## Inside The J-2X Doghouse: Performance Measurement, Part 1 of 2

I’ve had jobs of some flavor almost continuously since I was fourteen years old.  From delivering newspapers to cutting grass to flipping burgers to showing movies to mixing giant vats of coleslaw to instructing an aerodynamics laboratory course to hand counting vehicular traffic to researching the derivation of combustion stability equations, before I finally settled into something resembling a career (and while I was winding my way through the maze of secondary and higher education), I did all kinds stuff.  But when I did get my first job after graduate school it was doing test data reduction and performance calculations for the Space Shuttle Main Engine (SSME).

How cool is that?  Trust me: very cool. I consider myself to be extraordinarily lucky in this regard.  This article is going discuss engine performance measurements and so it’s going to reach back to my very roots in this business. And that sounds like fun!

In past articles we’ve discussed rocket engine performance. We’ve talked about the “Big Three” operational points and performance measures that characterize an engine: thrust, mixture ratio, and specific impulse. Okay, but how do we know what these values are for any given engine?  I mean, we can do calculations with analytical equations, formulas, algorithms, or models that tell us what these parameters ought to be for a given rocket engine design; but when I’ve got an actual rocket engine sitting in front of me — a big, shiny, complex hunk of metal standing ten feet high and weighing thousands of pounds — how do I know how it actually functions and performs?

Well, duh, you test it. Of course!  But making smoke and fire (and steam) does not, by itself, give you any data. The most that you could say from just watching an engine test is that it’s really, really noisy and that it makes a really, really big exhaust plume.  So, more than just observe, you have to take measurements during the test.  That’s how you get data.  As I’ve said many times, there are only two reasons to conduct rocket engine tests:  (1) to impress your friends, and (2) to collect data.  In order to get data on the “Big Three,” you need to measure thrust and you need to measure propellant flowrates. For this article, we’re going to focus on propellant flowrates. I will talk about thrust measurement in the next article.

Propellant flows are measured on a rocket engine test stand with “propellant flowmeters.”  Makes sense, right?  But calling something a “meter for flow” doesn’t tell you how it works. That’s like saying, “How do I make popcorn?”  Answer: “With a popcorn maker.”  No kidding. Thank you for playing and you’ve conveyed no useful or interesting information.

There are a number of different ways to measure fluid volumetric flow.  The units that we use for the very large flowrates feeding an engine are turbine flowmeters.  Have you ever blown into a small fan that’s turned off or perhaps a pinwheel?  If you blow hard enough, you can make the fan or pinwheel spin.  That is, quite simply, how a turbine flowmeter works: it’s a fan, i.e., turbine, stuck in a tube that spins as fluid flows through it. The faster the fluid flows, the faster the turbine spins. The thing that we measure is the speed of the turbine spinning.  The turbine has a number of blades (just like that small fan that you blow into).  We pick a spot on the tube in which the turbine sits and count how many blades pass by.  If we count, say, ten blade passes in a second, then there is more flow than if we’d only counted eight blade passes in a second.

So, how do we count blade passes?  Well, there’s a little window in the side of the propellant duct and we sit a young college co-op in front of the window with a little hand clicker and scream “GO!” from the blockhouse… Okay, I’m fibbing.  We don’t treat our co-ops nearly that bad.  Usually.  Besides, there would be no way that the human eye and brain could keep up since we’re talking about hundreds of blade passes per second.  Instead, we measure it electronically.  Each blade contains a magnet in the tip.  The sensor on the outside of the tube is activated by the magnet.  Each magnet pass generates an electronic pulse or blip — what we call a “pip” — and we keep a continuous count of these accumulated pips.  The pip count is then recorded with each time step in the data collection process.  Then, after the test, we can translate this ever-increasing pip count into a pip-rate based upon these recorded times.  Mathematically speaking, the pip-rate at any given point is the slope of the pip-count plotted against time.

In order to translate a pip-rate into a volumetric flowrate, such as gallons per minute (gpm), the flowmeter needs to be calibrated.  We need to know how much flow is required to generate a blade passage, i.e., a pip.  If, for example, we knew that the passage of one gallon was enough to move the turbine exactly one blade pass of rotation, then a measured 100 pips-per-second would equal 100 gallons-per-second, or 6,000 gpm.  Thus, calibration of a flowmeter consists of flowing a known volume of fluid through the meter and counting the pips read:

The truth is that it’s a bit more complicated in that the calibration varies with the speed of the turbine due to kinetic and mechanical issues of the rotating hardware and due to fluid dynamic effects of the fluid interacting with the turbine blades.  However, these are secondary effects as compared to the simple notion of figuring how much a pip is worth in terms of volume.

Luckily enough (or, really, strategically enough), the engine test stands are themselves set up to function as a calibration facility for the flowmeters.  This is because the propellant tanks have a known geometry and are equipped with fluid level sensors.

As shown in the figure above, if we know at a particular time the height of the fluid in the tank and then, at a later time, we know a lower height of the fluid, then, using tank geometry, we know the volume of fluid that exited the tank and ran through the flowmeter.  In practice, we actually perform this calibration during an engine test.  That way we can be assured that the flowmeter rotor is spinning at a speed representative of where we’ll need measurements.

An observant reader would note here that if we know the volume consumed over time just from the level sensors in the tank, then we don’t need a flowmeter in the middle.  All you need is volume divided by time, right?  The problem is one of fidelity.  Because the level sensors are discrete points on the pole submerged in the tank, the measures of volume used for calibration are relatively big chunks, as in enough propellant to run the engine tens of seconds.  In order to get a decent calibration across several discrete level sensors, we typically need to run between 100 and 150 seconds of steady, mainstage engine conditions. The use of a calibrated flowmeter allows you to see variations in flowrate at much smaller time increments and this allows us to collect and observe more data with regarding to engine characterization at different conditions. You can almost think of the flowmeter as a useful interpolation tool between large chunks of time and consumed propellant.

