NUCLEAR PULSE DRIVES


The Nuclear-Pulse-powered DAEDALUS interstellar probe.

Project Orion Nuclear Pulse Drive
Tech Level: 12
Super Orion Interstellar Craft
Tech Level: 13
Helios Nuclear Pulse Drive
Tech Level: 14
Medusa Nuclear Pulse Drive
Tech Level: 14
Daedalus Interstellar Probe
Tech Level: 15
VISTA Nuclear Pulse Drive
Tech Level: 15

Nuclear Pulse Drives are one of those ideas that sound completely insane when you first hear it, yet for decades it was THE most popular and widely-supported method of deep space propulsion among prominent space scientists and pundits, and underwent a decade of development by the US government in the 1950s and 60s.

Why insane? Basically, Nuclear Pulse Propulsion uses nuclear detonations exploding behind a spacecraft to push it forward, much like a firecracker exploding under an empty soupcan can send it skyward.

Nuclear Pulse Drives were seriously studied by private interests and the US government in the late 50s and early 60s, and a great many physicists and aerospace engineers still support them as a very desirable space propulsion system, the Orion scheme in particular. Project Orion-like ships have been seen in science fiction sources, most significantly in the novel Footfall, by Larry Niven and Jerry Pournelle, where its used to fight alien invaders, and in the movie "Deep Impact" where it is used to deliver astronauts to an Earth-threatening comet.


PROJECT ORION NUCLEAR PULSE DRIVE
Tech Level: 12

The Orion scheme for Nuclear Pulse Propulsion was first proposed by Stanislaw Ulam and Cornelius Everett in a classified 1955 paper. According to popular lore, Ulam was inspired by an experiment that involved suspending two graphite-covered steel spheres about thirty feet from ground zero of an atomic explosion. The spheres were later found fully intact miles away, with only a thin layer of graphite vaporized away by the explosion.

Ulamís idea was that the spaceship would eject a specialized atomic bomb a few hundred meters behind the ship, followed by solid-propellant disks. The explosion would vaporize the disks, and the resulting cloud of rapidly expanding plasma would impinge upon a pusher plate. In order to mediate the tremendous force on crew and cargo created by the explosion, the rest of the ship would be separated from the pusher plate by enormous shock absorbers.

Theodore Taylor came aboard Project Orion in 1958 after it was officially began at General Atomics, now a subsidiary of defense contractor General Dynamics. Taylorís main contribution was re-conceptualizing the bombs and propellant disks into a single pulse unit, coating the nuclear bomb with layers of plastic that would serve the same function as the disks. The plastic of choice was polyethylene, which was good at absorbing stray neutrons from the explosion, cutting down on radiation risk to the crew, and broke down into light-weight atoms such as hydrogen and carbon which can move at high speed when agitated. He also proposed techniques to "tamp" the pulse detonation, focusing as much energy from the explosion to the pusher plate as possible.

A basic diagram of the Orion concept.

Because of the open pusher-plate design and the lack of a combustion chamber, Orion vessels have very high upper limits to the amount of heat or thrust the ship can endure using its nuclear pulse drive. The specific pulse generated by Orion vessels can vary from 10,000 to 1,000,000 seconds. Compare this to the 450 seconds of modern chemical rockets and 5000 seconds of the most advanced proposed closed fission rockets.

A number of scale models were built, called Put-Puts or Hot Rods, and a test flight, carried out with conventional explosives, was made in 1959. Though it only achieved an altitude of 100 meters, it demonstrated that sustained stable flight was possible with pulse drives.

Durability of the pusher plate was at first a great concern, but it was found during experiments that it would be exposed to extreme temperatures for only about one millisecond per pulse, and that heat would not penetrate very far into the main body of the plate. Steel or aluminum, not any exotic material as previously thought, proved strong and durable enough to serve as the pusher plate material. The plates were designed in such a way that they would ablate away millimeter by millimeter every pulse, but be designed thick enough to be able to last the length of any voyage. A graphite-laced oil spray was also proposed to be used on the pusher plate between pulses in order to minimize ablation.

Orion Nuclear Pulse Drives are enormously powerful compared to modern rockets, and even compared to most other near-future Deep Space Propulsion schemes. The original Orion designs, envisioned as launch vehicles, could have put 10,000 tons (!) into orbit (compared to the "mere" 30 ton capability of the Space Shuttle), using 0.1 kiloton bombs at initial lift-off every second or so, then increasing to 20-kiloton explosions as the ship cleared the lower atmosphere. Enormous transplanetary vehicles were also envisioned, including an 8,000,000 ton vehicle that would have carried a crew of 150 to Saturn and back in three years using 1.4 megaton pulse bombs.

Orion was effectively killed by the Nuclear Test Ban Treaty of 1963, which made the use of any nuclear weapons in space illegal, and the general hostility toward nuclear technology in the ensuing decades have kept it from reviving in any serious form. Because it relies on frequent nuclear explosions for propulsion, it would be impossible in todayís political climate to even consider Orion as a possible launch vehicle as its original framers envisioned. Still, Orion has a great many proponents and converts to this day among the aerospace and physics community, and recent changes in US nuclear policy could perhaps re-open the door for it to be used as a means of interplanetary propulsion in the future.


