Editing Bussard ramjet

{{Use American English|date=January 2014}}
{{Short description|Proposed spacecraft propulsion method}}
[[Image:Bussard Interstellar Ramjet Engine.jpg|thumb|280px|Artist's conception of a Bussard ramjet. A major component of an actual ramjet &ndash; a miles-wide electromagnetic field &ndash; is invisible.]]
[[File:Bussard ramjet.gif|thumb|Bussard ramjet in motion. {{ordered list|[[Interstellar medium]] |Collect and compress [[hydrogen]] |Transport hydrogen beside the payload |[[Thermonuclear fusion]] |Engine nozzle |Flue gas jet}} ]]

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]].{{fact|date=August 2023}}

The Bussard ramjet was proposed in 1960 by the physicist [[Robert W. Bussard]].<ref>{{cite conference|last=Bussard |first=Robert W. |year=1960 |title=Galactic Matter and Interstellar Flight |url=http://www.askmar.com/Robert%20Bussard/Galactic%20Matter%20and%20Interstellar%20Flight.pdf |url-status=dead |journal=Astronautica Acta |volume=6 |pages=179–195 |archive-url=https://web.archive.org/web/20180417180616/http://www.askmar.com/Robert%20Bussard/Galactic%20Matter%20and%20Interstellar%20Flight.pdf |archive-date=2018-04-17 |access-date=2014-10-04}}</ref>

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 [[Cosmos (Sagan book)|book]] ''[[Cosmos: A Personal Voyage|Cosmos]]''.{{fact|date=August 2023}}

==Feasibility==
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.<ref>{{cite journal |last=Fishback |first=J. F. |year=1969 |title=Relativistic interstellar spaceflight |journal=Astronautica Acta |volume=15 |pages=25–35 |bibcode=1969AsAc...15...25F}}</ref> 

In 1978, [[T. A. Heppenheimer]] analyzed Bussard's original suggestion of fusing [[protons]], but found the [[Bremsstrahlung]] losses from compressing protons to [[nuclear fusion|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.<ref>{{cite journal |first=T.A. |last=Heppenheimer |title=On the Infeasibility of Interstellar Ramjets |journal=Journal of the British Interplanetary Society |volume=31 |page=222 |year=1978|bibcode = 1978JBIS...31..222H }}</ref> However, Daniel P. Whitmire's 1975 analysis<ref name="Whitmire1975">{{cite journal |first=Daniel P. |last=Whitmire |title=Relativistic Spaceflight and the Catalytic Nuclear Ramjet |journal=Acta Astronautica |volume=2 |issue=5–6 |date=May–June 1975 |pages=497–509 |doi=10.1016/0094-5765(75)90063-6 |url=http://www.askmar.com/Robert%20Bussard/Catalytic%20Nuclear%20Ramjet.pdf |bibcode=1975AcAau...2..497W |citeseerx=10.1.1.492.6775 |access-date=2009-08-30 |archive-url=https://web.archive.org/web/20181031134647/http://www.askmar.com/Robert%20Bussard/Catalytic%20Nuclear%20Ramjet.pdf |archive-date=2018-10-31 |url-status=dead }}</ref> indicates that a ramjet may achieve net power via the [[CNO cycle]], which produces fusion at a much higher rate (~10<sup>16</sup> times higher) than the [[proton–proton chain]].{{fact|date=August 2023}}

[[Robert Zubrin]] and Dana Andrews analyzed one hypothetical version of the Bussard ramjet design in 1988.<ref>{{Cite conference|last1=Andrews |first1=D.G. |last2=Zubrin |first2=R.M. |date=1988 |conference=39th [[International Astronautical Congress]], Bangalore|title=Magnetic sails and interstellar travel |article-number=IAF Paper IAF-88-533}}</ref> They determined that their version of the ramjet would be unable to accelerate into the solar wind.{{fact|date=August 2023}}

A 2021 study found that, while feasible in principle, the practical construction of a useful Bussard ramjet would be beyond even a civilization of [[Kardashev scale|Kardashev type II]].<ref>{{cite journal |title=The Fishback ramjet revisited |journal=Acta Astronautica |volume=191 |date=February 2022 |pages=227–234 |first1=Peter |last1=Schattschneider |first2=Albert A. |last2=Jackson |doi=10.1016/j.actaastro.2021.10.039|bibcode=2022AcAau.191..227S |doi-access=free }}</ref><ref name="ars-20220106">{{Cite news |last=Ouellette |first=Jennifer |date=January 6, 2022 |title=Study: 1960 ramjet design for interstellar travel—a sci-fi staple—is unfeasible |url=https://arstechnica.com/science/2022/01/study-1960-ramjet-design-for-interstellar-travel-a-sci-fi-staple-is-unfeasible/ |url-status=live |archive-url=https://web.archive.org/web/20240123142943/https://arstechnica.com/science/2022/01/study-1960-ramjet-design-for-interstellar-travel-a-sci-fi-staple-is-unfeasible/ |archive-date=January 23, 2024 |work=[[Ars Technica]] }}</ref>

