One of the most important analytical tools used in development of a rocket engine is called a "power balance."  A power balance is, stated simply, a simulation of the steady-state, internal conditions and functioning of the engine.  It can, on one extreme, be accomplished with a spreadsheet or, on the other extreme, take the form of a complex computer program with hundreds of theoretical calculations bolstered by dozens upon dozens of embedded, empirical relationships customized for a particular hardware configuration.  But first of all, let's talk about what a power balance is from a purely conceptual point of view.  You start with a schematic of the engine:

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

Next, you break down these pieces of the engine, the components, into descriptions with regards to how they relate to power, pressure, and temperature:

• Pumps:  Convert shaft power into fluid power in the form of elevated pressure
• Turbines:  Extract power from the turbine drive gases and converts it to shaft power
• Gas Generator and Main Combustion Chamber:  Generate power from combustion
• This power is in the form of elevated temperature (combustion = fire = hot)
• Ducts, Valves, and Injectors:  Control fluid movement in order to get propellants and combustion products where they need to be, i.e., plumbing.  Each of these items reduces pressure in the fluids flowing through them
• Cooling Jackets:  Here too pressure is lost as the fluid flows through the cooling passages, but temperatures are elevated as heat carried away (i.e., as cooling takes place)

Thus, in terms of the most significant power considerations, here is what is going on with the rocket engine:

You'll note that all of the power stuff happening in the engine is happening up on the top portion of the original schematic (and I've chopped away everything else).  In other words, the major power transfer stuff happening in the components that make up what we call our "powerpack" testing.  See?  That's why and that's where the name comes from.  Pretty clever, huh?  The whole idea is to get power to pumps so that they can makes lots and lots of fluid pressure so that they can push lots and lots of propellants through the system and into the combustion chamber.  That's the whole point of the rocket engine, push stuff to the combustion chamber to make thrust. 

So, how much pressure do you need?  That's a matter of how much stuff you've got to push the propellants through and how much pressure you want in the chamber at the end.  I sometimes think of it like that great old board game Monopoly ®.  You pass "Go" and get $200.  Remember that? 

Well, in a rocket engine, your pump is "Go" and at that point you get an allotment of pressure.  Then, as the fluid goes through the system, from component to component -- ducts, valves, cooling jackets, injectors -- you have to pay rent in the form of a loss of pressure.  That's like landing on the various squares around the board.  Paying all that rent is just fine.  You can't really avoid it.  But you have to make sure that you save enough money to stay at the hotel on Boardwalk in the end without going bankrupt.  In other words, you need to get your propellants into the chamber at the residual pressure that you desire.  Here's a representation of this pressure management process within the J-2X on both the fuel side and on the oxidizer side:

The explosion-looking symbols in that diagram represent combustion zones.  One is the gas generator, where you make the power to drive the pumps, and the other is, of course, the main combustion chamber, where you make your thrust.  The gray lines represent combustion products coming out of those combustion zones.

One last question that needs to be considered is this: How much combustion chamber pressure do you want (and/or need)?  In other words, when your propellants arrive at the main combustion zone, at what residual pressure do you want that combustion to take place?  Sounds like a simple question, right?  Well, of course, you want it to happen at the "optimal" pressure.  But what does that mean?  That is not an easy question to answer.  In terms of energy release, within certain bounds, the chamber pressure does not much matter (or, at most, it's a secondary factor).  What it really comes down to, believe it or not, is engine size and weight and a handful of manufacturing considerations. 

In the drawing above, I have tried to show two combustion chamber and nozzle combinations where the one on top has a throat diameter and nozzle exit diameter twice as large as the respective measurements in the lower version.  Thus, both engines using these combustion chambers and nozzles would have the same ratio of nozzle exits area to throat area.  It's just that the one on the top would have a throat with four times as much area (area being proportional to the square of the diameter).  Would it surprise you to learn that these two engines could generate the same thrust if the one on the bottom had four times as much chamber pressure as compared to the one on top?  Yep, it's true.  If the top engine has, say, 500 psi (pounds per square inch) chamber pressure and the bottom one has 2,000 psi, then these two rockets are -- to first order estimates -- operating at the same performance level. 

