Nuclear photonic rocket

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.

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 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 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 neutrinos 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 m_i/m_f of 1.0008. Using the rocket equation, we find v_f = ln 1.0008 × c where c = 299,792,458 m/s. v_f then may be 239,930 m/s which is about 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.

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