Inside the LEO Doghouse: Nuclear Thermal Engines

Perhaps it’s a Midwestern thing, I don’t know.  I grew up outside Chicago (although my family is mostly all from back east) so that’s where my theory originates regarding it being a Midwestern thing.  After all, when I was growing up, nobody around me seem to think that it was odd so my assumption is that they all – we all – pronounced it this same way:  “new-que-lar.”  It was only later, when removed from the wayward influences of my isolated rustic upbringing was it pointed out to me – sometimes amidst unsuppressed laughter – that the word is spelled “n-u-c-l-e-a-r”.  Or, in other words, it is pronounced: “new-clear.”  Okay, so, whatever.

nuclearplantIf I’m talking about ‘nuclear this’ or ‘nuclear that,’ what’s shown in the picture above is probably what popped into your head.  And that’s fair.  This is the way that nuclear power most commonly impacts our daily existence, i.e., through the light switches and electrical outlets in our houses that are ultimately traceable back to a power plant, some of which are based on nuclear fission reactors.  Below are a couple of other applications of nuclear power with which we are familiar.

shipsOkay, so what does this have to do with rockets?  Well, there are ways to use nuclear power to create rocket propulsion.  And, by the way, this is not some newfangled idea out of the blue.  Did you know that one of the original plans for the third stage of what would become the Saturn V rocket was for that stage to use nuclear-thermal propulsion?  That plan was eventually dropped and a configuration using a J-2 engine was chosen instead, but going all of the way back to the late 1950’s people were thinking of ways to use the extraordinary power of nuclear fission to enable and enhance space exploration.

There are two basic classes of rockets that use nuclear fission.  One is called “nuclear-electric” and the other is called “nuclear-thermal.”  In a nuclear-electric rocket, you use the reactor to generate electricity (like a small power plant) and then use that electricity to make high-velocity ions.  The latter portion of the sequence is called “ion propulsion” and there are different schemes and ideas out there, some of which have been used on unmanned spacecraft in the past using other sources for electrical power.


Nuclear-electric propulsion is extremely efficient.  In the past we’ve talked about specific impulse being a measure of rocket efficiency.  Well, a nuclear-electric propulsion system is on the order of ten or twenty times more efficient than your typical high-performance liquid hydrogen / liquid oxygen chemical propulsion rocket such as J-2X or RS-25.  BUT (and this is a really, really big “but”), for all that efficiency, they don’t generate much thrust.  AND they are very heavy.  Thus, the only place where using nuclear-electric propulsion makes any sense is in space.  Even there in “weightless” space, the extremely low thrust-to-weight ratio means that this propulsion system is only appropriate for missions where you’re willing to be very patient and get to wherever you’re going quite slowly.  That’s not really an appropriate approach for missions with humans on board.

The other class of nuclear power rocket engines, and the one that I really want to tell you about, is nuclear-thermal rockets.  It is appropriate that we discuss nuclear-thermal rockets in an article immediately following an article discussing expander cycle engines since they are actually closely related.  Almost cousins.  Below is a schematic for a nuclear-thermal rocket in the same general format as the various expander cycle engines were shown in the previous article.


What you don’t have here is any oxidizer.  Why?  Because there is no combustion.  In a normal rocket engine we use fuel and oxidizer in a chemical reaction to create hot combustion products.  It is the ejection of those hot combustion products generate the engine thrust.  For a nuclear-thermal rocket engine we use the reactor to make the hot stuff.  You can think of the reactor, when operating, as a really, really powerful heat source, even more powerful than a chemical reaction.  Thus, I can use that heat source to generate turbine drive gas, just as in an expander cycle engine, and I can also use that heat source to make the hot gas that generates the engine thrust.  In terms of configuration, the reactor has built into it flow passages where the fuel picks up heat as it goes along.  These passages can be along the outside, which I’ve shown here as feeding the turbine, and they are throughout the innards of the core.  There are different ways of accomplishing this.  One way is to make extrude the core rods with passages – “coolant channels” – through the length of the rods.  This is shown in an old sketch from the NASA archives below.  Another way to achieve this is to make the core out of pellets or “pebbles” trapped in little cages.  Doing this, you’d get what’s called a “pebble-bed reactor” and such a configuration provides for lots and lots of heat exchange surface area between the core pellets and the working fluid flowing through.


