Multi Stage Raptor Engine

[davamanra]

Here's a question about the vacuum raptor engine:  Are there any major problems with using a vacuum raptor engine at sea level?  The only possible issues that I can think of are combustion instability and reduced thrust.

The reason I ask this question is that it seems to me that if combustion instability can be addressed, then the three sea level engines could basically be made redundant.

The sea level engines are for landing and they also perform gimbal function.

The RVac is probably pretty close to flow sep, I imagine ISP at sea level is probably only like 290. Might be less of a case with Raptor 2 if it runs at a little higher pressure and they don’t alter ER.

This is why I was looking at this.  If you were to first, control the flow separation (obviously the key problem) and then eliminate the sea level engines and then mount gimballed vacuum engines.  Even if the ISP is greatly reduced, you could use all three of these engines to provide enough thrust for landing a Starship with nearly empty propellent tanks.  Also, by eliminating three engines you would reduce the overall mass and complexity of a Starship which would increase the amount of payload it could take into into space as well as reduce its landing mass.

Issues:

Even if flow separation is avoided or controlled this is most likely only the case for high throttle settings so you lose the ability to throttle down during landing.

Gimbaling of a larger/heavier RVac is much slower unless you beef up the actuators (and possibly reinforce the nozzle as well).

You can only fit 1 RVac in place of the three SL engines, so you lose some of the thrust you just added with the 9 Raptor version and you lose landing engine redundancy. If the 9 Raptor version had gained a limited low altitude abort capability you have now lost it again.

The longer center RVac means that you have to extend the skirt and move the outer ones down to avoid plume impingement, adding quite a bit of mass.

Summary: You now have a lower thrust and most likely heavier Starship that can only do suicide burn landings with reduced or no redundancy.

Issue #1:  This is a real possibility and would need to be addressed.

#2:  Although gimballing might be slower, this can be compensated by faster response to attitude inputs, so the nozzle might be moving slower but it will respond earlier.  In this way you wouldn't have to beef up the actuators or reinforce the nozzle.

#3:  I don't know what you are referring to with a 9 Raptor version.  The present design has only 3 SL Raptors and 3 RVAC's.  With the removal of the SL Raptors, there would be plenty of room for the 3 RVAC's.

#4:  There wouldn't be a center RVAC, in this design.  There would only be three RVAC's.  They would be spread out on a new thrust puck where there would be plenty of room, and they would be on the same plane so there would be no plume impingement.  Also less mass because three less engines 

I think you might be thinking of a Falcon 9 with it's 9 Merlin engines and not the Starship with it's 3 SL Raptors and it's 3 RVAC's.

#5:  As it is right now, Starship only needs 2 of it's three SL engines to land.  If you were to use 3 RVAC engines, even with less thrust, the overall thrust would still be more than 2 SL engines, giving you enough thrust and gimballing ability to land.

What do you mean by TWC responding earlier? It already reacts instantly (control loops are measured in milli- if not microseconds) so a slower moving nozzle means that the TVC has a larger time constant.

I thought you were talking bout the recently re-suggested 6 vacuum Raptor + 3 SL Raptor version. If you you just remove the SL Raptors on the 6 engine version you will drastically lower your payload.

Landing on 3 vacuum Raptors will be riskier (lower thrust, longer lever-arm, likely smaller throttle range and slower TVC response).

This 6 RVAC configuration is new news to me.  I was referencing the current 3 RVAC configuration.  Having said that, if they go with this 6 RVAC configuration Starship is gonna have a LOT of thrust!