Part 2 of 2
"The short-lived Uranium Age will see the dawn of space flight; the succeeding era of fusion power will witness its fulfillment." --Arthur C. Clarke
The Solar System is vast beyond our ability to easily envision it. Between the Sun and Pluto's orbit exist trillions of cubic kilometers of absolute emptiness. Crossing these vast gulfs will be one of the greatest challenges the human race will ever face.
However, the instruments we have used so far to traverse this wilderness, chemical rockets, have been ill-suited for the task, being slow, clunky, and inefficient. Future planetary explorers will need vehicles that are up to the task, vehicles that can eat away the millions of kilometers between destinations quickly and cost-effectively.
In Part One of this article, we looked at the Cold War-era Projects Orion and Nerva, as well as other propulsion concepts based on nuclear fission. In Part 2, we will look at even more advanced nuclear propulsion technologies based on emerging technologies and cutting-edge research.
Fusion is the process that powers all the stars in all the galaxies and gives light to the universe.
Cheap and practical fusion power has been the holy grail of nuclear researchers for well over 50 years. Cleaner and far more efficient than the currently available fission power, it promises to lift human civilization into a new age. Fusion chain reactions have been created and sustained in laboratories -- but not yet efficiently enough to be used as a commercially viable power source.
Basically, fusion smashes together atomic nuclei to form new chemical elements, most commonly hydrogen or hydrogen isotopes to create helium. This reaction converts some of the mass of the hydrogen atoms into energy. This energy creates a superhot, high-velocity plasma, which can be ejected for thrust.
Theoretically, a fusion reaction can allow a very high specific impulse. A specific impulse measures how long a rocket can produce one pound of thrust using one pound of fuel; basically it's a measure of rocket motor efficiency. A modern liquid hydrogen/oxygen rocket, such as one of the Space Shuttle's main engines, can produce a specific impulse of 450 seconds. A fusion rocket is thought to be able to produce a specific impulse of about 130,000 seconds at extremely high thrust.
The normal conditions needed for this process are extreme to say the least -- millions of degrees of temperature and millions of newtons of pressure. Fusion also comes in a variety of reactions, some easier to achieve than others.
Many schemes for fusion rockets involve deuterium and tritium. Deuterium and tritium are both isotopes of hydrogen, deuterium having one extra neutron in the nucleus, and tritium having two. Deuterium-tritium (DT) fusion is the easiest reaction to obtain, requiring the lowest temperature and pressure, but it unfortunately produces a large number of high-speed neutrons as a byproduct. Neutrons are electrically neutral and therefore are not easily contained in magnetic fields. This presents a serious radiation hazard, requiring very heavy physical shielding. Worse yet, the shielding itself becomes radioactive after extended use and has to be disposed of. DT fusion rockets would therefore have to be fitted with heavier, much more expensive shielding than other fusion spacecraft for any extended mission.
Helium-3 is an isotope of helium missing one neutron from its nucleus. Deuterium-helium-3 (DHe3) fusion is also a fairly "easy" fusion reaction to obtain, and has the added benefit of not producing neutron radiation. However, helium-3 is rare on Earth and would have to be mined either from the moon's surface or the atmospheres of gas giants in order to be used in any quantity. DHe3 fusion may therefore not come into its own until the moon or the rest of the Solar System is fairly well trafficked.
Deuterium-deuterium (DD) fusion is harder to obtain, requiring much higher temperatures and pressure than the previous two reactions, but it releases more energy. Also, deuterium is much easier to obtain than tritium or helium-3, and many proposals call for mining it from the oceans.
Hydrogen-Hydrogen (HH) fusion, more properly called proton-chain fusion, uses plain old atomic hydrogen, requiring the insane temperatures and pressures found at the heart of stars. It also releases the most energy of all the fusion reactions.
Fusion rockets may also take advantage of muon-catalyzed fusion, where the orbital electron of hydrogen fuels is replaced with a muon. A muon carries the same electrical charge but is 207 times more massive than an electron, and therefore reduces the classical Bohr radius of an atom by a like amount. Thus, atomic nuclei are able to approach each other more closely and this enhances the likelihood of overlapping wave functions, increasing the probability of fusion.
Containing and directing fusion reactions in a spacecraft engine is almost always envisioned as involving powerful magnetic containment fields, also called magnetic bottles, to contain the reaction and direct the resulting plasma for exhaust.
One of the main drawbacks of fusion rockets is that the fusion reaction always produces radiation (in addition to the stray neutrons in DT fusion), requiring heavy shielding for the crew. The plasma exhaust can also prove highly radioactive, making the use of fusion rockets in or around life-bearing environments problematic.
