Category:Command Module (CM)

Command Module Splashdown [image info]

The three-seater Apollo Command Module (CM) was the conical "space capsule" at the front end of the Apollo spacecraft, that detached at the end of the mission and underwent rentry. This was the only part of the Apollo spacecraft that returned to Earth.

Atmosphere and the Apollo 1 disaster

Normal air is around four-fifths inert nitrogen and one-fifth oxygen. Rather than deal with three different gases, nitrogen, oxygen and the carbon dioxide breathed out by the astronauts ... NASA decide to reduce the pressure of the environment inside the capsule and omit the inert nitrogen (which didn't do anything). In theory, if the capsule was at one fifth normal atmospheric pressure, but filled with pure oxygen, a lungful of air would still supply the same amount of O₂, and the reduced pressure would make air-leaks less likely, and slow their effects.

Unfortunately the flammability of various materials increases radically in a pure oxygen environment, to the point that some materials actually spontaneously combust. In Apollo One, astronauts were testing the pre-launch environment, in pure oxygen at standard pressure, when the capsule interior ignited. The heat increased the internal pressure further so that the complex hatch mechanism hatch (designed not to open unless the pressure was the same inside and out) could not be released in time, and all three astronauts perished.

Lessons learned=

As a result of the disaster, the hatch design underwent such a rigorous redesign that it ended up arguably being one of the most specialised parts of the entire craft. NASA also wen out of their way to make sure that all paints and materials used in the craft were solvent-free, and that there were no "problem" plastics involved. The slightly wierd pale green colour of the instrument panels as supposedly achieved not by using organic pigments, but by using tiny glass spheres in the paint whose size affected the wavelengths of light reflected to make it greenish.

Colours

• On the launchpad, the Apollo capsule seemed to be white ... but this was because of a white protective shield that also held the emergency escape system.
• In space, with the shield gone, the upper surface of the capsule was shiny, almost mirror-finish silver, achieved by laying layers of metallised tape over the surface. This was made easier by the capsule's conical shape: a cone is topologically equivalent to a flat sheet (before we bend it), so the strips of tape, butted up against each other, didn't need and special shaping ... just simple rectangular strips taken directly from a reel of tape would do the job (with the exception of some of the "bulging" parts of the capsule, such as the hatch area).
• After return, the capsule looked rather singed and more of a bronze colour due to the heat of reentry, and also because of the smoke from the sacrificial lower heatshield material that was designed to burn off when it got too hot.
• Examples that we've seen in preservation look different again: They look slightly singed and brownish, but the metallic tape has been removed to allow the material beneath to be checked for problems.

From photos, the similar-but-updated capsule used for the later Skylab missions does seem to have been white rather than silver -- they may have decided that the silver metal tape was unnecessary.

Heatshields

The CM was covered with a honeycomb mesh with thousands of segments, each of which was filled with the heatshield compound (by hand) and allowed to set solid. Once the capsule was clad in its heat shielding (thicker on the base), it was x-rayed to try to find air-pockets that might expand during the heat of re-entry. These spots were marked, the section of heatsinking drilled out, and the holes refilled.

This meant that the bolts for key parts of the spacecraft were initially hidden: access to these bolts was achieved by drilling down to the bolts, and then plugging the holes with special bungs. If needed the bung could be removed with a special tool. Protecting the top of each bung was a piece of cork -- a rather low-tech solution that worked: cork is lightweight, is a good heat insulator, makes a good flexible stopper, and responds to heat by charring rather than burning.

Access

There were two ways into and out of the capsule: the main side hatch, and also a complex "tunnel"-type arrangement set into the peak of the cone, which allowed the capsule to dock with the Lunar Excursion Module.

Landing

The capsule was designed to land on water: the capsule had an orange inflatable flotation skirt, and three parachutes were packed into the the region around the docking part, along with asymmetrically-distributed spherical orange balloons that improved the visibility of the craft for retrieval, and also helped to right it if it somehow managed to end up upside-down in the water.

In preservation

Since every lunar capsule that was sent into space ended up returning, there are now fifteen of these capsules (including those built for Skylab) in various museums around the world:

External links

• Apollo: Where are they Now? (nssdc.gsfc.nasa.gov)