Apollo (spacecraft) - Spacecraft Lunar Module Adapter (SLA)

Spacecraft Lunar Module Adapter (SLA)

The Spacecraft Lunar Module Adapter (SLA), built by North American Aviation (Rockwell), was a conical aluminum structure which supported the Service Module above the Saturn S-IVB rocket stage. It protected the Lunar Module (LM), the Service Propulsion System engine nozzle, and the launch vehicle to Service Module umbilical during launch and ascent through the atmosphere.

The SLA was composed of four fixed 7-foot-tall (2.1 m) panels bolted to the Instrument Unit on top of the S-IVB stage, which were connected via hinges to four 21-foot-tall (6.4 m) panels which would open from the top similar to flower petals.

The SLA was made from 1.7 inches (43 mm) thick aluminum honeycomb material. The exterior of the SLA was covered by a thin (0.03–0.2 inch or 0.76–5.1 millimetres) layer of cork and painted white to minimize thermal stresses during launch and ascent.

The Service Module was bolted to a flange at the top of the longer panels, and power to the SLA multiply redundant pyrotechnics was provided by an umbilical. Because a failure to separate from the S-IVB stage could leave the crew stranded in orbit, the separation system used multiple signal paths, multiple detonators and multiple explosive charges where the detonation of one charge would set off another even if the detonator on that charge failed to function.

Once in space, the astronauts pressed the 'CSM/LV Sep' button on the control panel to separate the Command and Service Module (CSM) from the launch vehicle. Detonating cord was ignited around the flange between the Service Module and SLA, and along the joints between the four SLA panels, releasing the Service Module and blowing apart the connections between the panels. Dual-redundant pyrotechnic thrusters at the lower end of the SLA panels then fired to rotate them around the hinges at 30-60 degrees per second.

On all flights through Apollo 7, the SLA panels remained hinged to the S-IVB and opened to a 45 degree angle, as originally designed. But as the Apollo 7 crew practiced rendezvous with the S-IVB/SLA containing a dummy docking target, concerns surfaced about the possibility of collision between the spacecraft and the SLA panels during docking and extraction of the Lunar Module in a lunar mission. This led to a redesign using a spring-loaded hinge release system which released the panels at the 45 degree angle and pushed them away from the S-IVB at a velocity of about five miles per hour, putting them a safe distance away by the time the astronauts pulled the Command/Service Module away, rotated it through 180 degrees, and came back for docking.

The Lunar Module was connected to the SLA at four points around the lower panels. After the astronauts docked the CSM to the LM, they blew charges to separate those connections and a guillotine severed the LM to Instrument Unit umbilical. After the charges fired, springs pushed the LM away from the S-IVB and the astronauts were free to continue their trip to the Moon.