In sci.space.policy message , Fri, 8
Jul 2011 04:16:11, Val Kraut posted:



It's true that after Apollo NASA thought it would need both a space
station and a Shuttle to lay the groundwork for manned flights to Mars.
How strange than that almost all subsequent designs for a manned Mars
mission called for a Direct trajectory from Earth to Mars, with nothing
being assembled in orbit. I doubt that even now ISS could be used to
assemble anything worthwhile, it's simply not designed for that. It's
nothing more than a trailerpark in space!

The space station is in the wrong orbit to support a Mars mission. Plane
change to support such a mission is essentially impossible. The ISS is a
dead end in itself.


Since the ISS orbit is at 52 degrees to the Equator, and the Equator is
23 degrees to the Ecliptic, and Mars orbits near the Ecliptic, ISTM that
twice in each orbit - more than once an hour - the ISS's velocity is
parallel to the plane of Mars' orbit.

The ISS orbit precession rate is once every 72 days, so ISTM that once
in every 36 days one of those occasions will have the velocity
approximately parallel to the Earth's velocity.

The launch window for a reasonably near-Hohmann orbit to Mars is, I
believe, over a month.

So it should be possible with chemical fuel to launch from ISS to Mars,
making good use of ISS's velocity.

Clearly, to be ready to go to Mars, we need rockets that can reliably be
started at the pre-planned moment - the Russians are already fairly good
at that.

But ISTM that chemical propulsion to Mars and back is very difficult;
propulsion with higher Isp is needed, in which case optimising the
starting conditions becomes less important.

The need, therefore, is to press on as fast as can be usefully afforded
with VASIMR and rival technologies.

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