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

Testing at night is always so much more dramatic.

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

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

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

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

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

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

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

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

Okay, with me so far?

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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.

Five hundred seconds is exactly eight minutes and twenty seconds.  Nope, that's not rocket science.  But that was what I had to keep in mind as I watched the stopwatch application on my smart phone during the last J-2X test.  Eight minutes and twenty seconds.  That seems like a really long time when you're counting every second.

Let me set the scene.

At the NASA Stennis Space center you have collected the directors from seven of the ten NASA field centers around the country.  You have representatives from the NASA headquarters in Washington, DC.  You have a live feed being picked up by NASA TV and broadcast into the living rooms of thousands or millions of dedicated NASA TV junkies.  You have dignitaries in suits and technicians and test conductors in jeans and Hawaiian shirts (test-day tradition), reporters with notepads and cameras from every paper and television station in the greater New Orleans and southern Mississippi area, and, sitting in his ceremonial throne, the Grand High Exalted Mystic Ruler of the International Order of Friendly Sons of the Raccoons.

Well, okay, that last part about the Exalted Mystic Ruler is just fictional (bonus points to anyone who gets the 20th-century cultural allusion without Google help), but that's the way that it felt.  This was test A2J008, the seventh planned hot-fire test of the very first development engine and it was time to play show-and-tell.

Does everyone remember show-and-tell in elementary school?  You bring in something that you think is neato or special and, by getting up in front of class and talking about it you reveal something about yourself and you accidentally practice public speaking and presentation.  Once, when I was seven years old, I brought in my new baby brother, or, well, my mother did so at my behest.  I wish that I could remember what I said about him.  I imagine it was something like, "He's short, cranky, and smells funny."  Today, at least he can say, he's taller than me.

J-2X is our new baby brother -- of a sort to carry forward the analogy -- and we're showing him off to the world.  Through the first six hot fire tests of engine E10001, we accumulated a total of 225 seconds of test time.  For test A2J008, on November 9th, our show-and-tell for the world, we scheduled a test lasting 500 seconds, which is the mission duration requirement for the engine.  Here is what I saw during the test, while holding the stopwatch, standing out in the field in front of the test control center:

Can't see anything?  Okay, I’ll expand the picture in pieces starting on the left.

This is the hydrogen burn stack.  All of the excess hydrogen coming from the facility or from the engine before, during, and after the test needs to be burned off.  This is all bleed flows and waste flows that you cannot avoid when dealing with a cryogenic propellant.  If you let hydrogen accumulate anywhere around the facility, then "BOOM" you're eventually going to have an explosion.  Talk to the guys who work out in the test areas and they'll tell you plenty of tales of such things.  What is amazing as you're standing out in that field to watch the test is the radiation heat coming off that thing.  It was a chilly day and yet you almost feel like you're going to end up with a sun-tanned face.  It feels like the sun while you're on the beach except that as warm is it makes your front side, your back side is still chilly from the blustery November breeze.  Kind of an odd sensation being both overheated and chilly at the same time.

In the middle of the picture is a sign for anyone who was born without that instinctual reflex for self-preservation.  While it would seem obvious to me to not walk in front of a roaring rocket engine throwing out a plume reaching hundreds of feet in the air, the fact that they have a sign like this suggests to me that for someone, somewhere, at some time, this was not so obvious.  An unfortunate thought…

And, on the right-hand side of the picture, in the distance, is test stand A-2 with the engine firing.  In the middle of the picture below, there is a tiny, very white spot in the middle of the test stand.  That's the flame coming directly out of the engine nozzle.  In the bottom right corner of the picture below you can just see the edge of one of the liquid hydrogen barges.  For both liquid oxygen and liquid hydrogen, for extended duration tests, the propellant tanks on the test stand are not quite big enough to hold all of the necessary propellant.  So, during the test you actually transfer propellants from these barges into the test stand tanks.  So, the engine is draining the test stand tanks while you are simultaneously re-filling them from the barges.  With all of this going on, you start to appreciate the coordination necessary to pull off one of these tests.