So, what’s the “fuel” in the rocket schematic, i.e., the working fluid shown in red?  The typical answer is hydrogen.  One of the reasons that we use hydrogen in a chemical engine is because when we run fuel-rich, we get lots of hot, unreacted hydrogen as part of the exhaust.  Hydrogen is very light.  When it gets hot and energetic – and hydrogen picks up heat wonderfully – it moves very fast.  If you think back to the rocket equation, fast moving exhaust means high performance.  In this case for the nuclear-thermal rocket, the exhaust is pure hydrogen, so performance can be quite high.  How high?  Well, it’s not as high as the nuclear-electric options discussed above, but specific impulse values two times that of J-2X or RS-25 are entirely plausible.  Further, despite the fact that nuclear-thermal engines are quite heavy, their thrust-to-weight ratio is generally much better than the nuclear-electric options.  In other words, a nuclear-thermal engine has some good “oomph,” enough oomph to make it potentially usable for human spaceflight.  And that’s why it was seriously contemplated in the earliest planning for the mission to the moon over fifty years ago.  That’s also why, in my humble opinion, it is a prime candidate for any future human mission to Mars.

As mention above, this is not a new idea.  It reaches all of the way back to the 1950’s.  There was a series of active programs all throughout the 1960’s falling under the general heading of NERVA, Nuclear Engine for Rocket Vehicle Application.  Below is a picture of an actual test of one of these engines.


So, with all this history behind us and with all this potential for performance, why on earth haven’t we been pursuing this technology first and foremost?  Because, well, nothing is free and nothing is ever as easy as it seems at first.

The biggest struggle with nuclear-thermal rockets is that whole radiation thing.  Okay, yes, I said it.  Radiation is bad.  Deadly.  And very long-lasting.  While rocket engines of any type always pack a punch in terms of power density and, therefore, the possibility for catastrophe, with the added spice of radiation, you’ve got quite the potential for a noxious stew.  Does this mean that we ought to simply avoid it altogether?  That’s a valid question and one that’s been debated for about 50 years.  It would be presumptuous of me to suggest that I could resolve the issue definitively, but we can discuss the constituent elements rather than just falling back on the “radiation is scary” answer.


First, let’s talk about whether it could be used on a vehicle.  The reactor is going to generate radiation.  Internally, that’s how it works and that radiation in different forms overflows the boundaries of the reactor.  It just does.  So, what do you do?  Well, you provide shielding.  The truth is that space is chock full of radiation.  If not for our little pocket of safety thanks to the magnetic poles of planet earth, we’d be cooked to the crisp by the radiation pouring out of the sun.  When you’re in space, particularly if you’re going beyond our little planetary pocket of safety and traveling to the moon or to Mars, you’re going to get bombarded by radiation so no matter what, shielding is necessary.  Shielding is heavy because in order for it to be effective, you need big, heavy molecules to catch gamma rays (my very simplistic explanation).  Lead and tungsten are two common shielding materials for this purpose.  With a fission reactor, you are also going need something for neutron flux moderation.  The typical material for this is Lithium Hydride but the propellant tank itself containing hydrogen also works well for this.

A means for minimizing the weight impact for the shielding used to protect the astronauts from the reactor radiation is to use the notion of a shadow.  In the sketch below, you have a reactor on the back end of the vehicle, a shield in between, and the spacecraft up front.  Between, connecting everything and not shown, would be the propellant tank and the usual shiny structural trusses.  As you can see, the shielding creates a shadow from the radiation within which the spacecraft sits.  Now, it’s not always this simple because you sometimes need holes through the shield for functional reasons or you could get reflected/scattered radiation effects from structural elements, but this is the most common general scheme for dealing with a reactor on a spacecraft.  Stick the reactor out a ways from everything, place the shield close, and cast a long, broad shadow.


