Tag Archives: RL10 rocket engine

Inside the LEO Doghouse: The Art of Expander Cycle Engines

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If you go back several generations on my mother’s side of the family, you will find a famous artist named Charles Frederick Kimball.  Also on my mother’s side of the family, in a different branch, a couple of generations later, there was a professional commercial artist.  On my father’s side, my grandmother was a wonderful artist who painted mostly landscapes of the Mohawk and Hudson River valleys in upstate New York.  And, of course, I’m married to an extremely talented artist.  You would think with those bloodlines and that much exposure, I’d have a just bit of artistic ability myself.  You would be wrong.  I love art.  I just can’t make it.

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The closest thing that I come to visual expression is confined to Microsoft PowerPoint creations.  However, within that narrow arena, particularly when it comes to engineering subjects, there is still fun to be had.  What we’re going to do for this article is undertake one of my favorite pseudo-artistic hobbies and play with expander cycle engine schematics.

So, let’s start with a simple, happy little cycle called the Closed Expander Cycle.  Most of what you need to know about this cycle is in the name.  First, it is closed.  That means that all of the propellants that come into the engine leave by going through the throat of the main combustion chamber thereby yielding the greatest chemical efficiency available.  Later, we’ll see that the opposite of “closed” is “open.”  Second, it is an expander.  That means that turbomachinery is driven by propellants that picked up heat energy from cooling circuits in the main combustion chamber and nozzle.  Typically, expander cycle engines use cryogenic propellants so that when these propellants are heated they change from liquid-like fluids to gas-like fluids.  Turbines very efficiently make use of gas-like drive fluids.  (Note that I keep referring to “fluids” rather than simply liquids and gases.  That’s because it’s usually a good idea to deal with supercritical fluids in cooling tubes or channels.  Phase changes can be unpredictable and lead to some odd pressure profiles.)

closedexpander

Above is a Microsoft PowerPoint masterpiece illustrating the Closed Expander Cycle rocket engine.  Fuel and oxidizer come in from the stage and are put through pumps to raise their pressure.  On the fuel side, the pump discharge is routed through the main fuel valve (MFV) to the nozzle and the main combustion chamber (MCC) cooling jackets.  I’ve not shown the actual routing here.  Typically, the MCC is cooled first and then, the now warmer fuel is used to cool the nozzle.  The heat loads in the MCC are significantly higher than those in the nozzle.  But whatever is the exact routing of the cooling fluid, the discharge, now full of energy picked up from the process of cooling, is fed into the turbines.  The oxidizer turbine bypass valve (OTBV) shown in the diagram is a means for controlling mixture ratio by moderating the power to the oxidizer turbine.  In some cases, if you have only one mixture ratio setting for the engine, you might be able to put an orifice here rather than a valve.  The turbines are driven by the warm fuel and then the discharge of the turbines is fed through to the main injector and then into the combustion zone.  On the oxidizer side, the routing is much simpler.  The oxidizer pump discharge is plumbed through the main oxidizer valve (MOV) directly into the main injector.  Within the MCC, you have the combustion of your propellants, the resultant release of energy, the generation of high-velocity combustion products, and the expulsion of these products through the sonic MCC throat and out the supersonic nozzle.  Ta-da, thrust is made!

The closed expander is one of the most simple engine cycles that has ever been imagined.  The venerable RL10 engine first developed in the 1950s and still flying today is based on this cycle (with the slight twist that there is only one turbine and the pumps are connected through a gear box – thereby eliminating the need for the OTBV).  This simplicity is both the strength of the cycle and also it’s limiting feature.  Consider the fact that all of the fuel – hydrogen in the case of most expanders – gets pushed all of the way through the engine to finally end up getting injected into the combustion chamber.  All that pushing translates to pressure drops.  It means that the turbines don’t have that much pressure ratio to deal with in terms of making power for the pumps.  In other words, the downstream side of the turbine is the lowest pressure point in the cycle and that’s the combustion chamber.  The result is that your chamber pressure can’t be very high.  That means that the throat of your MCC is relatively large and then that means the expansion ratio of your nozzle and nozzle extension start to get limited simply by size and structural weight.

