Assuming SpaceX can recover boosters and fairings, the largest remaining cost is the second stage. If a Falcon costs $50m to build, and 70% of the cost is the booster, and 10% is the fairing, then 20% is the second stage, or about $10 million.Recovering the second stage looks hard. But what about reducing the cost? A lower cost stage will have less performance, but particularly to LEO this may still result is very useful capacity. SpaceX could reduce the cost in at least two ways - use a regular merlin 1D (not vacuum optimized, potentially used) as the engine...
Assuming SpaceX can recover boosters and fairings, the largest remaining cost is the second stage. If a Falcon costs $50m to build, and 70% of the cost is the booster, and 10% is the fairing, then 20% is the second stage, or about $10 million.Recovering the second stage looks hard. But what about reducing the cost? A lower cost stage will have less performance, but particularly to LEO this may still result is very useful capacity. SpaceX could reduce the cost in at least two ways - use a regular merlin 1D (not vacuum optimized, potentially used) as the engine, and use cheaper (but heavier) construction for the stage.How much would this hurt performance? SpaceX quotes 22.8 tonnes to LEO. Using the usual assumptions of 4.5t empty mass and 111.5t of fuel, the second stage delta V is 348*9.8*ln((111.5+4.5+22.8 )/(4.5+22.8 )) = 5546 m/s. If we reduce the ISP to 311, and assume double the empty mass, then we get a similar delta-V for a 12.5 tonne payload: 311*9.8*ln((111.5+9+12.5)/(9+12.5) = 5554 m/s. 12.5 tonnes of payload seems like plenty for most LEO missions.
Considering spacex's internet constellation plans in LEO and VLEO, that orbital regime will probably take up at least 75% of their yearly launches. So the economy of scale would be present.
It might make sense to once in a while use an old sea-level Merlin. Presumably, Merlins would not be infinitely reusable. (<-- This comes from "rockets are legos" dept, which does not know whether MVac-to-stage plumbing is compatible with sea-level Merlins)
Maybe SpaceX should bring back the Kestrel with an electric pump and solar-electric power system.
M1D and M1DVac are not interchangeable. This idea comes up every now and then, but apparently it needs to be repeated: They are very different.
Quote from: Lars-J on 04/26/2017 06:11 pmM1D and M1DVac are not interchangeable. This idea comes up every now and then, but apparently it needs to be repeated: They are very different.OK I believ we all know now that a M1D can not be converted to a M1DVac.But that is not the point raised here. The idea is to install a M1D in place of a MVac. Are the mount points so different?