Part 1 of 2
Chemical rockets got humans to the moon. But no further.
Chemical rockets possess a number of inherent limitations, including their enormous fuel requirements and limited exhaust velocity. They're mighty impressive around the local orbital neighborhood, but on the open highways of the greater Solar System, they prove little better than a toddler on a tricycle.
In order to truly open up the other planets and beyond for both exploration and economic exploitation, humanity needs the space-borne equivalent of V8 engines. Fortunately, technologies capable of producing them are already being actively researched. The first true power vehicles of the space age will be not be based on chemical combustion, but on nuclear chain reactions.
Nuclear rockets are already a part of history. Two nuclear propulsion projects, Orion and NERVA, were actively pursued by the United States government during the Cold War, and even got to the point of test flights and engine firings.
Project Orion is the more well known of the two nuclear rocket projects, and has a number of proponents to this day. It is also one of those ideas that, at first, sounds completely insane. Even the most wide-eyed space enthusiasts tend to stutter and cough a bit at the prospect of getting into orbit by having a very real nuclear bomb touch off a hundred meters under their feet.
Yet, this is exactly what Project Orion envisioned: a spacecraft using nuclear bombs exploding under it to push it into space, much like a firecracker exploding under an empty soupcan can send it skyward, but on a far vaster scale.
This launch system was studied by private interests and the US government in the late '50s and early '60s. Project Orion-like ships have also been seen in many science fiction sources, most significantly in the novel Footfall, by Larry Niven and Jerry Pournelle, where it's used to fight alien invaders, and in the movie "Deep Impact," where it is used to deliver astronauts to an Earth-threatening comet.
This scheme for space travel, more formally called Nuclear Pulse Propulsion, was first proposed by Stanislaw Ulam and Cornelius Everett in a classified 1955 paper. According to 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.
Ulam's 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 begun at General Atomics, now a subsidiary of defense contractor General Dynamics. Taylor's main contribution was reconceptualizing 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 would be good at absorbing stray neutrons from the explosion, cutting down on radiation risk to the crew, and would break down into lightweight 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.
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 impulse (a unit used to measure rocket engine efficiency, basically how long a rocket can produce one pound of thrust using one pound of fuel) generated by Orion vessels can vary from 10,000 to 1,000,000 seconds. Compare this to the 450 seconds of modern hydrogen-fueled chemical rockets, and you have an enormously powerful spacecraft the likes of which modern-day NASA engineers can only salivate over.
A number of scale models were built, called Put-Puts and Hot Rods, and a scaled-down 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 explosive 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 thick enough to be able to last the length of any voyage. A graphite-laced oil spray was proposed for the pusher plate, in order to minimize ablation between pulses.
Orion Nuclear Pulse Drives are enormously powerful compared to modern rockets, and even compared to most other nuclear 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 eight million ton monster that would have carried a crew of 150 to Saturn and back in three years using 1.4 megaton pulse bombs.
The exact upper limits of an Orion vessel's velocity remains a matter for debate. 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.
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 today's political climate to even consider Orion as a possible launch vehicle as its original framers envisioned. Yet, Orion still has a great many fans within the aerospace and physics community, and recent changes in US nuclear policy could perhaps reopen the door for it to be used as a means of interplanetary propulsion in the future.
Project Orion Variants
Project Orion has had such an influence on space propulsion proponents that a number of offshoot ideas have emerged:
Helios: A parallel 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, about two and a half times that of a modern chemical rocket. However, a number of technical problems arose, most prominently how to keep the combustion chamber from exploding from the vast pressures of the atomic detonations. Also, producing fission bombs with yields as small as 1 kiloton or less is problematic with current known techniques.
Daedalus: In the 1970s, the British Interplanetary Society, as part of an effort to jump-start interest in interstellar exploration, conducted a design study on the feasibility of a process called inertial confinement fusion (ICF), which uses an array of simultaneously-fired, crossed laser or particle beams to implode pellets of fuel to fusion temperatures at the beams' 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, or a combination thereof. Detonation of about 250 pellets per second would enable, in the British Interplanetary Society's vision, a 55,000 ton vehicle to achieve about 10% lightspeed after 2 years of constant acceleration during a journey to one of the nearby stars.
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, as with the more modern VISTA design below.
VISTA is an acronym for Vehicle for Interplanetary Space Transport Applications, and is under preliminary study by NASA. This 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.
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 detonations' 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 less than 60 days.
Somewhat more conventional nuclear-powered rockets had been studied in one form or another for over 50 years, going back to the musings of the scientists struggling to build the first atomic bombs at Los Alamos. A program called NERVA (Nuclear Engines for Rocket Vehicle Applications) tested solid-core nuclear rocket concepts from 1961 to 1971, under a joint venture by the Atomic Energy Commission and NASA.
