Can a pressure fed engine be powerful enough to augment the landing of Starship on the Moon? How much thrust is needed to land SS if it’s brought to zero velocity at an altitude of 10 meters?

A debate exists whether SS is will kick up debris harmful to itself and others. Do not want to engage that debate here. But to address the possible need for alternatives: Posit the Raptors will bring SS to zero velocity at 10 meters altitude, instead of zero altitude, keeping the worst of the exhaust plume from the surface. Can a hot gas thruster, or cluster of them, be powerful enough to ease the final 10 meter descent? This will be the simplest additional system since SS will have an RCS of methalox hot gas thrusters. (The proposed thrusters to be larger but similar.) Assume a wet mass on landing of 240t; a modest payload and fuel to reach lunar orbit.

Know this is not the forum for the many issues surrounding Starship lunar missions, but want to know if this one aspect is at all workable.

(10 meters is an arbitrary starting point, but if this won’t work there, or lower, the question is moot.)

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    $\begingroup$ I do not understand the question at all. The Apollo LM landed using pressure fed engines. $\endgroup$
    – ikrase
    Dec 6 '19 at 9:29
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    $\begingroup$ Uhoh Thanks, title edited per your suggestion. $\endgroup$ Dec 6 '19 at 20:27
  • $\begingroup$ ikrase The LM had a mass of about 7t in the last 10 meters of descent, it's engine putting out less than 7 N. My question concerns a ship massing 240t. Can a pressure fed engine be made large enough, considering the mass is at barely more than zero velocity when the engine is needed for a 10 meter descent. $\endgroup$ Dec 6 '19 at 20:47
  • $\begingroup$ One of the largest rockets ever designed, the Sea Dragon, had a extremely large pressure-fed engine. Never built though. $\endgroup$ Dec 6 '19 at 22:33
  • $\begingroup$ Then you just add more engines. Still I don't see why the Starship would benefit from using other than the Raptors, except as verniers/RCS. $\endgroup$
    – ikrase
    Dec 8 '19 at 23:13

The question here is not so much about pressure fed engines as about thrust.

If you need to land 240 tonnes of space craft on the moon you need to overcome at least the force of lunar gravity on that mass with engine thrust or it stops being a landing. So there is a fixed lower amount of thrust needed to land a given craft on a given body and it scales with vehicle mass.

What does make a difference is how much above that lowest possible thrust your engine produces during landing. Landing Falcon stages are running at a higher thrust than needed for hovering because the engines cannot throttle that far down.

If the Starship engines cannot throttle down far enough to hover in lunar gravity and need a similar hover slam/suicide burn landing profile than there is certainly room for improvement. In the Starship case that would probably be a tuned engine using the existing fuels and pumps rather than a separate pressure feed system. Alternatively they could use opposed set of engines vectored outwards, losing thrust through cosine loses and making the blast craters either side rather than under the lander.

If the planned engines can already throttle down far enough to hover at landing weight then it just becomes a question of does the trust needed for Starship mass concentrated in the area under a starship produce problems. If so it becomes a case of finding a way to either reduce mass or increase the area it is applied over.

The decision to use a pressure fed engines for Apollo was much more about reliability than being inherently less prone to FOD during a lunar landing than a pump fed engine with the same capabilities.

There are certainly many valid reasons for using hypergolic pressure feed engines for a lunar landing but most of them apply to a dedicated lander design that will only have one engine/fuel system.

  • $\begingroup$ "Landing Falcon stages are running at a higher thrust than needed for hovering because the engines cannot throttle that far down." Not quite; they run at higher that 1:1 TWR because it's more fuel-efficient to do so. SpaceX sometimes uses 3-engine terminal burns, in fact. $\endgroup$ Dec 7 '19 at 4:20
  • $\begingroup$ As I understand it Starship will be using the hover slam method, due to the Raptor's high thrust even at low throttle. Thus the idea of switching to modestly powered engines briefly. Am considering pressure fed methalox engines because Starship has that system in place for the RCS. Hypergolics would add a whole new fuel system, so a no go. The idea is to reduce the amount of FOD kicked up, not have a less damage prone engine. Pressure fed also preferable to keep development costs of a new engine minimal. $\endgroup$ Dec 7 '19 at 5:25
  • $\begingroup$ With a mass of 240t in 1/6 gravity, the engines need to provide just over 40 kN and throttle a bit below it, correct? Or are my concepts totally off? $\endgroup$ Dec 7 '19 at 5:33
  • $\begingroup$ @Russel Borogove, had been under the impression that spaceX had been forced into the hover slam by not being able to hover, and having done so went all in with three engine burns to reduce gravity losses. There are have been enough iterations of stages and hardware that we both may have been right at various times. $\endgroup$ Dec 7 '19 at 6:23
  • $\begingroup$ @SpaceInMyHead. There is certainly a lot of sense in a landing stage that can touch down at 'just enough' thrust for a number of reasons. Unsure if adding a whole new engine to Starship is the way to do it. Wonder if you could just let gas pressure build in the at most 50% full tanks during approach and actually touch down using the raptors as cold gas engines? $\endgroup$ Dec 7 '19 at 6:40

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