The final three Moon-bound Apollo missions, Apollos 15 to 17, had one special task to perform prior to arrival in lunar orbit. Sector 1 of their service modules contained a scientific instrument module, or SIM bay for short. It housed a variety of cameras and instruments to investigate the Moon and its environment, and would be operated by the CMP during his lonely vigil while his crewmates explored the lunar surface.
Hidden as it was behind the external skin of the service module, the SIM bay had to be exposed to space by removing one of the spacecraft's panels. Rather than implementing door-like mechanisms with latches and hinges, engineers decided that a more reliable solution was to jettison the bolt-on panel by blowing it clear with pyrotechnic charges. This occurred before the spacecraft entered lunar orbit so that the jettisoned door would not enter lunar orbit and become a possible collision hazard. Explosive cord within a groove around the door's edge was detonated to cleanly cut the aluminium skin, while further charges were set off to push the severed door clear. While the spacecraft then eased itself into lunar orbit, the door coasted around the Moon to return to the vicinity of the Earth and, in all likelihood, be thrown into solar orbit indefinitely.
Apollo 15 provided the first occasion when these fireworks were set off and Capcom Joe Allen made light of the situation: "By the way, is that the manoeuvre where the SIM bay door jettisons the spacecraft?"
In the Newtonian environment of space, it was as valid to say that the door was jettisoning the spacecraft as the other way around. Al Worden agreed: "It has been variously known as that kind of a manoeuvre, yes.''
In fact, just as the spacecraft had pushed the door away, the door also pushed the spacecraft away, and engineers on Earth could detect this tiny trajectory change in their tracking. "15, just out of interest, we saw a good healthy jolt in our Doppler data down here during jett time,'' informed Allen.
"Gee, that's very interesting," replied Dave Scott, "because I would say that the jolt in here was very minor.''
As a precaution, the crew of Apollo 15 put their suits on in case the shock of the explosives caused a breach in the cabin for some reason. This reflected wariness by programme managers in the aftermath of a tragedy a month before the flight. When the Soviet spacecraft, Soyuz 11, departed an early Soviet space station, Salyut 1, a ventilation valve for use after landing was opened in space by the shock of jettisoning the orbital and service modules of the spacecraft immediately following the de-orbit manoeuvre. Both the primary and secondary pyros had fired venting the cabin's air and quickly asphyxiating the crew. Although their automatic systems brought them to a pinpoint landing on Earth, the ground personnel found that the crew were dead when the hatch was opened. When the pyros were fired, the crew were not wearing suits.
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