PTC: SPACECRAFT ON A SPIT

Space is a strange place for those of us who are used to the warmth of Earth. Here on our planet, the air. the oceans and the land absorb the heat from the Sun and give it up at night, thereby moderating temperatures. We know instinctively the importance of air in the transportation of heat, whether it is between the sea and land, within the rooms of our houses or inside the equipment we possess. In space, things are very different.

Imagine placing an object in cislunar space, not too near Earth, sitting motionless. The side facing the Sun will become warm. How much depends on its characteristics but as it gradually warms, it also radiates heat. The warmer it gets, the more heat it radiates until it reaches a point where it radiates as much heat as it receives. At this point, it is at thermal equilibrium and its surface temperature, probably quite high, is constant. The side of the object opposite the Sun will also radiate whatever heat it had. but this will not be replenished. The surface temperature will gradually fall until the minimal sources of heat available to it become comparable to the heat it is losing. Given time, and assuming that little heat leaks through the object from the sunward side, this area will become extremely cold. These extremes of temperature easily coexist in an environment where there is no air to transport heat.

In the Apollo spacecraft, there were various reasons why it was undesirable to allow these temperature extremes to exist for long. For example, tests had shown that the heaishicld material around the command module would crack and flake if it were allowed to become too cold, while the tanks for the RCS thrusters had to be kept at moderate temperatures at all times to prevent freezing or overpressurisation. The simple solution was to rotate the spacecraft gently around its long axis, side-on to the Sun. This technique was formally known as passive thermal control (PTC) but for many commentators, a far more descriptive term was the ‘barbecue’ mode.

Apollo 8 was the first to try to set up a PTC roll. Mission control gave Frank Borman an initial altitude that would place the spacecraft side-on to the Sun whilst avoiding gimbal lock and maintaining good communications. Once aligned, he began a constant, slow roll about the spacecraft’s longitudinal axis of only 0.1 degree per second which Look an hour to make a full rotation. However, physics abhors such a rotation, at least in the long term, and especially when large quantities of fluid are involved. With Lime, the rotation axis itself began to rotate so that the long axis sw’ept out a cone with an ever-increasing angle – a motion appropriately known as coning.

It was soon found that this simple method of initiating and maintaining PTC would not be appropriate for later missions. The addition of a lunar module would lengthen the stack further and make the simple roll manoeuvre even more difficult to maintain. Instead, use was made of the tracking programs in the command module’s computer to carefully control the overall attitude as the rotation progressed. Another change for later missions was to generate a reference orientation for the platform, a REFSMMAT, which was particularly suited to the manoeuvre.