Gas Dynamic Mirror Fusion Propulsion
The Gas Dynamic Mirror Fusion propulsion concept is currently undergoing research and development as part of NASA's Breakthrough Propulsion Project sponsored through the Marshall Space Flight Center.
One of the main problems with containing a fusion reaction in a confined space is the necessity of using curved magnetic fields, such as those used in tokamak reactors, to shape the reaction. Plasma, though electrically charged and reactive to magnetic fields, is still extremely volatile and difficult to control under the extreme pressures needed for fusion to take place. The curving of spherical magnetic fields with current technology inevitably leaves unforeseen weakened spots in the field through which the plasma can often escape, or the weakened spots skew the geometry of the plasma too much to sustain a useful reaction.
The solution is to use a long, thin, tubular magnetic bottle where most of the fusion reaction is contained by straight, easily-aligned magnetic field lines, generated by toroidal shaped superconducting magnets running the length of the reaction chamber. At either end of the reaction chamber are stronger "mirror" magnetic fields, which help focus the plasma and prevent it from escaping except for select apertures in the rear magnet, where exhaust is vented for thrust.
The plasma is heated with a powerful microwave antenna when it is first injected and the magnetic fields of the bottle further compress and heat it as it travels along its length in order to achieve a fusion reaction.
One of the signature secondary systems of the Gas Dynamic Mirror vehicles being developed is the enormous radiators attached to the long, thin reaction chamber, designed to keep both the containing walls cool and toroidal superconducting magnets cold enough to function optimally. Whether these will be necessary on future fusion-powered vessels remains to be seen.
The Gas Dynamic Mirror Fusion rocket is a sister project to the VASIMR plasma rocket, also being developed by NASA's Breakthrough Propulsion Project, and the two use many similar technologies and techniques.
Deep Plasma Focus
Much further up the technological development ladder is deep plasma focus fusion, basically a far more refined and powerful version of the Gas Dynamic Fusion rocket. While still mostly theoretical, its potential versatility and power could make it a very desirable option for researchers to pursue in the coming decades.
Deep Plasma Focus fusion is relatively simple in concept but difficult to achieve thanks to the power of the magnetic fields needed. Basically, plasma is forced into a long, cone-like magnetic "funnel" that continually compresses the plasma until, at the cone's apex, the pressure of the magnetic fields becomes greater than the particle pressure of the plasma, forcing the atomic nuclei together into a fusion reaction. The flow of the plasma forced into the magnetic funnel at ultra-high pressures keeps the entire system firing continuously.
These fusion rockets are also sometimes called spine drives, as the engine section would resemble a long, thin, tapering metal spine. Spine drives have been seen most notably in some of Larry Niven's Known Space stories.
Electron Beam Fusion
Another long-range proposal, Electron Beam Fusion postulates using a high-energy, relativistic (sped up to near-light speed) electron beam to catalyze a fusion reaction in a magnetically-bottled plasma, which is then released for thrust. The beam produces the fusion reaction explosively fast, requiring very powerful magnetic fields to contain it, and the system would in all likelihood have to be operated in rapid pulse mode.
Plasma rockets are currently being researched by NASA, specifically at Johnson Space Center's Advanced Propulsion Laboratory. Offshoots of earlier nuclear engine technologies, plasma rockets are capable or producing thrust as hot and energetic as nuclear rockets, but without much of the excessive radiation or radioactive exhaust. They may therefore ultimately prove to be cheaper than nuclear rockets, as they do not require heavy shielding to protect human crews from the drive reaction and, as the exhaust is not radioactive, they could be used as launch vehicles within Earth's atmosphere without fear of environmental contamination.
The hotter the exhaust of a rocket, the more thrust it will generate per unit of fuel. Plasma rockets heat their fuel to a plasma state, a state of matter where atoms are stripped of their electrons. Plasma exists at temperatures many thousands, even millions, of degrees beyond the burning gasses used in most conventional rocket exhausts. Plasma usually occurs in environments of high pressure and temperature, such as the Sun.
Since most standard materials cannot withstand such incredible temperatures, one of the primary features of a plasma rocket is containing the ignition reaction and directing the exhaust by means of electromagnetic fields and bottles. Since plasma is by definition electrically charged, it can be readily shaped and directed by fields of sufficient strength.
The key phrase above, of course, is "sufficient" strength. Containing and channeling the kind of material fueling the stars is no easy task, and is probably the single greatest technological hurdle to making high-temperature plasma rockets a reality.
Plasma rockets are thought to be able to eventually produce specific impulses in excess of 100,000 seconds, compared to a modern chemical rocket's specific impulse of 450 seconds.
