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?


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 '20 at 15:41
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    $\begingroup$ 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. $\endgroup$ – Russell Borogove Mar 30 '20 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? $\endgroup$ – Carl Witthoft Mar 31 '20 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. $\endgroup$ – Russell Borogove Mar 31 '20 at 16:39
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    $\begingroup$ @CarlWitthoft if you adjust flow rates while maintaining a constant MR, that's called throttling $\endgroup$ – Organic Marble Mar 31 '20 at 17:19

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