You will note that so far we’ve just talked about volumetric flowrates and yet, when we talk about engine performance we refer to mass flowrates.  The difference between the volume and the mass of something is its density.  For our very pure propellants, fluid density is simply a function of fluid temperature and static pressure. So, we take temperature and pressure measurements immediately downstream of the flowmeter and, using either an interpolated look-up table or empirical curves, we can get density.  So, you put it all together and you end up with something along the lines of the following:

That is how you measure and calculate the mass flowrate of the propellants flowing through the feedlines and going into the engine using a turbine flowmeter.  The item from the “Big Three” to which this can be applied directly is the engine inlet mixture ratio, which is defined as the oxidizer mass flowrate divided by the fuel mass flowrate.

However, depending on the engine and vehicle design, not all of the propellants that go into an engine go overboard.  Often, warmed propellants are returned from the engine to the stage to act as pressurizing gases for the stage propellant tanks.  On the Space Shuttle, both gaseous oxygen and gaseous hydrogen were flowed back to the stage for this purpose. The rocket equation that essentially defines the parameter we know as specific impulse is only concerned with propellants that leave the vehicle so for specific impulse calculations you need to use inlet mass flow minus pressurization flow.

As compared to the engine inlet mass flowrates, which for large rocket engines can amount to hundreds of pounds-mass per second, the pressurization flowrates are typically less than one or two pound per second.  Flows this small are more effectively measured using flowmeters different from the turbine flowmeters I’ve described above.  For our engine testing we use Venturi meters for these small flows. Venturi meters use a variable flow area coupled with pressure measurements to feed Bernoulli Equation relationships between pressure and fluid velocity. Once you know the fluid velocity, fluid density, and fluid flow area at any point, you can then calculate mass flowrate (for now, at least, I’ll not go any further with Venturi meter calculations).

This, then, wraps up the story with regards to propellant mass flow measurements and calculations on the engine test stands.  In the next article, we’ll go into the measurement of and calculation of thrust.  All of this discussion reminds me so much of my first days/weeks/months on the job working with SSME test data.  At first, it was just a bunch of bewildering numbers and data reduction tools and rules and calibration factors and work procedures.  I had no idea what was going on.  But gradually, as I dug into the data and talked to people and dissected the computer codes and tools we used, I began to piece it all together as to what these measurements and calculations actually meant. Seemingly every day brought a new epiphany in understanding.  Boy oh boy, that was fun!

## J-2X Progress: Engine Assembly, Volume 5

The first car that I ever owned was a “goldenrod” 1974 Ford Pinto. My uncle Johnny was a car salesman for Buick at the time. He sold new, bright, and shiny machines in his showroom, but, of course, they took in trades of all shapes and sizes and conditions, many of which they would not dare put back on their own lot. My little Pinto was just such a vehicle. I bought it from the obscure and hidden back lot for \$500 with absolutely, positively no guarantee that it would continue running until the end of the week. While it was only eight years old, back then (sad to say) autos simply didn’t last as long as they seem to today, particularly if driven hard in the weather of the northeast. So I spent months and months of afterschool hours and weekends replacing rusted panels and floorboards with broad swaths of fiberglass. Also, despite the fact that I was never a true gear-head, I was able to open up the hood and do some work on the engine. It was a straight four cylinders and as simple as could be. You could reach into the engine compartment and easily find and touch just about every greasy major component.

Just a couple of years later, my mother bought a new Saab.  When I opened the hood of that sleek machine, I couldn’t find anything.  The engine was turned sideways, apparently, and the compartment was completely stuffed to the gills with overly clean, boxed-up things that I couldn’t identify, and lines zigging and zagging in all directions.  It was fuel-injected and front-wheel-drive and all kinds of other jet-engine craziness that my cozy little, oil-burning Pinto was not.  So I closed the hood of that pretty blue Saab and I decided that I’d never make it as a modern mechanic.

Why am I bringing this up?  Because some of the latest pictures from the assembly of J-2X Engine 10001 reminded me of my confusion in looking under the hood of my mother’s car.  Only much worse.  Yikes.

I’ll get to the progress we’ve made with the big pieces, but first I want to talk briefly about what looks like a chaotic explosion of confusing stuff all over the engine pictures. I will begin by telling you a secret:  There are two reasons that we test rocket engines. First, we test rocket engines so as to impress our friends, most of whom are geek engineers like ourselves or people otherwise excited by loud noises made by neato, exotic machines (i.e., lots of NASCAR fans).

Second, we test rocket engines to gather data. Lots and lots of data. Many gigabytes of data. We measure pressures, temperatures, rotational speeds, flow rates, and dynamic vibrations both in terms of acceleration and in terms of strain. As part of the development program, we test in order to prove that what we thought was true when we created and analyzed the design is in fact true with the real hardware. And anywhere where we were mistaken, we need to know as quickly as possible so that we can recalculate our margins of safety and, if necessary, make adjustments to the design for the next development engine to be tested. Testing rocket engines is not cheap, and, of course, neither are the engines themselves, but we need to acquire this thorough, unassailable understanding of the engine long before we would ever imagine strapping a human life to something with such awesome power.

This need for data explains much of the “bloom” of seemingly ten thousand little lines and wires snaking hither and yon all over the engine.  Some of these small lines carry pneumatic pressure for actuating valves or providing purges, but lots of them are related to the many, many measurements we’ll be taking during the upcoming hot-fire test series.  In fact, these early development engines will be far more heavily instrumented than any engine we would ever fly because, again, their whole purpose to generate useful data (well, that and to impress our friends). What is truly amazing to realize is the fact that every one of these lines has a specific shape and route; every one of these lines is designed, not randomly configured; and every one of these lines represents a documented step in the assembly process.

Now to the big stuff.