==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 consists of three subsystems: a fusion reactor, a scoop field, and a plasma accelerator. Fuel is launched ahead of the ship with the accelerator.<ref>{{Cite web |title=Innovative Technologies from Science Fiction for Space Applications |url=https://www.esa.int/esapub/br/br176/br176.pdf |url-status=live |archive-url=https://web.archive.org/web/20231228070739/https://www.esa.int/esapub/br/br176/br176.pdf |archive-date=December 28, 2023 |access-date=May 2, 2023 |website=esa.it |page=13 }}</ref> 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 [[Working mass|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.<ref>{{cite journal |last=Bond |first=A. |year=1974 |title=An Analysis of the Potential Performance of the Ram Augmented Interstellar Rocket |journal=Journal of the British Interplanetary Society |volume=27 |pages=674–688|bibcode = 1974JBIS...27..674B }}</ref><ref>{{cite journal |last=Powell |first=C. |year=1976 |title=System Optimization for the Ram Augmented Interstellar Rocket |journal=Journal of the British Interplanetary Society |volume=29 |issue=2 |pages=136|bibcode = 1976JBIS...29..136P }}</ref><ref>{{cite journal |last=Jackson |first=A. |year=1980 |title=Some Considerations on the Antimatter and Fusion Ram Augmented Interstellar Rocket |journal=Journal of the British Interplanetary Society |volume=33 |pages=117–120|bibcode = 1980JBIS...33..117J }}</ref><ref>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</ref>

===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.<ref>{{cite journal |last=Whitmire |first=D. |author2 = Andrew Jackson |year=1977 |title= Laser Powered Interstellar Ramjet |journal=Journal of the British Interplanetary Society |volume=30 |pages=223–226|bibcode = 1977JBIS...30..223W }}</ref>

===Magnetic sail===
The calculations (by [[Robert Zubrin]] and Dana Andrews) inspired the idea of a [[magnetic sail|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.<ref>{{cite journal |last=Perakis |first=N. |author2 = Andreas M. Hein |year=2016 |title=Combining Magnetic and Electric Sails for Interstellar Deceleration |journal=Cornell University |volume=128 |pages=13–20 |doi=10.1016/j.actaastro.2016.07.005 |arxiv=1603.03015 |bibcode=2016AcAau.128...13P |s2cid=17732634 }}</ref>

===Dyson swarm-based stellar engine (Caplan thruster)===
{{anchor|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 10<sup>15</sup> grams per second of solar material to produce a maximum acceleration of 10<sup>−9</sup> m/s<sup>2</sup>, yielding a velocity of 200&nbsp;km/s after 5 million years, and a distance of 10 [[parsec]]s 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.<ref>{{cite journal |last=Caplan |first=Matthew |date=December 17, 2019 |title= Stellar engines: Design considerations for maximizing acceleration |url= https://sites.google.com/view/m-caplan-stellar-engines/startseite |journal=Acta Astronautica |volume=165 |pages=96–104 |archive-url= https://web.archive.org/web/20191223070252/https://sites.google.com/view/m-caplan-stellar-engines/startseite |archive-date=December 23, 2019 |access-date=December 22, 2019 |doi=10.1016/j.actaastro.2019.08.027 |bibcode=2019AcAau.165...96C |s2cid=203111659 }} [https://doi.org/10.1016/j.actaastro.2019.08.027 Alt URL]</ref>

===Pre-seeded trajectory===
Several of the obvious technical difficulties with the Bussard ramjet can be overcome by setting out solid pellets of fuel along the spacecraft's trajectory in advance.<ref name=":0">Discussed on {{cite book |last=Gilster |first=P. |year=2004 |title=Centauri Dreams: Imagining and Planning Interstellar Exploration |url=https://archive.org/details/springer_10.1007-978-1-4757-3894-0 |publisher=Springer |isbn=978-0-387-00436-5 |pages=[https://archive.org/details/springer_10.1007-978-1-4757-3894-0/page/n155 146]–8}} Also in the entry [http://www.centauri-dreams.org/?p=315 'A Fusion Runway to Nearby Stars'] from centauri-dreams.org.</ref> This could be done using a different "tanker" spacecraft dropping fuel pellets<ref name=":0" /><ref name=":1">{{Cite book |last=Matloff |first=Gregory L. |url=https://books.google.com/books?id=qFQSbAR4fMcC |title=Deep Space Probes: To the Outer Solar System and Beyond |date=2006-08-31 |publisher=Springer Science & Business Media |isbn=978-3-540-27340-0 |pages=118–120 |language=en}}</ref> or using laser propulsion.<ref name=":2">{{Cite web |title=Bussard buzz-bomb (Jordin Kare) |url=https://yarchive.net/space/exotic/bussard_buzz-bomb.html |access-date=2024-01-22 |website=yarchive.net}}</ref> The method has been referred to as the "fusion runway",<ref name=":0" /><ref name=":2" /> ramjet runway<ref name=":1" /> or "forward resupply runway".<ref name=":3">{{Cite journal |last1=Lenard |first1=Roger X. |last2=Lipinski |first2=Ronald J. |date=January 19, 2000 |title=Interstellar rendezvous missions employing fission propulsion systems |url=https://pubs.aip.org/aip/acp/article/504/1/1544-1555/578760 |journal=AIP Conference Proceedings |publisher=[[American Institute of Physics]] |volume=504 |issue=1 |pages=1544–1555 |doi=10.1063/1.1290979 |bibcode=2000AIPC..504.1544L |issn=1551-7616 }}</ref>