Solid-core fission rockets, the heart of NERVA, are fairly simple in concept. A reactive fluid, typically hydrogen, is pumped through numerous narrow channels in an active nuclear reactor, which heats the hydrogen into high-energy plasma. This plasma is then ejected from the ship, creating thrust. Solid-core fission rockets achieved specific impulses of 850 during the NERVA tests. Because these reactors were cooled with their own hydrogen fuel, they were able to achieve power densities ten times that of their more conventional water-cooled cousins.
The amount of radioactive exhaust released into the atmosphere from the numerous open-air NERVA engine tests was said to be "negligible" in official reports at the time, but what would be considered an acceptable amount of radioactive emissions in the late '60s might be a far different animal from what would be acceptable in today's political climate.
However, the reason for NERVA's cancellation had much more to do, ironically, with the space program's success at the time than with any potential political or environmental issues. Basically, at the time NASA was flush with prestige from the moon landings and none of its ongoing projects or long-range plans required a nuclear rocket for its success, so NERVA was shut down so money could be redirected toward more immediate pursuits.
In 1992, the first Bush administration launched the short-lived Space Exploration Initiative, which would have eventually led to a manned expedition to Mars. Interest in the old NERVA tests was revived and many related fission rocket schemes were studied.
Numerous space conferences and independent studies in the decades since the project's cancellation have yielded over a dozen different configurations and schemes for solid-core nuclear engines, based primarily on variations of existing nuclear reactors. Details on many of these can be found with the links at the end of this article.
PBR Rockets: The most promising of the current crop of solid-core designs has turned out to be the Particle Bed Reactor (PBR), a concept actively being pursued by the US Department of Defense as a space-based power source for missile defense.
Basically, nuclear fuel particles are suspended in a solid medium between two porous rotating drums called frits. The outer cold cylinder is made of stainless steel; the inner hot cylinder is made of tungsten and rhenium. The power output of the reactor can be controlled by the rates of spin on both the inner and outer cylinders.
Particle Bed Reactors are considered more desirable for fission rockets than other solid-core designs primarily because they can operate at lower temperatures but without the loss of efficiency because of their greater surface area to allow heat transfer. Also, they require much less fuel-injection pressure than other solid-core schemes, greatly reducing the potential stress on the engine.
Hydrogen fuel is pumped into the reactor in an inner radial direction, basically swirling at high speeds through the reactor until it reaches a superheated plasma state and is expelled for thrust. PBR fission rocket engines are estimated to have specific impulses of about 1300 seconds.
Gas Core Fission Rockets: One of the limitations of NERVA-like solid-core schemes is that they can only get so hot before they melt down, limiting the amount of energy they can impart to exhaust plasma. One way around this is to eliminate the solid core and replace it with a liquid or gaseous one. Various proponents have at times proposed liquid-core reactors, but most designers seem content to bypass them in favor of more efficient and powerful gas-core models.
Like solid-core fission rockets, there are numerous gas-core concepts based on different configurations and assumptions. The design currently showing the most promise, and actively being pursued at the Los Alamos National Laboratory, uses a toroidal vortex of uranium plasma. The fuel is shot through the center of the spinning "donut" of radioactive plasma, where it is superheated and most of it is ejected out of the spacecraft. However, a significant portion of the fuel is diverted to recirculate around the uranium plasma toroid, keeping the gaseous core under enough pressure to remain in a critical (i.e., undergoing nuclear fission) state. Fission rockets using this scheme are thought to have specific impulses in the range of 3000 to 5000 seconds.
However, several design problems have arisen, including how to keep the walls containing the reactor from melting down from the extreme heat, and how to efficiently inject more uranium plasma into the rapidly-spinning toroidal core once the engine is engaged to make up losses. Research is continuing.
Just the Beginning
Orion, NERVA, and their more modern intellectual descendants are far from the last word in nuclear space propulsion. Indeed, they may be only the first wave in what might eventually become a fleet of nuclear spaceships plying the Solar System. Fusion, plasma, and antimatter rockets, which will be discussed in in the next article, are also in various stages of design and development, and may yet take the lead in the path to space that their fission-based cousins helped to blaze.
Copyright © 2004 Paul Lucas
Paul Lucas is a freelance writer and artist hailing from the wilds of Erie, Pennsylvania. His previous publication in Strange Horizons can be found in our Archive.
An extensive links site to many articles and sources on nuclear space technologies
An in-depth discussion of the evolution of thought regarding nuclear power and propulsion in space
An article on using gas-core rockets on a manned mission to Mars