Okay, you say, you’ve protected the astronauts, great.  But what about the six or seven billion people back here on planet earth?  After all, in order to fire up a nuclear-thermal rocket in space you first have to get it into space and that means that you have to launch it from the surface of the planet.  Launch always involves risk.  What happens if the launch vehicle blows up?  If the launch vehicle blows up, then the reactor blows up.  Wow.  Now, how dangerous is that?  I will not pretend that I can answer that question with my limited background.  But I can tell you that prior to and during launch, the reactor is “cold.”  While you probably wouldn’t want to use enriched uranium to make wallpaper for your house, it’s not that horrifically dangerous prior to use in an active reactor.  It is only after the reactor gets going that the innards get all juiced up and seriously radioactive.  The plan would be to launch the reactor never having been “juiced up” and only start it when it is at a safe distance from earth thereby eliminating as much as possible the potential of reentry of a hot, radioactive reactor into the atmosphere.

[Note that the fact that you need something like tungsten for a shield (very heavy metal) and you’ve got bundled up uranium in your reactor (another even heavier metal) are big reason as to why a nuclear-thermal rocket engine is typically so heavy as compared to a chemical rocket engine.]

The next issue to deal with for a nuclear-thermal rocket is probably one of the most difficult: testing.  On the one hand, we’ve got lots of places where we can test rockets.  On the other hand, we have certain places where we test reactors (mostly under the expert supervision of the Department of Energy in coordination with the U.S. Navy).  But putting those two pieces together and playing with them as a unit, now that’s really tough.  Why?  Because of that darn radiation thing again.

After a typical J-2X or RS-25 test, after we’ve cleared residual propellants and bled away any excessive pressures, we’ve got technicians all over those engines.  They’re inspecting this, examining that, taking things apart, putting them back together.  The whole point of a development program is to get data and a lot of that data comes in the form of post-test inspections.  With a nuclear-thermal rocket, that wouldn’t be possible unless you really, really didn’t like your techs (please note that’s not serious, just a joke in poor taste).  Once the reactor has been fired up, it’s hot.  Yes, you can dial it back down so that it’s no longer at fully throttle, but both it and the surrounding stuff are contaminated to some extent with radiation.  And you don’t just wipe radioactivity away with a damp rag.  After that first initiation of self-sustaining chain reaction (i.e., “critical”), everything needs to be handled very differently.  Also, in addition to this, the hydrogen working fluid that we push through the reactor, it too picks up some level of radiation.  No, not a lot.  But under modern safety restrictions, all of that hydrogen would have to be captured and scrubbed clean before release.  Capturing rocket exhaust is not an easy job.  It’s possible but it requires some extraordinary test facility capabilities.


With all this difficulty, how can we conceive of getting through a development program?  A rocket engine development program requires testing because, frankly, we are demonstrably not smart enough to do without it.  One answer:  Split the engine into two pieces.  If you do the rocket part separate from the reactor part, then you can keep the two pieces blissfully in their natural environments, i.e., the rocket part on NASA test stands and the reactor part in the Department of Energy labs.  Focusing on the rocket side (not surprising for me, eh?), the difficulty then becomes in simulating the heat source that is the reactor.  There has been some work done here at NASA MSFC at creating reactor simulators specifically for the purpose of testing subsystem separate from reactors whether those subsystems are rocket engines or power generation systems.  Below is a picture of one such reactor simulator.


In this manner you can minimize or possibly even eliminate for the combined rocket/reactor testing that is so difficult to pull off.

Before nuclear-thermal rockets can be used on missions of the future, there are a number of challenges to overcome, but the potential gains in vehicle and mission performance are impressive.  While this topic doesn’t fall entirely within the realm of liquid rocket engines consistent with the title of this blog, I thought that the similarity of the schematic to expander cycle engines would be of interest.  In this case, rather than a chemical reaction, you have nuclear fission, yet the engine cycle is still a matter of driving a fluid into a place where it gets hot and, from there, is ejected at high velocities.  In this way, a rocket is a rocket is a rocket, even if it is “nu-que-lar.”