The first big assembly news to share was the arrival of the regeneratively-cooled portion of the nozzle (usually called the “regen nozzle” around here). In the pictures above, on the left you can see the assembly technicians preparing to lift the regen nozzle out of its shipping container. On the right is a picture of some preparations being made to the nozzle before the rest of the engine is stacked on top. If you’ll remember, the rest of the engine was previously being assembled on a simulator of the nozzle. Interesting little side note: In order for the technician to get into the position you see in the picture on the right, she had to crawl under the elevated assembly platform and stand up inside the nozzle. In the picture below you can see the stack of the engine on the actual regen nozzle.

Now, with the regen nozzle in place, lots more stuff could be added to the assembly. In the collage below, five big items are shown in the process of installation: (1) the liquid oxygen pump discharge duct that carries liquid oxygen from the pump to the main oxidizer valve, (2) the oxidizer turbine discharge duct that contains within it the helium heat exchanger, (3) the turbine cross-over duct that carries hot turbine drive gas from the hydrogen turbopump turbine outlet to the oxidizer turbopump turbine inlet, (4) the main fuel valve, and (5) the fuel pump discharge duct that carries liquid hydrogen from the fuel pump to the main fuel valve.

And that’s just a small sampling of the flurry of assembly activity over the last few weeks. I like the fact that I was able to include pictures showing some of the folks working hard to get this thing put together. They’re doing an extraordinary job. The result of all this work is something that is really, truly starting to look like a rocket engine. The pictures below show what you’d see if you took a casual stroll around the engine. Note that the regen nozzle is covered with a black, protective tarp and there are other protective coverings all over the engine, so this isn’t quite as gorgeous as it could be, but for us rocket geeks it’s awfully compelling.

So, that’s where the engine stands. Almost all of the big stuff is installed, with just a couple of notable exceptions. Additionally, lots and lots of little stuff has also been installed. Over the past few weeks, it’s come a long, long way. As a reminder, take a look at this:

But, then, I guess everything eventually comes a long way with a little bit of hard work.  Take, for example, how I’ve been blessed to evolve through the years:

With all due respect to the Ford Motor Company and their venerable line of Pinto automobiles, that’s one heck of a leap even if it did take a long time. I just bet that my Uncle Johnny would be proud to think that he was there at the start of the whole thing.

## J-2X Progress: Valves, Commands into Action

Everyone seems to like analogies between the composition of a rocket engine and that of the human body.  These are often colorful but not always helpful.  In some cases, however, they work pretty well.

Okay, so let’s start with your body as it is.  Now, imagine removing all of your bones.  Guess what?  You’re an immobile lump.  Even if your brain is sending signals and your muscles are contracting, you’re not really moving anywhere.

This time, let’s instead start with your body as it is, but now imagine removing all of the muscles and tendons that connect the muscles to the bone.  You’ve got a central nervous system and you’ve got bones, but with nothing to flex, the chain is broken and you’re stuck where you sit (assuming that you can still actually sit).

And, of course, if you instead start with your whole self and imagine removing your brain and/or your central nervous system that connects your brain to your muscles, again, you’ve achieved perfect immobility (i.e., you look like me on Saturday afternoons during college football season).

The point is that in order for you to be up and about, shoveling snow, doing laundry, playing pool, typing, whatever, you need both the command center that figures out what signals to send — your brain — and you need things that turn those signals into action — your muscles and tendons and bones.  In a rocket engine, the analogue for the brain is the engine controller.  It is a computer that receives instructions from the vehicle and sends out commands to the engine pieces so as to fulfill those instructions.  The analogue for the muscles are the valve actuation systems.  These are the things that “flex” and cause movement.  And the analogue for the bones, the final effectors that make things happen, are the valves.

The controller sends out signals and then the actuation system responds by shuttling pressurized working fluid — helium for J-2X though some engines use hydraulic fluid instead — where it needs to go so that the valves move and the engine comes to life.  The engine goes from being a lump of inert, shiny metal to a “living” beast of flowing propellants, spinning turbomachinery, lots of fire, and thundering, rumbling thrust.

On the J-2X, there are 42 valves.  Most of this number is made up of small valves like check valves, solenoid valves, and valves in small lines like the bleed lines.  There are also a handful of big valves — the primary valves — that directly control the flow of propellant and, in one case, combustion products along the plumbing of the engine.  Each of these primary valves is connected to a valve actuator, i.e., the muscle.  These valve actuators convert the energy of high pressure helium gas into mechanical rotation of the valve.  This is accomplished by pressurizing cavities and moving pistons and, in this way, the valve is pushed opened or closed.  I’ve used this schematic shown below before, but it is useful here as well since it illustrates the primary J-2X valves: Main Fuel Valve (MFV), Main Oxidizer Valve (MOV), Gas Generator Fuel Valve (GGFV), Gas Generator Oxidizer Valve (GGOV), and the Oxidizer Turbine Bypass Valve (OTBV).

The control logic for J-2X is relatively simple.  The whole subject of different kinds of control logic is a good topic for a future article, but suffice it to say that for normal operation the J-2X: starts on command, can change between two power levels on command, and shuts down on command.  The control system is designed to do other things as well, including monitoring the health of the engine, but these operations are the commanded functions.  Start and shutdown can be simplistically thought of as: the valves open and the valves close.  It’s a bit more complicated since the timing of opening and closing is extremely important, but the open/close notion is basically true.  The oddball action is the one consisting of changing power levels.  That is accomplished by controlling the power to the oxidizer turbine via the OTBV.  This bypass valve effectively allows for limited, independent control of the two turbopumps.  By altering the power to the oxidizer turbopump (OTP), you can control the engine thrust level (and, simultaneously, mixture ratio).