What does that mean?  That means that you could have a great big, bulky rocket engine or you could have a small, "tight" one.  It would seem that the small one feels more efficient except that with that high chamber pressure you have to generate all that extra pressure in your pumps.  That takes a lot of pump power and therefore turbine power.  And containing all of that pressure throughout the engine system means thicker walls on your ducts and valves and everything else.  Thicker walls mean heavier pieces.  So maybe that "tight" engine is really more wasteful.  So instead, maybe the big bulky engine sounds like a good idea since it’s easier on your turbopumps.  Except then you realize that it's too big to fit on your vehicle and, by the way, that monstrously big nozzle weighs a ton and nobody has machining tools large enough to produce the thing.  So maybe the bulky one isn't right either.  Blah, blah, blah…  It's enough to give you a headache!  But those kinds of discussions back and forth are what are known as trade studies and they are the foundation for what your engine will eventually become.  There is rarely a simple, obvious answer since everything has impacts on everything else.

So, how does all of this get back to the power balance?  Well, you take all of those notions discussed above and start applying the following:

• Calculations that describe how much energy is released by the combustion of your propellants.
• Calculations that relate pump speed and pump design features to fluid pressure increases.
• Calculations that relate turbine-drive gas conditions and turbine design features to power extraction.
• Calculations that describe pressure losses for fluid flowing through ducts, valves, cooling jackets, and injectors.
• Calculations that relate fluid flow and fluid conditions to heat transfer processes in cooling jackets

Once you have all of these relationships, then you can perform a power balance.  You use your power balance to inform your trade studies.  Bigger or smaller?  Faster or slower?  You just have to realize in using it that you can't get anything for free.  The power that you generate in your gas generator uses up some of your propellants (for a gas generator cycle engine) so they can't go through main injector with the purpose of generating thrust.  You cannot perfectly extract the power from the turbine drive gases.  And, you also cannot pump with perfect efficiency.  These considerations all have to be taken into account in your calculations.  But the result will be an analytical model that can tell you the pressure and temperature of the propellants throughout their journey through the engine.  It will tell you shaft speeds of the turbopumps.  And it will give you overall performance of your rocket engine.

So, let's say that you've been given the job of designing an engine from scratch.  You have a thrust requirement and a specific impulse requirement.  Let's say, further, that you know what your propellants are supposed to be and let's even go so far to say that you've been told that it ought to be a gas generator cycle engine.  Okay, so now what do you do?

Here's one approach (…one of many, many possible):

• Pick a chamber pressure.
• Because of your thrust requirement and specific impulse requirement, you can start with a pretty good guess as to your propellant flow rates.
• Next, generate your schematic layout of the engine and the various components and piece together your simulation of the system.
• Then, figure out how much pressure your pumps need to generate and, therefore, how much power you need your gas generator to create.
• Balance that pump power needed with turbine power to be extracted; you've now set your gas generator conditions.
• Based upon how much propellant that you're "losing" down the gas generator / turbine drive leg, you can figure out how much nozzle expansion ratio you need to get to your specific impulse requirement.
• You'll probably go around a bit in circles with the previous few steps -- also known as iterating -- until you get a completely self-consistent set of answers (It's essentially a process of making educated guesses, seeing if everything balances out, making new guesses based upon any lack of balance, and again seeing if everything balances.  With a good solution scheme, you'll eventually arrive at a place where all your guesses work and your system is balanced.)
• You now have a rocket engine design.

But, is it what you want?  Can you build it?  Does it fit with the vehicle?  Will it be too heavy?  Are the component performance factors within reasonable expectations (i.e., rules of thumb carried around by the various component experts)?  Is the design close enough to a legacy design so that you might be able to leverage previous, related experience?  Or, perhaps, is the design all so new and different that the necessary development program will be quite extensive (and therefore expensive)?  It may be that there are a whole bunch of reasons why your design, frankly, stinks so you need to go through the whole process again.  In the end, after several cycles through, you almost never come up with a design that makes everyone happy from every perspective, but you come up with one that is sufficient, acceptable, and reasonable.  So that's the design that you go and design, develop, and test.

