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

director of development operations at the Army Ballistic
Missile Agency, and then as deputy to von Braun at the
Marshall Space Flight Center. In 1970, he succeeded
von Braun as Marshall SFC’s director before retiring in
1973.
regenerative cooling
The cooling of part of an engine by the propellant. A
regeneratively cooled engine is one in which the fuel or
oxidizer, or both, is circulated around the combustion
chamber or nozzle.
regimes of flight
Ranges of speed defined relative to the local speed of
sound. They are subsonic, transonic, supersonic, and
hypersonic.
regressive burning
In solid-propellant rocket motors, the burning of the fuel
in such a way that the chamber pressure steadily decreases
throughout the burn time, thus delivering steadily
decreasing thrust.
relativistic effects
Several peculiar effects, predicted by Einstein’s special
theory of relativity, that come into play when objects—
such as spacecraft—travel at speeds that are a substantial
fraction of the speed of light. These effects include time
dilation, mass increase, and length contraction. Time
dilation refers to the slowing down of the passage of time
at very high speeds, effectively making it possible to
spend energy to buy time. A potentially important consequence
follows for interstellar flight: simply by traveling
fast enough, an astronaut can reach any destination
within a specified amount of (shipboard) time. The table
(“Shipboard Travel Times to Arcturus”) shows the travel
times, as measured by onboard clocks, to the star Arcturus,
which is located 36 light-years away, for a spacecraft
traveling at various speeds.
Shipboard Travel Times to Arcturus
Speed (fraction of
light-speed) 0.1 0.5 0.9 0.99 0.999
Travel time (years) 35.8 31.2 15.7 5.1 1.6
While time dilation enables, in principle, a spacecraft
to reach anywhere in the Galaxy (or the Universe) within
a human lifetime simply by traveling fast enough, there is
a serious downside. In exploiting this effect, astronauts
would age less than everyone else, including their friends
Relay 343
and family, who remained behind. For very long journeys
at high fractions of the speed of light, the time dislocation
would be so great that many generations, and even
millennia, might pass on the home planet before the
interstellar travelers returned. For example, an excursion
from Earth to Rigel—900 light-years away—and back, at a
constant 99.99% of light-speed, would take 1,800 years as
measured on Earth but only about 28 years as experienced
by those on the spacecraft.
The other major consequence for space travel at relativistic
speed is the increase in mass of a spacecraft. This
would make it more and more difficult to continue to
accelerate the vehicle. The factor that determines the
amount of mass increase and time dilation is called
gamma (γ). For an object moving with speed v relative to
an observer considered to be at rest (for example, on
Earth), γ is given by
γ=1 / �(1 �− v 2 /c 2 �)
where c is the speed of light.
The relativistic mass, m, of a body moving at velocity v
is then
m =γm0 = m0 / �(1 �− v 2 /c 2 �)
where m0 is the rest mass. Note that when v = 0, this
reduces to the non-relativistic result m = m0. The impossibility
of accelerating an object up to the speed of light is
shown by the fact that when v = c, m becomes infinite.
Similarly, the relativistic time dilation is given by:
t = t0 / γ=t0 �(1 �− v 2 /c 2 �)
Relay
Two spacecraft, designed and built by RCA, to test intercontinental
satellite communications technology. Relay
was a communications satellite of the active repeater
type, in which signals from one ground station were
picked up and rebroadcast to another station by the satellite’s
internal equipment. Relay 1 remained operational
for more than two years; Relay 2 was used in thousands
of tests and experiments and in some 40 public demonstrations
through September 1965. See Telstar. (See
table, “Relay Missions.”)
Launch
Vehicle: Delta B
Site: Cape Canaveral
Mass: 78 kg
Relay Missions
Spacecraft Launch Date Orbit
Relay 1 Dec. 13, 1962 1,319 × 7,440 km × 47.5°
Relay 2 Jan. 21, 1964 1,961 × 7,540 km × 46.4°