What is so hard about making a throttleable large rocket engine?

Question

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.

Answer 1

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:

• http://yarchive.net/space/rocket/deep_throttling.html
• http://yarchive.net/space/rocket/lunar_module_throttling.html
• http://yarchive.net/space/rocket/injectors.html

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:

• The RD-180 used on the Atlas V throttles down to 47%-50%;
• RS-68 on Delta IV throttles down to 57.5%;
• YF-100 on Long March 6 throttles down to 65%;
• RD-171 on Zenit throttles down to 56%.