Roof Support

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Architecture as Mole Hills, Standard dorm room

One serious lunar problem will be your roof falling in on you.

Using the roof for radiation and heat shielding

The need for radiation shielding for lunar settlement occupants means that there will be significant mass on the roof of all buildings used for long term occupancy. Much of the electronic equipment will need shielding too. This shielding could be in the form of lunar regolith as suggested in Architecture as Mole Hills and Architecture as Tent City or material brought from Earth, but it must be provided on any structure where people will stay longer than a few weeks.

Also, if your massive radiation shielding roof falls in on you, you will be in real trouble, so inflated room systems must allow for emergency exit even will all the air pressure lost.

Inflatable Housing

Since the interior living space must be at considerably higher air pressure than the exterior, it is only logical to use the force this pressure exerts on the outside walls to help support the weight of the roof. The rooms then become like balloons getting much of their strength from their internal pressure. The amount of mass an inflated roof will support is directly related to the internal pressure.

A major problem comes when you lose the pressure and the massive roof falls in on you. Loss of pressure, or blow out, is a real possibility due to meteorites, ejecta, landing accidents, or industrial accidents. Even if you can get into an environmental suit, you will have to get to safety before your air supply runs out.

Pressure Considerations

One of the most important considerations in the design of a lunar settlement will be the internal air pressure.

High Pressure

High air pressure, that is Earth normal 101 kpa (14.2 PSI, makes the living space more Earth like. People and plants accommodate to living on the Moon easily and food is easy to cook.

But, spacesuits must be at low pressure for the joints to work with acceptable amounts of efforts. Any time a persons moves from a high pressure to a low pressure environment, you risk nitrogen forming bubbles in your blood; a conditions called the bends. For the body to accommodate from normal air pressure to a low spacesuit pressure can take as long as an overnight stay in a low pressure chamber.

Low Pressure

Low air pressure, with adequate oxygen content, allows the human body to accommodation to spacesuit pressures in only a few minutes or just seconds in an emergency. Ranges from 74 kPa (10.2 psi) down to 33.5 kpi (4.7 psi) have been discussed for lunar settlements. The human body will simply never fully accommodate pressures below about 30 kPascal (4.2 psi).

At low pressures you can use 100% oxygen which greatly simplifies the entire life support system. Testing this type of system is very dangerous and has resulted in two serious fires. The key safety concept is that if the partial pressure of oxygen exceeds Earth normal of 22 kPascal (3.0 psi) then substancial fire control efforts are required.

The long term health effects of low pressure are not fully known for people or for plants. Any agricultural areas may need additional CO2, humidity, and nitrogen compared the people living areas.

Low pressure also saves the cost of shipping a large mass of bulk nitrogen from Earth. This could be a very important cost consideration in early lunar settlements.

Water boils at such a low temperature at low atmospheric pressure that cooking is difficult. You simply cannot get things hot enough to really taste right. A cup of tea that you can stir with your finger is simply not worth drinking.

Also low air pressure will not support as thick a layer of protecting regolith over inflated buildings.

Supporting Regolith

Here are some of present ideas for lunar settlement air pressures and how much regolith they will support:

Pressure Pressure Boiling Point Regolith Shield Regolith Supported Comment
kPascal PSI C Meters Meters
101.3 14.2 100 5.4 32 Earth standard, ISS body
84 12.17 ? 4.5 27 Denver, a high altitude city
81.4 11.74 ? 4.3 26 Mexico City, a high altitude city
74.0 10.2 4.0 24 Open airplane, ISS ports
59.1 8.3 19 ISS spacesuit
33.5 4.7 10 Apollo spacesuit
30.6 4.3 9.9 Shuttle spacesuit
26.0 3.65 69.0 1.4 Top of Mount Everest
10.0 1.5 .5 1/10 Atm, 16,000 m, unconscious in 10 sec

The Regolith Shield column shows how much lunar regolith is needed to provide radiation shielding equivalent to the air above your head at these locations on Earth. The Regolith Support column shows how much lunar regolith can be supported on the Moon by that level of internal pressure.

These calculations are based on the following parameters:

density of packed regolith 1.9 g/cm^3 Used for this calculation
density of loose regolith 1.5 g/cm^3 just poured in a pile
Lunar gravity 1.63 m/s^2 about 1/6 Earth
Human body temperature 37.0 C 98.6 F

It is important to note that the internal pressure inside a living area has plenty of force to support a substantial thickness of lunar regolith above it for radiation and thermal shielding even if low pressures are used.

Safety rule

The roof cave in problem will require a strong safety rule that will greatly affect the design of lunar buildings. One possible rule is this:

Roof Support Rule
With the internal building pressure completely lost, a person wearing an environmental suit must have enough clearance to crawl to safety even if a second person is lying immobile in the evacuation path.

Rule's effects

Such a rule, combined with the very high cost of bringing mass from Earth, will make large open rooms very rare on the Moon. Rooms and even halls will require internal columns or bearing walls to hold up the roof with a loss of pressure. Most rooms will have to be narrow in at least one direction and will have to have central support for the roof.

What will be very difficult to build will be very large areas such as meeting rooms and mess halls. They will probably need internal supports.

The absence of large rooms will make living in a lunar settlement even more claustrophobic. This effect might be countered with panoramic vistas of open space on external monitors, via pariscope style windows, or with viewing doom rooms.

Alternative Roof Supports

Once even some industrial capacity is established on the Moon, all lunar material structures will be preferable to inflated structures that require large amounts of materials from Earth.

Elevated roof concept

Hard Roof

If a practical lunar cement can be developed, then we can avoid pressure supported roof problems completely.

Earth cements cannot be used as they are water based and the minerals that they are made from are not available on the Moon.

Alternate Roof Support

There is an alternate form of roof support discussed within an article on Sintered Brick Construction.