Rising speed

The velocity of a homew’ard-bound Apollo spacecraft was quite low’ during much of its coast. It dipped to a minimum of about 850 metres per second at the point where Earth’s gravity overcame that of the Moon. As the spacecraft continued to approach, the increase in its velocity was painfully slow until the final few’ hours of the mission w’hen Earth’s increasing pull ramped it up markedly. For example, on a typical mission it w’ould take over two days for the velocity to rise to half its highest value, yet it took less than two hours to make up the other half. This steep increase simply reflected the fact that the spacecraft w’as falling into a deep gravity well.

It w’as during the final hours of the flight that mission control had the last of their seven planned opportunities to carefully track the spacecraft, refine their knowledge of its trajectory and have the crew adjust the approach velocity for a perfect entry. On some flights, guidance was so good that this mid-course correction was not required, while on others only a very minor firing of the RCS thrusters was needed to correct for earlier unbalanced thrusting or the tiny thrust imparted w’hen gases and liquids w’ere vented from the spacecraft.

Diagram of the approach flight path towards and through Barth’s atmosphere. Definitions: entry interface and the 0.05-g event

To aid their calculation of the spacecraft’s entry trajectory, mission planners adopted a height of 400,000 feet or 121.92 kilometres, at which the returning command module was deemed to have left space and begun re-entry. This was entry interface. The Retro flight controller’s task was to shape their approach trajectory to ensure that when they reached this altitude, the flight path would form an angle to the horizontal of 6.5 degrees, with a leeway of about 1 degree to help to cope with weather or unfavourable trajectory conditions.

Entry interface, while being handy for the trajectory analysts, was an entirely arbitrary point that had little to do with the real atmosphere and its properties. It was therefore of little use in the conduct of the re-entry itself because it did not take into account the variations that the outer atmosphere would present to the spacecraft. A means of referring to the physical atmosphere was required to aid coordination and timing, something that meant the spacecraft had truly entered the atmosphere. NASA chose the moment when the tenuous gases of the upper atmosphere exerted a drag equivalent to 0.05 g. When the spacecraft’s acceler­ometers detected this level of deceleration, they signalled to the relevant instrumentation that re-entry was underway. Most aspects of the entry were measured with respect to this 0.05-g event. For the sake of calculation prior to entry, just as for entry interface, it was taken to occur at an altitude of 90.66 kilometres. When it was reached, two important things occurred: the computer began to fly the re-entry, and the entry monitor system (EMS; of which more later) began to monitor the progress of the flight path.