Somewhere, something incredible is waiting to be known. – Sagan


Is it time to reconsider the nuclear option?

Blame the Russians. Had the Soviet Union continued to apply pressure on the United States by maintaining it’s lead in space endeavors, we could have been riding nuclear powered space craft to travel back and forth to one of our moon-bases or perhaps sign up for one of the pilgrimages to Mars in order to join the colony’s efforts in that dusty red world. Alas, the competition was fierce, they over-extended themselves. Their devastating and catastrophic launch accidents forced them out of the game, giving the American space program time to… languish.

In all seriousness (let’s lay off the Russians; they are still sending up huge payloads for us), current deep space missions still rely on chemical and electric ion propulsion. We went to the moon using chemical propulsion and electric ion propulsion is used for small, light probes. We’re only starting to play with solar sails on inner planet missions. But for truly interplanetary and interstellar missions, missions that need to transverse great distances in a reasonable amount of time, something new needs to get on the drawing board. Clearly, we need a new way of getting about space.

Let’s go over some of the early propulsion designs that use nuclear energy in it’s varied forms.

Early History of Nuclear Propulsion

Nuclear propulsion, in it’s varied forms, has been proposed as a means to travel in space for decades. The Department of Energy started it’s research into nuclear rockets in 1952 and by the early 60s, Project Rover led to the formation of the Space Nuclear Propulsion Office. From this, the NERVA rocket was born. The design of the engine works similarly to a chemical rocket but uses nuclear fuel and liquid hydrogen. The hydrogen is heated up in a nuclear reactor and expanded through a nozzle to create thrust. This design is typically called a nuclear thermal rocket and outperforms chemical rockets by at least two-fold.

However, plans for the 75,000 pound-thrust (with a specific impulse1 of 825 seconds) NRX rocket were put on hold during the Nixon administration, but not before all test objectives being accomplished successfully. The test rocket performed for nearly 2 hours, exceeding all expectations. A series of political and budgetary setbacks in the late 60s and early 70s basically shelved the NERVA engine with the program being officially disbanded by 1972.

Project Orion

Project Orion spacecraft

Project Orion was officially started in 1958, but the basic idea was concocted in the 40s and early 50s. The official project was headed up by Ted Taylor and physicist Freeman Dyson while at Los Alamos Laboratory. At first, the project promised a revolutionary solution for space travel; it combined high thrust and high Isp (specific impulse). With many designs you have to balance the thrust and specific impulse requirements and suffer the tradeoffs. Orion offered the best of both worlds.

So, how does it work? Well, imagine a spaceship with a cargo hold filled with thousands of mini nuclear bombs called “pulse units”. Pulse units would be ejected out the back of the ship, detonated at a specific distance, and the resulting shockwave would push against a pusher-plate separated by shock-absorbers connected to the rest of the ship. With such a design, the size of the ship was only limited to what we could build. An ambitious 8 million ton city-sized ship was even contemplated by engineers at the time.

While missions to the moon, Mars, and the outer planets were being proposed, Freeman Dyson was thinking interstellar. His 1968 paper “Interstellar Transport” outlined a design that would use deuterium fusion instead of fission pulse units to propel a large ship out of the solar system with Alpha Centuri being it’s destination.

Orion had to overcome some big engineering problems. The pusher plate would suffer from erosion, or ablation, if not constantly coated with some substance like a lubricating oil. But the main issue remained nuclear fallout. Any nuclear explosion in the Earth’s magnetosphere would be funneled back to Earth unless the ship was launched from one of the poles.

Either way, Orion was shelved because of the time the idea was conjured up in. Nuclear anything was unpopular with the public and international community. Ultimately, the signing of the Partial Test Ban Treaty in 1963 and decision to proceed with using chemical rockets to reach the moon ended Orion’s prospects at ever seeing the light of day again.

Fusion or Fission?

