Nuclear photonic rocket
Encyclopedia
In a nuclear photonic rocket, a nuclear reactor
would generate such high temperatures that the blackbody radiation from the reactor would provide significant thrust. The disadvantage is that it takes a lot of power
to generate a small amount of thrust
this way, so acceleration
is very slow. The photon radiators would most likely be constructed using graphite
or tungsten
. Photonic rockets are technologically feasible, but rather impractical with current technology.
power sources would be required to provide reasonable thrust without unreasonable weight. The specific impulse of a photonic rocket is harder to define, since the output has no (rest) mass and is not expended fuel; if we take the momentum per inertia of the photons, the specific impulse is just c, which is impressive. However, considering the mass of the source of the photons, e.g., atoms undergoing nuclear fission
, brings the specific impulse down to 300 km/s (c/1000) or less; considering the infrastructure for a reactor (some of which also scales with the amount of fuel) reduces the value further. Finally, any energy loss not through radiation that is redirected precisely to aft but is instead conducted away by engine supports, radiated in some other direction, or lost via neutrino
s or so will further degrade the efficiency. If we were to set 80% of the mass of the photon rocket = fissionable fuel, and recognizing that nuclear fission converts about 0.10 % of the mass into energy: then if the photon rocket masses 300,000 kg then 240,000 kg of that is atomic fuel. Therefore the fissioning of all of the fuel will result in the loss of just 240 kg of mass. Then 300,000/299,760 kg = an mi/mf of 1.0008. Vf = ln 1.008 × c where c = 300,000,000 m/s.
Vf then may be 240,096 m/s which is 240 km/s. The nuclear fission powered photon rocket may accelerate at a maximum of perhaps 1/10,000 m/s² (0.1 mm/s²) which is 10−5g. The velocity change would be at the rate of 3,000 m/s per year of thrusting by the photon rocket.
If a photon rocket begins its journey in low earth orbit, then one year of thrusting may be required to achieve an earth escape velocity
of 11.2 km/s if the vehicle is already in orbit at a velocity of 9,100 m/s, and 400 m/s additional velocity is obtained from the east to west rotation of the earth. The photon thrust will be sufficient to more than counterbalance the pull of the sun's gravity, allowing the photon rocket to maintain a heliocentric velocity of 30 km/s in interplanetary space upon escaping the Earth's gravitational field. Eighty years of steady photonic thrusting would be then required to obtain a final velocity of 240 km/s in this hypothetical case. At a 30 km/s heliocentric velocity, the photon ship would recede a distance of 600,000,000 miles (1 Tm) from the Sun per year.
It is possible to obtain even higher specific impulse; that of some other photonic propulsion devices (e.g., solar sail
s) is effectively infinite because no carried fuel is required. Alternatively, such devices as ion thruster
s, while having a notably lower specific impulse, give a much better thrust-to-power ratio; for photons, that ratio is
, whereas for slow particles (that is, nonrelativistic; even the output from typical ion thrusters counts) the ratio is 
, which is much larger (since 
). (This is in a sense an unfair comparison, since the photons must be created and other particles are merely accelerated, but nonetheless the impulses per carried mass and per applied energy—the practical quantities—are as given.) The photonic rocket is thus wasteful when power and not mass is at a premium, or when enough mass can be saved through the use of a weaker power source that reaction mass can be included without penalty.
A laser could be used as a photon rocket engine, and would solve the reflection/collimation problem, but lasers are absolutely less efficient at converting energy into light than blackbody radiation is—though one should also note the benefits of lasers vs blackbody source, including unidirectional controllable beam and the mass and durability of the radiation source.
). This could perhaps provide interplanetary spaceflight capability from Earth orbit. Nuclear fusion
reactors could also be used, perhaps providing somewhat higher power.
A design proposed in the 1950s by Eugen Sänger
used positron
-electron
annihilation to produce gamma ray
s. Sänger was unable to solve the problem of how to reflect, and collimate the gamma rays created by positron-electron annihilation; however, by shielding the reactions (or other annihilation
s) and absorbing their energy, a similar blackbody propulsion system could be created. An antimatter
-matter powered photon rocket would (disregarding the shielding) obtain the maximum c specific impulse; for this reason, an antimatter-matter annihilation powered photon rocket could potentially be used for interstellar
spaceflight.
