Monthly Archives: December 2011

J-2X Extra: From Concept to Hot Fire

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Around here, like most everywhere else, we’re winding down to the end of the year, into the holiday season, and towards the promise of the coming New Year.  This has been one heck of a year!

As a final treat from the realm of J-2X development, I have the video below to share.  The author/director of this creative piece is Paul Gradl, a friend of mine, a coworker on the J-2X development effort, and a superb engineer with a technical background in combustion devices design and analysis.  He came up with the notion of stringing together the J-2X development process starting with conceptual design, then detailed design and analysis, through fabrication and assembly, and finally into full-scale hot-fire testing.  Working with the local NASA video specialists, Paul assembled this piece and the result is truly excellent.  Thank you, Paul.


Happy holidays to everyone

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Welcome to the J-2X Doghouse: You Dropped a Bomb on Me, Baby!

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The Gap Band, 1982!  Yep, just recalling those wise lyrics and the electrofunk music of my youth and I’m up and dancing and strutting around in my office like a goofball.  Luckily, nobody is watching.  So, is there a point to this opening other than imposing on you the painful image of a middle-aged bureaucrat getting down and getting funky in his office?  Yep, there is.  Here it goes:  The next J-2X Engine 10001 will be a bomb test.

You read that correctly: Bomb Test.

Now, I could probably weave a complex and fanciful tale explaining how “bomb test” is really just a creative government euphemism, but we’ve been straight with each other before, right?  So, the truth is that for this next test we will be mounting into the main combustion chamber a 100% genuine bomb, a small explosive device.  And, yes, we will detonate that device to set off an explosion.

Other than proving once again that we’ve got a cool job and that we’re really like a bunch of 14-year-olds who like to make loud smoke and fire, there is actually a technical reason for doing this.  Way, way back in February, some 20 articles ago (“”-2X Extra: Shiny Metal Pieces”), I briefly mentioned the possibility of combustion instabilities in the gas-generator.  I likened them to the melodious sounds from a pipe organ although combustion instabilities in rocket engines are far, far from melodious.  Indeed, they can be dangerous and destructive.  Our bomb test is a means for characterizing the combustion stability of the J-2X engine.

In order to understand combustion instabilities in a very general way, we have to take about a dozen steps backwards and get to some really basic physics.  The issue comes down to one of natural frequencies and resonance. 

Have you ever tried to push a kid on a swing?  Almost immediately, instinctively, you know that there is a particular rhythm with the swing and if you push in concert with that rhythm, the magnitude of the swinging motion will be increased.  If you try to push at a different rhythm, the kid just kind of sits there in the middle, bobbing around and getting annoyed.  The swing has a natural frequency and if you match that frequency, you get a big response.  That’s resonance.  If you fight against it, your energies are dissipated without much else happening.  Everyone can picture that, right?  That’s a simple, common, shared experience.  Good.

Next, we’re still going to use a rope, but in a little different manner.  In the diagram below, you see someone – i.e., you – holding onto a rope that is secured to a wall.  If you jiggle the rope with no rhythm (i.e., if you jiggle it kind of like how I dance to The Gap Band), not much happens.  It will wiggle and move, but with no organized pattern.  But very quickly, again almost instinctively, you can find the rhythm to make the rope define, back and forth, a single, graceful arc of movement.  With a little practice, you can then jiggle the rope at exactly twice that first speed and get it to make the shape shown in the middle.  And, if you’re really good, you can jiggle it at three times the speed of the first one and get the shapes shown on the bottom. 

If you do this, you will have demonstrated the first, second, and third natural modes of the standing waves for that length and properties of rope.  You cannot get those shapes by inputting any random jiggle on the end.  You have to input a specific forcing function, tuned to a specific frequency, and you will get the desired results.  Your forcing function must resonate with the mode.  Just like with the kid on the swing, if you input the wrong forcing function, nothing much happens.

Another point to consider — in addition to considering the forcing function — is that the rope has particular characteristics that define its natural modes.  But anyone who has ever picked up a guitar knows this, right?  Each string is a different thickness, each is pulled tight to a particular tension, and by putting your fingers on different frets, you alter the effective length of the string.  So, each string, when made the correct length and plucked, vibrates in its first natural mode to yield a particular note.  Because the guitar string is fixed on both ends, what you get when you pluck it is like the top picture in the jiggled rope discussion, the first natural mode.

Now, we’re going to make the jump from wave shapes in ropes to pressure waves in air.  Imagine rather than a string showing wave patterns, pressure variations in air.  Can’t image that?  Okay, then imagine someone talking to you.  Sound travels via fluctuations of pressure in the air.  When you talk, you tighten or loosen your vocal chords, make them vibrate like a guitar string, and sound emanates from that physical vibration turned into pressure variations in the air.  This is the jump from structural vibrations to acoustics but it’s still dependent on the notion of waves and frequencies. 

Okay, so rather than thinking about your vocal chords, imagine playing a trumpet.  You make your lips vibrate in the mouthpiece and, at certain particular frequencies, you can make the trumpet sing clear, bright notes.  You supply the forcing function; the forcing function matches a natural frequency of the column of air within the trumpet; and you create standing pressure waves that sound like music to everyone in the vicinity.  Quite simple.  And different instruments have different natural frequencies.  A tuba will never sound like a trumpet.  Why?  Because it has a different natural frequency and responds differently to different forcing functions. 

