View source for Project Orion (nuclear propulsion)


{{Short description|Discontinued US research program on the viability of nuclear pulse propulsion}}
{{About|the 1950s nuclear propulsion project|other spaceflight vehicles called Orion|Orion (disambiguation)#Space}}
{{Distinguish|Orion (spacecraft)}}
{{Use mdy dates|date=April 2012}}
{{Use American English|date=April 2012}}
[[File:NASA-project-orion-artist.jpg|thumb|right|NASA [[Space art|artist rendering]], from 1999, of the Project Orion pulsed [[nuclear fission]] spacecraft]]

'''Project Orion''' was a study conducted in the 1950s and 1960s by the [[United States]] [[United States Air Force|Air Force]], [[DARPA]],<ref>{{Cite web |title=Past Visionaries: von Braun and Project Orion {{!}} APPEL Knowledge Services |url=https://appel.nasa.gov/2010/03/04/aa_3-1_f_past-html/ |access-date=2023-08-03 |website=appel.nasa.gov|date=March 4, 2010 }}</ref> and [[NASA]] into the viability of a [[nuclear pulse propulsion|nuclear pulse]] spaceship that would be directly propelled by a series of [[Nuclear weapon#Fission weapons|atomic explosions]] behind the craft.<ref name=":0">{{Cite web |title=The Case For Orion |url=https://www.spacedaily.com/news/nuclearspace-03h.html |access-date=2023-08-03 |website=www.spacedaily.com}}</ref><ref name=":1">{{Cite web |title=Nuclear Pulse Propulsion: Gateway to the Stars |url=https://www.ans.org/news/article-1294/nuclear-pulse-propulsion-gateway-to-the-stars/ |access-date=2023-08-03 |website=www.ans.org |language=en}}</ref> Following preliminary ideas in the 1940s,<ref name=":2" /> and a classified paper co-authored by physicist [[Stanisław Ulam]] in 1955,<ref name="astronautica2" /> DARPA agreed to sponsor and fund the program in July 1958.<ref name="astronautica3" /><ref name=":5" />

Early versions of the vehicle were designed for [[Launch vehicle|ground launch]], but later versions were intended for use only in space. The design effort took place at [[General Atomics]] in San Diego,<ref name="astronautica2">{{cite encyclopedia |title=Project Orion: Its Life, Death, and Possible Rebirth |encyclopedia=Encyclopedia Astronautica |url=http://www.astronautix.com/articles/probirth.html |access-date=2012-01-11 |last=Flora |first=Michael |archive-url=https://web.archive.org/web/20111109175529/http://www.astronautix.com/articles/probirth.htm |archive-date=November 9, 2011 |url-status=dead}}</ref> and supporters included [[Wernher von Braun]],<ref name=":4" /> who issued a [[white paper]] advocating the idea.<ref name=":0" /><ref>{{Cite book |last=Dyson |first=George |url=https://www.google.com/books/edition/Project_Orion/4S2KocYp8AkC?hl |title=Project Orion: the true story of the atomic spaceship |date=2002 |publisher=[[Henry Holt and Company]] |isbn=978-0-8050-5985-4 |edition= |location=New York, NY |pages=254}}</ref> NASA also created a [[Human mission to Mars|Mars mission]] profile based on the design, proposing a 125 day round trip carrying eight astronauts with a predicted development cost of $1.5 billion.<ref name=":4" /> Non-nuclear tests were conducted with models, with the most successful test occurring in late 1959,<ref name=":5">{{Cite web |last=Ferguson |first=Reuben David |date=2002 |title=Brief history of Project Orion {{!}} fau.digital.flvc.org |url=http://fau.digital.flvc.org/islandora/object/fau:9755 |access-date=2023-08-03 |website=fau.digital.flvc.org |pages=26–28}}</ref> but the project was ultimately abandoned for reasons including the 1963 [[Partial Test Ban Treaty]],<ref name=":3">{{Cite web |title=Model, Spacecraft, Orion Nuclear Pulse {{!}} National Air and Space Museum |url=https://airandspace.si.edu/collection-objects/model-spacecraft-orion-nuclear-pulse/nasm_A19790892000 |access-date=2023-08-03 |website=airandspace.si.edu |publisher=[[Smithsonian]] |language=en}}</ref> which prohibited [[Nuclear testing in space|nuclear explosions in space]] amid concerns over [[Nuclear fallout|radioactive fallout]].<ref name=":0" />

Physicists [[Ted Taylor (physicist)|Ted Taylor]] and [[Freeman Dyson]] led the project, and Taylor has been described as the "driving force behind Orion."<ref name="astronautica3" /> In 1979, [[General Dynamics]] donated a 26 inch (56 cm) tall wooden model of the craft to the [[Smithsonian Institution|Smithsonian]], which displays it at the [[Steven F. Udvar-Hazy Center]] in [[Fairfax County, Virginia|Fairfax County]], [[Virginia]].<ref name=":3" />

== Background and planning ==
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Physicist [[Stanislaw Ulam]] proposed the general idea of nuclear pulse propulsion in 1946,<ref name=":2" /> and preliminary calculations were made by [[F. Reines|Frederick Reines]] and Ulam in a [[Los Alamos National Laboratory|Los Alamos]] memorandum dated 1947.<ref name=":2">{{cite web |author1=Everett, C.J. |author2=Ulam S.M. |title=On a Method of Propulsion of Projectiles by Means of External Nuclear Explosions. Part I |publisher=Los Alamos Scientific Laboratory |date=August 1955 |page=5 |url=https://apps.dtic.mil/sti/pdfs/ADA306631.pdf |archive-url=https://web.archive.org/web/20120725080421/http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA306631&Location=U2&doc=GetTRDoc.pdf|url-status=live |archive-date=2012-07-25}}</ref><ref name=":1" /><ref>{{cite web |date=2008–2009 |title=History of Project Orion |url=http://www.oriondrive.com/p1_story_history.php |website=The Story of Orion}}</ref> In August 1955, Ulam co-authored a classified paper proposing the use of nuclear fission bombs, "ejected and detonated at a considerable distance", for propelling a vehicle in outer space.<ref name="astronautica2" /><ref name=":2" /> The project was led by [[Ted Taylor (physicist)|Ted Taylor]] at General Atomics and physicist [[Freeman Dyson]] who, at Taylor's request, took a year away from the [[Institute for Advanced Study]] in [[Princeton, New Jersey|Princeton]] to work on the project.<ref name="astronautica3">{{cite encyclopedia |title=Project Orion: Its Life, Death, and Possible Rebirth |encyclopedia=Encyclopedia Astronautica |url=http://astronautix.com/p/projectoriossiblerebirth.html |access-date=2012-01-11 |last=Flora |first=Michael |archive-url=https://web.archive.org/web/20111109175529/http://www.astronautix.com/articles/probirth.htm |archive-date=November 9, 2011 |url-status=dead}}</ref>

