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The Bussard ramjet method of spacecraft propulsion was proposed in 1960 by the physicist Robert W. Bussard and popularized by Carl Sagan in the television series and subsequent book Cosmos as a variant of a fusion rocket capable of fast interstellar spaceflight. It would use a large scoop (on the order of miles in diameter) to compress hydrogen from the interstellar medium and fuse it. This mass would then form the exhaust of a rocket to accelerate the ramjet.

Design discussion

A workable ramjet design could in principle accelerate indefinitely until its mechanism failed. Such a ramjet could theoretically accelerate arbitrarily close to the velocity of light, and would be a very effective interstellar spacecraft. How long it will take a ramjet driven starship to obtain a given fraction of the velocity of light is determined by the thrust to mass ratio of the interstellar ramjet.

The velocity actually attained by a ramjet-driven starship arises by summing over time the acceleration supplied by the ramjet. If a ramjet accelerates at 10 m/s2, slightly more than one Earth gravity, it can obtain 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.

How fast a Bussard ramjet could actually go depends on four things:

1. How much mass is actually scooped up from space each second 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 (Ve).
3. The thrust to mass ratio of the ramjet which is: A = thrust divided mass (N/kg = m/s2)
4. How long the ramjet is actually able to remain under thrust before it breaks down.

The collection-radius of such an ionic ramscoop is the distance in meters from the ramjet at which the ramscoop's electric field is greater than the galactic electric field of 1.6×10−19 volt, 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/(D2) in distance from the ramscoop generator.

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. This means that the ramjet scoop must sweep 1018 cubic meters of space to collect one gram of ions per second.

A massive fuel system is likely to be far harder to accelerate. Therefore, the specific power,(A), is crucial. This is the joules of energy the starship's reactor generates per kilogram of fuel. This depends on the fuel's energy density and the mass of the scoop.

The obvious fuel source, the one proposed by Bussard, is hydrogen fusion. Hydrogen is believed to be the most common component of interstellar gas. However an interstellar ramjet might be more easily powered by other nuclear reactions. Protium fusion based on the P+P + P+P fusion sequence has not been achieved in power reactors. It might be even harder in an interstellar ramjet. A fusion reactor used to power a ramjet starship could be a steady state magnetic fusion reactor based on the following nuclear fusion reactions . 2H + 2H → 3He + 1n0 + 18 meV, or 2H + 3H4He + 1n0 + 20 meV .It could also be an inertial confinement fusion reactor in which pellets of lithium 6, or lithium 7 deuteride, undergo Teller-Ulam radiation implosion by high energy laser beams, maser beams, or proton or antiproton particle beams. This will heat and compress the fusion fuel pellet until its temperature is more than 100,000,000 degrees Celsius, and increase the density of the fusion plasma by up to 30 times. This will ignite nuclear fusion in the fusion fuel pellet.

To save structural mass, some people have suggested using an electromagnetic field, or alternatively using an electrostatic field for an ion scoop. Such an ion scoop will use electromagnetic funnels, or electrostatic fields to collect hydrogen gas from space for use as propellant by ramjet propulsion systems. An electric field can electrostaticly 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. A magnetohydrodynamic generator drawing power from the exhaust could power the scoop.

Discussions of feasibility

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 was infeasible by calculation. However, in their calculations they assumed that: 1, the exhaust velocity of their interplanetary ion propulsion ramjet could not exceed 100,000 m/s which is 100 km/s; and, 2, that 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 (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 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 (mv2) was of course 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 mv1 > mv2. This condition of course resulted in the drag force exceeding the thrust of the hypothetical ramjet in the Zubrin/Andrews version of the design.

These assumptions may have been valid for the specfic version of the ramjet that was examined by Zubrin/Andrews. Many other serious researchers however have recognized that the assumptions made by Zubrin and Andrews were faulty, and they can not be applied to all other possible ramjet designs, and ion scoop designs.

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 mv2 > mv1 must be true.

• mv1 is the drag force experienced by the ramjet during its actual operation; mv1 is the mass of collected propellant times the velocity of the scooped ions relative to the ramjet starship.
• mv2 is the thrust produced by the ramjet; mv2 is the mass of the collected ramjet propellant multiplied by the exchaust velocity at which it is expelled from the Ramjet engine to generate thrust.

Example

For example, a ramjet might collect 1 gram of incoming ions per second from interstellar space beyond the heliopause, at a velocity of 50 km/s relative to the ramjet driven spacecraft. In this case mv1 is (0.001 kg/s)(50,000 m/s), yielding a drag force of 50 newtons.

If the gram of ions is then accelerated to 500,000 m/s then mv2 is (0.001 kg/s)(500,000 m/s) = 500 N.

Therefore, -50 newtons + 500 newtons yields a net force forward of 450 newtons.

The typical velocity of the solar wind within the solar system is 500 km/s. The typical velocity of the interstellar wind is 50 km/s beyond the heliopause. In the solar system, if the exhaust velocity of the ramjet exceeds 500 km/s there will be a net thrust that will accelerate the ramjet.

If the example were set in the solar system, the drag force, mv1, would be about (0.001 kg/s)(500,000 m/s), or 500 newton.

If the exhaust velocity of the ramjet were 1,000,000 m/s then mv2 = (0.001 kg/s)(1,000,000 m/s) = 1000 N of thrust, and -500 newtons + 1000 newtons = net thrust of 500 newtons to accelerate the ramjet forward.

If the Zubrin/Andrews assumption were correct then mv1 = 500 N, and mv2 = 100 N, and the drag forces would exceed the thrust of the ramjet.

Related inventions

The calculations (by Robert Zubrin and an associate) inspired the idea of a magnetic parachute or sail. This could be important for interstellar travel because it means that deceleration at the destination can be performed with a magnetic parachute rather than a rocket.

Carl Sagan called the construction of a ramjet propelled star ship "engineering on the scale of small worlds".

There may be other practical modifications of this concept. For example, perhaps one could shoot nuggets of fuel in front of a spacecraft from a fixed base, and then the spacecraft would not have to accelerate its own fuel. More speculatively, if the hydrogen was somehow fed into the engine and fused without being accelerated to the spacecraft's current velocity first, there would be no drag. A problem that must be overcome is that most interstellar hydrogen is ordinary protium, instead of the easier-to-fuse deuterium and tritium isotopes, and so makes a poor fusion fuel; it is possible that this could be overcome by using a carbonnitrogenoxygen catalysed nuclear cycle. Potential relative velocities of such a ship are theorized to exceed 16 per cent (0.16) of the speed of light.