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Also, note that all of the power to drive the entire cycle is provided by the heat picked up by the fuel in the MCC and nozzle cooling channels.  This then becomes a limiting factor in terms of the overall power and thrust-class of the engine.  As an engine gets bigger, at a given chamber pressure, the thrust level increases to the second power of the characteristic throat diameter, but the available surface area to be used to pick up heat to power the cycle only increases by that characteristic diameter to the first power.  In other words, thrust is proportional to “D-squared” but, to a first order, turbine power is proportional to “D.”  Thus, you can only get so big before you can’t get enough power to run the cycle.  One means for overcoming this is to make the combustion chamber longer just to give yourself more heat transfer surface area.  The European engine called the Vinci follows this approach.  But even this approach is limiting if taken too far since a chamber that is too long makes for less efficient combustion and, of course, a longer combustion chamber also starts to get awfully darn heavy.

So, how big can a closed expander cycle rocket engine be?  Well, that’s a point of recurring dispute and debate.  I can only give my opinion.  I would say that the closed expanding cycle engine most useful and most practical when kept to a thrust level of less than approximately 35,000 pounds-force.

Getting back to the notion of artistic expression, what then are the possible variations on the theme of the expander cycle engine?  Well, the themes and variations are used to explore and potentially overcome perceived shortfalls in the Closed Expander Cycle.  The first in this series is the Closed Split Expander, the portrait of which is below:

closedsplitexpander

The shortfall being addressed here is the fact that in the Closed Expander Cycle all of the fuel was pushed all over the engine resulting in large pressure losses.  In this case, some – usually most – of the fuel is pumped to a lower pressure through a first stage in the pump and then another portion is pumped to a higher pressure.  Thus, the fuel supply is “split” and that’s the origin of the name.  It is this higher pressure stream, routed through the fuel coolant control valve (FCCV) that is pushed all over the engine to cool the MCC and nozzle and to drive the turbines.  The lower pressure stream is plumbed directly into the main injector.  The theory is that by not requiring all of the fuel to be pumped up to the highest pressure, you relieve the power requirements for the fuel turbine.  It is always the hydrogen turbopump that eats up the biggest fraction of the power generated in the cycle so this is an important notion.

Does this cycle help?  Yes, some.  Maybe.  The balance of how much to split, what that split does to the efficiency of the heat transfer (less flow means possibly lower fluid velocities, lower velocities means lower heat transfer, lower heat transfer means less power…) makes it not always clear that you gain a whole lot from the effort of making the cycle more complex.  The portrait, however, is nice, don’t you think?  It has a realistic flair, a mid-century industrialist-utilitarian feel.

Next, wishing to express yourself, you can address the age-old issue of the intermediate seal in the oxidizer turbopump.  Take a good look at the first two schematics presented here.  You will see that the oxidizer pump is being driven by a turbine using fuel as a working fluid.  This is a very typical situation with rocket engines, whether they’re expander cycle engine or other cycles.  For example, this is the situation that you have in the RS-25 staged-combustion cycle engine and in the J-2X gas-generator cycle engine.  What that situation sets up, however, is a potential catastrophic failure.  You have fuel and oxygen in the same machine along with spinning metal parts.  If the two fluids mix and anything rubs, then BOOM, you have a bad day.  So, inside oxidizer pumps you usually have a complex sealing arrangement that includes a continuous helium barrier purge to keep the two fluids separate.  For the next expander cycle schematic, however, we can eliminate the need for this complex, purged seal.

 closeddualexp

This is a Closed Dual Expander Cycle.  It is still “closed” in that everything that comes into the engine leaves through the MCC throat.  The new part is that it is “dual” in that we now not only use the fuel to cool, but we also use the oxidizer.  Thus, we use heated fuel to drive the fuel turbopump and heated oxidizer to drive the oxidizer turbopump.  For this sketch, I’ve used a split configuration on the oxidizer side with a portion of the flow being pumped to a lower pressure and routed directly to the main injector and another portion pumped to a higher pressure, routed through the oxidizer coolant control valve (OCCV), to be pushed through the regeneratively cooled nozzle jacket and then through the oxidizer turbopump turbine.  I’ve done this since you’re likely running the engine at a mixture ratio (hydrogen/oxygen) of between 5 and 6.  You wouldn’t want to push that much oxidizer through the nozzle cooling channels or tubes.  Now, if you’re designing an expander with something like methane as your fuel so your mixture ratio lower, then maybe you can consider a non-split oxidizer side.

