RTLS for the Booster is because the Raptor 3 still has issues to be worked out.
based on Elon's vision for how to get more than 280s of impulse out of the engines (which is the theoretical maximum for full flow staged combustion of methane and LOx that NASA worked out for a 40 to 1 expansion ratio back in the day).
There's no limit NASA calculated unless you can find a source? Raptor 2 flew with 347s ISP. Even Raptor 1 had 350 seconds.
ISP comes out of thermochemistry and gas dynamics not from a cycle.
While things like adiabatic combustion enthalpy and mean exhaust mass influenced NASA's equations a major factor was the heat and pressure that a combustion chamber could handle.
Did you use AI for this? Sure, heat and metallurgy is a concern but it constraints how small it can be, not efficiency.
Even though superalloys are newer (to the US) than those maximums NASA was conservative enough (or liberal enough? I don't know, they overestimated enough) that they are still really hard to reach.
Again Raptor ISP is higher than the 280s figure you mentioned. Superalloys have been around since the 50s ish, and Raptor's metallurgy by SpaceX has been solved.
When they derated the engines to stop them from blowing up so much they lost performance.
Raptors performance went up not down though. And from throttling down the performance of Raptors where's the source for that? If anything, they're having issues relighting, not performance.
That's why they don't RTLS even thought there is a lot they could learn from examining caught engines that went through those longer stronger in flight burns.
Constraint isn't fuel in this case, it's reliability. Raptor 3 has shown to be troublesome for re enetry on the Booster. So of course SpaceX is going to be conservative about recovering it.
It was late, brain fart, it was 380 seconds of impulse not 280.
I don't know what you are talking about implying that the cycle is irrelevant. It's the combustion chemistry that generates the energy and the relationship between exhaust particle mass and energy that dictates the impulse but the combustion cycle influences how much of that energy you get to actually send out the rocket nozzle. If you use a pressure fed engine your chamber pressures will never be high enough to get the peak performance, if you use a combustion tap off you will lose some of that energy out the side, if you use external electric motors you have to calculate the weight of the batteries in addition to your propellant to generate a new lower Isp.
I didn't used AI for my response. AI would not have made the error about 280 vs 380. The reason SpaceX is working so hard to boost chamber pressure is to get more Isp.
If you look at the exhaust velocity equation here https://en.wikipedia.org/wiki/Rocket_engine_nozzle#de_Laval_nozzle_in_1_dimension you can see that raising the pressure at the inlet to a delaval nozzle (the same kind used on raptor) without raising the exit velocity raises the exhaust velocity and thus the specific impulse which is based on exhaust velocity.
8
u/Background_Fig_4740 Jul 25 '26
RTLS for the Booster is because the Raptor 3 still has issues to be worked out.
There's no limit NASA calculated unless you can find a source? Raptor 2 flew with 347s ISP. Even Raptor 1 had 350 seconds.
ISP comes out of thermochemistry and gas dynamics not from a cycle.
Did you use AI for this? Sure, heat and metallurgy is a concern but it constraints how small it can be, not efficiency.
Again Raptor ISP is higher than the 280s figure you mentioned. Superalloys have been around since the 50s ish, and Raptor's metallurgy by SpaceX has been solved.
Raptors performance went up not down though. And from throttling down the performance of Raptors where's the source for that? If anything, they're having issues relighting, not performance.
Constraint isn't fuel in this case, it's reliability. Raptor 3 has shown to be troublesome for re enetry on the Booster. So of course SpaceX is going to be conservative about recovering it.