Bussard Ramjet - Design Discussion

Design Discussion

A major problem with using rocket propulsion to reach the velocities required for interstellar flight is the enormous amounts of fuel required. Since that fuel must itself be accelerated, this results in an approximately exponential increase in mass as a function of velocity change at non-relativistic speeds, tending to infinity as it approaches the speed of light. In principle, the Bussard ramjet avoids this problem by not carrying fuel with it. An ideal ramjet design could in principle accelerate indefinitely until its mechanism failed. Ignoring drag, a ship driven by such an engine could theoretically accelerate arbitrarily close to the speed of light, and would be a very effective interstellar spacecraft. In practice, since the force of drag produced by collecting the interstellar medium increases approximately as its speed squared at non-relativistic speeds and tends to infinity as it approaches the speed of light (taking all measurements from the ship's perspective), any such ramjet would have a limiting speed where the drag equals thrust. To produce positive thrust, the fusion reactor must be capable of producing fusion while still giving the incident ions a net rearward acceleration (relative to the ship).

An object's velocity can be calculated by summing over time the acceleration supplied (ignoring the effects of special relativity, which would quickly become significant at useful interstellar accelerations). If a ramjet could accelerate at 10 m/s2, slightly more than one Earth gravity, it would attain 77% of light velocity within a year. However, if the ramjet has an average acceleration of 0.1 m/s2, then it needs 100 years to go as fast, and so on.

The top speed of a ramjet-driven spaceship depends on five things:

1. The rate at which mass is collected from space by the ion scoop.
2. The ramjet's exhaust velocity, and the net thrust level obtained from the exhaust jet. The generated thrust can be calculated as the mass of ions expelled per second multiplied by the ramjet exhaust velocity (V_e), adjusted for relativistic effects.
3. The drag produced by collecting the interstellar medium, which will be a function of velocity.
4. The thrust to mass ratio of the ramjet, which is: A = thrust divided mass (N/kg = m/s2) adjusted for relativistic effects.
5. How long the ramjet is actually able to remain under thrust before it breaks down.

The collected propellant can be used as reaction mass in a plasma rocket engine, ion rocket engine, or even in an antimatter-matter annihilation powered rocket engine. Interstellar space contains an average of 10−21 kg of mass per cubic meter of space, primarily in the form of non-ionized and ionized hydrogen, with smaller amounts of helium, and no significant amounts of other gasses. This means that the ramjet scoop must sweep 1021 cubic meters of space (approximately the volume of the Earth) to collect one kilogram of hydrogen.

A large energy source adds more mass to the ramjet system, and this makes it harder to accelerate. Therefore, the specific power, (A) of the ramjet's energy source is crucial. The specific power A is the number of joules of energy the starship's reactor generates per kilogram of its mass. This depends on the ramjet fuel's energy density, and on the specific design of the ramjet's nuclear power reactors.

The obvious fuel source, the one proposed by Bussard, is fusion of hydrogen, the most common component of interstellar gas. Unfortunately, the proton-proton fusion rate is close to zero for this purpose: protons in the Sun on average survive for a billion years or more before reacting. Accordingly, an interstellar ramjet would have to be powered by other nuclear reactions, but the required isotopes are rare in the interstellar medium. A fusion reactor used to power a ramjet starship might be a steady state magnetic fusion reactor based on the following nuclear fusion reactions. 2H + 2H → 3He + 1n0 + 4 MeV, or 2H + 3H → 4He + 1n0 + 17.8 MeV.

This problem was solved, in principle, according to Dr. Bussard by use of the stellar CNO cycle in which carbon is used as a catalyst to burn hydrogen via the strong nuclear reaction. This cycle occurs in the sun (<4%) and is dominant in higher mass stars. The power improvement over the slow PPI chain is by a factor of 1016.

Bussard ramjet designs that use the collected hydrogen only as reaction mass are sometimes referred to as ram-augmented interplanetary or interstellar rockets (RAIR) to distinguish them from the designs that use the collected hydrogen as fuel.

The mass of the ion ram scoop must be minimized on an interstellar ramjet. The size of the scoop is large enough that the scoop cannot be solid. This is best accomplished by using an electromagnetic field, or alternatively using an electrostatic field to build the ion ram scoop. Such an ion scoop will use electromagnetic funnels, or electrostatic fields to collect ionized hydrogen gas from space for use as propellant by ramjet propulsion systems (since much of the hydrogen is not ionized, some versions of a scoop propose ionizing the hydrogen, perhaps with a laser, ahead of the ship.) An electric field can electrostatically attract the positive ions, and thus draw them inside a ramjet engine. The electromagnetic funnel would bend the ions into helical spirals around the magnetic field lines to scoop up the ions via the starship's motion through space. Ionized particles moving in spirals produce an energy loss, and hence drag; the scoop must be designed to both minimize the circular motion of the particles and simultaneously maximize the collection. Likewise, if the hydrogen is heated during collection, thermal radiation will represent an energy loss, and hence also drag; so an effective scoop must collect and compress the hydrogen without significant heating. A magnetohydrodynamic generator drawing power from the exhaust could power the scoop.

The collection-radius of such an ionic ramscoop is the distance from the ramjet at which the ramscoop's electric field is greater than the galactic electric field of 1.6×10−19 V/m, or the ramscoop's electromagnetic field is greater than the natural galactic magnetic field of 0.1 nanotesla (1×10−6 gauss). The strength of the ramscoop collection field would decline proportionately to 1/d3 in distance from the ramscoop generator.