Fusion rocket
Encyclopedia
A fusion rocket is a theoretical design for a rocket
Rocket
A rocket is a missile, spacecraft, aircraft or other vehicle which obtains thrust from a rocket engine. In all rockets, the exhaust is formed entirely from propellants carried within the rocket before use. Rocket engines work by action and reaction...

 driven by fusion power
Fusion power
Fusion power is the power generated by nuclear fusion processes. In fusion reactions two light atomic nuclei fuse together to form a heavier nucleus . In doing so they release a comparatively large amount of energy arising from the binding energy due to the strong nuclear force which is manifested...

 which could provide efficient and long-term acceleration in space
Spacecraft propulsion
Spacecraft propulsion is any method used to accelerate spacecraft and artificial satellites. There are many different methods. Each method has drawbacks and advantages, and spacecraft propulsion is an active area of research. However, most spacecraft today are propelled by forcing a gas from the...

 without the need to carry a large fuel supply. The design relies on the development of fusion power technology beyond current capabilities, and the construction of rockets much larger and more complex than any current spacecraft
Spacecraft
A spacecraft or spaceship is a craft or machine designed for spaceflight. Spacecraft are used for a variety of purposes, including communications, earth observation, meteorology, navigation, planetary exploration and transportation of humans and cargo....

. A smaller and lighter fusion reactor might be possible in the future when more sophisticated methods have been devised to control magnetic confinement and prevent plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...

 instabilities.

For space flight, the main advantage of fusion would be the very high specific impulse
Specific impulse
Specific impulse is a way to describe the efficiency of rocket and jet engines. It represents the derivative of the impulse with respect to amount of propellant used, i.e., the thrust divided by the amount of propellant used per unit time. If the "amount" of propellant is given in terms of mass ,...

, the main disadvantage the (probable) large mass of the reactor. In addition, a fusion rocket may produce less radiation than a fission
Nuclear fission
In nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts , often producing free neutrons and photons , and releasing a tremendous amount of energy...

 rocket, reducing the mass needed for shielding. The surest way of building a fusion rocket with current technology is to use hydrogen bombs as proposed in Project Orion
Project Orion (nuclear propulsion)
Project Orion was a study of a spacecraft intended to be directly propelled by a series of explosions of atomic bombs behind the craft...

, but such a spacecraft would also be massive.

Electricity generation vs. direct thrust

Many spacecraft propulsion methods such as ion thruster
Ion thruster
An ion thruster is a form of electric propulsion used for spacecraft propulsion that creates thrust by accelerating ions. Ion thrusters are categorized by how they accelerate the ions, using either electrostatic or electromagnetic force. Electrostatic ion thrusters use the Coulomb force and...

s require an input of electric power to run but are highly efficient. In some cases their maximum thrust is limited by the amount of power that can be generated (for example, a mass driver
Mass driver
A mass driver or electromagnetic catapult is a proposed method of non-rocket spacelaunch which would use a linear motor to accelerate and catapult payloads up to high speeds. All existing and contemplated mass drivers use coils of wire energized by electricity to make electromagnets. Sequential...

). An electric generator that ran on fusion power could be installed purely to drive such a ship. One disadvantage is that conventional electricity production requires a low-temperature energy sink, which is difficult (i.e. heavy) in a spacecraft. Direct conversion of the kinetic energy of the fusion products into electricity is in principle possible and would mitigate this problem.

An attractive possibility is to simply direct the exhaust of fusion product out the back of the rocket to provide thrust without the intermediate production of electricity. This would be easier with some confinement schemes (e.g. magnetic mirror
Magnetic mirror
A magnetic mirror is a magnetic field configuration where the field strength changes when moving along a field line. The mirror effect results in a tendency for charged particles to bounce back from the high field region....

s) than with others (e.g. tokamak
Tokamak
A tokamak is a device using a magnetic field to confine a plasma in the shape of a torus . Achieving a stable plasma equilibrium requires magnetic field lines that move around the torus in a helical shape...

s). It is also more attractive for "advanced fuels" (see aneutronic fusion
Aneutronic fusion
Aneutronic fusion is any form of fusion power where neutrons carry no more than 1% of the total released energy. The most-studied fusion reactions release up to 80% of their energy in neutrons...

). Helium-3 propulsion is a proposed method of spacecraft propulsion that uses the fusion of helium-3
Helium-3
Helium-3 is a light, non-radioactive isotope of helium with two protons and one neutron. It is rare on Earth, and is sought for use in nuclear fusion research...

 atoms as a power source. Helium-3, an isotope
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...

 of helium with two proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....

s and one neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...