The OTBV for J-2X is designed and built by Pratt & Whitney Rocketdyne (PWR), the prime contractor for the whole engine.  In addition to being responsible for the “oddball action” on the engine of changing power levels, it represents a challenging design due to the range of operating conditions.  Unlike the other primary valves on the engine that see, essentially, one narrow range of environmental conditions, the OTBV has to function in temperatures approaching 420 degrees below zero Fahrenheit (liquid hydrogen conditions) immediately prior to start and then, suddenly, within 1 second of ignition of the gas generator, see temperatures approaching 750 degrees above zero Fahrenheit (combustion products).  That broad range of operating conditions requires special design considerations and special materials.  Not only do you have to worry about wear and tear under such harsh conditions, but you also have to think about simple operation under the extremes of thermal expansion.

The original, Apollo-era J-2 engine also had an OTBV, but it was used slightly differently and was designed much differently.  It was a butterfly valve whereas the J-2X OTBV is a ball valve.

No, the valves shown in the picture are NOT rocket engine valves.  I can’t show any internal workings of rocket engine valves.  In fact, I am not even allowed to describe the general design details that make the J-2X OTBV kind of unique.  However, the basic elements of rocket engine valve functionality for butterfly and ball valves are essentially the same as these water valves.  The biggest difference is the replacement of the handles with pneumatically driven actuators.  Back during the Apollo era it would seem that butterfly valves were most frequently used, but after many years of usage on the Space Shuttle Main Engine, ball valves are often preferred these days.  They generally require less torque to move and they generate better flow characteristics and flow rate control capability.

The first OTBV unit for use on the upcoming development engine testing for J-2X is in the later phases of manufacturing at the PWR in Los Angeles.  All of the individual piece parts are schedule to be complete by the beginning of February and assembly will begin the middle of February.  The valve then will be integrated the actuator and shipped to the NASA Stennis Space Center to be put on the first engine.

## J-2X Extra: The Faces Behind J-2X, NASA MSFC, Part 1

Years ago, I worked in support of the Space Shuttle Program, specifically on the Space Shuttle Main Engine (SSME) from the engineering side of the house.  I was an analyst and sometimes Datadog.  By happenstance, I learned from another relative that whenever a launch occurred my mother, back home in Pennsylvania, would routinely tell folks at work or at church or wherever that her son was responsible for that Shuttle launch.  I said, “Mom, don’t tell people that.  I’m just one of hundreds or, really, thousands of people behind the whole thing.”  She replied, with biased motherly wisdom, “Yes, but you’re all responsible for doing a good job so that the whole thing works, right?”

Lesson learned #1:  Don’t argue with Mom.

Lesson learned #2:  She was right.  Any venture as big and complex as Shuttle, or even as big and complex as the development effort for J-2X really does require that everyone does their job well.  In that way, we’re all responsible.

So, I’m going to introduce you now to just a few of the people (out of hundreds) responsible for making J-2X a reality.  Right now I’ll focus on the top leaders Upper Stage Engine Element office here at NASA Marshall Space Flight Center.  In another posting, I’ll tell you all about the subsystem managers.  And perhaps, later if I’m lucky, I’ll sneak some pictures of the good people out at Pratt & Whitney Rocketdyne as well.

So, here we go…

First, we have our Element Manager, Mike Kynard.  He is a graduate of the University of Alabama [Roll Tide!], grew up not far from Tuscaloosa, and, to put it mildly, is a devoted fan of Crimson Tide football.  He started working at NASA MSFC as a co-op in college back and accepted a full-time job in 1985.  He spent a good spell supporting SSME from the resident office at NASA Stennis Space Center in Mississippi in the 1990s and eventually rose to become the deputy project manager for SSME back in 2004.  He has two beautiful little girls and, in addition to everything else, somehow finds the time to play on a team with me in a billiards league.  Sometimes, on rare occasions such as a blue moon, he even manages to beat me in a game of 8-ball.

Mike’s deputy is Tom Byrd.  He is a graduate of Memphis State University who, once upon a time long, long ago was a competitive bicyclist.  He started working here at NASA MSFC in 1983 in the area of valves and actuators, specifically supporting SSME (yes, you’ll see SSME as a recurring theme both here and when later tell you about the subsystem managers).  Along the way since then, he was a subsystem manager for the Fastrac engine development effort (a NASA MSFC in-house project), was the NASA chief engineer for COBRA engine development effort, and he supported the Shuttle program in the area of systems engineering and integration.  Plus, back in 1994, representing the NASA engineering community, he spent a total of four weeks in Russia studying their space transportation technologies.  Tom has one young son who, given the uncanny resemblance, we suspect might be the product of a direct cloning experiment … but we don’t talk about that too much.

Our Chief Engineer is Eric Tepool.  His job is to function as the balance point between technical and programmatic aspects of engine development.  He also indirectly functions as a balance in another way, relative to our element manager’s tendencies, in that he is a graduate of Auburn University [War Eagle!].  After making his mark as a star high school athlete, just as his father was a star high school and collegiate athlete, Eric turned down athletic scholarship offers to settle into the pursuit of engineering.  He started at NASA in 1990 in the turbomachinery branch, supported development and certification of the two new turbopumps for the Block 2 SSME configuration, moved on to be a subsystem manager for the COBRA engine development effort, resident manager for the Fastrac project, and the NASA-side lead systems engineer for the Integrated Powerhead Demonstrator project.  He has two kids currently in college (one at Alabama, one at Auburn … yikes!) and one soon to be on her way to college.

The other “technical balance point,” in accordance with the NASA governance model, is our Chief Safety Officer Phil Boswell.  He’s been working here at NASA MSFC since 1985.  He started right after graduating from the University of Alabama at Birmingham (UAB).  While he started in the Safety and Mission Assurance Directorate, and he’s back there now, in the interim he spent many years working within the Engineering Directorate on such projects as microgravity experiments for Shuttle and the MIR space station, the Orbital Space Plane program, and just about every propulsion element of the Shuttle itself including engines, tanks, and boosters.  Phil played on the college tennis team, plays tennis still, and is an excellent golfer.  His son, who will be attending UAB starting in the autumn (pre-med) follows in his father’s footsteps and is also an excellent golfer.  Little known fact about Phil: he loves swing dancing and even took a year of lessons with his wife.

