- Ultimately, the unsolved problem of radioactive fallout killed ORION for good. Any ORION launch would have led to public protests, and the Atmospheric Test-Ban Treaty, signed in August 1963, basically made an ORION launch illegal. ORION was finally canned for good in December 1965. It remains an interesting footnote to a time when people were crazy about space, crazy about the atom, and thought they could accomplish anything. ORION proved to be a dead end, but NTR remains attractive. It is basically a simple idea, involving no more than heating up a gas to use for an exhaust jet. The biggest drawback is that the means that it uses to heat up the gas, nuclear power, is unfashionable these days. NASA has considered an alternative approach that is much trendier, in which solar power is used to heat up a propellant instead of a nuclear reactor. The idea is not new, having been proposed by German-American space flight engineer Krafft Ehricke in 1956, but little was done on the concept until recently. In 1997, NASA launched a study of such a “solar thermal rocket (STR)” engine named “Shooting Star”. Shooting Star was envisioned strictly as a test system, using a Fresnel lens 1.8 meters in diameter supported by an inflatable structure to focus sunlight into the rocket engine, which would then generate about 250 grams of thrust by heating hydrogen to 1,650 degrees Celsius.
An operational STR engine would have one or two mirrors, supported by an inflatable structure made of a foam that hardens on exposure to ultraviolet. The large concentrators would focus sunlight on the exhaust chamber to produce temperatures of 2,750 degrees Celsius, producing thrust with a specific impulse about three times that of LOX-RP propulsion. The engine could be used to transfer payloads from low Earth orbit to geostationary orbit, or to power an unmanned lunar shuttle. The idea is very elegant, though it seems unlikely that it could be scaled up to any great size. It would also not be very useful for exploring the outer Solar System, where the intensity of sunlight is low, although a high-powered laser station could be used as an alternative source of energy for such missions.
The Shooting Star study was supposed to have produced the test system for launch as a space shuttle secondary payload, but little more was said about it after the original announcement, and status of the effort is uncertain.
In the spring of 1998, the US Air Force awarded a contract to Boeing to perform initial studies for an orbital transfer vehicle, or “space tug”, powered by an STR engine, with the designation “Solar Orbital Transfer Vehicle (SOTV)”. The contract followed Boeing tests of an Earth-based static-test STR demonstration system in 1997. Boeing’s concepts for the SOTV envision a spacecraft built around a liquid hydrogen tank, with a spacecraft bus at one end and an STR engine at the other. A mirror is directed by a sunlight-tracking system to focus light onto a graphite “receiver-absorber-converter (RAC)” block, which contains channels for heating the hydrogen, as well as four thermionic converters to provide electricity for spacecraft systems. The thermionic converters use radiators mounted on the spacecraft to dissipate heat.
Fully fuelled, the SOTV is 3.35 meters long and 1.22 meters in diameter, a size that allows it to be launched by an Orbital Sciences Pegasus or Taurus light launch vehicle, and weighs no more than 620 kilograms. At present, there has been no commitment to full development and flight test.