Variable Specific Impulse Magnetoplasma Rocket (VASIMR)
VASIMR, being researched and developed in parallel to the Gas Dynamic Mirror Fusion project, uses powerful microwaves to heat its hydrogen fuel into a plasma state, and manipulates it using three powerful superconducting magnets, which further ionize and heat the plasma as it flows through.
The VASIMR system consists of three major magnetic cells, denoted as "forward," "central" and "aft." This particular configuration of electromagnets is called an asymmetric mirror.
The forward cell involves the main injection of gas to be turned into plasma and the ionization subsystem.
The central cell acts as an amplifier and serves to further heat the plasma up to 50,000 degrees Celsius with microwaves, operating similarly to the microwave oven in your home, but on a far larger scale.
The aft cell is more accurately a two-stage hybrid nozzle that ensures that the plasma will efficiently detach from the magnetic field, by greatly speeding it up and expelling it for thrust. Without the aft cell, the plasma would tend to follow the magnetic field and provide little thrust. With this configuration, the plasma can be guided and controlled over a wide range of plasma temperatures and densities.
Since the hydrogen fuel used is kept at very low temperatures, it can be used to not only help cool the engine as a whole but also to keep the superconducting materials within the magnets at proper operating temperature.
Designers also foresee the powerful magnetic fields of the engine acting as a supplementary radiation shield during interplanetary flight. Scaled-down versions of the VASIMR design may eventually be used for satellite station-keeping.
Combined Cycle Plasma/Fusion Rockets
One of the great advantages of the VASIMR scheme is that it can easily be modulated for many different power levels. Down the line, very advanced versions may be combined with fusion technologies, creating a combined-cycle plasma/fusion rocket. A hybrid plasma/fusion rocket would have a dynamically controlled thrust system, where the magnetic fields and/or catalyzing reaction are easily adjustable so the same engine can function as both a plasma or a fusion rocket, depending on circumstances.
This can prove very advantageous in spacecraft designed to operate within a planet's atmosphere as well as in deep space, as the vessel can use the plasma rocket mode for travel on or near inhabited planets and not have to worry about radioactive exhaust threatening a biosphere or human population. Once away from the planet, however, it can kick up the power to fusion mode for more rapid deep space travel.
Antimatter particles have the same mass as normal matter particles, but opposite electrical charges. Matter and antimatter mutually annihilate each other on contact and are converted to pure energy. This energy takes the form of gamma rays, neutrinos, antineutrinos, and/or pions. This total energy conversion makes forms of antimatter very attractive as a spacecraft fuel. Antimatter rockets are thought to be able to provide specific impulses of up to 10 million seconds.
Proton-antiproton collisions (as opposed to electron-positron or hydrogen-antihydrogen collisions) are preferred for propulsion, as the reaction produces a large percentage of charged particles (pions) that can be contained and directed for thrust with electromagnetic fields.
Antimatter reactions produce radiation as their byproduct, including gamma rays and pions, making heavy shielding an absolute necessity on all missions that would use the technology.
Antimatter responds as readily to magnetic fields as normal matter, so containing and directing antimatter for use in a spaceship engine does not represent as huge a problem as many assume. No, the true problem with using antimatter is with obtaining and storing it.
Antimatter is very rare and short-lived in nature, so it must be manufactured artificially. Today, it can only be produced in the amount of nanograms (billionths of a gram) per year at about $62.5 trillion per gram, making it the most expensive substance on Earth. This could be unfortunate, as dozens of kilograms may have to be made to make interplanetary flights possible. Production methods for creating and storing antimatter would have to be increased a billionfold while its cost would have to decrease on a similar scale before antimatter propulsion could be considered practical.
ICAN Micro Fusion/Fission Propulsion
ICAN stands for Ion Compressed Antimatter Nuclear. Very similar in principle to the Daedalus and VISTA concepts discussed in the first part of this article, the ICAN engine uses fuel pellets ignited to a fusion state by crossed lasers or particle beams. The resultant explosion is partially channeled by a concave magnetic nozzle to provide thrust.
The ICAN scheme uses pellets that contain uranium fission fuel as well as a fusion fuel mix of deuterium. The pellet is bombarded by the triggering beams, and at the moment of peak compression the pellet is also bombarded with a stream of antiprotons to catalyze the fission process. Ordinary uranium fission produces 2 to 3 neutrons per fission; by contrast, antiproton-induced uranium fission produces about 16 neutrons per fission. The released energy from the fission process ignites a high-efficiency fusion burn, resulting in the rapidly-expanding plasma used for thrust. Each reaction produces about as much energy as 20 tons of TNT. Pulsed as efficiently as possible, the ICAN scheme would produce a specific impulse of up to 17,000 seconds and a maximum velocity of 166,600 meters per second.