, could be fused with deuterium
Deuterium
Deuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...

 in a reactor. The resulting energy release could be used to expel propellant out the back of the spacecraft. Helium-3 is proposed as a power source for spacecraft mainly because of its abundance on the moon. Currently, scientists estimate that there are 1 million tons of helium-3 present on the moon, mainly due to solar wind colliding with the moon's surface and depositing it, among other elements, into the soil. Only 20% of the power produced by the D-T reaction could be used this way; the other 80% is released in the form of neutrons which, because they cannot be directed by magnetic fields or solid walls, would be very difficult to use for thrust. Helium-3 is also produced via beta decay
Beta decay
In nuclear physics, beta decay is a type of radioactive decay in which a beta particle is emitted from an atom. There are two types of beta decay: beta minus and beta plus. In the case of beta decay that produces an electron emission, it is referred to as beta minus , while in the case of a...

 of tritium
Tritium
Tritium is a radioactive isotope of hydrogen. The nucleus of tritium contains one proton and two neutrons, whereas the nucleus of protium contains one proton and no neutrons...

, which in turn can be produced from deuterium, lithium, or boron.

Even if a self-sustaining fusion reaction cannot be produced, it might be possible to use fusion to boost the efficiency of another propulsion system, such as a VASIMR
Variable specific impulse magnetoplasma rocket
The Variable Specific Impulse Magnetoplasma Rocket is an electro-magnetic thruster for spacecraft propulsion. It uses radio waves to ionize and heat a propellant and magnetic fields to accelerate the resulting plasma to generate thrust...

 engine.

Confinement concept

To sustain a fusion reaction, the plasma must be confined. The most widely studied configuration for terrestrial fusion is the tokamak
Tokamak
A tokamak is a device using a magnetic field to confine a plasma in the shape of a torus . Achieving a stable plasma equilibrium requires magnetic field lines that move around the torus in a helical shape...

, a form of magnetic confinement fusion
Magnetic confinement fusion
Magnetic confinement fusion is an approach to generating fusion power that uses magnetic fields to confine the hot fusion fuel in the form of a plasma. Magnetic confinement is one of two major branches of fusion energy research, the other being inertial confinement fusion. The magnetic approach is...

. Currently tokamaks weigh a great deal, so the thrust to weight ratio would seem unacceptable. NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

's Glenn Research Center
Glenn Research Center
NASA John H. Glenn Research Center at Lewis Field is a NASA center, located within the cities of Brook Park, Cleveland and Fairview Park, Ohio between Cleveland Hopkins International Airport and the Cleveland Metroparks's Rocky River Reservation, and has other subsidiary facilities in Ohio...

 has proposed a small aspect ratio spherical torus reactor for its "Discovery II" conceptual vehicle design. "Discovery II" could deliver a manned 172 000-kilogram payload to Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

 in 118 days (or 212 days to Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...

) using 861 metric tons of hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

 propellant, plus 11 metric tons of Helium-3
Helium-3
Helium-3 is a light, non-radioactive isotope of helium with two protons and one neutron. It is rare on Earth, and is sought for use in nuclear fusion research...

-Deuterium
Deuterium
Deuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...

 (D-He3) fusion fuel. The hydrogen is heated by the fusion plasma debris to increase thrust, at a cost of reduced exhaust velocity (348–463 km/s) and hence increased propellant mass.

The main alternative to magnetic confinement is inertial confinement fusion
Inertial confinement fusion
Inertial confinement fusion is a process where nuclear fusion reactions are initiated by heating and compressing a fuel target, typically in the form of a pellet that most often contains a mixture of deuterium and tritium....

, such as that proposed by Project Daedalus
Project Daedalus
Project Daedalus was a study conducted between 1973 and 1978 by the British Interplanetary Society to design a plausible unmanned interstellar spacecraft. Intended mainly as a scientific probe, the design criteria specified that the spacecraft had to use current or near-future technology and had to...

. A small pellet of fusion fuel (with a diameter of a couple of millimeters) would be ignited by an electron beam or a laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...

. To produce direct thrust, a magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...

 would form the pusher plate. In principle, the Helium-3 - Deuterium reaction or an aneutronic fusion
Aneutronic fusion
Aneutronic fusion is any form of fusion power where neutrons carry no more than 1% of the total released energy. The most-studied fusion reactions release up to 80% of their energy in neutrons...

 reaction could be used to maximize the energy in charged particles and to minimize radiation, but it is highly questionable whether it is technically feasible to use these reactions. Both the detailed design studies in the 1970s, the Orion drive and Project Daedalus
Project Daedalus
Project Daedalus was a study conducted between 1973 and 1978 by the British Interplanetary Society to design a plausible unmanned interstellar spacecraft. Intended mainly as a scientific probe, the design criteria specified that the spacecraft had to use current or near-future technology and had to...

, used inertial confinement. In the 1980s, Lawrence Livermore National Laboratory
Lawrence Livermore National Laboratory
The Lawrence Livermore National Laboratory , just outside Livermore, California, is a Federally Funded Research and Development Center founded by the University of California in 1952...

 and NASA studied an ICF-powered "Vehicle for Interplanetary Transport Applications" (VISTA). The conical VISTA spacecraft could deliver a 100-tonne payload to Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

 orbit and return to Earth in 130 days, or to Jupiter orbit and back in 403 days. 41 tonnes of deuterium
Deuterium
Deuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...

