SpaceX to NASA quote : simplified mission architecture : Technical discussion

[Vultur]

I think that has been said at some point, but the recent Moon and Beyond" update refers to "Raptor lunar landing throttle test demonstrating a representative thrust profile that would allow Starship to land on the lunar surface".

That looks to me like SpaceX is still hoping to land on the Moon (not just Mars) with Raptors, and may be trying to demonstrate this at least partly with the intent to offer "no landing thrusters needed" to NASA as a potential simplification.

It's absolutely a requirement to use Raptors to land on the Moon.  What's at issue is whether to use Raptors for the last hundred meters or so.  But there's still a throttle profile that's required.

Even if you're using thrusters, you need the Raptors to deliver the HLS to a specific height with a specific downward velocity.  If the velocity is too low, the thrusters have to use a lot of extra prop, and if they get close to running out, that's an abort.  If the velocity is too high, then the thrusters' thrust may not be adequate to do a safe hoverslam, and that's also an abort--or a crash.

That makes sense.

So do you think SpaceX has given up on Raptor-only landing?

[TheRadicalModerate]

So do you think SpaceX has given up on Raptor-only landing?

I don't know.  There were methox or methalox thruster tests at McGregor a couple of years ago but, AFAIK, nothing since then.  That doesn't sound like something that's going into HLS.  So that seems to leave two possibilities:

1) They're almost sure that Raptor landing will work.  I don't know how they would have come to that conclusion, so I suspect it's not the answer.

2) They're planning on using SuperDracos for landing and ascent,¹ as a temporary measure.  This would require fixing the SD's problem with backflow through the check valves.  (For the D2, the check valves have been replaced with a burst disc, which makes the system good for only one burn.  That's adequate for D2 launch escape, but not for what would have to happen with Starship.)

SDs would be a pain, and they won't work for a reusable HLS (no way to refuel them and replenish their pressurant), but they're well-understood and, oddly, about the right thrust for Starship's purposes.  But they're pressure-fed, and they'd need quite a bit more prop.²

I view the thruster situation as a non-trivial source of schedule risk.

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¹Raptors can be used for trajectory correction maneuvers, at least in theory.  However, I'm skeptical that they'd be particularly accurate doing burns for 1-5m/s delta-v.  I'd expect thrusters to fill that role, as well as for coarse attitude control, where cold gas would not be very responsive in the control system.  But this may be one of those "best part is no part" decisions on SpaceX's part, while they wait for some data from the refueling tests, which should exercise both of these functions.

²For your dancing and dining pleasure, here's a spreadsheet that shows how much prop is needed for various combinations of Raptor shutdown height, shutdown velocity, and startup height during ascent.  Note!  If you copy it to play with it, it needs iterative calculation turned on, because the prop needed to land isn't factored into the wet mass that's entered as a parameter.

Note that thruster prop mass gets really out of hand if you make the shutdown happen too high, or the shutdown velocity too small.  For comparison, SD prop capacity is about 1.4t on the D2.