Bussard ramjet

A fast moving spacecraft scoops up hydrogen from the interstellar medium using an enormous funnel-shaped magnetic field (ranging from kilometers to many thousands of kilometers in diameter); the hydrogen is compressed until thermonuclear fusion occurs, which provides thrust to counter the drag created by the funnel and energy to power the magnetic field.

In 1969, John Ford Fishback made an important contribution, describing the details of the required magnetic field.

[citation needed] Robert Zubrin and Dana Andrews analyzed one hypothetical version of the Bussard ramjet design in 1988.

[citation needed] A 2021 study found that, while feasible in principle, the practical construction of a useful Bussard ramjet would be beyond even a civilization of Kardashev type II.

However it greatly enhances its performance by scooping the interstellar medium and using this as extra reaction mass to augment the rocket.

Astrophysicist Matthew E. Caplan of Illinois State University has proposed a type of stellar engine that uses a Dyson swarm of mirrors to concentrate stellar energy onto certain regions of a Sun-like star, producing beams of solar wind to be collected by a multi-ramjet assembly which in turn produces directed jets of plasma to stabilize its orbit and oxygen-14 to push the star.

[15] Several of the obvious technical difficulties with the Bussard ramjet can be overcome by setting out solid pellets of fuel along the spacecraft's trajectory in advance.