40 years ago: U of T scientists key to Apollo 13 rescue

(PhysOrg.com) -- On April 16,1970, we were absorbed in the minutiae of a departmental meeting when a secretary interrupted, informing us that Martin Marietta, the builder of the lunar capsule, had called Barry French to help with the rescue of Apollo 13. The meeting broke up and French assembled a team of advisers from the University of Toronto Institute of Aerospace Studies (UTIAS) faculty.

The Apollo craft comprised three modules: a service module providing both life support and rocket thrust for most of the voyage, a lunar excursion module (LEM) to land on the moon and a module for both the voyage and terrestrial re-entry. But when an explosion completely disabled the service module, the LEM became a lifeboat, with its life support and rocket thrust -- intended only for lunar landing and return to lunar orbit -- becoming essential to the rescue.

Normally the LEM would have been jettisoned just after completing its mission by severing the tube connecting it to the re-entry module. This tube, which also served as the LEM access tunnel, was to be cut by a ring of explosive located just four inches from the re-entry module's hatch.

To ensure that shock waves from the explosion did not damage the hatch, before detonation the 5 psi (pound-force per square inch) oxygen atmosphere in the tunnel would have been evacuated. The service module's rockets would then have been used to back away from the LEM. But because these rockets were inoperative, NASA's engineers proposed using the oxygen pressure as a spring to jettison the LEM just before re-entry.

A previous incident suggested that retaining the full 5 psi in the tunnel could cause shock damage to the hatch. It was on this point that an engineer at the LEM manufacturer called UTIAS for advice.

With a telephone line held open to allow us immediate access to data on spacecraft geometry, masses and other quantities, we worked in two groups. One used Newton's laws of mechanics to estimate LEM separation speeds attainable with various tunnel pressures. The second group estimated the strength of the pressure pulse generated by the explosive charge. They adapted formulas verified in the first instance by comparisons with photographs of the first atomic explosion at Alamogordo, New Mexico. We concluded that a tunnel pressure of 2 psi would provide sufficient separation speed while minimizing the risk of damage to the re-entry module.

We assumed that other groups were consulted but we subsequently learned that our advice was the main basis for a decision to lower the tunnel pressure and thus to complete a successful rescue.

Provided by University of Toronto