.

It was always believed that the toughest letter to receive was the dreaded “Dear John” letter (as in, “Dear John, I’ve fallen in love with someone else…”). However, I think t’at the hardest letter to write is the “it’s been awhile” letter. This one starts, “Well, it’s been awhile since I’ve written. Sorry.” This blog article is just like one of those letters. It’s been awhile since I’ve written one of these articles and I’m sorry about that. I could give you a big long list of all the really, really serious stuff that I’ve been doing instead, but that’s just a bunch of feeble excuses so I’ll keep them to myself. Instead, I’ll just get down to business and give you a status report on the J-2X development effort.

Engine #1 (E10001) Testing is Complete!
Over fourteen months and across the span of twenty-one tests, more than 2,700 seconds of engine run time was accumulated and recorded, including nearly 1,700 seconds of hot fire with an instrumented nozzle extension. With this engine we achieved stable 100% power level operation by the fourth test and full mission duration by the eighth test. While we don’t have any official statistics on the issue, most folks around here believe that we accomplished those milestones faster than has ever been done on a newly developed engine. We learned how to calibrate the engine and the sensitivities that the engine has to different calibration settings, i.e., orifice sizes and valve positions. We were able to estimate performance parameters for the full-configuration of the engine at vacuum conditions and the calculations suggest strongly that all requirements are met by this design and met with substantial margin. This is significant considering that we’ve long considered our performance goals to be pretty aggressive. Well, our little-engine-that-could showed us that it did just fine with those goals, thank you very much.

One of the truly unique and successful aspects of the E10001 testing was the testing of a nozzle extension. This component is a key feature that allows J-2X performance to far exceed that of the J-2 engine from the Apollo Program era. While it is true that we cannot test the full-length nozzle extension without a test stand that actively simulates altitude conditions, we did test a highly instrumented “stub” version that allowed us to characterize the thermal environments to which the nozzle is exposed during engine hot fire and it demonstrated the effectiveness and durability of the emissivity coating that was used. This stub-nozzle configuration is actually the current baseline for the in-development Space Launch System vehicle upper stage.

Another key success for E10001 was the demonstration of both primary and secondary power levels with starts and shutdowns from each power level and with smooth in-run transitions back and forth between them. That smoothness was thanks, in part, to demonstrating our understanding of the control of the engine. From the very first test it was clear that we understood pretty well how to control the engine in terms of proper control orifices for the various operating conditions. What we did not entirely understand — in other words the fine-tuning details — we successfully learned via trial-and-error throughout the E10001 test series. All of this learning has been fed back into further anchoring our analytical tools and models so that we can move forward with J-2X development with a great deal of confidence.

Okay, so that’s a brief description of just some of the good stuff. We had lots and lots of good stuff with the E10001 testing, far more than just that I’ve discussed here (see previous blog articles). The somewhat unfortunate part was the way in which the E10001 test series came to an end. On test A2J021, we had a disconnection between the intent for test and the detailed planning that led to the actual hardware configuration we ran for the test. That disconnection led to an ill-fated situation. Let me explain…

The J-2X gas generator has ports into which solid propellant igniters are installed. These igniters are like really high-powered Estes® rocket motors that light off when supplied with a high-energy electrical pulse. The flame from the igniter lights the fire of the hydrogen-oxygen mixture during the engine start sequence. It’s essentially the kindling for the fire of mainstage operation. The igniters perform this function at a very specific time during this sequence. If you try to light the fire too early, then you may not have enough propellant available in a combustible mixture so you get a sputtering fire. If you try to light too late, then you may have too much propellant built up such that rather than getting a good fire, you get an explosion instead. But here’s a key fact: You have to plug them in or they don’t work.

Have you ever stuck bread in the toaster, pushed down the plunger, gone off to make the coffee, and come back only to find that your darn toaster is broken? You curse a little because you’re already late for work and this is the last darn thing you need. You would think that somebody somewhere could make a toaster that lasts more than six months or a year or whatever. For goodness sake! We put a man on the moon and yet we can’t … oh, wait … um … ooops, it’s not plugged in. My bad.

In a nutshell, that’s what happened on test A2J021. The electronic ignition system sent the necessary pulse, but because of the uniqueness of our testing configuration as opposed to our flight configuration the wires carrying the pulse weren’t hooked up to the little solid propellant igniters in the gas generator. In the picture below you can see the external indication that something was not entirely good immediately after the test. The internal damage was more extensive to both the gas generator and the fuel turbopump turbine.

Many years ago, I met an elderly engineer who was still on the job well into his 80’s because he loved his work. His entire career had been dedicated to testing. He’d actually been there, out in the desert, in the 1940’s testing our very earliest rockets as part of the Hermes Project. One day, they had a mini disaster on the launch pad. He told me that the rocket basically just blew up where it sat. Boom and then a mess. And, it was his job to assemble the test report. Being a conscientious, ambitious, young engineer, he recorded the facts and offered a narrative abstract and extensive, annotated introduction that categorized the test as, well, a failure. Not long after submitting his report, one of the senior German engineers in the camp came into his office, put the test report down on the desk, and said that the tone of the report was entirely wrong. He said, “Every test report should begin with: ‘This test was a success because…'” The purpose of testing is to gather data and learn. If you learn something, then your test was, by definition, a success on some level. I’ve tried very hard to remember this very important bit of wisdom.

So, A2J021 was a success because we learned that we had some deficiencies in our pre-test checkout procedures. It was a success because it was an extraordinary stress test on the gas generator system. No, it didn’t recover and function properly, but neither did the engine come apart. While that might seem like a minor detail, when you’re hundred miles from the surface of the earth, you would much rather have a situation where an abort is possible than a failure that could result in collateral vehicle damage and make safe abort impossible. We have a stout design. Good. Also, this test failure was due to a unique ground test configuration. In flight, it’s not really plausible just because we would never fly in this configuration.

