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Passive Thermal Control (PTC) of Spacecraft Heating

Click here to see a movie comparing a rotisserie grill to the Apollo spaceships. (Morphed photo is by Jerry Woodfill from original Wayne T. Mitchell photograph.)


DISCUSSION

Without the barbeque rotisserie passive thermal control system on an Earth-Moon journey, one side of the ship's systems would cook while the other side's systems froze, not a good thing. In space there are no clouds or atmosphere to filter the Sun's rays. This makes the heat from the Sun very harmful to exposed surfaces in the vacuum of space.

Often cartoons, science fiction movies, and comic books about space exploration ignore a very important technique used to heat and cool spaceships. NASA uses a system called Passive Thermal Control or PTC to evenly heat critical space hardware. The principle used is exactly how a barbeque grill uses a rotisserie to evenly cook a pair of chickens.

If a spaceship rocketed to the Moon with the Sun always on the same side of the craft, the ship would suffer. It would be like a barbeque grill's rotisserie motor jamming. One side of the meat would be burned with the other side remaining raw. In either case, the food would not be edible. Having a pair of spacecraft or chickens in tandem rotating about a horizontal center axis provides even exposure to the Sun or a grill's burners.

The Apollo Lunar Module and Command and Service Modules had a PTC of one revolution per minute. The vehicles rotated about a common centerline. An automatic computer program set up the PTC operation when the astronaut entered the code for the maneuver on the computer's keyboard. The guidance system and thrusters served to keep the PTC motion constant as the journey to the Moon progressed.

Examining this barbeque technique demonstrates engineering principles used on Apollo. A list of suggestions from a barbeque manual speaks to space science:

1. In using a rotisserie, security and balance are very important. Position the meat on the middle of the rotisserie skewer. Next fasten the meat as firmly as possible. If you are cooking chickens, remember to tie the wings and legs in as tightly as possible. If parts are loose to flop around, as the rotisserie turns, you can get burning of parts of the birds. Also, an off balance condition might arise. Some rotisserie units have a counter balance device to assist the chef in getting it right.

When Apollo 13 exploded, Commander Jim Lovell had to restore PTC control to the assemblage. The initial imbalance of the explosion caused the vehicles to sway and wrobble like a pair of barbeque chickens whose wings and legs were not secured. The automatic system was struggling to bring the movement back to stability. Taking over manual control failed to fix things. Lovell went to the spacecraft viewing window and saw something venting. The oxygen leaking from the ruptured cryogenic system was producing an unexpected thrust. Compensating for this thrust made PTC very difficult.

2. As for cooking times, (the barbeque book suggests) a thermometer is best. Though charts can be fine, differences in wind, air temperature, equipment, can make them fail to work. The thermometer gives the best indication that the food is cooked. Guessing just won't work.

Just as the cooking manual suggests using a thermometer to assure rotisserie performance, the Apollo astronauts had thermal measuring devices to assure the PTC mode was working properly. Yes, there were charts and rules of thumb, but the meters, instruments, sensors, and telemetered data seen by mission control worked best.

Animated PTC Picture

Click here (1639 K .avi movie) to see how cooking chickens on a rotisserie compares to cooling the Apollo spaceships in route to the Moon.


Last modified:Friday, 14-Jul-00- 09:00:00 CDT


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