LEO Progress: RS-25 Adaptation

I love dictionaries (yes, I know, you’re shocked; shocked!).  I have several at home and at work including a two-volume abridged Oxford English Dictionary (OED) that was a wonderful gift from my mother several years ago. 

The definitions and word origin above comes from the OED on-line site.  Embedded within our language is so much condensed history and accumulated knowledge that it’s amazing.  While I have no doubt that this is true of every language, I only know my own to any significant degree.  Indeed, I’ve been babbling my own language for a long time now — forty-some years — but I can always pick up a dictionary and learn something new with just the flick of a page or two.  You really can’t say that about too many other things.

As the title for the article and the definitions above suggest, for this article we’re going to talk about “adaptation,” specifically about adaptation of the RS-25 engine.  As part of the Space Launch System Program, we are undertaking something a bit unusual for the world of rocket engines.  We are taking engines designed for one vehicle and finding a way to use them on another vehicle.  Now, this is not a completely unique circumstance.  The Soviets/Russians really were/are masters of this kind of thing.  But for us, it is not something that we do very often.  In terms of the big NASA program rocket engines, I can only think of the RL10 that was originally part of the Saturn I vehicle as an engine design that has had a long second and even third career on other vehicle systems.  The truth is that engines and vehicles are, for us, generally a matched set and the reason is that we just don’t frequently enough build that many of either.  Note that, technically speaking, we are also adapting the J-2X from a previous program, Constellation/Ares, but that’s obviously a bit different in scope and scale given where we are in the development cycle.

The name “RS-25” is, as I’ve mentioned in past articles, the generic name for the engine that everyone has known for years as the “SSME,” i.e., the Space Shuttle Main Engine.  There is much to talk about the RS-25.  Lots and lots of stuff.  More stuff that I could possibly fit into a single article.  Here are just some of the RS-25 topics that we’ll have to defer to future articles:

       •  History and evolution
       •  A tour of the schematic
       •  Engine control, performance, and capabilities

For this article, I want to just talk about the scope of work that will be necessary to adapt RS-25 to suit the Space Launch System Program. 

Clear Communication
The most significant thing that has to be done to the RS-25 to make suitable for the new program is that it has to be able to respond to and talk to vehicle.  Remember, the Space Shuttle was developed in the 1970s and first flown in the 1980s.  Yes, many things were updated over the years, but given the lightning-fast speed of computer evolution and development, it is not surprising that what RS-25 is carrying around a controller basically can’t communicate with the system being developed today for the SLS vehicle.  The SLS vehicle would say, “Commence purge sequence three,” and the engine would respond, “Like, hey dude, no duh, take a chill pill” and then do nothing (my lame imitation of 1980s slang as best I remember it).

But here are the neato things that we’ll be able to do: We can use almost all of the work now completed on the J-2X engine controller hardware to inform the new RS-25 controller and we can use the exact same basic software algorithms from the SSME.  Because the RS-25 has a different control scheme from the J-2X, we cannot use the exact controller unit design from J-2X, but we can use a lot of what we’ve learned over the past few years.  And, because we can directly port over the basic control algorithms, we don’t have to re-validate these vital pieces from the ground up.  We just have to validate their operation within the new controller.  That’s a huge savings.

This is work that is happening right now, as I’m typing.  Pratt & Whitney Rocketdyne, the RS-25 developer and manufacturer, is working together with Honeywell International, the electronic controller developer, on this activity.  Within the next couple of months, they will have progressed beyond the point of the critical design review for both the hardware and software. 

In the long term, it is our hope that we will evolve to the utopian plain of having one universal engine controller, a “common engine controller,” that can be easily fitted to any engine, past, present, or future.  Such a vision has in mind a standardization of methods and architecture such that we could largely minimize controller development efforts in the future to the accommodation of obsolescence issues.  The simple truth is that development work is always expensive.  It would be nice to avoid as much as that cost as possible.  With the new RS-25 controller, we’re getting pretty close to that kind of situation.

Under Pressure
Have you ever spent much time thinking about water towers?  As in, for instance, why are they towers in the first place?  As you might have guessed, this is something that I wondered about many years ago as a pre-engineering spud.  They seemed to be an awfully silly thing to build when you could just turn on the faucet and have water spurt out whenever you want.  Ah, the wonderful simplicity of childhood logic:  Things work just because they do and every day is Saturday.