So, E10001 completed its test program with a bang. Kinda, sorta literally. But it was nearly the end of its design life anyway, so we didn’t lose too many test opportunities, and, as I said, even with test A2J021 the way it happened we learned a great deal. Overall, the E10001 test series was an outrageous success. Rocketdyne, the J-2X contractor, ought to be darn proud and so should the outstanding assembly and test crews at the NASA Stennis Space Center and our data analysts here at the NASA Marshall Space Flight Center. Bravo guys! Go J-2X!

Power-Pack Assembly 2 (PPA-2) Testing is Complete!
Over ten months and across the span of thirteen tests, nearly 6,200 seconds of engine run time was accumulated and recorded on the J-2X Power Pack Assembly 2. That’s over 100 minutes of hot fire. Three of the tests were over 20 minutes long (plus one that clocked in at 19 minutes) and these represent the longest tests ever conducted at the NASA Stennis Space Center A-complex. But more than just length, it was the extraordinary complexity of the test profiles that truly sets the PPA-2 testing apart.

Because PPA-2 was not a full engine with the constraints imposed by the need to feed a stable main combustion chamber, and because we used electro-mechanical actuators on the engine-side valves and hydraulic actuators on the facility side valves, we could push the PPA-2 turbomachinery across broad ranges of operating conditions. These ranges represented extremes in boundary conditions and extremes in engine conditions and performance. On several occasions we intentionally searched out conditions that would result in a test cut just so that we could better understand our margins. As the saying goes: It’s only when you go too far do you truly learn just how far you can go. We successfully (and safely) applied that adage several times. In short, we gathered enough information to keep the turbomachinery and rotordynamics folks blissfully buried in data for months and months to come.

On an interesting and instructive side note, the PPA-2 testing also showed us that we needed to redesign a seal internal to the hydrogen turbopump. In the oxygen turbopump, you have an actively purged seal between the turbine side and the pump side. After all, during operation you have hydrogen-rich hot gas pushing through the turbine side and liquid oxygen going through the pump side. You obviously don’t want them to mix or the result could be catastrophic. That’s why we have a purged seal. But for the hydrogen turbopump you don’t have such an issue. During operation, at worst should the two sides mix you could get some leakage of hydrogen from the pump side into the turbine side that is already hydrogen rich. In order to maintain machine efficiency, you don’t want too much leakage, but a little is not catastrophic (and can be used constructively to cool the bearings). What could be dangerous at the vehicle level, however, is if you have too much hydrogen floating around prior to liftoff. This is especially true for an upper-stage engine like J-2X that’s typically sitting within an enclosed space until stage separation during the mission. You could have the engine sitting on the pad for hours chilling down and filling the cryogenic systems and you don’t want gobs and gobs of hydrogen leaking through the turbopump since any leakage ends up within the closed vehicle compartment housing the engine. That’s just asking for an explosion and a bad day.

To avoid this, within the J-2X hydrogen turbopump we have what is called a lift-off seal. And, as the name applies, it’s a seal that actively lifts off when we’re ready to run the engine. When the engine is just sitting there chilling down, not running, with liquid hydrogen filling the pump end of the hydrogen turbopump, the seal is, well, sealed. Then, when we’re ready to go, it unseals and allows the turbopump to operate nominally.

During the PPA-2 test series we found that we formed a small material failure within the actuation pieces for our lift-off seal. Then, upon analysis of the test data and a reassessment of the design, we figured out what was most likely the cause and Rocketdyne proposed a redesign to mitigate the issue. Again, going back to that important piece of wisdom: This testing was a success because, in part, we learned that we needed a slight redesign of the lift-off seal. That’s the whole purpose of development testing! Everything always looks great when it’s just in blueprints. It’s not until you hit the test stand do you truly learn what’s good and what need to be reconsidered. In the end, this sort of rigor and perseverance is what gives you a final product that you feel good about putting in a vehicle carrying humans in space. And that, truly, is what it’s all about.

As with E10001, the PPA-2 test series was simply an outrageous success. Rocketdyne should be proud and so should the outstanding assembly and test crews at the NASA Stennis Space Center and the data analysts at the NASA Marshall Space Flight Center. Bravo guys! Go J-2X!

Engine #2 (E10002) Assembly is Underway
Our next star on the horizon is J-2X development Engine 10002. It is being assembled right now, as I’m typing this article. It is slated for assembly completion in January 2013 and it will be making lots of noise and very hot steam in the test stand soon after that. While our current plans are to first test E10002 in test stand A2, we will later be moving it to test stand A1. This, then, will be the first engine then to see both test stands. The more important reason for the A1 testing, however, is because that will give us the opportunity to hook up some big hydraulic actuators and gimbal the engine, i.e., make it rock and tilt as though it were being used to steer a vehicle. Now that will be some exciting video to post to the blog! I can’t wait.

Happy New Year!
So, this has been my “it’s been awhile” letter. Hopefully this will bring everyone up to speed with where we stand with J-2X development. In my next article, I will share with you some of what’s been keeping me from my J-2X article writing over the last several months. And, hopefully, it won’t be several months in the making. So, farewell for now and Happy New Year! On to 2013 and another great year full of J-2X successes. Go J-2X!

Inside The J-2X Doghouse: Performance Measurement, Part 2 of 2

In the last article, we talked about the measurement of propellant flow during a test. Propellant is the stuff we put into the rocket engine. What we get out of the rocket engine is thrust. We get propulsion. Or, in the immortal words of Salt-n-Pepa, 1987, we get push… “Push it real good.”

