The first is a more traditionally designed metal shell akin to most spacecraft and the other is a radical design of fabric and composite materials that is inflated once in orbit.
The Common Module Hab is a Boeing-built aluminum structure much like the rest of the ISS. It fits inside the cargo bay of a space shuttle and is based upon subsystems known as "racks." These refrigerator-sized units are the standard for installing equipment in the ISS.
Boeing Co. completed the modules outer shell and little else by 1997, when NASA ordered a halt while another concept was evaluated the Transhab an inflatable design that can stow inside a space shuttle cargo bay and expand to a larger volume during inflation in orbit.
The Transhab is composed primarily of multi-layered fabric, foam and Kevlar, the major component of most bulletproof vests. A shell of carbon composite ribs maintains the shape of the module.
Transhab's original concept came from a possible shelter design for human exploration of Mars.
A prototype of Transhab has been tested at Johnson Space Center, while its hard body competitor sits at Marshall Space Flight Center in Huntsville, Ala.
One advantage of the Transhab is it offers more room than the Common Hab and actually has "floors" much like a multistory house. The Common Hab layout is much like the rest of the station and slightly resembles a submarines interior.
Current plans call for the Common Hab to sleep four and six in Transhab.
Both modules would provide living space to include sleeping, meeting and dining areas along with workout space and showers.
One advantage of the Common Hab is that its metal-skinned surface and technology has been used since man first ventured into space.
Both are estimated to cost about $200 million to complete with a proposed launch date of sometime in 2004.
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