The Extravehicular Mobility Unit worn during spacewalks by NASA's
Space Shuttle astronauts represents more than 60 years of development
and testing of pressure suits in the U.S., Russia, France, Italy,
Germany, and other countries. It all began with high-altitude flyers,
and one of the first was an American, Wiley Post. Post, an aviation
pioneer of the 1930s, was seeking to break high-altitude and speed
records. Post, as well as others, knew that protection against low
pressure was essential. Through experience, aviators had learned
that Earth's atmosphere thins with altitude.At 5,500 meters, air
is only one-half as dense as it is at sea level. At 12,200 meters,
the pressure is so low and the amount of oxygen is so small that
most living things perish. For Wiley Post to achieve the altitude
records he sought, he needed protection. (Pressurized aircraft cabins
had not yet been developed.) Post's solution was a suit that could
be pressurized by his airplane engine's supercharger.
First attempts at building a pressure suit failed since the suit
became rigid and immobile when pressurized. Post discovered he couldn't
move inside the inflated suit, much less work airplane controls.
A later version succeeded with the suit constructed already in a
sitting position. This allowed Post to place his hands on the airplane
controls and his feet on the rudder bars. Moving his arms and legs
was difficult, but not impossible. To provide visibility, a viewing
port was part of the rigid helmet placed over Post's head. The port
was small, but a larger one was unnecessary because Post had only
one good eye!
During the next 30 years, pressure suits evolved in many ways and
technical manufacturing help was gained from companies that made
armor, diving suits, galoshes, and even girdles and corsets. Designers
learned in their search for the perfect suit that it was not necessary
to provide full sea-level pressure. A suit pressure of 24.13 kilopascals
(sea level- 101 kilopascals) would suffice quite nicely if the wearer
breathed pure oxygen. Supplying pure oxygen at this low pressure
actually provides the breather with more oxygen than an unsuited
person breathes at sea level. (Only one-fifth of the air at sea
level is oxygen.)
Various techniques were used for constructing pressure garments.
Some approaches employed a rigid layer with special joints of rings
or cables or some other device to permit limb movements. Others
used non-stretch fabrics- laced-up corset fashion.
With the advent of pressurized aircraft cabins, comfort and mobility
in the suit when it was unpressurized became prime objectives in
suit design. The suit could then be inflated in the event that the
aircraft cabin lost pressure.
By the time NASA began the Mercury manned space flight program,
the best full-pressure suit design consisted of an inner gas-bladder
layer of neoprene-coated fabric and an outer restraint layer of
aluminized nylon. The first layer retained pure oxygen at 34.5 kilopascals;
the second layer prevented the first from expanding like a balloon.
This second fabric restraint layer directed the oxygen pressure
inward on the astronaut. The limbs of the suit did not bend in a
hinge fashion as do human arms and legs. Instead, the fabric arms
and legs bent in a gentle curve, which restricted movement. When
the astronaut moved one of his arms, the bending creased or folded
the fabric inward near the joints, decreasing the volume of the
suit and increasing its total pressure slightly. Fortunately for
the comfort of the Mercury astronauts, the Mercury suit was designed
to serve only as a pressure backup if the spacecraft cabin decompressed.
No Mercury capsule ever lost pressure during a mission, and the
suits remained uninflated.
|Project Mercury astronauts.
(Front row, left to right) Walter M. Schirra, Jr., Donald K. Slayton,
John H. Glenn, Jr., M.Scott Carpenter. (Back row, left to right) Alan
B. Shepard, Jr., Virgil I. Grissom, L. Gordon Cooper, Jr.