When we first split the atom in the 30s, it paved the way for nuclear fission reactors, the type of reactor that still power much of the world. Ever since then, we’ve been looking for a way to harness a much cleaner type of reaction, the fusion reaction. Instead of splitting apart heavy atoms to release monstrous amounts of energy, fusion combines simple nuclei, such as helium or hydrogen, to release energy. In order to fuse nuclei together, you need two things: heat and pressure. And it’s not as easy as it sounds.The sun does it by applying a tremendous amount of pressure and some 15,000,000 C to fuse hydrogen into helium. But the Sun has help from the gravity it’s tremendous mass affords it. We on Earth (or in space) have to find other ways to match those conditions.

Fission reactors have the advantage of being a proven technology that has made a lot of advances. However, perceived safety has always been an issue. In the past, catastrophic meltdowns have been the cause of great concern, the worst being the incident at Chernobyl. But with the proper safeguards in place, operating a fission reactor in space should not be a problem. Launching it, though, will present it’s own issues. Needless to say, an launch accident high enough from the ground would result in serious fallout if the proper precautions were not in place.

The ITER tokamak. Image: ITER

Fusion relies on the fusing together of non-radioactive fuel. Therefore, in the event of a launch accident, the damage would be restricted to the launch vehicle and shrapnel. Generating a stable, self-sustaining fusion reaction is another matter. Efforts in this field are being pursued and a breakthrough is supposedly around the corner. One such effort is a new 15 billion euro plan to build ITER, the International Thermonuclear Experimental Reactor near Marseille in France. Another domestic effort in the works is based off of Dr. Richard Bussard’s life work in the pursuit of nuclear fusion, which received $8 million in funding back in 2009.

So what is the best option for a deep-space craft? In the next few years we will know if designers should be swapping their fission reactors for fusion ones, but the preferred option would be fusion. It’s cleaner and fuel for it is much more abundant in the interstellar medium than uranium or another fissionable material would be. The good news is, for electricity production, the reactor type is interchangeable, even late in the design process.

Project Daedalus and It’s Offshoots

Like the Phoenix, Project Daedalus rose from the ashes of Orion in an effort to resurrect the idea behind using nuclear technology to reach the stars. Project Daedalus was designed by the British Interplanetary Society (B.I.S.) in the mid 70s and was envisioned as a robotic interstellar probe to Barnard’s Star. Barnard’s Star is relatively close, only about 6 light years away. It’s an old star, and at the time, was thought to have some sort of planetary system. Recent data suggests that there is actually no planets orbiting Barnard’s Star, however, Daedalus is flexible enough for other interesting destinations not too far from home.

Project Daedalus proposed to work around the restriction of the Partial Test Ban Treaty by using a fusion micro explosion rocket. The ship would use one gram deuterium/helium-3 pellets, at a rate of 250 per second, that would be ignited by particle or laser beams and powered by a closed cycle fusion reactor to produce very small fusion explosions. The resulting plasma would be directed by a magnetic nozzle to drive the ship forward. Daedalus’ top speed would be about .12c.

Daedalus would have to be constructed in orbit, launched from Earth piece by piece. The Helium-3 fuel necessary could be harvested from Jupiter’s upper atmosphere or the lunar surface, where it is abundant.

In 1989, a similar concept to Daedalus was studied by the U.S. Navy and NASA called Project Longshot. Since the project was designed to use only existing technology, a nuclear fission reactor was at the heart of Longshot. The reactor powered several lasers that would ignite the helium-3 pellets, similar to Daedalus. However, Longshot’s top speed would be limited to about .045c (4.5% the speed of light). The status of Project Longshot is unknown.

Artist

The dream of Daedalus is still alive, even if it goes by another name. The concept is sound and the technology that could make it a reality is within our grasp. The British Interplanetary Society still heads up research into Daedalus and it’s concepts, and it was a hot topic at the 100 Year Starship Symposium earlier this month. In fact, in 2009, the BIS initiated a volunteer study called Project Icarus, inspired by Daedalus, would last 5 years with the objective of studying all aspects of what would be involved in designing an interstellar spacecraft. Their goal is to design, not only a spacecraft, but a mission that is based on credible science2.

