Nuclear photonic rocket
In a nuclear photonic rocket, a nuclear reactor would generate such high temperatures that the blackbody light from the reactor would provide thrust. Think of a nuclear light-bulb, with a reflector. The big advantage is that negligible reaction mass leaves the spacecraft, so only nuclear fuel is depleted. The disadvantage is that it takes a lot of power to generate a small amount of thrust this way, so acceleration is very low. The reactor would be most likely be constructed using graphite or tungsten. Photonic rockets are technologically feasible, but they are very low thrust devices that require immense amounts of energy to operate. In essence they demonstrate the upper limit of specific impulse. The exchaust of a photon rocket engine can be a laser beam, but lasers are less efficient at converting energy into light then are photon radiators such as tungsten and graphite.The exhaust of a photon rocket is made of photons of light. The exchaust of other devices such as fission fragment rocket engines are made of sub atomic particles, and of the atoms of liter elements that are created when heavy atoms undergo nuclear fission. Such particles as these are called "baryons" in nuclear physics. Non-nuclear energy sources can not generate enough energy to energize practical photon roclet motors. Other kinds of electric rocket motors deliver much more thrust for the same amount of energy expenditure then due photon rocket motors. A nuclear light bulb type photon rocket motors avoids the use of electricity to generate light, and reflects all of its energy off of mirrors to generate photonic thrust. The photons are emitted by the nuclear reactors radiators before they are reflected into space by the mirrors.The hotter the temperature of the radiators the more energy is emitted in the form of light by them.
Fission powered photonic propulsion
In a nuclear fission powered photon engine the electric power generated by a nuclear reactor can be used to heat up tungsten coils, or graphite blocks until they glow white and emit photons of light. These photons can be reflected by a parabolic mirror, or a convex mirror into space to generate thrust. The thrust of a photon rocket is . In this equation c is the speed of light, 300,000,000m/s. P is the light emission power, i.e. how many watts of light is emitted. R is the percentage reflectivity of the mirror-collimator inside of the photon rocket motor.
A nuclear fission reactor actually generates at least 5 or 10 times more thermal (heat) energy, than the amount of electrical energy it can generate. This heat energy will have to be emitted into space by radiator pipes to cool the nuclear fission reactor in any case. This infra-red light, however can also be reflected into space by mirrors to generate thrust. This means that most of the total thermal, and electric energy output of the reactor can be used to generate photonic thrust.
Feasible current, or near term fission reactor designs can generate up to 2.2kW of electricity per kilogram of reactor mass. A nuclear fission powered photon rocket can thus achieve accelerations of 10 -5G to 10 -4G.
This is sufficient to achieve a minimum interplanetary spaceflight capability if the photon rocket begins its journey in earth orbit. A nuclear fission reactor may fission all its atomic fuel within a period of 10-25 years of reactor operation.
The used up reactor core will remain intensely radioactive for 10s of thousands of years, but a used up fission reactor can still be used as a RTG (radioactive decay powered electric generator) to generate 800 watts of electric power per kilogram of reactor weight for many thousands of years afterwards.
This could allow the photon rocket motor to continue firing for thousands of years at up to 40% of the maximium power output that was achieved during the initial operation of the nuclear fission reactor.
Antimatter powered photonic propulsion
A design proposed in the 1950s by Eugen Sänger used positron-electron annihilation to produce gamma rays. Sänger was unable to solve the problem of how to reflect, and collimate the gamma rays created by positron-electron annhilation.
A workable variation of this is to annihilate protons and antiprotons inside of hollow block of graphite or tungsten. The gamma ray and neutron energy thus released by heats up the block until it glows white. This light would be reflected into space by a mirror to generate photonic thrust. An antimatter-matter powered photon rocket would have 1000 times more energy available for each kilogram of fuel. For this reason, an antimatter-matter annhilation powered photon rocket could potentially be used for interstellar spaceflight.
The specific impulse would be 300,000,000 m/s or, expressed as a time, one year. , i.e. the net power needed to accelerate 1 kg 1 would be 300 MW.
