4 ms·
If the first stage reusability works fine and cost per flight goes down a lot, then it starts making sense to optimize the upper stage and spacecraft for lower
by Gravityloss 13y ago
If the first stage reusability works fine and cost per flight goes down a lot, then it starts making sense to optimize the upper stage and spacecraft for lower cost per flight as well. (Because now, even if the first stage flight was free, the launch would still cost a huge amount of money.)
In manned flights to low earth orbit, since the spacecraft reenters, at least that part could be reused.
The second stage is hard to reuse because it flies so far downrange horizontally and reenters at very high speed. The engine also can't run low in the atmosphere, meaning somehow different recovery than for the first stage.
The parts of the spacecraft that are not heat shielded (service module) will be sacrificed, but in the future the whole spacecraft might be a monolithic entity or even part of the second stage and do its mission and reenter and land as a whole.
- jcampbell1 13y agoThe first stage has 9 motors, and the second stage has 1. Recovering the 2nd stage seems incredibly difficult involving lots of compromises for a relatively small benefit. I'd invest engineering effort into making the first stage motors not require much re-manufacturing between flights. The cost savings are going to come from a first stage that is re-fill and go, vs completely disassemble the motors and rebuild and replace parts.
- Crito 13y agoAlso, although the first stage is quite large and has 9 engines, when it finally touches down it uses only the center engine, throttled down. Even using just that one engine, it produces too much thrust to hover (grasshopper can hover, but grasshopper is heavier than a returning F9 first stage). The second stage is meant for use in a vacuum so it's expansion nozzle is much larger, so perhaps that extra weight would balance things out a bit, but I suspect that the second stage would have a massive thrust/weight ratio when reentering almost empty. Makes things more challenging, even if the engine does work in atmosphere correctly.
- Gravityloss 13y agoAFAIK the second stage merlin with the big nozzle would have flow separation and would shake itself apart if fired at low altitude. IIRC The concept videos have shown separate smaller DRACO landing engines. Also used with Dragon for soft touchdown?
- Crito 13y agoAh yes, you're right. The concept video shows them using the second stages main engine for a deorbiting burn, then using a heat shield for reentry, then using [dracos/superdracos?] to touch down.
- Gravityloss 13y agoThat's a good approach. If the first stage engines last for 9 flights, then it's one engine per flight and usage is at equilibrium point with the upper stage. AFAIK the upper and lower stage engines are almost identical bar nozzle. This ignores refurbishment cost. It is very interesting to think about the economics.