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The Bussard ramjet is a theoretical method of spacecraft propulsion proposed in 1960 by the physicist Robert W. Bussard, popularized by Poul Anderson's novel Tau Zero, Larry Niven in his Known Space series of books, Vernor Vinge in his Zones of Thought series, and referred to by Carl Sagan in the television series and book Cosmos. Bussard ramscoops are also seen in Star Trek, where they are situated at the glowing tips of the warp drive nacelles of spacecraft, although the hydrogen is not used as nuclear fuel.
Bussard proposed a ramjet variant of a fusion rocket capable of reasonable interstellar travel, using enormous electromagnetic fields (ranging from kilometers to many thousands of kilometers in diameter) as a ram scoop to collect and compress hydrogen from the interstellar medium. High speeds force the reactive mass into a progressively constricted magnetic field, compressing it until thermonuclear fusion occurs. The magnetic field then directs the energy as rocket exhaust opposite to the intended direction of travel, thereby accelerating the vessel.
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Since the time of Bussard's original proposal, it has been discovered that the region surrounding the sun has a much lower density of interstellar hydrogen than was believed at that time (see Local Interstellar Cloud). T. A. Heppenheimer analyzed Bussard's original suggestion of fusing protons, but found the bremsstrahlung losses from compressing protons to fusion densities was greater than the power that could be produced by a factor of about 1 billion, thus indicating that the proposed version of the Bussard ramjet was infeasible. However Daniel P. Whitmire's 1975 analysis indicates that a ramjet may achieve net power via the CNO cycle, which produces fusion at a much higher rate (~1016 times higher) than the proton-proton chain.
Robert Zubrin and Dana Andrews analyzed one hypothetical version of the Bussard ramscoop and ramjet design in 1985. They determined that their version of the ramjet would be unable to accelerate into the solar wind. However, in their calculations they assumed that:
- The exhaust velocity of their interplanetary ion propulsion ramjet could not exceed 100,000 m/s (100 km/s);
- The largest available energy source could be a 500 kilowatt nuclear fission reactor.
In the Zubrin/Andrews interplanetary ramjet design, they calculated that the drag force d/dt(mv1) equals the mass of the scooped ions collected per second multiplied by the velocity of the scooped ions within the solar system relative to the ramscoop. The velocity of the (scooped) collected ions from the solar wind was assumed to be 500,000 m/s.
The exhaust velocity of the ions when expelled by the ramjet was assumed not to exceed 100,000 m/s. The thrust of the ramjet d/dt(mv2) was equal to the mass of ions expelled per second multiplied by 100,000 meters per second. In the Zubrin/Andrews design of 1985, this resulted in the condition that d/dt(mv1) > d/dt(mv2). This condition resulted in the drag force exceeding the thrust of the hypothetical ramjet in the Zubrin/Andrews version of the design.
Ram Augmented Interstellar Rocket (RAIR)
Due to the potential for drag caused by a ramjet attempting to accelerate the hydrogen it captures up to its speed before the hydrogen gas can undergo fusion, a concept had been proposed which uses a ramjet in partnership with a fusion rocket. An onboard hydrogen fuel supply is used to provide power to a fusion reactor but a ramscoop is used to provide propellant. By this method the hydrogen entering the ramscoop need not be accelerated up to the ship's speed before fusion can occur, because the hydrogen being collected is not used for fusion. Instead this hydrogen can continue to travel relative to the ship at high speed as it passes through the engine and has some energy transferred to it from the reactor on board. This hydrogen propellant leaves the rear of the vessel travelling relative to the vessel at its own initial velocity relative to the vessel plus the velocity provided to it by energy transfer as it passed the reactor. The fusion reactor itself may provide some thrust by release of fusion products but the majority of the thrust on such a vehicle will come from the interstellar sourced propellant hydrogen.
The calculations (by Robert Zubrin and an associate) inspired the idea of a magnetic parachute or sail. This could be important for interstellar travel because it means that deceleration at the destination can be performed with a magnetic parachute rather than a rocket.
Several of the obvious technical difficulties with the Bussard Ramjet can be overcome by prelaunching fuel along the spacecraft's trajectory using something like a magnetic rail-gun.
The advantages of this system include
- Launching only ionized fusion fuel so that either magnetic or electrostatic scoops can more easily funnel the fuel into the engine. The drawback is this will cause the fuel to disperse due to electrostatic repulsion.
- Launching the fuel on a trajectory so that the fuel velocity vector will closely match the expected velocity vector of the spacecraft at that point in its trajectory. This will minimize the "drag" forces generated by the collection of fuel.
- Launching optimized isotope ratios for the fusion engines on the spacecraft. A conventional Bussard ramjet will mostly collect hydrogen with an atomic weight of 1. This isotope is harder to fuse than either the deuterium or tritium isotopes of hydrogen. By launching the ideal ratio of hydrogen isotopes for the fusion engine in the spacecraft one can optimize the performance of the fusion engine.
- Although the prelaunched fuel for the ramjet negates one advantage of the Bussard design (collection of fuel as it moves through the interstellar medium) it retains the advantage of not having to accelerate the mass of the fuel and the mass of the rocket at the same time.
- The prelaunched fuel would provide some visibility into the interstellar medium – thus alerting the trailing spacecraft of unseen hazards (e.g. brown dwarfs).
The major disadvantages of this system include
- The spacecraft could not deviate from the precalculated trajectory unless it was critical to do so. Any such deviation would separate the spacecraft from its fuel supply and leave it with only a minimal ability to return to its original trajectory.
- Prelaunched fuel for deceleration at the destination star would not be available unless launched many decades in advance of the spacecraft launch. However, other systems (such as the Magnetic sails) could be used for this purpose.
- Heppenheimer, T.A. (1978). "On the Infeasibility of Interstellar Ramjets". Journal of the British Interplanetary Society 31: 222.
- Whitmire, Daniel P. (May–June 1975). "Relativistic Spaceflight and the Catalytic Nuclear Ramjet" (PDF). Acta Astronautica 2 (5-6): 497–509. doi:10.1016/0094-5765(75)90063-6.
- Further information on this RAIR concept can be found in the book "the star flight handbook" and at http://www.projectrho.com/public_html/rocket/slowerlight.php
- Discussed on Gilster, Paul (2004). Centauri Dreams: Imagining and Planning Interstellar Exploration. Springer. pp. 146–8. ISBN 978-0-387-00436-5. Also in the entry 'A Fusion Runway to Nearby Stars' from centauri-dreams.org.
- For more on ramjet math calculations see The Star Flight Handbook. 1989. ISBN 0-471-61912-4.
- John Ford Fishback; "Relativistic Interstellar Spaceflight"; Astronautica Acta 15 (November, 1969):25-35.
- The original paper is : Bussard, Robert W. (1960). "Galactic Matter and Interstellar Flight" (PDF). Astronautica Acta 6 (4): 179–194.