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.

J-2X Progress: The Next Phase for E10001

In January, the Chinese people celebrated their traditional New Year and formally initiated the year of the Dragon.  I was born in the year of the Dragon (it comes up every twelve years) and I started thinking about previous Dragon years and where I was when they occurred.  My first year of the Dragon after my birth happened to be the 200th birthday of our great country and I was starting sixth grade.  My second year of the Dragon was the year that I got married so that was kind of important to me on a personal level.  My third year of the Dragon was the year that I started working for NASA after spending a decade working for defense and space industry contractors.  It is interesting looking at one’s life in such a series of widely separated snapshots.  Things move on.

The same is true for J-2X.  Last year was momentous for our project.  We assembled and tested our first development engine, E10001.  We celebrated and received well-deserved (if I do say so myself) kudos and pats on the back.  But now things move on and the life of our good friend E10001 enters its next phase.  And the next phase for E10001 involves changes to its nozzle configuration.  So, before I tell you specifically what we’re doing to E10001, we need to discuss how a supersonic nozzle works.

Below is a schematic of what, on a rocket engine, would be called the thrust chamber assembly or the main injector plus main combustion chamber plus the nozzle.  Within the realm of compressible flow this is known as a convergent-divergent nozzle, or as a “de Laval nozzle” after a late 19th-century Swedish engineer, Gustaf de Laval, who pioneered using such shapes as part of steam engines […and you woke up this morning not realizing that you’d learn something historical today!].  How it works is simple.  Fluid flows from high pressure at the head end on the left towards the low pressure at the exhaust on the right.  In between, the flow area of the “pipe” in which the fluid flows is manipulated to accelerate the fluid.  The most narrow point in the flow is called the throat.  Fluid flow to the left, upstream, of the throat is subsonic, i.e., traveling at less than the speed of sound.  If the ratio of “high” to “low” pressure at the two ends is large enough, then fluid flow to the right, downstream, of the throat is supersonic, i.e., traveling at greater than the speed of sound.  Under such conditions, the velocity at the throat itself is exactly that of the speed of sound.  In other words, the fluid is traveling at “Mach 1” at the throat [the term named for Ernst Mach, an Austrian scientist and philosopher also from the late 19th century].  Oh, and all of this only works if your “fluid” is compressible, or in other words a gas like air or, in a rocket, combustion products.

How and why this happens gets a little heavy on the thermodynamics, so please just trust me for now.  But the really neato thing that Mr. De Laval learned when playing with convergent-divergent nozzles like this is that: (1) for subsonic flow, as the flow area gets smaller, the flow velocity goes up, (2) for supersonic flow, as the flow area gets larger, the flow velocity goes up.  In other words, they act the opposite of each other.  For a rocket, this is absolutely fantastic since the whole idea of a rocket is to fling stuff out the back end at very, very high velocity and this cool device accomplishes that with just a little bit of creative geometry. 

Okay, with me so far?

Then, here’s another thing to think about regarding supersonic flow: You can’t shout upstream.  Sound is nothing more than pressure waves traveling through a fluid.  A gas has a characteristic speed at which pressure waves are conveyed within it.  That, then, is the speed of sound.  So, if the gas is traveling at greater than the speed of sound, then pressure waves cannot travel upstream.  Think of it this way: imagine yourself to be a gas molecule.  Normally, when traveling less than the speed of sound, you can receive signals from all directions.  Your motion can be impacted by pressure waves both upstream and downstream of where you sit at any given time.  However, now imagine that you are that gas molecule hurtling along in a supersonic flow.  Now, because you’re traveling faster than the ability of pressure waves to get back upstream, you can have no idea what’s going on downstream.  You’re flying along blindly. 

Thus, the bottom line is that once the ratio of high and low pressures are sufficient to cause this situation of supersonic flow in the divergent portion of the nozzle (a term that we use is that the throat is “choked”), then the nozzle flow is the nozzle flow.  In other words, it is largely independent of what happens beyond the exit plane.  Largely, but not entirely.  I’ll explain below.  Hold on.

Next, we’re going to talk about the Bernoulli Equation [developed by an 18th-century father and son team of Swiss professors Johann and Daniel Bernoulli].  No, we’re not going to do any math.  All that we have to do is understand the concept of the Bernoulli Equation and how it relates to the flow in the divergent portion of our nozzle.  Here it is:  Absent other factors, when fluid is accelerated, its pressure drops.  You can think of this in terms of energy.  Pressure is like stored energy, as in electrical energy in a battery.  Velocity is active energy, as in electrical energy spinning a fan.  Absent any other input or output, when you show more active energy (velocity), you then have less stored energy (pressure). 

Just for fun, here are some pictures of the men I’ve mentioned so far.  Oh, and I tossed in a friend of Daniel Bernoulli’s named Leonhard Euler.  Anyone who knows anything about mathematics or fluid dynamics knows all about Mr. Euler.  He was truly a genius on par with Sir Issac Newton.  (BTW, I kinda like the white, powdered wig thing the Bernoulli guys had going there.  Maybe I’ll adopt it myself…)

Back to the topic at hand.  Where do we stand once we combine compressible fluid flow through the divergent portion of a de Laval nozzle, traveling at speed greater than Mach 1 (meaning that pressure waves cannot travel upstream), and with the application of the Bernoulli Equation and the effect on pressure?  I will attempt to show you in a picture…

So, if I make my nozzle longer and longer and longer, with a larger and larger exit size, my exhausting gas goes faster and faster and faster.  Again, that’s why rocket engines have big divergent nozzles.  Ta-da!  But, there are limits.  There always are.  Nothing is free.

The first limit is weight.  As your nozzle gets bigger and bigger, your nozzle structure gets heavier and heavier.  As some point, any gain in engine performance is offset by the loss of vehicle performance because your engine is too heavy to lift.

The second limit is due to what’s on the other side of the exit plane.  What’s outside the nozzle is, well, the ambient environment.  If you’re sitting at the NASA Kennedy Space Center in Florida, where we usually launch our rockets, the ambient conditions are known as “sea level” conditions, meaning that the atmospheric pressure averages about 14.7 pounds per square inch.  On the other hand, if you’re floating around in space and in orbit around the earth, then your ambient conditions are, to a pretty good approximation, a vacuum, meaning 0.0 pounds per square inch pressure. 