The reason that you go to the trouble of sticking water way up in a tower is so that you have a reliable source of water pressure that can absorb the varying demands on the overall system.  In the background, you can have a little pump going “chug, chug, chug” twenty-four hours per day pushing water all of the way up there, but by having this large, reserve quantity always in the tower, the system can respond with sufficient pressure when, at six in the morning, everyone in town happens to turn on the shower at the same time to start their day.  The pressure comes from the elevation of tower, the height of the column of water from top to bottom.

On the Space Launch System vehicle we have something of a water tower situation except that, in this case, we’re dealing with liquid oxygen.  In the picture below, you can see roughly scaled images of the Space Shuttle and the Block 2 Space Launch System vehicle.  On both of these vehicles, the liquid oxygen tank is above the liquid hydrogen tank.  This means that for the very tall SLS vehicle, the top of oxygen tank is approximately fifty feet higher relative to the inlet to the engines (as illustrated).  This additional fifty feet of elevation translates to more pressure at the bottom just as if it were a taller water tower.  However, in this case liquid oxygen is heavier than water (meaning more pressure) and the SLS vehicle will be flying, sometimes, at accelerations much higher than a water tower sitting still in the Earth’s gravitational field.  Greater acceleration also amplifies the pressure seen by the engines at the bottom of the rocket.

“So what?” you’re saying to yourself as you read this.  After all, higher pressure is a good thing, right?  If you have more pressure at the inlet to the engine, then you don’t need as much pumping power.  So, life should be easier for the engines with this longer, taller configuration.  There are two reasons why this is not quite the case.

First, think about when the vehicle is sitting on the pad at the point of engine start.  The pumps aren’t spinning so all the pressure you’re dealing with is coming from the propellant feed system.  And now, simply, for the SLS vehicle it will be different than before.  One of the tricky things about a staged-combustion engine, in general, is that the start sequence (i.e., the sequence of opening valves, igniting combustion, getting turbopumps spun up) is touchy.  Given that the RS-25 has two separate pre-burners — and therefore three separate combustion zones — and four separate turbopumps, the RS-25 start sequence especially touchy.  You have to maintain a very careful balance of combustion mixture ratios that allow things to light robustly, but not too hot, and a careful balance of pressures throughout the system so as to keep the flow headed in the right direction and keep the slow build to full power level as smooth as possible.  We have an RS-25 start sequence that works for Space Shuttle.  Now, for the SLS vehicle, we will have to modify it to adapt to these new conditions.

The second issue to overcome with regards to the longer vehicle configuration and the liquid oxygen inlet conditions is due total range of pressures that the engines have to accommodate.  When sitting on the launch pad, you have the pressure generated by the acceleration of gravity.  During flight, as you’re burning up and expelling propellants, the vehicle is getting lighter and lighter and you’re accelerating faster and faster, you can reach the equivalent of three or four times the acceleration of gravity.  So that’s the top end pressure. 

On the low end, you have the effect of when the boosters burn out and are ejected at approximately two minutes into flight.  When this happens, the acceleration of the vehicle usually becomes less than the acceleration of gravity meaning that the propellant pressure at the bottom of the column of liquid oxygen can get pretty low.  Momentarily, the vehicle seems to hang, almost seemingly falling, despite the fact that the RS-25 engines continue to fire.  Pretty quickly, however, the process of picking up acceleration begins again.  (In the movie Apollo 13, they illustrated a similar effect with the separation of the first and second stages of Saturn V.  The astronauts are pushed back in their seats by the acceleration until, boom, first stage shutdown and separation happens and they’re effectively thrown forward.  With the Space Shuttle and the SLS vehicle, however, the return to acceleration is not as abrupt as it was on Saturn V where they show the astronauts slammed back into their seats with the lighting of the second stage.)  The point is that the RS-25 has to accommodate a very wide range of inlet pressures while maintaining a set thrust level and engine mixture ratio.  While this has always been the case for the SSME/RS-25, the longer SLS vehicle configuration simply exacerbates the situation.