SUPER ORION INTERSTELLAR CRAFT
Tech Level: 13

Super-Orion craft, such as the 8,000,000 ton monster referred to above, are often cited as being the one type of ship with possible interstellar capabilities that could be built with todayís technology. Exactly how fast such a vessel could traverse the interstellar void remains a matter for debate, however. The most optimistic estimates claim it can achieve 10% to 15% of lightspeed after several years of constant acceleration, but some cite limitations in the efficiency of energy transfer from the nuclear pulses, wear and tear on the pusher plate and shock absorber, and other factors would limit even the best Orion drive to under 5% lightspeed.


HELIOS NUCLEAR PULSE DRIVE
Tech Level: 14

A precursor concept to Orion proper, Helios postulated detonating small, 0.1 kiloton nuclear bombs into a chamber roughly 130 feet in diameter. Water would be injected into the chamber, super-heated by the explosion and expelled for thrust. Like Orion, it would have achieved constant acceleration through rapid "pulsed" operation.

This design would have yielded a specific impulse of about 1150 seconds (compared to a modern chemical rocketís 450 seconds). However, a number of technical problems arose, most prominently how to keep the combustion chamber from exploding from the great pressures of the atomic detonations. Also, producing fission bombs with yields as small as 1 kiloton are problematic with current known techniques, and miniaturizing them for significantly smaller yields may not be possible.

The combustion chamber problem could perhaps be solved with powerful magnetic bottle technology currently being developed for plasma, fission, and fusion rockets.


MEDUSA NUCLEAR PULSE DRIVE
Tech Level: 14

This is an idea that Iíve seen only in bare-bones form. Basically, like Orion, its uses nuclear explosions for thrust, but instead of a pusher plate the explosion pushes on an enormous solar sail, which in turn drags the payload behind it. Unlike Orion, the pulse bombs in this scheme would not have a plastic coating to produce propellant plasma; the radiation pressure from the bomb alone would do the job, pushing on the relatively fragile, micron-thin sails.

I am unaware how the designers of this scheme planned to deal with some engineering problems that are sure to arise, such as the fact that the "dragged" payload would endure both the brunt of the pulse detonation and being dragged, at high speed, through the explosionís resultant vapor cloud. Also, the tethers connecting the payload and the sail seems like they too would undergo undo stress from both the explosions and the constant pulsed accelerations. However, since solar sails can be made to enormous proportions (thousands of miles across) and the nuclear explosions would be limited in scope to a few kilotons at most, its possible these effects could be minimized.


DAEDALUS INTERSTELLAR PROBE
Tech Level: 15
A size comparison between the Daedalus and the Space Shuttle

In the 1970s, the British Interplanetary Society conducted a design study on the feasibility of a process called inertial confinement fusion (ICF), which uses an array of crossed laser or particle beams to implode pellets of fuel to fusion temperatures at the beamsí locus point. The resultant nuclear detonation pulse pushes against either a pusher plate as in the Orion scheme or a combustion chamber reinforced with magnetic fields.

Fuel for the pellets is usually cited as deuterium, tritium, helium-3, some other easily fusible material, or a combination thereof. Detonation of about 250 pellets per second would enable, in the British Interplanetary Societyís vision at least, a 55,000 ton vehicle to achieve about 10% lightspeed after 2 years of constant operation.

Daedalus itself was envisioned as a two-stage design, shedding all but about 6000 tons of its mass after its initial two-year burn. Designed for an interstellar fly-by only, it would spend more than fifty years en route for a few days worth of data gathering. However, the technology could eventually be refined for manned vehicles or scaled down for in-system use (see VISTA below.)


VISTA NUCLEAR PULSE DRIVE
Tech Level: 15

VISTA (Vehicle for Interplanetary Space Transport Applications) is a scaled-down and reconfigured Daedalus-like craft, meant for interplanetary instead of interstellar travel. Rather than direct-fire induced implosion, the VISTA craft uses mirrors to redirect over a dozen high-powered lasers onto the ignition focal point. Shaped like an inverted cone, initial designs call for it to measure 100 meters high and 170 meters across along the widest point of the cone "base."

A superconducting ring magnet halfway up the cone is used to produce the magnetic fields that contain and direct the pulse detonations that propel the craft. This ring magnet also has the added feature of being able to act as a supplementary radiation shield to help protect the crew from pulse detonationsí radiation.

In one conceptual study, a 5800 ton VISTA vessel was cited as being able to deliver a 100-ton payload to Mars and back in under 60 days.

A diagram of VISTA's Inertial Confinement Fusion engine.

RELATED INFORMATION

A site dedicated to the development of nuclear space drives:

>http://www.nuclearspace.com/

An extensive links site to many articles and sources on nuclear space technologies:

http://www.nuclearspace.com/nuclear_links.htm

A link site dedicated to Project Orion:

http://www.angelfire.com/stars2/projectorion/

An article on the development of Project Orion:

http://www.astronautix.com/articles/probirth.htm

A site dedicated to the development of nuclear space drives:

http://www.nuclearspace.com/

A page dedicated to Inertial Confinement Fusion propulsion:

http://www.islandone.org/APC/Nuclear/11.html#DAEDALUS



Article added 2005

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