Bussard Ramjet - Discussions of Feasibility

Discussions of Feasibility

Since the time of Bussard's original proposal, it has been discovered that the region surrounding the sun has a much lower density of interstellar hydrogen than was believed at that time (see Local Interstellar Cloud). T.A. Heppenheimer analysed Bussard's original suggestion of fusing protons, but found the bremsstrahlung losses from compressing protons to fusion densities was greater than the power that could be produced by a factor of about 1 billion, thus indicating that the proposed version of the Bussard ramjet was infeasible. However Daniel P. Whitmire's 1975 analysis indicates that a ramjet may achieve net power via the CNO cycle, which produces fusion at a much higher rate (~1016 times higher) than the proton-proton chain.

Robert Zubrin and Dana Andrews analyzed one hypothetical version of the Bussard ramscoop and ramjet design in 1985. They determined that their version of the ramjet would be unable to accelerate into the solar wind. However, in their calculations they assumed that:

1. The exhaust velocity of their interplanetary ion propulsion ramjet could not exceed 100,000 m/s (100 km/s);
2. The largest available energy source could be a 500 kilowatt nuclear fission reactor.

In the Zubrin/Andrews interplanetary ramjet design, they calculated that the drag force d/dt(mv₁) equals the mass of the scooped ions collected per second multiplied by the velocity of the scooped ions within the solar system relative to the ramscoop. The velocity of the (scooped) collected ions from the solar wind was assumed to be 500,000 m/s.

The exhaust velocity of the ions when expelled by the ramjet was assumed not to exceed 100,000 m/s. The thrust of the ramjet d/dt(mv₂) was equal to the mass of ions expelled per second multiplied by 100,000 meters per second. In the Zubrin/Andrews design of 1985, this resulted in the condition that d/dt(mv₁) > d/dt(mv₂). This condition resulted in the drag force exceeding the thrust of the hypothetical ramjet in the Zubrin/Andrews version of the design.

Consider also the case of a vessel leaving a star system, or heading to the outer planets. In this case, the force produced by the solar wind is beneficial. Since the values for drag are based on relative velocity, using the scoop as a form of electromagnetic sail will provide additional thrust as long as the vessel is travelling at less than 500,000 m/s away from a star. While interstellar matter is relatively scarce, this abundance of high-energy ions in the neighborhood of stars has potential for initial acceleration and braking on arrival.

The key condition that determines whether or not an interstellar ramjet will accelerate forward in the direction of its thrust is that the thrust of the ramjet must exceed drag that results from scooping up ions from space. Or, as discussed above, the condition d/dt(mv₂) > d/dt(mv₁) must be true.

• d/dt(mv₁) is the drag force experienced by the ramjet during its actual operation; d/dt(mv₁) is the mass of collected propellant per unit time times the velocity of the scooped ions relative to the ramjet starship.
• d/dt(mv₂) is the thrust produced by the ramjet; d/dt(mv₂) is the mass of the collected ramjet propellant per unit time multiplied by the exhaust velocity at which it is expelled from the Ramjet engine to generate thrust.