Did the Lunar Module Have an Airlock?

Overview of the Apollo Lunar Landers' Design

The Apollo lunar landers, including the Lunar Module (LM), did not feature an airlock as one might expect. This design decision was the result of a careful balance between crew safety, mission efficiency, and vehicle weight.

The Lunar Module's Hatch System

The LM utilized an overhead crew transfer hatch for movement between it and the docked Command Module (CSM). This hatch, along with a larger, squared-off forward hatch for exiting the lander while wearing the PLSS (Portable Life Support System) backpack, served as the primary means of egress and transfer. Both hatches featured purge valves that could be used to vent cabin air, and in case of emergencies, the LM could even be used as an airlock.

In practice, however, the process of opening the hatch typically involved venting the cabin and prying open the seal to release the last bit of pressure. This approach minimized the need for additional design complexity but came with obvious drawbacks, such as dust being blasted across the lunar surface.

Design Trade-offs and Cabin Size

The entire pressurized living space of the LM was compact, with a volume of approximately 235 cubic feet. This small pressurized area, roughly the size of two telephone booths, was precisely engineered to meet the mission's needs while minimizing weight. The cabin atmosphere was pure oxygen at 5 psi, which had a mass of about 5.5 pounds. Given the limited space, astronauts in bulky space suits couldn't afford much additional volume for air.

To further illustrate the trade-offs, consider the Apollo 11 astronauts. They spent 20 hours on the Moon but were unable to use seats, resting on the chilly floor during their downtime. Later missions addressed this by adding hammocks, which could be attached to the cabin walls to provide a more comfortable sleep environment.

No Airlock, but Plenty of Oxygen

While there wasn't an airlock, the cabin atmosphere was vented to space before any Extravehicular Activity (EVA) took place. This was not only due to the cabin's small size but also because the LM carried excess oxygen in tanks to replace the vented atmosphere.

The descent stage of the LM featured two 2700 psi tanks with 48 pounds of oxygen each, providing about 2 pounds of oxygen per day for a human. While this might seem sufficient, the designers had to weigh the benefits of carrying extra oxygen against the added weight and complexity of an airlock.

Device comparisons further illustrate this point. The International Space Station (ISS) has a much larger pressurized volume and vents the whole system, resulting in significant atmospheric loss. To mitigate this, ISS uses proper airlocks, which can handle the influx of air more efficiently and without the loss of atmosphere.

Conclusion

The absence of an airlock in the Apollo lunar landers was a result of design choices that optimized for weight, efficiency, and mission safety. While the process of venting and opening the hatches may have seemed less than ideal, it was a practical solution that allowed for the successful execution of the Apollo missions.