/tritium
Tritium
Tritium is a radioactive isotope of hydrogen. The nucleus of tritium contains one proton and two neutrons, whereas the nucleus of protium contains one proton and no neutrons...

 (D-T) fusion fuel would be required, plus 4124 tonnes of hydrogen expellant. The exhaust velocity would be 157km/s.

Magnetized target fusion
Magnetized target fusion
Magnetized target fusion is a relatively new approach to producing fusion power that combines features of the more widely studied magnetic confinement fusion and inertial confinement fusion approaches. Like the magnetic approach, the fusion fuel is confined at lower density by magnetic fields...

 (MTF) is a relatively new approach that combines the best features of the more widely studied magnetic confinement fusion (i.e. good energy confinement) and inertial confinement fusion (i.e. efficient compression heating and wall free containment of the fusing plasma) approaches. Like the magnetic approach, the fusion fuel is confined at low density by magnetic fields while it is heated into a plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...

, but like the inertial confinement approach, fusion is initiated by rapidly squeezing the target to dramatically increase fuel density, and thus temperature. MTF uses "plasma guns" (i.e. electromagnetic acceleration techniques) instead of powerful lasers, leading to low cost and low weight compact reactors. The NASA/MSFC
Marshall Space Flight Center
The George C. Marshall Space Flight Center is the U.S. government's civilian rocketry and spacecraft propulsion research center. The largest center of NASA, MSFC's first mission was developing the Saturn launch vehicles for the Apollo moon program...

 Human Outer Planets Exploration (HOPE) group has investigated a manned MTF propulsion spacecraft capable of delivering a 163 933-kilogram payload to Jupiter's moon Callisto
Callisto (moon)
Callisto named after the Greek mythological figure of Callisto) is a moon of the planet Jupiter. It was discovered in 1610 by Galileo Galilei. It is the third-largest moon in the Solar System and the second largest in the Jovian system, after Ganymede. Callisto has about 99% the diameter of the...

 using 106-165 metric tons of propellant (hydrogen plus either D-T or D-He3 fusion fuel) in 249–330 days. This design would thus be considerably smaller and more fuel efficient due to its higher exhaust velocity (Isp=700 km/s) than the previously mentioned "Discovery II", "VISTA" concepts.

A popular confinement concept for fusion rockets is inertial electrostatic confinement
Inertial electrostatic confinement
Inertial electrostatic confinement is a concept for retaining a plasma using an electrostatic field. The field accelerates charged particles radially inward, usually in a spherical but sometimes in a cylindrical geometry. Ions can be confined with IEC in order to achieve controlled nuclear fusion...

, such as in the Farnsworth-Hirsch Fusor or the Polywell
Polywell
A polywell device is a type of fusion reactor that was originated by Robert Bussard under a U.S. Navy research contract. It traps electrons in a magnetic confinement inside its hollow center. The negatively charged electrons then accelerate positively charged ions for the purpose of achieving...

 variation being researched by the Energy-Matter Conversion Corporation. Dr. Robert Bussard published a series of technical articles discussing its application to spaceflight throughout the 1990s. His work was popularised by an article http://torsatron.tripod.com/fusor/fusor.html in the Analog Science Fiction and Fact
Analog Science Fiction and Fact
Analog Science Fiction and Fact is an American science fiction magazine. As of 2011, it is the longest running continuously published magazine of that genre...

 publication, where Tom Ligon (who has also written several science fiction stories) described how the fusor would make for a highly effective fusion rocket. It was also featured in this role in the science fiction novel The Wreck of the River of Stars, by Michael Flynn. However, there are theoretical difficulties with the concept, related to energy losses via bremsstrahlung
Bremsstrahlung
Bremsstrahlung is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typically an electron by an atomic nucleus. The moving particle loses kinetic energy, which is converted into a photon because energy is conserved. The term is...

 radiation and maintaining a highly non-Maxwellian ion energy distribution.

A still more speculative concept is antimatter catalyzed nuclear pulse propulsion
Antimatter catalyzed nuclear pulse propulsion
Antimatter catalyzed nuclear pulse propulsion is a variation of nuclear pulse propulsion based upon the injection of antimatter into a mass of nuclear fuel which normally would not be useful in propulsion...

, which would use tiny quantities of antimatter to catalyze a fission and fusion reaction, allowing much smaller fusion explosions to be created.

External links


See also

  • Helium-3
    Helium-3
    Helium-3 is a light, non-radioactive isotope of helium with two protons and one neutron. It is rare on Earth, and is sought for use in nuclear fusion research...

  • Nuclear propulsion
    Nuclear propulsion
    Nuclear propulsion includes a wide variety of propulsion methods that fulfil the promise of the Atomic Age by using some form of nuclear reaction as their primary power source.- Surface ships and submarines :...

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