In July 1958, DARPA agreed to sponsor Orion at an initial level of $1 million per year, at which point the project received its name and formally began.<ref name="astronautica3" /><ref name=":5" /> The agency granted a study of the concept to the [[General Dynamics|General Dynamics Corporation]],<ref name=":3" /> but decided to withdraw support in late 1959.<ref name="astronautica3" /> The U.S. Air Force agreed to support Orion if a military use was found for the project, and the NASA Office of Manned Spaceflight also contributed funding.<ref name="astronautica2" /> The concept investigated by the government used a blast shield and shock absorber to protect the crew and convert the detonations into a continuous propulsion force.<ref>{{Cite web |date= |title=9906378 - Pulsed Fission Propulsion Concept |url=http://mix.msfc.nasa.gov/abstracts.php?p=704 |archive-url=https://web.archive.org/web/20091102051639/http://mix.msfc.nasa.gov/abstracts.php?p=704 |archive-date=November 2, 2009 |access-date=2023-08-03 |website=msfc.nasa.gov |publisher=[[Marshall Space Flight Center]]}}</ref><ref>{{Cite web |title=NASA Image and Video Library |url=https://images.nasa.gov/details/9906395 |access-date=2023-08-03 |website=NASA Image and Video Library |language=en}}</ref> The most successful model test, in November 1959, reached roughly 100 meters in altitude with six sequenced chemical explosions.<ref name=":5" /> NASA also produced a Mars mission profile for a 125 day round trip with eight astronauts, at a predicted development cost of $1.5 billion.<ref name=":4" /> Orion was canceled in 1964, after the United States signed the Partial Test Ban Treaty the prior year; the treaty greatly reduced political support for the project.<ref name=":3" /><ref name=":4">{{Cite web |last1=Schmidt |first1=G.R. |last2=Bonometti |first2=J.A. |last3=Morton |first3=P.J. |date=2000 |title=Nuclear Pulse Propulsion - Orion and Beyond |url=https://ntrs.nasa.gov/api/citations/20000096503/downloads/20000096503.pdf |access-date=2023-08-03 |website=NTRS - NASA Technical Reports Server}}</ref> NASA had also decided, in 1959, that the civilian space program would be non-nuclear in the near-term.<ref name="astronautica3" /> 

The Orion concept offered both high thrust and high [[specific impulse]], or propellant efficiency: 2,000 pulse units ([[Specific impulse|Isp]]) under the original design and an Isp of perhaps 4,000 to 6,000 seconds according to the Air Force plan, with a later 1968 [[Fusion Bomb|fusion bomb]] proposal by Dyson potentially increasing this to more than 75,000 Isp, enabling velocities of 10,000 km/sec.<ref name=":4" /> A moderate-sized nuclear device was estimated, at the time, to produce about 5 or 10 billion horsepower.<ref name="astronautica3" /><ref>{{Cite book |last=Pedersen |first=Erik S. |title=Nuclear Propulsion in Space |publisher=[[Prentice-Hall, Inc.]] |year=1964 |location=Englewood Cliffs, NJ |pages=276}}</ref>

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The extreme power of the nuclear explosions, relative to the vehicle's mass, would be managed by using external detonations, although an earlier version of the pulse concept did propose containing the blasts in an internal pressure structure, with one such design prepared by [[Glenn L. Martin Company|The Martin Company]].<ref name=":4" /><ref name="astronautica3" /> As a qualitative power comparison, traditional [[Rocket engine#Terminology|chemical rockets]], such as the [[Saturn V]] that took the [[Apollo program]] to the Moon, produce high thrust with low specific impulse, whereas electric [[ion engine]]s produce a small amount of thrust very efficiently. Orion, by contrast, would have offered performance greater than the most advanced conventional or nuclear rocket engines then under consideration. Supporters of Project Orion felt that it had potential for cheap [[interplanetary travel]].<ref name="''Cosmos'' by Carl Sagan">Sagan, Carl; Druyan, Ann; Tyson, Neil deGrasse (2013). ''Cosmos''. New York: Ballantine Books. {{ISBN|978-0-345-53943-4}}.</ref>

From [[Project Longshot]] to [[Project Daedalus]], [[Mini-Mag Orion]], and other proposals which analyze thermal power dissipation, the principle of external [[nuclear pulse propulsion]] to maximize survivable power has remained common among serious concepts for interstellar flight without external power beaming and for very high-performance interplanetary flight.<ref name=":4" /> Such later proposals have tended to modify the basic principle by envisioning equipment driving detonation of much smaller fission or fusion pellets, in contrast to Project Orion's larger nuclear pulse units (full nuclear bombs).

==Basic principles==
[[File:ProjectOrionConfiguration.png|thumb|upright=2|The Orion Spacecraft – key components<ref>[https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19770085619_1977085619.pdf Nuclear Pulse Space Vehicle Study] Vol IV – Conceptual Vehicle Designs and Operational Systems, Fig 2.1, pp 4., NASA</ref>]]

The Orion nuclear pulse drive combines a very high exhaust velocity, from 19 to 31&nbsp;km/s (12 to 19&nbsp;mi/s) in typical interplanetary designs, with [[newton (units)|meganewton]]s of thrust.<ref>Ross, F.W. – Propulsive System Specific Impulse. General Atomics GAMD-1293 8 Feb. 1960</ref> Many spacecraft propulsion drives can achieve one of these or the other, but nuclear pulse rockets are the only proposed technology that could potentially meet the extreme power requirements to deliver both at once (see [[spacecraft propulsion]] for more speculative systems).

[[Specific impulse]] (''I''<sub>sp</sub>) measures how much thrust can be derived from a given mass of fuel, and is a standard figure of merit for rocketry.  For any rocket propulsion, since the [[kinetic energy]] of exhaust goes up with velocity squared ([[kinetic energy]] = {{sfrac|1|2}}mv<sup>2</sup>), whereas the [[momentum]] and thrust go up with velocity linearly ([[momentum]] = mv), obtaining a particular level of thrust (as in a number of [[G-force|g]] acceleration) requires far more power each time that exhaust velocity and ''I''<sub>sp</sub> are much increased in a design goal. (For instance, [[Mass driver#Spacecraft-based mass drivers|the most fundamental reason]] that [[Electrically powered spacecraft propulsion|electric propulsion]] systems of high ''I''<sub>sp</sub> tend to be low thrust is due to their limits on available power. Their thrust is actually inversely proportional to ''I''<sub>sp</sub> if power going into exhaust is constant or at its limit from heat dissipation needs or other engineering constraints.)<ref>{{cite web |first=Anthony |last=Zuppero |url=http://neofuel.com/optimum/ |title=Physics of Rocket Systems with Separated Energy and Propellant |access-date=2012-04-24}}</ref> The Orion concept detonates nuclear explosions externally at a rate of power release which is beyond what nuclear reactors could survive internally with known materials and design.

Since weight is no limitation, an Orion craft can be extremely robust. An uncrewed craft could tolerate very large accelerations, perhaps 100 [[g-force|''g'']]. A human-crewed Orion, however, must use some sort of [[Inertia damper|damping system]] behind the pusher plate to smooth the near instantaneous acceleration to a level that humans can comfortably withstand – typically about 2 to 4 ''g''.

The high performance depends on the high exhaust velocity, in order to maximize the rocket's force for a given mass of propellant. The velocity of the plasma debris is proportional to the square root of the change in the temperature (''T<sub>c</sub>'') of the nuclear fireball. Since such fireballs typically achieve ten million degrees Celsius or more in less than a millisecond, they create very high velocities. However, a practical design must also limit the destructive radius of the fireball. The diameter of the nuclear fireball is proportional to the square root of the bomb's explosive yield.

The shape of the bomb's reaction mass is critical to efficiency. The original project designed bombs with a reaction mass made of [[tungsten]]. The bomb's geometry and materials focused the [[X-ray]]s and plasma from the core of nuclear explosive to hit the reaction mass. In effect each bomb would be a nuclear [[shaped charge]].

A bomb with a cylinder of reaction mass expands into a flat, disk-shaped wave of plasma when it explodes. A bomb with a disk-shaped reaction mass expands into a far more efficient cigar-shaped wave of plasma debris. The cigar shape focuses much of the plasma to impinge onto the pusher-plate.<ref name="auto">{{cite book |last=Dyson |first=George |title=Project Orion – The Atomic Spaceship 1957–1965 |publisher=Penguin |isbn=978-0-14-027732-6|date=2003}}</ref> For greatest mission efficiency the [[rocket equation]] demands that the greatest fraction of the bomb's explosive force be directed at the spacecraft, rather than being spent [[Isotropy#Physics|isotropically]].

