View source for Nuclear pulse propulsion

{{Short description|Hypothetical spacecraft propulsion through continuous nuclear explosions for thrust}}
{{Distinguish|Pulsed nuclear thermal rocket}}
[[File:NASA-project-orion-artist.jpg|thumb|right|300px|An artist's conception of the [[Project Orion (nuclear propulsion)|Project Orion]] "basic" spacecraft, powered by nuclear pulse propulsion.]]
'''Nuclear pulse propulsion''' or '''external pulsed plasma propulsion''' is a hypothetical method of [[spacecraft propulsion]] that uses [[nuclear explosion]]s for [[thrust]].<ref>{{cite web |title=External Pulsed Plasma Propulsion (EPPP) Analysis Maturation |last=Bonometti |first=Joseph A. |author2=P. Jeff Morton |publisher=NASA Marshall Space Flight Center |url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20000097368_2000138015.pdf |access-date=December 24, 2008}}</ref> It originated as [[Project Orion (nuclear propulsion)|Project ''Orion'']] with support from [[DARPA]], after a suggestion by [[Stanisław Ulam]] in 1947.<ref>{{cite web |title=History of Project Orion |website=The Story of Orion |date=2008–2009 |url=http://www.oriondrive.com/p1_story_history.php}}</ref> Newer designs using [[inertial confinement fusion]] have been the baseline for most later designs, including [[Project Daedalus|Project ''Daedalus'']] and [[Project Longshot|Project ''Longshot'']].

==History==

===Los Alamos===
{{Main|Los Alamos National Laboratory}}

Calculations for a potential use of this technology were made at the laboratory from and toward the close of the 1940s to the mid-1950s.<ref>{{cite journal |last1=Nance |first1=JC |title=Nuclear-Pulse Propulsion – II |journal=Annals of the New York Academy of Sciences |date=December 1966 |volume=140 |issue=1 |pages=396–406 |bibcode=1966NYASA.140..396N |s2cid=85148140 |doi=10.1111/j.1749-6632.1966.tb50975.x |url=https://nyaspubs.onlinelibrary.wiley.com/doi/abs/10.1111/j.1749-6632.1966.tb50975.x |access-date=2 July 2021 |quote=Research discussed in this paper was sponsored by Air Force Special Weapons Center, Kirtland Air Force Base, New Mexico, Air Force Systems Command, USAF, under Contract AF29(601)-6214.|url-access=subscription }}</ref>

===Project Orion===
{{Main|Project Orion (nuclear propulsion)}}
[[File:Orion pulse unit.png|thumb|A nuclear pulse propulsion unit. The explosive charge [[radiation implosion|ablatively vaporizes]] the propellant, propelling it away from the charge, and simultaneously creating a plasma out of the propellant. The propellant then goes on to impact the pusher plate at the bottom of the Orion spacecraft, imparting a pulse of 'pushing' energy.]]

Project Orion was the first serious attempt to design a nuclear pulse rocket. A design was formed at [[General Atomics]] during the late 1950s and early 1960s, with the idea of reacting small directional nuclear explosives utilizing a variant of the [[Teller–Ulam]] two-stage bomb design against a large steel pusher plate attached to the spacecraft with shock absorbers. Efficient directional explosives maximized the momentum transfer, leading to [[specific impulse]]s in the range of {{convert|6,000|isp|abbr=on}} seconds, or about thirteen times that of the [[Space Shuttle main engine]]. With refinements a theoretical maximum specific impulse of {{convert|100,000|isp|abbr=on}} (1 MN·s/kg) might be possible. Thrusts were in the millions of [[ton]]s, allowing spacecraft larger than 8{{X10^|6}} tons to be built with 1958 materials.<ref name="GeneralDynamics1964NuclearPulseVehicleCondensed">{{cite web |title=Nuclear Pulse Vehicle Study Condensed Summary Report (General Dynamics Corp.) |date=January 1964 |author=[[General Dynamics]] Corp. |publisher=U.S. Department of Commerce National Technical Information Service |url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19760065935_1976065935.pdf |access-date=December 24, 2008}}</ref>

The [[reference design]] was to be constructed of steel using submarine-style construction with a crew of more than 200 and a vehicle takeoff weight of several thousand [[ton]]s. This single-stage reference design would reach Mars and return in four weeks from the Earth's surface (compared to 12 months for NASA's current chemically powered reference mission). The same craft could visit Saturn's moons in a seven-month mission (compared to chemically powered missions of about nine years). Notable engineering problems that occurred were related to crew shielding and pusher-plate lifetime.

