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How can a single spacecraft namely Crew Dragon aka Dragon V2, accommodate 8 SuperDraco thrusters along with sufficient propellant required for a precision propulsive landing and/or other such manuevers?

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  • $\begingroup$ It can't. Or at least it hasn't been done yet. It's safe to say that's a large part of the reason SpaceX isn't doing propulsive landing. In terms of 'other such maneuvers,' the answer is essentially: "with very complex, highly engineered systems of propellant tanks that are extremely proprietary." $\endgroup$ – randomUsername Apr 9 '18 at 21:34
  • $\begingroup$ @randomUsername the same system is used for launch abort and was already tested. $\endgroup$ – jkavalik Apr 10 '18 at 5:51
  • $\begingroup$ The crew dragon isn't that small--its designed to ferry up to 7 people to orbit with cargo to spare. It just looks small in many photos because it's always next to big things. $\endgroup$ – Dragongeek Apr 10 '18 at 6:46
  • $\begingroup$ @jkavalik yeah kinda. That configuration could never have flown with anything resembling acceptable mass margins for propulsive landing. I meant that it can't accommodate the full propulsion system mentioned along with the all the ECLSS stuff and other crew accommodations. $\endgroup$ – randomUsername Apr 10 '18 at 19:25
  • $\begingroup$ @randomUsername it has to accomodate the full launch abort propulsion system - if you can propell the capsule at 5g for escape, you can sure land it using the same engines (well, if they are throttleable enough) and propellant. $\endgroup$ – jkavalik Apr 10 '18 at 19:34
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Firstly, available space for engines isn’t really a problem. A spacecraft can be made exactly as big as the needed cargo space plus engines. Capsules only look tiny because you usually see them next to an absolutely enormous launch vehicle.

Secondly, landing doesn’t require that much fuel. Even the apollo capsules returning from a very fast orbit had slowed to around 230mph before the parachutes opened. That’s way less than the 20,000mph that the launch stages need to achieve, and well within the capability of something with fuel tanks sized for a regular aircraft.

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The SuperDraco engines themselves are quite compact. Liquid rocket engines consist of a relatively small combustion chamber and a nozzle which may be large or small in proportion depending on the ambient air pressure where it's intended to be used. Vacuum engines work best with very large nozzles, but for an engine that is only intended to work at low altitude, like SuperDraco, the optimum nozzle size is very small -- in this case, the end of the nozzle is about 20cm inches in diameter. The combustion chamber is estimated to be about 25cm long x 15cm diameter.

Dragon V2 only carries 1700 liters (450 gallons) of propellant -- roughly 2 spheres of 1-meter diameter each.

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