Solar Boilers use large parabolic mirrors to focus sunlight on a fluid fuel, most often cited as water or liquid hydrogen, superheating it and expelling the resulting steam for thrust. The sunlight is collected and focused using large, lightweight parabolic mirrors. These mirrors are either silvered balloon-like inflatable structures or thin sheets of silvered plastic supported by lightweight inflatable trusses. These focus the sunlight on either a window or open area of the engine, either directly on the fuel or into a graphite block which in turn heats the fuel. The graphite has superior heat-absorption properties as compared to water or hydrogen, the two fuels most associated with
solar boilers, but it also results in some energy loss due to the indirect heat transfer.
In and around Earth’s orbit, the sun provides enough power to impel a specific impulse of about 800 to 1000 seconds, with medium thrusting power. This gradually tapers off as the ship moves away from the sun, and solar boilers may not be a very practical method of propulsion in the outer solar system.
Solar Boilers are designed to be lightweight, inexpensive craft, and are primarily cited as obtaining fuel cheaply from in-space sources like comets. Water is the most likely choice for fuel, as it is readily available on many comets and it can be stored easily as ice or liquid. Also, the craft would not need the heavy, pressurized tanks needed to carry hydrogen.
Though some experimental solar
boiler craft may be launched in the next decade or two, they probably will not come into their own until a permanent infrastructure in space is well established, where they can be used as cheap "workhorse" vehicles around the inner solar system.
