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From: fcrary@lightning.Berkeley.EDU (Frank Crary)
Newsgroups: sci.space,sci.space.shuttle
Subject: Re: space news from March 18 AW&ST
Message-ID: <1991May20.034518.6909@agate.berkeley.edu>
Date: 20 May 91 03:45:18 GMT
References: <1991May10.034743.29351@zoo.toronto.edu> <1991May18.231158.24883@milton.u.washington.edu>
Sender: root@agate.berkeley.edu (Charlie Root)
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In article <1991May18.231158.24883@milton.u.washington.edu> wiml@milton.u.washington.edu (William Lewis) writes:
>   What does a "fast trajectory" mean? Faster than a Hohmann orbit?
>Or faster than some other orbit I'm not familiar with? How much time
>would this hypothetical trajectory take, and how picky is it about 
>planetary alignments? Inquiring minds want to know ...
>
A fast trajectory means faster than a hohmann orbit. In fact a specific
transfer orbit. While I don't recall the details, I seem to remember that 
it is the "next step down" e.g. all the other orbits, except a hohmann 
orbit, are less efficient. In the case of a Earth to Orbit transfer
the fast, or "opposition" class, orbit still takes months (about 3 or 4)
as opposed to the 260 days of the Hohmann transfer. The big problems are 
fuel, about 150% the C(3) energy of a hohmann is required (how 50% more
energy translates into fuel mass depends on the design, but a 200 - 400%
increase might be a good ball park guess.) Also, a "opposition" class
orbit reaches Mars only a week or two before the launch window for an 
Earth return. As a result, there are two possibilites: 

1) Four months in transit to Mars, 2 weeks on the surface, four months
   returning to Earth.

or
2) Four months in transit to Mars, 26 months on the surface, four months
   returning to Earth. (E.g. miss the first return window and wait for
   the next.)

Neither is too appealing, the first allows not enough time on the surface
to justify a manned mission (e.g. the work that men could do in this short
time could be just as well done by robots. Men need to be there a while
for the advantages of a manned mission to matter.) The second alternative
is just TOO LONG for a safe, productive mission (almost 3 years).
As a result, the Case For Mars IV, Launch, Assembly and Transportation 
Workshop recomended that "fast" missions be ignored. The 1000 day hohmann
orbit requires not only less fuel, but provides a reasonable ammount of
time on the surface (about 460 days.)

				Frank Crary
				UC Berkeley