The maximum effective specific impulse, ''I<sub>sp</sub>'', of an Orion nuclear pulse drive generally is equal to:

:<math>I_{sp} = \frac{C_0 \cdot V_e}{g_n}</math>

where ''C<sub>0</sub>'' is the collimation factor (what fraction of the explosion plasma debris will actually hit the impulse absorber plate when a pulse unit explodes), ''V<sub>e</sub>'' is the nuclear pulse unit plasma debris velocity, and ''g<sub>n</sub>'' is the standard acceleration of gravity (9.81&nbsp;m/s<sup>2</sup>; this factor is not necessary if ''I<sub>sp</sub>'' is measured in N·s/kg or m/s). A collimation factor of nearly 0.5 can be achieved by matching the diameter of the pusher plate to the diameter of the nuclear fireball created by the explosion of a nuclear pulse unit.

The smaller the bomb, the smaller each impulse will be, so the higher the rate of impulses and more than will be needed to achieve orbit. Smaller impulses also mean less ''g'' shock on the pusher plate and less need for damping to smooth out the acceleration.

The optimal Orion drive bomblet yield (for the human crewed 4,000 ton reference design) was calculated to be in the region of 0.15&nbsp;kt, with approx 800 bombs needed to orbit and a bomb rate of approx 1 per second.<ref>{{cite book |last=Pedersen |first=Erik Storm |title=Nuclear Energy in Space |publisher=Prentice-Hall |date=1964 |page=275}}</ref>

==Sizes of vehicles==
The following can be found in [[George Dyson (science historian)|George Dyson]]'s book.<ref name="auto"/> The figures for the comparison with Saturn V are taken from [[Payload (air and space craft)#Space craft|this section]] and converted from metric (kg) to US [[short ton]]s (abbreviated "t" here).
[[File:Project Orion Saturn-V compatibility.png|thumb|upright=2|Image of the smallest Orion vehicle extensively studied, which could have had a payload of around 100 tons in an 8 crew round trip to Mars.<ref>{{cite conference |title=Nuclear Pulse Propulsion – Orion and Beyond |url=http://large.stanford.edu/courses/2013/ph241/micks1/docs/aiaa-2000-3856.pdf |id=AlAA 2000-3856 |author1=G.R. Schmidt |author2=J.A. Bunornetti |author3=P.J. Morton |conference=36th AIAA / ASME / SAE / ASEE Joint Propulsion Conference & Exhibit, Huntsville, Alabama, 16–19 July 2000 |quote=Two or possibly three Saturn V's would have been required to put this vehicle into orbit, and some on-orbit assembly would be required. Several mission profiles were considered -the one developed in greatest detail was for a Mars mission. Eight astronauts. With around 100 tons of equipment and supplies. could have made a round trip to Mars in 125 days (most plans call for one-way times of at least nine months). Another impressive figure is that as much as 45% of the gross vehicle in Earth orbit could have been payload.}}</ref> On the left, the 10 meter diameter [[Saturn V]] "Boost-to-orbit" variant, requiring in-orbit assembly before the Orion vehicle would be capable of moving under its own propulsion system. On the far right, the fully assembled "lofting" configuration, in which the spacecraft would be lifted high into the atmosphere before pulse propulsion began. As depicted in the 1964 [[NASA]] document "Nuclear Pulse Space Vehicle Study Vol III – Conceptual Vehicle Designs and Operational Systems."<ref>{{cite book |chapter=Nuclear Pulse Space Vehicle Study |chapter-url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19770085619_1977085619.pdf |author=Shipps, P. R. |display-authors=etal |date=2005-07-04 |title=Vol III – Conceptual Vehicle Designs and Operational Systems |id=Contract:NASA-CR-60653; GA-5009-VOL-III |orig-year=Sep 19, 1964 |url-status=dead |archive-date=2006-01-03 |archive-url=https://web.archive.org/web/20060103021446/https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19770085619_1977085619.pdf }}</ref><ref>{{cite web |url=http://www.lepp.cornell.edu/~seb/celestia/orion/index.html |title=Project Orion |first=Selden |last=Ball |location=Wilson Lab}}</ref>]]

{| class="wikitable" style="text-align:center"
!
!Orbital<br />test
!Interplanetary
!Advanced<br />interplanetary
!Saturn V
|-
!Ship mass
| 880 t || 4,000 t || 10,000 t ||3,350 t
|-
!Ship diameter
| 25 m || 40 m || 56 m ||10 m
|-
!Ship height
| 36 m || 60 m || 85 m ||110 m
|-
!Bomb yield<br />(sea level)
| 0.03 kt || 0.14 kt || 0.35 kt ||n/a
|-
!Bombs<br />(to 300&nbsp;mi [[Low Earth Orbit]])
| 800 || 800 || 800 ||n/a
|-
!Payload<br />(to 300&nbsp;mi LEO)
| 300 t || 1,600 t || 6,100 t ||130 t
|-
!Payload<br />(to Moon soft landing)
| 170 t || 1,200 t || 5,700 t ||2 t
|-
!Payload<br />(Mars orbit return)
| 80 t || 800 t || 5,300 t || –
|-
!Payload<br />(3 yr Saturn return)
| – || – || 1,300 t || –
|}

In late 1958 to early 1959, it was realized that the smallest practical vehicle would be determined by the smallest achievable bomb yield. The use of 0.03&nbsp;kt (sea-level yield) bombs would give vehicle mass of 880 tons. However, this was regarded as too small for anything other than an orbital test vehicle and the team soon focused on a 4,000 ton "base design".

At that time, the details of small bomb designs were shrouded in secrecy. Many Orion design reports had all details of bombs removed before release. Contrast the above details with the 1959 report by General Atomics,<ref name="GAMD-784">{{cite book| year=1959| publisher=General Atomics| title=Dimensional Study of Orion Type Spaceships| last=Dunne|author2=Dyson and Treshow | id=GAMD-784}}</ref> which explored the parameters of three different sizes of [[hypothetical]] Orion spacecraft:

{| class="wikitable"
!
!"Satellite"<br />Orion
!"Midrange"<br />Orion
!"Super"<br />Orion
|-
!Ship diameter
| 17–20 m || 40 m || 400 m
|-
!Ship mass
| 300 t || 1000–2000 t || 8,000,000 t
|-
!Number of bombs
| 540 || 1080 || 1080
|-
!Individual bomb mass
| 0.22 t || 0.37–0.75 t || 3000 t
|}

The biggest design above is the "super" Orion design; at 8 million tons, it could easily be a city.<ref name=george>{{cite book| title=Project Orion: The True Story of the Atomic Spaceship| first=George| last=Dyson| year=2002| isbn=978-0-8050-7284-6| publisher=Henry Holt and Co.| location=New York}}</ref> In interviews, the designers contemplated the large ship as a possible [[interstellar ark]]. This extreme design could be built with materials and techniques that could be obtained in 1958 or were anticipated to be available shortly after.

Most of the three thousand tons of each of the "super" Orion's propulsion units would be inert material such as [[polyethylene]], or [[boron]] salts, used to transmit the force of the propulsion units detonation to the Orion's pusher plate, and absorb neutrons to minimize fallout. One design proposed by Freeman Dyson for the "Super Orion" called for the pusher plate to be composed primarily of uranium or a [[transuranium element|transuranic element]] so that upon reaching a nearby star system the plate could be converted to nuclear fuel.

