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

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

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

So, let me tell you a little about MCCs.  

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

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

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

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

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

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

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

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

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

Welcome back to the J-2X Doghouse.  The last time that we met here, we discussed the fundamentals of what exactly makes something a rocket.  As I explained, on the conceptual level, rockets aren’t really “rocket science.”  You get the propellants together, light them on fire, and eject them out the back end of the vehicle.  Simple enough.
Okay, but how do you move that much propellant and make that much smoke and fire, enough to propel something as big as, say, the Saturn V that was over 300 feet tall and weighed millions of pounds?  That’s where things get interesting and technically difficult.  As I said before it is all a matter of power. And to get power you use an engine.

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

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


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

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

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


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

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

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

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

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

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

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

J-2X Extras: Rebuilding the Past

Several years ago, I was determined and ready to buy a new vehicle.  I happened to be at my grandparents place at the time in upstate New York and my grandfather saw me perusing the local paper for dealerships making good deals.  I told him that I was interested in getting a new pickup truck, something that I could use to go back and forth to grad school and carry all my stuff.


“Well, I’ll tell you what,” he said, “the best darn vehicle I ever had was a 1937 Ford Pickup.  That thing just ran forever, it seems to me.”  Then he winked, smiled, and added, “And, even better, I met your grandmother while I was driving that thing.”


So I went on down to the local Ford dealership and announced to the salesman wearing a plaid jacket and striped tie that I wanted to buy a pickup truck.  My new best friend smiled a huge smile, shook my hand, and led me over to the part of the showroom dedicated to their latest line of beautiful F-150s.


“No, no,” I said, “I want to buy a 1937 pickup.”


“But we don’t sell used cars here, son, and certainly not classics like that.”


“I don’t want a used 1937 pickup,” I replied.  “I want a new 1937 pickup.”


“There is no such thing,” he said, in an obvious state of confusion or maybe annoyance.


I scratched my head.  “I don’t understand.  I mean, you guys still have the drawings and such, right?  And if you can build these big, shiny new things, then you can certainly go back and build something more simple, right?  My granddad told me not to get suckered in.  He said that I don’t need all these new-fangled bells and whistles.”


For the next hour, the salesman, named Pete by way (Pistol Pete, he chuckled to himself), tried to talk me into buying one of the current year models.  He showed me everything, explaining with fast-talking expertise the dramatic advantages that his trucks had over the competition and even, he tossed in for me, far older models.  But I was not totally convinced.  Pistol Pete just shrugged and gave me his business card with a scribbled phone number for someone at their corporate headquarters who might be better able to help me.  I left thinking that might have just lost the best friend I’d had for the last ninety minutes.


The next day, I called the corporate headquarters and tried to make clear what I wanted.  I got bumped from department to department several times until I finally got someone named George seemingly willing to indulge me. 


I told George what I wanted, but I also told him that I was impressed with what Pistol Pete the salesman had shown me.  I said, “I’d really like to get that 1937 pickup with an automatic transmission, with overdrive, and cruise control.  I would really like more speed and better handling.  Better gas mileage too.  Also, I’m thinking that I need more safety stuff, so I’d like that pickup to have air bags, modern crushable bumpers, and the latest auto glass.  Plus, I’d like a bit more life and reliability, so building in that self-diagnostic system and computer would be good.  And I read that some new body materials are less prone to corrosion, so build it out of new stuff.”


“So,” said George, who sounded perpetually half asleep when he spoke, “you want a 1937 pickup truck, with all modern features, built to all modern standards, with more performance, with better reliability, and with greater safety.  Do I have that right?”


“Finally,” I said, “someone who understands!  You’ve got it!  That’s exactly what I want!”


“Right, then we will have to design you a new vehicle from scratch.  That will take about two years of design effort, building a few prototypes, and then another couple of years of road testing and then certification from the federal highway authorities — which, by the way, will result in a bunch more changes unless you only want to drive it on Sundays and holidays like an antique car.  Overall then I’m projecting that we’re talking, maybe, forty or fifty millions dollars as a starting point.”


“Huh?  What are you talking about?  That’s outrageous,” I yelled into the phone in dismay.  “You built this thing 50 years ago, didn’t you?  It didn’t cost that much then for goodness sake, even with inflation.  Surely you’ve got the drawings just lying around somewhere, right?”


“No, actually we don’t,” said George in his tired monotone.  “And even if we did and we had to build exactly a 1937 pickup, as it was built back then, it would be a project.  To start, we would have to rebuild all of the tooling, recreate the materials we used back then, and reconstitute suppliers who have long since gone out of business.  Add to that all of your new requirements and, well, you’ve got a whole new vehicle, right?  So, basically, we’ll just have to start from scratch.”


I hung up the phone in an utter daze.  Several weeks later, I bought a little Toyota pickup.  I drove it for lots of years.  I plan to someday tell my grandson that it was the greatest thing ever on four wheels and see where that leads…


One of the first questions that I got when I started this blog was why we didn’t just dust off the drawings of the old J-2 used for the Apollo Program and use that rather that launching into the J-2X development effort.  Hopefully this little story provides a bit of insight by way of analogy.  As we go along, I will tell you about the actual changes between the Apollo-era J-2 and the J-2X of today.