[The Following is an excerpt from a letter to Allen Meece]

[Updated 5 September 2002]

Here's a rough schematic of my idea for the plastic hab design's thermal control sytem.

I'm afraid it didn't translate too well, but you can still discern the basics.

There are three sections -- the crew cabin, the airlock, and the thermal panels. The airlock and thermal panels are all linked to the crew cabin by at least one feed line and one return line. These two get their antifreeze supply from the crew cabin, although the airlock is able to pump its own antifreeze as long as its lines aren't turned off. There are at least two thermal panels in this scenario, one illuminated to warm antifreeze and one shaded to cool it. The thermal panel return lines are joined and the flow from each panel can be adjusted to control the return temperature just like the tap on a sink.

While there is a large antifreeze reservoir in the crew cabin which serves as part of the hab thermal mass, it is not necessary for the transportation of heat from point to point in the system. With proper adjustment of the valves, the flow of antifreeze can be routed entirely around the reservoir. This allows rapid adjustment of the hab temperature without having to first change the temperature of the thermal mass. For example, if the hab becomes too hot during the day, the return feed from the hot panel can be turned off and the flow routed around the reservoir. This sends a stream of cold antifreeze through the heaters (actually heat exchangers) and/or through the fuel cells, cooling the hab. Alternately, the thermal panels can be placed in a closed loop with just the reservoir to heat it up, and the remainder of the system can continue to function using the heat of the fuel cells. If you trace the path of the antifreeze, four closed loops are possible, one for each pump: one for the cabin heaters, one for the airlock, one for the fuel cells, and one for the reservoir. The temperature is adjusted by varying the flow rates between these main loops. And, of course, the whole thing can function temporarily with just one working pump.

There are four cabin heaters: one on each end of the crew cabin and one on each side of the airlock. These heaters consist of what is essentially a car radiator with a fan and air filter attached, supplemented with an electric heating element.

Other sources of heat in the hab include the dehumidifier, the reserve battery banks, and cabin electronics. Alternate cooling can be provided by carefully balanced use of the airlock venting system.

A system like this could provide variable temperature control, heating or cooling, with a relatively short response time. It can also be adjusted in minutes to provide the temperature stability of a system based on thermal mass. And it can also be adjusted to provide any range of response between the two. This versatility is useful for switching between day and night thermal behavior.