So, we started with a kid on a swing and now we’re talking about trumpets and tubas.  And what does any of that have to do with a bomb in a rocket engine?

Let’s review.  In our discussion we’ve learned that things have natural frequencies.  A rope has a natural frequency.  The space within a tuba has a natural frequency.  Everything around you has natural frequencies.  And we’ve learned that if you input a correct forcing function to a system, we can get organized results by working in conjunction with the natural frequencies.  We can get resonance.  A child swings high on the swing.  A rope makes neato patterns when jiggled.  A trumpet blares a high-C over the cheering crowd.  Okay, so here’s the kicker: all this stuff that we’ve discussed is exactly what we DON’T want to happen in the rocket engine combustion chamber.

Just like any other semi-enclosed space, a combustion chamber has natural frequencies.  These frequencies relate to all three dimensions in space (longitudinal, radial, and circumferential since chambers are typically cylindrical) and to the “stuff” in the space (for a tuba that “stuff” is air, in a combustion chamber it’s the propellants and, primarily, the combustion products).  These characteristics together define many, many potential natural frequencies, or modes, where the chamber could “sing.”  The forcing function is the combustion itself, which makes lots and lots of noise at many, many different frequencies and at extraordinarily great magnitudes all jumbled together.  So, you have lots of potential modes and lots of very powerful, very high-energy forcing functions.  Should these combine such that one feeds the other, then you could get resonance.  Again, resonance is what happens when you push the kid in the swing at the right rhythm.  But, taken to the extreme, a situation of resonance in an environment like a combustion chamber can continue to grow out of control until it becomes destructive.

Everyone’s favorite example of destructive resonance in practice was the Tacoma Narrows Bridge collapse in 1940.  The bridge was a suspension bridge, which means that it was kind of like a long, heavy, hanging piece of rope made of concrete and steel.  Well, it turns out that when the wind blew across the bridge at the right speed, it excited a natural mode of the hanging roadway.  As the forcing function blew, the bridge oscillated in response, more and more, quite violently, until, ultimately, the structure crumbled into Puget Sound.  These were not tornado-like winds.  They weren’t even unusually high winds.  They just happened to tune into the natural frequency of the bridge and the bridge responded by tearing itself apart.

Lesson:  Big vibrations can be destructive.  The bridge was fine when it was built.  It was fine for several months thereafter.  But when the right forcing function came along, it was disaster. 

This is as true in combustion chambers as it is for suspension bridges.  In order to avoid this, we build into combustion chambers such things as acoustic cavities, which are sized cavities that are  tuned so as to damp known natural frequency vibrations should they arise.  We also use physical barriers across the faceplate of the injector so as to disrupt the establishment of radial or circumferential pressure wave patterns.  These are features that we build into the design to help ensure that the space within the combustion chamber is not excited into any organized pattern that could build up to destructive levels.  We simply don’t want the chamber to sing. 

And this, finally, is where the bomb test comes in.  We use this kind of test to help prove that the features we’ve included in the combustion chamber do indeed suppress the formation of destructive oscillations.  During the test, we will set off the bomb.  It will act as a broad spectrum forcing function with sudden input of energy.  We need something as extreme as a bomb explosion to perform this energy input because there’s already so much energy being released in the combustion chamber.  It’s not like we could toot a horn at it and try to find some particular frequency.  That would be like trying to whisper to the person next to you while sitting in the fifth row of a rock concert.  It ain’t gonna get through.  So, we set off the bomb and if we have a mode lurking in the chamber that is not sufficiently suppressed by our design features, it ought to poke its head out of the noisy response that follows the explosion.  We will analyze the pressure oscillation and structural vibration data and look for notes that might “sing.”  We don’t expect any to be destructive based upon many years of design experience, but even if we identify any that don’t die down quickly we will have cause for further assessment. 

To wrap this up, I will use one more image that helps me whenever we talk about “instabilities.”  Through the whole discussion here, I’ve talked about vibrations and oscillations and natural modes and forcing functions and resonance, but what does that have to do with stability or instability? 

In the image above, I have drawn two situations of stability.  In both cases there is a ball sitting at rest between two hills.  For the ball on the left, if you perturb the ball slightly to the left or to the right, it will roll back to the middle and sit there peacefully.  However, for the ball on the right, if I perturb it much in either direction, the ball will crest one of the hills and fall into oblivion.  Thus, both situations shown are stable, but the one on the left is more intrinsically stable than the one on the right.

Similarly, the Tacoma Narrows Bridge was stable when it was built.  But given the right perturbation, it was knocked out of valley of stability and became destructively unstable.  Also, we know that the engine is stable.  We’ve already run several tests and it’s been fine.  While we don’t expect anything dramatic or destructive to happen on our upcoming test, this notion of “how stable” is what we’re examining with the bomb.  We are using the bomb to knock the ball off the center of the valley and then we’ll measure how quickly it comes back to rest, i.e., how deep and steep is the valley of our stability.  This is a measure and demonstration of the robustness of our engine design.