Newsgroups: sci.space.history
From: Henry Spencer <henry@zoo.toronto.edu>
Subject: Re: Apollo 18 and 19
Date: Tue, 5 Aug 1997 15:56:47 GMT

In article <5s5v9k$23a@lace.colorado.edu>,
Frank Crary <fcrary@rintintin.Colorado.EDU> wrote:
>...The real problem wasn't getting into a polar orbit,
>but _not_ getting into it: If you target for an equatorial orbit, it's
>reasonably easy to put the spacecraft on a free return trajectory (i.e.
>if the rockets fail and you can't make the burn to enter lunar orbit,
>the spacecraft would be on a trajectory that takes it back to the Earth.)
>But it's more difficult (or impossible) to target a lunar, polar
>orbit and also be on a free return trajectory.

Actually, it's impossible to fly anything but an equatorial orbit using a
free-return trajectory.  In fact, Apollo 11 was the last lunar mission to
use a pure free-return trajectory.  The "H" missions (12-14) started out
in free-return trajectories but shifted to non-return ones after S-IVB
separation and CSM-LM docking, with the shift done progressively earlier
in the later flights.  The "J" missions injected directly into non-return
trajectories, again getting increasingly aggressive about it as time went
by -- in the event of an SM engine failure, Apollo 15 could have moved
back to a free-return trajectory using its RCS thrusters, while Apollo 17
would have had trouble doing it even with the LM descent engine.
--
Committees do harm merely by existing.             |       Henry Spencer
                           -- Freeman Dyson        |   henry@zoo.toronto.edu