Note that with the dual expander approach I’ve gotten rid of the need for the purged seal package in the oxidizer pump and thus I’ve eliminated a potential catastrophic scenario (in the event of seal package failure).  However, I’ve accomplished that at the cost of some cycle complexity.  Also, cooling with oxidizer does not always make everyone happy.  Whenever you have a cooling jacket (either smooth wall or tubes), you always have the potential for cracking and leaking.  If you’re cooling with hydrogen, then a little leakage of extra hydrogen into a fuel-rich environment is a relatively benign situation.  It happens all of the time.  But what if you leak oxidizer into that fuel-rich combustion product environment?  Well, some studies have suggested that you’ll be fine, but it makes me just a little uneasy.  Then, also, you’re using heated oxidizer to drive your turbine.  It can be done, but using something like oxygen to drive spinning metal parts requires great care.  Under the wrong circumstances, a pure oxidizer environment can burn with just about anything as fuel, including most metals.  So, for all your effort to eliminate the seal package in the oxidizer turbopump, it’s not clear to me that you’ve made the situation that much safer.  However, despite these potential drawback, the schematic portrait itself has a certain baroque feel to it with the oxidizer side being positively rococo.

So, you’ve gone this far.  Why not take the final plunge?  Introducing the “Closed Dual Split Expander:”

closeddualsplit

By now, having stepped through the progression, you understand how it is “closed,” how it is “dual,” and how it is “split” (on both sides this time).  It’s not practical in terms of being a recipe for a successful rocket engine design for a variety of reasons balancing complexity versus intended advantages, but it’s an impressive schematic.  To me, it has a gothic feel, almost like a medieval cathedral with glorious flying buttresses and cascading ornamentation that just leaves you dazzled with details.

So, we’ve wondered off and into the weeds of making expander cycle portraits for the sake of their beauty rather than necessarily their useful practicality.  Let’s return to the more practical realm and question that which has been common to every cycle thus far presented.  It’s been the word “closed.”  Does an expander cycle engine have to be a closed cycle?  Of course not!  Once we’ve made that observation, we come to a very practical option.  Introducing the “Open Expander Cycle:”

open

 This biggest difference between this and every other previous schematic is the fact that the working fluid driving the turbines is dumped into the downstream portion of the nozzle.  This is a much lower pressure point than the main combustion zone.  The first thing that most people think when they see this cycle is that it must be a lower performance engine.  After all, you’re dumping propellant downstream of the MCC throat.  And, yes, that is an inherent inefficiency within this cycle.  Whenever you expel propellants in some way bypassing the primary combustion, you lose efficiency.  However, here is what you gain:  lots and lots of margin on your pressure budget.  Because I don’t have to try to stuff the turbine bypass into the combustion chamber, I can make my chamber pressure much higher.  In a practical sense, I can make it two or three times higher than in a simple closed expander cycle engine.  What that allows me to do is make the throat very small and that, in turn, provides for the opportunity for a very high nozzle expansion ratio within reasonable size and structural weight limits.  The very high expansion ratio means more exhaust acceleration and, in this way, I can get almost all of the way back to the same kind of performance numbers as a closed cycle despite the propellant dump.