You’ll note that I’ve not talked about the liquid hydrogen here.  That’s because, as I’ve mentioned in the past, liquid hydrogen is very, very light.  Think of fat-free, artificial whipped cream.  Yes, the top of the hydrogen tank is much higher, but due to the lightness of the liquid, it doesn’t make much difference at the engine inlet even when the vehicle is accelerating at several times the acceleration of gravity.

Some Like it…Insulated
Look again at the pictorial comparison of the Space Shuttle and the Space Launch System vehicles shown above.  Do you see where the SSME/RS-25 engines are relative to the big boosters on the sides of the vehicles?  On the Space Shuttle, the engines were on the Orbiter and they were forward of the booster nozzles.  On the SLS vehicle, there is no Orbiter so the engines are right on the bottom of the tanks and their exit planes line up with the booster nozzle exit planes.  In short, the engines are now closer to those great big, loud, powerful, and HOT boosters.  We are in the process now of determining whether this poses any thermal environments issues for the RS-25.  Thus far, based upon analyses to date, there do not appear to be any thermal issues that cannot be obviated through the judicious use of insulation.

Other environments also have to be checked such as the dynamic loads transmitted to the engine through the vehicle or the acoustic loads or whatever else is different for this vehicle.  The point is not that all of these environments are necessarily worse than what they were on the Space Shuttle.  It is only that they all need to be checked to make sure that our previous certification of the engine is still valid for all of these considerations.

The Tropic of Exploration
So those are the three most obvious and primary pieces of the RS-25 adaptation puzzle: a new engine controller, dealing with different propellant inlet conditions, and understanding the new vehicle and mission environments.  Each of these pieces carries with it analysis and testing and the appropriate documentation so there is plenty of work scope to accomplish.  We are extremely lucky to be starting with an engine of such extraordinary pedigree, performance, and flexibility. 

Henry Miller once said, “Whatever there be of progress in life comes not through adaptation but through daring.”  It is our intent to prove Mr. Miller wrong in this case.  We will make progress by using the adaptation of RS-25 to enable the daring of our exploration mission.

Liquid Engines Extra — Introducing LEO

The following picture is a test.  Find me amidst the mess…

That’s right.  I’m the handsome chap with the snazzy specs.  See, I’m just oozing with exactly the vitality that you’d expect from your typical government-trained civil servant!  Well, okay, so maybe I’m not exactly Milton Waddams.  All personal resemblance and charm aside, I don’t actually have quite that much paper clutter in my office.  No.  Instead, I just have electronic clutter.  Indeed, if someone spent the time to actually print out all the stuff on my computer here at work, then we wouldn’t need a rocket to get beyond the moon.  We could just build a staircase from the resulting gargantuan pile of paper. 

Why is this the case?  It could be that I’m just a data hoarder.  Some people hoard clothes (that’s not me).  Some people hoard automobiles (that’s not me).  Some people hoard books (okay, that is me).  And some people hoard data.  All data.  All of the way back to class notes from Aerodynamics I (AERSP 311, junior year, professor Bill Holl — great teacher, great guy).  Yes, okay, so maybe I’m a little guilty of all that.  In my defense, however, I will simply say that there’s a lot that goes into making rocket engines and much of it is quite removed from the exciting cutting-metal stuff or the making-smoke-and-fire stuff.  And I get to stick my nose into much of it.  For this article, I am going to reveal super-duper, deep-and-dark secrets that they won’t even teach you in college.  I am going to tell you a little about …

…wait for it…

…project management.  Yes, it’s true:  I am going to invite you into our little piece of office space and I will do so to tell you about how the office has recently evolved.  Also, towards the end as a reward, I’ll give you an update of J-2X testing progress to date.

