Newsgroups: sci.space.policy
From: henry@zoo.toronto.edu (Henry Spencer)
Subject: Re: Imponderables About Space
Date: Sat, 16 Dec 1995 06:12:12 GMT
Lines: 28

In article <rpfg2ema4nx.fsf@unicorn.bbn.com> Chris Jones <clj@bbn.com> writes:
>...the early Soviet Lunas (1 through 3, and various unnumbered failures) as
>well as the the US lunar attempts prior to the Ranger series used direct
>ascent.  I think you lose by having a much shorter launch window, and gain by
>having a somewhat higher payload which can be accelerated to escape velocity.

Correct.  The key observation is that if you're heading out and not coming
back, any energy spent raising your *perigee* is wasted; you want to put
the energy into raising the apogee.  But a parking orbit's perigee must
clear the atmosphere, so it wastes some energy.  The payoff is longer
launch windows (just spend less time in parking orbit if you're late
launching) and greater accuracy (because you essentially get to stop
halfway up, measure your exact trajectory at leisure, and crank in some 
corrections as part of the final burn).

You can have your cake and eat it too, to a limited extent, by making the
parking orbit as low as possible.  The later Apollo missions -- which
couldn't linger long in parking orbit in any case -- used a parking orbit
altitude of about 150km, so low that friction heating was important in the
thermal calculations. 

When push comes to shove, though -- for example, when you're trying to
launch a Halley probe on a booster that's just barely up to the job --
you may have to accept a price in flexibility and accuracy for the sake
of just getting the damn thing up there.
-- 
Look, look, see Windows 95.  Buy, lemmings, buy!   |       Henry Spencer
Pay no attention to that cliff ahead...            |   henry@zoo.toronto.edu