What happens if you’re that gas molecule hurtling along in the flow at supersonic velocity down the nozzle and then you’re suddenly flung into ambient conditions?  Well, if you’re in the main part of the flow, not much.  You eventually slow down through a series of oblique shocks external to the nozzle.  As I said above, if you’re moving supersonically within the nozzle, then you’re not affected by what’s downstream.  But what if you’re not in the main flow but instead along the wall?  Here’s a secret: The flow along the wall is slower than the main, core flow.  Indeed, exactly at the wall, in the limit, the velocity is zero.  That changes things.

So, exactly at the wall, the velocity is zero, and just fractions of an inch into the flow the velocity is supersonic.  This transition zone is known as the “boundary layer” and the fluid dynamics complexity here can be nearly mind boggling and it has to do with viscous friction between the fluid and the wall.  But the important point is that there is a thin layer that is not supersonic.  Below is a typical textbook-like representation of boundary layer flow. 

Remember when I said that what happens beyond the exit plane largely doesn’t affect the fluid flow in the nozzle?  The boundary layer is the exception.  Because the flow here is subsonic, pressure conditions downstream can influence things upstream.  And here is the source of the other limit on your nozzle size. 

If the ambient pressure is much, much higher than the pressure of the nozzle flow, then this pressure can slow up the subsonic portion along the wall.  If you slow it up enough, you can make the boundary layer thicker and thicker until it’s no longer just fractions of an inch thick.  Having a thick boundary layer means that your nozzle is not flowing “full.”  The flow can become “detached” from the wall and such a situation is inherently unstable.  All around the nozzle, in local pockets, the boundary can grow and collapse and grow again causing localized pressure variations.  Shock waves start bouncing around.  Then the nozzle structure itself, usually not built very stiff so that it doesn’t weigh too much, starts to respond to these local pressure variations and shock waves and it wobbles and ripples and buckles.  To put is more succinctly, if your nozzle expands the rocket exhaust flow too much for the ambient conditions, you have an “over-expanded” condition and this can literally tear the nozzle apart.  Below is a picture that tells the story of the impact of ambient pressure on nozzle flow.

Now, finally, we’ll get back to J-2X E10001.

For all of the tests conducted to date, the nozzle that we’ve tested on E10001 has had an expansion ratio of 35 to 1, meaning that the area of the exit plane is thirty-five time larger than the area of the throat.  With this kind of expansion ratio for this engine, the nozzle flow is not over expanded.  The nozzle “flows full” at sea level conditions like those seen at the NASA Stennis Space Center (SSC) where we test the engines and all is good.  But the J-2X is intended to be an upper stage engine in flight, meaning that when it fires during the mission, it will be at over 100,000 feet in the altitude where the ambient pressure is much less than sea level conditions.  Because of that, we designed the engine to use a larger nozzle, get more performance from greater exit velocity, and not over expand the exhaust flow at THOSE conditions way up in the upper atmosphere, practically in space.

But then how do we test it?  If we have a nozzle that flows full at altitude, but does not flow full (i.e., it’s over expanded) at sea level, then how do we perform a test showing that the nozzle works?  We can’t exactly build a test stand at 100,000 feet in the sky.  Instead, we make the test stand simulate these high-altitude conditions.  Below is a picture of NASA SSC test stand A-2.  What you see there in the middle, the big tube several stories tall surrounded by structures, is the passive diffuser.

The diffuser, combined with a clam-shell enclosure structure around the bottom portion of the engine, uses Bernoulli effects (see, they come into play again!) such that when the engine is firing, it does so into an ambient environment that “appears” to be like that at high altitude.  By doing this, for the next phase of J-2X E10001 development, we will be able to do testing with a nozzle extended to an expansion ratio of 59 to 1.  That is one step closer to the ultimate flight configuration for the J-2X as part of the exploration mission and therefore one step closer to fulfilling that mission.  It takes a bit of explaining to understand why all this is necessary, but the bottom line truly is that we are getting closer and closer to our exploration goals.

So, enjoy come on along with us to celebrate the Year of the Dragon with the generation of lots of smoke and fire from the J-2X.  It’s going to be fun.  But first, maybe a few traditional Chinese New Year’s treats…

Inside The J-2X Doghouse: A2J006 &Turbopump Thrust Balance

Okay, before I even get to the subject of thrust balance, I want to report that J-2X development engine E10001 got back into the stand and that the next test, A2J006, was conducted.  The test went the full planned duration of 40 seconds and the issues that caused us to remove the engine after the previous test all appear to be fixed.  In other words, it was a complete success.  Here’s a video for your viewing pleasure:




Now, speaking of issues on E10001 that appear to be resolved, I want to talk about turbopump thrust balance.  While I hadn’t previously mentioned it, the data from the first several tests showed that there was a thrust balance issue with the oxidizer turbopump (OTP).  So, that naturally leads to the question:  What is “thrust balance?”


Here is a crude drawing of a turbopump for this discussion (And, please, save your cards and letters telling me that I am a terrible artist.  I will already catch plenty of grief from the turbo guys over this drawing.).  The arrows are intended to illustrate fluid flows.



I’ve drawn this in three colors because a turbopump can be thought of as having three fundamental elements.  The first element, the one in blue, is the part that spins.  This is the rotor onto which is attached the pumping stuff (i.e., the inducer and the impeller) on the pump side and the turbine stuff (i.e., the disks and blades) on the turbine side.


The second element, shown in green, is the part that doesn’t spin.  This is the housing.  It is the shell within which the rotor spins.

The third element, shown in red, is the bearings. 


Whenever you’ve got one thing spinning and another thing stationary, you’ve got to have some way of communicating between the two pieces.  Maybe the rotor just spins and slides around in some kind of restraint holding it in place.  Well that might work except that you can’t put grease into a cryogenic oxygen environment and, should too much heat build-up due to friction, then you’ve got a very combustible situation.  Mix heat with a pure oxygen environment and anything and everything will burn, including metal. 


What we do instead is typically use ball bearings between the rotor and the housing.  In some other pumps we use roller bearings that look like little cylinders rather than little balls, and there are other solutions as well.  These bearings do two things, they allow the rotor to spin and they hold the rotor in place.  So they need to be loose enough to spin yet tight enough to maintain control of a heavy hunk of metal spinning at over 10,000 rpm.  How much force the bearing put on the rotor to maintain control and, conversely, how much the rotor puts back on the bearings is called the bearing load. 