Although the system appeared to be workable, the project was shut down in 1965, primarily because the [[Partial Test Ban Treaty]] made it illegal; in fact, before the treaty, the US and Soviet Union had already separately detonated a combined number of at least nine nuclear bombs, including thermonuclear, in space, i.e., at altitudes of over 100&nbsp;km (see [[high-altitude nuclear explosion]]s). [[Nuclear ethics|Ethical]] issues complicated the launch of such a vehicle within the Earth's [[magnetosphere]]: calculations using the (disputed) [[linear no-threshold model]] of radiation damage showed that the [[fallout]] from each takeoff would cause the death of approximately 1 to 10 individuals.<ref>{{cite book |last=Dyson |first=George |title=Project Orion: the atomic spaceship, 1957–1965 |date=2003 |publisher=Penguin |isbn=0-14-027732-3 |location=London |oclc=51109229 }}</ref> In a threshold model, such extremely low levels of thinly distributed radiation would have no associated ill-effects, while under [[hormesis]] models, such tiny doses would be negligibly beneficial.<ref>{{cite journal |author=Heyes |display-authors=etal |title=Authors' reply |date=1 October 2006 |journal=British Journal of Radiology |volume=79 |issue=946 |pages=855–857 |doi=10.1259/bjr/52126615 |url=http://bjr.birjournals.org/cgi/content/citation/79/946/855 |access-date=27 March 2008|url-access=subscription }}</ref><ref>{{cite report |author=Aurengo |display-authors=etal |title=Dose-effect relationships and estimation of the carcinogenic effects of low doses of ionizing radiation |publisher=Académie des Sciences & Académie nationale de Médecine |date=30 March 2005 |citeseerx=10.1.1.126.1681 |type=PDF |url=https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.126.1681&rep=rep1&type=pdf |access-date=27 March 2008 |url-status=live |archive-url=https://web.archive.org/web/20220205043705/http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.126.1681&rep=rep1&type=pdf |archive-date=5 February 2022}}</ref> The use of less efficient [[nuclear weapon design#Clean bombs|clean nuclear bombs]] for achieving orbit and then more efficient, higher yield [[dirty bomb#Other uses of the term|dirtier bombs]] for travel would significantly reduce the amount of fallout caused from an Earth-based launch.

{{Anchor|deflect2016-01-30}}One useful mission would be to deflect an asteroid or comet on collision course with the Earth, depicted dramatically in the 1998 film ''[[Deep Impact (film)|Deep Impact]]''. The high performance would permit even a late launch to succeed, and the vehicle could effectively transfer a large amount of [[kinetic energy]] to the asteroid by simple impact.<ref>{{cite journal |last=Solem |first=J. C. |year=1994 |title=Nuclear explosive propelled interceptor for deflecting objects on collision course with Earth |journal=Journal of Spacecraft and Rockets |volume=31 |issue=4 |pages=707–709 |bibcode=1994JSpRo..31..707S |doi=10.2514/3.26501}}</ref> The prospect of an imminent asteroid impact would obviate concerns over the few predicted deaths from fallout. An automated mission would remove the challenge of designing a shock absorber that would protect the crew.

Orion is one of very few interstellar space drives that could theoretically be constructed with available technology, as discussed in a 1968 paper, "Interstellar Transport" by [[Freeman Dyson]].

===Project Daedalus===
{{Main|Project Daedalus}}
Project Daedalus was a study conducted between 1973 and 1978 by the [[British Interplanetary Society]] (BIS) to design an interstellar uncrewed spacecraft that could reach a nearby star within about 50 years. A dozen scientists and engineers led by [[Alan Bond (engineer)|Alan Bond]] worked on the project. At the time [[nuclear fusion|fusion]] research appeared to be making great strides, and in particular, [[inertial confinement fusion]] (ICF) appeared to be adaptable as a rocket engine.