 

When you're standing there being halfway cooked by the burn stack, several hundred feet away, the roar from the engine is nearly deafening.  Many people wear hearing protection.  Others of us are aging rock-n-roll fans.  I honestly don't think that anyone gets a complete sense of how powerful these machines are until they see, hear, and feel one of these tests.  All of the performance numbers in the world simply do not have the same visceral impact as when the engine lights and the initial sound wave runs over, around, and through you and you watch the flame bucket fill with billowing, thick, white steam.  Even twenty years after having seen my first test in person, I still cannot help but stand there like a bedazzled goof and say to myself, "Wow."

Here, below, is a picture of the test from the other side of the stand.  Why is this important (other than the sign on the fence clearly advertising what you're looking at)?  Because this is the side from which all of the non-NASA folks, some of the NASA dignitaries as well – including an astronaut representative – and the local press corps watched the test.  

So as to not keep you in any more suspense, the test came off perfectly.  The full, planned duration of 500 seconds was achieved thereby effectively tripling out test experience to date.  The coverage on NASA TV was good.  The bigwigs clapped and cheered with infectious excitement right along the rest of us.  And the press corps wore out their thesauruses trying to capture just a slice of the actual experience.  It was a complete success on all fronts.

Congratulations to the Pratt & Whitney Rocketdyne J-2X development team, the NASA SSC test crew, and the NASA Marshall Space Flight Center project management team.  While I had every bit of confidence that we'd be successful, with so many people watching our show-and-tell exercise, those 500 seconds -- eight minutes and twenty seconds -- ticking away on my stopwatch seemed like a whole lot longer.  Whew! and Yahoo!

I just got back into the office a few days ago after a long weekend in Philadelphia. My wife's niece got married. It was a beautiful venue and a moving service and good food, fabulous band, great party, and celebratory beverages flowed freely. Our niece looked gorgeous and her new husband was suitably handsome. A good time was had by all! Congratulations Ashley and Carmello!

And in the midst of all these festivities, an analogy came to mind regarding J-2X (well okay, perhaps not truly in the midst of the festivities, but certainly as part of the next-day hangover).  It's not a perfect analogy, but it kind of works on a couple of levels.  I am referring to engine-to-vehicle integration.  Here, follow my thinking...

The wedding itself is a great big project.  Everything needs to be figured out, from the biggest stuff (Where?  When?  Who to invite?) to the finest details (What food is to be served at the cocktail hour?  What are the different lighting schemes for the service and for the dinner?).  So too is the development and launch of a great big launch vehicle.  When the engine and the stage come together and the mission comes off as planned, it's beautiful.  Launch day is just like a well planned, well coordinated wedding.

Also, beyond just the singular event of the wedding day, there it the issue of everything that follows, i.e., the marriage.  And that is a matter of compatibility.  The most spectacular venue for the service and the best food for dinner and the grooviest band for the reception doesn't guarantee happily ever after.  Things have to work together on many levels in order for success to be found in a match, whether that's two people married or the engine and the stage coming together and successfully fulfilling a mission.

(Okay, so how's that analogy working for me?  Not bad, huh?)

So what's "vehicle integration"?  Well, it's lots of stuff.  On the one hand, it's the basic engine requirements.  After all, who says that J-2X ought to generate 294,000 pounds-force thrust at vacuum conditions?  It’s not as if us engine folks get to randomly pick a power level requirement out of thin air.  It comes from an integrated, comprehensive mission analysis of the vehicle.  While we like to think that the engine folks run the world, the truth is that without a vehicle and a mission to dictate requirements, we’d be nothing more than an expensive science project.

But beyond this, how do we interact with the stage?  I would suggest that there are four essential categories of interaction:
• Integrated analysis
• Boundary conditions
• Induced environments
• Operations

The first area, I've already discussed in part.  Integrated vehicle/mission analysis is used to establish the basic requirements for the engine.  Beyond that, though, you have other analyses such as contingency and hazards analyses that examine what happens if something goes wrong.  How should the vehicle respond if there is an issue with the engine or the stage or with how the engine and stage interact with each other?  So, in addition to defining upfront what the pieces should do, integrated analysis looks at how the actual, designed parts will interact under different circumstance.  Note that an output of integrated analysis often leads to the category of induced environments discussed below.