When you are designing a turbopump, you design it so that the bearings have a specific load, and that means more than just a value, it also means a direction as shown in the sketch above.  You design the whole bearing package assuming that you know the magnitude and direction of the forces loaded through the package.  This then allows you to do some calculations that tell you, “Yep, I’ve got the rotor properly and sufficiently controlled.”  Those calculations are part of what is often referred to as rotordynamics (and, by the way, NASA MSFC has some of the world’s best experts in that area).


Question: How do you get different bearing loads in a turbopump?  Answer: Pressures.


Here is a nifty little fact: Pressure is all around us.  We don’t feel the atmospheric pressure of 14.7 pounds-force over every square inch of your body because, well, it’s always been there.  That’s the environment within which we’ve evolved.  Now, go to the bottom of the ocean and you’ll find a whole other pressure environment, one that would crush our delicate bodies into mess.  The basic principle to remember is this: Pressure applied over an area equals force.


Within a turbopump, you are explicitly manipulating pressure.  That’s the whole purpose of a pump: to move fluid through the use of pressure.  So, in the pump end you have a low pressure coming in at the inlet and a very high pressure at the outlet.  On the turbine end, it’s the opposite since on that end you’re extracting energy (and therefore pressure) from the fluid.  So, on the turbine end you have high pressure at the inlet and lower pressure at the outlet.  Thus, throughout the turbopump, you’ve got pressures of all sorts and those pressures are pushing on the different pieces of the rotor.  This means that you’ve got all kinds of forces pushing on the rotor.  Some of the forces push the rotor towards the turbine end.  Some of the forces push the rotor towards the pump end.  Getting the right balance of forces is called your thrust balance.




So, what’s the “right” thrust balance?  It is the one to which you’ve designed your bearings.  You design your bearings to have a certain load and a big part of that load is determines by the thrust balance on the rotor.  So, if your thrust balance is off, i.e., not what you intended, then your bearing loads are off and you’re therefore not controlling the rotor the way that you wanted.  In the worst of circumstances, an out-of-control rotor represents a catastrophic situation.  In less-than-worst circumstances, operating with a thrust balance significantly different than the design intent could lead to unexpected wear on components and therefore limited operational life. 


The next question is this: How do you create the thrust balance that you want and need to accomplish all this? 


That’s a matter of designing the internal guts of the turbopump and analytically modeling all of the internal flows within these guts.  In addition to the big flows coming into and out of the pump and turbine, you’ve got internal flows around seals, between pump or turbine stage, and through the bearings (as coolant).  When operating, the whole turbopump is full of fluid.  And, everywhere you have fluid, you have a pressure; and everywhere you have pressure applied to an area, you have a force.  So you calculate pressure drops through small passages or through seals and figure out a whole detailed map of different pressures and forces throughout the whole unit and you can understand your thrust balance.  And then, as necessary, you manipulate your clearances and seals and passages designs until you achieve the thrust balance you need.




Getting back to the E10001 OTP, what appears to have been the issue was that we had some seals that were too tight.  The tight seals overly restricted internal flows and, basically, messed up the intended force balance on the rotor.  This could be seen in the data collected from the testing.  Remember when I said several articles ago that this engine is instrumented to the max so that we can learn all about how it is working.  Here is a case where special instrumentation identified an issue.  The interesting part is that these seals were kinda sorta fine in terms of design but the processing of the material of which they are made — it feels like hard plastic — was not quite refined enough so that when subjected to operational conditions it changes dimensions and actually tightened up. 


Prior to test A2J006, while we had the engine out of the stand, our contractor Pratt & Whitney Rocketdyne was able to partially disassemble the engine and the OTP just enough to change out the seals in question and then get the whole thing buttoned up and ready for test in record time.  They truly did some excellent work here.  And the success of test A2J006 is the proof!


J-2X Progress: Engine 10001 Testing Status

When last I discussed with y’all the status of testing on the first J-2X development engine, E10001, I showed you a video of our first mainstage test, A2J003. That article was posted just over a month ago. So, what’s been happening since then? Hopefully, this brief article will catch you up to where we are.

First, we had test A2J004 in early August. This test went the full, planned duration of 7 seconds. This was our first test with any sustained duration at mainstage conditions. Lots and lots of good data. The Datadogs and the analysts were absolutely giddy for days and days afterwards.

We then had test A2J005 in mid August. This test was scheduled for 50 seconds duration but it was cut off at 32.2 seconds due to a not-totally-unexpected redline cut. This redline cut was similar to the one described for A2J003. In fact, it was the same parameter although the test conditions were different. You can stomp your foot, or kick at the air, moan, whine, whatever; it doesn’t really make any difference (…which is not to say that we did any of this … well, okay, maybe a little whining…). This is all simply part of the learning process with a new engine. It is expected, necessary, and educational. Deep breath and all is well.

Here is a video of test A2J005: J-2X Fifty Second Test

However, after test A2J005 shutdown — several seconds after shutdown — we had a “pop.” These are not uncommon when ground testing rocket engines so let me use yet another automobile analogy (since I use them so often) to explain.


Just a few years ago, I was still driving my 19-year-old pickup that I bought just before grad school. Towards the end of its long and glorious life, my little red truck developed a habit of rumbling and grumbling to a stop long after I’d turned the key to “off.” Residual gasoline fumes and air and hot metal combined into a sequence of “blurrr, blurrr, cough, cough, blurrr … POP” (and the neighbors just loved that!). In essence, a similar thing happened on A2J005.

In flight, the shutdown environments are quite different than on the ground. When you’re way, way up in the atmosphere — or even outside of the atmosphere — the surrounding environment is effectively a vacuum, which is a very useful means of sucking out any residual propellants from the engine after shutdown. On the ground, residual stuff (leftover propellants and fuel-rich combustion products) doesn’t get pulled out as efficiently. We can and do push it out with inert gas purges, but they’re not always as effective as we’d like. And so, on A2J005, we didn’t get everything pushed out as well as we’d like and we had a “pop” — or, actually, a “POP.” Technically speaking, it was a detonation in the “lox dome,” i.e., the oxidizer manifold volume feeding the main injector.