Nuclear reactor
A nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. Most commonly they are used for generating electricity and for the propulsion of ships. Usually heat from nuclear fission is passed to a working fluid , which runs through turbines that power either ship's...
would generate such high temperatures that the blackbody radiation from the reactor would provide significant thrust. The disadvantage is that it takes a lot of power
Power (physics)
In physics, power is the rate at which energy is transferred, used, or transformed. For example, the rate at which a light bulb transforms electrical energy into heat and light is measured in watts—the more wattage, the more power, or equivalently the more electrical energy is used per unit...
to generate a small amount of thrust
Thrust
Thrust is a reaction force described quantitatively by Newton's second and third laws. When a system expels or accelerates mass in one direction the accelerated mass will cause a force of equal magnitude but opposite direction on that system....
this way, so acceleration
Acceleration
In physics, acceleration is the rate of change of velocity with time. In one dimension, acceleration is the rate at which something speeds up or slows down. However, since velocity is a vector, acceleration describes the rate of change of both the magnitude and the direction of velocity. ...
is very slow. The photon radiators would most likely be constructed using graphite
Graphite
The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...
or tungsten
Tungsten
Tungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as...
. Photonic rockets are technologically feasible, but rather impractical with current technology.
Energy requirements and comparisons
The power per thrust required for a perfectly collimated output beam is 300 MW/N (half this if it can be reflected off the craft); very high energy densityEnergy density
Energy density is a term used for the amount of energy stored in a given system or region of space per unit volume. Often only the useful or extractable energy is quantified, which is to say that chemically inaccessible energy such as rest mass energy is ignored...
power sources would be required to provide reasonable thrust without unreasonable weight. The specific impulse of a photonic rocket is harder to define, since the output has no (rest) mass and is not expended fuel; if we take the momentum per inertia of the photons, the specific impulse is just c, which is impressive. However, considering the mass of the source of the photons, e.g., atoms undergoing nuclear 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...
, brings the specific impulse down to 300 km/s (c/1000) or less; considering the infrastructure for a reactor (some of which also scales with the amount of fuel) reduces the value further. Finally, any energy loss not through radiation that is redirected precisely to aft but is instead conducted away by engine supports, radiated in some other direction, or lost via neutrino
Neutrino
A neutrino is an electrically neutral, weakly interacting elementary subatomic particle with a half-integer spin, chirality and a disputed but small non-zero mass. It is able to pass through ordinary matter almost unaffected...
s or so will further degrade the efficiency. If we were to set 80% of the mass of the photon rocket = fissionable fuel, and recognizing that nuclear fission converts about 0.10 % of the mass into energy: then if the photon rocket masses 300,000 kg then 240,000 kg of that is atomic fuel. Therefore the fissioning of all of the fuel will result in the loss of just 240 kg of mass. Then 300,000/299,760 kg = an mi/mf of 1.0008. Vf = ln 1.008 × c where c = 300,000,000 m/s.
Vf then may be 240,096 m/s which is 240 km/s. The nuclear fission powered photon rocket may accelerate at a maximum of perhaps 1/10,000 m/s² (0.1 mm/s²) which is 10−5g. The velocity change would be at the rate of 3,000 m/s per year of thrusting by the photon rocket.
If a photon rocket begins its journey in low earth orbit, then one year of thrusting may be required to achieve an earth escape velocity
Escape velocity
In physics, escape velocity is the speed at which the kinetic energy plus the gravitational potential energy of an object is zero gravitational potential energy is negative since gravity is an attractive force and the potential is defined to be zero at infinity...
of 11.2 km/s if the vehicle is already in orbit at a velocity of 9,100 m/s, and 400 m/s additional velocity is obtained from the east to west rotation of the earth. The photon thrust will be sufficient to more than counterbalance the pull of the sun's gravity, allowing the photon rocket to maintain a heliocentric velocity of 30 km/s in interplanetary space upon escaping the Earth's gravitational field. Eighty years of steady photonic thrusting would be then required to obtain a final velocity of 240 km/s in this hypothetical case. At a 30 km/s heliocentric velocity, the photon ship would recede a distance of 600,000,000 miles (1 Tm) from the Sun per year.
