THE LUNAR GAS GUN MASS DRIVER
The importance of obtaining the mass of material from the Moon has been emphasized earlier. Overall probability of success of the entire system is substantially improved by having an alternative to the transport linear accelerator (TLA) and the mass catcher that has been selected as the primary or baseline system for transporting 10 million tonnes of lunar material to L5 over a period of 10 yr.
The lunar gas gun is a fundamentally different concept to that of the TLA. Where the TLA launches small payloads with very high repetition rate onto a precisely determined trajectory, and has the small individual loads caught in a localized active net or passive catcher, the gas gun launches large payloads with a much lower repetition rate onto a less precisely determined trajectory and has these large payloads collected by remote controlled interceptor rocket engines.
The gas gun has four primary elements in its system:
The equations that govern the mass of the launching barrel are:
These equations together yield the following approximate expression for the barrel mass which depends only on the properties of the barrel material, the muzzle velocity, and the mass of the projectile:
For hydrogen at 200 degrees C, when the thermodynamic properties of motion near the speed of sound are taken into account, this expression becomes:
For lunar escape velocity of 2370 m/s, and for a boron or graphite filament epoxy such as PRD-49 of density 1.38 X l0^3 kg/m^3 and an allowable working stress of 1650 MPa, the mass ratio of barrel to projectile is 24.5.
Given a desired mass flow rate of 106 t/yr the projectile mass varies inversely as the launch repetition rate for a single barrel. Since large payloads are desired at low repetition rates, the largest projectile that can be handled on the lunar surface represents the best solution to this element of the system taken alone. Since a 10-t shell is a reasonable size for a shell on the Earth's surface, a 57-t projectile is assumed to be manageable on the lunar surface. With this value the desired mass flow rate can be achieved with a repetition rate of 2 launches per hour. A cylindrical projectile of this mass sintered from lunar material and having a density presumed to be 2.5 can be about 2 m in diameter and 4 m long. The mass of the barrel for a projectile of this size is about 1400 t.
The compressed gas can be stored in a deep sublunar hole which can have a diameter of approximately 30 m and can, if necessary, be lined with a heavyweight plastic film. The mass of the film is not expected to exceed 5 t.
The average power required to launch 1 million tonnes at lunar escape velocity over the course of a year is 89 MW. The energy for this comes from a nuclear turbine and gas compressor. For a nuclear electrical power generator the study assumes a mass-to-power ratio of 45 Mt/MW (exclusive of shielding). Although an electrical generator is inherently more efficient than a gas compressor, perhaps by a factor of 2, the compressor has a much lower ratio of mass to power throughput, a factor of from 5 to 10. The combined effect on the overall energy source is assumed to reduce the mass to power ratio to 27 Mt/MW. Thus the mass of the nuclear compressor may be taken as approximately 2400 t.
Remote Controlled Interceptor Rockets
The large blocks of lunar material arrive in the vicinity of L5 at an average rate of one every 1/2 hr. With an anticipated launch velocity error of +/-0.5 m/s, the radius of the scatter circle is approximately 1000 km. If 50 interceptor rockets are used to collect the lunar material the rotation time to intercept and dock is 24 hr. If this maneuver is carried out with as small velocity change as possible, the average power required to collect the lunar material is 1.2 kW, or 37.8 J/kg of material.
A more accurate study of this part of the problem is desirable. Perhaps the interceptors could rendezvous and correct the trajectory at a point closer to the Moon. It might also be possible to schedule the timing of launches and the direction of trajectories to take advantage of the relative velocities of the Moon and the L5 processing location which is in a large orbit around the L5 point.
Lunar Gas Gun Summary
Many aspects of this system need to be studied further. For example, the thermodynamic efficiency should be determined with greater accuracy. It is now merely assumed to be nearly 1 percent because the temperature of the gas never differs from the ambient temperature by more than about 20 degrees C and then only for a short time. The process is, therefore, considered to be a quasi-isentropic, adiabatic, expansion/compression. No losses at the valves are taken into account. The valves can be seen in the schematic of the launcher, figure 4-27, which indicates that the launching gas is not allowed to escape.
Some indication is also given in the figure that fine velocity control might be developed to reduce the scatter circle at L5.
For comparative purposes, the component masses of the lunar gas gun are compared in table 4-16 with the corresponding masses of the transport linear accelerator.
TABLE 4-16 (gif format)
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