Initial Habitat Orientation

The Reference Mission calls for a vertical landing of the stack, consisting of the integrated ascent/descent stage and the habitation module, with the two lined up vertically. The habitation module is then lowered, as can be seen in the agenda for Day 1.

The obvious thought when trying to trim unnecessary structural mass from the stack is to forego the added strcture and lowering mechanisms and land the habitation module in a horizontal fashion, eliminating the need to move it from an original vertical state.

There are numerous avantages to this arrangement:

• The weight and expense of lowering mechanisms and the controlling gears or circuitry can be removed
• It eliminates the risks involved in lowering the 12,000 lb, mission-critical habitat
• Redundant sets of trusses can be removed
• Stereo-optical cameras recording footage and governing the landing would be more effective if farther separated
• Thrusters are more powerful if farther separated, and exhaust will affect less surface area of the hull
• It remains possible for the crew to move about in the cabin during the many lengthy descent burns
• The descent will push the crew towards the floor instead of the rear wall of the habitat
• There may be a lower development cost if a motor is used to lower the habitat

However, on close inspection one realizes that the advantages are more then countered by additional mass required in structure and other complications.

• External structure would be needed to stabilize the hab module, mount the engines, and distribute the stress of burns (more than is removed from the lowering apparatus)
• There would be less stability during descent
• Thrusters have less torque, use more fuel and operate more slowly at a time when 10ths of seconds count
• The jacks to level the module after landing would have to be stronger and need more gearing to lift the full weight of the module vs the lower weight of the bracing
• More assembly in space would be required to attach the rocket to the center of the module, with its associated costs, and potential for failure
• The stack would be more volumnous at launch with the uncollapsible extra trusses and framework

The vertical alternative obviously wins. A useful alternative might be to build lowering gear that can be salvaged from the first module and used for subsequent modules, and/or put to other uses.