So, let me introduce you to LEO —


No, none of those LEOs.  Instead, let me introduce you to the Space Launch System (SLS) Program Liquid Engines Office (LEO).  The LEO is responsible for development and delivery of the liquid rocket engines for the core stage and upper stage of the SLS vehicle.  For more information about the SLS Program in general, I highly recommend the following site:

(Oh, and I want to say something briefly about the term “liquid engines.”  Probably every single person who would bother to read a blog about engines or rockets or space travel in general knows that this is just a shorthand term.  No, we are not talking about engines made of liquid — although that would be really cool.  “Liquid engines” is a quick and easy way to denote “liquid-propellant rocket engines.”  In case I’ve disappointed anyone, I’m sorry.  If ever we are able to make an engine out of liquid, I promise to be the first to report it.  Probably the most far-out thing that I once heard was the suggestion to make a hybrid rocket motor using solid hydrogen and liquid oxygen.  I cannot even imagine what the infrastructure would be to make the use of solid hydrogen plausible, but you never know…)

For the SLS vehicle, the upper-stage engine is the rocket engine so near and dear to our hearts after several years of design and development and fabrication and assembly and test:  J-2X.  The core-stage engine is the RS-25.  No, the RS-25 is not a brand new engine.  Rather, it is the generic name for that workhorse of the last thirty years, the Space Shuttle Main Engine (SSME). 

At the end of the Space Shuttle Program, there were fourteen SSMEs that had flown in space on the Shuttle and that still had usable life remaining.  I’m not sure that everyone knows this, but rocket engines have limited useful lives.  I guess that most things do, but with rocket engines it’s often pretty short.  Think of them like cherry blossoms (popular motif in Japanese tattoos): amazingly beautiful and quickly gone too soon.  The stresses within an operating rocket engine are tremendous.  For example, the J-2X has an official, useful life of only four starts and less than 2,000 seconds of operational run time after the engine has been delivered for use as part of the vehicle.  No, the engine doesn’t crumble into dust after that, but based upon our certification strategy and on our analysis of margins, that is the official life for our human-rated launch system.  After that point, depending on the proposed usage and risk considerations, and based on the likely reassessment of our margins with the proverbial “sharper pencil,” we can and do routinely talk ourselves into longer active lives for engine hardware.  On the test stand, we can test the J-2X upwards of 30 times and for lots of run time, but that is a lower risk situation.  Nobody is riding the test stand into space. 

Thus, when you come to the end of a program and you have fourteen engines with remaining, usable life, then you’ve got one heck of a residual resource.  In addition, there was one SSME assembled and ready to go, but it never made it to the test stand or the vehicle.  So it’s brand new.  And, on top of that, there were enough leftover pieces and parts lying around of flight-quality hardware to cobble together yet another engine.  And, there’s more! (Yes, I feel like the late-night infomercial guy, “and if you call in the next 10 minutes you will get this special gift!”)  There are also two development SSMEs.  These are not new enough to fly, but they are useful for ground testing and issues resolution.  That means that there are a total of sixteen RS-25 flight engines and two RS-25 development engines available to support the SLS Program. 

However, before your excitement bubbles over, you have to understand that when you see a sign for “free puppies,” you probably shouldn’t take that whole notion of “free” too literally.  As in, well, not at all.  Yes, we still have an extraordinary asset in the residual RS-25 engines.  No question.  But, we have work to do to integrate them into the SLS Program.  In a future article, I will discuss the multiple facets of this work.  By the way, I cannot claim to be immune from the “free puppies” thing myself.  Meet Ruugie –

The Liquid Engines Office (LEO) was formed to manage both the J-2X and the RS-25.  This office will also manage other liquid rocket engines used to support the SLS Program as it matures.  It was decided from a project management perspective that it would be best to have one office manage both engines.  In this way, we can be more efficient by leveraging the expertise across various disciplines and components.  For example, do we really need two turbomachinery subsystem managers?  No, Gary Genge is our turbomachinery subsystem manager and in that position he can understand and evaluate the relative programmatic and technical risk across all of the various turbomachinery pieces under his purview.  If in some utopian future our office responsibilities expands to three or four or eight different engine development or production efforts, we would, in theory, maintain the same structure but provide Gary with the support necessary to effectively manage turbomachinery across so many activities. 