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Here, however, is the really cool part of the open expander cycle: I can leverage the high pressure ratio across the turbines such that I can get more power out of a given heat transfer level in the cooling jackets.  Up above, earlier in this article, I suggested that there was a practical thrust limit for closed expanders of approximately 35,000 pounds-force (my opinion) and this was due to the geometric relationships between thrust and heat transfer surface area.  For an open expander, I can design high-pressure-ratio turbines for which I don’t need as much heat pick up to drive the pumps.  Thus, I can make a higher thrust engine.  How high?  Well, my good friends from Mitsubishi Heavy Industries (MHI) and the Japanese Space Exploration Agency (JAXA) have designed a version of this cycle that gets up to 60,000 pounds-force of thrust and I’ve seen other conceptual designs that go even higher.  The folks in Japan already fly a smaller version of this cycle in the LE-5B engine that generates 32,500 pounds-force.  Note that they often refer to this cycle by another name that is very common in the literature and that’s “expander bleed cycle,” with the “bleed” portion describing the overboard dump into the nozzle.  I prefer the designation of “open” since it clearly distinguishes it from the “closed” cycles illustrated earlier.

We have just about reached the end of this article but we have not reached the end of possibilities with expander cycle engine schematics.  That’s what makes them fun and, in my mind, kind of like playing with art.  You can come up with all kinds of combinations and additions.  For example, what if you took an expander cycle and added a little burner?  Over and over I’ve said that the limiting factor for a closed expander is the amount of heat that you pick up in the cooling jackets.  Well, okay then, let’s add a small burner that has no other purpose than to make the turbine drive gas hotter.  The result looks something like this:

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This cycle has a gas generator but is not a gas generator cycle since the combustion products from that GG are not used to drive the turbines directly.  Rather, the GG exhaust is piped through a heat exchanger and then dumped overboard.  Yes, you lose a little of your performance efficiency because it’s no longer a closed cycle, but the GG flows can be small and what you get out of it is a boost in available turbomachinery power and therefore potential thrust.  That’s my own little piece of artwork to demonstrate and anyone can do it.

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Remember Bob Ross from Public Broadcasting?  I loved watching his show and, as I’ve said, I can’t paint worth a lick.  But his show was relaxing to watch and listen to and he was always so relentlessly supportive.  There never were any mistakes.  Everything could be made all right in the end.  And anyone could make pretty mountains and happy little trees.  I’d like to suggest that the same is true about my little hobby of assembling happy little expander cycle schematics.  No, most will probably never be built or fly and the schematic portraits will probably never grace the walls of MOMA, but that’s okay.  My artist grandmother used to tell me that sometimes the purpose of doing art was not necessarily found in the end product, but instead as part of the journey of creation.

Inside The J-2X Doghouse: Beyond the Gas Generator Cycle

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Okay, I admit it: I’m a sucker for the Olympics.  I watch with rapt attention to sporting events that I would otherwise never consider viewing other than under the once-every-four-years heading of the Olympics.  Why is that?  Perhaps that is somehow a measure of my shallowness as a sports fan.  Nevertheless, I was truly on the edge of my seat watching the women’s team archery semi-finals and finals.  Great drama.  Wonderful competitors.  Exceptional skills.  Bravo ladies!

Another thing that I find fascinating about the Olympics is the fact that it brings together such a broad range of people.  No, I’m not going to trail off in a chorus of Kumbayah.  You simply cannot deny, however, that during the opening ceremonies you see people of every possible color and shade, from every corner of the planet, straight hair, curly hair, black hair, blonde hair, red hair, eye colors to fill a rainbow, and most startlingly, such an amazing collection of body types.  These are all world-class athletes and yet they’re often so different from each other.  I like seeing the six-foot-seven volleyball player walking next to the four-foot-ten gymnast.  I like seeing the contrast of the marathoner and the shot-putter.  We’re all the same species, but, my goodness, we come in an amazing array of shapes and sizes and various accoutrements.

The Pivot to Topic
Rocket engines, too, come in an array of shapes and sizes and various accoutrements (…I bet that you were wondering when or how I’d turn the conversation on topic).  I know that this is a blog dedicated nominally to J-2X development, but I think that it’s important to understand where the J-2X fits in this family of rocket engines.  So, let’s start with a table of top-level engine parameters:

Note that is list is nowhere close to being comprehensive.  There are lots and lots of rocket engines out there including those currently in development or in production and many that have been retired (like the F-1A in the table).  And if you open the window a little wider to include engines originating from beyond our shores, then you’ve got many more Soviet/Russian, European, Japanese, and Chinese engines to consider.  All I want to do here is expose you to some basic yet significant differences between this small set of examples.  Interestingly, if you can understand these few engines, then you can understand most of rest of the ones out there as variations on these basic themes.