Hopefully, I've shown you that a power balance, an analytical simulation of the internal workings of an engine, is an integral tool in the conceptual design of a rocket engine.  Once you've got some general idea of some key parameters you need, the power balance fills in the details, sets the necessary parameters for your turbopumps, captures your fluid splits and conditions, and establishes the general sizing for your main combustion chamber and nozzle.  It uses physics and physics-based empirical relationships -- combining the disciplines of fluid dynamics, heat transfer, combustion science, and hardware mechanics -- for all of the major components of the engine to balance the power generated against the power used and, in so doing, describes conditions throughout the engine.

(This, by the way, is my favorite kind of analytical modeling simply because it combines so many different disciplines and yields such a broad and useful tool.  I was lucky enough to be assigned to power balance modeling activities for the Space Shuttle Main Engine when I started working.  And that experience has informed everything else I've done for the last 20+ years.)

It's been awhile since I've had the opportunity to update what we've been doing for the J-2X development test campaign.  So, everyone is probably wondering where we stand.  Well, if possession is nine-tenths of the law, then J-2X IS THE LAW for the NASA Stennis Space Center A-complex!  Right now, the J-2X development effort has our PowerPack Assembly 2 in test stand A-1 and Engine 10001 has been reinstalled on test stand A-2.

Below are two pictures of the J-2X PowerPack Assembly 2 (known as PPA2) taken from different perspectives.  In the second one, you can see that several pieces are coated with ice.  That's obviously a picture with cryogenic propellants loaded in the ducts and turbomachinery.  In other words, to use our local jargon, in the second picture PPA2 is chilled down.

Well, you saw in a previous blog article that we spun up the PPA2 and we demonstrated ignition of the gas generator.  Beyond that, however, we've had a few hiccups.  For the first test intended to get to mainstage operation, we didn't get very far.  We effectively demonstrated again the spin start and ignition of the gas generator.  Immediately beyond that, just a few tenths of a second in fact, the test shut down due to an issue on the facility side.  As I've described before, the PPA2 is kind of an odd beast in that it's a half-engine and half-facility test article.  In this case, a facility valve did not function the way that it was supposed to.  It was sluggish.  A subsequent investigation into the facility hydraulic system identified and fixed the issue so we were again all ready to go.

On the next test we got a little farther but just before getting to mainstage, we busted an engine-side redline limit and had to shut down early.  The reason for that early cut was actually quite analogous to the early cut we had on our first attempt at a mainstage test for Engine 10001.  We didn't quite understand the characteristics of the engine components and so, as we powered up the system, we were headed towards an operating point different than we'd intended.  In other words, our calibration was a bit off.  The redline system identified this situation and, properly, cut off the test before anything damaging might occur.  While early cuts are sometimes a pain in the neck, we have those safety systems built in there for a reason.  There is always a substantial and meaningful difference between a nuisance and something potentially worse. 

Over the course of the next couple of PPA2 tests we once again proved that hydrogen is a pernicious rascal.  This is something that has been proven on many former occasions throughout the history of rocket engine development.  If you give hydrogen any opportunity to leak, any at all, it will.  And sometimes, it will only leak when the system is chilled down so that when you're checking out the system before a test, when you're searching for potential leaks, you don’t see a thing.  But then, when you are all set up and get the test going, ta-da, you suddenly have a fire.  Why a fire?  Because with a hydrogen leak around all the rest of the hot stuff going on with the test, a leak almost always becomes a fire.  And, because pooled, un-burnt hydrogen is a potential detonation hazard, we also have devices all around the vicinity of the test article designed to make sure that any leaked hydrogen gets burnt.  So, quite simply: hydrogen leak on engine test = hydrogen fire on engine test.  The fires that we saw on these two tests were not on the "engine" half of the PPA2 test article per se.  Instead, we got fires on the facility half.  The emergency systems in place for such issues include cameras and temperature probes so that there was practically no damage and our hardware is just fine.  But the fires did mean that we've accumulated only a limited amount of mainstage data so far.

Undaunted, we have investigated and, we believe, solved the issue and will once again be ready for testing in the near future.

On the other test stand, specifically stand A-2, the folks at the NASA Stennis Space Center have been darn busy.  If you go back a couple of months in these blog articles you'll find a discussion about the next phase of testing for J-2X development engine 10001 (E10001 for short).  In that article, I tell you all about the test stand passive diffuser and the engine nozzle extension that we’ll be testing.  Well, the first thing that we had to do to make this next phase for E10001 possible was to modify the test stand.  In order to make the passive diffuser function properly, you have to effectively seal off the top.  

