Skipjack wrote:happyjack27 wrote:
If one is only launching one satellite into orbit, it is more economical to use a two-stage reusable system, scaled to the approximate payload.
unless you use air-breathing engines for a dramatic increase in isp, and have a good total reuses / total cost ratio to properly amortize the initial investment.
No, it is not. The economy of RLVs is driven by:
- Development cost
- Cost to build
- Cost of refurbishment
- Flight rate
- number of reuses
Small two stage compared to Massive (ITS tanker) SSTO:
- Development cost - well clearly there'd be some, but it's not new territory here.
- Cost to build - smaller, fewer engines, less materials, means lower cost to build. also lower stage doesn't need heat shields
- Cost of refurbishment - as above, lower costs
- Flight rate - once per satelite, either way
- number of reuses - the lower stage would have a lot more reuse, flight stresses are much less - lower speed, doesn't need heat shields
so... tie,win,win,tie,win.
Isp is not a major contributor to cost. It enables more payload and a higher delta-v but that is a comparably minor factor when you already have at least 20 t to LEO.
...or you could use it to build a smaller rocket. reducing both cost to build and cost to refurbish.
Air breathing only potentially(!) enables more payload and higher orbits without an orbital tug or some other form of second stage (e.g. more fuel in the satellite so it can get there on its own).
Everything "only potentially" does anything. A rocket "only potentially" makes it to LEO.
Like the Merlins, the Raptor engines might experience a slight increase in Isp (and thrust) over time as they figure out their margins and other ways to optimize them. Though I doubt that the improvements will be as big as they have been with Merlin. Still, this is something that will potentially benefit the proposed SSTO in the future.
Benefit how? Isp is not a major contributor to cost. It enables more payload and a higher delta-v but that is a comparably minor factor when you already have at least 20 t to LEO.