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In the old days, most rocket engines weren't throttleable. I think the first one was actually the Apollo Lunar Module descent engine.

But I'm interested in large, turbopump-fed engines using RP-1/LOX or LH2/LOX. Why is it so hard to makes these throttleable? The way I see it, the preburner running the turbopump should be throttleable just by varying the intake flow with some valves.

(I could have a mistaken impression, of course. Maybe it's not all that hard, but they just never needed this for rocket stages and thus didn't want to add unnecessary complexity?)

Note: I don't need a huge range like 1% to 100% of thrust. More like 50% to 100%, or even 75% to 100% would probably be fine.

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  • $\begingroup$ You should also do some basic research on your own as to why booster engines, in general, do not need to be throttleable; the difference between static and dynamic stability; the problems encountered when starting and shutting down liquid rocket engines (which, after all, is much like deep throttling); the evolution of engine controllers; why STS needed throttling and Saturn didn't.... then you will have invested some time in gaining background knowledge of the issues involved. $\endgroup$ Jul 25 '15 at 2:12
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    $\begingroup$ Take a look into the the engines that powered the X-15, the XLR-99 This was the first large, throttleable, restartable liquid-propellant rocket engine. There are several good books that detail it's development and the problems with creating a man rated engine of this type. It's performance and reliability was amazing considering the state of the art at the time and the weight of only 413 kg $\endgroup$ Sep 18 '15 at 22:35
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For a large bipropellant rocket engine to fire safely and stably, the fuel and oxidizer have to mix very thoroughly at high flow rate and pressure before they ignite. Otherwise, there will be sputtering and popping, which is very bad at that scale.

Also, in many engines, a "film" of unburned propellant flowing along the walls of the nozzle is critical for cooling. You need that film to be even or you'll get burn-through. When exhausting into atmosphere at low power levels, the exhaust flow tends to separate one side of the nozzle and adhere to the other, causing off-center thrust and undesirable spot heating, instead of moving freely down the middle. To avoid this flow separation you either need a very short nozzle (reducing specific impulse) or an aerospike, expansion-deflection, or other compensating nozzle; those generally increase weight and haven't been thoroughly researched and developed.

Therefore, the design of the fuel injection system is critical, and it's very sensitive to flow rates. The common injector designs for large liquid engines are referred to as "showerhead" injectors and like common showerheads, if the propellant valves are partially shut, you'll get an unsteady dribble out of it instead of a fine, predictable spray.

So early engines in particular were designed for stability in a narrow regime around full throttle.

Pintle injectors like that used in the SpaceX Merlin are apparently less sensitive than showerhead injectors, and the state of the art in fluid dynamic simulation is way better today than it was in the 60s, so the Merlin is more throttlable than some, and can throttle over a wide range. The October 2015 revision of the Falcon 9 user's guide claims 70%-100% throttling for the first-stage Merlins, but 39%-100% for the second stage Merlin Vacuum; this suggests that nozzle flow issues (vastly reduced out of atmosphere) are the limiting factor rather than chamber stability.

Henry Spencer's usenet posts, as always, have some good information:

These indicate that 70%-100% throttling is relatively easy to accomplish, with the challenge being to get much below that.

However, this NASA study on deep throttling (2010?) is more optimistic, suggesting that 4:1 ranges (i.e. 25%-100%) are straightforward to achieve with small modifications to fixed-thrust engines. It mentions in particular:

  • The RL-10-derived CECE with better than 10:1 range (i.e. <10%-100%)
  • The SSME, designed to throttle down to 65%, but stable combustion demonstrated as low as 17% of rated power (this is somewhat misleading, because the pumps, and therefore the engine as a whole, have various problems at low power).

A bunch of modern launchers use engines with substantial throttling capability, used to reduce aerodynamic stress around max-Q and/or excessive g-force at the end of the first-stage run when fuel tanks are near-empty:

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  • $\begingroup$ IIRC Merlin can throttle down to 40%. $\endgroup$ Jul 25 '15 at 22:14
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    $\begingroup$ Got a reference? I've only seen ambiguous statements where it was unclear if a 40% range was meant (I.e. 60%-100%) or 40%-100%. $\endgroup$ Jul 25 '15 at 23:09
  • $\begingroup$ Not an authoritative one sadly (I mean, with SpaceX, who does? They release pretty small amounts of information), but yes I think I'm referring to the same thing you are where it was unclear if 60% or 40% was meant. However, someone over at r/SpaceX did the math on the CRS-6 barge landing and determined that the throttle must've been below 60% for it to experience the sort of deceleration seen. $\endgroup$ Jul 25 '15 at 23:26
  • $\begingroup$ ! "Otherwise, there will be sputtering and popping, which is very bad at that scale." Hey, what's that sound? Oh, that's just the engines popping. It'll be fine. $\endgroup$ Jun 25 '18 at 20:58

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