NASA SP-413 Space Settlements - Saint Ann's School

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NASA SP-413 — SPACE SETTLEMENTS — A Design Study
One method is to launch large payloads, of about 60 t, by
firing them from a large gas gun. The gun is operated by
using nuclear power to compress hydrogen gas and then
permitting the gas to expand the length of the launch tube.
Because hydrogen must be obtained from Earth, its
replacement is expensive, and consequently after each
launch the gas is recovered through perforations in the end
section of the launch tube which is encased in an enclosed
tube. Further details are given in appendix K.
The system is of interest because of its conceptual simplicity
and light weight. But the principal drawback of the gas gun
system is the difficulty of collecting the payloads once they
have been launched because their dispersion is large.
Collection needs a fleet of automated interceptor rockets.
The propellant requirement for interception is about 1
percent of the total mass launched. In terms of technology
that may be available in the near future, these interceptor
rockets have to use chemical propulsion with hydrogen as
fuel. The second drawback is that the gas gun requires the
development of sliding seals able to withstand high pressures
and yet move at high velocities and still maintain acceptable
leakage rates. Despite the uncertainties about precision of
aim, the difficulties of automated rendezvous and
interception, and the associated propulsion requirements,
the concept appears fundamentally feasible and worthy of
more study. However, the uncertainties are sufficient to
make another alternative more attractive at this time.
The alternative method, which is the one chosen for this
design, involves an electromagnetic mass accelerator. Small
payloads are accelerated in a special bucket containing super
conducting coil magnets. Buckets containing tens of
kilograms of compacted lunar material are magnetically
levitated and accelerated at 30 g by a linear, synchronous
electric motor. Each load is precisely directed by damping
the vibrations of the bucket with dashpot shock absorbers, by
passing the bucket along an accurately aligned section of the
track and by making magnetic corrections based on
measurements using a laser to track the bucket with great
precision during a final drift period. Alignment and
precision are the great problems of this design since in order
to make efficient collection possible, the final velocity must
be controlled to better than 10 -3 m/s. Moreover, the system
must launch from 1 to 5 buckets per second at a steady rate
over long periods of time, so the requirements for reliability
are great. This system is considerably more massive than
the gas gun. More details about it are given in the next
chapter.
The problem of catching the material launched by the
electromagnetic mass driver is also difficult. Three possible
ways to intercept and gather the stream of material were
devised. Two so-called passive catchers (described in more
detail in appendix L), involve stationary targets which
intercept and hold the incoming material. The other is an
active device which tracks the incoming material with radar
and moves to catch it. The momentum conveyed to the
catcher by the incident stream of matter is also balanced out
by ejecting a small fraction of the collected material in the
same direction as, but faster than, the oncoming stream.
An arrangement of catching nets tied to cables running
through motor-driven wheels permits rapid placement of the
catcher anywhere within a square kilometer. By using a
perimeter acquisition radar system, the active catcher tracks
and moves to intercept payloads over a considerably larger
area than the passive catchers. Unfortunately this concept,
described in more detail in the next chapter, has the defects
of great mechanical complexity. Nevertheless, although
many questions of detail remain unanswered and the design
problems appear substantial, the active catcher is chosen as
the principal means of collecting the material from the mass
launcher on the Moon.
Despite possible advantages it seems desirable not to place
the catcher at the site of the colony at L5. For three reasons
L2 is chosen as the point to which material is launched from
the Moon.
First, the stream of payloads present an obvious hazard to
navigation, posing the danger of damage if any of the
payloads strike a colony or a spacecraft. This danger is
particularly acute in view of the extensive spacecraft traffic
to be expected in the vicinity of the colony. The payloads,
like meteoroids, may well be difficult to detect. Hence, it
appears desirable to direct the stream of payloads to a target
located far from the colony.
Second, L2 is one-seventh the distance of L5, permitting use
of either a smaller catcher or a less-accurate mass-driver.
Third, to shoot to L5 requires that the mass-driver be on the
lunar farside. For launch to L2, the mass-driver must be on
the nearside. By contrast, a nearside location for the
mass-driver permits use of our knowledge of Moon rocks
brought back in Apollo flights, and there are a number of
smooth plains suitable for a mass-launcher. The nearside
also permits line-of-sight communications to Earth.
Catching lunar material at L2 means that transport must
then be provided to L5. It appears most practical to use
mechanical pellet ejectors powered by an onboard nuclear
system of 25 MW. This same system is used to offset the
momentum brought to the catcher by the payloads arriving
at up to 200 m/s.
THE TRANSPORT SYSTEM
The transportation requirements of a colony are much more
extensive than merely getting material cheaply from the
Moon to the factories of the colony. There must be a
capability for launching about 1 million tonnes from the
Earth over a total period of 6 to 10 years. There must be
vehicles capable of traversing the large distances from Earth
to L5 and to the Moon. There must be spacecraft that can
land equipment and people on the Moon and supply the
mining base there. Fortunately, this is a subject to which
NASA and the aerospace industry have given considerable
thought; the study group relied heavily on this work. A
schematic representation of the baseline transportation
system is shown in figure 4-13.
Chapter 4 — Choosing Among Alternatives