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The Super Draco rocket motor was originally developed for propulsive landings on earth, right. It has performed great in the test where crew dragon escaped from the Falcon 9 last year. It is supposed to be throttle-able from full to very low power and able to go through a number of start/stop cycles.

Why didn't we see a SpaceX lunar lander proposed that would use it?

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  • $\begingroup$ I could be wrong, but it looked to me like their Starship-derived CLS proposal had some higher-mounted engines that were likely to be SuperDracos or similar (MegaDracos?) $\endgroup$ – Anton Hengst Jun 20 at 19:50
  • $\begingroup$ Landing on the moon requires more delta v than on earth $\endgroup$ – Antzi Jun 21 at 2:59
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To begin with, SpaceX bid Starship as their lunar lander. Starship used methalox as their propellant for their Raptor engines. However, Starship does need landing engines higher up on the hull for the moon landing to avoid digging a crater in the moon surface with the Raptor exhaust. So why don't they use SuperDracos here? Well, SuperDracos uses as propellants monomethylhydrazine and nitrogen tetroxide. So for them to be used in Starship, seperate tanks for the storage of the propellants would need to be created. This also makes refuelling operations more difficult, as the refuelling craft needs to carry two different oxidisers and fuels. So likely in light of this, SpaceX decided to instead use a raptor derived methalox based thrusters.

In the render below, the methalox landing thrusters are shown being used to land Starship.

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    $\begingroup$ Also worth noting that superdraco fuel is highly toxic, though it may be used for the RCS. $\endgroup$ – ikrase Jun 21 at 9:40
  • $\begingroup$ @ikrase The RCS is (or was, the last time anything was said about it) planned to be methox (not methalox, using gaseous propellants). The problem with hypergolics isn't the quantity, but just the fact that they're another set of fluids that would require plumbing and tankage and which would complicate the logistics of orbital resupply. Ideally, the only fluids Starship will need are liquid methane and liquid oxygen. It's not impossible that hypergolics would be used early on, but it'd only be a temporary stopgap to get things working while the methox thrusters are developed. $\endgroup$ – Christopher James Huff Jun 21 at 18:59
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First of all, the SuperDracos were designed from the start primarily as a launch escape system. Although using them for propulsive landing was explored extensively, it was never implemented.

For landing on Earth or Mars, the atmosphere is used to lose most of the craft's speed, with the thrusters (and/or parachute) used only after that. The moon has no atmosphere so the entire landing process must be propulsive, which would require a sustained burn (and a lot of only moderately-efficient propellant) that SuperDracos weren't designed for. Maybe they could take it as is, but I wouldn't count on it.

SuperDracos aren't actually very powerful compared to a booster's main engines. A Falcon 9 coming in for landing uses 1-3 of its Merlin 1D engines, each of which are 1.5x as powerful as all 8 SuperDracos on a Dragon2. Landing a Starship even on the moon is likely going to require even more power, due to the high "dry mass" of the craft, the return fuel it might be carrying, and the payload it will be carrying. With enough SuperDracos it could probably be done, but that seems silly compared to just using a few more-powerful landing engines.

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  • $\begingroup$ 2 other things worth noting. The Super Dracos are pressure fed engines. This means that the tanks and piping all needs to be in higher pressure than the engine combustion chamber. This is bound to complicate refueling. Also they are designed to operate for just a few seconds. A whole new cooling system would have to be developed for sustained thrust. $\endgroup$ – Johnny Robinson Jun 21 at 23:23
  • $\begingroup$ @JohnnyRobinson I do mention that they're not designed for sustained operation, but the pressure-fed thing is a good point. I don't know if the eventual landing engines will be pump-fed or not - depends how highly they can pressurize the propellants, and keep them at pressure, I guess - but I'd be a bit surprised if they just use the autogenously-pressurized main tanks.. $\endgroup$ – CBHacking Jun 22 at 8:42

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