Next, you have boundary conditions and these are the most straightforward consideration.  In order to figure out what you need here, all you have to do is draw a box around the engine and see what stuff has to go into or out of the box to make the engine-vehicle combination work.  In fact, that's basically how we started in creating the Interface Control Document (ICD) for J-2X.  That's where you capture all of the agreements between the engine and the stage.  Here's a piece of that "what's crossing the box" diagram:

This diagram shows the fluids (liquids, gases) that cross the interface with the stage.  You have, of course, the propellant flows of liquid hydrogen and liquid oxygen, but then you also have the propellant tank pressurization flows that are used by the stage to keep the tanks pressurized during flight.  There are also gases used for pneumatic control of the valves and to perform purges through different phases of the flight.  There is a dedicated line that handles high-pressure helium for spin-starting the engine.  And then there are drain flows back to the stage for disposal of excess hydrogen and oxygen.  This latter category is necessary for safety reasons since, for an upper stage engine, it's usually enclosed within the vehicle for much of the mission and you don’t want to build up an explosive mixture of fuel and oxidizer in the intertank area.

For each of these interfaces, we have to define throughout the different phases of the mission acceptable pressure ranges, temperature ranges, flowrates, and fluid qualities (purity, particulate contamination, etc.).  Both sides have to agree that these values at this interface will happen during the mission or else someone might make an erroneous assumption and either the engine or the stage could fail to perform.  Sometimes, we need to specify even more detail to ensure mission success such as the two-dimensional velocity profile of the propellants as they enter the engine.  Something like this can drive significant design effort on one side or the other (or both) so such details are rarely trivial. 

Now, add to this one set of interface just for fluids additional interfaces for electrical power, control and data transmissions, and then the actual physical connections (including not just the forces and moments applied to these connections but the actual physical designs themselves in terms of dimensions and materials, bolt-hole patterns, and seal configurations).  After you’ve done all that -- fully negotiated and agreed to by both sides -- you then have an ICD, one of the bedrock documents in the life of any engine.  It's like a really, really detailed marriage license that goes on and on between the engine and the stage: who cuts the grass, who does the laundry, who sleeps on what side of the bed, who cleans the litter boxes, who opens the pickle jars, who has the remote control come football season…

The next area of consideration with regards to vehicle integration is induced loads.  In truth, these are really just another boundary condition, but we often break them out separately for convenience of tracking and documentation.  What we're talking about here are loads: structural dynamics, acoustics, and thermal loads.  Rocket engines and launch vehicles make lots of rumbling, roaring noise and lots of smoke and fire.  That’s part of what makes them kinda cool (right?!).  But it's also the kind of stuff that can cause damage if not properly accounted for in the design. 

Above is the output from an integrated analysis looking at thermal conditions of the engine during a stage separation event.  In this case, depending upon the design of the stage separation system, there were situations where the engine was getting exposed to damaging thermal loads.  In other words, the stage was imposing a load on the engine that jeopardized mission success, so the stage design was altered.  All of the elements of the vehicle have to live with the environments created by everyone else.  So, this is not too much unlike figuring out how to live together after getting married.  You learn, for example, that the combined environment of stogie smoke, an overgrown lawn, and blaring NASCAR on television apparently do not constitute the most congenial, constructive induced environment at home…

The last category in my simplified breakdown of vehicle integration is that of operations.  This comes down to who does what, when, and how.  Bringing together a whole vehicle requires quite a detailed set of instructions.  It's a lot more than "Insert tab A into slot B."  And the pieces that you're assembling come from several different project office and different contractors located all over the country.  So, on the one side of the issue is the technical matter of how you do the whole thing, but on the other side, just as importantly, you have the issue of who is responsible for performing the tasks.  With tasks come manpower, roles and responsibilities for facilities and tooling and, before you know it, meaningful expenses.  Thus, (ta-da!) you've got more negotiations and agreements and documentation.

So, engine-to-vehicle integration is, in the end, like a long, complex, heavily negotiated, analyzed, and documented marriage.  Perhaps then, other than the documentation part, it's probably like most successful marriages over the long haul.