[AR] Re: SpaceX Single Stage to Orbit
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Sat, 25 May 2019 19:13:21 -0400 (EDT)
On Sat, 25 May 2019, William Claybaugh wrote:
I cannot rule out someone trying to build a dedicated TSTO for high
inclination launches only... More likely, in my view, is the use of
same technology strapon’s on the SSTO.
Or use a LEO-based high-Isp tug to do the plane change. If we're doing
this sort of thing much, investing in orbital transport infrastructure is
an alternative to investing in launcher variants.
Going to an inclination of 30-40deg will not hurt an SSTO's payload too
badly -- the gain from Earth's rotation is a cosine function of
inclination, so at 30deg you still get ~87% of the benefit. From there,
about a 60deg further plane change will get you to the worst-case orbit of
interest (sun-synch at about 97deg).
Doing 60deg the naive way requires delta-V equal to the orbital velocity,
about 8km/s, which sounds bad but is not that big a deal for ion thrusters
with an Isp of 5000s (not difficult for ion): only about 15% of the
initial mass is expended if you're patient. The tug then has to come
back, but that uses rather less because the heavy payload is gone.
Impatience incurs a delta-V penalty, but you can probably get some of that
back by using a less-naive maneuvering plan -- big plane changes are
cheaper at higher altitude, so almost any major plane change is best done
by boosting higher first, doing it, and then coming back down. (With high
thrust, a total delta-V of about 6km/s goes to *infinity* and back, so you
can do any plane change for that cost -- maybe rather less with lunar
gravity assist and aerobraking. With low thrust it's complicated, but
there is generally still a gain.)
Henry
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