Section 2.8 - Photon Engines
There are a variety of propulsion techniques that use either internally generated or externally supplied photons for propulsion. Stars, of course, are a natural supply of photons, but a purpose-made light source of sufficient beam intensity can also be used. Photons, being massless, travel at the maximum possible speed. Thus they are theoretically the highest possible exhaust velocity. By the same token, being massless, they do not transfer much momentum.
A. Photon Sails
As noted above, light does not transfer much momentum. So the following methods reach useful thrust level by maximizing area to mass ratio. The large area increases thrust relative to mass. The resulting structure resembles a terrestrial sail.
66 Solar Sail
Alternate Names: Lightsail
Type: Solar Flux by Photon Deflection
Description: Starlight, for example from the Sun, reflecting off a large area sail produces a force because the momentum of the photons is reversed. The force for perpendicular (normal) reflection is
- (1+r)(E/c) ,
where r is the reflectivity of the sail, E is the incident light power, and c is the speed of light. Thus even perfectly absorbing surfaces see light pressure, but reflecting ones perform up to twice as well. No real surface is perfectly reflecting, so analysis of sails needs to take into account the actual optical properties. At sail angles other than perpendicular to the light source the net force has a direct component from the incident light, and a reflected or diffuse component depending on the surface properties of the sail. For reflection the incident and reflected angles are the same amount relative to the plane of reflection. The diffuse component is light scattered by a rough surface into a wide range of angles. The force is reduces by a cosine factor due to the spread of directions.
The absorbed component of light is eventually re-radiated by blackbody radiation. If this happens equally from both sides of the sail, it produces no net thrust. Coatings can modify the emissivity of each side and therefore give a net thrust. For high performance sails coatings add too much mass. It ends up being better to use bare metal foil of high reflectivity and just ignore the small part not reflected. Thickness so low as to be partially transparent, and micro-perforations smaller than typical light wavelengths can lower weight even further without significantly reducing reflectivity. Manufacturing, installation, and operation with extremely thin foil sails will be a challenge. The best mass-to-reflectivity material for visible light is an Aluminum/Magnesium alloy. Despite higher mass, refractory metal foils like Tungsten can be used at much higher temperatures. Therefore they can be used much closer to stars where light intensity is higher, and develop more thrust. This can be used for initial kick to outer system missions. The sail shape can be maintained by struts and tension wires, or by the balance of rotation and light pressure, or some combination.
At the distance of the Earth from the Sun, the incident power is 1370 MW per square kilometer. This produces about 8 Newtons/square kilometer for high-reflectivity sails. The attractiveness of solar sails is they use no fuel, and in principle the reflector can be made of local metals in the case of asteroids. The disadvantages are low thrust unless you build them very large, limited angular control, and inverse square dependence of thrust with distance from the source star.
Status: As of 2012 a few test flights of sails have been attempted, with varying success. Any object exposed to sunlight will see a light pressure force, but most spacecraft do not have a low enough mass per area for the force to be more than a minor correction to it's motion.
- 66a Gravity Tractor - In this variation, the sail is not attached to a cargo such as an asteroid you are trying to move. The mutual gravity of the objects anchors the sail. Reasons to do this are not having to devise methods to attach the sail, and allowing the sail to maneuver itself with less restrictions.
- Wikipedia article - Solar sail
- Marchal, C., Solar Sails and the ARSAT Satellite - Scientific Applications and Techniques, L'Aeronautique et L'Astronautique, no 127, pp 53-7, 1987.
- Friedman, Louis, Starsailing: Solar Sails and Interstellar Travel , Wiley, New York, 1988, 146 pp.
67 Laser Lightsail
Type: Laser by Photon Deflection
Description: A high powered laser is aimed at a target sail. The beam of photons are reflected off the sail material. Reflection of the photons reverses their momentum vectors' component which is normal to the sail. By conservation law, the sail gains momentum. Laser sails can have higher performance than solar sails because the laser beam intensity is not as limited as the local star light, and can be focused over longer distances. The sail can be designed to optimize reflectivity at the laser wavelength. This method is still by the overheating of the sail and lack of sufficiently powerful lasers for useful missions.
Advanced interstellar mission concepts have proposed very large phase plate type lenses for long range focus. Another advanced concept is to use two sails to slow down a vehicle, by releasing the first sail and using that to reflect light back to the second, smaller one.
Status: Not tested as of 2012 mainly due to lack of powerful lasers. Megawatt class lasers are not powerful in this context.
- Forward, Robert L., Roundtrip Interstellar Travel Using Laser-Pushed Lightsails Journal of Spacecraft and Rockets , vol. 21, pp. 187-95, Jan.-Feb. 1984
68 Microwave Sail
Alternate Names: Starwisp
Type: Beamed Microwave by Photon Deflection
Description: In this method microwaves are reflected off a very thin, open wire mesh. The momentum change of microwave photons bouncing off the mesh provides thrust. Because an open mesh of thin wires can have a very low weight, in theory this propulsion method can give high accelerations. With feasible power levels, the cargo mass will still be small. A good use for such would be delivering nanofactories, which can then build larger scale infrastructure. The beam can supply power to operate the factory at the destination. This is related to the non-transport energy delivery by microwave concept.
Status: Generating lots of microwave power is well understood. Keeping it focused over useful distances in space, and building the lightweight sail material are not.
