May 19, 2017

[2.70] Seek and Geek #11: Apollo Command Module Door Locking Mechanism

Spaceflight is the pinnacle of precision engineering - once it's gone, everything better work. Even if your component only needs to work once before maintenance, it needs to work every time. The crew access hatch (side door) on the Apollo 14 Command Module "Kitty Hawk" uses the Unified Hatch design, a door design implemented on every Apollo mission post-Apollo 1 disaster.

Apollo 14 Command Module Door - NASA Engineering
Apollo 14 Command Module Side Door - NASA Engineering
Photo by Christopher Campbell
© Christopher Campbell Photography

Apollo 14 command module with Edgar Mitchell's memorial wreath
Apollo 14 Command Module "Kitty Hawk" at the Saturn V Center, Kennedy Space Center
Photo by Allen McGregor
From Wikipedia

On January 27, 1967, the Apollo 1 spacecraft caught fire during a manned-practice countdown. Sudden fire in the pure-oxygen atmosphere of the cabin created a negative pressure differential (opposite to usual operating conditions like space) and made the door impossible to open. The entire crew died within minutes.

From HistoricSpaceSystems (1996) Issue 2 [1]

The original hatch had three layers; unlocking the door required manually removing the internal layers and using a plunger to access the outer latches. In ideal conditions, the crew could unlock/remove the doors and exit the hatch in 60-90 seconds. Post-accident, egress-hatch requirements were drastically changed. Crew needed to be able to open the hatch in 3 seconds and exit in 30. And to fit the Apollo launch schedule, there needed to be minimal modifications to the existing command module structure. No welding allowed. [1]

Apollo Command Module
From NASA Apollo Spacecraft News Reference p39 [2]

The chosen "Unified Hatch" solution combined the inner two layers, and only slightly modified the outer layer. This design fulfilled all new functional requirements, and was used in every subsequent Apollo mission.

From NASA Apollo Spacecraft News Reference p46 [2]

Crew could open the door by hand; levers and gearing incorporated into the hatch allow 12 latches (on the side door; the forward hatch has 6 latches) around the perimeter to be simultaneously operated by a single ratcheting mechanism. Opening the hatch in the presence of gravity is assisted by nitrogen-bottles and a piston-based counterbalance assembly. Both side and forward doors had air-vent valves for equalizing pressure if necessary[2].

Assembled (left) and exploded view (right) of operating lever and gearbox mechanism.
Digital model & images by HistoricSpaceSystems
Used with permission

In normal operation, pressure against the hatch increases locking pressure of the latches, but each latch can be opened/closed manually from the inside (or outside, using an emergency allen-wrench carried with ground crew) in the event of mechanical failure. This latch design was also used on the Space Shuttle hatches[3].

Apollo-11 latch assembly on display at the Smithsonian National Air & Space Museum (left)
and latch assembly digital model by HistoricSpaceSystems (right)
From HistoricSpaceSystems, used with permission

If some malfunction prevented the latches from engaging, three jackscrews included in the crew's toolkit could hold the door closed. These jackscrews were made to engage against paired catches built into the hatch and interior structure. When tightened, the jackscrews pulled the hatch closed and could thermally withstand reentry [2].

[1] J. Fongheiser. Apollo hatch redesign: a matter of urgency. HistoricSpaceSystems Info Sheet, (1996) Issue 2.
[2] Apollo Spacecraft News Reference. North American Rockwell Corp., Space Division. (1972).
[3] J. Fongheiser. Apollo spacecraft crew side hatch (2015). HistoricSpaceSystems, web. Accessed May 2017.


  1. My dad made the parts for this door. He worked for a sub-contractor of NAA. He was a precision thread grinder. If you are interested in my dad's story let me know.

    1. Thanks for the reply Jack, and your dad must have done really cool work - dealing with precision threads that could still handle thermal expansion and other quirks! I'd like to hear his story. You're welcome to either post it in a comment, or directly via the contact form on my about-me page.

  2. The way the latch works is amazing, I am trying to find more information on the way the link works and who invented this. Was this mechanism designed specially for the latch, or was it a preexisting mechanism?

  3. 12 latches? I count at least 14 in the image on this site. Should be one hidden behind stuff at the bottom right to have even latch spacing.

    1. Fair; the source I pulled from specifies 12 latches on the side (exit) CM hatch and did not specify the number for the forward (docking) hatch.