The bottom line is that we had some damage. It wasn’t anything that couldn’t be fixed and so, we’re fixing it. We didn’t bend any metal or anything like that. It’s more of breaking something that’s kind of like really tough plastic. Throughout this article you’ll see pictures of the engine being pulled out of the test stand because we couldn’t do the repair in place. After removal, we then took the engine back across the NASA Stennis Space Center to the assembly facility. In other words, we took it back to the garage for a tune up and we’ll be back in business soon. In fact, E10001 will be reinstalled by mid-September and we’ll be back into testing right around the start of October. Oh, and we’ll be doing a better job of avoiding pops based on what we’ve learned about purge rates and durations.

So, that’s where we stand. Two things to look forward to in the future: first, the upcoming report on test A2J006 after it happens, and, second, a discussion of another change we made to the engine while we had it in the shop. That latter discussion is an interesting technical tidbit regarding the internals of turbomachinery. So, “Don’t touch that dial!” and keep your set tuned to this station…

(I think that my grandparents had this exact set! Ahh, the good old days of vacuum tubes and horizontal hold.)

Inside The J-2X Doghouse: Engine Control — Open versus Closed Loop

As I was driving to work this morning, I came up over a rise and saw suddenly appear in my windshield, over towards the left on the other side of the road, a police cruiser with a radar gun mounted in the window.  Even before I could think about it, the pressure of my right foot on the accelerator lessened.  I then instinctively looked at the speedometer and found that I was traveling 48 miles per hour on a road for which the speed limit is 45 mph.  Thankfully, the officer apparently forgave me the 3 mph violation and continued to wait where he was for a better opportunity to serve and protect the community. 


What I find interesting about that little episode was the immediate, unthinking response I made in response to seeing the cruiser.  In terms of control systems, that could be called a feedback loop.  My senses received data, my brain rapidly processed that data and then sent a signal to react to the data, and then my calf muscles responded by easing up on the throttle.  The car, in turn, slowed to respond to the lower throttle.  Never mind, of course, that if I’d been really, really speeding all this would have been too late (and I would have arrived at work very angry), there was nevertheless a closed-loop response that is not too dissimilar to what we do for rocket engine control … in some cases.


First, let’s get familiar with a couple of terms –


Open-loop: We typically refer to something as open-loop when we have instrumentation that measures conditions in the engine but the engine itself does not respond to those measurements.


Closed-loop: We typically refer to something as closed-loop when we have instrumentation that measures conditions in the engine and then, potentially, the engine takes action based upon those measurements.


It is based upon those definitions that I would call my response to seeing the cruiser closed-loop since I responded and did something with the data.  Another example would be the more modern systems that are used to monitor and control automobile systems.  It used to be that you had a temperature measurement stuck in the coolant loop.  You could watch the temperature rise, but until the system went kaput and boiled over leaving you stranded alongside the road, there was no active, closed-loop control.  Nowadays, if the computer in my pickup truck sees that the engine temperature is too high, it will take action to try and protect itself.  For example, it will inhibit the use of the air conditioning system since that represents an additional power requirement on the engine.

 So, what do we control on a rocket engine?


One thing that we control is power level.  On the Space Shuttle Main Engine (SSME), power level is controlled in a closed-loop manner.  This means that the main combustion chamber pressure is measured as an indication of thrust level and in response to that measurement a valve is opened or closed to increase or decrease the engine power level.  On the J-2X, power level is controlled in an open-loop manner.  This means that we measure the main combustion chamber pressure but we don’t have any feedback loop where we control a valve to ensure that we’re on target.  Instead, should we happen to be off on power level, we have to physically change an orifice in the engine between tests.  The “feedback loop” is data analysis and a guy with a wrench.  Which approach you choose to take are dependent upon your requirements of performance and affordability. 


Another thing that we control on a rocket engine is the mixture ratio (i.e., the ratio of oxidizer to fuel).  Given that on a rocket you are carrying both your oxidizer and fuel with you in the vehicle, you certainly want to make sure that you consume your propellants in the correct ratio to get the most uumph out of them.  Again, on SSME we control mixture ratio in a closed loop manner.  There is actually a small flowmeter on the SSME and, using the data from that flowmeter (and some associated calculations), we move a valve on the engine to dial in the correct mixture ratio.  It’s a pretty nifty system.  Also again, on the J-2X, we have an open-loop system for mixture ratio just like we have for power level.  We test the engine, look at the results, and, if necessary, make a physical change to the engine in the form of an orifice.


Because of these two areas, power level and mixture ratio, SSME is usually referred to as a “closed-loop engine” and J-2X is usually referred to as an “open-loop engine.”  Now, this terminology is not entirely correct since there are some closed feedback loops within the J-2X control system pertaining to engine health and status diagnostics, but we all know how enduring shorthand designations can be.  Also, engines don’t have to be one or the other.  They can be half-and-half.  The engine used on the Delta IV vehicle, the RS-68, sort of falls in this category. 


How you choose to design your engine control system is driven by your requirements.  Put real simply:  The SSME is all fancy-schmancy because it had extremely tight power level and mixture ratio precision requirements and because it was a reusable engine.  The J-2X is intentionally more simplistic because it has looser precision requirements and because it is expendable (and throwing away orifices is a whole lot cheaper than throwing away valves if your requirements will let you get away with it).  Requirements drive design.


Note that I will save the fun topic of engine diagnostics — and the potential for long philosophical meanderings within that realm — for future posting. 




Let’s end this posting with a fun little exercise.  Above is a simplified schematic of a gas-generator cycle engine kind of like a J-2X.  I have shown in the schematic two orifices #1 and #2 (highlighted in yellow).  With those two orifices, we can calibrate the engine.


Scenario:  Power level too low, i.e., measured main combustion chamber pressure too low.

·         Solution:  Increase the size of orifice #1.

·         Explanation:  By increasing the size of orifice #1, I will deliver more oxidizer to the gas generator.  This will deliver more power to both turbines thereby increasing how much propellant gets pumped into the engine.  More propellants pumped in equals more thrust and greater overall power level.


Scenario: Power level too high, i.e., measured main combustion chamber pressure too high.

·         Solution:  Decrease the size of orifice #1.

·         Explanation:  The exact opposite of the previous scenario.


Scenario:  Mixture ratio too low, i.e., the flow of oxidizer is too low in proportion to the flow of fuel, as measured by the test facility.

