The Complete Book of Spaceflight: From Apollo 1 to Zero Gravity

10 aerobraking
was one of two rockets developed by the U.S. Navy in the
1940s—the other being the Viking—to loft scientific
instruments into the upper atmosphere. An unguided
two-stage vehicle, the Aerobee was launched by a solidpropellant
booster of 80,000-newton (N) thrust that
burned for two and a half seconds. After the booster was
spent, the rocket continued upward, propelled by a liquid-fueled
sustainer engine of 18,000-N thrust. Its fins
were preset to give a slight spin to provide aerodynamic
stability during flight. Rockets in the Aerobee family were
7.6 to 17.4 m long and carried payloads of 90 to 360 kg to
altitudes of 160 to 560 km. Between 1947 and 1985, hundreds
of Aerobees of different designs were launched,
mostly from the White Sands Missile Range, for both
military and civilian purposes.
On May 22, 1952, in one of the earliest American
physiological experiments on the road to manned spaceflight,
two Philippine monkeys, Patricia and Mike, were
enclosed in an Aerobee nose section at Holloman Air
Force Base, New Mexico. Patricia was placed in a sitting
position and Mike in a prone position to test the effects
on them of high acceleration. Reaching a speed of 3,200
km/hr and an altitude of 58 km, these monkeys were the
first primates to travel so high. Two white mice, Mildred
and Albert, also rode in the Aerobee nose, inside a slowly
rotating drum in which they could float during the
period of weightlessness. The section containing the animals
was recovered by parachute with the animals safe
and sound. Patricia died about two years later and Mike
in 1967, both of natural causes, at the National Zoological
Park in Washington, D.C.
aerobraking
The action of atmospheric drag in slowing down an object
that is approaching a planet or some other body with
an atmosphere. Also known as atmospheric breaking, it
can be deliberately used, where enough atmosphere
exists, to alter the orbit of a spacecraft or decrease a vehicle’s
velocity prior to landing. To do this, the spacecraft
in a high orbit makes a propulsive burn to an elliptical
orbit whose periapsis (lowest point) is inside the atmosphere.
Air drag at periapsis reduces the velocity so that
the apoapsis (highest point of the orbit) is lowered. One
or more passes through the atmosphere reduce the
apoapsis to the desired altitude, at which point a propulsive
burn is made at apoapsis. This raises the periapsis out
of the atmosphere and circularizes the orbit. Generally,
the flight-time in the atmosphere is kept to a minimum
so that the amount of heat generated and peak temperatures
are not too extreme. For high-speed aeromaneuvering
that involves large orbit changes, a heat-shield is
needed; however, small orbit changes can be achieved
without this, as demonstrated by the Magellan spacecraft
at Venus. In Magellan’s case, the aerobraking surfaces
were just the body of the spacecraft and its solar arrays.
Aerobraking and aerocapture are useful methods for
reducing the propulsive requirements of a mission and
thus the mass of propellant and tanks. This decrease in
propulsion system mass can more than offset the extra
mass of the aerobraking system.
aerocapture
A maneuver similar to aerobraking, but distinct in that it
is used to reduce the velocity of a spacecraft flying by a
planet so as to place the spacecraft in orbit around the
planet with a single atmospheric pass. Aerocapture is very
useful for planetary orbiters because it allows spacecraft to
be launched from Earth at high speed, resulting in a short
trip time, and then to be decelerated by aerodynamic drag
at the target. Without aerocapture, a large propulsion system
would be needed to bring about the same reduction
of velocity, thus reducing the amount of deliverable payload.
An aerocapture maneuver begins with a shallow approach
to the planet, followed by a descent to relatively
dense layers of the atmosphere. Once most of the needed
deceleration has been achieved, the spacecraft maneuvers
to leave the atmosphere. To allow for inaccuracy of the
entry conditions and for atmospheric uncertainties, the
vehicle needs to have its own guidance and control system,
as well as maneuvering capabilities. Most of the maneuvering
is done using the lift that the vehicle’s aerodynamic
shape provides. Upon exit, the heat-shield is jettisoned and
a short propellant burn is carried out to raise the periapsis
(lowest point of the orbit). The entire operation requires
the vehicle to operate autonomously while in the planet’s
atmosphere.
aerodynamics
The science of motion of objects relative to the air and
the forces acting on them. Related terms include: (1) aerodynamic
heating, which is heating produced by friction
when flying at high speed through an atmosphere, and
(2) aerodynamic vehicle, which is a vehicle, such as an airplane
or a glider, capable of flight when moving through
an atmosphere by generating aerodynamic forces.
aeroembolism
(1) The formation of bubbles of nitrogen in the blood
caused by a change from a relatively high atmospheric
pressure to a lower one. These bubbles may form obstructions,
known as emboli, in the circulatory system. (2) The
disease or condition caused by this process, characterized
by neuralgic pains, cramps, and swelling, which in extreme
cases can be fatal. Also known as decompression
sickness or the bends.