But how do you measure “push,” or in other words, force? The simple answer can be found through one of the most frightening household appliances any of us own: The dreaded bathroom scale…

A bathroom scale works by pushing back at your weight when you stand on it. Your weight is a force caused by your body mass and the Earth’s characteristic acceleration of gravity. The scale pushes back with a spring system but deflects slightly under the load. The scale is then measuring the deflection allowed by the springs at the equilibrium where the spring force exactly counteracts your weight. More weight pushing down results in more deflection to the equilibrium point and that thereby results in a bigger reading (i.e., think: the Monday after Thanksgiving).

The way that we measure rocket engine thrust is basically the same thing except that instead of measuring something between zero and — as in the bathroom scale picture above — 300 pounds, we’re measuring hundreds of thousands of pounds of force. Or, in the case of very large engines like the F-1 or the RD-170, we’re measuring over a million pounds of thrust. That requires a system just a bit more rugged even if the principles remain the same. What we use rather than bathroom scale and springs are things called “load cells.” Below is an example of a generic load cell design:

The gray object with the funky cut through it is a metal piece. As you can imagine, when forces are applied as shown, that slot on the right-hand side will tend to close slightly. In turn, that would cause the metal on the left-hand side to stretch slightly as the whole thing bends a very little amount. We measure that slight stretching of the metal on the left-hand side with a strain gauge bonded to the surface of the metal. A strain gauge is a small electrical device that changes resistance when stretched. Using an electrical circuit known as a Wheatstone Bridge, we measure small changes in electrical resistance caused by the slight deformations of the load cell. The amount of stretch can be astonishingly small and yet good strain gauges and good electrical interpretation of the output can yield very accurate data. The load cell is then calibrated using a known applied load and measuring the resulting strain (i.e., metal stretch). You now have a more rugged version of a bathroom scale. Apply a load, get a reading, and, ta-da, you’ve measured push.

Actual load cells used for rocket engines can take different forms from the generic cell shown here. Any way that you can get an applied load to result in a slight, measureable stretching of metal (while obviously avoiding yielding or buckling) is a valid load cell design.

Above is a picture of a vertical load cell arrangement on test stand A-2 at the NASA Stennis Space Center, where we’ve been testing J-2X development engine E10001. There are two pieces in series. The bottom piece, the big chunk of metal with a bunch of crazy holes and slashing cuts through it, is called a “flexure,” which, to me, seems to be a silly name since it doesn’t look very flexible at all. What it does, however, is effectively make sure that the load entering the load cell is properly directed through the intended vector. Any skewing of the input off from the intended axis and your results could be erroneous. The brown cylindrical thing above the flexure is the actual load cell. You can see the strain gauge wires coming out of it that are fed into the data acquisition system. This two-piece combination is effectively analogous to a spring in your bathroom scale.

The next item to discuss is how you put load cells into the structure of the test stand so that they can do their job. On a bathroom scale, the thing that you step onto is essentially a platform “floating” above the base. It has to be free to move so that the springs can compress honestly. If there was some interference with this movement, then the reading would be wrong. The same is true on the test stand when measuring engine thrust. It is necessary to use a free-floating platform. The picture below is a drawing of the platform used on test stand A-2.

The engine has a single input point as shown — the gimbal bearing that we’ve discussed before in previous articles — and there are three load cells above the platform. This is not the only possible way to do it. Other test stands use a rectangular pattern of four cells. Or, if it’s a smaller system, you might be able to use just a single load cell. The important point is that the load cells are in between the pushing engine and the resisting test stand. Put into the structure of the test stand, and viewed from the side as in the picture below, you can see the whole stack up. On the bottom is where you attach the engine. In the middle is the platform into which the engine pushes. And then the platform is connected to the structure of the stand through the load cells. The structure of the stand has to be strong enough to absorb the thrust of the engine without distorting. It has to be fixed (the mythical “immovable object” from physics class). So, as you can imagine, when you’re talking about hundreds of thousands or even millions of pounds of force, the test stand structure is pretty darn stout. We usually refer to the primary structure responsible for resisting the force of the engine as the “thrust take-out” structure.

A final subject to mention is that matter of tares. Tares are corrections to measured data. For example, when you step on the bathroom scale and you’re dressed, do you subtract off an estimate for the weight of your clothing and your shoes? If so, then you’re making a correction for a particular tare. Of course, you have to do this accurately (honestly). If I assume that my clothes and shoes are made of lead, for instance, then I can declare that I weigh the same as when Salt-n-Pepa were releasing their first albums. But that’s not quite the truth. Getting your tares correct is important for interpreting your data correctly.

When measuring rocket engine thrust you have lots and lots of corrections to the raw data that you measure with your load cells. This is because, in truth, the gimbal bearing is not the only connection between the engine and the test stand. While you’d really like to have that perfectly free-floating platform situation, you’ve got to have, for example, propellant feedlines hooked up to the engine. Flexible bellows are built into the line so that they’re not completely stiff and thereby interfering with the movement of the platform, but they still absorb some of the thrust load and, therefore, make the raw thrust reading skewed. There are a number of other such corrections that need to be made such that the whole calculation process related to tares can get a bit cumbersome with all its many pieces, but nobody ever said that developing rocket engines was supposed to be easy, right?

Now, between this article and the previous one, you have a good idea of how we get basic performance data from rocket engine testing and also the necessary configuration of the test stands that allow us to gather this information. The smoke and fire and rumbling roar of an engine test is all very impressive, but for us Datadogs, it’s the data that matters most. We get lots and lots of data from every test, but propellant flow rates and engine thrust are the most important in terms of understanding how an engine fits into a vehicle and a mission.