·         Solution:  Decrease the size of orifice #2 and decrease size of orifice #1.

·         Explanation:  By decreasing the size of orifice #2, I decrease the amount of flow that is diverted around the oxidizer turbopump turbine.  I therefore increase the flow through the turbine thereby increasing pumping power of the oxidizer side.  So I increase oxidizer flow to the engine.  However, by increasing oxidizer flow to the engine and doing nothing else, I’ve probably messed up my overall engine power level so I’ve got to back down a little bit by decreasing the size of orifice #1.


Scenario:  Mixture ratio too high, i.e., the flow of oxidizer is too high in proportion to the flow of fuel, as measured by the test facility.

·         Solution:  Increase the size of orifice #2 and increase size of orifice #1.

·         Explanation:  The opposite of the rationale for the scenario immediately above.


See, being a rocket scientist isn’t that difficult, really.  Now you too can calibrate an open-loop rocket engine.



P.S., I read in the paper this morning that NASCAR racer Kyle Busch had his civilian driver’s license revoked for 45 days for doing 128 mph in a 45 mph zone.  Well, at least I wasn’t going that fast when I came over the rise this morning and saw the police cruiser.  Then again, I wasn’t driving a $400,000 Lexus LFA sports car like Mr. Busch was…





J-2X Doghouse: Okay, So We Ain't That Smart — Yet

Welcome back to the J-2X Doghouse.  We’re going talk about some test results and test data, exactly what Data Dogs love most to do.

Back in the day — back before I had the carefully regulated, federally mandated, and strictly enforced lobotomy that allows people into the ranks of management — I was once an analyst.  And, since it seems so long ago that it doesn’t sound like bragging anymore, I will admit that I was pretty good at it.  I absolutely loved the process of using fundamental physics or empirical correlations for fluid dynamics, thermodynamics, and heat transfer all together to simulate in computer coding how things function in the real world.  Whereas many people enter the field engineering because they like mechanical things or electronic things, there are some of us who relish the seeming purity of problem solving in abstraction.

 Over the years, working on many diverse projects and building many diverse mathematical models to simulate many diverse systems, I came to the realization that my models always appeared most unassailable and brilliant when there was no test data against which to compare them.  To put it bluntly, test data always proves that you simply ain’t as smart as you thought you were.  But, that’s okay.  If that wasn’t the case, then you wouldn’t bother to test.  The whole point of testing to gather data and learn more.

With all due respect to the Serenity Prayer, this ought to be the analyst’s prayer relative to testing:

Grant me —
— the results to validate that which I do understand
— the data to explain that which I did not understand
— and the openness to accept that I can always understand better

That last line is critical.  Ignoring data contrary to what your model output is a seductive, addictive, and dangerous path to follow.  We don’t/won’t do that.

That brings us to the subject of test A2J003 of the J-2X development engine E10001.  This was our first test to mainstage operation.  The planned duration was to be seven seconds.  On Tuesday 26 July, right around five in the afternoon, the test ran for 3.72 seconds and then shutdown.  We did not accomplish the full duration.  Why?  Basically because we ain’t as smart as we thought we were.  We had analytical models telling us that performance would be X, but the hardware knew better and decided on its own to perform to Y.  Here is a cool video of the test:

J-2X Engine Test A2J003

A more detailed explanation of what happened is that the engine shutdown due to the measurement of a pressure too high in the main combustion chamber.  The measurement crossed a pre-set “redline” and the test controller unit took the automatic (and autonomous) action of shutting down the engine in a safe and controlled manner.  The high pressure in the main chamber was caused by higher than expected power coming out of the oxidizer pump.  This, in turn, was due to more power being delivered to the turbine side than expected.  It comes down to a fluid dynamics phenomenon (pressure drops) and what we have is not inherently bad, just different than expected.  So, in essence, we used our models to predict that the pressure in the main chamber would be at a certain level — indicating a certain power level — but the different performance of the hardware resulted in pushing us away from our analytical prediction.

  • Here is the good part:  We learned something.  We learned that our model needs to be updated and we collected the data that will allow that to happen.
  • Here is another good part:  We got enough data, despite the short duration, to recalibrate the engine for the next test thereby making it far more likely that we will hit our target. .
  • Here is yet another good part:  We had a successful demonstration of the test controller redline system by safely shutting down the engine.  The engine looks fine.  The controller did exactly what it was supposed to do and protected the hardware.  In fact, for these early tests we have the redlines clamped down pretty tight specifically to protect the hardware as we learn more about the engine..
  • And here is, finally, yet another good part:  Other than the power applied to the oxidizer turbopump, most of our other predictions with regards to hardware performance appear to be awfully darn good.  So, we’ve got a preliminary validation for much of our understanding of the engine.  Indeed, this is a brand new engine and we have just accomplished mainstage operation in the second hot-fire test.  That is truly unprecedented..
  • Here is the bad part: We have to spend a few minutes explaining to folks not directly involved that despite not achieving full duration, the test was in reality a total success.

If that, then, is the bad part, I can live with it.  I can live with admitting that we ain’t as smart as we thought were.  Why?  Because now, after the test, we are indeed smarter.  And we will continue to get smarter and smarter about the J-2X design until, one day, we will be smart enough to say that, yes, we understand this engine so well that it is safe enough to propel humans into space.

J-2X Progress: Pardon Me

I’ve lived in the South for about 20 years now.  Over that time, I’ve heard and learned all kinds of quirks of regional dialect and colloquial sayings.  I’m quite sure that I’ve even picked up a few myself.  Oh well.  One particular expression shared with me by a former training partner in the gym was a compliment.  She told me that I had good “home training.”  That is a shorthand way of saying that my parents taught me to have good manner. 

With that in mind, and in consideration of what we’ve been calling the first hot-fire test of J-2X development engine E10001 (i.e., the “burp test”), I respectfully and bashfully declare:


In the early evening of Thursday, 14 July, E10001 generated a burst of ignition and thrust with something like a 30,000 pounds of force — enough to toss five or six crew-cab pickup trucks into the air.  That’s one heck of a burp.  Yep, we were successful.  NASA Stennis Space Center in Mississippi, coordinating with the Upper Stage Engine office at the NASA Marshall Space Flight Center and with Pratt & Whitney Rocketdyne in Los Angeles, California conducted a full-duration, 1.9-second test.  Here is a picture of what a burp that big looks like:



Here are a couple of pictures of the engine prior to the test.  These are taken from a deck inside the test stand basically looking down on the engine.  The nozzle extends through the deck to the next level below.



