How much Monomethylhydrazine + Dinitrogen tetroxide (MMH/NTO) hypergolic propellant does the Dragon crew module carry and how much burn time does that afford them during launch escape if one is triggered?
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For the abort tests in early 2020, according to documents filed with the FAA over a year prior, the Dragon flew with 2565 kg of propellant (1590 kg of NTO and 975 kg of MMH), but only about half of that was fired during the abort.
Consumption rate for a single SuperDraco is about 31 kg/s (= 71kN thrust / 2300 m/s exhaust velocity), so for the suite of 8 thrusters, 2565 kg of propellant would last about 10 seconds.
The actual current figures could be quite a bit different from that.
answered Apr 23, 2021 at 22:52
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$\begingroup$ Thank you, Russell! This is super helpful. So I take it the combustion ratio would be 1.59 NTO to 0.975 MMH by mass? $\endgroup$– user39728Commented Apr 24, 2021 at 1:14
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1$\begingroup$ Seems like, though at 1.63:1 that's a richer mix than the 2.17:1 WP suggests. $\endgroup$ Commented Apr 24, 2021 at 1:21
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$\begingroup$ Ah! You know, 1.63 is super close to the 1.65 NTO/MMH density ratio, meaning you'd burn 1:1 NTO/MMH by volume, so you'd burn almost exactly one tank of NTO for every tank of MMH if the tanks were identical (as they seem to be from pictures I've seen). $\endgroup$– user39728Commented Apr 24, 2021 at 5:28
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1$\begingroup$ Interesting. Most spacecraft using similar propellant combinations have asymmetrical tanks to optimize the mixture ratio for performance (Transtage, and the Apollo LM ascent stage's "cheeks" being two cases where it's externally apparent). It would be a little odd if volume symmetry won out in Dragon. $\endgroup$ Commented Apr 24, 2021 at 5:41
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1$\begingroup$ I don't know anything about its tankage. Shared propellant would make sense; in a nominal mission plan, they don't use the Superdracos at all, while in an abort, little or no RCS would be needed. During the escape burn, the ship is passively stabilized by the fins on the trunk, but they might need a few seconds RCS to reorient the ship tail-first for parachute deployment and splashdown, after the trunk is dropped. It might be worth hunting down newer documentation for the Crew Dragon. $\endgroup$ Commented Apr 24, 2021 at 7:47