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FUSION ROCKETS


Gas Dynamic Mirror Fusion Propulsion
Tech level: 14
Deep Plasma Focus Fusion Propulsion
Tech Level: 15
Electron Beam Fusion Propulsion
Tech Level: 15
Contained Inertial Confinement Fusion Propulsion
Tech Level: 16
Combined Cycle Plasma/Fusion Rockets
Tech Level: 16
Pulsed Magnetic Compression Fusion Propulsion
Tech level: 17

"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


One type of fusion propulsion, Inertial Confinement Fusion, is discussed at length in the Nuclear Pulse Drive section. Antimatter-assisted and Gravitics-assisted fusion rockets will be covered in their appropriate sections.

Fusion is the process that powers most of the stars in all the galaxies and gives light to the universe. An uncontrolled nuclear fusion chain reaction is what makes hydrogen bombs so much more powerful than "mere" fission bombs. It is one of the most fundamental sources of power in creation.

Cheap and practical fusion power has also been the holy grail of nuclear researchers for well over 50 years now. Cleaner and far more efficient than currently-available fission power, it promises to lift modern-day earth into a new age. Fusion chain reactions have been created and susutained in laboratories--but not long enough or efficient enough to be used as a commercially viable power source.

Fusion propulsion of one form or another is also used everywhere in science fiction, from Star Trek and Star Wars to the "Known Space" stories of Larry Niven and the Foundation series of novels by Isaac Asimov.

BASICS

Fusion smashes together atomic nuclei to form new chemical elements, most commonly hydrogen or isotopes thereof are fused to create helium. This reaction converts some of the mass of the former hydrogen atoms into energy. This energy creates a superhot, high velocity plasma, which is then ejected for thrust. Fusion rockets are thought to provide specific impulses of up to 130,000 seconds at extremely high thrust.

The normal conditions needed for this process are extreme to say the least--millions of degrees of temperatures and millions of pounds of pressure. Fusion also comes in a variety of reactions, some easier to achieve than others.

Deuterium-tritium (DT) fusion is the easiest fusion reaction to obtain, but it unfortunately produces high-speed neutrons as a byproduct of the reaction. 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 retrofitted with new shielding after any extended mission.

Deuterium-Helium-3 (DH3) fusion is also a fairly "easy" fusion reaction to obtain, and has the added benefit of not producing neutronic radiation. However, Helium-3 is rare on Earth and would have to be mined either from the moonís surface or the atmospheres of gas giants in order to be used in any quantity. DH3 fusion will therefore probably not come into its own until outposts on the moon or the rest of the Solar System are well established.

Deuterium-deuterium (DD) fusion is harder to obtain, requiring much higher temperatures and pressure than the previous two reactions, but intrinsically releases more energy. Also, deuterium is much easier to obtain than tritium or Helium-3.

Hydrogen-Hydrogen (HH) fusion, or more properly 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 of heavier elements is of course possible, and is going on all the time in the heart of supermassive stars and in stars leaving the main sequence. However, they are usually not considered practical as a means of power or propulsion.

Fusion rockets may also take advantage of Muon Catalyzed Fusion, where the electron of hydrogen fuels is replaced with a muon. A muon is 207 times larger than an electron, and therefore reduces the classical Bohr radius of an atom by 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 and plasma in a spacecraft engine is almost always envisioned as involving powerful magnetic containment fields, also called magnetic "bottles," to contain the reaction and direct ther resulting plasma for exhaust.

One of the main drawbacks of fusion rockets is that the fusion reaction always produces radiation (in addition to the neutrons in DT fusion), requiring heavy shielding for the crew. The plasma exhaust is also highly radioactive, making their use on or around inhabited worlds problematic.


GAS DYNAMIC MIRROR FUSION PROPULSION
Tech Level: 14

One of the main problems with containing a fusion reaction in a confined space is the necessity of using curved magnetic fields to shape the reaction. Plasma, though reactive to magnetic fields, is still extrememly energetic 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 skews the geometry of the plasma too much to sustain a useful reaction.

The solution is to use a long, thin 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 and the magnetic fields of the bottle further compress and heat it in order to achieve a fusion reaction.

Cutaway diagram of Gas Dynamic Mirror system.