So, that’s the top leadership group for the J-2X development effort.  Good guys, hardworking engineers and managers, all around.

## J-2X Progress: The Main Combustion Chamber — the Heart of the Fire

My background is analytical modeling.  There are all kinds of modeling in use today, including some extraordinary computational fluid dynamics and structural dynamics work that is amazing, but back when I did more hands on work I did zero/one-dimensional, system-level modeling of fluid systems, thermodynamics, and heat transfer.  Rather than looking at the micro-level, my work was usually a step back at the macro-level.

I’ve told you all that because, over the years I found that one of things that I found most challenging and most enjoyable to model is combustion devices.  Take the main combustion chamber (MCC) as the prime example.  Its job is to contain the combustion, squeeze the combustion products to sonic velocity, and then direct them towards the nozzle.  It’s just a big, hourglass-shaped tube.  It all sounds simple until you realize that those combustion products are at about 6,000 degrees Fahrenheit and, for the J-2X, at a pressure of over 1,300 pounds per square inch.  How it is that the MCC doesn’t melt or burst – or both – is amazing.

So, let me tell you a little about MCCs.

The only way to build one where the walls don’t melt during operation of a large liquid hydrogen / liquid oxygen engine is to actively cool them.  This means flowing cold hydrogen inside the walls.  On one side of the wall you have combustion 6,000 degrees and on the other side, you have hydrogen that is typically entering at less than 100 degrees above absolute zero.  Now you see why this is a delight for someone who enjoys modeling thermodynamics, fluid flow, and heat transfer.

How do you make walls that allow for active cooling? Well, years ago, back during the era of the Apollo Program the walls were made of tubes. They took a whole bunch of tubes, bent them into the profile shape of an MCC, stacked them up into a circular pattern, lay in or pack in binder metal, and then brazed them together. Brazing is essentially a welding process where you stick the whole thing into an oven, make it really hot, and the thing melts together (as someone who enjoys cooking, I tend to think of it kind of like a stiff metal stew). In this case what melts together is binder metal stuck between the tubes so that when you take it out of the oven, you’ve turned several hundred tubes into a single piece. Note that the regeneratively-cooled portion of the J-2X nozzle is made in this manner using tubes, but when doing this with an MCC where the pressures are higher and the temperatures are hotter: we’ve advanced a long way since the 1960’s.

First of all, in order to make the tube-wall MCC structurally sound, you were kind of forced to use steel tubes (steel of some sort at least). What you’d really like to use is something like a copper alloy that has higher thermal conductivity properties. Copper tubes might be possible, but you would have to put behind them a steel support structure, a jacket of some sort. That, however, introduces potential issues of hot stuff or high-pressure stuff seeping behind the tubes and causing all kinds of problems including catastrophic failure of the engine. The hourglass geometry required for the combustion products flow path complicates nearly all of the structural issues.

Then, along came the development effort for the Space Shuttle Main Engine (SSME).  Here you’re dealing with MCC pressures over 3,000 pounds per square inch so you needed a whole new approach since tubes just weren’t going to cut it.  What they came up with, conceptually, is the same thing used by J-2X and most other large rocket engines, although the means for fabrication vary.  They used a liner and jacket concept.  The liner replaced the hundreds of tubes.  It is a single piece with the hourglass shape within which the combustion takes place.  Around the liner is fitted a structural jacket.  Thus, the liner can be made of something like a copper alloy to deal with the heat transfer issues and the liner can be made of some steel or nickel alloy to deal with the structural issues.  Where before the tubes provided the coolant flow path, here groove are cut into the backside of the liner so that when the liner and the jacket are fit together, these grooves become channels.  Ta-da!

The first step in manufacturing an MCC involves hot spin-forming a copper-alloy forging.  The resulting piece is then machined to a precise contour.  The liner is then slotted, meaning that the grooves for the coolant flow passages are cut into the backside away from where the combustion products will flow past.

The really tricky part about this kind of MCC is making the whole thing fit together and stay together given the different metals being used.  The different metals have different properties and different structures at the micro level so a variety of methods including electro-plating and brazing and welding methods are used to make a single unit.  It is these details that are proprietary and export-controlled technologies that cannot be revealed but that make the whole thing possible.  Luckily, this humble analyst knows little about metallurgy so there is almost no danger of me exposing something useful-but-sensitive to the world at large.  In the past, developments for methods of producing a robust bond between the liner and the structural jacket resulted in cheaper, more reliable, yet heavier design.  The J-2X design, however, has allowed for significant weight decreases by optimizing the design for post-bond machining.

Another recent innovation in MCC fabrication, and one being pursued for the J-2X MCC design, is the use of castings for the manifolds that distribute the coolant to the flow passages and collects the coolant at the end of the passages.  These pieces of hardware see very high fluid pressures due to their function as manifolds but also are used by a number of other components for structural mounting into the engine system.  Thus, they are heavy and complex pieces.  In the past, the typical practice was to machine these parts out of wrought forgings.  The J-2X cast manifolds offer a significantly more cost efficient way to produce these parts.

The MCC for the first J-2X development engine is currently planned to complete fabrication in early February 2011.  All of the major parts for the second MCC are already in work including the manifolds, the liner, and the jacket.  It is slated for completion in July.  So now, analytical modelers, like me, will just have to sit back and wait for the testing to commence in a couple of months.  Then the process of reconciling models to test results, understanding the clash of approximations and standard correlations and actual, particular physics, starts in earnest.  That’s the truly fun part!

## Inside The J-2X Doghouse: The Gas-Generator Cycle Engine

Welcome back to the J-2X Doghouse.  The last time that we met here, we discussed the fundamentals of what exactly makes something a rocket.  As I explained, on the conceptual level, rockets aren’t really “rocket science.”  You get the propellants together, light them on fire, and eject them out the back end of the vehicle.  Simple enough.

