THE ROUTE TO THE MOON Translunar injection

Flying to the Moon when you don’t have a lot of propellant to hose around is like a stone throw – a ballistic lob across 400,000 kilometres of space between two worlds. The impulse for this ‘throw’ came from the S-IVB stage of the Saturn V which added an additional three kilometres per second to their speed with a burn that was nearly six minutes long. As this translunar injection burn progressed, it modified the spacecraft’s circular orbit into an increasingly long, stretched elliptical orbit whose apogee reached further and further into space. By the time the S-I VB had shut down, it had set the Apollo spacecraft on an elongated orbit around Earth that had a perigee of only 170 kilometres, but whose apogee would have taken it to an altitude of over half a million kilometres except for the intervention of the Moon!

The precise details of Apollo’s throw to the Moon, its duration, direction and timing, depended on a collection of constraints. These were often contradictory, but they narrowed the possible options for the S-IVB‘s burn to a unique but very useful trajectory. One constraint was propellant, which was a very expensive commodity by virtue of the fact that it had to be lifted off Earth’s surface. Consequently, planners tended to prefer trajectories that did not demand long burns of rocket engines. As a result, flying to the Moon was not going to be a quick affair. In modern Limes, means have been found of reaching the Moon that require very little propellant, but these result in complex trajectories that can take weeks or months to complete. The consumables carried by the Apollo spacecraft would never last long enough, and the physical endurance of the crew in the confined space would be sorely tested. For a manned mission, there comes a point where the advantages of reduced propellant requirements become more than matched by the increases in food, power and radiation shielding required by the crew.

Another constraint was the landing site. When Apollo reached the Moon and inserted itself into lunar orbit, that orbit had to pass over the landing site. Therefore, to save propellant, their Moonward trajectory had to be shaped to reach the best position near the Moon from which to achieve this orbit, and do so travelling in the same direction as the orbit. Planners also had to consider the lighting conditions at the time of landing and the thermal conditions on the Moon’s surface. By landing in the lunar morning, crew’s could benefit both from the low-angle sunlight which made the surface topography clear, and from the benign thermal environment that exists between the chill of the night and the heat of the day.

The choice of burn was further constrained by crew’ safety considerations. This was paramount in the minds of planners in view of Kennedy’s stipulation that a crew be returned safely to Earth. Any option that maximised NASA’s ability to bring an endangered crew home was eagerly adopted. The engineering mantra was: if there is a problem with which you cannot deal directly, then do as little as possible lest you make things w’orse. What the planners really wanted w’as an option that would still allow the crew’s safe return even if the spacecraft’s main propulsion system had completely failed. Fortuitously, a suitable option existed – the frec-return trajectory.