J-2X Progress: Once Upon a Time at Stennis…

I enjoy movies. I don’t get to watch much television due to other endeavors that consume much of my time, but if I do it’ll almost always be one of four things on the screen: some news program, a sporting event, a history program, or a movie. And I like lots of different kinds of movies. Some of my favorites include: The Hustler, Singing in the Rain, Rocky, Schindler’s List, Barfly, Hannah and Her Sisters, Fargo, The Apartment, The Godfather, Leaving Las Vegas, The Deer Hunter, Hoosiers, Nobody’s Fool (the Paul Newman one). I don’t believe that one could decipher a pattern from that list other than the fact they all follow the classic narrative structure:

Think of the classic “stranger comes to town” story. (1) It’s a quiet little town and all is peaceful. (2) Then a stranger comes to town and stirs up all kinds of trouble. (3a) In the end, the stranger marries and settles down with the prom queen and everyone learns to live with one another. Or, (3b) in the end, the stranger ends up mysteriously dead and lying in the gutter along the road leading out of town and they secretly bury him promising never to mention it to anyone from out of town. Or, (3c) in the end, the stranger ends up mayor of the town by exposing and driving out the secretly corrupt sheriff. Obviously, the possibilities are endless and that’s why there are thousands and thousands of stories to be told. But the root of all of this is the middle block, “…something disturbs that situation and troubles ensue…” Nobody ever tells an interesting story where nothing happens. And with no “troubles” of some sort, nobody cares about the resolution.

So, that brings me to rocket engine testing and the fact that it is always interesting. This article is intended to bring you up to date on the status of our J-2X test campaign at the NASA Stennis Space Center in southern Mississippi. Remember, we last left our heroes on test stand A1 with PowerPack-2…

Test A1J015, J-2X PowerPack-2: It ran 340 seconds of a planned 655 seconds duration. The test profile called for simulated primary mode and secondary mode (i.e., throttled) operation. Also, throughout the test, turbomachinery speed sweeps were planned meaning that we systematically varied turbine power, increasing and decreasing, to force the pumps through a broad range of conditions. It was during one of these sweeps that the fuel turbopump crossed a minimum speed redline and the test was cut short. Before the test, we knew that it would be close and the analytical prediction was just enough off from reality to cause the early cut. Nevertheless, most of the primary objectives were achieved and the test was a success.

One of the things that we often talk about when discussing an engine test is the “test profile” or sometimes the “thrust profile.” The test/thrust profile is the plan for what you’re going to do during the test. When we say that we had a planned duration of 655 seconds, that value comes from the test profile that is agreed upon prior to the test. Usually a test/thrust profile is a single page showing engine power levels and propellant inlet conditions, but for these complex PPA-2 tests, the test profile can be expanded to include such things as these turbomachinery speed sweeps. To give you an idea of what an engine test/thrust profile looks like, here is one for a Space Shuttle Main Engine (SSME) test performed back in 2001. It contains a wealth of knowledge about the test to be run.

Test A1J016, J-2X PowerPack-2: It ran 32 seconds of a planned 1,130 seconds duration. In this case, unlike the previous test, because we cut so early we can’t really say that it was mostly a success. However, every time that you chill an engine, successfully get it started, and shut it down safely, you have accomplished something significant and you are always collecting data and learning. The early cut in this case had nothing to do with the PowerPack-2 performance. Rather, it was a facility issue, a hydrogen fire due to a leak. As I’ve said before, the PowerPack-2 is an oddball test article in that it is half engine and half facility. That makes the interfaces technically difficult in some cases due to thermal and structural loads. The leak and fire in this case was on the facility side near one of these difficult interfaces.

Below is a picture captured off a video taken during the test and behind the structure and the piping you can see the bright orange flame that resulted in the early cut. This issue of hydrogen leaks and fires has been somewhat recurring so a team of NASA and contractor folks stepped forward to work towards a resolution of the issue.

Test A1J017, J-2X PowerPack-2: It ran the full, planned 1,150 seconds duration. That’s over 19 minutes of continuous rocket engine operation and that’s pretty amazing. It was the longest, most complex engine test ever conducted across the long history of the NASA Stennis Space Center A Complex. We did some wacky stuff on test stand A1 during the XRS-2200 (linear aerospike engine) development effort and there were a couple of longer SSME tests in the B Complex twenty-some years ago, but test A1J017 stands out for the combination of complexity and duration. The test profile contained over a dozen unique, steady state “set points,” i.e., prearranged combinations of engine operational conditions and facility boundary conditions. The objectives of this test included speed sweeps for the oxidizer turbopump and an examination of cavitation performance for both the oxidizer pump and the fuel pump. Pulling off this test was a dazzling success with many people deserving credit.

So, trouble ensues (hydrogen fire on test #16) and the combined team of NASA Stennis, NASA Marshall, test operations and support contractors, and Rocketdyne worked through to a resolution of the issue and a new situation of unprecedented success has been achieved. It’s easy to write a blog like this when reality lines up so conveniently in the narrative form.

But back at the ranch, our heroes find J-2X development engine E10001 on test stand A2…

To refresh your memory, we’d last tested E10001 on stand A2 back in December of last year. Back then, we were testing the engine without a nozzle extension and not using the passive diffuser system on the stand. This year, we were going to get back to testing E10001 but now with a nozzle extension so that necessitated use of the passive diffuser. The Stennis folks installed a clamshell and seal apparatus that connects the engine to the diffuser thereby allowing the diffuser to “suck down” to pressures lower than sea level ambient. In my crude sketch below, I try to show you how this fits together.