ICF uses small pellets of fusion fuel, typically [[Lithium hydride#Lithium deuteride|lithium deuteride]] (<sup>6</sup>Li<sup>2</sup>H) with a small [[deuterium]]/[[tritium]] trigger at the center. The pellets are thrown into a reaction chamber where they are hit on all sides by [[laser]]s or another form of beamed energy. The heat generated by the beams explosively compresses the pellet to the point where fusion takes place. The result is a hot [[Plasma (physics)|plasma]], and a very small "explosion" compared to the minimum size bomb that would be required to instead create the necessary amount of fission.

For Daedalus, this process was to be run within a large [[electromagnet]] that formed the rocket engine. After the reaction, ignited by electron beams, the magnet funnelled the hot gas to the rear for thrust. Some of the energy was diverted to run the ship's systems and engine. In order to make the system safe and energy efficient, Daedalus was to be powered by a [[helium-3]] fuel collected from [[Jupiter]].

===''Medusa''===
[[File:MedusaNuclearPropulsionConceptDrawing.png|thumb|left|upright=1.36|Conceptual diagram of a Medusa propulsion spacecraft, showing: '''(A)''' the payload capsule, '''(B)''' the winch mechanism, '''(C)''' the optional main tether cable, '''(D)''' riser tethers, and '''(E)''' the parachute mechanism.]]
[[File:MedusaNuclearPropulsionOperatingSequenceDrawing.png|thumb|right|Operating sequence of the ''Medusa'' propulsion system. This diagram shows the operating sequence of a ''Medusa'' propulsion spacecraft '''(1)''' Starting at moment of explosive-pulse unit firing, '''(2)''' As the explosive pulse reaches the parachute canopy, '''(3)''' Pushes the canopy, accelerating it away from the explosion as the spacecraft plays out the main tether with the winch, generating electricity as it extends, and accelerating the spacecraft, '''(4)''' And finally winches the spacecraft forward to the canopy and uses excess electricity for other purposes.]]

The ''Medusa'' design has more in common with [[solar sail]]s than with conventional rockets. It was envisioned by [[Johndale Solem]]<ref>{{cite book |last=Gilster |first=Paul |title=Centauri Dreams: Imagining and Planning Interstellar Exploration |publisher=Copernicus Books, Atlanta Book Company |year=2004 |isbn=978-0387004365 |page=86 |url={{google books|plainurl=y|id=57HbBwAAQBAJ}} }}</ref> in the 1990s and published in the ''[[Journal of the British Interplanetary Society]]'' (JBIS).<ref>{{cite journal |last=Solem |first=J. C. |title=Medusa: Nuclear explosive propulsion for interplanetary travel |journal=Journal of the British Interplanetary Society |volume=46 |number=1 |pages=21–26 |date=January 1993 |bibcode=1993JBIS...46R..21S |issn=0007-084X}}</ref>

A ''Medusa'' spacecraft would deploy a large sail ahead of it, attached by independent cables, and then launch nuclear explosives forward to detonate between itself and its sail. The sail would be accelerated by the plasma and photonic impulse, running out the tethers as when a fish flees a fisher, generating electricity at the "reel". The spacecraft would use some of the generated electricity to reel itself up toward the sail, constantly smoothly accelerating as it goes.<ref>{{cite journal |last=Solem |first=J. C. |title=Nuclear explosive propulsion for interplanetary travel: Extension of the ''Medusa'' concept for higher specific impulse |journal=Journal of the British Interplanetary Society |volume=47 |number=6 |pages=229–238 |date=June 1994 |bibcode=1994JBIS...47..229S |issn=0007-084X}}</ref>

In the original design, multiple tethers connected to multiple motor generators. The advantage over the single tether is to increase the distance between the explosion and the tethers, thus reducing damage to the tethers.