B. Photon Rockets
This group of methods involve emitting photons from inside the vehicle, rather than reflecting photons from an external source. Like photon reflection, photons do not carry much momentum, so it is a low thrust set of concepts, although the exhaust velocity is the maximum possible. It can be considered an addition to other propulsion methods by directional emission of radiator waste heat.
69 Thermal Photon Reflector
Alternate Names: Nuclear Photonic Rocket
Type: Nuclear Reactor via Photon Emission
Description: A heat generating device, such as a nuclear reactor, is at the focus of a parabolic reflector. The thermal photons are focused into a near parallel beam, which propels the vehicle. Another high-energy source is a matter-antimatter reaction, which is absorbed by a blanket of heavy metals and converted to heat. This method is probably not practical, since emitting photons gives less momentum than emitting the reaction products of the energy source directly. It might be worth using directional radiators to get rid of surplus heat in addition to the reaction products. In that case it increases the total efficiency.
Status: Not tested as of 2012
70 Stellar Photon Engine
Alternate Names: Stardrive
Type: Solar Flux by Photon Emission
Description: This is similar to the previous Thermal Photon Reflector method, except an entire star is at the focus of a cloud of lightsails which are balanced by gravity vs light pressure. Since both gravity and light intensity vary as the inverse square of distance from the star, a specific thickness of sail will be balanced at any distance. The ratio of the star's luminosity to it's mass will determine the thickness. The star's spectrum and desired sail distance will determine what to make the sail out of. By directing the light in one particular direction rather than symmetric in all directions as natural stars do, the sails convert the star into an unbalanced light emitter, and so create a net thrust for the combined star + sail combination.
Sails with a slight dihedral angle oriented perpendicular to the star will direct the light to miss the star itself, and will be stable in orientation. Flat sails which do not reflect the light directly back to the star are slightly more efficient since none will be absorbed by the star, but will accelerate themselves sideways. So that type of sail must change it's orientation periodically to maintain position. Sails which are not flat, such as a shallow cone, can direct the light to miss the star without drifting.
To give a numerical example, a sail at the Earth's distance from the Sun produces about 8 newtons/km^2. Solar acceleration at that distance is 0.006 m/s^2. Therefore if your sail mass is 1330 kg/km^2, light pressure will balance gravity. If made of aluminum, then it needs to be 0.5 microns thick. Foils that thick are commercially sold today. A sail cloud reflecting 10% of the Sun's output will have an area of 28 x 10^15 km^2 and a mass of 37.5 x 10^15 tons (3.7% of the mass of the largest asteroid, 1 Ceres), Such a sail cloud would produce a thrust of 225 x 10^15 Newtons, and accelerate the Sun by 3.5 meters per second per million years.
Uses for this type of engine are adjusting orbits of binary or multiple star systems, escaping future supernovae, and generally moving stars within a galaxy or between galaxies. The physics of this method are simple. The challenge is one of scale, and the extremely slow accelerations it produces because stars are very massive. Note that moving a star does not generally bring along objects in orbit about the star, they would need their own propulsion.
- Moving Planets - A more practical (relatively) use for this method is moving planets, because of their much lower mass. The sails would be anchored by the planet's rather than the star's gravity. A larger secondary cloud of sails would direct more sunlight to the anchored sails than they would collect on their own. A reason to do this for the Earth is the Sun is increasing it's brightness by 10% per billion years due to increasing fusion rate in the core. Thus if you want to keep the Earth habitable, you would want to move it slowly outwards. Alternately, if you wanted to make Venus more habitable, you could move it into the Asteroid belt, where the excess greenhouse effect would be an advantage rather than a detriment. This would still likely be a slow method. Using gravity assist maneuvers with large asteroids or outer Solar System bodies would be faster. Unlike gravity assist for spacecraft, in this case the object doing the flyby is massive enough to affect the orbit of the planet. If a secondary body such as a gas giant is used to absorb the orbit changes, the flyby object could be used multiple times to shift the small planet without expending too much energy.
Status: Very far from testing. Possibly not even used in science fiction stories.
71 Gamma Ray Thruster
Alternate Names: redshift rocket
Type: Antimatter by Photon Emission
Description: Gamma rays produced by antimatter annihilation behind a vehicle can be absorbed by a thick layer of heavy metals. The momentum of the gamma ray photons produces thrust. Like other photon engines, it is likely more effective to use the decay particles directly, since they have more mass and can be directed better, but using the gamma rays as a supplement makes sense.
Status: Currently theoretical, as antimatter is only produced in tiny amounts in particle accelerators.
C. Amplified Photon Pressure
In this approach, a reflecting cavity is used to multiply the photon pressure. In one arrangement, a laser emits a particular frequency, and the walls of an enclosing cavity and the vehicle back end are designed to have a high reflectivity for that frequency. In another arrangement, there is no enclosing cavity, and a laser gain medium is placed in front of one mirror. The vehicle carries a second mirror, and the pair of mirrors plus gain medium form a laser amplifier which happens to have a large vacuum gap between the energy source and vehicle. In both cases the light will bounce multiple times until finally absorbed or dispersed, each bounce adding a momentum increment. The wavelength can be adapted to the physical dimensions, for example using microwaves instead of a laser if it creates an efficient reflection. The multiple reflections will extract energy from the photons as they repeatedly bounce off the moving vehicle by redshift. If sufficient bounces at sufficient projectile velocity happen, the light would be redshifted to a wavelength not efficiently reflected, or merely lose the majority of their energy, thus ending the momentum transfer.