Fission sail
Encyclopedia
The fission sail is a type of spacecraft propulsion
proposed by Robert Forward
that uses fission
fragments to propel a large solar sail
-like craft. It is similar in concept to the fission-fragment rocket
in that the fission by-products are directly harnessed as working mass
, and differs primarily in the way that the fragments are used for thrust.
In the fission sail, the "rocket" is built in the form of a two-layer sheet, with some sort of absorber on one side, and nuclear fuel on the other. Atoms in the fuel that decay will release their fragments in random, but opposite, directions. In the simple case where the decay releases the fragments "front" and "rear", the rearward moving fragment generates thrust directly, while the frontward moving one is absorbed in the front half of the sail, also contributing thrust. The system must be built very large in order to ensure that those fragments that are not moving directly to the front of rear nevertheless tend to hit the sail somewhere. A sail several kilometers on a side would have fairly good "capture ratios".
The sail is not a nuclear reactor
, and relies on natural decay rates for energy release. The thrust from such a system will always be very low, albeit extremely efficient. Another problem is that the sail receives the energy equally over the entire sail surface, and thus can't be steered. This differs from a solar sail, where tilting the sail to the sunlight can produce a sideways thrust for steering.
Forward proposed the system as an "add on" to existing solar sails. Close to stars where the light density is high the sails work fine, but as they move past about 2 AU
their thrust is too low to be useful. Forward suggested that coating the sail with fissionable material would provide thrust in this region, where maneuvering is no longer as important. Such a system would accelerate and maneuver based on solar energy for the start of its flight, and then continue to accelerate at a lower rate for long periods of time.
Spacecraft propulsion
Spacecraft propulsion is any method used to accelerate spacecraft and artificial satellites. There are many different methods. Each method has drawbacks and advantages, and spacecraft propulsion is an active area of research. However, most spacecraft today are propelled by forcing a gas from the...
proposed by Robert Forward
Robert Forward
Robert Lull Forward — known as Robert L. Forward — was an American physicist and science fiction writer...
that uses fission
Nuclear fission
In nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts , often producing free neutrons and photons , and releasing a tremendous amount of energy...
fragments to propel a large solar sail
Solar sail
Solar sails are a form of spacecraft propulsion using the radiation pressure of light from a star or laser to push enormous ultra-thin mirrors to high speeds....
-like craft. It is similar in concept to the fission-fragment rocket
Fission-fragment rocket
The fission-fragment rocket is a rocket engine design that directly harnesses hot nuclear fission products for thrust, as opposed to using a separate fluid as working mass...
in that the fission by-products are directly harnessed as working mass
Working mass
Working mass is a mass against which a system operates in order to produce acceleration.In the case of a rocket, for example, the reaction mass is the fuel shot backwards to provide propulsion. All acceleration requires an exchange of momentum, which can be thought of as the "unit of movement"...
, and differs primarily in the way that the fragments are used for thrust.
In the fission sail, the "rocket" is built in the form of a two-layer sheet, with some sort of absorber on one side, and nuclear fuel on the other. Atoms in the fuel that decay will release their fragments in random, but opposite, directions. In the simple case where the decay releases the fragments "front" and "rear", the rearward moving fragment generates thrust directly, while the frontward moving one is absorbed in the front half of the sail, also contributing thrust. The system must be built very large in order to ensure that those fragments that are not moving directly to the front of rear nevertheless tend to hit the sail somewhere. A sail several kilometers on a side would have fairly good "capture ratios".
The sail is not a nuclear reactor
Nuclear reactor
A nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. Most commonly they are used for generating electricity and for the propulsion of ships. Usually heat from nuclear fission is passed to a working fluid , which runs through turbines that power either ship's...
, and relies on natural decay rates for energy release. The thrust from such a system will always be very low, albeit extremely efficient. Another problem is that the sail receives the energy equally over the entire sail surface, and thus can't be steered. This differs from a solar sail, where tilting the sail to the sunlight can produce a sideways thrust for steering.
Forward proposed the system as an "add on" to existing solar sails. Close to stars where the light density is high the sails work fine, but as they move past about 2 AU
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
their thrust is too low to be useful. Forward suggested that coating the sail with fissionable material would provide thrust in this region, where maneuvering is no longer as important. Such a system would accelerate and maneuver based on solar energy for the start of its flight, and then continue to accelerate at a lower rate for long periods of time.