Please allow me to introduce you to the engines listed in the table. 
• Of course, the J-2X needs no further explanation for anyone who reads this blog regularly. 
• The RL10 is a small engine that has been the product of Pratt & Whitney since the late 1950’s.  Over the past sixty years it’s evolved and matured.  It was actually used on a NASA vehicle back in the 1960’s, the Saturn I launch vehicle upper stage (S-IV).  Today it’s used, in different variants, as an upper stage and in-space engine for both the Atlas V and Delta IV launch vehicles. 
• The RS-25 is another name for the Space Shuttle Main Engine (SSME).  The development of the SSME began with research efforts in the late 1960’s, using a great deal of knowledge gathered from the development of the original J-2, and it was first tested in 1975 and first flew on STS-1 in 1981.  The RS-25 engine is now designated to be the core stage engine for the next generation of launch vehicles under the Space Launch System (SLS) Program. 
• The F-1A was an upgraded version of the F-1 engine that powered the first stage (S-IC) of the mighty Saturn V launch vehicle that first took man to the Moon.  The F-1A was a more powerful version of the F-1 with a handful of design changes intended to make it cheaper yet more operable and safe.

The Key is in the Power
In a blog article here over a year and a half ago, I introduced you to the gas generator cycle engine.  The key philosophical point discussed in that article about what makes a rocket engine an engine is the fact that it feeds and runs itself.  It does this by finding a means for providing power to the pumps that move the propellants.  The origin for this power is the key to any rocket engine cycle.  In a gas generator engine, this power is generated by having a separate little burner that makes high-temperature gases to run turbines that makes the pumps work.  Below is a schematic for such a system.  You’ve seen this schematic before and it is very much like J-2X.

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

Behold Now Behemoth
The F-1A power cycle is similar to the gas generator cycle shown above in that it is still a gas generator cycle, but rather than two separate turbopump units, there was only a single (huge) unit that contained both pumps.  So, a single turbine was used to power both pumps rather than having two separate turbines like J-2X.  Going back to the table, you will see that the F-1A was different from the J-2X also in the fact that the propellants were different.  The J-2X uses hydrogen for fuel and the F-1A used RP-1 (FYI, RP-1 stands for “rocket propellant #1” and is actually just highly purified, high quality kerosene).  The chief difference between hydrogen and kerosene is chemistry.  A hydrogen-fuel engine will get higher specific impulse than a kerosene-fuel engine but kerosene engines have the distinct advantage of being able to generate more thrust for a given engine size.  With a kerosene engine, you are simply throwing overboard more massive, high-velocity propellants in the form of combustion products.  Hydrogen is light and efficient from a “gas mileage” perspective but kerosene gets you lots and lots of oomph.  That’s why you typically use it for a first stage application like on the Saturn V vehicle.  You want to have lots of oomph to get off the ground.  Later, on the upper stages, you can better use the greater gas mileage afforded by hydrogen.

Note, however, that you could theoretically build a hydrogen engine as large as the F-1A in terms of thrust.  The RS-68 (also a gas generator cycle engine) on the Delta IV vehicle puts out around three quarters of a million pounds-force thrust so that’s pretty big.  Also, back in the 1960’s, there was conceptual design work performed on an enormous hydrogen fuel, gas generator cycle engine called the M-1.  On paper, that behemoth put out 1.5 million pounds-force of thrust just like the F-1 on the Saturn V.  But that project was abandoned and here’s why: hydrogen is very, very light so if you want to carry any appreciable amount, you need to have truly huge tanks.  Huge tanks mean huge stages.  Huge means heavy.  Eventually it becomes a game of diminishing returns at the vehicle level.

What this discussion of J-2X and F-1A (and RS-68 and even M-1) shows you is the extreme versatility of the gas generator cycle.  It can be used with nearly any reasonable propellant combination and it can be scaled from pretty darn small to absolutely enormous.

