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What are the limitations for the 1st stage liquid fueled rocket engines that are currently in widespread use, what are the factors that limit their total thrust? Why can't you just inject more and more fuel in the same sized engine to produce more thrust for example??

By limitations, I mean the largest factors that stop these engines from producing more thrust. I would assume that it's something like the heating of the engine, or the rate at which the fuel can be burnt / passed through into the combustion chamber.

Could you also explain how these factors limit the thrust produced?

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  • $\begingroup$ Do you mean the limit on the thrust of each individual engine by itself, or a limit on total thrust of all engines together on a first stage? $\endgroup$
    – uhoh
    Aug 13, 2016 at 3:47
  • $\begingroup$ I meant a single engine, however, I'd like to hear about limitations on total thrust of all engines too. $\endgroup$
    – Boris Deletic
    Aug 13, 2016 at 4:58
  • $\begingroup$ OK good that makes the most sense. Maybe edit you question a little to make it clearer there, including the title? That way when people read the question in the future they'll have a better idea what they can expect to find here. Also welcome to SX SE! $\endgroup$
    – uhoh
    Aug 13, 2016 at 5:01
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    $\begingroup$ I'm not asking specifically about what limits the size of rocket engines, although I am still interested in this. I'm more interested to know what limits regular sized liquid engines, from producing more thrust. Why can't you just inject more and more fuel in the same sized engine? $\endgroup$
    – Boris Deletic
    Aug 13, 2016 at 11:16
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    $\begingroup$ Thanks for the idea and help with clarifying what I meant. The community here is really great! $\endgroup$
    – Boris Deletic
    Aug 13, 2016 at 13:22

3 Answers 3

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To move more propellant faster into the combustion chamber increases the chamber pressure; this requires a larger, more powerful turbopump.

High pressure, high volume turbopumps are hard to design - most of the problems encountered in developing the space shuttle main engines were pump failures, some catastrophic. There are some terrifying numbers associated with the SSMEs -- each engine's pumps, turning at about 30,000 RPM, produce something like 100,000 HP of mechanical power just to move propellants. (https://en.wikipedia.org/wiki/Space_Shuttle_main_engine#Turbopumps )

The SpaceX Merlin series, while using a much more conservative pump design, is still likely pump-limited; they want to be able to reuse those engines for a long time without the total overhaul required of the SSME, so they can't risk damaging the pumps by driving them too hard.

At some point it's easier to gain performance by making a larger, lower pressure engine. Taken to the limit, this approach gives you Sea Dragon -- an extremely large, powerful, low-pressure engine. How realistic would the Sea Dragon engine be to produce given today's technology?

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  • $\begingroup$ If the only limitation of thrust is the rate at which turbopumps can drive fuel into the combustion chamber, then why wouldn't they use huge F1 turbopumps for Merlin engines to produce more thrust? $\endgroup$
    – Boris Deletic
    Aug 15, 2016 at 12:32
  • $\begingroup$ It's not the only limitation; per Hobbes, temp and pressure are factors as well. The F-1s turbopump probably weighs more than an entire Merlin engine. You're rapidly approaching "why not put a Ferrari engine in a Honda Civic" territory here. $\endgroup$
    – Russell Borogove
    Aug 15, 2016 at 13:52
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    $\begingroup$ If you don't scale the engine itself (nozzle and combustion chamber) with the fuel throughput, you'll be losing ISp rapidly, as increasingly more gas expansion occurs outside the engine - energy is wasted on the gas expanding sideways instead of pushing the engine. And if you keep increasing the combustion chamber size, soon burn stability problems appear. For that reason it may be beneficial to use more smaller nozzles, even if they are fed by the same pump (like Soyuz). $\endgroup$
    – SF.
    Aug 19, 2016 at 1:24
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Several issues come to mind:

  1. Temperature and pressure of the combustion chamber (increase them enough and the walls will deform or melt). Can be mitigated by cooling the chamber walls.

  2. Combustion instability: the larger the engine, the more chance you have to get instability. This was a big issue in development of the F-1 engine (Saturn V first stage). The issue was solved through lots of testing of different injector configurations.

  3. Injection density. Higher pressures require more propellant so you need more injectors until most of the chamber wall consists of injector holes.

The F-1 injector had to have what was described as an "extraordinarily high injection density," approximately 5 pounds of propellant per square inch per second.

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  • $\begingroup$ Thank you for the answer! As a side question, would these also be the factors that would limit a smaller engine producing more thrust? If for instance, you were to just inject more propellant into a Merlin engine, which factor would most likely cause a failure? Would combustion stability still be a likelyhood? $\endgroup$
    – Boris Deletic
    Aug 13, 2016 at 8:25
  • $\begingroup$ In smaller engines it's probably the temperature/pressure that will limit you. $\endgroup$
    – Hobbes
    Aug 13, 2016 at 9:53
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    $\begingroup$ If I think simple-mindedly, I'd guess that a small engine - having more surface area to volume - could actually manage temperature and pressure better than a large one if designed for it. But for the question of an existing engine - it could be that combustion would be incomplete - it may be harder to get sufficient reaction time before it's blown out the nozzle. $\endgroup$
    – uhoh
    Aug 13, 2016 at 10:42
  • $\begingroup$ Can you link me to any articles with evidence of this? $\endgroup$
    – Boris Deletic
    Aug 13, 2016 at 13:13
  • $\begingroup$ @BorisDeletic I didn't add anyone's name - I'm thinking that it is directed at (at)Hobbes who wrote this answer. I'm thinking that the mixing (at the molecular scale) and the combustion both need time to take place, and if you push stuff in too fast, it will just end up blowing out the other end and reacting later. It is just a guess/comment. $\endgroup$
    – uhoh
    Aug 13, 2016 at 16:25
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Ultimately there is the limit of total available energy. There are just so many calories in a pound of hydrogen and oxygen (or whatever fuels) Rockets are actually extremely efficient engines, turning more of the latent energy into movement than most engines. (I wish I had the numbers.) That's why some people argue that we need nuclear rockets, the energy density is higher by order of magnitudes.

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    $\begingroup$ That limits the Specific Impulse but not thrust. $\endgroup$
    – SF.
    Aug 20, 2016 at 16:18

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