While the commander was Lending to the LM’s guidance needs, the LMP continued his checks with the activation of the communications system. Communications to the CSM were handled by two VHF antennae mounted fore and aft. Communica­tions to Earth, as well as the same ranging and tracking functions as found on the CSM, used either of two low-gain S-band antennae, also mounted fore and aft. For higher data rates, a steerable, high-gain dish was mounted high on the right-hand side of the ascent stage and it included systems to keep itself aimed towards Earth.

For the descent, the electrical power for the LM came from batteries mounted in the descent stage. Originally, the lunar module’s manufacturer, Grumman, had intended to power it with fuel cells, in a manner similar to the CSM and most of the Gemini spacecraft. Managerial and technical difficulties, mostly concerned with the interdependence of the power system with other systems in the already exotic LM, conspired with the race to get the spacecraft ready on time to force a switch to using batteries as the power source. Though heavy, batteries had the enormous advantage of simplicity, and since the LM was intended to be powered for only a few days, their weight penalty was no worse than the fuel cells. As part of the checkout of the LM, their health was closely studied, as were the extra set required for the ascent stage. The ascent stage carried a separate small set of batteries because it had to operate on its own for only a few hours for the trip from the surface of the Moon up to the CSM.

Another major item in the LMP’s checklist was the cooling system. Whereas the CSM used radiators and evaporators to lose heat from a water/glycol coolant, the LM relied on a sublimator to achieve the same task. These devices cooled by having ice directly sublimate to waiter vapour in a vacuum, in the process taking heat away from the coolant. The lunar spacesuits used the same cooling technique. Because the LM had no source of water available as a by-product of fuel cell operation, a large water tank was included in the descent stage, with a smaller supply in the ascent stage. The water/glycol coolant was pumped between the LM’s electronics and the sublimator by redundant sets of pumps. The pressures delivered by these pumps were checked before committing the LM to the surface.

Manoeuvring the LM was effected by a set of thrusters similar to the RCS jets mounted on the service module. Where each cluster of jets on the service module had independent propellant supplies, those on the lunar module had a common propellant system that, if the need arose, could be topped up from the propellant used by the ascent stage’s main rocket engine. As a further difference, the service module’s RCS quads w’ere mounted on the spacecraft axes, while those for the LM were set at the corners of a square around the spacecraft to keep the windows and hatchway clear. Before it could be used, the propellant system had to be pressurised. An explosively operated valve was opened to allow helium gas into the propellant tanks, at which point the crew and mission control could verify that the pressures within the system were as expected.

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