A propulsion system for interstellar travel proposed in 1960 by Robert W. Bussard.

By scooping up atoms (mostly hydrogen) and stray particles in the path of the spacecraft and fuel a fusion reactor with the collected matter, thereby generating thrust, the need for onboard fuel will be minimized.

In order to keep the fusion reactor running, enough matter has to be collected. By using a toroidal coil mounted in the front of the craft, a magnetic field is generated that draws matter into the collector, thereby lowering the speed needed by the ramjet to sustain reactor operation. Still, approximately 6-10% of c is considered the minimum initial speed for the ramjet drive.

To attain this speed and to ensure that the ramjet can return to orbiting speeds, fuel has to be stored aboard the ramjet.

Typical flight schedule for a craft with this type of propulsion is:

  1. Acceleration to 6-10% of c using onboard fuel.
  2. Activation of the fuel scoop. Further acceleration, max speed depends on the density of the surrounding. A low density allows a higher maximum speed but a slower acceleration due to lack of fuel, and vice versa.
  3. Initial brake. The fusion reactor is deactivated but the fuel scoop is left running, serving a dual purpose of acting as a brake (using the inertia of particles in the semi-vacuum) and gathering fuel for the final brake and a re-acceleration.
  4. Final brake. As the speed of the spacecraft decreases, the efficiency of the initial brake diminishes. To bring the spacecraft to orbiting speed, the thrusters are activated, consuming fuel scooped during the initial brake.

The main benefit from this form of propulsion is that little fuel is needed to be stored for direct propusion. Theoretically, it could accelerate for its entire trip, reaching as close to c as possible. Unfortunately for Bussard, there are a few problems with this "free" propusion.

Firstly, the proposed fusion process (proton-proton fusion)is difficult to sustain. As Rollo says above, you would need to travel at 6-10% lightspeed to maintain proper fuel intake. I don't know how slow that sounds to you, but at 5% c, you can get to Alpha Centauri in less than 100 years. That's a pretty good clip to be moving at, and that's what is said to be minimum. Might be nice for those further stars, or cross-galaxy journies (assuming there is enough fuel along the way).

Speaking of enough fuel, the craft would have to be able to deal with the possibility of not having enough fuel to sustain the fusion process. Again, as Rollo mentioned, fuel could be stored for the bare patches where there isn't much hydrogen around. Also, it has been found that certain "impurities" such as carbon and other more complex atoms can be added to the intersteller hydrogen to lower the difficulty of fusion. Modest calculations say it would be some 10 orders of magnitude easier to keep the process going this way.

An alternative to a fusion engine is to use plasma instead, energized by an onboard fission powerplant. This makes it much easier to create propulsion, makes it easier to turn on and off, would have power to spare for other functions, and would use the fuel almost as efficiently as fusion. However, plasma drives aren't terribly powerful, and my next point makes this fact more important.

The scoop causes drag as the craft sails along, its magnetic fields pushing against other fields, solar wind and cosmic rays. If our spacecraft was heading past a star, it might be able to pick up more fuel, but it would encounter a greater amount of drag. Such drag would easily outpower a plasma drive, and would impact a fusion drive to a marginal extent.

The idea sounds great, since it means that materials are used when they are needed rather than stored up. I'm sure that one day this technology will be used in some form, more likely the scoop design would be used as a brake rather than to gather fuel. The human race has a lot of work to do before these become practical to take to the stars and back, technologically, economically, and socially.

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