Roof Support

Architecture as Mole Hills, Standard dorm room

Introduction

Structural support in lunar habitats will be quite different from earth based buildings. One primary difference is that, due to the need for lunar habitats to maintain a sizable internal pressure, most of the standard operating stresses will be internal rather than external.

Safety considerations

Most proposals for long term lunar habitats call for the use of a thick blanket of lunar regolith to be piled atop the structure, providing protection from temperature swings, meteorite impacts, and cosmic radiation for the inhabitants, as well as any electronic equipment (see Architecture as Mole Hills and Architecture as Tent City). Most of the thickness called for in this blanket is to protect against cosmic radiation.

Most of the experience mankind has accumulated concerning radiation deals with nuclear radiation. The cosmic radiation encountered beyond earths magnetic field is much less understood, as it can only be replicated on earth by means of a particle accelerator. As such, the question of how much regolith is needed to protect the inhabitants is currently little more than educated guesswork, and will likely remain so until more thorough field study is performed.

One common estimation is to provide enough regolith to equal earths atmosphere in shielding potential. This is based on the fact that the earths magnetic field has collapsed several times in geologic history, and said collapses did not seem to have any major effect on the life on earth. As such, it is believed that the mass of earths atmosphere is by itself sufficient to guard against cosmic radiation. Assuming that air and lunar regolith have similar shielding properties according to their mass (as they do for nuclear radiation), this gives several different thicknesses, depending on what altitude is used for comparison.

Location	Atmospheric Pressure		Regolith Needed to Equal Atmospheric Shielding		Miniumum Internal Pressure Required
	kPa	psi	m	f	kPa	psi
Sea Level	101.3	14.2	5.4	17.9	16.9	2.4
Denver, a high altitude city	84	12.17	4.5	14.8	14	2
Mexico City, a high altitude city	81.4	11.74	4.4	14.3	13.5	2
Open airplane	74.0	10.2	4.0	13	12.3	1.8
Top of Mount Everest	26.0	3.65	1.4	4.6	4.3	0.6

(assuming a regolith density of 1.9 grams/cm^3 and lunar gravitational acceleration of 1.63 m/s^2)

As seen from the chart, the minimum habitat pressure level needed to support the regolith shield on pressure alone is quite low, even for sea level equivalent shielding. Even if a structure was only pressurized to equal the peak of mount everest, it would be able to support sea level radiation shielding and still have a net outward force. The reason for this is that, though the regolith piled atop the structure is the same mass per unit area as earths atmosphere would be, lunar gravity is only around one sixth of earths, resulting in much less force pushing downward, but the same pressure pushing upward. If the same structure was constructed in earth-like gravity (but still in a vacuum), the pressure balance would be quite different.

As a result, a lunar habitat functioning under normal parameters would not require any internal support for its main structure, as the internal forces of pressure are greater. A lunar habitat could therefore be essentially a giant, reinforced balloon, covered in lunar regolith.

Depressurization

One safety consideration for such a structure is to plan for operation in case of pressure loss, also known as a blow out. Some mechanism of coping with a blow out would be required.

One method of dealing with a blowout is to make the outer structure sufficiently strong that it will not only keep in the pressure under normal conditions, but will also hold its own in a depressurized state. As seen in the chart, a structure capable of supporting a sea level equivalent regolith shield under depressurized conditions would be subject to just under 3 psi of pressure across its roof. This is approximately equivalent to designing an earthbound structure capable of maintaining standard atmospheric pressure six feet underwater in an emergency, as well as containing twice atmospheric pressure under the same conditions normally. A steel or titanium structure could be made sufficiently strong to withstand these forces, as could a properly designed lunar brick structure reinforced with steel cable.

A structure which was supported only by internal pressure would require additional mechanisms to ensure the safety of its inhabitants in case of a blowout. One way of coping with this is to compartmentalize the structure, designing it so that separate areas could be sealed off if needed, so that a breach in one area would not affect the entire habitat. Multiple air bladders around the outsize, also compartmentalized, would also aid in reducing the chance of a blowout. An additional safety measure that could be installed is for each section of the habitat to have its own backup gas tank, which would open in case of a pressure loss, feeding the leak until an patch job / evacuation could be achieved. Since the minimum pressure that needs to be maintained is quite low (as discussed previously), this approach could be utilized with a modestly sized backup tank.

Hazards
Physiological/Psychological/Human Factors  Zero or Low Gravity  Physical Environments  Medical Treatment  Fire/Explosion  Toxins  Corrosives  Temperature Extremes  Non-Ionizing Radiation  Ionizing Radiation  Lack of Breathable Atmosphere  Inadequate Food/Water  Improper Pressure