ICAN is significant in that it needs only a very modest amount of antimatter in order to work, on the order of the amount that can be produced and stored with today's technology.
The ICAN scheme is being studied by Penn State University and is being considered for a possible manned Mars missions. The most recent engine configuration, called ICAN-II, could theoretically make a trip to the red planet and back again in only 100 days.
AIMStar Antimatter Rocket
The AIM in AIMStar stands for Antimatter Initiated Microfusion. Like the ICAN scheme, the AIMStar is being developed by Penn State University, specifically for an interstellar "precursor" mission that would carry a probe well beyond the heliopause to a distance of 10,000 AUs from the sun. Also like the ICAN scheme, the AIMStar engine tries to make use of existing or near-term antimatter technology, specifically penning traps, and apply it to space propulsion.
A penning trap is basically a powerful magnetic bottle with specific electrical fields used to hold anti-protons. In the AIMStar scheme, pellets of fusion fuel are "shot" through the trap, compressed onto the outer layer of the antiparticle mass suspended in the trap as it passes through. The energy of the antimatter annihilations heats the plasma to a fusion state, which is then expelled for thrust.
After each such "burn" the antiprotons in the penning trap are allowed to reset back to their original configuration, minus about 0.5% of their original number, which was used up in the burn cycle annihilations. After every 50 burns, new antiprotons are injected into the magnetic bottle to reload the trap. The AIMStar engine would fire at about 200 burns per second.
Fuels being considered for the AIMStar are a deuterium-tritium (DT) mix and a deuterium-helium-3 (DHe3) mix. The DT fuel provides much more energy and higher thrust, but the tritium for the DT mix is much harder to obtain than helium-3 and the reaction produces far more radiation than the DHe3 fuel.
The AIMStar engine has an upper specific impulse of about 61,000 seconds.
Advanced Antimatter Rocket Concepts
ICAN and AIMStar are just the tip of the iceberg of the potential of antimatter technology. NASA has put forth a number of possible concepts.
A solid core antimatter rocket would function very similarly to NERVA solid-core nuclear rockets. Antiprotons annihilate protons, heating a tungsten or graphite heat exchanger. Hydrogen fuel is pumped through narrow channels between the heat exchangers, heating the hydrogen to a plasma state, which is then expelled for thrust. Because of the material limitations of the system, solid-core antimatter rockets would be capable of specific impulses of only 1000 or so seconds.
A gas-core antimatter rocket would inject antiprotons directly into the hydrogen fuel stream. Magnetic fields would be used to contain only the energetic charged pions which spiral into the hydrogen gas to heat it. The resultant plasma could then be expelled through a conventional rocket nozzle. Gas core antimatter rockets are less efficient than solid core models, but because they are less constrained by the melting points of their material components, they can achieve specific impulses of up to 2500 seconds.
A plasma core antimatter rocket is similar to the Gas Dynamic Mirror Fusion Propulsion engine. The gas core system uses a relatively small amount of antimatter to heat the hydrogen; the plasma core injects a much larger amount of antimatter into the hydrogen fuel, using powerful magnetic fields to contain the high energy pions that result from the annihilation reactions to heat the resultant plasma to a superheated state. This plasma is then expelled for thrust. This engine is not limited by the material melting points of its components, and thus can achieve specific impulses in excess of 100,000 seconds at significant thrust levels.
The beam-core antimatter thruster employs a converging magnetic field just upstream of the annihilation point between the antimatter and low-density hydrogen. The magnetic field then directly focuses the exhaust, which consists of energetic charged pions, which serve as propellant. Since the charged pions are traveling close to the speed of light, the specific impulse of the device could possibly range as high as 10 million seconds, but at very low thrust levels.
The Nuclear Future
The last century saw the birth of the Space Age, an era dominated by chemical rockets. In this century, the Chemical Space Age will run its course and fade into history, as the Nuclear Space Age will begin its ascendancy when humanity begins truly challenging the vast expanses of the universe. And in doing so, it will need spacecraft adequate to the task, vehicles far more powerful than even the Saturn V rockets that took man to the moon. Vehicles such as Orion, NERVA, VISTA, VASIMR, ICAN, AIMStar, and many more that we can only imagine . . . for now.
Lecture Notes on Fusion Propulsion
A site dedicated to various Fusion methods
Copyright © 2004 Paul Lucas
Paul Lucas grew up on the shores of Lake Erie just a few snow drifts away from Buffalo in the sleepy little town of Dunkirk, NY. He currently resides in Erie, PA, where he freelances as a writer and artist. His previous publication in Strange Horizons can be found in our Archive.