For heavy payloads, performance could be improved by taking advantage of lunar materials, for example, wrapping the explosive with lunar rock or water, stored previously at a stable [[Lagrangian point|Lagrange point]].<ref>{{cite journal |last=Solem |first=J. C. |title=The Moon and the Medusa: Use of Lunar Assets in Nuclear-Pulse-Propelled Space Travel |journal=Journal of the British Interplanetary Society |volume=53 |number=1 |pages=362–370 |date=2000 |bibcode=2000JBIS...53..362S |url=http://www.jbis.org.uk/paper.php?p=2000.53.362 |access-date=2015-12-02 |archive-date=2019-02-27 |archive-url=https://web.archive.org/web/20190227060241/http://www.jbis.org.uk/paper.php?p=2000.53.362 |url-status=dead }}</ref>

''Medusa'' performs better than the classical Orion design because its sail intercepts more of the explosive impulse, its shock-absorber stroke is much longer, and its major structures are in tension and hence can be quite lightweight. ''Medusa''-type ships would be capable of a [[specific impulse]] of {{convert|50,000|-|100,000|isp|abbr=on}} (500 to 1000&nbsp;kN·s/kg).

''Medusa'' became widely known to the public in the BBC documentary film [[To Mars By A-Bomb (film)|''To Mars By A-Bomb: The Secret History of Project Orion'']].<ref>{{Citation|last=Sykes|first=Christopher|title=To Mars by A-Bomb: The Secret History of Project Orion|date=2003-03-26|url=https://www.imdb.com/title/tt1039992/|type=Documentary|others=Jaromir Astl, Jeremy Bernstein, Arthur C. Clarke, Ed Creutz|publisher=British Broadcasting Corporation |access-date=2021-06-04}}</ref> A short film shows an artist's conception of how the ''Medusa'' spacecraft works "by throwing bombs into a sail that's ahead of it".<ref>Archived at [https://ghostarchive.org/varchive/youtube/20211211/mT4ZdPRADEw Ghostarchive]{{cbignore}} and the [https://web.archive.org/web/20141016072034/http://www.youtube.com/watch?v=mT4ZdPRADEw Wayback Machine]{{cbignore}}: {{Citation|title=The Medusa: An advanced nuclear pulse spacecraft|url=https://www.youtube.com/watch?v=mT4ZdPRADEw|language=en|access-date=2021-06-04|last=Stevens|first=Nick|year=2014}}{{cbignore}}</ref>

===Project Longshot===
{{Main|Project Longshot}}
Project Longshot was a [[NASA]]-sponsored research project carried out in conjunction with the [[US Naval Academy]] in the late 1980s.<ref>{{cite web |last=Beals |first=Keith A. |display-authors=etal |title=Project Longshot An Unmanned Probe To Alpha Centauri |publisher=[[NASA]] |url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19890007533_1989007533.pdf |access-date=March 14, 2011}}</ref> ''Longshot'' was in some ways a development of the basic Daedalus concept, in that it used magnetically funneled ICF. The key difference was that they felt that the reaction could not power both the rocket and the other systems, and instead included a 300&nbsp;kW conventional [[nuclear reactor]] for running the ship. The added weight of the reactor reduced performance somewhat, but even using [[Lithium hydride|LiD]] fuel it would be able to reach neighboring star [[Alpha Centauri]] in 100 years (approx. velocity of 13,411&nbsp;km/s, at a distance of 4.5 light years, equivalent to 4.5% of light speed).

===Antimatter-catalyzed nuclear reaction===
{{Main|Antimatter-catalyzed nuclear pulse propulsion}}

In the mid-1990s, research at [[Pennsylvania State University]] led to the concept of using [[antimatter]] to catalyze nuclear reactions. [[Antiproton]]s would react inside the nucleus of [[uranium]], releasing energy that breaks the nucleus apart as in conventional nuclear reactions. Even a small number of such reactions can start the [[chain reaction]] that would otherwise require a much larger volume of fuel to sustain. Whereas the "normal" [[Critical mass (nuclear)|critical mass]] for [[plutonium]] is about 11.8 kilograms (for a sphere at standard density), with antimatter catalyzed reactions this could be well under one gram.

Several rocket designs using this reaction were proposed, some which would use all-fission reactions for interplanetary missions, and others using fission-fusion (effectively a very small version of Orion's bombs) for interstellar missions.