Okay, but how do you move that much propellant and make that much smoke and fire, enough to propel something as big as, say, the Saturn V that was over 300 feet tall and weighed millions of pounds?  That’s where things get interesting and technically difficult.  As I said before it is all a matter of power. And to get power you use an engine.

What makes a rocket engine an engine is the fact that it contains more than just a combustion chamber where the propellants mix.  It is an arrangement of machinery that, once started, feeds and powers itself.  During operation, a rocket engine uses some cycle – some circuit of piping and thermodynamics and combustion and valves and control system and rotating machinery – to keep itself up and running and generating thrust.

Think about your car engine.  You turn the key, the engine gets up and going, and then it can sit there for hours idling, running happily all by itself, converting gasoline and air into mechanical energy, with no additional input from you.  You don’t have to manually pump the gas into the injectors (or the carburetor).  You don’t have to plug it into an outlet to feed it more electrical energy.  It’s self-sufficient until you turn it off or until you run out of gas.  That’s what truly makes it an engine.  It’s similar with a rocket engine except that the product is not mechanical energy; the product is very fast moving gases generating lots of thrust.

For rocket engine conceptual design, in terms of making it an engine, the goal is always, “How do you keep the pumps pumping?”  These are extremely powerful pumps moving lots and lots of fluid, so you need some powerful energy source to drive them.  The answer is to use what you’ve already got in the engine: the propellants.  There are different ways to do this and thus you have different engine “cycles,” i.e., component arrangements.  The most common rocket engine cycles are the gas-generator cycle (examples include J-2X, J-2, F-1, RS-68, and Vulcain 2 – see pictures above), the expander cycle (examples include RL10 and Vinci), and the staged-combustion cycle (examples include Space Shuttle Main Engine and RS-170/180).  In addition to these, there are many other cycles and variations as well.  Each different cycle has advantages and disadvantages and, usually, constraints linked to physics.  Choosing the right cycle to fit the mission application is generally the first decision that an engine designer has to make.  Because this is a blog dedicated to J-2X, I will focus on the gas-generator cycle engine.

Ideally, what you would want to do with a rocket engine is use all of your propellants in as efficient manner as possible meaning that you would want to use all them in the production of thrust.  In a gas-generator engine, however, you concede right up front to a loss of some efficiency to achieve greater engine simplicity.  You use a certain amount of the propellants brought into the engine almost entirely to keep the engine running rather than for generating thrust.  In practice what this means is that you have a separate, small combustion chamber within the engine that does nothing but produce gases to drive the turbines connected to the propellant pumps.  As compared to the large quantities of propellants being pumped through the whole engine, the amount going to the gas generator is small (less than 3% for J-2X), but once used to drive the turbomachinery, the exhaust is drained of much of its thrust-generating energy.

Below is a simplified schematic of a gas-generator cycle rocket engine like the J-2X.  The propellants, liquid hydrogen (fuel) and liquid oxygen (oxidizer), enter the engine and go immediately into the pumps: the fuel turbopump (FTP) and the oxidizer turbopump (OTP).  There, the mechanical energy of the spinning pumps is turned into high pressures in the liquid propellants.

After exiting the pumps, a small amount of each propellant is tapped off to supply the gas generator (GG).  The GG is, in essence, a small rocket engine embedded within the larger rocket engine.  It makes hot, high-pressure combustion products, steam and gaseous hydrogen, that are used to drive first the turbine connected to the fuel pump and then the turbine connected to the oxidizer pump.  After driving the two turbines, this still-warm gas is used first to warm the helium flowing through the heat exchanger (HEX) that is used to pressurize the oxygen tank of the stage and is then dumped along the walls of the nozzle extension to keep that relatively cool.  The video below is a component test of the J-2X GG performed at NASA MSFC.  Even with the relatively small amount of propellant that the GG burns, an enormous amount of energy is released to drive the turbopumps.

The rest of the liquid oxygen coming out of the oxidizer pump, meaning that which is not going to the GG, is directed through the main injector and into the main combustion chamber (MCC).  The main injector is analogous to a fuel injector in a car engine except that here it injects two propellants through hundreds of injector elements.  The effectiveness of this injection and the mixing of the propellants are crucial for overall engine performance.

The hydrogen circuit after the fuel pump is more complicated.  This is because the hydrogen is used to cool the nozzle and combustion chamber walls.  The walls of these two components are essentially hollow.  They contain hundreds of passages for the hydrogen to flow thereby keeping the walls from melting due to the extreme high temperatures of the contained combustion zone.  After doing its job as coolant, the hydrogen is then directed through the main injector and into the MCC.  Not shown on the diagram is the fact that a very small amount of the warm hydrogen gas is tapped off prior to entering the main injector and is routed back to the stage to pressurize the hydrogen tank (like the helium through the HEX on the oxygen side).

It is in the MCC where the mixed hydrogen and oxygen combust to make steam and residual hydrogen gas.  The temperature of that combustion is approximately 6,000 degrees Fahrenheit and in the J-2X the pressure is approximately 1,300 pounds per square inch.  These combustion products are then accelerated to sonic velocity at the converging throat of the MCC and then to supersonic velocities down the diverging nozzle and nozzle extension.  As discussed previously, it is the high-velocity expulsion of these hot gases that produces thrust.

Note that the turbine exhaust gases dumped along the nozzle extension still generate some thrust, but not as effectively as the combustion products that are accelerated through the nozzle throat.  This loss of effectiveness is the price that you pay for this relatively simple engine cycle.  As a comparison to a more complex engine cycle, do a web search for the schematic for Space Shuttle Main Engine (SSME).

FYI, the other items denoted on that GG-cycle schematic above are the control valves: the main fuel valve (MFV), the main oxidizer valve (MOV), the gas-generator fuel valve (GGFV), and the gas-generator oxidizer valve (GGOV).  These primary valves, along with several other minor ones, are used to control the engine during the start and shutdown of the engine.

