Monthly Archives: January 2011

J-2X Progress: Valves, Commands into Action

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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

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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

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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!