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

436 thrust chamber
At first glance, it might seem that a constant thrust would
lead to a constant acceleration, but this not the case.
Even when the propellant flow rate and exhaust velocity
are constant, so that the thrust is constant, a rocket will
accelerate at an increasing rate because the rocket’s overall
mass decreases as propellant is used up. The total
change in velocity of a rocket due to a specific thrust, acting
in a straight line, is given by an important formula
known as the rocket equation. In some situations, as of a
rocket rising from Earth’s surface, a large thrust acting
over a relatively short period is essential. But in other situations,
as of a probe on a deep space mission, the key
factor is not so much the amount of thrust, which determines
only the acceleration, but the final velocity. A high
final velocity can be achieved by a propulsion system
that produces a low thrust but expels material over long
periods at a high velocity—for example, an ion engine.
Equation (1) holds true only when the pressure of the
outgoing exhaust exactly equals the ambient (outside)
pressure. If this is not the case, then an extra term comes
into play, and the thrust is given by:
F = m pv e + ( p e − pa )A e
(2)
where pe is the exhaust pressure, pa the ambient pressure,
and Ae the area of the exit of the rocket nozzle. The first
term in this equation is called the momentum thrust and
the second the pressure thrust.
thrust chamber
The heart of all liquid propellant rocket engines. In its
simplest form, the thrust chamber accepts propellant
from the injector, burns it in the combustion chamber,
accelerates the gaseous combustion products, and ejects
them from the chamber to provide thrust.
thrust commit
The time, when all engines of a launch vehicle on the
launch pad have been running for a designated period of
time (typically about three seconds) and all other parameters
are normal, that is the start of the final launch
sequence.
thrust decay
The progressive decline of propulsive thrust, over some
fraction of a second, after a rocket motor burns out or is
cut off.
thrust equalizer
A safety device that prevents motion of a spacecraft if its
solid-fuel rocket motor ignites accidentally. The device is
usually a vent at the top of the thrust chamber that is left
open until launch time. If the fuel ignites accidentally
before launch, the gases of combustion will blow out
from both the top and bottom of the motor, thus equal-
izing thrust on both sides and preventing the spacecraft
from launching prematurely.
thrust generator
A device that produces motive power. In an electric
propulsion system, for example, it is composed of an
electric power source and a device that expels a high
velocity flow of the propellant.
thrust misalignment
Thrust directed accidentally in an undesired direction.
Thrust misalignment can have serious consequences,
especially during the initial stages of a spacecraft’s ascent
into orbit.
thrust vector control
Controlling the flight of a launch vehicle or spacecraft by
controlling the direction of thrust.
thruster
A small rocket used by a spacecraft to control or change
its attitude.
thrust-to-Earth-weight ratio
A quantity used to evaluate engine performance,
obtained by dividing the thrust developed by the vehicle
by the engine dry weight.
tidal force
A force that comes about because of the differences in
gravitational pull on an object due to a large mass around
which the object is moving. In the case of a space station
in Earth orbit, parts of the station that are further away
from the Earth are pulled less strongly, so that the centrifugal
force of the orbit is not quite balanced by gravity,
and there is a net upward tidal force. Similarly, for
parts closer to the Earth, there is a downward tidal force.
These opposing forces try to stretch the station along a
line that passes through Earth’s center. One effect is that
tidal forces will make any elongated object tend toward
an orbit with its long axis pointing to the Earth’s center.
Either the space station has to be designed to orbit in this
way, or it must have an orientation correction system to
counter the orientation drift that the tidal forces will produce.
Another effect will be on objects within a space
station. Tidal forces are one of the reasons it is impossible
to have perfectly zero-gravity conditions in orbit.
The fact that microgravity always exists has important
consequences for some experiments and manufacturing
processes in space.
Dangerous tidal effects would be most evident near
highly condensed objects, such as black holes. Tidal forces
are proportional to d/R 3 , where d is the density of the