Ram augmented interstellar rocket The problem of using the interstellar medium as the sole fuel source led to study of the ram augmented interstellar rocket (RAIR). The RAIR carries its nuclear fuel supply and exhausts the reaction products to produce some of its thrust. However it greatly enhances its performance by scooping the interstellar medium and using this as extra reaction mass to augment the rocket. The propulsion system of the RAIR consists of three subsystems: a fusion reactor, a scoop field, and a plasma accelerator. Fuel is launched ahead of the ship with the accelerator. The scoop field funnels the fuel into another accelerator (this could for example be a heat exchange system transferring thermal energy from the reactor directly to the interstellar gas) which is supplied power from a reactor. One of the best ways to understand this concept is to consider that the hydrogen nuclear fuel carried on board acts as a fuel (energy source) whereas the interstellar gas collected by the scoop and then exhausted at great speed from the back acts as a propellant (the
reaction mass), the vehicle therefore has a limited fuel supply but an unlimited propellant supply. A normal Bussard ramjet would have an infinite supply of both. However, theory suggests that where a Bussard ramjet would suffer drag from having to pre-accelerate interstellar gas to its own speed before intake, a RAIR system would be able to transfer energy via the "accelerator" mechanism to the interstellar medium despite velocity differences, and so would suffer far less drag.
Laser powered interstellar ramjet Beamed energy coupled with a vehicle scooping hydrogen from the interstellar medium is another variant. A laser array in the solar system beams to a collector on a vehicle which uses something like a linear accelerator to produce thrust. This solves the fusion reactor problem for the ramjet. There are limitations because of the attenuation of beamed energy with distance.
Magnetic sail The calculations (by
Robert Zubrin and Dana Andrews) inspired the idea of a
magnetic parachute or sail. This could be important for interstellar travel because it means that deceleration at the destination can be performed with a magnetic parachute rather than a rocket.
Dyson swarm-based stellar engine (Caplan thruster) 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. Using rudimentary calculations that assume maximum efficiency, Caplan estimates the Bussard engine would use 1015 grams per second of solar material 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. The Bussard engine would theoretically work for 100 million years given the mass loss rate of the Sun, but Caplan deems 10 million years to be sufficient for a stellar collision avoidance.
Pre-seeded trajectory 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. This could be done using a different "tanker" spacecraft dropping fuel pellets or using laser propulsion. The method has been referred to as the "fusion runway", While most proposals use fusion power, as with conventional Bussard ramjets, fission has also been suggested. A conventional Bussard ramjet will mostly collect
protium (hydrogen-1), as this is the most abundant isotope of hydrogen in nature. • Launching only ionized fusion fuel so that either magnetic or electrostatic scoops can more easily funnel the fuel into the engine. The drawback is this will cause the fuel to disperse due to electrostatic repulsion. • Launching the fuel on a trajectory so that the fuel velocity vector will closely match the expected velocity vector of the spacecraft at that point in its trajectory. This will minimize the "drag" forces generated by the collection of fuel. • Although the prelaunched fuel for the ramjet negates one advantage of the Bussard design (collection of fuel as it moves through the interstellar medium, saving the cost to launch the fuel mass), it at least retains the advantage of not having to accelerate the mass of the fuel and the mass of the rocket at the same time. • The prelaunched fuel would provide some visibility into the interstellar medium – thus alerting the trailing spacecraft of unseen hazards (e.g.,
brown dwarfs). The major disadvantages of this system include: • The need to precisely intercept the fusion runway pellets with a spacecraft moving at high speed. Laser pulses could be used to guide the pellets. • If using impact fusion, the spacecraft must be moving at a minimum speed in order to achieve fusion. One suggestion is for the spacecraft to use a Sun-skimming maneuver to reach 600 km/s. • The spacecraft could not deviate from the precalculated trajectory unless it was critical to do so. Any such deviation would separate the spacecraft from its fuel supply and leave it with only a minimal ability to return to its original trajectory. • Prelaunched fuel for deceleration at the destination star would not be available unless launched many decades in advance of the spacecraft launch. However, other systems (such as the
magnetic sails) could be used for this purpose. ==See also==