guest blogger john hare
I fail to understand the attraction of the complex nuclear and electric engines for deep space maneuvering when solar thermal promises to be so much simpler and higher performance. A few simple modifications to the normal methods of heating the working fluid and nuclear thermal becomes an under performing, over politicized dead end.
There has been some work in laser launch on letting the beam hit particles in the exhaust rather than the solid portions of the engine itself. By constantly reheating the expanding gasses with the heat pumped into the smog particles, the natural tendency of expanding gasses to cool is counteracted so that a relatively cool exhaust through the throat has an unlimited expansion ratio. Unlimited by physics, not by optimum mass and performance. The smog/water propellant for laser launch could exceed the Isp and T/W of the SSMEs by a good margin. Smog /hydrogen Isp can be toward 2,000.
Laser and other beamed propulsion has to contend with the financial aspects of beating established techniques for Earth launch. The business case doesn’t close unless extremely high launch volumes are projected due to the high capital costs. Solar in space can use the same techniques without the same problems faced by lasers. Propulsion mass and propellant must be lifted anyway, the most efficient means of using them should score high in a trade study, and hardware costs should be well below most of the competitors.
The sunlight cannot be focused to a tight spot right into the ‘combustion’ bell as a laser could. Even if the mirrors were dead astern, they would be in the exhaust path which would be most unfortunate. By using one side of the linear aerospike concept, sunlight can be focused on the expansion plume without the mirrors being close to the exhaust path at all. Most of the light will hit the smog particles in the expanding plume for some really intense afterburning effects. The exhaust can be in the thousands of degrees even in the exit plane of the nozzle.
The nozzle will get some heating from the sunlight that ‘leaks’ through the smog as well as from the exhaust gasses. Enough heat will be absorbed to run the expander cycle propellant pump, as well as enough heat for the initial expansion through the throat to supersonic velocities. The solid portions of the engine will never be subjected to the temperatures of nuclear thermal or most competing solar thermal concepts. With propellant use in the grams per second, a large number of very small throats will be requires to feed the large expansion ratio. This is the one side linear aerospike layout with perhaps dozens of throats.
Most desired thrust directions in deep space missions will be at close to 90 degrees from the sun. Either accelerating or decelerating will be modification of a solar orbit. This allows the mirror to be in a constant relative position to the ship off to one side and slightly behind the engine section. While very large, the mirror is not necessarily massive. It is however, awkward and off center. With the low thrusts that will be used, less than 1 m/s, the mirror will never see more than 1/10 gee. The supporting truss can be a gossamer affair with almost fishing line size cables for mirror support. The counterweight can be almost anything useful to the mission.
If coasting periods are sufficiently long, the truss might be adaptable for a centrifugal arm to provide some gravity to a small module.