==Theoretical applications==
The Orion nuclear pulse rocket design has extremely high performance. Orion nuclear pulse rockets using nuclear fission type pulse units were originally intended for use on interplanetary space flights.

Missions that were designed for an Orion vehicle in the original project included single stage (i.e., directly from Earth's surface) to Mars and back, and a trip to one of the moons of Saturn.<ref name=george/>

Freeman Dyson performed the first analysis of what kinds of Orion missions were possible to reach [[Alpha Centauri]], the nearest star system to the [[Sun]].<ref>{{cite journal |title=Nuclear Pulse Propulsion: A Historical Review of an Advanced Propulsion Concept |last1=Martin |first1=Anthony R. |last2=Bond |first2=Alan |name-list-style=amp |journal=Journal of the British Interplanetary Society |volume=32 |date=August 1979 |bibcode=1979JBIS...32..283M |page=301 }}</ref> His 1968 paper "Interstellar Transport" (''[[Physics Today]]'')<ref>{{cite journal |url=http://galileo.phys.virginia.edu/classes/109.jvn.spring00/nuc_rocket/Dyson.pdf |first=Freeman J. |last=Dyson |title=Interstellar Transport |journal=[[Physics Today]] |date=October 1968 |volume=21 |issue=10 |pages=41–45|doi=10.1063/1.3034534 |bibcode=1968PhT....21j..41D }}</ref> retained the concept of large nuclear explosions but Dyson moved away from the use of fission bombs and considered the use of one megaton [[deuterium]] fusion explosions instead. His conclusions were simple: the debris velocity of fusion explosions was probably in the 3000–30,000&nbsp;km/s range and the reflecting geometry of Orion's hemispherical pusher plate would reduce that range to 750–15,000&nbsp;km/s.<ref>{{cite book |title=The Starflight Handbook |last1=Mallove |first1=Eugene F. |last2=Matloff |first2=Gregory L. |publisher=John Wiley & Sons |year=1989 |isbn=978-0-471-61912-3 |page=[https://archive.org/details/starflighthandbo0000mall/page/66 66] |url=https://archive.org/details/starflighthandbo0000mall/page/66 }}</ref>

To estimate the upper and lower limits of what could be done using 1968 technology, Dyson considered two starship designs. The more conservative ''energy limited'' pusher plate design simply had to absorb all the thermal energy of each impinging explosion (4×10<sup>15</sup> joules, half of which would be absorbed by the pusher plate) without melting. Dyson estimated that if the exposed surface consisted of [[copper]] with a thickness of 1&nbsp;mm, then the diameter and mass of the hemispherical pusher plate would have to be 20 kilometers and 5 million tonnes, respectively. 100 seconds would be required to allow the copper to radiatively cool before the next explosion. It would then take on the order of 1000 years for the energy-limited [[heat sink]] Orion design to reach Alpha Centauri.

In order to improve on this performance while reducing size and cost, Dyson considered an alternative ''momentum limited'' pusher plate design where an ablation coating of the exposed surface is substituted to get rid of the excess heat. The limitation is then set by the capacity of shock absorbers to transfer momentum from the impulsively accelerated pusher plate to the smoothly accelerated vehicle. Dyson calculated that the properties of available materials limited the velocity transferred by each explosion to ~30 meters per second independent of the size and nature of the explosion. If the vehicle is to be accelerated at 1 Earth gravity (9.81&nbsp;m/s<sup>2</sup>) with this velocity transfer, then the pulse rate is one explosion every three seconds.{{sfnp|Martin|Bond|1979|p=302}} The dimensions and performance of Dyson's vehicles are given as:

{| class="wikitable"
!
!"Energy Limited"<br />Orion
!"Momentum Limited"<br />Orion
|-
!Ship diameter (meters)
| 20,000&nbsp;m || 100&nbsp;m
|-
!Mass of empty ship (tons)
| 10,000,000&nbsp;t (incl. 5,000,000&nbsp;t copper hemisphere) || 100,000&nbsp;t (incl. 50,000&nbsp;t structure + payload)
|-
!+Number of bombs = total bomb mass (each 1&nbsp;Mt bomb weighs 1 ton)
| 30,000,000 || 300,000
|-
!=Departure mass (tons)
| 40,000,000&nbsp;t || 400,000&nbsp;t
|-
!Maximum velocity (kilometers per second)
| 1000&nbsp;km/s (=0.33% of the speed of light) || 10,000&nbsp;km/s (=3.3% of the speed of light)
|-
!Mean acceleration (Earth gravities)
| 0.00003&nbsp;g (accelerate for 100 years) || 1&nbsp;g (accelerate for 10 days)
|-
!Time to Alpha Centauri (one way, no slow down)
| 1330 years || 133 years
|-
!Estimated cost
| 1 year of U.S. [[GNP]] (1968), $3.67 trillion || 0.1 year of U.S. GNP $0.367 trillion
|}

Later studies indicate that the top cruise velocity that can theoretically be achieved are a few percent of the [[speed of light]] (0.08–0.1c).<ref name="Cosmos by Carl Sagan">Cosmos by Carl Sagan</ref>{{Verify source|date=March 2024}} An atomic (fission) Orion can achieve perhaps 9–11% of the speed of light. A nuclear pulse drive starship powered by fusion-[[antimatter catalyzed nuclear pulse propulsion]] units would be similarly in the 10% range and pure [[Relativistic rocket|Matter-antimatter annihilation rockets]] would be theoretically capable of obtaining a velocity between 50% and 80% of the [[speed of light]]. In each case saving fuel for slowing down halves the maximum speed. The concept of using a [[magnetic sail]] to decelerate the spacecraft as it approaches its destination has been discussed as an alternative to using propellant; this would allow the ship to travel near the maximum theoretical velocity.<ref>{{cite journal |url=http://www.space-nation.org/images/a/a1/Mini-Mag_Orion_and_superconducting_coils_for_near-term_interstellar_transportation_LenardAndrews.pdf |title=Use of Mini-Mag Orion and superconducting coils for near-term interstellar transportation |first1=Roger X. |last1=Lenard |first2=Dana G. |last2=Andrews |journal=Acta Astronautica |volume=61 |issue=1–6 |date=June–August 2007 |pages=450–458 |via=Science Direct |doi=10.1016/j.actaastro.2007.01.052 |bibcode=2007AcAau..61..450L |access-date=November 9, 2013 |archive-date=June 17, 2014 |archive-url=https://web.archive.org/web/20140617053903/http://www.space-nation.org/images/a/a1/Mini-Mag_Orion_and_superconducting_coils_for_near-term_interstellar_transportation_LenardAndrews.pdf |url-status=dead }}</ref>

At 0.1''c'', Orion thermonuclear starships would require a flight time of at least 44 years to reach Alpha Centauri, not counting time needed to reach that speed (about 36 days at constant acceleration of 1''g'' or 9.8&nbsp;m/s<sup>2</sup>). At 0.1''c'', an Orion starship would require 100 years to travel 10 light years. The astronomer [[Carl Sagan]] suggested that this would be an excellent use for stockpiles of nuclear weapons.<ref>Cosmos series, Episode 8</ref>
{{Further|Interstellar travel}}

