The Bussard ramjet is a theoretical method of spacecraft propulsion for interstellar travel. A fast moving spacecraft scoops up hydrogen from the interstellar medium using an enormous funnel-shaped magnetic field (ranging from kilometers to many thousands of kilometers in diameter); the hydrogen is compressed until thermonuclear fusion occurs, which provides thrust to counter the drag created by the funnel and energy to power the magnetic field. The Bussard ramjet can thus be seen as a ramjet variant of a fusion rocket.
The concept was popularized by Poul Anderson in his novel Tau Zero, Larry Niven in his Known Space series of books, Vernor Vinge in his Zones of Thought series, and Carl Sagan, as referenced in the television series and book Cosmos.
Since the time of Bussard's original proposal, it has been discovered that the region surrounding the Solar System has a much lower density of hydrogen than was believed at that time (see Local Interstellar Cloud). In 1969, John Ford Fishback made an important contribution, describing the details of the required magnetic field.
In 1978, 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 ramjet design in 1988. They determined that their version of the ramjet would be unable to accelerate into the solar wind.
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 consists of three subsystems: a fusion reactor, a scoop field, and a plasma accelerator. Fuel is launched ahead of the ship with the accelerator. The scoop field funnels the fuel into another 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 on board 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 Bussard ramjet would have an infinite supply of both. However, theory suggests that where a Bussard ramjet would suffer drag from having to pre-accelerate interstellar gas to its own speed before intake, a RAIR system would be able to transfer energy via the "accelerator" mechanism to the interstellar medium despite velocity differences, and so would suffer far less drag.
Laser Powered Interstellar Ramjet
Beamed energy coupled with a vehicle scooping hydrogen from the interstellar medium is another variant. A laser array in the solar system beams to a collector on a vehicle which uses something like a linear accelerator to produce thrust. This solves the fusion reactor problem for the ramjet. There are limitations because of the attenuation of beamed energy with distance.
The calculations (by Robert Zubrin and Dana Andrews) 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.
Dyson swarm-based stellar engine (Caplan thruster)
Astrophysicist Matthew E. Caplan of Illinois State University has proposed a type of stellar engine that uses a Dyson swarm of mirrors to concentrate stellar energy onto certain regions of a Sun-like star, producing beams of solar wind to be collected by a multi-ramjet assembly which in turn produces directed jets of plasma to stabilize its orbit and oxygen-14 to push the star. Using rudimentary calculations that assume maximum efficiency, Caplan estimates the Bussard engine would use 1015 grams per second of solar material to produce a maximum acceleration of 10−9 m/s2, yielding a velocity of 200 km/s after 5 million years, and a distance of 10 parsecs over 1 million years. The Bussard engine would theoretically work for 100 million years given the mass loss rate of the Sun, but Caplan deems 10 million years to be sufficient for a stellar collision avoidance.
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, saving the cost to launch the fuel mass), it at least 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.
- Bussard, Robert W. (1960). Galactic Matter and Interstellar Flight (PDF). Astronautica Acta. Vol. 6. pp. 179–195. Archived from the original (PDF) on 2018-04-17. Retrieved 2014-10-04.
- Fishback, J. F. (1969). "Relativistic interstellar spaceflight". Astronautica Acta. 15: 25–35. Bibcode:1969AsAc...15...25F.
- Heppenheimer, T.A. (1978). "On the Infeasibility of Interstellar Ramjets". Journal of the British Interplanetary Society. 31: 222. Bibcode:1978JBIS...31..222H.
- 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. CiteSeerX 10.1.1.492.6775. doi:10.1016/0094-5765(75)90063-6. Archived from the original (PDF) on 2018-10-31. Retrieved 2009-08-30.
- Andrews, D.G.; Zubrin, R.M. (1988). Magnetic sails and interstellar travel. 39th International Astronautical Congress, Bangalore. Art. IAF Paper IAF-88-533.
- Schattschneider, Peter; Jackson, Albert A. (February 2022). "The Fishback ramjet revisited". Acta Astronautica. 191: 227–234. Bibcode:2022AcAau.191..227S. doi:10.1016/j.actaastro.2021.10.039.
- "Study: 1960 ramjet design for interstellar travel—a sci-fi staple—is unfeasible". arstechnica. 2022-01-06.
- "Innovative Technologies from Science Fiction for Space Applications" (PDF). esa.it. p. 13. Retrieved May 2, 2023.
- 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.
- Powell, C. (1976). "System Optimization for the Ram Augmented Interstellar Rocket". Journal of the British Interplanetary Society. 29 (2): 136. Bibcode:1976JBIS...29..136P.
- 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.
- 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
- Whitmire, D.; Andrew Jackson (1977). "Laser Powered Interstellar Ramjet". Journal of the British Interplanetary Society. 30: 223–226. Bibcode:1977JBIS...30..223W.
- Perakis, N.; Andreas M. Hein (2016). "Combining Magnetic and Electric Sails for Interstellar Deceleration". Cornell University. 128: 13–20. arXiv:1603.03015. Bibcode:2016AcAau.128...13P. doi:10.1016/j.actaastro.2016.07.005. S2CID 17732634.
- Caplan, Matthew (December 17, 2019). "Stellar engines: Design considerations for maximizing acceleration". Acta Astronautica. 165: 96–104. Bibcode:2019AcAau.165...96C. doi:10.1016/j.actaastro.2019.08.027. S2CID 203111659. Archived from the original on December 23, 2019. Retrieved December 22, 2019. Alt URL
- 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.
- For more on ramjet math calculations see Mallove, Eugene F.; Matloff, Gregory L. (1989). The Star Flight Handbook. Wiley. ISBN 978-0-471-61912-3.
- Mallove, Eugene (1989). The Starflight Handbook. John Wiley & Sons, Inc. ISBN 978-0-471-61912-3.
- Matloff, Gregory (August 5, 2005). Deep Space Probes: To the Outer Solar System and Beyond. Springer Praxis Books. ISBN 978-3540247722.
- Anderson, Poul (2006). Tau Zero. Gollancz. ISBN 978-1407239132.