Return to Earth: "Apollo Atmospheric Entry Phase" 1968 NASA JSC-345; Mission Planning & Analysis Division (MPAD)

more at http://quickfound.net/

"Demonstrates the simulation of reentry into the Earth's atmosphere by the Apollo spacecraft, explaining the problems involved and showing the hardware used."

NASA film.

Originally a public domain film from NASA, slightly cropped to remove uneven edges, with the aspect ratio corrected, and mild video noise reduction applied.

The soundtrack was also processed with volume normalization, noise reduction, clipping reduction, and/or equalization (the resulting sound, though not perfect, is far less noisy than the original).

http://en.wikipedia.org/wiki/Atmospheric_entry

Wikipedia license: http://creativecommons.org/licenses/by-sa/3.0/

Atmospheric entry is the movement of human-made or natural objects as they enter the atmosphere of a celestial body from outer space—in the case of Earth from an altitude above the Kármán Line, (100 km). This article primarily addresses the process of controlled reentry of vehicles which are intended to reach the planetary surface intact, but the topic also includes uncontrolled (or minimally controlled) cases, such as the intentionally or circumstantially occurring, destructive deorbiting of satellites and the falling back to the planet of "space junk" due to orbital decay.

Most objects entering the atmosphere are not released from rest just above it, but rather are entering at hypersonic speeds because they are on suborbital (e.g. ICBM reentry vehicles), orbital (e.g. the Space Shuttle), or unbounded (e.g. meteors) trajectories. Therefore, controlled atmospheric entry often requires special methods to protect against severe aerodynamic heating. Various advanced technologies have been developed to enable atmospheric reentry and flight at extreme velocities...

The concept of the ablative heat shield was described as early as 1920 by Robert Goddard: "In the case of meteors, which enter the atmosphere with speeds as high as 30 miles per second, the interior of the meteors remains cold, and the erosion is due, to a large extent, to chipping or cracking of the suddenly heated surface. For this reason, if the outer surface of the apparatus were to consist of layers of a very infusible hard substance with layers of a poor heat conductor between, the surface would not be eroded to any considerable extent, especially as the velocity of the apparatus would not be nearly so great as that of the average meteor."

In the United States, H. Julian Allen and A. J. Eggers, Jr. of the National Advisory Committee for Aeronautics (NACA) made the counterintuitive discovery in 1951 that a blunt shape (high drag) made the most effective heat shield. From simple engineering principles, Allen and Eggers showed that the heat load experienced by an entry vehicle was inversely proportional to the drag coefficient, i.e. the greater the drag, the less the heat load. Through making the reentry vehicle blunt, air cannot "get out of the way" quickly enough, and acts as an air cushion to push the shock wave and heated shock layer forward (away from the vehicle). Since most of the hot gases are no longer in direct contact with the vehicle, the heat energy would stay in the shocked gas and simply move around the vehicle to later dissipate into the atmosphere...

The ablative heat shield functions by lifting the hot shock layer gas away from the heat shield's outer wall (creating a cooler boundary layer) through blowing providing the best protection against high heat flux. The overall process of reducing the heat flux experienced by the heat shield's outer wall is called blockage. Ablation causes the TPS layer to char, melt, and sublime through the process of pyrolysis. The gas produced by pyrolysis is what drives blowing and causes blockage of convective and catalytic heat flux...

AVCOAT is a NASA-specified ablative heat shield, a glass-filled epoxy-novolac system.

NASA originally used it for the Apollo capsule and then utilized the material for its next-generation beyond low Earth-orbit Orion spacecraft. The Avcoat to be used on Orion had been reformulated to meet environmental legislation that has been passed since the end of Apollo...