===Magneto-inertial fusion===
{{infobox rocket engine
|name            = MSNW magneto-inertial fusion driven rocket
|image           = The Fusion Driven Rocket powered spacecraft.jpg
|image_size      = 300px
|caption         = Concept graphic of a fusion-driven rocket powered spacecraft arriving at Mars
|country_of_origin=
|date            =
|first_date      =
|last_date       =
|designer        = MSNW LLC
|manufacturer    =
|purpose         = Interplanetary
|associated      =
|predecessor     =
|successor       =
|status          = Theoretical
|specific_impulse= 1,606 s to 5,722 s (depending on fusion gain)
|burn_time       = 1 day to 90 days (10 days optimal with gain of 40)
|references = <ref name="sloughetal">{{cite web |last1=Slough |first1=John |last2=Pancotti |first2=Anthony |last3=Kirtley |first3=David |last4=Pihl |first4=Christopher |last5=Pfaff |first5=Michael |title=Nuclear Propulsion through Direct Conversion of Fusion Energy: The Fusion Driven Rocket |publisher=NASA |date=September 30, 2012 |pages=1–31 |url=https://www.nasa.gov/pdf/716077main_Slough_2011_PhI_Fusion_Rocket.pdf |access-date=December 20, 2015 |archive-date=July 31, 2022 |archive-url=https://web.archive.org/web/20220731111638/https://www.nasa.gov/pdf/716077main_Slough_2011_PhI_Fusion_Rocket.pdf |url-status=dead }}</ref>
|notes           = {{plainlist|
* '''Fuel''':           Deuterium-tritium cryogenic pellet
* '''Propellant''': Lithium or aluminum
* '''Power requirements''': 100 kW to 1,000 kW}}
}}
NASA funded MSNW LLC and the [[University of Washington]] in 2011 to study and develop a [[fusion rocket]] through the NASA Innovative Advanced Concepts [[NASA Innovative Advanced Concepts|NIAC]] Program.<ref>{{cite web |title=Nuclear Propulsion Through Direct Conversion of Fusion Energy |last=Hall |first=Loura |date=July 13, 2017 |website=NASA |url=http://www.nasa.gov/directorates/spacetech/niac/2011_nuclear_propulsion}}</ref>

The rocket uses a form of [[magneto-inertial fusion]] to produce a direct thrust fusion rocket. Magnetic fields cause large metal rings to collapse around the [[deuterium]]-[[tritium]] plasma, triggering fusion. The energy heats and ionizes the shell of metal formed by the crushed rings. The hot ionized metal is shot out of a magnetic rocket nozzle at a high speed (up to 30&nbsp;km/s). Repeating this process roughly every minute would accelerate or decelerate the spacecraft.<ref>{{cite conference |title=Nuclear Propulsion based on Inductively Driven Liner Compression of Fusion Plasmoids |last1=Slough |first1=J. |last2=Kirtley |first2=D. |conference=AIAA Aerospace Sciences Conference |year=2011 |url=http://msnwllc.com/Papers/FDR_AIAA_2011.pdf}}</ref> The fusion reaction is not self-sustaining and requires electrical energy to explode each pulse. With electrical requirements estimated to be between 100&nbsp;kW to 1,000&nbsp;kW (300&nbsp;kW average), designs incorporate solar panels to produce the required energy.<ref name="sloughetal"/>

Foil Liner Compression creates fusion at the proper energy scale. The proof of concept experiment in Redmond, Washington, was to use aluminum liners for compression. However, the ultimate design was to use lithium liners.<ref>{{cite conference |title=Mission Design Architecture for the Fusion Driven Rocket |last1=Pancotti |first1=A. |last2=Slough |first2=J. |last3=Kirtley |first3=D. |conference=AIAA Joint Propulsion Conference |year=2012 |url=http://msnwllc.com/Papers/FDR_JPC_2012.pdf}}</ref><ref>{{cite news |title=Scientists develop fusion rocket technology in lab – and aim for Mars |work=NBC News |date=April 5, 2013 |last=Boyle |first=Alan |url=http://www.nbcnews.com/science/scientists-develop-fusion-rocket-technology-lab-aim-mars-1B9235633}}</ref>

Performance characteristics are dependent on the [[fusion gain|fusion energy gain factor]] achieved by the reactor. Gains were expected to be between 20 and 200, with an estimated average of 40. Higher gains produce higher exhaust velocity, higher specific impulse and lower electrical power requirements. The table below summarizes different performance characteristics for a theoretical 90-day Mars transfer at gains of 20, 40, and 200.

