This week’s image comes to you from the U. S. Air Force Space and Missile Museum at Cape Canaveral Air Force Station, Florida. It’s an early “heatsink” style test re-entry vehicle, designed to be carried on the nose of an Atlas ICBM:
In the 1950’s, the U.S. military (and its industrial partners) faced a daunting task in the design of a capsule that could successfully re-enter the Earth’s atmosphere after a trip halfway across the globe. In a typical ICBM flight, the atmosphere ahead of the warhead would be heated to temperatures in excess of 12,000 deg. F — some 2,000 degrees hotter than the surface of the Sun. A number of different approaches to dealing with this heating were tested:
- Heat sink — a thick plate of metal with good thermal conductivity (like copper) would be used to absorb the heat of re-entry, while insulation behind the heat sink protected the warhead and its electronics
- Transpiration cooling — water would be injected at the front of the vehicle, absorbing heat as it flashed into steam
- Ablative cooling — a shield made of a resinous material would char from re-entry heating, resulting in effects similar to transpiration (from gasses generated during the charring process), as well as cooling resulting from the absorption of heat by the charring shield
- Hot structure — tiles made from metal with a high melting point covered the surface of the vehicle; insulation behind the tiles then kept the vehicle structure down to a relatively cool 200 F
Heat sink technology was simple, if very heavy — so it provided thermal protection for the first ICBM warheads that the U.S. fielded. But testing soon proved the superiority of ablative cooling of re-entry vehicles, rapidly becoming the predominant technology for protecting both warheads and (later) manned spacecraft from the heat of atmospheric re-entry.