As part of the development of Project Orion, to garner funding from the military, a derived "space battleship" space-based nuclear-blast-hardened nuclear-missile weapons platform was mooted in the 1960s by the United States Air Force. It would comprise the USAF "Deep Space Bombardment Force".<ref name=9783030280802-234p235>{{cite book |title= Rocket Science: From Fireworks to the Photon Drive |first1= Mark |last1= Denny |first2= Alan |last2= McFadzean |year= 2019 |isbn=978-3-030-28080-2 |publisher= Springer International Publishing |chapter= Chapter 7: Into the Future |pages= 234–235 }}</ref><ref name=0140066845-50p>{{cite book |title= Nukespeak |first1= Stephen |last1= Hilgartner |first2= Richard C. |last2= Bell |first3= Rory |last3= O'Connor |isbn= 0140066845 |year= 1983 |publisher= Penguin Books |page= 50 }}</ref><ref name=9781250021014-138p>{{cite book |title= Maverick Genius: The Pioneering Odyssey of Freeman Dyson |author= Phillip F. Schewe |year= 2013 |publisher= St. Martin's Publishing Group |isbn= 978-1-250-02101-4|chapter= Death of Orion |page= 138}}</ref>

==Later developments==
[[File:Modern Pulsed Fission Propulsion Concept.jpg|thumb|Modern pulsed fission propulsion concept]]
A concept similar to Orion was designed by the [[British Interplanetary Society]] (B.I.S.) in the years 1973–1974. [[Project Daedalus]] was to be a robotic interstellar probe to [[Barnard's Star]] that would travel at 12% of the speed of light. In 1989, a similar concept was studied by the U.S. Navy and NASA in [[Project Longshot]]. Both of these concepts require significant advances in fusion technology, and therefore cannot be built at present, unlike Orion.

From 1998 to the present, the nuclear engineering department at Pennsylvania State University has been developing two improved versions of project Orion known as [[ICAN-II|Project ICAN]] and [[AIMStar|Project AIMStar]] using compact [[antimatter catalyzed nuclear pulse propulsion]] units,<ref>{{cite web |url=http://www.engr.psu.edu/antimatter/introduction2.html |title=Introduction |first=Kirby J. |last=Meyer |website=Antimatter Space Propulsion |publisher=Penn State University |date=2001-02-27 |access-date=2013-07-20 |url-status=dead |archive-url=https://web.archive.org/web/20121101033843/http://www.engr.psu.edu/antimatter/introduction2.html |archive-date=November 1, 2012 |df=mdy-all }}</ref> rather than the large [[inertial confinement fusion]] ignition systems proposed in Project Daedalus and Longshot.<ref>{{cite web |url=http://www.engr.psu.edu/antimatter/documents.html |title=Documents |website=Antimatter Space Propulsion |publisher=Penn State University |date=2001-02-27 |access-date=2009-11-15 |url-status=dead |archive-url=https://web.archive.org/web/20100107140516/http://www.engr.psu.edu/antimatter/documents.html |archive-date=January 7, 2010 |df=mdy-all }}</ref>

==Costs==
The expense of the fissionable materials required was thought to be high, until the physicist Ted Taylor showed that with the right designs for explosives, the amount of fissionables used on launch was close to constant for every size of Orion from 2,000 tons to 8,000,000 tons. The larger bombs used more explosives to super-compress the fissionables, increasing efficiency. The extra debris from the explosives also serves as additional propulsion mass.

The bulk of costs for historical nuclear defense programs have been for delivery and support systems, rather than for production cost of the bombs directly (with warheads being 7% of the U.S. 1946–1996 expense total according to one study).<ref>{{cite web |publisher=Brookings Institution |date=1998 |url=http://www.brookings.edu/projects/archive/nucweapons/figure1.aspx |title=Estimated Minimum Incurred Costs of U.S. Nuclear Weapons Programs, 1940–1996 |access-date=2012-01-12 |url-status=dead |archive-url=https://web.archive.org/web/20120310070123/http://www.brookings.edu/projects/archive/nucweapons/figure1.aspx |archive-date=March 10, 2012 |df=mdy-all }}</ref> After initial infrastructure development and investment, the marginal cost of additional nuclear bombs in mass production can be relatively low.  In the 1980s, some U.S. thermonuclear warheads had $1.1 million estimated cost each ($630 million for 560).<ref name=astronautica>{{cite encyclopedia |encyclopedia=Encyclopedia Astronautica |url=http://www.astronautix.com/articles/probirth.htm |title=Project Orion: Its Life, Death, and Possible Rebirth |first=Michael |last=Flora |url-status=dead |archive-url=https://web.archive.org/web/20111109175529/http://www.astronautix.com/articles/probirth.htm |archive-date=November 9, 2011 |access-date=2012-01-11}}</ref> For the perhaps simpler fission pulse units to be used by one Orion design, a 1964 source estimated a cost of $40,000 or less each in mass production, which would be up to approximately $0.3 million each in modern-day dollars adjusted for inflation.<ref name="astronautica"/><ref>[http://146.142.4.24/cgi-bin/cpicalc.pl?cost1=40000&year1=1964&year2=2011 CPI Inflation Calculator] {{Webarchive|url=https://web.archive.org/web/20130626184036/http://146.142.4.24/cgi-bin/cpicalc.pl?cost1=40000&year1=1964&year2=2011 |date=June 26, 2013 }} retrieved 2012-01-11</ref>

[[Project Daedalus]] later proposed fusion explosives ([[deuterium]] or [[tritium]] pellets) detonated by electron beam inertial confinement. This is the same principle behind [[inertial confinement fusion]]. Theoretically, it could be scaled down to far smaller explosions, and require small shock absorbers.

==Vehicle architecture==
[[File:Project-Orion propulsion-module section.png|thumb|upright=2|A design for the Orion propulsion module]]

From 1957 to 1964 this information was used to design a spacecraft propulsion system called Orion, in which nuclear explosives would be thrown behind a pusher-plate mounted on the bottom of a spacecraft and exploded. The shock wave and radiation from the detonation would impact against the underside of the pusher plate, giving it a powerful push. The pusher plate would be mounted on large two-stage [[shock absorber]]s that would smoothly transmit acceleration to the rest of the spacecraft.

During take-off, there were concerns of danger from fluidic shrapnel{{what|date=April 2023}} being reflected from the ground. One proposed solution was to use a flat plate of conventional explosives spread over the pusher plate, and detonate this to lift the ship from the ground before going nuclear. This would lift the ship far enough into the air that the first focused nuclear blast would not create debris capable of harming the ship.
[[File:Orion pulse unit.png|thumb|left|A design for a pulse unit]]

A preliminary design for a nuclear pulse unit was produced. It proposed the use of a shaped-charge fusion-boosted fission explosive. The explosive was wrapped in a [[beryllium oxide]] channel filler, which was surrounded by a [[uranium]] radiation mirror. The mirror and channel filler were open ended, and in this open end a flat plate of [[tungsten]] propellant was placed. The whole unit was built into a can with a diameter no larger than {{convert|6|in}} and weighed just over {{convert|300|lb}} so it could be handled by machinery scaled-up from a soft-drink vending machine; Coca-Cola was consulted on the design.<ref name="Jacobsen">{{cite book |last=Jacobsen |first=Annie |year=2012 |title=Area 51: An Uncensored History of America's Top Secret Military Base |publisher=Back Bay Books |isbn=978-0-316-20230-5 |page=305}}</ref>

At 1&nbsp;microsecond after ignition the gamma bomb plasma and neutrons would heat the channel filler and be somewhat contained by the uranium shell. At 2–3&nbsp;microseconds the channel filler would transmit some of the energy to the propellant, which vaporized. The flat plate of propellant formed a cigar-shaped explosion aimed at the pusher plate.

The plasma would cool to {{cvt|14,000|°C|order=flip}} as it traversed the {{convert|25|m|order=flip}} distance to the pusher plate and then reheat to {{cvt|67,000|°C|order=flip}} as, at about 300&nbsp;microseconds, it hits the pusher plate and is recompressed. This temperature emits ultraviolet light, which is poorly transmitted through most plasmas. This helps keep the pusher plate cool. The cigar shaped distribution profile and low density of the plasma reduces the instantaneous shock to the pusher plate.

