The CSM was a streamlined and sturdy craft, designed to ascend through the atmosphere of Earth, and, in the case of the command module, withstand a punishing re-entry. In complete contrast, the lunar lander was a true spacecraft because it was entirely incapable of flight in an atmosphere.
Known as the lunar module (LM), its construction was entrusted to the Grumman Aircraft Engineering Corporation. This was a truly exotic ship in which every aspect of its major systems pushed the know-how of the engineers who designed it. When originally conceived, it was called the lunar excursion module and therefore received the acronym LEM. However, managers decided in 1965 that the use of excursion was too flippant as it suggested that the crews were going on a vacation. The name was shortened to lunar module but the pronunciation as 'lem' stuck.
The LM needed to be sturdy enough to withstand the acceleration and vibration of a launch from Earth and the shock from a rough landing on the lunar surface. It also had to be as light as could humanly be achieved in order not to outweigh the ability of both the Saturn V and the CSM to deliver it to lunar orbit. Its largest engine had to be throttleable to provide adequate control of the astronauts' descent to the surface of another world without the aid of wings or runways. Its flight path was controlled by two small computers in an age when entire floors of buildings were given over to such machines. Its propellant systems operated at extreme pressures, yet its engines had to be completely reliable.
Prior to Apollo, no one had dealt with the realities of designing a lunar module which meant that Grumman could start with a clean sheet, but even before they won the LM contract, their engineers had produced preliminary designs. They then worked through a number of iterations before settling on the final design. The idea of the LM being a two-part craft was adopted immediately as an essential requirement of the LOR concept. It operated as one vehicle until the moment of departure from the lunar surface. The form and layout of less fundamental aspects of the LM, like the number of legs and the seating arrangements, required some extra thought. Three legs would have been the lightest arrangement and most adaptable to an undulating terrain, but a failure of any leg would be bad news. Five legs provided excellent stability and safety but the layout conflicted with the arrangement of the tanks for the propellant, and would have necessitated more structure and more mass. Four legs proved to be a suitable compromise. The lower or descent stage was a cross-frame carrying an engine in its centre surrounded by four propel-lant tanks. At each end of the cross-frame, a landing leg was mounted, one of which included a ladder. The bays of the frame between the landing gear were used as stores for the equipment the crews would need when their roles changed from that of spacecraft pilots to lunar explorers, and, on later flights, would provide somewhere to carry a fold-up electric car.
The upper stage of the LM was the crew quarters. Since it would lift the crew off the Moon, it was known as the ascent stage. A pair of propellant tanks protruded like cheeks on either side of a horizontally mounted cylindrical pressure hull, and a small rocket engine was set in the centre of the stage. Early designs for the cockpit included seats and large, high-visibility windows, as in a helicopter. In spacecraft design, there is a tendency for the mass of a spacecraft to rise as engineers go from initial concepts and estimates to final hardware. The Apollo LM could not afford such increases and the constant pressure to minimise the spacecraft's mass continued up to and beyond its first successful mission. Engineers conceived the innovative idea of removing the seats because they realised that a crewman's legs would make excellent shock absorbers for the low g-forces encountered during descent. Also, in the low gravity of the Moon, standing would be effortless. This change had a profound effect on the layout of the ascent stage. Had the crew been seated, their heads would have been placed well away from the windows, entailing huge areas of heavy glass to give an adequate field of view. A better solution was to have the two crewmen stand close to the front wall of the spacecraft where they could look out of two small downward-tilted triangular windows from which they could see an approaching landing site and steer towards it. This arrangement saved a large amount of mass. Major electronics systems were placed to the rear to balance the crew, quad packages of thrusters were placed at each corner for attitude control, and a collection of antennae were mounted on the roof, where function dictated. The result was a remarkable manned spacecraft that was perhaps aesthetically ugly, yet perfectly designed for the function it had to perform.
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