A key piece in this arrangement is the clamshell seal. Whereas the engine is obviously metal and the clamshell and diffuser and big pieces of structural metal, the clamshell seal is a fibrous/rubber-ish piece that has to provide the seal that allows the whole thing to work together and simulate altitude operation when the engine is running. It has to be strong yet compliant so as to accommodate movements of the nozzle during hot fire. To give you an idea of how strong it needs to be, let’s calculate the force imposed on the seal during operation. Ambient sea level pressure is 14.7 psia (pounds per square inch, absolute). Let’s say that in the diffuser, during operation, it will be about 10 psi lower than sea level ambient. In reality, the pressure will be slightly lower than that, but 10 is a nice round number to work with. Let’s further say that the diameter of the nozzle at which the seal is attached is about five feet (or, 60 inches). That’s pretty close to reality, give or take a bit. And, let’s say that the seal itself is about six inches in width. So, the total area of the seal is:

So, if the pressure differential across the seal is 10 pounds per square inch and you have 1,244 square inches of surface area, then that makes for over 12,000 pounds of force — or more than 6 tons! Wow, so that seal and the brackets that holds it in place still needs to be pretty darn tough.

Test A2J011, J-2X E10001: It ran 3 seconds of a planned 7 seconds duration. The early cut was due to a facility redline violation; specifically, the measured pressure within the clamshell did not drop down the way that it was supposed to. Post-test inspections quickly revealed why this redline violation occurred. The clamshell seal was torn up. If the seal doesn’t seal, then the pressure differential is not maintained and so, appropriately, we tripped a redline.

An informal team was assembled of NASA, contractor, and Rocketdyne folks and the design deficiency was quickly identified. New parts were designed and fabricated and, in a matter of just a couple of weeks, we were once again ready for test.

Test A2J012, J-2X E10001: It ran the full, planned 7 seconds duration. The objectives for this test were to demonstrate that the clamshell, seal, and diffuser arrangement was properly working and to perform a bomb test in the main chamber. The testing arrangement worked perfectly and the bomb test did not reveal any combustion stability issues.

Test A2J013, J-2X E10001: It ran the full, planned 40 seconds duration. This was yet another bomb test and again there was no combustion stability issue uncovered. The neato thing on this test was that while the engine started to primary mode operation (i.e., 100% throttle), it switched to secondary mode operation (i.e., throttled) mid-test. This was the first operation of the complete J-2X engine (as opposed to just the powerpack portions) in secondary mode.

Test A2J014, J-2X E10001: It ran the full, planned 260 seconds duration. This test represented several more “firsts” for J-2X. This was the first time that the J-2X was started directly to secondary mode. It was the first time that the J-2X switched, in run, from secondary mode to primary mode. This was the first J-2X test with a stub nozzle extension that offered the opportunity to perform an in-run calibration of the facility flow meters and, in so doing, provide for a good estimation of engine performance. It turns out that E10001 is, to our best understanding, exceeding expectations in terms of required performance.

Again, the old narrative structure holds: New guy comes to town (the stub nozzle extension). The situation changes (new test stand configuration to accommodate the stub). Troubles ensue (the clamshell seal gets torn up). Resolution is found (new design for clamshell seal attachments). And a new situation is achieved (we’re knocking off successful test after successful test).

But, there is a twist (literally) to our denouement. I’ll explain this twist by starting with a picture:

Can you see it? This is a picture of the fuel inlet duct. Remember, this duct has an inner and an outer shell (or bellows as we call them) so that in between there will be vacuum to keep the hydrogen cold, like a Thermos® bottle. Between tests, one of the customary inspection techniques used to ensure that you’re good to go for the next test is to do a series of helium leak checks. You systematically pressurize different portions of the engine and make sure that everything is still sealed up tight. Well, when they pressurized this portion of E10001, they got what we’re calling “squirm.” If you look closely at the duct you’ll see that on the left-hand side the convolutions are bunched together and on the right-hand side they’re spread apart. This indicated that there was leak in the inner bellows of the duct so that the cavity between the two bellows was pressurizing with the leak-check helium. The squirm effect was due to the outer shell was deforming — squirming — due to that pressurization of the vacuum cavity.

Now, there are several important things to note about this. First, this particular duct is a heritage piece of hardware. It was not made for E10001. It was made during the Apollo era for J-2 and J-2S, forty years ago. It had seen its fair share of hot-fire history long before it reached E10001. Second, the new ducts being built for J-2X have a design modification that ought to mitigate this kind of failure. Third, we can see in the test data, with perfect hindsight, exactly when the leak occurred in test A2J014 and the engine ran for some time with the leak and nothing catastrophic happened. Thus, while nobody is happy when something breaks, in this case there’s no need for overreaction.

Getting back to the narrative structure and this little twist at the end, I kind of think of this like a teaser — a cliff-hanger — that leads to a sequel. Will our intrepid heroes dig their way out of this situation? Will the test program recover and move ahead to new successes and glory? Or will the monster creep up from the dark, dank Pearl River swamps and terrorize the test crew…?

…oops, wrong movie.

[Hint: We’ll be fine. Already moving out at full speed. In the immortal words of Journey (i.e., Jonathan Cain, Steve Perry, and Neal Schon) “Oh the movie never ends. It goes on and on and on and on…”]

J-2X Progress: Two Stands Occupied

It’s been awhile since I’ve had the opportunity to update what we’ve been doing for the J-2X development test campaign. So, everyone is probably wondering where we stand. Well, if possession is nine-tenths of the law, then J-2X IS THE LAW for the NASA Stennis Space Center A-complex! Right now, the J-2X development effort has our PowerPack Assembly 2 in test stand A-1 and Engine 10001 has been reinstalled on test stand A-2.

Below are two pictures of the J-2X PowerPack Assembly 2 (known as PPA2) taken from different perspectives. In the second one, you can see that several pieces are coated with ice. That’s obviously a picture with cryogenic propellants loaded in the ducts and turbomachinery. In other words, to use our local jargon, in the second picture PPA2 is chilled down.