Shaving with Occam’s Razor
Occam’s Razor is the notion that one should proceed with simplicity until greater complexity is necessary.  Along these lines, I will introduce you to a simpler engine cycle: the expander cycle.  For this engine cycle, you do not use a gas generator to drive your turbine(s) so you don’t have a second, separate combustion zone apart from the main combustion chamber.  That makes everything simpler.  Instead, you use only the heat gathered in the cooling the thrust chamber assembly (i.e., the main combustion chamber walls and that portion of the nozzle regeneratively cooled).  See the schematic below.

See?  I got rid of not just the gas generator but also the two valves that fed the gas generator.  That’s huge in terms of simplification.  And whenever you can make an engine simpler you’ve usually made it cheaper and more reliable just because you have fewer things to build and fewer things that could break.  Cool!

Here, however, is the problem: How much power do you really have just from the fluid cooling the walls?  The answer can be found by looking at the table and seeing, for example, the RL10 thrust output is less than one-tenth of J-2X.  You just can’t pull that much energy through the walls.  There have been attempts to increase heat transfer by various means including making the main combustion chamber longer than typical so that you have more heat transfer area or even by adding nubs or ridges onto the wall to gather up more heat.  Using the longer chamber notion, the European Space Agency is working on an engine called the Vinci that almost doubles the thrust output from the RL10, but getting much further beyond that is darn tough.  Also note that hydrogen is a wonderful coolant based upon its thermodynamic properties.  Being a wonderful coolant means that it picks up a lot of heat.  It is difficult to imagine using the expander cycle engine with another fuel beside hydrogen (though maybe methane might work … haven’t examined it). 

On the plus side, in addition to the simplicity, what the cycle shown offers is what is called a “closed cycle” meaning that no propellants are thrown overboard other than through the main injector.  In a gas generator cycle engine, after the gas generator combustion gases pass through the turbine(s), it’s dumped into the nozzle (or, in other schemes, dumped overboard in other ways).  Any propellants or combustion products that do not exit the rocket engine through the main injector and through the main combustion chamber throat represent an intrinsic loss in performance.  “But,” you’ll say, “the specific impulse for the RL10 and the J-2X in the table are the same.”  Well, that’s a little bit of apples and oranges because it’s based upon the nozzle expansion ratio.  Another model of the RL10, the B-2, has a much larger nozzle extension and the vacuum specific impulse for that model is over 462 seconds (minimum).  The European Vinci engine that I mentioned above has a projected vacuum specific impulse of about 465 seconds.  Those are darn impressive numbers that make the mouths of in-space stage and mission designers drool.

A couple of final notes about the expander cycle engine.  First, the RL10 is not quite like the schematic shown.  It only has one turbine with one pump driven directly and the other pump driven through a gear box.  Thus, the OTBV goes away (making it even simpler!).  Second, there are versions of the expander cycle engine concept that are not closed cycles.  In these versions, you dump the turbine drive gas overboard in a manner similar to what you do in a gas generator cycle.  You are still using the heat from the chamber walls to drive the turbine(s), so it’s still an expander, but with an overboard dump you can also leverage a larger pressure ratio across the turbine(s) and thereby get a bit more oomph out of the cycle.  You sacrifice a bit of performance for more oomph.  The Japanese LE-5B engine is an open expander cycle engine like this (also called an “expander bleed” cycle).

“We do these things not because they are easy…”
So, you’ve seen the incredibly versatile gas generator cycle engine.  And, you’ve seen the simple yet limited expander cycle engine.  So what do you do if you say, “The heck with it, I want the Corvette”?  What if you want a closed cycle, high performance engine not limited to lower thrust levels and you’re willing to accept consequent greater complexity?  The answer is staged combustion.  Below is a simplistic schematic for a staged-combustion engine.

Where:
      CCV = Coolant-Control Valve
      PBOV = Preburner Oxidizer Valve

In a staged combustion cycle engine, we rename the gas generator and call it the “preburner.”  The biggest difference between a gas generator cycle and a staged combustion cycle is what you do with the turbine exhaust gases.  In a gas generator cycle, the turbine exhaust gases effectively get dumped overboard.  In a staged combustion cycle, the turbine exhaust gases get fed back into the main injector and get “burned again.”  This is possible since the combustion in the preburner is off from stoichiometric conditions, meaning that in addition to combustion products you also have lots of leftover propellant (either fuel or oxidizer depending on the scheme). The leftover propellants from the turbine exhaust then become part of the mix of propellants in the main combustion chamber.

