4 ms·

The 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 sm

The 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 ago
Also, 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 ago
  AFAIK 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 ago
    Ah 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 ago
That'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.

Crito 13y ago

Also, 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 ago
AFAIK 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 ago
  Ah 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 ago

That'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.