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

nozzle The Space Shuttle main engine test firing in 1981. NASA
tion at the exit, the higher the speed and the lower the
pressure that the gas can achieve at the exit. For optimum
thrust, the gas pressure at the nozzle exit should be
exactly equal to the outside air pressure. In the vacuum of
space the outside pressure is zero, so it is impossible for
this optimum to be achieved. The bigger the exit area of
the nozzle, the closer the thrust will be to optimum; however,
there is a point at which gains in thrust are offset by
the extra mass needed to make the nozzle wider. Even
during the atmospheric portion of a rocket’s flight it is
impossible to achieve theoretical optimum performance,
because the outside air pressure changes as the vehicle
climbs. All designers can do is target the performance of a
nozzle to some average outside pressure.
When a nozzle ends before the gas reaches the pressure
of the outside air, it is called an under-expanded nozzle.
In the under-expanded case, the rocket design is not getting
all the thrust that it can from the engine. When a
nozzle is too large and keeps trying to expand the gas
flow, at some point the rocket plume will separate from
the wall inside the nozzle. This is called an over-expanded
nozzle. The performance from an over-expanded nozzle
is worse than in the under-expanded case, because the
nozzle’s large exit area results in extra drag.
nuclear power for spacecraft 299
Like the combustion chamber, the nozzle throat gets
very hot. Nozzle throats are often cooled by circulating
fuel directly behind the pressure wall to cool it. The fuel
passes through while it is still cool on its way to the combustion
chamber. This has the added bonus of preheating
the fuel before combustion, making more energy available
from the combustion to provide thrust. Recently,
new materials have become available, including ceramics
and composites, that can withstand extremely high temperatures.
These materials often slowly ablate under the
extreme conditions in the nozzle throat of a highperformance
rocket; however, the ablation rates may be
tolerable in a rocket that is only fired once. In the case of
a rocket designed for multiple firings, either in the same
mission or in multiple missions, cooling rather than ablation
is likely to be the method of choice.
nozzle area ratio
The ratio of a nozzle’s throat area to the exit area. The ideal
nozzle area ratio allows the gases of combustion to exit the
nozzle at the same pressure as the ambient pressure.
nozzle efficiency
The efficiency with which the nozzle converts the potential
energy of a burned fuel into kinetic energy for thrust.
nozzle exit angle
The angle of divergence of a nozzle.
nuclear detection satellites
Spacecraft designed to detect nuclear explosions on the
ground or in the atmosphere. Although primarily intended
to monitor nuclear treaty compliance, these satellites have
also been used to detect and observe galactic events such as
supernovae. See Vela and Advanced Vela.
nuclear fission
See fission, nuclear.
nuclear fuel
Fissionable material of reasonably long life, used or
usable in producing energy in a nuclear reactor.
nuclear fusion
See fusion, nuclear.
nuclear power for spacecraft
Nuclear power has essentially three applications to spaceflight:
to provide a source of heat to keep equipment
warm (see radioisotope heater unit [RHU]), to provide
a source of electricity to power equipment (see radioisotope
thermoelectric generator [RTG]), or to provide
a means of propulsion either directly (see nuclear