{| class="wikitable"
|+ |FDR parameters for 90 Mars transfer burn<ref name="sloughetal"/>
|-
! Total gain !! Gain of 20 !! Gain of 40 !! Gain of 200
|-
| Liner mass (kg) || 0.365 || 0.365 || 0.365
|-
| Specific impulse (s) || 1,606 || 2,435 || 5,722
|-
| Mass fraction || 0.33 || 0.47 || 0.68
|-
| Specific mass (kg/kW) || 0.8 || 0.53 || 0.23
|-
| Mass propellant (kg) || 110,000 || 59,000 || 20,000
|-
| Mass initial (kg) || 184,000 || 130,000 || 90,000
|-
| Electrical power required (kW) || 1,019 || 546 || 188
|}

By April 2013, MSNW had demonstrated subcomponents of the systems: heating [[deuterium]] [[plasma (physics)|plasma]] up to fusion temperatures and concentrating the magnetic fields needed to create fusion. They planned to put the two technologies together for a test before the end of 2013.<ref name="sloughetal"/><ref name=ps20130408>{{cite news |last=Diep |first=Francie |title=Fusion Rocket Would Shoot People To Mars In 30 Days |work=Popular Science |date=2013-04-08 |url=http://www.popsci.com/science/article/2013-04/fusion-rocket-idea-would-shoot-people-mars |access-date=2013-04-12}}</ref><ref>{{cite conference |title=The Fusion Driven Rocket |last1=Slough |first1=J. |last2=Pancotti |first2=A. |last3=Kirtley |first3=D. |last4=Pfaff |first4=M. |last5=Pihl |first5=C. |last6=Votroube |first6=G. |conference=NASA NIAC (Phase II) Symposium |date=November 2012 |url=http://www.msnwllc.com/Papers/NIAC_PhaseII_FDR.pdf}}</ref>

===Pulsed fission-fusion propulsion===
Pulsed fission-fusion (PuFF) propulsion is reliant on principles similar to magneto-inertial fusion. It aims to solve the problem of the extreme stress induced on containment by an Orion-like motor by ejecting the plasma obtained from small fuel pellets that undergo autocatalytic fission and fusion reactions initiated by a [[Z-pinch]]. It is a theoretical propulsion system researched through the NIAC Program by the [[University of Alabama in Huntsville]].<ref>{{cite web |last=Adams |first=Robert B. |date=2013 |title=Pulsed Fission-Fusion (PuFF) – Phase I Report |website=nasa.gov |url=https://www.nasa.gov/sites/default/files/files/Adams_2013_PhI_PuFF_inProgress.pdf |access-date=7 February 2021 |url-status=live |archive-url=https://web.archive.org/web/20220414171413/https://www.nasa.gov/sites/default/files/files/Adams_2013_PhI_PuFF_inProgress.pdf |archive-date=14 April 2022}}</ref> It is in essence a fusion rocket that uses a Z-pinch configuration, but coupled with a fission reaction to boost the fusion process.

A PuFF fuel pellet, around 1&nbsp;cm in diameter,<ref>{{cite web |last1=Adams |first1=Robert B. |first2=Jason |last2=Cassibry |first3= Kevin |last3=Schillo |title=The Pulsed Fission-Fusion (PuFF) Concept for Deep Space Exploration and Terrestrial Power Generation |website=nasa.gov |url=https://www.nasa.gov/sites/default/files/atoms/files/the_pulsed_fission_-_fusion_puff_concept_for_deep_space_exploration_and_terrestrial_power.pdf |url-status=live |archive-url=https://web.archive.org/web/20220414171503/https://www.nasa.gov/sites/default/files/atoms/files/the_pulsed_fission_-_fusion_puff_concept_for_deep_space_exploration_and_terrestrial_power.pdf |archive-date=14 April 2022}}</ref> consists of two components: A deuterium-tritium (D-T) cylinder of plasma, called the ''target'', which undergoes fusion, and a surrounding [[U-235]] sheath that undergoes fission enveloped by a lithium liner. Liquid lithium, serving as a moderator, fills the space between the D-T cylinder and the uranium sheath. Current is run through the liquid lithium, a [[Lorentz force]] is generated which then compresses the D-T plasma by a factor of 10 in what is known as a Z-pinch. The compressed plasma reaches criticality and undergoes fusion reactions. However, the fusion energy gain (''Q'') of these reactions is far below breakeven (''Q'' < 1), meaning that the reaction consumes more energy than it produces.

