What factors can change the mixture ratio of propellant? I think they can be changed to control the thrust of a rocket.Stay safe. EDIT:What about considering only one stage? Do they change or can they be altered?


1 Answer 1


As described in this QA, the upper stages of the Saturn V could fire their J-2 engines at different mixture ratios.

The second stage would switch from 5.5:1 (oxidizer mass to fuel mass) for higher thrust, to 4.5:1 for higher fuel efficiency partway through the ascent.

The third stage would normally start at 4.5:1 and switch to 5:1 partway through the burn, if the trans-lunar injection maneuver was executed at the first opportunity. It was possible to delay it by one orbit, during which time some hydrogen fuel would boil off and be lost; in that case the entire burn would be done at 5:1, which would ensure that a minimum of oxidizer was left unburned when the fuel ran out.

The thrust was about 35% higher at 5.5:1 than at 4.5:1 and the specific impulse (fuel efficiency) was only a couple of percent lower.

Similar mix ratio controls appear on some other engines (RL10, RD-180), but I'm not sure how universal they are. Their main use is to balance usage of fuel and oxidizer so you don't end up with a dead-weight surplus of one component or the other at the end of a burn.

  • $\begingroup$ What about considering only one stage? Do they change or can they be altered? $\endgroup$
    – Auberron
    Mar 30, 2020 at 15:41
  • 1
    $\begingroup$ Yes, in the normal case the second stage would switch from 5.5:1 to 4.5:1, and the third stage would switch from 4.5:1 to 5:1. Edited to clarify. $\endgroup$ Mar 30, 2020 at 17:19
  • $\begingroup$ OK, but the Saturn V engine design dates back to before the Apple-IIe :-). Do modern engines have the capability to adjust flow rates, and is that even more efficient than mixture adjustments? $\endgroup$ Mar 31, 2020 at 14:31
  • $\begingroup$ I don't know how many modern engines have variable mix ratios; RL10 and RD-180 do, and are in use today. I don't know what distinction you're trying to make between "adjust flow rates" and "mixture adjustments"; it's the same thing. $\endgroup$ Mar 31, 2020 at 16:39
  • 1
    $\begingroup$ @CarlWitthoft if you adjust flow rates while maintaining a constant MR, that's called throttling $\endgroup$ Mar 31, 2020 at 17:19

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.