One of the signature secondary systems of a Gas Dynamic Mirror vehicle are the enormous radiators attached to the long, thin reaction chamber, designed to keep both the containing walls cool and toroidal 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, along with the VASIMR plasma rocket, is one of the two propulsion schemes undergoing serious research for use on a manned mission to Mars.


DEEP PLASMA FOCUS FUSION PROPULSION
Tech Level: 15

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 coneís apex, the pressure of the magnetic field becomes greater than the particle pressure of the plasma, forcing the atomic nucleii 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 resembles a long, thin, tapering metal spine. Spine drives have been seen in some of Larry Nivenís Known Space stories.

By adjusting the magnetic field strength as well as the width of its fusion apex, this scheme can be scaled down somewhat for use as a plasma rocket.


ELECTRON BEAM FUSION PROPULSION
Tech level: 15

This scheme 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.

The system can be scaled down in power somewhat for use as a Plasma Rocket (q.v.)


CONTAINED INERTIAL CONFINEMENT FUSION
Tech Level: 16

Both the Daedalus and VISTA projects envision using Inertial Confinement Fusion (ICF), where ejected pellets of fuel are shot by crossed-focus high energy beams of lasers, electrons, or heavier particles. The alignment and force of the beams creates both the pressures and temperatures needed for fusion to take place.

However, both Daedalus and VISTA use an open ICF system, where the detonations created by the fusing material take place in space behind the vehicle, propeling the craft forward Orion-like. However, using powerful, advanced magnetic fields, it may one day possible to contain the reaction completely, and thus utilize much more of the detonationsí energy for propulsion. Like electron beam fusion, the system would by necessity have to operated in rapid pulse mode.


COMBINED CYCLE PLASMA/FUSION ROCKETS
Tech Level: 16
Though often referred to as using "ion drives", sublight engines in the Star Wars universe seem more akin to combined cycle plasma/fusion rockets. Image (c) Lucasfilm.

Both the Deep Plasma Focus and the Electron Beam schemes can be scaled down for use as a plasma rocket. A combined cycle plasma/fusion rocket has a dynamic controlled thrust system, where the magnetic fields and/or catalyzing electron beam are easily adjustable so the same engine can function as both a plasma and a fusion rocket of varying outputs, depending on circumstances.

This can prove very advantageous in craft designed to operate both within a planetís atmosphere as well as in deep space, as the vessel can use the plasma rocket mode for travel on or near planets and not have to worry about radioactive exhaust threatening a biosphere or human population. Once away fom the planet, however, it can kick up the power to fusion mode for rapid deep space travel.

Most of the unnamed "thruster" systems seen on many science fiction TV shows and movies (particularly Star Wars and its many clones) behave very similarly to combined cycle plasma/fusion rockets.


PULSED MAGNETIC COMPRESSION FUSION PROPULSION
Tech Level: 17

This is a very straight-forward--and powerful--system, using collapsing magnetic fields of unbelievable strength to near-instantly squeeze fuel plasma into a fusion state. Used in rapid pulse mode, from several hundred to several thousand reactions per second, it can provide extremely high thrust.

It can also be scaled down for plasma rocket use, and can therefore be used as a combined cycle plasma/fusion rocket.


RELATED INFORMATION

In Print:

Fire Fusion, and Steel:The Traveller Technical Architecture by Frank Chadwick, Dave Nilsen, and many others.

Writerís Guide to Creating a Science Fiction Universe, by George Ochoa and Jeffrey Ossier

On the Web:

Lecture Notes on Fusion Propulsion:

http://rigel.neep.wisc.edu/~jfs/neep533_lect32_99_fusionProp.html

An extensive article on fusion and fusion propulsion:

http://www1.msfc.nasa.gov/STD/propulsion/research/fusion/gdm/fusiongen.html

Using helium-3 as a fusion fuel source:

http://www.totse.com/en/technology/space_astronomy_nasa/new_fuel.html

A site dedicated to various Fusion methods:

http://www.aa.washington.edu/AERP/Propulsion/index4.htm

An article on Electron Beam Fusion:

http://www1.msfc.nasa.gov/STD/propulsion/research/fusion/fusion_ebeam.html




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