You’ll note that there’s some misty fog hanging around the engine in those pictures.  While it is true that most of the time you can almost see the thickly humid air of southern Mississippi in July, this is something different.  This fog is being created by the presence of cryogenic propellants in the lines.  These lines are so cold that they effectively condense water in the air around them, even at a distance, to create a fog.  Here are some close-up frosty pics of lines chilled down prior to test:



Out of one end comes smoke and fire while the other end is frosty cold.  This is a good illustration of the broad span of environments that exist within a rocket engine.

So, what’s next?

As exciting at this brief test was, what we didn’t do is light the gas generator and get the turbomachinery up to full speed.  That’s the goal for the next test.  In addition to spinning up the turbopumps with helium and lighting the propellants in the main chamber, as we did for the burp test, we’ll take the next step in the start sequence and light off the propellants in the gas generator.  This will provide the turbopumps with the power necessary to reach mainstage, steady-state operation.  And that — those initial few seconds of mainstage —  will be our first glimpse at genuine engine operation like that which will propel spacecrafts into orbit and then outwards across our solar system.

J-2X Progress: The Burp Test

On my very first home computer, I had a silly little program — made by the marketers for the Monty Python brand name I believe — that turned the keyboard into collection of funny or disgusting or borderline obscene simulated sounds of bodily functions.  Several keys triggered a variety of sneezing sounds.   Another set of keys activated a broad range of burping sounds.  Another set of keys set off sounds inappropriate for further discussion within a NASA blog.  And, of course, there were handful of sounds that simply left you scratching your head.  I guess that one should feel heartened by the notion that even at a time when the sterile realm of machines seem to be taking over our lives, we still revert to our childish fascination and amusement with the functions of our quirky bodies. 


And so, in that light, I give to you the J-2X Burp Test.  No, that is of course not the official name.  The official name is the “J-2X Ignition Test” or, even more formally: Test A2J001.  That really rolls off the tongue doesn’t it? 

Etymological dissection of “A2J001”
     • “A2” because the test is happening on NASA Stennis Space Center test stand A-2.
     • “J” to distinguish this from 30 years of Space Shuttle Main Engine Testing data records related 
        to test stand A-2.
     • “001” because, well, it’s the first test

The first test of the first J-2X development engine will have a duration of 1.9 seconds between the time that the engine receives a command to start and the time that the engine receives a command to shutdown.  That is not a long time.  It is, indeed, not a whole lot more than an extended, impolite belch considering that the engine is designed to ultimately roar for a full eight or ten minutes for full-duration tests.

So, why do such a seemingly silly little test?  That’s a valid question and the answers back are just as valid.  We have a wide range of objectives for this test.  For example, this will be the first time that cryogenic propellants (liquid oxygen, liquid hydrogen) will have been loaded into the engine and the lowest reaches of the facility feedlines.  Remember, these fluids are so cold that they make metal shrink.  You have to design the engine and the facility to resist or accommodate this thermal stress.  And while you continuously check for leaks as you assemble the engine and the facility, nothing is ever quite like a good cryogenic chill for finding where seals might separate. 

Also, during a chill test you want to make sure that you can get the engine cold.  I know that that sounds funny, but it is possible to have enough ambient heat going back into the metal of the hardware such that it overwhelms the capacity of the cold fluids to take it away.  Essentially what happens is the cryogenic fluids boil when they hit warmer metal.  Boil?  Like water in a pot on the stove?  Yes, but remember that liquid hydrogen boils at about 420 degrees below zero and liquid oxygen boils at about 300 degrees below zero (both Fahrenheit).  What you want is for the hardware to get so cold that the boiling stops.  This is accomplished by continuously flowing new, fresh, cold stuff through the hardware via a bleed line.  During the chill test, you monitor the conditions within the engine and of the fluid coming out of the bleed line.  When you get to a suitably cold, steady state situation, then you’ve successfully chilled the engine.

Why is this important?  First, you run this test to make sure that you actually can chill the engine.  Not only do you design the engine to run, you have to design it to be able to get to this chilled state during a launch sequence.  Second, the engine needs to be chilled because if you have any boiling liquid in the pumps when it is time to start, that boiling represents voids in the fluid.  The movement of the pump will exacerbate these voids and potentially convert even more of the liquid into gas.  The pumps are not designed to pump gas and so the result is that the engine could go off mixture ratio, or it could fail to start, or it could even head rapidly towards a far more exciting failure situation.  Like a good martini, really chilled is really good.

Next, after the long chill, like a long, filling meal, comes the … BURP …

The 1.9 ignition test will demonstrate: the use of the helium spin-start system, ignition of the augmented spark igniter and the main injector, and the functioning of the start continue logic software.  Now, explaining one at a time — 

In your automobile, you’ve got an electric motor that, when you turn the key (or push a button these days in some fancy cars), spins the motor to life.  We’ve got essentially the same thing on the J-2X.  There are different ways that this could be accomplished, but one of the cleanest and simplest is to use the inherent functionality of the turbines and provide a burst of power in the form of high-pressure helium.  The helium flows through the turbines, spins up the pumps, and thereby builds pressure throughout the engine making it primed for the rest of the start sequence.  The important features that will be demonstrated with the planned short test is the careful timing of the sequence and the tailoring of the pressure profile supplied to the turbines to yield the desired pressure build up on the other end, in the pumps.

The augmented spark igniter (ASI) is, effectively, a torch lit off by a pair of spark plugs.  This small hydrogen-oxygen torch resides in the center of the main injector and it is what lights off the propellants in the main chamber during the start sequence.  Just like you don’t start a campfire by holding a match against the biggest log in the pile, the ASI provides the kindling to get the main fire going.  The burp test will demonstrate the effective discharging of our high-tech spark plugs and achieve ignition of the ASI and the main chamber.  They will not be lit for long, but just long enough to characterize the process.