The Bussard Fusion Ramjet

In 1960, the physicist Robert W. Bussard proposed a method of propulsion for spacecraft that involved using hydrogen fusion. The resultant exhaust would be channeled out a rocket nozzle producing thrust. This method of propulsion wasn’t a ground-breaking revolutionary idea at the time, but the method the craft used to collect fuel was. Instead of carrying it’s fuel, the Ramjet would collect the necessary hydrogen fuel from interstellar space. The exact method if does this varies, but most feasible designs rely on a huge electromagnetic scoop that channels hydrogen nuclei from the interstellar medium into the reaction chamber.

Over time, the Bussard Ramjet fell out of favor, mainly because of some of the flaws that were pointed out early on. Drag from the interstellar medium was a big concern because the necessary scoop size for a 1,000 ton craft was nearly 10,000km3! A solution to this problem involves using a high powered laser to ionize the hydrogen ahead of the ship and collected via a relatively large electromagnetic scoop. The scoop area necessary would be huge, about the diameter of Jupiter, meaning it would need to be a huge electromagnetic field, and the coils needed to produce such a huge magnetic field that could withstand the stresses involved are not yet technically feasible.

Another issue is that in order for the ship to be able to run on it’s own collecting hydrogen from the medium, it needs to be traveling at .06c, or about 6% the speed of light. Not an easy feat, but not a supremely difficult one, either. What it does mean is that the ship will have to have enough fuel to get it up to speed. Most likely a secondary propulsion method in order to jumpstart the process.

Bussard ramjet. Illustration courtesy Popular Science (June 1999)

Renewed interest in the Bussard Ramjet came in 1974, when rocket engineer Alan Bond proposed a variant of the Ramjet referred to as the RAIR, or Ram-Augmented Interstellar Rocket. Instead of using the collected hydrogen as fuel, it would instead be used as reaction mass. It involved using the incoming ionized hydrogen nuclei stream, basically protons, and bombarding it against lithium or boron. This type of fusion is much easier to induce and releases more energy. A refined method of RAIR, catalyzed-RAIR, adds a tiny bit of antimatter to the compressed stream. This method does rely on quite a bit of antimatter, something we only have experience in producing minuscule amounts of, but offers a fantastic energy/mass ratio ship. It basically lets us bring more cargo with less fuel. Deceleration of the craft can be handled by the drag created by accumulating hydrogen from the medium with a wide electromagnetic scoop. Barring some technology that lets us create abundant amounts of anti-matter or we discover a new source of anti-matter, this solution may not be quite feasible at the moment.

Another alternative is to use the collected ions as reaction mass in an electric ion or plasma engine, two engine designs we do have some experience with.

Bussard fusion coils (WB6)

In 2009, an IEC (inertial electrostatic) fusion reactor design received $8m in funding, and if it works well, would have far reaching applications, including a clean reactor design suitable for spacecraft. The project is based off a 2007 paper2 by Dr. Richard Bussard. Dr. Richard Nebel is leading the IEC/Bussard Fusion project, and a full-scale demonstration of it’s feasibility should be made by 2015. IEC fusion uses magnets to contain an electron cloud in the center of the reaction chamber. Deuterium, lithium or boron fuel is then injected into the chamber in the form of positive ions. The positive ions get attracted to the high negative charge of the electron cloud at a speed that starts the fusion reaction.

This type of fusion reactor would have ramification beyond space exploration, but in terms of advancing space travel, it would be a boon. The fact that it is a clean reactor that can be launched from Earth without any worry of radioactive contamination being spread across a land area in case of an accident is in itself an enormous win.

VASIMR – A Nuclear Powered Plasma Rocket

Work on the VASIMR, Variable Specific Impulse Magnetoplasma Rocket, started in 1977 and uses radio waves to ionize and heat up the propellant through magnetic fields creating a plasma that is then accelerated out the back generating thrust. How much thrust are we talking about here? As it turns out, not much. Only about … . But where it lacks in thrust, it makes up in Isp (specific impulse). 5,000 seconds as a high end right now, possibly higher in the future. To put it in terms easier to understand, this technology has the ability to shorten a mission to Mars to a few weeks versus almost 9 months using conventional chemical rockets. Because of it’s low thrust, a ship based off the VASIMR will need to be launched into deep space using traditional methods, probably even be built in orbit.

