Editing Nuclear photonic rocket (section)

==Energy requirements and comparisons==
The power per thrust required for a perfectly [[collimated]] output beam is 300 [[megawatt|MW]]/[[Newton (unit)|N]] (half this if it can be reflected off the craft); very high [[energy density]] 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&nbsp;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&nbsp;kg then 240,000&nbsp;kg of that is atomic fuel. Therefore, the fissioning of all of the fuel will result in the loss of just 240&nbsp;kg of mass. Then 300,000/299,760&nbsp;kg = an ''m''<sub>i</sub>/''m''<sub>f</sub> of 1.0008. Using the [[rocket equation]], we find ''v''<sub>f</sub> = ln 1.0008 × ''c'' where ''c'' = 299,792,458&nbsp;m/s.
''v''<sub>f</sub> then may be 239,930&nbsp;m/s which is about 240&nbsp;km/s. The nuclear fission powered photon rocket may accelerate at a maximum of perhaps 1/10,000&nbsp;m/s² (0.1&nbsp;mm/s²) which is 10<sup>−5</sup>''g''. The velocity change would be at the rate of 3,000&nbsp;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&nbsp;km/s if the vehicle is already in orbit at a velocity of 9,100&nbsp;m/s. Upon escaping the Earth's gravitational field the rocket will have a heliocentric velocity of 30&nbsp;km/s in interplanetary space. Eighty years of steady photonic thrusting would be then required to obtain a final velocity of 240&nbsp;km/s in this hypothetical case.

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 <math>1/c</math>, whereas for slow particles (that is, nonrelativistic; even the output from typical ion thrusters counts) the ratio is <math>2/v</math>, which is much larger (since <math>v\ll c</math>). (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&mdash;the practical quantities&mdash;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&mdash;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. The limitations posed by the [[Tsiolkovsky rocket equation|rocket equation]] can be overcome, as long as the reaction mass is not carried by the spacecraft. In the Beamed [[Laser Propulsion]] (BLP) concept, the photons are beamed from the photon source to the spacecraft as coherent light. [[Robert L. Forward]] pioneered interstellar propulsion concepts including photon propulsion and [[antimatter rocket]] propulsion. However, BLP is limited because of the extremely low thrust generation efficiency of photon reflection. One of the best ways to overcome the inherent inefficiency in producing thrust of the photon thruster by amplifying the momentum transfer of photons by recycling photons between two high reflectance mirrors.