Newsgroups: sci.space.history
From: Henry Spencer <henry@zoo.toronto.edu>
Subject: Re: Apollo 8 - first high speed re-entry into Earth's atmosphere?
Date: Sat, 16 May 1998 14:44:08 GMT

In article <355cb6dc.0@news.vphos.net>,
Mike Humphreys <mhumphre@awinc.com> wrote:
>Were there ever any unmanned tests performed on high speed re-entry prior to
>Apollo 8?

Apollos 4 and 6, in addition to being test flights of the Saturn V, were
high-speed reentry tests -- one for the worst-case shallow reentry, long
and drawn out with maximum total heat load, and one for the worst-case
steep reentry, short and sharp with maximum heating rate.  The Apollo 6
engine problems compromised the plans a bit, but they still managed an
approximation to the intended test.

>Or, were Borman, Lovell and Anders relying on the theoreticians being
>correct?

Aside from the tests, BL&A could also rely on the fact that their
heatshield was greatly overbuilt.

The CM heatshield was one of the two items of unproven technology that
went into the Apollo design (the other being supercritical helium for LM
descent-stage tank pressurization).  At the time of initial design, both
the size of the heatshield and the speed of the reentry were well beyond
practical experience, and the details of the scaling laws were not fully
understood... so the designers guessed and then added a big safety margin.
The resulting heatshield was, in fact, massive overkill -- only a fraction
of its thickness was ever used.  Had there ever been an Apollo Mark 2, the
heatshield would probably have been trimmed down to reduce weight.
--
Being the last man on the Moon                  |     Henry Spencer
is a very dubious honor. -- Gene Cernan         | henry@zoo.toronto.edu