the design and prototype units was successive layers of Nextel, a material commonly used as insulation under the hoods of many cars, spaced between several-inches-thick layers of open cell foam, similar to foam used for chair cushions on Earth. The Nextel and foam layers cause a particle to shatter as it hits, losing more and more of its energy as it penetrates deeper. Many layers deep in the shell was a layer of superstrong woven Kevlar to hold the module's shape. The air was held inside by three bladders of Combitherm, a material commonly used in the food-packing industry. The innermost layer, forming the inside wall of the module, was Nomex cloth, a fireproof material that also protected the bladder from scuffs and scratches. TransHab layout TransHab was intended to be composed of four levels/floors. Level 4 The Pressurized Tunnel Area was intended to provide a passageway between TransHab and any vehicle to which it is attached. The original tunnel design had two standard International Space Station hatches and housed critical equipment required during module expansion, such as avionics and power equipment. Level 3 Level Three was to house an exercise area with a treadmill and stationary bicycle. The design also contained a complete health care system with all types of medical and emergency equipment and a type of "space bath" area provided for body cleansing. In addition, this level contained a soft stowage area identical to level one. Crew Health Care Area -- The design for the crew health care area incorporated two ISS Crew Health Care System racks, a Full Body Cleansing Compartment, changing area, exercise equipment (treadmill and ergometer), a partitionable area for private medical exams and conferencing, as well as an Earth-viewing window. The Earth-viewing window was to have been 51 centimeters (20 inches) in diameter and was located near the exercise area. The window design had four panes totaling 10.2 centimeters (4 inches) thick and a hard frame around each window that attaches to all layers of the shell. Stowage -- Soft-sided cabinets was to have provided stowage for spare parts, supplies, clothing and other equipment. If built, TransHab would have doubled the stowage room available on the International Space Station. Level 2 Crew Quarters -- The crew quarters area design had six individual crew quarters, sized at 2.3 cubic meters (81 cubic feet) of volume per crew quarter, and a central passageway located within