Just like everything these days, the J-2X and the entire test facility element are run by software.  Streams of 1s and 0s are taking over everything.  And while J-2X does not have an exceptionally complex control system, there are a handful of absolutely critical feedback loops that must function properly.  The “start continue logic” is composed of a set of criteria that tell you, as the engine progresses through the start sequence, that it is okay to continue with the process.  Being not “okay” in this case means that you could be facing a catastrophic situation and that you must halt the engine starting process in order to ensure safety (of the engine, of the test stand, and, in flight, of the vehicle and crew).  Now, for this short test, it is extremely unlikely that we would be building up enough energy to damage much of anything even if things did go awry, but what is important is the demonstration of the closed loop of imposed software limits, measured parameters, the application of software logic, and the confirmation that all is well.  Considering that the engine and test facility is being entirely controlled by a group of people in a secured building that is hundreds of yards away, making sure that you’ve got complete control of the test facility and test article, and complete insight into what’s going on via instrumentation, is pretty darn important.

So, that explains the strategy behind the first test of J-2X E10001.  What we will not be doing is lighting the gas generator.  That would be the next step in the start sequence: spin up the pumps, open the valves, light the main chamber, and then light the gas generator.  We will then be just one step away from ramping up to full power.  We’ll save that step for next test. 

To be entirely frank, this first won’t be very impressive for uninvolved bystanders, it probably won’t even be as much fun for a lot of people as would be the silly/disgusting bodily function sounds on my very first computer, but for those of us down in details, this burp test is a vital full — dress rehearsal before the real fun begins of genuine, mainstage engine testing.  It represents yet another significant milestone on our path towards completing J-2X development.  Go J-2X!


J-2X Extra: The Rocket Engine Development Life Cycle

Everyone has in their personal histories certain events that they can look back on and say, “That’s where things changed for me.”  Some of these could be planned rites of passage like the first day of kindergarten, joining a little league team, getting your driver’s license, and high school graduation.  Others are not necessarily foreseen or planned but seem in retrospect like inevitable events such as a championship football game, falling in love, or your child being born.  Everyone has a history, a story.  It is the narrative of how we come to be who we are.  Believe it or not, rocket engines also have a life story of how they come to be.

Below is a timeline of a rocket engine development life cycle (and, yes, that is actually what we call it: “life cycle”).


The first thing that you’ll notice from the drawing is that there are a bunch of NASA-typical three-letter abbreviations.  Sorry for that, but I promise that I will explain them.  Generally, think of these three-letter abbreviations as milestones or as gates through which the engine development project much pass.  For the rocket engine, these are its planned rites of passage.  The gray bars below the timeline of milestones describe the general activities during different phases of the development effort.  Note, however, that these are general notions and every engine development project is different.  For example, if what you’re doing is a change to an existing engine or if you have components already developed lying around and waiting to be incorporated into an engine, then perhaps you could do earlier component testing.  Also, not shown on here are such things as subscale or laboratory testing.  These too can often occur earlier in the life cycle thereby informing the design of the system.  On the other hand, if you’re starting entirely from scratch, then you’ve got to do a good bit of work before you would even know what to wring out in the laboratory.  In that case, everything can get pushed outwards to the right.

The first milestone gate in the life cycle is Authority to Proceed (ATP).  For activities as substantial as rocket engine development, we don’t go off willy — nilly and decide for ourselves when to start such a thing.  While that might be fun for a little while, it is a generally accepted and overarching rule — of — thumb that prison is something we should to endeavor to avoid.  There is a whole chain of command that ultimately flows down from Congress and the Administration.  Thus, at ATP we essentially get a formal charter to fulfill certain objectives such as, in this case, develop an engine.  Also, with that charter comes the authority to spend time and money in pursuit of the objectives.  The authority part is key.  And so, not much can happen until ATP.

Next, you have the milestone of the System Requirements Review (SRR).  Before you can design something, you need to know what it is that you need and expect it to do.  Now, some requirements development had better have happened at a higher level before you even began the project — otherwise, how would you even know that you need to start developing a rocket engine versus, say, a rocking chair? —  but getting a set of valid, system-level functional, performance, physical, and safety requirements is extremely important.  In addition to the system requirements pertaining to the engine, the other stuff that is part of the SRR is a whole slate of the programmatic documentation that forms the infrastructure for the organization responsible for the development effort.  This review therefore establishes the foundation of the product and the processes for the development effort to follow.

For all of these “reviews,” what these represent is, usually, one great — big meeting, sometimes taking several days, multiple smaller meetings, detailed review, comment, and response periods on documentation and drawings and analyses and test data, and a final, formal series of technical and programmatic board meetings airing any issues found, citing accomplishments identified, and declaring the success or failure of passing through the gate.  They are periods of intense activity and high internal and external scrutiny.

The next milestone in the timeline, the System Definition Review (SDR), is sometimes combined with the SRR.  The objective of the SDR is to demonstrate that you have a concept for the rocket engine that is plausible, feasible, and achievable given programmatic constraints.  However, since you need to have something in your head that is plausible, feasible, and achievable as part of the validation of your requirements set, separating SRR and SDR does not always make sense.  For J-2X, for example, we conducted the two of them as a joint review.

The first true design review is the Preliminary Design Review (PDR).  The question at this review is, at its root, pretty simple:  Do we have the right design?  Now that we’ve spent some time doing analyses, performing trade studies between subsystem design concepts, perhaps running component or subscale tests, laying out the initial drawings, can we say with sufficient confidence that we have a functional design that meets our technical requirements within the limits of the time and money we’ve been allocated?  To me, more than at any other moment in the development life cycle, this is that pivotal point.  If you fail here, everything stops, as it should.  It means that you’ve been working on the wrong design.  Start over.  However, if you are successful, then you start ordering materials to start building the engine and you commit to completing the design.  The stakes are very, very high.

The next milestone is an interesting one.  For many, many development efforts other than rocket engines, the Critical Design Review (CDR) is The Design Review.  For these other projects, it represents a true completion point for the final, to-be-flown design and it often takes place after prototype fabrication and testing.  But for a rocket engine development effort, the CDR takes on a somewhat different flavor.  This is because just building a rocket engine for testing can take several years and the necessary extensive test program that can also consume a couple of more years of activity.  Thus, for a rocket engine, the CDR is focused on getting the right design into the test stand.  The question to be answered is this:  Are we still on the right path, with the design essentially finished, to commit to the rest of the effort?  Therefore, you review not only the design (and associated matured analyses) but also all of the planning documentation that explains how the engines that you are already building will be used to demonstrate that the design meets all of the imposed requirements.  In essence, you have to prove that all of your ducks are in a row because now you’re getting into some serious welding, grinding, shaping, and cutting of metal. 

