The utility of 10-8-10-7 meter (m) thick space manufactured solar sails for interstellar mission launch is considered. An interstellar solar sail is assumed to be partially or completely deployed behind a much more massive chunk of asteroidal debris with similar dimensions to the sail and utilized as an occulter. The sail is released from the occulter and exposed to sunlight, during a 1.5 x 109 m perihelion pass in a near-parabolic or hyperbolic orbit. Robot probes would be deposited as thin films on the spaceward side of the sails; larger habitats with human crews would be suspended by diamond or copper cables behind the sails. Stress problems in the cable and sail, the applicability of giant-planet rebounds, sail deployment strategies, thermal problems and preliminary mission design are all considered. Near term technology may be capable of launching small robot probes that could reach Alpha Centauri within about 350 years. Much larger human-occupied habitats, constrained to lower peak accelerations, may eventually be able to utilize the solar sail to reach Alpha Centauri within 900-1400 years. Eventually, when our Sun enters its giant phase about 4 x 109 years in the future, the performance of the solar sail as an interstellar booster will greatly increase.
This paper discusses the use of solar system-based lasers to push large lightsail spacecraft over interstellar distances. The laser power system uses a 1000-km-diam. lightweight Fresnel zone lens that is capable of focusing laser light over interstellar distances. A one-way interstellar flyby probe mission uses a 1000 kg (1-metric-ton), 3.6-km-diam. lightsail accelerated at 0.36 m/s2 by a 65-GW laser system to 11% of the speed of light (0.11 c), flying by a Centauri after 40 years of travel. A rendezvous mission uses a 71-metric-ton, 30-km diam. payload sail surrounded by a 710-metric-ton, ring-shaped decelerator sail with a 100-km outer diam. The two are launched together at an acceleration of 0.05 m/s2 by a 7.2-TW laser system until they reach a coast velocity of 0.21 c. As they approach a Centauri, the inner payload sail detaches from the ring sail and turns its reflective surface to face the ring sail. A 26-TW laser beam from the solar system, focused by the Fresnel lens, strikes the heavier ring sail, accelerating it past a Centauri. The curved surface of the ring sail focuses the laser light back onto the payload sail, slowing it to a halt in the a Centauri system after a mission time of 41 years. The third mission uses a three-stage sail for a roundtrip manned exploration of t Eridani at 10.8 light years distance.
A new concept, the magnetic sail, or "Magsail", is proposed which propels spacecraft by using the magnetic field generated by a loop of superconducting cable to deflect interplanetary or interstellar plasmas winds. A description is given of the computer code used to model the performance of such a device and results of a series of investigations are presented. It is found that a Magsail sailing on the solar wind at a radius of one astronautical unit (A.U.) can attain accelerations on the order of 0.01 m/s2, much greater than that available from a conventional solar lightsail. When used as a brake for an interstellar spacecraft, the Magsail can reduce spacecraft velocity by a factor of e every five years. A systems performance code was used to analyze the utility of the Magsail when used in conjunction with either fusion rocket or laser lightsail accelerated interstellar spacecraft. It is found that the Magsail can reduce flight times by forty to fifty years and propellant requirements by thirty percent for fusion rocket propelled ten lightyear missions. The Magsail also provides an efficient method for decelerating laser lightsail propelled missions that are otherwise simply impossible.
The basis of a novel space propulsion system called a Nuclear Salt Water Rocket (NSWR) is outlined. NS WR is constructed of a bundle of boron-carbide coated pipes, each containing an aqueous solution of uranium or plutonium salt These pipes empty into a single long cylindrical plenum pipe of larger diameter, which terminates in a rocket nozzle. When the rocket is to be fired, the aqueous solution held in the pipe bundles is emptied into the plenum. When the plenum has filled to a certain point, the fluid assembly within it exceeds critical mass and goes prompt supercritical, with the neutron flux concentrated on the downstream end due to neutron convection. Enormous amounts of energy are generated within this region, flashing the solution to steam which then streams down the plenum pipe towards the nozzle, convecting the exponentially growing fission chain reaction with it. As the solution continues to pour into the plenum from the borated storage pipes, a steady-state condition of a moving detonating fluid can be set up within the plenum.
Assuming a solution of 2 atoms of 20% enriched uranium per 100 molecules of water and a fission yield of 0.1% is obtained, the specific impulse produced will be about 7,000 seconds. This is comparable to that available from electric propulsion (EP). However, unlike an EP system, a NSWR is not power limited (since waste heat is eliminated with the propellant) and systems with jet power ratings of thousands of megawatts are obtainable. The NSWR can thus deliver its very high specific impulse at thrust levels of the same magnitude as chemical engines, or about 4 orders of magnitude greater than that of a multimegawatt EP system. The NSWR system is simple and lightweight. Shielding considerations are minimized by the fact that almost no radioactive inventory travels with the vehicle. On the other hand, the exhaust stream from the NSWR is extremely radioactive, which limits the use of the device to orbit transfer propulsion, despite its high thrust to weight.