Well, you saw in a previous blog article that we spun up the PPA2 and we demonstrated ignition of the gas generator. Beyond that, however, we’ve had a few hiccups. For the first test intended to get to mainstage operation, we didn’t get very far. We effectively demonstrated again the spin start and ignition of the gas generator. Immediately beyond that, just a few tenths of a second in fact, the test shut down due to an issue on the facility side. As I’ve described before, the PPA2 is kind of an odd beast in that it’s a half-engine and half-facility test article. In this case, a facility valve did not function the way that it was supposed to. It was sluggish. A subsequent investigation into the facility hydraulic system identified and fixed the issue so we were again all ready to go.

On the next test we got a little farther but just before getting to mainstage, we busted an engine-side redline limit and had to shut down early. The reason for that early cut was actually quite analogous to the early cut we had on our first attempt at a mainstage test for Engine 10001. We didn’t quite understand the characteristics of the engine components and so, as we powered up the system, we were headed towards an operating point different than we’d intended. In other words, our calibration was a bit off. The redline system identified this situation and, properly, cut off the test before anything damaging might occur. While early cuts are sometimes a pain in the neck, we have those safety systems built in there for a reason. There is always a substantial and meaningful difference between a nuisance and something potentially worse.

Over the course of the next couple of PPA2 tests we once again proved that hydrogen is a pernicious rascal. This is something that has been proven on many former occasions throughout the history of rocket engine development. If you give hydrogen any opportunity to leak, any at all, it will. And sometimes, it will only leak when the system is chilled down so that when you’re checking out the system before a test, when you’re searching for potential leaks, you don’t see a thing. But then, when you are all set up and get the test going, ta-da, you suddenly have a fire. Why a fire? Because with a hydrogen leak around all the rest of the hot stuff going on with the test, a leak almost always becomes a fire. And, because pooled, un-burnt hydrogen is a potential detonation hazard, we also have devices all around the vicinity of the test article designed to make sure that any leaked hydrogen gets burnt. So, quite simply: hydrogen leak on engine test = hydrogen fire on engine test. The fires that we saw on these two tests were not on the “engine” half of the PPA2 test article per se. Instead, we got fires on the facility half. The emergency systems in place for such issues include cameras and temperature probes so that there was practically no damage and our hardware is just fine. But the fires did mean that we’ve accumulated only a limited amount of mainstage data so far.

Undaunted, we have investigated and, we believe, solved the issue and will once again be ready for testing in the near future.

On the other test stand, specifically stand A-2, the folks at the NASA Stennis Space Center have been darn busy. If you go back a couple of months in these blog articles you’ll find a discussion about the next phase of testing for J-2X development engine 10001 (E10001 for short). In that article, I tell you all about the test stand passive diffuser and the engine nozzle extension that we’ll be testing. Well, the first thing that we had to do to make this next phase for E10001 possible was to modify the test stand. In order to make the passive diffuser function properly, you have to effectively seal off the top.

In the picture above you’ll see what’s called the clamshell. This two-piece device rotates out of the way for access to the engine between tests but during a test wraps around the nozzle of the engine on the top side and connects to the diffuser on the bottom side. We’ll use a rubber-ish seal in the gap between the clamshell and the nozzle to maintain the seal while accommodating movement of the nozzle during hot fire testing. Getting this thing designed, built, and into the stand was a heck of a lot of work. The folks who accomplished this deserve mucho kudos.

So, that’s the test stand side. Next, there is the test article side, i.e., the engine itself. Because the nozzle extension is not structurally beefy enough to support the rest of the engine, the installation of the test article into the stand has to be performed in two steps. First, you install the main part of the engine and then, once that’s in place, you install the nozzle extension.

By the way, while it sounds easy enough to simply bolt the nozzle extension into place on the end of the nozzle, it’s actually a bit more complicated. While both pieces are designed to be exactly round, nothing is truly exactly round, especially not pieces of hardware this large. We have to use special “rounding” tools during the mating process. It’s sometimes amazing to think about all of the specialized tools and equipment that you need, in addition to the engine itself of course, just to make the engine work.

So, that’s where we stand in terms of our development test campaign. As if southern Mississippi isn’t hot enough in the summer, J-2X will soon be adding even more heat from two active test stands very, very soon and for several months to come. Elsewhere, FYI, we’re working on various stages of fabricating and/or assembling J-2X development engines 10002 and 10003. They will be what follows PPA2 and E10001 into the test stands. In other words, there’s lots of excitement yet to come.

Welcome to the J-2X Doghouse: Old Dogs, New Tricks

A couple of articles back, I asked the following question:

“The whole orange-flame thing is not something I entirely understand…Any ideas from anyone else?”

I was talking about the flame stack during a night test at the NASA Stennis Space Center. It was a legitimate question. Combustion chemistry is really not my specialty. Lots of things are not my specialty. Try as I might, I’ve found that I can’t know everything about everything. Indeed, considering the many brilliant and knowledgeable people with whom I have the privilege of working here at NASA, I’ve come to accept the conclusion that there is a lot more stuff to know than can ever be learned. But that can never stop you from learning something new. And so I have with this.

In response to my blog question, we received a number of comments on the blog and those are posted. Thank you for your inputs and interest.

However, behind the scenes (so to speak), a coworker of mine, Robin Osborne, who does have experience with this kind of stuff read the blog and starting poking around amongst her notes and amongst her fellow experts in the field of flame spectroscopy. Below is a picture taken of igniter testing at MSFC using a gaseous hydrogen-oxygen mixture. Here too you can see a red-orange flame although it takes a distance for that colored portion to show itself.