In the picture above you'll see what’s called the clamshell.  This two-piece device rotates out of the way for access to the engine between tests but during a test wraps around the nozzle of the engine on the top side and connects to the diffuser on the bottom side.  We'll use a rubber-ish seal in the gap between the clamshell and the nozzle to maintain the seal while accommodating movement of the nozzle during hot fire testing.  Getting this thing designed, built, and into the stand was a heck of a lot of work.  The folks who accomplished this deserve mucho kudos.

So, that's the test stand side.  Next, there is the test article side, i.e., the engine itself.  Because the nozzle extension is not structurally beefy enough to support the rest of the engine, the installation of the test article into the stand has to be performed in two steps.  First, you install the main part of the engine and then, once that’s in place, you install the nozzle extension. 

By the way, while it sounds easy enough to simply bolt the nozzle extension into place on the end of the nozzle, it's actually a bit more complicated.  While both pieces are designed to be exactly round, nothing is truly exactly round, especially not pieces of hardware this large.  We have to use special "rounding" tools during the mating process.  It's sometimes amazing to think about all of the specialized tools and equipment that you need, in addition to the engine itself of course, just to make the engine work. 

So, that's where we stand in terms of our development test campaign.  As if southern Mississippi isn't hot enough in the summer, J-2X will soon be adding even more heat from two active test stands very, very soon and for several months to come.  Elsewhere, FYI, we're working on various stages of fabricating and/or assembling J-2X development engines 10002 and 10003.  They will be what follows PPA2 and E10001 into the test stands.  In other words, there's lots of excitement yet to come.

A couple of articles back, I asked the following question:

"The whole orange-flame thing is not something I entirely understand...Any ideas from anyone else?"

I was talking about the flame stack during a night test at the NASA Stennis Space Center.  It was a legitimate question.  Combustion chemistry is really not my specialty.  Lots of things are not my specialty.  Try as I might, I've found that I can’t know everything about everything.  Indeed, considering the many brilliant and knowledgeable people with whom I have the privilege of working here at NASA, I've come to accept the conclusion that there is a lot more stuff to know than can ever be learned.  But that can never stop you from learning something new.  And so I have with this.

In response to my blog question, we received a number of comments on the blog and those are posted.  Thank you for your inputs and interest. 

However, behind the scenes (so to speak), a coworker of mine, Robin Osborne, who does have experience with this kind of stuff read the blog and starting poking around amongst her notes and amongst her fellow experts in the field of flame spectroscopy.  Below is a picture taken of igniter testing at MSFC using a gaseous hydrogen-oxygen mixture.  Here too you can see a red-orange flame although it takes a distance for that colored portion to show itself.

According to Dr. Robert Pitz from Vanderbilt University, "Pure hydrogen (with no sodium) -- air flames will glow red in a dark room due to the water vapor emission lines."  Both Dr. Joseph Wehrmeyer working in support of the Air Force and Richard Eskridge from NASA concur, noting that water vapor generates an orange-red-infrared continuum in such flames.  However, all of these individuals also noted that there is a strong orange coloration in such flames due to sodium contamination within the hydrogen.  The sodium is present as sodium hydride within the liquid hydrogen which decomposes at high temperatures to generate the vibrant color.   The sodium contamination is a byproduct of how large, industrial quantities of hydrogen are made for uses such as, for example, flying the Space Shuttle.  Dr. Christopher Dobbin of NASA noted that in the 1990 timeframe he was engaged in an analysis of the flame plumes ejecting from the Space Shuttle Main Engines (SSME).  He said, "The (time) average sodium concentration we measured in the SSME exit plane was 0.091 parts per billion."  That doesn’t sound like much, and it's not enough to impact engine performance or operation, but it's still enough to measure based upon spectral analysis of the plume.  Another possible contaminant, according to Richard Eskridge, is potassium and that can further contribute red emissions.

So, there you go.  It's a matter of water vapor at the right temperature and pressure (and therefore density) and a couple of key contaminants in the fuel.  It's "common knowledge" around here amongst us Datadogs that the plume of a Lox/Hydrogen rocket engine is clear.  But that's not entirely correct.  It's nearly clear.  It still has the characteristic red-orange tint, but it's at a density where the emission is too low to see.  On the other hand, for the flame stacks at the test facility -- the origin of this whole discussion -- we're talking combustion at atmospheric pressure so the water vapor products are denser as are the relative contamination levels since it's a fuel-rich environment.  And that's why they show up at those brilliant colors in the nighttime pictures.