That sounds simple, right?  It’s just a twist on the gas generator cycle theme, right?  Well, there are larger implications.  First, think about the pressure drops through the system.  On a gas generator cycle engine, the pressure in the gas generator can be lower than the main chamber.  After all, the downstream side of the turbine(s) is effectively ambient, external conditions.  In a staged combustion cycle, the preburner pressure has to be substantially higher than the main chamber pressure sitting downstream of the turbine(s) or you don’t get enough flow to power the turbine(s).  Insufficient turbine power and the cycle doesn’t work.  So, in general, a staged-combustion cycle engine has higher system pressures than a gas-generator cycle engine of comparable size.  Next, think about starting the system.  In a gas generator cycle engine, the two combustion zones are effectively disconnected.  In a staged combustion cycle engine, the two combustion zones are on either side of the turbine(s) so there is effectively communication between these two zones.  Now, try to imagine getting these two combustion zones ignited and up to pressure and the turbine(s) spun up to speed in an orchestrated manner during the start sequence.  It ain’t easy.

So, what do you get for this complexity and higher operating conditions?  Well, you get a closed cycle, high performance, and high thrust engine design choice.  The RS-25 (SSME) is the American example of such an engine.  If you put a higher expansion ratio nozzle on the RS-25, just as with the RL10 discussion, the specific impulse value would be as much as ten seconds higher than J-2X.  However, if you go out and find a schematic of an SSME, what you’ll see is a heck of a lot more complexity than even I’ve shown in my simplified sketch.  Because the pressures are so high, there are actually four separate turbopumps and a boost pump in the SSME.  The design relies on putting pumps in series to achieve the necessary pressures and fluid flow rates through system.  And, the SSME has not one but two separate preburners, one for the high pressure fuel turbopump and one for the high pressure oxidizer turbopump.  It’s a very complex engine, but it has extraordinary capabilities.

The RS-25 (SSME) is a staged combustion cycle engine with hydrogen as the fuel.  The preburners are run fuel-rich such that the generated gases contain excess hydrogen for injection in the main chamber.  Back in the days of the Soviet Union, they developed a whole series of staged combustion cycle engines that instead used kerosene as the fuel.  In these engines, the preburner is run oxidizer-rich so that the gases run through the turbines and then through the main injector have excess oxidizer to be used for final combustion in the chamber.  The Russian-supplied RD-180 that is currently used for the Atlas V launch vehicle is an example of such an engine.  It too is an extremely complex, high pressure, and high performance engine.

So, staged combustion cycle engines are not easy.  Their complexity and operating conditions suggest, generically, greater expense and lower reliability.  But if you can make the trade-off between high performance and the adverse issues, then they can function quite impressively.  Nearly thirty years of Space Shuttle flights are an indisputable demonstration of this fact.

Just One Bolt
Can you imagine opening a hardware store and selling just one kind of bolt?  That would be it.  One brand.  One diameter.  One length.  And just one bin full of identical versions of this one bolt in your store.  It sounds really kind of stupid.  The unavoidable truth is that you need different bolts for different applications.  It’s kind of like trying to imagine telling the Olympic gymnastics team that they now had to play basketball and the basketball players to do gymnastics.  I don’t know about you, but I’d love to see Lebron James have a go at the pommel horse.

Well, over the last fifty-plus years, we’ve developed different rocket engines and rocket engine concepts for a variety of different applications.  Just one design does not fit all applications.  Each design has advantages and disadvantages.  If you can understand the basics of what I’ve discussed in this article, however, then you will have a fundamental understanding of at least 90% of the engines spanning that fifty-plus years of history.  And that, in turn, might help you better appreciate why one bolt is chosen over another or why, for example, shot-putters tend to be a bit more beefy than cyclists.