Space engine!

Here's an interesting application that I'd like to share: Knudsen compressors as propulsion devices. The Knudsen compressor is a solid state pump (free of working fluids or lubricants) that operates due to thermal transpiration, which is a rarefied gas effect. The results of thermal transpiration are most easily illustrated by considering a simple Knudsen compressor configuration: a channel with solid walls connecting two reservoirs in the presence of a temperature gradient.

So that molecule-molecule collisions are not significant and molecule-surface collisions dominate, the characteristic length of the channel is smaller than the molecular mean free path of the gas. Thermal transpiration is then observed as a flow of gas along the channel's inner surfaces from cold to hot regions (black arrows in the diagram above). Note the opposing pressure-driven flow (white arrow in the diagram above) in the central portion of the channel that develops as the pressure in the hot reservoir increases.

Since they facilitate the compression or transport of a gas, Knudsen compressors may be used for the propulsion of small spacecraft in low Earth orbit. In such a rarefied environment, the molecular mean free path is large. Temperature variations are also large; without thermal controls, the temperature of the Sun-facing side of the International Space Station would hit ~390 K, while thermometers on the dark side would plunge to ~120 K. As harsh as these conditions may seem to us they are in fact favourable for the efficient operation of Knudsen compressors.