Because the momentum transferred by the plasma is greatest in the center, the pusher plate's thickness would decrease by approximately a factor of 6 from the center to the edge. This ensures the change in velocity is the same for the inner and outer parts of the plate.

At low altitudes where the surrounding air is dense, [[compton scattering|gamma scattering]] could potentially harm the crew without a radiation shield; a radiation refuge would also be necessary on long missions to survive [[solar flare]]s. Radiation shielding effectiveness increases exponentially with shield thickness, see [[gamma ray]] for a discussion of shielding. On ships with a mass greater than {{convert|1,000|t|lb kg|order=out}} the structural bulk of the ship, its stores along with the mass of the bombs and propellant, would provide more than adequate shielding for the crew. Stability was initially thought to be a problem due to inaccuracies in the placement of the bombs, but it was later shown that the effects would cancel out.<ref>Teichmann, T. – The angular effects due to asymmetric placement of axial symmetric explosives: GAMD-5823, 26 Oct 1963</ref><ref>David, C. V. – Stability study of Nuclear Pulse Propulsion (Orion) Engine System. GAMD-6213, 30 Apr 1965</ref>

Numerous model flight tests, using conventional explosives, were conducted at [[Point Loma, San Diego]] in 1959. On November 14, 1959 the one-meter model, also known as "Hot Rod" and "putt-putt", first flew using [[RDX]] (chemical explosives) in a controlled flight for 23&nbsp;seconds to a height of {{convert|56|m|order=flip}}. Film of the tests has been transcribed to video<ref>Archived at [https://ghostarchive.org/varchive/youtube/20211211/E3Lxx2VAYi8 Ghostarchive]{{cbignore}} and the [https://web.archive.org/web/20111016180948/http://www.youtube.com/watch?v=E3Lxx2VAYi8 Wayback Machine]{{cbignore}}: {{cite web|date=August 6, 2007 |url=https://www.youtube.com/watch?v=E3Lxx2VAYi8 |title=Project Orion |publisher=YouTube |access-date=2009-11-15}}{{cbignore}}</ref> and were featured on the BBC TV program "To Mars by A-Bomb" in 2003 with comments by [[Freeman Dyson]] and [[Arthur C. Clarke]]. The model landed by parachute undamaged and is in the collection of the Smithsonian National Air and Space Museum.

The first proposed shock absorber was a ring-shaped airbag. It was soon realized that, should an explosion fail, the {{convert|500|–|1000|t|lb kg|adj=on|order=out}} pusher plate would tear away the airbag on the rebound. So a two-stage detuned spring and piston shock absorber design was developed. On the reference design the first stage mechanical absorber was tuned to 4.5 times the pulse frequency whilst the second stage gas piston was tuned to 0.5 times the pulse frequency. This permitted timing tolerances of 10 ms in each explosion.

The final design coped with bomb failure by overshooting and rebounding into a center position. Thus following a failure and on initial ground launch it would be necessary to start or restart the sequence with a lower yield device. In the 1950s methods of [[Variable yield|adjusting bomb yield]] were in their infancy and considerable thought was given to providing a means of swapping out a standard yield bomb for a smaller yield one in a 2 or 3 second time frame or to provide an alternative means of firing low yield bombs. Modern variable yield devices would allow a single standardized explosive to be tuned down (configured to a lower yield) automatically.

The bombs had to be launched behind the pusher plate with enough velocity to explode {{convert|20|–|30|m|order=flip}} beyond it every 1.1&nbsp;seconds. Numerous proposals were investigated, from multiple guns poking over the edge of the pusher plate to rocket propelled bombs launched from roller coaster tracks; however, the final reference design used a simple gas gun to shoot the devices through a hole in the center of the pusher plate.

==Potential problems==
{{Refimprove section|date=July 2023}}
Exposure to repeated nuclear blasts raises the problem of ''ablation'' (erosion) of the pusher plate. Calculations and experiments indicated that a steel pusher plate would ablate less than 1&nbsp;mm, if unprotected. If sprayed with an oil it would not ablate at all (this was discovered by accident: a test plate had oily fingerprints on it and the fingerprints suffered no ablation). The absorption spectra of [[carbon]] and [[hydrogen]] minimize heating. The design temperature of the shockwave, {{cvt|67,000|°C|order=flip}}, emits [[ultraviolet]] light. Most materials and elements are opaque to ultraviolet, especially at the {{cvt|340|MPa|order=flip}} pressures the plate experiences. This prevents the plate from melting or ablating.

One issue that remained unresolved at the conclusion of the project was whether or not the turbulence created by the combination of the propellant and ablated pusher plate would dramatically increase the total ablation of the pusher plate. According to Freeman Dyson, in the 1960s they would have had to actually perform a test with a real nuclear explosive to determine this; with modern simulation technology this could be determined fairly accurately without such empirical investigation.

Another potential problem with the pusher plate is that of [[spall]]ing—shards of metal—potentially flying off the top of the plate. The shockwave from the impacting plasma on the bottom of the plate passes through the plate and reaches the top surface. At that point, spalling may occur, damaging the pusher plate. For that reason, alternative substances—plywood and fiberglass—were investigated for the surface layer of the pusher plate and thought to be acceptable.

If the conventional explosives in the nuclear bomb detonate but a nuclear explosion does not ignite, shrapnel could strike and potentially critically damage the pusher plate.

True engineering tests of the vehicle systems were thought to be impossible because several thousand nuclear explosions could not be performed in any one place. Experiments were designed to test pusher plates in nuclear fireballs and long-term tests of pusher plates could occur in space. The shock-absorber designs could be tested at full-scale on Earth using chemical explosives.

However, the main unsolved problem for a launch from the surface of the Earth was thought to be [[nuclear fallout]]. Freeman Dyson, group leader on the project, estimated back in the 1960s that with conventional [[nuclear weapons]], each launch would statistically cause on average between 0.1 and 1 fatal cancers from the fallout.<ref name="disturbing">{{cite book |title=Disturbing the Universe |year=1979 |url=https://archive.org/details/disturbinguniver00dyso |url-access=registration |first=Freeman|last=Dyson|publisher=Harper & Row |isbn=978-0-06-011108-3 }}</ref> That estimate is based on [[Linear no-threshold model|no-threshold]] model assumptions, a method often used in estimates of statistical deaths from other industrial activities. Each few million dollars of efficiency indirectly gained or lost in the world economy may statistically average lives saved or lost, in terms of opportunity gains versus costs.<ref>{{cite book |chapter-url=http://www.phyast.pitt.edu/~blc/book/chapter8.html |chapter=Understanding Risk |title=The Nuclear Energy Option |first=Bernard L. |last=Cohen |url=http://www.phyast.pitt.edu/~blc/book/index.html |publisher=Plenum Press |date=1990 |access-date=2012-01-11}}</ref> Indirect effects could matter for whether the overall influence of an Orion-based space program on future human global mortality would be a net increase or a net decrease, including if change in launch costs and capabilities affected [[space exploration]], [[space colonization]], the odds of [[Space and survival|long-term human species survival]], [[space-based solar power]], or other hypotheticals.

Danger to human life was not a reason given for shelving the project. The reasons included lack of a mission requirement, the fact that no one in the U.S. government could think of any reason to put thousands of tons of payload into orbit, the decision to focus on rockets for the Moon mission, and ultimately the signing of the [[Partial Test Ban Treaty]] in 1963. The danger to electronic systems on the ground from an [[Nuclear electromagnetic pulse|electromagnetic pulse]] was not considered to be significant from the sub-kiloton blasts proposed since solid-state integrated circuits were not in general use at the time.

