Nuclear Photonic Rocket - Energy Requirements and Comparisons

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 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. V_f = ln 1.008 × c where c = 300,000,000 m/s. V_f 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 sails) is effectively infinite because no carried fuel is required. Alternatively, such devices as ion thrusters, 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.