So, for LEO we have subsystem managers for Engine Systems (effectively systems engineering and integration), Engine Assembly and Test (also includes asset management, logistics, and operations), Engine System Integration and Hardware, Valves and Actuators, Engine Control Avionics, Turbomachinery, and Combustion Devices.  LEO is supported by a Chief Engineer, a Chief Safety and Mission Assurance (S&MA) Officer, Program Planning and Control (i.e., the business office), and Procurement.  Plus, of course, we have support from the engineering and S&MA organizations across the many technical disciplines.  The structure is really quite similar to how we’ve been managing J-2X for these past several years.  We’ve just expanded our responsibilities.

So, that’s LEO and I’ll be talking more about RS-25 and SLS in the future.

Now, while I’ve been off doing my little part to get the foundation of LEO solid, including refreshing and getting into place our prime contracts for both J-2X and RS-25, how has J-2X been doing?  Well, in short, J-2X has been just cruising along.  E10002 has gone through six tests on NASA Stennis Space Center (SSC) test stand A-2.  Below are a series of images showing what an E10002 start looks like if you stood in view of the flame bucket (which I would very strongly advise against, by the way):

First, all you see is the facility water being pumped into the flame bucket.  Then you can see the ignition and everything glows orange.  Then the whole flame bucket is filled with exhaust.  And, finally, the exhaust coming barrelling down the spillway and eventually engulfs the camera.  The final step is not shown since there’s nothing to see but solid whitish grayness.

Here are the stats on the six tests:
     • Test:          A2J022          2/15/2013          35 seconds duration
     • Test:          A2J023          2/27/2013          550 seconds duration
     • Test:          A2J024          3/07/2013          560 seconds duration
     • Test:          A2J025          3/19/2013          425 seconds duration
     • Test:          A2J026          4/04/2013          570 seconds duration
     • Test:          A2J027          4/17/2013          16 seconds duration

So the total accumulated time is 2,156 seconds.  Tests #22, #25, and #27 all experienced early cuts, but all three were instigated by different flavors of instrumentation or monitoring system issues or oddities.  The engine is fine and running well.  Some of the key objectives included gathering additional data about the nozzle extension cooling characteristics, additional samples of the turbomachinery design, and main chamber combustion stability trials.  Something else that we did for this test series is that we tested a very special fuel turbopump port cover.  Here’s a picture of it:

Now, port covers are not something about which one usually says anything at all.  What makes this one special is that it was made by using a process known as Selective Laser Melting (SLM).  That is a fabrication method that is somewhat analogous to “3-D printing.”  A long time ago, I wrote a blog article about a gas generator discharge duct that we made for component-level testing using this technique.  This, however, is an engine test and this small, seemingly innocuous, piece of engine hardware may be the humble harbinger of a revolution in rocket engine fabrication.  The fact that we systematically stepped through the process of validating this port cover as a piece of hardware for an engine hot fire demonstration paves the way for pursuing other parts in the future, more complex parts, and, hopefully one day, regular production parts as part of a human-rated launch architecture. 

E10002 was removed from NASA SSC test stand A-2 on April 30th.  It is currently being retrofitted with instrumented inlet ducts and other hardware in preparation for the next phase of testing that will occur on NASA SSC test stand A-1.  As you’ll remember, in the past A-1 was used for the PowerPack Assembly testing.  Well, the talented and productive folks at NASA SSC remodeled the stand back to the configuration for engine testing.  The current plan is to install E10002 into A-1 by mid-May and to perform a series of five to seven tests through probably August.  The reason for using A-1 for the next series is because that stand does not have a diffuser.  That means that we can gimbal the engine, i.e., twist it around as if we were providing steering for a vehicle.  The thrust vector control (TVC) system composed of the hydraulic push-pull actuators that will be performing the gimballing is a component belonging to the stages element of the vehicle.  This testing will be providing those folks with data to inform their system design for the SLS Program.  See, it’s all win-win when we play nicely together.

And, finally, right on the heels of E10002, the assembly of E10003 will commence in June with scheduled installation into NASA SSC A-2 in September.  That’s my report for where things stand.  To finish up, I’ll leave you with a purely gratuitous glamor shot of the J-2X.  Isn’t she pretty?