According to Dr. Robert Pitz from Vanderbilt University, “Pure hydrogen (with no sodium) — air flames will glow red in a dark room due to the water vapor emission lines.” Both Dr. Joseph Wehrmeyer working in support of the Air Force and Richard Eskridge from NASA concur, noting that water vapor generates an orange-red-infrared continuum in such flames. However, all of these individuals also noted that there is a strong orange coloration in such flames due to sodium contamination within the hydrogen. The sodium is present as sodium hydride within the liquid hydrogen which decomposes at high temperatures to generate the vibrant color. The sodium contamination is a byproduct of how large, industrial quantities of hydrogen are made for uses such as, for example, flying the Space Shuttle. Dr. Christopher Dobbin of NASA noted that in the 1990 timeframe he was engaged in an analysis of the flame plumes ejecting from the Space Shuttle Main Engines (SSME). He said, “The (time) average sodium concentration we measured in the SSME exit plane was 0.091 parts per billion.” That doesn’t sound like much, and it’s not enough to impact engine performance or operation, but it’s still enough to measure based upon spectral analysis of the plume. Another possible contaminant, according to Richard Eskridge, is potassium and that can further contribute red emissions.

So, there you go. It’s a matter of water vapor at the right temperature and pressure (and therefore density) and a couple of key contaminants in the fuel. It’s “common knowledge” around here amongst us Datadogs that the plume of a Lox/Hydrogen rocket engine is clear. But that’s not entirely correct. It’s nearly clear. It still has the characteristic red-orange tint, but it’s at a density where the emission is too low to see. On the other hand, for the flame stacks at the test facility — the origin of this whole discussion — we’re talking combustion at atmospheric pressure so the water vapor products are denser as are the relative contamination levels since it’s a fuel-rich environment. And that’s why they show up at those brilliant colors in the nighttime pictures.

See, even old Datadogs can learn new tricks. Thank you to everyone who added their two cents, but especially to Robin Osborne for her inputs and insight.

J-2X Progress: Getting All Spun Up

If you go back through the J-2X Development Blog articles, you’ll find one about the “Burp Test” that we conducted last July on J-2X development engine E10001. In that case, we ran a very short test where we activated the helium spin start system and we ignited the main chamber, very briefly, before we shut down the whole thing. Well, here we are about six months later and we’re doing the equivalent thing on the J-2X PowerPack Assembly 2 (PPA2). Here is a video of the test:

Testing at night is always so much more dramatic.

For the PPA2, there is no main chamber to light, so this entire test was primarily focused on exercising the helium spin start system. The flames that you see are from flare stacks necessary to get rid of the hydrogen used in the test. Remember, the PPA2 is primarily a test article for turbomachinery and the gas-generator turbine-drive system. It doesn’t make thrust. All of that hydrogen that gets pumped by the fuel turbopump has to be disposed of in a controlled manner other than in the production of thrust. So, we burn it off. The liquid oxygen is disposed of as well, but it doesn’t require anything quite so gaudy as flare stacks.

Interestingly, when hydrogen burns, it usually burns clear. The whole orange-flame thing is not something I entirely understand, but it always looks that way at night. There’s some propane in the flame used as kind of like a pilot light, but not enough to cause that much color. It could be that burning hydrogen at such a low mixture ratio (i.e., not enough oxygen immediately available so you get afterburning effects) is the cause of this as compared to the usual white hot rocket engine exhaust. It’s also possible that it’s stuff in the air or somehow water vapor effects, or disassociation effects, but I honestly don’t know. Any ideas from anyone else? I’d love to hear some theories. I do know that if you’re standing anywhere where you can see the flame, you can feel the heat radiating from it. It’s quite an impressive experience.

Beyond exercising the helium spin start system, what this test also did is prove out the test stand subsystems, the test stand and test article control systems, demonstrates that the gobs and gobs of instrumentation is hooked up, working properly, and feeding back reasonable data, and that the proper procedures are in place to conduct a safe test. Every facet listed is a big, big deal and has to work in conjunction with everything else.

The folks at the Stennis Space Center — civil service, support contractors, and prime contractors alike — all deserve kudos for pulling this off successfully and, really, with minimal technical issues. Way to go guys! This test is yet another in a long string of demonstrations of the power of collaboration and the overall dedication and excellence of the J-2X team. We’re now ready to step into the meat of the test series and start putting the hardware through its paces. This is going to be exciting! Go J-2X!

J-2X Progress: A New Star on Our Horizon

For weeks and weeks (or months and months really), we’ve been going on and on about the star of our J-2X project, development engine E10001. And there is every reason to focus much of our attention on this first example of our new engine. It has really put on one a heck of a show, generating oodles of data, and we’re far from being finished with it.

So, E10001 is unquestionably a star. Beyond this, however, we have other potential stars waiting in the wings. I would liken this situation to “American Idol” except that I’ve never actually seen that show and, further, all of our test articles are not in competition with each other. Indeed, the whole point of a coordinated and integrated development plan is for all of the test plans and test articles to complement each other. One big star that will soon be making an important contribution is called “PowerPack Assembly 2” (or “PPA2”). Okay, you’re saying to yourself: I know what an engine is, but what is a “powerpack assembly”? And, why is this number two? Good questions. We’ll start with the first one…

A powerpack assembly — or simply a “powerpack” — is a subset of the total engine. Specifically, it is the engine minus the thrust chamber assembly (i.e., the main injector, main combustion chamber, and nozzle/nozzle extension). About a year ago, I wrote an article here in the J-2X development blog talking about what a gas-generator cycle rocket engine looks like. The schematic of that cycle is shown below for reference and comparison:

Where:
MCC = Main Combustion Chamber
GG = Gas Generator
MFV = Main Fuel Valve
MOV = Main Oxidizer Valve
GGFV = Gas Generator Fuel Valve
GGOV = Gas Generator Oxidizer Valve
OTBV = Oxidizer Turbine Bypass Valve

The lines and arrows in red denote fuel (hydrogen) flow; the green lines and arrows denote oxidizer (oxygen) flow; and the gray lines and arrows denote the flow of combustion products. Using the same abbreviations and same color schemes, here is the schematic for a gas-generator cycle powerpack:

See? As I said, you simply pull off the whole thrust chamber assembly and there you go: powerpack. If you think of the thrust chamber assembly as what you use to make thrust, then the powerpack portion of the engine is what you use to feed the thrust chamber assembly. In other words, to be particular, it’s the gas generator, the turbopumps, and the full set of major control valves…plus, of course, the lines and ducts that connect everything together.

