Ram Scoop Devices
To cross the gulfs between stars – even between next-door neighbours like Sol and Alpha Centauri or Barnard’s Star – a very fuel-efficient method of travel is needed if the vessel is not to be just a huge fuel supply with a tiny ship added to it. Robert Forward in his 1995 book Any Sufficiently Advanced Technology is Indistinguishable from Magic sets a reasonable velocity for an interstellar craft at 10% of the speed of light. If exploration should go further, to Epsilon Eridani or Tau Ceti, for instance, then 30% of the speed of light should be the goal as an attainable velocity. These velocities are within the limits of those of the fuelless craft, light sails and ramjets.
The ramjet or ram scoop is a device that uses a powerful magnetic field to collect interstellar hydrogen during flight. The greater the speed, the more efficient the collection will be.
The Bussard ramjet was proposed by R.W. Bussard in 1960. The original vehicle collects charged particles from interstellar space using a large magnetic scoop, and funnels them to the onboard H-He fusion reactor, where they are converted to fuel. According to Bussard's calculations, a 1000 ton starship with a 100% reactor efficiency, which collects fuel from a medium with 1 charged nucleon/cubic centimetre would accelerate almost indefinitely at 1g. In a year the craft would reach the speed of light and in the subjective lifetime of the crew it would also reach the end of the Universe. The diameter of the scoop would need to be 100 km for this 1000 ton vehicle, if it is to move through a space medium with a density of 1000 atoms/cm3.
The top speed of a Bussard ramjet is theoretically very close to the speed of light, but practically it may be hindered by the density of interstellar matter, the drag of the magnetic field and the braking of the incoming protons. The advantage of Bussard ramjets is that they do not need to bring fuel along with them. The downside is that the ramjet will not work from a standstill, but needs a velocity of 4-6% of the speed of light to get the right flux of charged particles to work.
Poul Anderson (Tau Zero) and especially Larry Niven (Tales of Known Space) use ramjets extensively in their science-fiction novels. Here interstellar hydrogen was trapped in nets of electromagnetic force, compressed and guided into a ring of pinched force fields, and there burned in a fusion fire. The maximum velocity of Niven's ramjets is reached when the speed of interstellar hydrogen coming in matches the speed of the exhaust at a significant percentage of the speed of light. In a chase between two ramjets in the Ethics of Madness, both ships accelerate out of the Milky Way. In Niven’s tales of the Man-Kzinn Wars there are many early encounters between ramjets and the alien ships. The earliest ramjets were unmanned, but later versions carried a crew and were often modified to increase thrust by the use of anti-matter/matter reactions. Most of these manned Bussard ramjets in Niven’s stories have two stages of propulsion. The first stage is a laser-propelled light sail launch or a more ordinary rocket launch, and when the right percentage of the speed of light is reached (about 4-6%), the ramjet kicks in. One of the ramjets encountered in Niven’s Man-Kzinn Wars carries its own supply of anti-matter, but scoops interstellar hydrogen to feed the matter/anti-matter drive using a standard Bussard ram scoop. This should increase the thrust enormously, and a fusion reactor would not be needed since heat is produced from the matter/anti-matter reaction alone.
Niven also considers the dangers of the powerful magnetic field to living organisms and puts some limitations on the use of Bussard ramjets. Modifications of the original ramjet concept include charging incoming neutral particles using a laser, getting the ramjet to ram speed using a light sail, boosting the thrust of the craft by anti-matter/ matter reactions, using an accelerator as an alternative reaction-mass drive – the last concept could be enhanced using fusion or anti-matter catalysation. An onboard or Earth-based laser could be used to heat the incoming plasma in the ramjet to further increase thrust.
The multi-cycle Ram Augmented Interstellar Ramjet (RAIR) is an idea conceived by Alan Bond in 1974. Like the Bussard ramjet, the RAIR scoops up interstellar matter using an electromagnetic collector. The RAIR consists of a fusion-powered electromagnetic accelerator running through the core of the ship. Matter ahead of the ship would be ionised with a laser and collected. Collected matter would not be burned as fuel, but only used as reaction mass. The accelerator is thus used to create a ram flow.
Laser-assisted ram scoops are also mentioned. An onboard laser could be used to heat and accelerate the plasma in the ram tube, maybe triggering fusion pulses. This could be used to enhance thrust, giving the craft greater accelerative power. The laser would need a fuel supply, though. Another alternative uses an Earth-orbit laser to beam energy to the craft, in the way of Niven’s light-sail-launched ramjets. But instead of using the beam for push, the craft focuses the beam to heat up the incoming ion stream. This would increase thrust and acceleration power, but the effect of the laser would decrease with the craft’s increasing distance from it, much in the way of the limitations of light sails.
Limitations of ram scoops include the fuel, since it is not known whether sufficient molecules would be available. Another major problem is the ram-scoop braking effect. Magnetic fields tend to catch particles, which resist the inward funneling of the scoop. Thus the craft would push a wide cone of matter in front of it, in effect braking it. Ramjets would also be large. NASA’s model is a 45-year mission to Alpha Centauri, using a 3000 ton craft with a 650 km-diameter ram intake. Science-fiction writers often mention magnetic fields extending many thousands of kilometres. Of course, interstellar craft that carry their own fuel would be much larger.