One thing to note - while Starship is a highly credible interplanetary
vehicle delta-V and landing wise, it's not the only credible architecture
SpaceX can consider.
Notably, the Falcon 9 second stage is already relightable, LOX is the only
propellant with any boiloff issues, battery and igniter fluid EDO packs are
already discussed, and the Starship tanker's payload is about one Falcon 9
refueling worth.
-george
On Tue, Aug 27, 2019 at 10:04 AM Henry Spencer <hspencer@xxxxxxxxxxxxx>
wrote:
On Tue, 27 Aug 2019, James Fackert wrote:
Maybe the orbiting fuel depot as a separate entity is not optimal. The
fuel depot implies that there is a requirement for a much larger supply
of fuel than a single tanker can bring up.
There is. We're talking high-mass-ratio vehicles here; even with generous
assumptions, refueling one ship takes maybe 8-10 ship payloads of fuel,
and 12+ would be more realistic.
This is simply fundamental to using chemical rockets without staging.
Going anywhere from LEO takes at least a few km/s of delta-V, and since
the exhaust velocity of chemical rockets is also in that vicinity, the
rocket equation says 2/3 or more of a fully-loaded ship's mass has to be
fuel. And much of the rest has to be things like structure and engines;
very aggressive design can give you 1/3 of that as payload (it has been
done), in an expendable, but 1/5 is more typical, and for an
aerobraking-capable reusable it'll probably be lower. Multiplying it out,
very optimistically the payload might be 1/8 of a full fuel load, probably
rather less.
You can improve on that a little by scavenging unused fuel from the
propulsion systems of your tankers. To reliably reach the refueling
orbit, the tankers need some fuel margin, but once they get there, any
still-unused fuel margin is surplus. However, for well-tested reusable
vehicles, the required margins won't be large.
When you have a big ship with lots of supplies and living space and
stuff for a real long term mission rather than a touch and go, you also
have a ship capable of being a tanker that can bring enough fuel in one
flight to refuel that ship for an extended lunar or interplanetary
mission and return.
Alas, while this sort of sounds plausible, there is no substitute for
doing the *numbers*, and they say it's not so.
Henry
