The main arguments against this seem to be (a) it's intuitively scary, and (b) that's not the way we've always done it. These are basically the same arguments against loading astronauts before fueling. But in both cases you need to run the numbers, and perhaps the engineering argument gives a different result than intuition.
(b) The impact of an engine (or even 2 or 3) that fails to re-light is small, assuming this is detected and the engine shut down. This follows from the rocket equation. In the absence of gravity losses, the delta-V is determined only by the initial mass, the final mass, and the ISP. Each of these is unchanged with one or more inoperative engines. The acceleration is less, but the stage burns longer, and these exactly cancel. Now with gravity included there will be some performance loss, but this is late in the first stage burn where the gravity losses are comparatively small, and the difference between 8 and 9 engines smaller yet.
For the Falcon Heavy, the most conservative choice may be to shut down the engines of the center core early in ascent, then use the saved fuel after the side boosters separate.Intuitively this seems like a bad idea. What if the engines don't restart? Then the mission would fail.The counter argument has 4 parts:(a) re-lighting Falcon engines in flight is by now a known quantity. The odds of a re-light failure are low. (b) The impact of an engine (or even 2 or 3) that fails to re-light is small, assuming this is detected and the engine shut down. This follows from the rocket equation. In the absence of gravity losses, the delta-V is determined only by the initial mass, the final mass, and the ISP. Each of these is unchanged with one or more inoperative engines. The acceleration is less, but the stage burns longer, and these exactly cancel. Now with gravity included there will be some performance loss, but this is late in the first stage burn where the gravity losses are comparatively small, and the difference between 8 and 9 engines smaller yet.(c) On the other hand, with complete shutdown you get considerably better margins (I get about 200 m/s better a wide range of assumptions). This is much more than the loss if one or two engines don'r restart.(d) Higher margins help overall mission success. The Delta-4 with the RL-10 leak succeeded due to margins. The Atlas to the station succeeded since the margins were enough to overcome engine troubles. Likewise with the Falcon to the station - and on this flight the secondary failed since the margins were not quite enough.You could even put some numbers on this argument. The above examples (and I'm sure there are more) show that margin matters in a few percent of all launches. Conversely, the SpaceX tests to date have had perhaps 50-60 air restarts, and as far as I know no engines have failed to re-light. So the odds of two or more engines failing to re-light should be well under a percent. (Plus, of course, the odds of some common glitch that affects multiple engines as they re-light. That's hard to quantify but seems small. Boost-back and re-entry burns start 3 engines at time routinely already.)The main arguments against this seem to be (a) it's intuitively scary, and (b) that's not the way we've always done it. These are basically the same arguments against loading astronauts before fueling. But in both cases you need to run the numbers, and perhaps the engineering argument gives a different result than intuition.Note that this argument does not apply to the Delta Heavy, for several reasons. The margins would indeed be better, for the same reasons as above. However, the core engine, as far as I know, cannot restart, much less have a history of many successful air restarts. Also it has only one engine on the center core, and if that fails to re-light, it's catastrophic.
You are ignoring that only 1-3 engines are set for restart. TEB tankage has to be increased and also lines have to run to all the engines. Also, a lot more He has to be carried to restart 9 engines.
Just out of curiosity here, totally random:If you want to spin up more than one turbine, could you route a bleed from an already started engine instead of an He line for that purpose?And I'm not even talking about "borrowing a light" for ignition
I also thought... put a separate RP1/LOX gas generator onboard (burner can) to generate CO/CO2 to pressurize the tanks... but that's crazy talk I am told...
Quote from: John Alan on 01/30/2017 02:33 pmI also thought... put a separate RP1/LOX gas generator onboard (burner can) to generate CO/CO2 to pressurize the tanks... but that's crazy talk I am told... It would be.CO/CO2 would condense and freeze in LOX. And there can't be any unused O2 in the RP-1 tank
Agreed... but how much would condense/freeze in the short time S1 is actually firing all 9...
About half the output of a gas generator is water, which would condense and freeze in both the LOX and RP-1 tanks, since SpaceX loads RP-1 at -8 degrees C. Also, carbon monoxide and other incomplete combustion products are fuels... which you do not want in the LOX tank.
Is the point of this idea to mitigate the need for cross-flow?
The most simple thing they could do is switch off the three engines they can relight without modifications. Together with throttling the other engines of the central core they could retain a lot of the fuel while the side cores burn full throttle.
You guys are overthinking it... The M1D can already throttle down pretty deep. It is far simpler and safer to run a low throttle than to restart all 9 engines.
Quote from: Lars-J on 01/30/2017 05:39 pmYou guys are overthinking it... The M1D can already throttle down pretty deep. It is far simpler and safer to run a low throttle than to restart all 9 engines.ISTR the center engine is the only one that can throttle down deep, which requires special hardware (a special valve?). That information may be old/outdated though.