Bussard ramjet

Artist's conception of a Bussard ramjet. A major component of an actual ramjet a miles-wide electromagnetic field is invisible.

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 [1] 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.

Feasibility

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). John Ford Fishback made an important contribution to the details for the Bussard ramjet in 1969,[2] 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.[3] However Daniel P. Whitmire's 1975 analysis[4] 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:

  1. The exhaust velocity of their interplanetary ion propulsion ramjet could not exceed 100,000 m/s (100 km/s);
  2. 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.

Related inventions

Ram Augmented Interstellar Rocket (RAIR)

The problem of using the interstellar medium as the sole fuel source led to study of the Ram Augmented Interstellar Rocket (RAIR). The RAIR carries its nuclear fuel supply and exhausts the reaction products to produce some of its thrust. However it greatly enhances its performance by scooping the interstellar medium and using this as extra reaction mass to augment the rocket. The propulsion system of the RAIR consist of three subsystems: a fusion reactor, a scoop field, and a plasma accelerator. The scoop field funnels interstellar gas into an "accelerator" (this could for example be a heat exchange system transferring thermal energy from the reactor directly to the interstellar gas)which is supplied power from a reactor. One of the best ways to understand this concept is to consider that the hydrogen nuclear fuel carried onbaord acts as a fuel (energy source) whereas the interstellar gas collected by the scoop and then exhausted at great speed from the back acts as a propellant (the reaction mass), the vehicle therefore has a limited fuel supply but an unlimited propellant supply. A normal Buassard ramjet would have an infinite supply of both, however theory suggests that where a bussard ramjet would suffer drag from the fact that the interstellar gas ahead of it would have to be accelerated to its speed before entering the fusion reactor a RAIR system would be able to transfer energy via the "accelerator" mechanism from the reactor to the interstellar gas without having to accelerate the interstellar gas up to the ship's speed before putting this gas through the "accelerator", so would suffer far less drag. [5][6][7][8]

Magnetic sail

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.

Pre-seeded trajectory

Several of the obvious technical difficulties with the Bussard Ramjet can be overcome by prelaunching fuel along the spacecraft's trajectory[9] using something like a magnetic rail-gun.

The advantages of this system include

The major disadvantages of this system include

References

  1. Bussard, Robert W. (1960). "Galactic Matter and Interstellar Flight" (PDF). Astronautica Acta 6: 179–195.
  2. Fishback, J. F. (1969). "Relativistic interstellar spaceflight". Astronautica Acta 15: 25–35. Bibcode:1969AcAau..15...25F.
  3. Heppenheimer, T.A. (1978). "On the Infeasibility of Interstellar Ramjets". Journal of the British Interplanetary Society 31: 222. Bibcode:1978JBIS...31..222H.
  4. Whitmire, Daniel P. (May–June 1975). "Relativistic Spaceflight and the Catalytic Nuclear Ramjet" (PDF). Acta Astronautica 2 (5-6): 497–509. Bibcode:1975AcAau...2..497W. doi:10.1016/0094-5765(75)90063-6.
  5. Bond, A. (1974). "An Analysis of the Potential Performance of the Ram Augmented Interstellar Rocket". Journal of the British Interplanetary Society 27: 674–688. Bibcode:1974JBIS...27..674B.
  6. Powell, C. (1976). "System Optimization for the Ram Augmented Interstellar Rocket". Journal of the British Interplanetary Society 29 (2): 136. Bibcode:1976JBIS...29..136P.
  7. Jackson, A. (1980). "Some Considerations on the Antimatter and Fusion Ram Augmented Interstellar Rocket". Journal of the British Interplanetary Society 33: 117–120. Bibcode:1980JBIS...33..117J.
  8. 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
  9. Discussed on Gilster, P. (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.

External links

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