Lunar Electrothermal Oxygen Rocket
An electrothermal oxygen rocket may be the first means by which product is exported from the moon for a profit. This is not intended to pay off the investment necessary to bring the lunar industrial base to the state necessary for this device, but early returns will give confidence that more substantial returns will be achieved later.
A conjectural example lunar electrothermal oxygen rocket flies parallel to the ground between two calcium filled steel tube electrically conductive rails. The rails are held 10 meters above the average ground level and extend forty miles (65 km) down range, straight, level, and parallel to each other. Beyond forty miles two steel rails without calcium fill extend in the same line to a point 60 miles (96 km) down range.
The rocket, made on Earth, has an electrically driven oxygen pump that forces oxygen at high pressure through the engine cooling system, a resistive heater, an arc gap and an expansion nozzle. The exhaust velocity should be 2000 meters per second. The rocket has 40 kg upper stage, 40 kg empty weight, and 120 kg oxygen in the propellant tank for a fueled loaded weight of 200 kg. The mission delta v is 1832 meters per second of which 229 meters per second is reserved for gravity loss and 1603 meters per second is stage separation velocity. Average downrange acceleration is 20.1 meters per second per second (2.05 g) for 79.9 seconds with stage separation at about forty miles (64) km downrange at which point the first stage begins to brake to a stop by friction, to be reused. Braking friction is provided by locally produced ceramic pads pressed against steel rails. Electric power for the rocket comes from the two rails that it flies between. The power required is 340,000 kilowatts for 80 seconds. The upper stage is a mini ion thrusting spacecraft made on Earth. The cargo in the upper stage is a canister of helium 3. Helium is not yet used for fusion power, but a small amount is used for research at about $4000 per gram. It is conceivable that at this price it would be worth exporting.
Since the steel rails must be straight and level to a close tolerance, they must be protected from thermal expansion stresses that result from the day/night temperature change of the surface of Luna. Fiberglass shades like tents would shade the entire length of the rocket launching rails. This would reduce the requirement for expansion joints in the rails.
Power could be supplied by about a dozen steel flywheels about 6.6 meters in diameter and 30 cm thick. These could be ganged 6 to each vertical shaft of two electric motor/generators and ride on gas bearings. Any other means of providing 27 gigajoules in 80 seconds would do. The rocket may use mainly microwave to heat the oxygen plasma with the power provided by spark gap conduction from steel rails along the flight path. To keep the heating chamber from oxidizing in the hot oxygen environment, the oxidation resistant structural metal of the heating chamber could be separated from the hot plasma by a layer of refractory ceramic scales in a fish-scale pattern. These could be supported by stems anchored in the structural metal but leaving a less hot layer of oxygen between the refractory scales and the cooled metal wall. At the end of the line a robot truck would pick up the first stage and haul it back for maintenance and refueling.
If there gets to be a massive enough industrial infrastructure on Luna that can cease operation for a couple minutes at a time so the electrical power is available to launch a rocket, the electric launch concept could be scaled up to launch a small manned craft.