What this configuration allows you to do, far more so than the complete engine configuration, is “play games” with turbomachinery conditions and operations. And here’s why. On the full engine configuration, you have to feed the thrust chamber assembly a pretty steady diet of fuel and oxidizer. If you deviate too far, things get too hot or too cold or you get too much pressure in the chamber or too little. The thrust chamber assembly is a wonderful piece of equipment, astonishingly robust when functioning in their normal regimes, but it’s basically static and, to be honest, a bit persnickety when it comes to significantly off-nominal operations.

So, you first get rid of the persnickety thrust chamber assembly to give yourself more flexibility and then, taking the next step, you get creative with the valves. On the complete engine configuration for flight, the J-2X engine has pneumatically actuated valves. As we’ve discussed in the past, this means that they have two positions to which they are actuated: open and close. We can’t partially open or close them and hold them in intermediate positions thereby altering or directly controlling the propellant flows through the engine. But for powerpack, we’re not so constrained. For powerpack, we will use electro-mechanical valve actuators for the two gas generator valves (the GGFV and the GGOV) and we will use hydraulically-actuated facility valves to simulate the two main valves (the MFV and the MOV). All four of these valves will then no longer be simply open/close. They can be held as partially open or closed and, using these as control tools, we can vary temperatures, pressures, and flowrates throughout the powerpack. We can vary the power with which we drive the turbines. We can vary the downstream resistances seen by the pumps thereby altering the flows and pressure-rise profiles through the pumps. The OTBV — the valve that we normally use to alter engine mixture ratio by applying differential power levels to the two turbines — will not be actively actuated for the powerpack testing, but it will be configured such that we can alter its fixed, incremental position from test to test. In that manner, we can use the OTBV position variations to explore inlet mixture ratio deviations on powerpack that the full engine configuration simply couldn’t tolerate.

Thus, the powerpack assembly configuration is first and foremost (though not exclusively) a test bed for the turbomachinery. Just as with the “bomb test” philosophy discussed in the previous article, we already know that the J-2X engine works, but now we need to further explore the detailed implications of the design. We need to anchor and validate our analytical models, demonstrate operations across the spectrum of boundary conditions and environments, better characterize our margins, and exercise the full slate of design features and operational capabilities. The powerpack assembly test series is one very important means for doing this.

Okay, so it’s a useful test article, but where does the actual Powerpack Assembly 2 stand? Well, while we’ve all been heavily (and appropriately) focused on the testing of J-2X development engine E10001, our contractor, Pratt & Whitney Rocketdyne, has been also quietly assembling Powerpack Assembly 2 back in the engine assembly area. Here is a picture of the complete Powerpack Assembly 2.

It kind of looks like an engine, almost, doesn’t it? Well, that’s because we assembled it kind of like an engine but used a “dummy” thrust chamber assembly. You should recognize the yellow thing that looks like a cage. That’s the nozzle simulator that we used early on in the assembly of E10001. Sitting on top of the nozzle simulator is a simulated main combustion chamber and a simulated main injector. By making it look so much like a regular J-2X engine, it allows us to install the PowerPack Assembly 2 into the test stand much like we do a regular engine. The only special adaptations are lines to catch the propellants coming out from the pumps and the discharge coming from the turbines. In a regular, full configuration engine all of these flows get routed through the thrust chamber assembly to produce thrust. For PowerPack Assembly 2 testing, these fluid streams are collected and disposed of off of the test stand.

Next is a picture of the PowerPack Assembly 2 being carefully loaded onto the truck to transport it out to the test stand. Road trip!

PowerPack Assembly 2 will be tested on test stand A-1, which is the sister test stand to A-2 where E10001 is currently being tested. Here, below, are a couple of pictures of PowerPack Assembly 2 being lifted onto and then sitting on “the porch” of A-1. In the background you can see a portion of the canals that weave in and around the big test stands at the NASA Stennis Space Center. Nowadays, these canals are mostly used just to transport barges full of propellants. But back in the Apollo era, these canals were used to transport whole rocket stages in and out of the test facilities since they were too big for trucking.

And here, is Power Pack Assembly 2 installed into the test position on stand A-1. Many kudos should be extended to our diligent contractor Pratt & Whitney Rocketdyne and our faithful partners at the NASA Stennis Space Center for making this milestone possible. Great work guys!

Now, getting back to that other question regarding the “2” part of “PowerPack Assembly 2.” That denotation is simply there because this is the second powerpack assembly we’ve tested as part of the J-2X development effort. PowerPack Assembly 1 testing was conducted about four years ago using residual hardware from the XRS-2200 (linear aerospike) development project. While that first PowerPack Assembly did not use any true J-2X hardware since that hardware was not yet designed or built, it did help inform the J-2X turbomachinery designs. It used what were essentially J-2S turbopumps to explore J-2X-like operating regimes. The J-2X turbopump designs then began with the J-2S designs and made the changes necessary to fulfill the J-2X mission. Another way of looking at this is that PowerPack Assembly 1 was used to inform the design and PowerPack Assembly 2 will be used to validate and characterize the design. To me, this sounds like a very nice pair of bookends on either side of the J-2X turbomachinery development effort.