Note that because CDR is not the final review of the flight design for a rocket engine, there can be a handful non-milestone check-point meetings during the years that follow the CDR.  For J-2X, we held one such meeting about a year after CDR in order to help come to closure on any actions lingering from the design phase and to review our maturing go-forward plans.  We may have another one prior to going into what we call certification testing with what we believe to be the true final, to-be-flown design of the engine.

The final, formal milestone for a rocket engine development project is the Design Certification Review (DCR).  At the end of this review process, the engine is declared to be certified for spaceflight.  So, what is reviewed?  It is a combination of several things.  The first part is known as a physical configuration audit.  This is basically a demonstration that you can (and have, and will continue to in the future) build what the design drawings prescribe.  The second part is known as the functional configuration audit.  This is a review of all of the collected evidence demonstrating that the engine fulfills all of the functional, performance, physical, and safety requirements imposed at the very beginning.  The evidence is a wide array of test data, analyses, and/or inspection results.  And the final portion of the DCR is a review and approval of all of the other products related to the engine design.  These include operational manuals and, most importantly, all of the reliability and safety analyses and assessments, plus an assessment of the quality and configuration management systems in place to ensure continued high standards during subsequent engine production.  Overall, when you complete a successful DCR, you are stamping the entire development effort as a success and pointing towards a future of successful launch operations.  It is therefore both an endpoint and a point of transition.

So, how long does all this activity take?  Well, that depends strongly on whether you’re just making limited modifications to an existing design or if you’re starting from scratch.  Using these scenarios as extremes, you usually can get to PDR in one or two years.  At PDR you start the process of test engine fabrication.  For very simple engines, fabrication can be as short as a couple of years.  For other, complex and large engines, fabrication for a first unit can take as long as five years or more.  Again, depending on how much new stuff you have to prove out, your test program could be as short as a year or as long as three years or more.  Thus, the duration between ATP and DCR could be as short as three or four years or as long as seven or eight (or, historically speaking for truly new and different stuff, even longer).  And that is all dependent on getting an appropriate funding stream…but that’s a discussion for another time…

If you’ve made it through to the end of this article, you now have a high — altitude perspective of what makes up the life cycle for a rocket engine development effort.  Successfully guiding that whole process from one rite of passage to another, from ATP through DCR (within budget and schedule), the entire development life cycle, sometimes seems like herding cats, but that’s the essence of project management.  Luckily, as you can see from the picture above, I’ve had some experience with the whole clowder of cats thing.

J-2X Progress: Road Trip, Baby!

It wasn’t too many years ago that there was this thing about asking sports heroes after winning the big game, “So, what’s next?”  They would always dutifully answer “I’m going to Disney World!”  I guess that that whole thing is passé since I’ve not heard it in awhile, so I am going offer an alternative.  Maybe it’ll catch on and be the BIG THING this summer…

…or, well, maybe not.

But that is what happens next.  Our little engine is pulled out of the air-conditioned confines of its assembly area and trucked across the NASA Stennis Space Center to its test stand.  No more pleasantly cool and dry air for you, E10001.  This is Mississippi in June.  Thus, in order to make this trip out in the open like this on the back of the truck (don’t try this at home!), the engine has to be sealed up tight against the humidity (and bugs) hanging in the air.  Anywhere where there is an opening, there is a cover, a closure, or a plug.  From the lot at the assembly building in picture (1) below, down the road towards the engine testing area in pictures (2) and (3), and finally arriving at the lot behind test stand A-2 in picture (4).  In picture (5), you can see that the truck backs in alongside the test stand for the next operation. 

The next operation is to get the engine up into the test stand.  Years ago, this test stand was built for testing the Apollo Program S-II stage (the second stage of the Saturn V vehicle that was powered by five J-2 engines).  Back then, they basically picked up the whole stage (from a canal barge, not a flatbed truck) high into the air and lowered it down from above into the stand.  When it was converted to be an engine-only test stand for Space Shuttle Main Engine testing in the early 1970’s, propellant tanks were added on top of the stand.  So you can no longer lower the test article in from way up above.  Rather, you lift it up about four or five stories and then pull it in laterally.  This is the “engine deck,” the level where the engine will be installed into the stand.  In the pictures below you can see the operation of pulling the engine off the transport truck and up to the engine deck level of test stand A-2.

After the engine is lifted to the correct height, it is brought laterally into the stand and set down on the “porch.”  That’s what the folks on the test stand call it: the porch.  The other day somebody (obviously from out of town) mistakenly referred to it as the “veranda.”  We’ll have none of that fancy talk around here!  The thing onto which the engine is set is the Engine Vertical Installer (EVI).  This is a hydraulic lift table that will be used to raise the engine into place when it is to be bolted to the test stand.  So, here is the sequence: you lift the engine up to the engine deck level, you pull it into the stand and set the engine down on the EVI sitting on the porch, then you slide the EVI horizontally into the heart of the test stand (the EVI is on rails for this purpose), you then raise the engine into the test position, bolt it in place, and then you slide the EVI back out of the way.  Ta-da!  Now you’ve installed an engine for test!

In the pictures below you can see the technicians positioning the engine onto the EVI on the porch.  In the bottom picture of the set, you can see in the background to the left test stand A-3 still under construction and, to the right, test stand A-1 where, early next year, J-2X powerpack testing will be conducted.

So, our little baby engine is all grown up and ready to see the great big world from high up in the test stand.  The next phase of our development program is now begun: the testing phase.  After the engine is installed and the test stand is readied for hot fire, J-2X development engine E10001 will be used to demonstrate basic operations such as start, mainstage, and shutdown, to verify main chamber combustion stability, and to provide initial validation of numerous systems-level simulations and models.

Okay, somebody go carefully poke the Datadogs because soon we’re going to have genuine, full-up rocket engine test data from J-2X.  And, as a final note, I offer an extra special tip of the hat to all of the folks at SSC (NASA, Pratt & Whitney, and support contractors) for doing an amazing job in terms of engine assembly and test stand readiness preparations.  Don’t ever think that your extraordinary efforts go unrecognized or unappreciated.  Bravo!