INTERLUNAR CYCLING STATION This station is made from 16 modified external tanks, 8 on each end.  Together with weightless observation chambers and floating rooms in the hub section, there are about 80 cubic meters of volume for each of the 400 passengers.  Station rotates to produce the equivalent of lunar gravity for passenger comfort and ease.  Silicon solar panels generate about 1.7 MWe and are mounted on rotating motorized platform for constant solar orientation.   Hub section contains LSS, storage, fuel cells, heavy gyros, water tanks, etc.

illustration by D.Martin based on design by D.Dietzler

Cycling stations will travel along their elliptical orbits in frictionless space forever with only small rocket thruster firings to make course corrections to counteract the effects of gravitational perturbation.  To reach the large cycling stations, taxis are necessary.  A taxi like the one below, derived from a Space Shuttle external tank, could haul 400 passengers on a one or two hour trip to rendezvous with an inter-lunar cycling station.  Taxis carry about ten tons of liquid oxygen for breathing; enough for 48 hours.  Taxis, as well as cyclers and taxiports, use GPS receivers in addition to inertial guidance for precise navigation.  There are also lunar GPS satellites in lunar orbit, of course, and constellations of satellites at very high altitudes like 100,000 miles and 300,000 miles for accurate navigation in Earth-Moon space.

This vehicle uses pressure fed monopropellant rocket motors; ISP 250-285 seconds.  The nozzles are very highly expanded for efficiency in the vacuum of outer space.  The  fuel consists of silicon and aluminum  dust mixed with liquid oxygen.  Power comes from solar panels and rechargable batteries in addition to fuel cells.  Auxillary power units use compressed oxygen to drive turbo-generators and hydraulics pumps. Thrusters use silane and oxygen.  Steering would require gimbals or thrust modulated  steering motors, would it not?  I opt for thrust modulated steering.

Painting by Dennis Martin  Space Taxi made from Shuttle external tank orbiting the Moon.  Note the highly expanded vacuum nozzles with black carbon-carbon external coating of  high emmissivity to radiate heat. Nozzles are lined with magnesia or titania.

A specific impulse or ISP of 250 to 285 seconds is hoped for.  The motor will use monopropellant stored in  aluminum tanks.  The nozzle will be of titanium lined with magnesia, a ceramic we can make plenty of on the Moon, which is important because nozzle linings will erode and replacement will required every so many burns.  The nozzle and perhaps the motor casing will be coated with carbon-carbon because it has high thermal emmissivity, like all dark substances, and will radiate heat thereby keeping things from overheating and melting.  Earlier, I was worried about flash-back with monopropellant and envisioned a hybrid motor with LOX and sintered aluminum beads; however, I have abandoned this in favor of monopropellant made from a suspension of ultrasonically mixed Si and Al powder and LUNOX.  This decision was made partly because of the statements made by Dr. Bruce Dunn of Dunn Engineering: Both solid propellant rockets and liquid propellant rockets require the development of large pressure vessels (the tanks for the liquid propellants, and the whole rocket for solid propellants).  The engineering for the liquid containing pressure vessel is considerably simpler.  The liquid tanks are under regulated pressure, while the casing of the solid rocket is hammered by pressure spikes from combustion instability.  The liquid tanks have the propellant in benign contact with the load carrying structure, while solid rockets require insulation to keep hot gases from burning through the walls.  Liquid tanks are in one piece, while in large sizes solids have jointed pressure vessels. Having filled a liquid propellant tank, there is nothing much to inspect.  Having filled a solid propellant rocket, one worries about voids and cracks in the propellant, batch to batch variation in the mechanical and burning properties of the propellant, the internal bonding of the propellant to insulation material and inhibitor coatings, the effect of the ambient temperature on propellant mechanical and burning properties, the age and prior storage conditions of the propellant grain etc. Also, it will be much easier to store monopropellant at taxiports and fuel up taxis than it would be to manufacture segmented solid casings loaded with solid fuel in the form of sintered aluminum beads that have to be removed, broken down and replaced after each flight to "refuel" the taxi. The taxiport does not have to do all this fabrication of hybrid-solid portions of the rocket and work simply to refuel the rocket. With monopropellant it is just a matter of pumping in the "rocket fuel" and the taxi is ready to go again.  It will only need major overhaul when the ceramic lining of the motor, throat and nozzle needs replacement.

Taxi steering will be accomplished with thrust modulation of small motors (not pictured above) and of the main engines. The lateral cylindrical tanks are 3.25m x 35m and have 290 cubic meters each for a total of 882 tons of monopropellant with a density of 1.52.  The 8.3m spherical tank has 300 cubic meters and holds 455 tons of "fuel."  The total is 1337 tons "fuel" which is close to estimate made in TAXI PROPELLANT 1.  The ET hull amasses 25 tons, the polyethylene rad shield 90 tons and the passengers 35 tons with 100 tons left for everything else; a total of 250 tons.  Passengers will not be on taxis very long so taxis will not carry much water, so polyethylene is used for rad shield. It is actually superior to water, besides it won't ever leak.  About 2 gr/cm2 Al hull and 8gr/cm2 polyethylene and spacesuits will protect passengers from solar flares when beyond the magnetosphere around L2.