§ 3.3.2 Steam Rockets and Asteroids
Probably the simplest and cheapest means of transportation of asteroidal resources will be by a simple steam rocket. This uses plain water as the propellant, and a solar or nuclear heat source, e.g., a large foil mirror array to create a super hot chamber to introduce the water into. Water is probably the most abundant volatile in most volatile rich asteroids near Earth. The steam rocket is lightweight, as is the equipment required to extract and store the water from the asteroid.
Solar Steam Rocket (aka Solar Thermal Propulsion, or STP) Source: NASA SP-509, "Space Resources - Energy, Power and Transport", 1992, p.161
Nuclear Steam Rocket. Source: Idaho National Energy and Environmental Lab (INEEL), from paper by Zuppero et al.
The chief advantages of steam rockets are:
The vehicle itself is not heavy. Water is easily stored as ice or liquid, so the mass of the tanks is reduced from that needed for the pressurized propellants used in chemical rocketry and today's electric ion drive. The solar mirrors can be foil thin, deposited on plastic-like (carbon composite) substrate and lightweight structure. The thrust of a steam rocket is low. In orbital space, you don't need high thrust to fight gravity. You can take your time using fuel efficient vehicles.
Dr. John S. Lewis of the University of Arizona/NASA Space Engineering Research Center shows in a paper that a steam rocket operating between high Earth orbit and a near Earth asteroid can achieve mass payback ratios of 50:1 to over 100:1, as compared to about 2.4:1 for the best scenarios for lunar missions using lunar-derived liquid oxygen and chemical rocket launch.
Dr. Lewis puts forth his analysis, including sample missions to known asteroid 1977 VA (which appears to be a good candidate for being a volatile rich asteroid, judging from spectroscopic data), in his paper "Logistical Implications of Water Extraction from Near-Earth Asteroids". The best economics probably comes from "armadas" of small vehicles.
At around the same time, Anthony Zuppero and his associates were making a case for using steam rockets to retrieve cometary materials. After discovery of ice at a lunar pole, Zuppero et al. have been making a case for lunar launch using a nuclear steam rocket. Remarkable are the detailed designs and parameters in much of the work by Zuppero et al.
Of course, in addition to retrieving asteroidal and lunar material, this method can also be applied to interorbital vehicles operating in Earth orbit with earth-launched payloads.
Extensive work has been performed on solar thermal propulsion by the U.S. Air Force Rocket Propulsion Laboratory (AFRPL) with support from Rocketdyne, L'Garde, and Spectra Research. Their objective has been to produce lightweight, efficient concentrators and simple, reliable thrusters for a solar rocket. Rocketdyne in California performed R&D on the engine itself and the orifice which receives the high temperature concentrated solar energy. Rocketdyne used hydrogen as the propellant, but modifying it to use water would not be very difficult. Regarding the large parabolic reflector mirrors for the solar ovens, these are relatively easy to set up. Notable is work in inflatable concepts by the California company LeGuard, which produced an inflatable reflector 3 meters in diameter with a surface accuracy of 2.8 milliradians (RMS).
Rocketdyne's heat exchanger thruster design. This was built, and using hydrogen produced an exhaust velocity of 7.9 km/sec at a temperature of 2700K. thrust was 3.7 newtons. Source: NASA SP-509, "Space Resources - Energy, Power and Transport", 1992, p.162
There are two ways in the literature for heating the propellant, so-called "indirect" and "direct". Indirect solar radiation has the propellant flow through only pipes or passages in the wall of a windowless heating cavity as shown below:
Indirect solar radiation absorption (steam goes through pipes or walls). Source: NASA SP-509, "Space Resources - Energy, Power and Transport", 1992, p.162
The other way is to let the steam flow through sandy material within the heat exchange cavity. We put holes in the pipes or walls of the indirect heat exchanger so that the gas flows directly into the heat cavity, which requires a window, as pictured below:
Direct solar radiation absorption (steam goes into windowed heating chamber). Source: NASA SP-509, "Space Resources - Energy, Power and Transport", 1992, p.163
In the direct concept, the cylindrical heating chamber rotates so that the centrifugal force keeps the sand, or "seeds", along the chamber wall, which is porous to let the gas in. The seeds are chosen for stability at high temperature and heat transfer properties. (Tantalum carbide and hafnium carbide are popular.)
Heat transfer is more efficient in the direct concept, i.e., it's more compact, but clouding of the window or eventual leakage around and other seals are serious concerns. The rotating chamber is considerably more complex.
The best place on the web to keep up with developments on steam rockets in the late 1990s has been the work led by Dr. Anthony C. Zuppero at www.neofuel.com. However, Dr. Zuppero left his post there in 1999 for an unrelated job, and it is not clear who will advance the ball next.
At the moment, the best materials are in the literature off of the net. PERMANENT has plans to bring the subset of public domain literature from the recent past onto the web, which otherwise wouldn't make it to the web. Further details are available in the PERMANENT paper library.