See, even old Datadogs can learn new tricks.  Thank you to everyone who added their two cents, but especially to Robin Osborne for her inputs and insight.

Put a little kid into the driver's seat of a (safely parked) car and what's the first thing that they do?  They grab the steering wheel and twist it back and forth.  Twisting the steering wheel back and forth is just about the most intuitive, intrinsic -- practically instinctive -- sense of "driving" that I can imagine.  Even the handlebars of a bicycle or a motorcycle fit into the same idea.  Can you think of driving a car or a boat or, well, anything, without a steering wheel (of some sort)?  It's tough, isn't it?  

Okay, now think of a launch vehicle blasting off the pad and upwards heading towards the sky.  Other than for some extreme, emergency conditions, there is not anything that stands in for the steering wheel on a launch vehicle during ascent.  The process of steering the vehicle requires such precision and responsiveness that it has to be automated.  Sorry Buck Rogers, the computer is flying the vehicle.  But, even without a steering wheel, per se, how does steering happen?

With a car, you point the front wheels and, thanks to friction between the tires and the road, you get pulled (or pushed for the sports car purist and NASCAR fans) in that direction.

With a boat, you use a rudder so that the water pushing against it points the boat in the direction you want to head.

With an airplane, you have to use a combination of aerodynamic surfaces since you're now dealing with steering in three dimensions, not just two as with an automobile or a boat.  But the idea is basically the same: the air through which you're moving pushes against the aerodynamic surfaces and points the plane in the direction you need to go.

What do you do with a launch vehicle?  Not long after the first couple minutes of flight, you're so high in the atmosphere that there's not enough air to effectively use aerodynamic surfaces.  In other words, you don't have a road and a rudder won't work.  So what do you use when you don’t have anything against which to push?  That's right: a rocket!

You could, if you chose to do it this way, use dedicated steering rockets.  We do use these when we're in space and we typically call them "retrorockets" or "reaction and control" rockets.  But during the ascent, you already have a big rocket engine pushing you along so you might as well use that if you can, but to do so, you need to twist it around…

[Yes, I can’t help myself.  I had to make a musical reference.  "Twist and Shout" (written by Phil Medley and Bert Russell) was originally recorded by the Top Notes, then the Isley Brothers, and, eventually by the Beatles (as so memorably replayed many years later in "Ferris Bueller's Day Off").  Lots and lots of people have done versions of this song, but probably the most bizarre was Mae West -- yes, THAT Mae West -- when she was 72 years old.  Who knew?]

What do I mean with regards to twisting a rocket engine?  Here's a video of what we call "gimballing" an engine on the test stand, in this case a Space Shuttle Main Engine (video provided by my friend and coworker Rick Ballard from his Liquid Rocket Engine class materials):

So, for a launch vehicle during ascent, you accomplish steering by pointing the thing pushing you, i.e., your main propulsion rocket engine.  That's a cool video, huh?  But how do we accomplish that?  The movement itself is provided by hydraulic actuators.  These are push/pull devices driven by fluid pressure.  The brakes on your car are hydraulically actuated, for example.  Another example of hydraulic actuators are those lifts at the garage they use to pick your car up off the ground.  In other words, they can be very powerful devices.  You can do a quick web search on "hydraulic actuators" and find all kinds of pictures and articles and even sales pitches from manufacturers.  

 
On the rocket engine we put just two connection points for the actuators at ninety degrees apart from each other.  This gives us what you can think of as full, two-dimensional coverage.  If you remember back to math class, everything on a flat page can be located via X-Y coordinates.  Thus, one actuator provides the X-direction and the other provides the Y-direction.  And, with that, we can point the engine to any location within a given, limited range of movement.

At the top of the engine, in order to allow the movement, we put in what amounts to a universal joint.  It's called the "gimbal bearing" and it's like the ball-and-socket joint in your shoulder except that this joint has to carry the full thrust load of the engine while maintaining its flexibility.  Because of the conditions seen by the engine, you can't use any typical lubrication like grease or anything like that.  Instead, we use a Teflon-impregnated fabric layer.