From many smaller detonations combined, the fallout for the entire launch of a {{convert|6,000|short ton|lb kg|order=out|adj=on}} Orion is equal to the detonation of a typical 10&nbsp;[[TNT equivalent|megaton]] (40&nbsp;[[Joule|petajoule]]) nuclear weapon as an [[air burst]], therefore most of its fallout would be the comparatively dilute [[Nuclear fallout|delayed fallout]]. Assuming the use of nuclear explosives with a high portion of total yield from fission, it would produce a combined fallout total similar to the surface burst yield of the [[Ivy Mike|''Mike'' shot]] of [[Operation Ivy]], a 10.4 Megaton device detonated in 1952. The comparison is not quite perfect as, due to its surface burst location, ''Ivy Mike'' created a large amount of [[nuclear fallout|early fallout]] contamination.  Historical above-ground nuclear weapon tests included 189&nbsp;[[TNT equivalent|megatons]] of fission yield and caused average global radiation exposure per person peaking at {{cvt|0.11|mSv/a|order=flip}} in 1963, with a {{cvt|0.007|mSv/a|order=flip}} residual in [[Background radiation#Overview|modern times]], superimposed upon other sources of exposure, primarily natural [[background radiation]], which averages {{cvt|2.4|mSv/a|order=flip}} globally but varies greatly, such as {{cvt|6|mSv/a|order=flip}} in some high-altitude cities.<ref>{{cite report |author=United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) |year=2008 |url=http://www.unscear.org/docs/reports/2008/09-86753_Report_2008_GA_Report.pdf |title=Sources and Effects of Ionizing Radiation |access-date=2012-01-11}}</ref><ref>{{cite web |url=http://www.aradnj.com/radiation2.html |title=Radiation – How much am I getting? How can I Lower it? |publisher=Associated Radiologists, P.A. |year=2009 |access-date=2012-01-11 |archive-url=https://web.archive.org/web/20110413061249/http://www.aradnj.com/radiation2.html |archive-date=2011-04-13 }}</ref> Any comparison would be influenced by how population dosage is affected by detonation locations, with very remote sites preferred.

With special designs of the nuclear explosive, Ted Taylor estimated that fission product fallout could be reduced tenfold, or even to zero, if a [[pure fusion weapon|pure fusion explosive]] could be constructed instead.  A 100% pure fusion explosive has yet to be successfully developed, according to declassified US government documents, although relatively clean PNEs ([[Peaceful nuclear explosions]]) were tested for canal excavation by the Soviet Union in the 1970s with 98% fusion yield in the ''Taiga'' test's 15 [[kiloton]] devices, 0.3 [[kiloton]]s fission,<ref name="disturbing"/><ref>{{cite journal |url=http://www.bibliotecapleyades.net/ciencia/ciencia_uranium27.htm |title=The Soviet Program for Peaceful Uses of ... Nuclear Explosions |first=Milo D. |last=Nordyke |journal=Science & Global Security |year=1998 |volume=7 |issue=1 |pages=1–117 |doi=10.1080/08929889808426448|bibcode=1998S&GS....7....1N }}</ref> which excavated part of the proposed [[Pechora–Kama Canal]].

The vehicle's propulsion system and its test program would violate the [[Partial Test Ban Treaty]] of 1963, as currently written, which prohibits all nuclear detonations except those conducted underground as an attempt to slow the arms race and to limit the amount of radiation in the atmosphere caused by nuclear detonations. There was an effort by the US government to put an exception into the 1963 treaty to allow for the use of nuclear propulsion for spaceflight but Soviet fears about military applications kept the exception out of the treaty.  This limitation would affect only the US, Russia, and the United Kingdom.  It would also violate the [[Comprehensive Nuclear-Test-Ban Treaty]] which has been signed by the United States and China as well as the de facto moratorium on nuclear testing that the declared nuclear powers have imposed since the 1990s.

The launch of such an Orion nuclear bomb rocket from the ground or [[low Earth orbit]] would generate an [[Nuclear electromagnetic pulse|electromagnetic pulse]] that could cause significant damage to [[computer]]s and [[satellite]]s as well as flooding the [[Van Allen radiation belt|van Allen belt]]s with high-energy radiation. Since the EMP footprint would be a few hundred miles wide, this problem might be solved by launching from very remote areas. A few relatively small space-based [[electrodynamic tether]]s could be deployed to quickly eject the energetic particles from the capture angles of the Van Allen belts.

An Orion spacecraft could be boosted by non-nuclear means to a safer distance only activating its drive well away from Earth and its satellites. The Lofstrom [[launch loop]], [[space elevator]], or other [[Non-rocket spacelaunch|alternative launch systems]] hypothetically provide excellent solutions; in the case of the space elevator, existing [[carbon nanotubes]] composites, with the possible exception of [[Colossal carbon tube]]s, do not yet have sufficient [[tensile strength]]. All chemical rocket designs are extremely inefficient and expensive when launching large mass into orbit but could be employed if the result were cost effective.

==Notable personnel==
{{unreferenced section|date=February 2021}}
{{div col|colwidth=20em}}
* [[Lew Allen]], contract manager
* [[Jerry Astl]], explosives engineer
* [[Jeremy Bernstein]], physicist
* [[Edward Creutz]], physicist
* [[Brian Dunne]], Orion's chief scientist
* [[Freeman Dyson]], physicist
* [[Harold Finger]], physicist
* [[Burt Freeman]], physicist
* [[Edward B. Giller]], USAF liaison
* [[Charles Clark Loomis]], physicist
* [[Harris Mayer]], physicist
* [[James C. Nance (scientist)|James Nance]], project director
* [[H. Pierre Noyes]], physicist
* [[Ronald F. Prater]], USAF liaison
* [[Don Prickett]], USAF liaison
* [[Kedar "Bud" Pyatt]], mathematician
* [[Morris Scharff]], physicist
* [[Ted Taylor (physicist)|Ted Taylor]], project director
* [[Mike Treshow|Micheal Treshow]], physicist
* [[Stanisław Ulam]], mathematician
{{div col end}}

==Operation Plumbbob==
{{main|Operation Plumbbob}}

A test that was similar to the test of a pusher plate occurred as an accidental side effect of a nuclear containment test called "[[Operation Plumbbob#Missing steel bore cap|Pascal-B]]" conducted on 27 August 1957.<ref>{{cite web |website=Nuclear Weapon Archive| url=http://nuclearweaponarchive.org/Usa/Tests/Plumbob.html#PascalB| title=Operation Plumbbob |date=July 2003| access-date=2006-07-31}}</ref> The test's experimental designer Dr. Robert Brownlee performed a highly approximate calculation that suggested that the low-yield nuclear explosive would accelerate the massive (900&nbsp;kg) steel capping plate to six times [[escape velocity]].<ref>{{cite web |website=Nuclear Weapon Archive| url=http://nuclearweaponarchive.org/Usa/Tests/Brownlee.html| title=Learning to Contain Underground Nuclear Explosions| first=Robert R.| last=Brownlee |date=June 2002| access-date=2006-07-31}}</ref> The plate was never found but Dr. Brownlee believes that the plate never left the atmosphere; for example, it could have been vaporized by compression heating of the atmosphere due to its high speed. The calculated velocity was interesting enough that the crew trained a high-speed camera on the plate which, unfortunately, only appeared in one frame indicating a very high lower bound for the speed of the plate.

