What if we use something else for reaction mass and use antimatter to energise that? And, instead of using it in a rocket, we use a magnetic scoop to draw in reaction mass from the interstellar medium? This is the Ram-Augmented Interstellar ‘Rocket’ – though technically a rocket carries all its reaction mass – and it promises high performance without all the disadvantages of exponentially rising mass-ratios. Mixing 1% antimatter into the matter flow could, in theory, produce an exhaust velocity of ~0.2 c. Scooping and energising the equivalent mass of ~100 times the mass of the starship would allow a top-speed of 0.999999996 c to be achieved, before braking to a halt using half that mass. This would allow, at 1 gee acceleration, a journey of ~20,000 light-years. The nearby stars would be accessible at a much lower antimatter budget.
Artwork of a Bussard ramjet in flight. Credit and ©: Adrian Mann
Ram-augmented interstellar rocket (information from David Darling)
A modified ramjet design, known as RAIR (ram-augmented interstellar rocket), proposed by Alan Bond in 1974, tackles the fusion-reaction problem by using the scooped-up interstellar hydrogen not as fuel but simply as reaction mass. The incoming proton stream is decelerated to about 1 MeV, then allowed to bombard a target made of lithium-6 or boron-11. Lithium-proton or boron-proton fusion is easy to induce and releases more energy than any other type of fusion reaction. The energy produced in this way is added to the mass stream which then exits the reactor. In the exhaust nozzle, the energy created by initially braking the mass stream is added back to it.
The so-called catalyzed-RAIR offers an even more efficient approach. After the incoming mass stream has been compressed, a small amount of antimatter is added. The reaction cross-section is not only enormous compared to fusion, it happens at much lower temperatures. According to one estimate, the energy release is such that the drive reactor of a 10,000-ton antimatter catalyzed-RAIR accelerating at 1g and maintaining 1018 particles per cubic cm within the reactor only has to be about 3.5 m in diameter.
Bond, Alan. “An Analysis of the Potential Performance of the Ram Augmented Interstellar Rocket.” Journal of the British Interplanetary Society, 27, 674-688, 1974.
Jackson, A. “Some Considerations on the Antimatter and Fusion Ram Augmented Interstellar Rocket,” Journal of the British Interplanetary Society, 33, 117-120, 1980.