It is possible to obtain even higher specific impulse; that of some other photonic propulsion devices (e.g., solar sail
Solar sail
Solar sails are a form of spacecraft propulsion using the radiation pressure of light from a star or laser to push enormous ultra-thin mirrors to high speeds....
s) is effectively infinite because no carried fuel is required. Alternatively, such devices 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, while having a notably lower specific impulse, give a much better thrust-to-power ratio; for photons, that ratio is
, whereas for slow particles (that is, nonrelativistic; even the output from typical ion thrusters counts) the ratio is , which is much larger (since ). (This is in a sense an unfair comparison, since the photons must be created and other particles are merely accelerated, but nonetheless the impulses per carried mass and per applied energy—the practical quantities—are as given.) The photonic rocket is thus wasteful when power and not mass is at a premium, or when enough mass can be saved through the use of a weaker power source that reaction mass can be included without penalty.A laser could be used as a photon rocket engine, and would solve the reflection/collimation problem, but lasers are absolutely less efficient at converting energy into light than blackbody radiation is—though one should also note the benefits of lasers vs blackbody source, including unidirectional controllable beam and the mass and durability of the radiation source.
Power sources
Feasible current, or near-term fission reactor designs can generate up to 2.2 kW per kilogram of reactor mass. Without any payload, such a reactor could drive a photon rocket at nearly 10−4 m/s² (10−5g; see g-forceG-force
The g-force associated with an object is its acceleration relative to free-fall. This acceleration experienced by an object is due to the vector sum of non-gravitational forces acting on an object free to move. The accelerations that are not produced by gravity are termed proper accelerations, and...
). This could perhaps provide interplanetary spaceflight capability from Earth orbit. Nuclear fusion
Nuclear fusion
Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...
reactors could also be used, perhaps providing somewhat higher power.
A design proposed in the 1950s by Eugen Sänger
Eugen Sänger
Eugen Sänger was an Austrian-German aerospace engineer best known for his contributions to lifting body and ramjet technology.-Early career:...
used positron
Positron
The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. The positron has an electric charge of +1e, a spin of ½, and has the same mass as an electron...
-electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
annihilation to produce gamma ray
Gamma ray
Gamma radiation, also known as gamma rays or hyphenated as gamma-rays and denoted as γ, is electromagnetic radiation of high frequency . Gamma rays are usually naturally produced on Earth by decay of high energy states in atomic nuclei...
s. Sänger was unable to solve the problem of how to reflect, and collimate the gamma rays created by positron-electron annihilation; however, by shielding the reactions (or other annihilation
Annihilation
Annihilation is defined as "total destruction" or "complete obliteration" of an object; having its root in the Latin nihil . A literal translation is "to make into nothing"....
s) and absorbing their energy, a similar blackbody propulsion system could be created. An antimatter
Antimatter
In particle physics, antimatter is the extension of the concept of the antiparticle to matter, where antimatter is composed of antiparticles in the same way that normal matter is composed of particles...
-matter powered photon rocket would (disregarding the shielding) obtain the maximum c specific impulse; for this reason, an antimatter-matter annihilation powered photon rocket could potentially be used for interstellar
Interstellar travel
Interstellar space travel is manned or unmanned travel between stars. The concept of interstellar travel in starships is a staple of science fiction. Interstellar travel is much more difficult than interplanetary travel. Intergalactic travel, or travel between different galaxies, is even more...
spaceflight.
External links
- Application of nuclear photon engines for deep-space exploration by Andrey V. Gulevich, Eugeny A. Ivanov, Oleg F. Kukharchuk, Victor Ya. Poupko, and Anatoly V. Zrodnikov. AIP Conference Proceedings
- "Interstellar rendezvous missions employing fission propulsion systems," Lenard, R.X., and Lipiniski, R.J., in Proceedings of the Space Technology Applications Int'l Forum, 2000
- On the conversion of infrared radiation from fission reactor-based photon engine into parallel beam, Gulevich, A. V.; Levchenko, V. E.; Loginov, N. I.; Kukharchuk, O. F.; Evtodiev, D. A.; Zrodnikov, A. V., in Proceedings of the Space Technology Applications Int'l Forum, 2002
- Long-life space reactor for photon propulsion, Sawada, T.; Endo, H.; Netchaev, A., in Proceedings of the Space Technology Applications Int'l Forum, 2002