In a PuFF design, the fast neutrons released by the initial fusion reaction induce fission in the U-235 sheath. The resultant heat causes the sheath to expand, increasing its implosion velocity onto the D-T core and compressing it further, releasing more fast neutrons. Those again amplify the fission rate in the sheath, rendering the process autocatalytic. It is hoped that this results in a complete burn up of both the fission and fusion fuels, making PuFF more efficient than other nuclear pulse concepts.<ref>{{cite journal |last=Winterberg |first=Friedwart |date=2000 |title=Autocatalytic fission–fusion microexplosions for nuclear pulse propulsion |journal=Acta Astronautica |volume=47 |issue=12 |pages=879–883 |bibcode=2000AcAau..47..879W |via=Elsevier Science Direct |doi=10.1016/S0094-5765(00)00136-3}}</ref><ref>{{cite journal |last=Adams |first=Robert B. |date=2014 |title=Developing the Pulsed Fission-Fusion (PuFF) Engine |journal=Propulsion and Energy Forum}}</ref> Much like in a magneto-inertial fusion rocket, the performance of the engine is dependent on the degree to which the fusion gain of the D-T target is increased.

One "pulse" consist of the injection of a fuel pellet into the combustion chamber, its consumption through a series of fission-fusion reactions, and finally the ejection of the released plasma through a magnetic nozzle, thus generating thrust. A single pulse is expected to take only a fraction of a second to complete.

==See also==
{{cmn|colwidth=30em|
* [[AIMStar]]
* [[Antimatter-catalyzed nuclear pulse propulsion]]
* [[Antimatter rocket]]
* [[Spacecraft electric propulsion]]
* [[Ion thruster]]
* [[Nuclear electric rocket]]
* [[Nuclear power in space]]
* [[Nuclear propulsion]]
* [[Nuclear thermal rocket]]
* [[Nuclear photonic rocket]]
* [[Project Pluto]]
* [[Pulsed nuclear thermal rocket]]
* [[Stellarator]]
}}

==References==
{{Reflist|30em}}

==External links==
{{Commons category|Nuclear pulse propulsion}}
* [https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20000096503.pdf G.R. Schmidt, J.A. Bunornetti and P.J. Morton, Nuclear Pulse Propulsion – Orion and Beyond, NASA technical report AlAA 2000-3856, 2000]
* [https://ieeexplore.ieee.org/document/4323511/ J. C. Nance, "Nuclear Pulse Propulsion," IEEE Trans. on Nuclear Science 12, 177 (1965) &#91;Reprinted as Ann. N.Y. Acad. Sci. 140, 396 (1966)&#93;.]
* [http://large.stanford.edu/courses/2012/ph241/klein2/docs/lams1955.pdf "Nuclear Pulse Space Vehicle Study, Vol III," Report on NASA Contract NAS 8-11053, General Atomics, GA-5009, 19 Sep 64.]
* [https://www.science.org/doi/abs/10.1126/science.149.3680.141 F. Dyson, "Death of a Project," Science 149, 141 (1965).]
* [https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19910017902.pdf W. H. Robbins and H. B. Finger, H.B., "An Historical Perspective of the NERVA Nuclear Rocket Engine Technology Program", NASA Contractor Report 187154, AIAA-91-3451, July 1991.]

{{Spacecraft propulsion}}
{{Nuclear propulsion}}
{{Emerging technologies|topics=yes|space=yes}}

[[Category:Nuclear spacecraft propulsion]]
[[Category:Plasma technology and applications]]