Bussard Ramjet
Tech Level: 18

The Bussard Ramjet was first conceived by Robert Bussard in 1960, and has since been used in countless literary science fiction stories and novels, such as Tau Zero by Poul Anderson, A Deepness in the Sky by Vernor Vinge, and numerous "Known Space" stories by Larry Niven.

The Bussard Ramjet concept relies on the fact that the vacuum of space is not quite as empty as we tend to think. While matter is spread incredibly thin in the depths between the stars, there still exists about one hydrogen atom per cubic centimeter (compared to the 10^18 atoms per cubic centimeter in Earth’s atmosphere at sea level.) The Bussard Ramjet uses an immense forward-facing conical magnetic field to scoop up this interstellar medium as it zooms through space, using its tremendous forward velocity to force the funneled hydrogen molecules to a fusion state at the apex of the converging magnetic fields. The super-hot fusion plasma is then expelled for thrust.

Used in this way, the Ramjet-equipped ship would never run out of fuel as long as it maintained a certain minimum velocity for the system to remain functioning. This exact minimum velocity is a subject for debate; some sources say 1% lightspeed or so, while others quote it as high as 6%. Whatever it may be, the ship would need a secondary drive system that would allow it to not only get up to these velocities without using the ramscoop but to maneuver around within a star system where significant fractions of lightspeed may prove impractical. Typical secondary drive sources from science fiction source include light sails, magnetic sails, fusion drives, and antimatter drives.

Various diagrams of Bussard Ramjet operations

Because it has a practically unlimited fuel supply, a bussard ramjet is a particularly powerful stardrive, as it can theoretically accelerate for any arbitrary interval of time, whether it be a few minute or many millennia. Very efficient ramjet drives can come to within a hairbreadth’s of the velocity of light, though some source say that a ramjet’s more practical limit may be between 50% and 85% lightspeed. Accelerating at a constant 1 g, a Bussard Ramjet could get to within a few percentage points of lightspeed within a year.

A wide-beam laser is shot ahead of the vehicle, imparting enough energy to any hydrogen atom in its path to force its electrons to fly off. The magnetic field projected by the ship attracts the positively charged ions and repels the now free electrons. The tremendous forward velocity of the ship and the tapering cone of the magnetic fields force the protons together with enough force to spark a fusion reaction. Alternately, the hydrogen can be catalyzed by an on-board antimatter supply to produce more efficient fusion reactions.

One of the main difficulties in building a bussard ramjet (aside from getting it up to the minimal operational velocity of one to six percent lightspeed) is creating magnetic fields large enough to gather enough fuel to be practical and strong enough to handle the stresses of scooping and fusing hydrogen at significant fractions of lightspeed. In order to obtain enough fuel for continual operation, the scoop would have to be thousands of miles wide and relatively narrow to aid in maintaining magnetic field strength. The strength of the field would also be immense, on the order of 10 million tesla, making them instantly deadly to any living creature. In one Larry Niven story, a bussard ramjet scoop field was used to threaten an entire planet with extinction.

Some concern has been expressed about the amount of drag the interstellar medium will induce on the ramjet. Moving at significant fractions of lightspeed, the repeated impacts of the interstellar hydrogen on the immense ramscoop field is thought by some to offset much of the acceleration produced by the fusion engines, greatly reducing the starship’s capabilities. If this is so, a ramjet’s top speed may be only 15% to 25% lightspeed. However, it has also been pointed out that the impacts would not necessarily produce nothing but waste energy, as the ramscoop uses the impacts as part of its scooping and fusion processes, so how much drag a bussard ramjet would actually experience is a matter for debate.

The relative sizes of a typical ramscoop field and planet Earth. Ship not to scale.







Article added 2005