==Notable appearances in fiction==
{{main|List of stories featuring nuclear pulse propulsion}}
The first appearance of the idea in print appears to be [[Robert A. Heinlein]]'s 1940 short story, "[[Blowups Happen]]."

As discussed by Arthur C. Clarke in his recollections of the making of ''[[2001: A Space Odyssey (film)|2001: A Space Odyssey]]'' in ''[[The Lost Worlds of 2001]]'', a nuclear-pulse version of the U.S. interplanetary spacecraft ''[[Discovery One]]'' was considered. However the ''Discovery'' in the movie did not use this idea, as [[Stanley Kubrick]] thought it might be considered parody after making ''[[Dr. Strangelove or: How I Learned to Stop Worrying and Love the Bomb]]''.<ref name="BBCToMars">{{cite web |title=BBC Four – To Mars by A-Bomb |date=15 November 2009 |url=https://www.bbc.co.uk/programmes/b0074p0z |website=[[BBC Four]]}}</ref>

An Orion spaceship features prominently in the [[science fiction]] novel ''[[Footfall]]'' by [[Larry Niven]] and [[Jerry Pournelle]]. In the face of an alien siege/invasion of Earth, the humans must resort to drastic measures to get a fighting ship into orbit to face the alien fleet.

The opening premise of the show ''[[Ascension (miniseries)|Ascension]]'' is that in 1963 President John F. Kennedy and the U.S. government, fearing the Cold War will escalate and lead to the destruction of Earth, launched the ''Ascension'', an Orion-class spaceship, to colonize a planet orbiting Proxima Centauri, assuring the survival of the human race.

Author Stephen Baxter's science fiction novel ''[[Ark (novel)|Ark]]'' employs an Orion-class generation ship to escape ecological disaster on Earth.

Towards the conclusion of his  [[The Merchant Princes|Empire Games]] trilogy, [[Charles Stross]] includes a spacecraft modeled after Project Orion. The crafts' designers, constrained by a 1960s level of industrial capacity, intend it to be used to explore parallel worlds and to act as a nuclear deterrent, leapfrogging their foes more contemporary capabilities.

In the horror novel Torment by [[Jeremy Robinson]] (written under the pseudonym Jeremy Bishop), the main characters escape from a global nuclear war in a [[nuclear pulse propulsion]] craft. The craft is among 3 others; part of the "Orion Protocol", an escape mechanism for members of the federal government. The craft are housed in a subterranean chamber below [[The Ellipse]].

In the science fiction novel "[[The Three-Body Problem (novel)|3 Body Problem]]" and its associated television shows, a probe is launched towards an approaching alien fleet using a variation of the Orion method.<ref>{{cite web | url=https://www.vox.com/culture/24108638/3-body-problem-science-physics-theory-explained-how-real-is-it | title=3 Body Problem, explained with the help of an astrophysicist | date=March 22, 2024 }}</ref>

==See also==
{{Portal|Space|Spaceflight}}
* [[AIMStar]]
* [[Antimatter-catalyzed nuclear pulse propulsion]]
* [[Helios (propulsion system)]]
* [[NERVA|NERVA (Nuclear Engine for Rocket Vehicle Application)]]
* [[Nuclear propulsion]]
* [[Peaceful nuclear explosion]]
* [[Project Pluto]]
* [[Project Prometheus]]
* [[Project Valkyrie]]

==References==
{{reflist|30em}}

==Further reading==
{{Refbegin |30em}}
* {{cite book |url=https://books.google.com/books?id=r_Gu4f0QxrkC|title=Project Orion: The True Story of the Atomic Spaceship |first=George |last=Dyson |date=2003 |publisher=Henry Holt and Company|isbn=978-0-8050-7284-6}}
* {{cite journal |title=The Blinding of Orion: The Problematic Promotion of Pulse Propulsion |last=Guimont |first=Edward |journal=[[Quest: The History of Spaceflight]] |volume=27| number=4|date=Winter 2020 |page=3-12}}
* {{cite book|last=McPhee|first=John|title=The Curve of Binding Energy|publisher= Farrar, Straus and Giroux|year=1994|isbn=978-0-374-51598-0}}
* "Nuclear Pulse Propulsion (Project Orion) Technical Summary Report" RTD-TDR-63-3006 (1963–1964); GA-4805 Vol.&nbsp;1:&nbsp;Reference Vehicle Design Study, Vol.&nbsp;2:&nbsp;Interaction Effects, Vol.&nbsp;3:&nbsp;Pulse Systems, Vol.&nbsp;4:&nbsp;Experimental Structural Response. (From the National Technical Information Service, U.S.A.)
* "Nuclear Pulse Propulsion (Project Orion) Technical Summary Report" 1 July 1963 – 30 June 1964, WL-TDR-64-93; GA-5386 Vol.&nbsp;1:&nbsp;Summary Report, Vol.&nbsp;2:&nbsp;Theoretical and Experimental Physics, Vol.&nbsp;3:&nbsp;Engine Design, Analysis and Development Techniques, Vol.&nbsp;4:&nbsp;Engineering Experimental Tests. (From the National Technical Information Service, U.S.A.)
* General Atomics, [https://web.archive.org/web/20070701004217/http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19650058729_1965058729.pdf ''Nuclear Pulse Space Vehicle Study, Volume I – Summary''], September 19, 1964
* General Atomics, [https://web.archive.org/web/20100513021916/https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19770085619_1977085619.pdf ''Nuclear Pulse Space Vehicle Study, Volume III – Conceptual Vehicle Designs And Operational Systems''], September 19, 1964
* General Atomics, [https://web.archive.org/web/20100512204710/https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19660072847_1966072847.pdf ''Nuclear Pulse Space Vehicle Study, Volume IV – Mission Velocity Requirements And System Comparisons''], February 28, 1966
* General Atomics, [https://web.archive.org/web/20100518130855/https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19660072846_1966072846.pdf ''Nuclear Pulse Space Vehicle Study, Volume IV – Mission Velocity Requirements And System Comparisons (Supplement)''], February 28, 1966
* {{cite book |title=Dynamic America: A History of General Dynamics Corporation and Its Predecessor Companies |author1=General Dynamics Corporation |first2=Erik |last2=Nitsche |editor=John Niven |publisher=General Dynamics |year=1960 |url=https://books.google.com/books?id=YFxEAQAAIAAJ |oclc=1284190}}
* NASA, [https://web.archive.org/web/20100511090218/https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19760065935_1976065935.pdf ''Nuclear Pulse Vehicle Study Condensed Summary Report (General Dynamics Corp)''], January 14, 1964
{{Refend}}

==External links==
{{Commons|Project Orion|Project Orion (nuclear propulsion)}}
* [http://www.spacedaily.com/news/nuclearspace-03h.html The case for Orion]
* [http://www.webofstories.com/people/freeman.dyson/116 Freeman Dyson talking about Project Orion]
* [http://www.u.arizona.edu/~tuvas/ Electromagnetic Pulse Shockwaves as a result of Nuclear Pulse Propulsion]; {{Webarchive|url=https://web.archive.org/web/20210224231605/http://www.u.arizona.edu/~tuvas/ |date=February 24, 2021 }}
* [http://www.ted.com/talks/view/id/221 George Dyson talking about Project Orion] at [[TED (conference)|TED]]

{{Nuclear propulsion}}
{{US human spaceflight programs}}
{{Authority control}}

[[Category:Engineering projects|Orion]]
[[Category:Freeman Dyson]]
[[Category:Hypothetical spacecraft]]
[[Category:Interstellar travel]]
[[Category:Nuclear spacecraft propulsion|Orion]]
[[Category:Single-stage-to-orbit]]
[[Category:Space access]]