While most proposals use fusion power, as with conventional Bussard ramjets, fission has also been suggested.<ref name=":3" />

The advantages of this system include:

* Fusion can be achieved by slamming two fuel pellets together at a speed of at least 200 km/s - this is known as impact fusion. Compared to inertial confinement fusion, impact fusion would not require spherically symmetrical pellets or application of force from all directions.<ref name=":0" />
* The fuel pellets could use isotopes such as [[deuterium]] (hydrogen-2) or [[tritium]] (hydrogen-3).<ref name="nbf-20170620">{{Cite web |last=Wang |first=Brian |date=June 20, 2017 |title=Nuclear fusion pellet runway |url=https://www.nextbigfuture.com/2017/06/nuclear-fusion-pellet-runway.html |url-status=live |archive-url=https://web.archive.org/web/20240123025436/https://www.nextbigfuture.com/2017/06/nuclear-fusion-pellet-runway.html |archive-date=January 23, 2024 |access-date=January 22, 2024 |website=nextbigfuture.com }}</ref> A conventional Bussard ramjet will mostly collect [[protium]] (hydrogen-1), as this is the most abundant isotope of hydrogen in nature.
* 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.{{fact|date=August 2023}}
* 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.{{fact|date=August 2023}}
* 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.{{fact|date=August 2023}}
* The prelaunched fuel would provide some visibility into the interstellar medium &ndash; thus alerting the trailing spacecraft of unseen hazards (e.g., [[brown dwarf]]s).{{fact|date=August 2023}}

The major disadvantages of this system include:{{fact|date=August 2023}}

* The need to precisely intercept the fusion runway pellets with a spacecraft moving at high speed. Laser pulses could be used to guide the pellets.<ref name=":0" />
* If using impact fusion, the spacecraft must be moving at a minimum speed in order to achieve fusion. One suggestion is for the spacecraft to use a Sun-skimming maneuver to reach 600 km/s.<ref name=":0" />
* 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.{{fact|date=August 2023}}
* 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 sail]]s) could be used for this purpose.{{fact|date=August 2023}}

==See also==
*[[Fusion rocket]]
*[[Ramjet]]

==References==
{{reflist|1}}

==Bibliography==
* For more on ramjet math calculations see {{cite book |title=The Star Flight Handbook |year=1989 |isbn=978-0-471-61912-3 |url=https://books.google.com/books?id=6ZtTAAAAMAAJ|last1=Mallove |first1=Eugene F. |last2=Matloff |first2=Gregory L. |publisher=Wiley }}
* {{cite book | last = Mallove | first = Eugene | title=The Starflight Handbook | url = https://archive.org/details/starflighthandbo0000mall | url-access = registration | publisher = [[John Wiley & Sons, Inc.]] | date = 1989 | isbn = 978-0-471-61912-3 }}
* {{cite book | last =  Matloff   | first = Gregory  | title=Deep Space Probes: To the Outer Solar System and Beyond | publisher = Springer Praxis Books  | date =August 5, 2005   | isbn = 978-3540247722   }}
* {{cite book | last = Anderson   | first = Poul  | title=Tau Zero | publisher =  Gollancz  | date = 2006  | isbn = 978-1407239132   }}

==External links==
*[http://www.astronomynotes.com/ismnotes/s3.htm Interstellar Medium]
*[http://casswww.ucsd.edu/public/tutorial/ISM.html Interstellar Hydrogen]
*[http://www.bisbos.com/space_n_bussard_gallery.html Images of Bussard Ramjet]

{{spacecraft propulsion}}
{{Nuclear propulsion}}

{{DEFAULTSORT:Bussard Ramjet}}
[[Category:Hypothetical spacecraft]]
[[Category:Interstellar travel]]
[[Category:Nuclear spacecraft propulsion]]
[[Category:Hydrogen technologies]]
[[Category:Hypothetical technology]]
[[Category:1960 in science]]