I like the picture above showing several guys working with typical engine gimbal bearings.  In the picture you can get a sense of how beefy these things are when assembled and you can clearly see the "ball" part of the ball-and-socket joint. 

Have we gotten to the really, really neato part yet?  Yes, we have (in my humble opinion).  Here it comes.  How is it that we can move around the engine?  I mean, besides the big ball-and-socket joint at the top that is meant to move around, all the rest of it is assembled out of all kinds of stiff metal pieces, right?  It’s not like you can stick cryogenic propellants through a flexible rubber garden hose.  So how do we get the compliance in the rest of the engine components that allow for the movement the actuators and gimbal bearing are providing?  With no compliance, the actuators would push and pull, and, assuming that they were powerful enough to do damage (and they usually are), the engine ducts would buckle and crush and, frankly, you'd have a crumpled mess.  What we do then is build the compliance into the engine with specific parts to provide this functionality.  This is accomplished in different ways on different engines.  Below is how this compliance is accomplished for J-2X for the main propellant lines:

That pretty piece of hardware is a propellant inlet duct.  In fact, that picture is of the first new propellant inlet duct fabricated for a J-2, J-2S, or J-2X engine in forty years.  This new duct is like the heritage design but better, safer, more robust.  It is an extremely difficult piece of hardware to make in that it involves some very highly specialized welding techniques.  So a big shout-out goes to Pratt & Whitney Rocketdyne and the guys on the shop floor.  Way to go guys!

How does it work?  The sections with the convolutions are called bellows.  Above is a cut-away of a metal bellows made by the same company as our propellant inlet duct, Gardner Bellows Corporation, but not our same design.  The bellows take advantage of the way that metal can act like a spring.  If it doesn't get bent too far, the metal will bounce back undamaged.  These dozens of convolutions in the bellows allow for enough movement that the whole thing acts like a stiff spring.  The hinged structures on the sides hold the bellows together and constrain the springy parts and make sure that they stay in their groove (so to speak). 

The next natural question about this duct is this:  Why does it appear to be in two pieces, an upper bellows and a lower bellows?  The answer is that it isn't in two pieces; it's in three pieces.  In between the upper bellows and the lower bellows is a third set of bellows that you can't see very well and that's because they're really flat.  This is the torsional bellows and it provides for a slight twist between the upper and lower sections.  When you're gimballing the engine, not only do you need these ducts to bend, you also need a bit of twist…

I think that the torsional bellows is even cooler than the bending bellows.  Have you ever tried to twist a long piece of wood, like maybe an eight-foot-long, one-by-two strip?  The longer the piece, the easier it is to get a few degrees of twist.  A short piece of wood, even with the same cross-sectional dimensions, won't allow for as much twist.  There is an "allowable twist per unit length" thing going on: longer = more twist, shorter=less twist.  Okay, now assume that the same is true for a metal pipe.  If you have a very long metal pipe and you apply a twisting force to it (torsion), you can get some movement, more movement than you’d get with a short pipe.  But there's no space on a rocket engine for a very long pipe, so how do you allow for some twist?  What we do is collapse the long pipe into shortness by making it into a very tight accordion-like package.  In other words, we add convolutions kind of like the bending bellows, but make them very tight, very flat.  So, all of the metal "length" is still there, just in a really compact, squashed package.  It kind of feels like cheating, somehow, but it works.  See?!  That's just neato!

In addition to the big ducts, the propellant ducts, you also have to take into account any other connections between the engine and the vehicle stage.  If you think back to the article about vehicle integration, you'll remember that we've got pneumatic lines and propellant pressurization lines and helium spin start lines connecting the engine to the stage.  In all of these lines we have to make provisions for compliance to engine gimballing motion.  As you can imagine, this makes the design for these pieces not simple.  But nobody ever said that rocket engines were supposed to be simple.  Also note that different rocket engines use different approaches for achieving the compliance necessary to accommodate gimballing, but they almost always use "springy" metal bellows in some sort of configuration.

The first J-2X engine that will see gimballing in the test stand will be development engine E10002.  That should be happening later this year.  Stay tuned.  I’ll certainly be posting some gee-whiz video after that happens.  Go J-2X!

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