The flow of this steam could be regulated by throttling the admission of its three constituents. It turned a turbine which provided power for the caterpillar.
The steam was condensed in a low-pressure condenser, cooled by a blower, after passing through the turbine. The carbon dioxide and carbon monoxide remained in the vapor phase and were drawn off and expelled by a second and smaller blower, while the water in liquid phase was recirculated from the condenser to the combustion chamber. The water loss of the system, therefore, was limited to the portions affected by dissociation of the peroxide and the combustion of the oil, by being ejected into the atmosphere as withdrawn from circulation.
The efficiency of this system was quite high in view of the necessity for providing artificially the oxygen which an ordinary engine would extract from the atmosphere, and considering that this oxygen formed part of the propellants and so tankage had to be provided for it. In order to conserve supplies, the cruising speed of the land vehicles was restricted to 13 mph.
The tracks of the caterpillars extended across their whole lower surface and their twelve foot length in order to diminish their surface loading and give maximum traction on the softest ground. In the weak Martian gravity, the vehicles weighted but 28% of their terrestrial tonnage. This reduced the danger of becoming mired, but it also diminished the traction to the same extent. Thus the maximum obtainable length tended to prevent non-propulsive “churning” of the tracks.
Directional control was obtained by braking one or the other of the tracks, their low ground pressures permitting this despite their great width.
The power plant was located between the tracks, and above it was an elongated cylindrical body which could be pressurized and which provided space for passengers and lading. Forward it had two large, oval windows through which the driver might view the ground, and several circular ports along the sides for the passengers. Two hemispherical, plastic gun turrets stood above the forward and after ends of the cylinder. Just behind it, mounted on the framework of the strange vehicle, was a small crane such as is used on wrecking tow cars. From the crane’s jib to the forward end of the cylinder ran the radio antenna which was to aid communication via the radio bombs.
When all three caterpillars had been lowered and given a short test run to insure that their power plants were working properly, Holt deployed them around the helpless landing boat in the untoward event that the hitherto invisible Martians might undertake some hostile action. But nothing happened.
After the removal of the three huge caterpillars, unloading began in earnest. Three folding trailers were dropped through the hatch and assembled on the snow beneath the belly of the boat. The first one completed, standing upon its wide wheels, was placed beneath the hatch while busy hands under the expert direction of Clark Winslow piled it with a vast assortment of cases and equipment. As each was hauled away with its load, the reserve fuel tanks to supplement the tankage of the caterpillars were filled by gravity hoses from wing tanks of the landing boat. The long voyage ahead precluded the tractor-caterpillars from carrying adequate fuel supplies in their own tanks.
For twelve hours the landing party bent its united energies to the accomplishment of the seemingly endless task, but when the work was done, no restful night came to induce sleep in their wearied limbs. It was Summer at the Martian south pole and the midnight Sun remained visible in undiminished splendor. It made but a sweep at the horizon, returning in a great circle in the sky to a point due south.
All hands were much relieved when Holt ordered the hatch closed on their boat and the air valves opened to bring up the pressure. Wearily they had trooped through the door and divested themselves of their space suits to seat themselves around a table hastily constructed from various bits and pieces of the stowage gear. One of the soldiers proved himself to be no mean cook, and it was a novel experience for them all to eat and drink in the old familiar fashion from open plates and glasses. That night, when the shades were drawn over the landing boat’s ports to keep out the brilliant glare reflected from the snow, the men retired to their acceleration couches, somehow grateful not to be floating in space, despite the sometimes painful pressure which even the light Martian gravity inflicted upon them.