So that’s how a gas-generator cycle engine like the J-2X works.  As this blog continues and as we head towards testing next year, I will continue to report on the progress of the components that make up the engine.

## J-2X Progress: The J-2X Test Stands

Okay, so now you’ve got a great big rocket engine.  What are you going to do with it?  Well, fire it, of course.  Make great big and noisy smoke and fire.  There’s really not much that is more thrilling than an engine test…although, I guess, launches qualify (says the old engine guy reluctantly).

Engine Test at NASA Marshall Space Flight Center

But where are you going to do this?  It’s not like you can do it in your garage.  You’d blow away your entire neighborhood in the matter of a few seconds and the authorities tend to frown on such antics.  Take a look (and listen) again at the video clip from the “What is a Rocket?” blog article to get an idea of what I’m talking about.  Also, it’s not even like you can simply hire a company that specializes in testing stuff and there are many fine companies that do just that for all kinds of products big and small.  No, rocket engine testing is an endeavor that requires its own dedicated facilities and infrastructure.

Over the past fifty years, NASA has developed a number of rocket engine test facilities, but by far the single largest and dedicated site is in southern Mississippi, Hancock County to be exact, today called the NASA Stennis Space Center (SSC).  This facility is just about an hour from New Orleans.  It is in a very secluded, woody bayou area far from any population centers.  And that was the point when it was established.  Given the size of the place needed to test rocket engines and rocket stages and given the noise that such testing makes, having no neighbors is basically a requirement.

Testing for the J-2X engine is currently planned in the “A-Complex” test area.  That area is composed of three test stands.  There are stands A1, A2, and A3 (no, it’s not an especially colorful naming scheme, I admit).

Stands A1 and A2 were designed to look like and function like the large test stand here at the NASA Marshall Space Flight Center.  They were built in the 1960’s and were originally stage test facilities to accommodate testing of the S-II stage, the second stage of the Saturn V launch vehicle that took humans to the moon.  The S-II stage was, of course, powered by the original J-2 rocket engine.  Then in the early 1970’s, these two stands were converted into single-engine test stands to facilitate the development of the Space Shuttle Main Engine (SSME).  Test stand A2 remained dedicated to SSME up until last year.  Test stand A1 over the last thirty-five years was used primarily for SSME, but it was also used in the late 1990’s for the XRS-2200 linear aerospike engine development (which used a number of heritage J-2 and J-2S component designs) intended to support the X-33 vehicle.

Test Stand A2 Under Construction, Early 1960’s

S-II Stage being Hoisted into A2 in 1967 and the First SSME Test on A1 in May 1975

Test Stand A2 Today

Test stand A3 is a new facility currently being built specifically to accommodate development of the J-2X engine.  It is unique in that it simulates the atmospheric pressures at high altitudes.  Because the J-2X is being designed for maximum performance and for engine start at high altitudes, it is only within such a test facility as A3 that the complete configuration of the J-2X engine can be tested.  The altitude simulation capability is produced by encapsulating the entire engine within a test chamber and using a system of steam ejectors to “suck down” the chamber using the Bernoulli effect familiar to students of fluid dynamics.  Basically, what you have on A3 is a series of rocket engines, powered by liquid oxygen and alcohol, used to make a huge amount of high-velocity steam that creates a low-pressure environment into which the J-2X fires (itself also making a huge amount of steam).  When A3 is up and running, the J-2X testing conducted there is going to be even more impressive than the usual engine tests.

Test Stand A3 Under Construction Today

The J-2X puts out approximately 300,000 pounds-force of thrust when fully configured and operating in space.  As currently rigged, each of these three test stands can handle 600,000 pounds-force of thrust and, with some modifications, significantly more (the current thrust measurement systems being the limiting factor).  Back when they were testing the S-II stages on A1 and A2, those stands were seeing nearly one million pounds-force of thrust with five J-2 engines firing simultaneously.

Within the last month, I had the opportunity to tour NASA SSC and see the progress of the work being done on these test stands to support the J-2X test campaign.  Below are a series of photos with some accompanying commentary.

Above is a picture looking up and into the flame bucket on Stand A2.  To give you an idea about dimensions, notice the person in the blue jacket and orange hardhat down on the right-hand wide.  During an engine test, this entire area is deluged with water for the purposes of cooling and sound suppression.  The flame bucket diverts the rocket exhaust from shooting downwards to shooting outwards and away from the stand.  The long tube-like structure in the middle is a feature unique to Stand A2.  It is a passive diffuser that creates simulated high-altitude conditions while the engine is running.  The difference between this passive diffuser and the active diffuser on A3 is the fact that A3 can simulate higher altitudes and can do so even when the engine is not firing.

This is a shot taken near the top of stand A3.  They have not yet built in the elevator so I know firsthand that the walk to the top is just about 23 flight of stairs, give or take a couple.  I’ve marked stand A2 and also stand B1, which is currently used for RS-68 engine testing that supports the Delta IV launch vehicle.  Stand A1 is off to the left, out of the frame of this picture.  The low white building in the middle of the picture is the control room from where they conduct engine tests on A1 and A2.  The control room for A3 will be in a different building.

Above is a picture of Jason Turpin (Liquid Engine Systems Branch, ER21, NASA MSFC) and Rick Ballard (Upper Stage Engine Element Systems Engineering and Integration Manager) standing on the Level 5 deck of stand A1 with the A3 construction site in the background.  The water that you see behind them is part of a canal system that runs throughout the test area.  On these canals they bring in barges filled with the propellants used for the testing.  Back in the day, these canals were used to float in the assembled Saturn stages.  This is not, however, necessary for engine testing since a single engine can be loaded onto a truck.

Overall, this tour of the facilities showed that NASA SSC is making tremendous progress in getting the test stands ready for the J-2X development test series campaign.  In only a few months, we will be making smoke and fire (mostly steam!) and rumbling the acres of swampy woodlands that surround the site.  I can hardly wait!

