Four classes of stellar engines have been defined.
Class A (Shkadov thruster) One of the simplest examples of a stellar engine is the
Shkadov thruster (named after Dr.
Leonid Shkadov, who first proposed it), or a
class-A stellar engine. Such an
engine is a stellar propulsion system, consisting of an enormous mirror/
light sail—actually a massive type of solar
statite large enough to classify as a
megastructure—which would balance gravitational attraction towards and
radiation pressure away from the star. Since the radiation pressure of the star would now be
asymmetrical, i.e. more radiation being emitted in one direction as compared to another, the "excess" radiation pressure acts as net
thrust, accelerating the star in the direction of the hovering statite. Such thrust and acceleration would be
very slight, but such a system could be stable for millennia. Any
planetary system attached to the star would be "dragged" along by its parent star. For a star such as the
Sun, with luminosity 3.85
W and mass 1.99
kg, the total thrust produced by reflecting half of the solar output would be 1.28
N. After a period of one million years this would yield an imparted speed of 20 m/s, with a displacement from the original position of 0.03
light-years. After one billion years, the speed would be 20 km/s and the displacement 34,000 light-years, a little over a third of the estimated width of the
Milky Way galaxy.
Class B A
class-B stellar engine consists of two concentric spheres around a star. The inner sphere (which may be assimilated with a Dyson shell) receives energy from the star and becomes hotter than the outer sphere. The difference of temperature between the two spheres drives thermal engines able to provide mechanical work. Unlike the Shkadov thruster, a class-B stellar engine is not propulsive.
Class C A
class-C stellar engine, such as the Badescu–Cathcart engine, based on a modification of the concept of “star lifting” proposed in 1985, the Caplan thruster proposed in 2019 and the Svoronos Star Tug proposed in 2020.
Caplan thruster Astronomer Matthew E. Caplan of
Illinois State University has proposed a type of stellar engine that uses concentrated stellar energy (repurposing the mirror statites from class A) to excite certain regions of the outer surface of the star and create beams of
solar wind for collection by a multi-
Bussard ramjet assembly. The ramjets would produce directed plasma to stabilize its orbit and jets of
oxygen-14 to push the star. Using rudimentary calculations that assume maximum efficiency, Caplan estimates that the Bussard engine would use 1012 kg of solar material per second to produce a maximum acceleration of 10−9 m/s2, yielding a velocity of 200 km/s after 5 million years and a distance of 10
parsecs over 1 million years. While theoretically the Bussard engine would work for 100 million years, given the mass loss rate of the Sun, Caplan deems 10 million years to be sufficient for a stellar collision avoidance. His proposal was commissioned by the German educational
YouTube channel
Kurzgesagt.
Svoronos Star Tug Alexander A. Svoronos of Yale University proposed the 'Star Tug', a concept that combines aspects of the Shkadov thruster and Caplan engine to produce an even more powerful and efficient mechanism for controlling a star's movement. Essentially, it replaces the giant parabolic mirror of the Shkadov thruster with an engine powered by mass lifted from the star, similar to the Caplan engine. However, instead of pushing a star from behind with a beam of thrust, as the Caplan engine does, it pulls the star from the front via gravity, like the Shkadov thruster. As a result, it only needs to produce a single beam of thrust (toward but narrowly missing the star), whereas the Caplan engine must produce two beams of thrust (one to push the star from behind and negate the force of gravity between the engine and the star, and one to propel the system as a whole forward). The result is that the Svoronos Star Tug is a much more efficient engine capable of significantly higher accelerations and max velocities. The Svoronos Star Tug can, in principle (assuming perfect efficiency), accelerate the Sun to ~27% the speed of light (after burning enough of the Sun's mass to transition it to a brown dwarf). == In fiction ==