Rocket Science for Earthlings
a continuing series for the gravitationally impaired. Rocket Science for Earthlings 7

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chapter the seventh


HEAT. This is the second half of my discussion on reentry. The heat generated by a vehicle reentering the Earth's atmosphere is equal to the reduction of the vehicle's kinetic energy produced by the drag. If you know how fast a vehicle is slowing down, and what it's mass is and it's frontal area are, you can approximately calculate how much heat is developed. The air close to any vehicle passing rapidly through the atmosphere is heated due to friction and compression. In the upper atmosphere, this heating is due to the molecules of the air impacting directly on the vehicle's surface. This heating occurs at the surface of the vehicle. Half of the heat energy is radiated away and half is absorbed by the vehicle.

In the lower atmosphere, a boundary layer of slow moving air forms just above the surface of the vehicle. The heating now occurs just above the surface of the vehicle in the region where the boundary layer, the air right next to the vehicle skin that is moving slowly, interacts with the high velocity air stream. However, because the point of heating is in the boundary layer not on the vehicle's surface, and the air is heated to an opaque luminous plasma, a very large portion of the heat is radiated away before it reaches the vehicle. The absorbed heat can be reduced by a factor of ten. The primary point to remember of course is that the total kinetic energy of the vehicle is converted to heat, the question is, where do you want to develop, absorb, and radiate that heat.

The earliest nuclear reentry systems used very heavy blunt copper disks ahead of the warhead to absorb the heat. The X-17 program developed light weight ablative reentry systems. It is rumored that the Soviets used plywood for their early reentry heat shields. In the Mercury, Gemini, and Apollo programs, small dense reentry capsules penetrated rapidly and deeply into the atmosphere to decelerate at high Gees in the lower atmosphere, thereby using a boundary layer to reduce the amount of heat absorbed by the vehicle. Later, several studies were done to design light weight large area reentry canopies which would decelerate slowly at high altitudes, none of these systems has been flown yet, but the Space Shuttles use their large wing areas to loose some of their energy in the upper atmosphere. "The Space Handbook" a text printed for Air Force officers, presents a good rule of thumb method to calculate reentry heating. (see the reading list on my web page, I'm going to try to work this into my reentry1.bas computer program to provide a general idea of the heat energy acting on a reentry vehicle.