## Inside the J-2X Doghouse: What is a Rocket?

For as long as anyone can remember here at NASA’s Marshall Space Flight Center, the collection of engineers who analyze and evaluate rocket engine test and flight data results have been called “Datadogs.”  However, that time-honored moniker is a title that must be earned.  It’s not automatic based upon your job assignment.  It is based upon your ability to create a coherent technical narrative derived from hundreds of pieces of data spanning pre-start purge schedules, through engine start to mainstage operation, through shutdown transients and, finally, post-test inspections.  With every engine firing we ask: What happened and, more importantly, why?  The Datadogs provide the answers.

So, as a regular part of the J-2X Blog, I will be inviting you into the J-2X Doghouse just to ramble a bit about rockets and rocket engines in preparation for the upcoming J-2X development testing next year.

The most basic question is, of course, what is a rocket?  Often, when lost in the mountain of ten thousand details of fabrication processes and assembly procedures and structural analyses and operational manuals and information of all flavors, even rocket scientists sometimes lose sight of the most basic concepts.  Yet any child who has ever blown up a balloon and then let it fly across the room as it deflates has experimented in rocketry.  A rocket is simply a vehicle that is self-contained and self-propelled.  It takes in nothing from its external environment and it achieves motion from Newton’s principle of a reaction resulting from every action.  A rocket effectively throws stuff out the back end while what remains in the rocket moves forward thereby balancing the net sum of inertia.

In technical terms, the balloon flying across the room — likely landing in your uncle’s soup thereby causing a minor family crisis — is a pressure-fed, mono-propellant rocket.  The stretchy plastic of the balloon supplies the pressure and the single propellant is the breath with which the balloon was filled.  The pressure from the plastic pushes the air out the back end.  The air goes one way rapidly and the balloon itself goes hurtling through space in the opposite direction.  Ta-da, a rocket!  And now you are privy to the NASA secret that rockets, at their most basic, conceptual level, are pretty darn simple.

So, what makes a rocket engine different than a child’s balloon?  Power.  In order to throw thousands of pounds of a launch vehicle into the sky and accelerate it to thousands of miles per hour, you need lots and lots of power.  Rather than relying on pressure to push the working fluid out the back end, a large rocket engine like J-2X uses very powerful pumps.  And, rather than relying on just the velocity generated by moving the fluids, a large rocket engine taps into the chemical energy released by combustion.

For example, during every second of operation the J-2X pumps hundreds of pounds of hydrogen and oxygen into a chamber not much bigger than a large spaghetti pot.  There, these fluids combust, making steam (and residual hydrogen gas) at blistering hot temperatures of thousands of degrees.  That tremendous amount of energy is then directed out the back end, accelerating the hot gases down the length of the nozzle to supersonic speeds, converting thermal energy to kinetic energy all along the way.

How much steam does this make?  Well, if you ever have the opportunity to see a J-2X engine test, bring an umbrella.  A full duration test will make enough steam to make its own rain cloud in the sky.  Below is a video of a Space Shuttle Main Engine test in stand A2 at NASA’s Stennis Space Center in Mississippi.  Tests of the J-2X will look quite similar.

Thus, the tough part about rocket engines is not their basic concept.  That’s simple.  The tough part is building a device that can harness the power necessary to make that simple concept useful.  As we go along, we’ll discuss that tough part in more detail.

## Welcome to the J-2X Blog!

Hello!  Welcome to the J-2X Blog.

I would say that it’s a pretty safe bet that a large (very large) majority of the American population is unaware that we stand on the brink of testing the first new, large, human-rated liquid rocket developed in this country since Gerald Ford was President.  I might even venture to suggest that a majority of the diverse and busy population supporting NASA also don’t know that this is the case.

Back then, during the Ford administration, the new engine was called the Space Shuttle Main Engine (SSME).  Its initial development at the conceptual level began in the late 1960’s.  The Space Shuttle itself wouldn’t fly until 1981, nearly six years after the first attempted engine test.  Today, the engine is called J-2X and this blog represents an attempt to inform those who want to follow the exciting progress of this development effort as we approach full engine testing in early 2011.

As the name suggests, the J-2X has its roots in the Apollo Program with the J-2 engine used for the second and third stages of the Saturn V rocket that first took humans to the moon.  In many ways, the original J-2 was the technological predecessor of the SSME.  The J-2X design is the beneficiary of over fifty years of rocket engine experience spanning the original J-2, the SSME, the experimental J-2S, and the RS-68 engine that today powers the Delta IV commercial rocket.

The J-2X is being developed by the NASA Marshall Space Flight Center in Huntsville, Alabama, the home of the propulsion systems for the Apollo Program and the Space Shuttle Program.  Our contracted partner in this development is Pratt & Whitney Rocketdyne located in Los Angeles, California.  Appropriately, Pratt & Whitney Rocketdyne, taking into account corporate name changes over the years, was the developer of the liquid rocket engines that powered the Apollo Program and still powers today the Space Shuttle Program.  Thus, we have assembled an experienced, formidable, and knowledgeable team for J-2X.

Your humble chronicler for this journey into the exciting final stages of J-2X development is William D. Greene.  I am currently the Upper Stage Engine Element Associate Manager.  The Upper Stage Engine Element is the NASA office responsible for J-2X engine design and development.  For the first three and a half years of this project, I was the Systems Engineering and Integration Manager for this office.  I have 22 years of experience, most of which has been in support of the NASA Marshall Space Flight Center and much of which has be dedicated to liquid rocket engine analysis, development, production, and testing.  I will be charting the progress of the J-2X development effort, introducing you to the extraordinary team responsible for this effort, and sharing what I know about both this activity as well as about rocket engines in general.

This is going to be fun!  C’mon along for the ride!  For more information about the J-2X project, see the link to the video starring some of the key people engaged in this historic effort.

NASA and Pratt & Whitney Rocketdyne managers discuss the design and
development of the J-2X engine.