Exploring the Unknown: Selected Documents in ... - The Black Vault

Exploring the Unknown 247
A series of flight tests and wind tunnel tests were conducted to get the
answers to some of the basic questions. First, would the ablation [3] principle
work in our application? Could we conduct heat away from the spacecraft body
by melting the fiberglass and resin material? How thick would the shield have to
be for our particular conditions? What temperatures would be encountered and
for what time period would they exist? Early wind tunnel test proved in theory
that the saucer shaped shield would protect the rest of the spacecraft from heat
damage. The flight test on the heat shield must prove the theory. In February
1961, we made a ballistic flight in which the spacecraft reentered at a sharper
angle than programmed and the heat shield was subjected to great than normal
heating. The test proved the heat shield material to be more than adequate.
The Mercury spacecraft did not start with the familiar bell shape. It went
through a series of design changes and wind tunnel tests before the optimum
shape was chosen. The blunt shape had proven best for the nose cone reentry. Its
only drawback was the lack of stability. We next tried the cone-shaped spacecraft,
but wind tunnel testing proved that heating on the afterbody would be too
severe, although the craft was very stable in reentry. After two more trial shapes,
the blunt bottom cylinder on cone shape came into being. It was a complete cycle
from the early concepts of manned space-craft, but it was only the first of a series
of changes in our way of thinking of the flight program and its elements.
A second part of design philosophy thinking came in connection with
the use of aircraft equipment in a spacecraft. We had stated at the start of the
program that Mercury would use as much as possible the existing technology
and off-the-shelf items in the design of the manned spacecraft. But in many cases
off-the-shelf equipment would just not do the job. Systems in space are exposed
to conditions that do not exist for aircraft within the envelope of the atmosphere.
Near absolute vacuum, weightlessness and extremes of temperatures makes
equipment react differently than it does in aircraft. We had to test equipment
in advance in the environment in which it was going to be used. It produced
an altered concept in constructing and testing a spacecraft. Although aircraft
philosophy could be adapted, in many cases, aircraft parts could not perform in
a spacecraft.
The third part of the design philosophy, and perhaps the most important
one in regard to future systems is the automatic systems contained in the
Mercury spacecraft. When the project started, we had no definitive information
on how Man would react in the spacecraft system. To insure that we returned the
spacecraft to Earth as planned, the critical functions would have to be automatic.
The control system would keep the spacecraft stabilized at precisely thirty-four
degrees above the horizontal. The retrorockets would be fired by an automatic
sequence under a grogramed [sic] or ground command. The drogue and main
parachutes would deploy when a barostat inside the spacecraft indicated that the
correct altitudes had been reached. The Mercury vehicle was a highly automatic
system and the man essentially was riding along as a passenger, an observer. At
all costs, we had to make sure that the systems worked.
[4] But we have been able to take advantage of Man’s capability in space.
It started from the first manned orbital flights. When some of the thrusters
became inoperative on John Glenn’s flight, he was able to assume manual control