Wait a minute. Hold on there for a second. There is no nuclear activity going on here, this is strictly an electrical reaction that is generating the thrust. Why is it in the purview of this article? You are correct, astute reader, however, to produce the copious amounts of electricity needed for intra-planetary travel, we need something like a nuclear reactor to power the craft. Therefore, in the context of manned or large payload space travel, VASIMR is usually teamed up with an on-board fission or fusion nuclear power plant.

Diagram of a VASIMR engine. Image: NASA

Project Prometheus

Project Prometheus was started by NASA in 2003 with the intent of designing a ship for long duration, deep space missions. Like VASIMR, the ship would use a nuclear reactor to power engines indirectly, in Prometheus’ case, an electric ion engine called a HiPEP (High Power Electric Propulsion). The project was initiated because using solar powered ion engines to explore the outer planets and their moons would be ineffective due to the distance from the Sun. Therefore, Prometheus required a small nuclear reactor at it’s core to increase longevity and power. The project was based on technology at our disposal, but budget cuts terminated Prometheus in 2005.

The Verne Gun – Getting Raw Material into Space, Cheap

The idea behind the Verne Gun is simple, yet terrifying: dig a mile deep shaft, install a set of guide rails along the length of the shaft, pump the chamber full of reaction mass (salt water), and detonate a nuclear explosive device in the middle of the reaction mass, thereby accelerating the poor craft at the top of the shaft at breakneck speed into space.

This is strictly a terrestrial launch system and suitable strictly for raw cargo. We’re talking about 1,000s of Gs acceleration. The advantages are huge in terms of cost of getting raw materials into space: you’re looking at $5 to $20 per pound. This cargo would be restricted to the most basic of cargo, mainly refined metal.

Personally, I think this method is not a suitable solution to the problem of getting large quantities of raw material into space. That’s why asteroid or lunar mining proposals are generally favored to lifting off every single pound of raw materials.

Practical Designs For the Near Future

I think society as a whole is on the verge of overcoming it’s stigma with regards to using nuclear energy in space. Eventually, fear and irrationality will give way to logic. We already use nuclear power to power our naval ships and subs. While having a nuclear launch system still poses significant issues with regards to fallout and radioactive exposure, I believe technology will overcome the hurdle of launching a spaceship capable of interplanetary or interstellar travel. Heavy lift rockets may be a solution, but commercial space entities are also having a lot of success using space planes to deliver payloads into LEO while using ion thrusters to nudge payloads into even higher orbits.

Safety will always be a concern, but with the advent of fusion type reactors, the risk to the public from even a catastrophic launch into space would be mitigated by the use of non-radioactive fuel sources. Even with a fission reactor, there are ways to protect it from causing any sort of fallout in the event of an accident during launch.

I look forward to seeing what Project Icarus proposes in 2014, when their 5 year study should be coming to an end, especially in light of some exiting developments in nuclear fusion technology in the last decade.

Ultimately, it will come down to mission objectives and funding which are inextricably tied. We have no problems coming up with mission objectives. For example, in another few years, Kepler will have given us an extensive list of Sun-like stars within 30 light years that have rocky planets in the habitable zone. Funding a spaceship of this magnitude will be of concern, of course. For strictly scientific missions, with little or no return on investment, you’re probably looking at government or internationally funded programs. For more profit driven missions, like exploratory missions to the outer planets and asteroid belt in order to mine their resources, a commercial entity would be more apt to invest in such a large venture. Despite being in an economic crisis right now, it’s well known in the space community that difficult and enormous projects like the ones in this article spark amazing technical innovations that not only apply to space exploration, but can help improve countless lives on this planet as well.

The technology is within our grasp for some of the ideas on the drawing board. It’s no surprise that some of the objectives of groups such as the Icarus Project or the Tau Zero Foundation is to educate and excite the public into wanting — scratch that — demanding we aim for the stars. Forward ho!

Footnotes

1. Described in seconds, specific impulse is the way we measure the efficiency of a rocket, electric ion engine, or other method of propulsion. It’s the thrust divided by the amount of propellant used per second.

2. For more information on what an Icarus inspired interstellar mission would look like, please see this Feb 2011 Discovery article on the subject.

3. For more information on Dr. Bussard’s paper, download the pdf.

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