Bussard ramjet

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 A Personal Voyage 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 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.

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

The rate at which mass is collected from space by the ion scoop.
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).
The thrust to mass ratio of the ramjet which is: A = thrust divided mass (N/kg = m/s²)
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. This means that the ramjet scoop must sweep 10¹⁸ cubic meters of space to collect one gram of ions per second.

A massive energy source adds more mass to the ramjet system, and this makes it harder to accelerate the ramjet . Therefore, the specific power, (A) of the ramjets 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 ramjets nuclear power reactors.

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 + ³H → ⁴He + 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.

The mass of the ion ram scoop must be minimized on an interstellar ramjet . 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 hydrogen gas from space for use as propellant by ramjet propulsion systems. 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. A magnetohydrodynamic generator drawing power from the exhaust could power the scoop.

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.

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 (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 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 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.

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.

Consider also the case of a vessel leaving a star system, or heading to the outer planets. In this case, the drag produced by the solar wind is beneficial. Since the values for drag are based on relative velocity, using the scoop as a form of solar sail will provide additional thrust as long as the vessel is travelling at less than 500,000 m/s away from Sol. 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 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 d/dt(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 d/dt(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. Figures here assume the spacecraft is travelling towards the sun (since the solar wind is directional), under the worst conditions for thrust.

If the example were set in the solar system, the drag force, d/dt(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 d/dt(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 d/dt(mv₁) = 500 N, and d/dt(mv₂) = 100 N, and the drag forces would exceed the thrust of the ramjet. Under those conditions, the ramjet would likely only function along vectors perpendicular to the solar wind.

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 carbon–nitrogen–oxygen catalysed nuclear cycle. Potential relative velocities of such a ship are theorized to exceed 16 per cent (0.16) of the speed of light.

One useful modification of the ramjet design is to use an electrostatic ion scoop, instead of an electromagnetic ion scoop to achieve the ion collection from space. In an electrostatic scoop a negative electric field on a forward grid electrostatically attracts the positive charged ions present in interstellar space and thus draws them into the ramjet engines. This can be a 100% electrostatic scoop in which an electromagnetic field is not used at all. There will be no converging electromagnetic field lines that can potentially generate drag effects by scooping the ions from interstellar space if this pure electrostatic approach is used. The scooped ions will however have an electric field-induced velocity when they are drawn inside of the ion ramjet engine. So long as the velocity of the ramjet engine exhaust jet is greater than the electric field-induced velocity of the incoming scooped ions there will be a net force in the direction of the ramjet's flight that will accelerate the spacecraft forward.

Furthermore, the net potential difference of the galactic electric field in interstellar space is only 1.6×10^-19 volt. The effective ion collection radius of an electrostatic ion ram scoop will be the range at which the ramscoop electric field has a greater potential difference from the galactic electric field. This potential difference declines proportionately, too: 1/d² for distance d measured in meters, from the source of the ram scoop electric field.

The flux of the interstellar galactic magnetic field is 1×10^-10 tesla. This means that electric ion ram scoop field will have an ion collection radius that is the square root of 10¹³ times (about three million times) greater than the ion collection radius of the electromagnetic ion ramscoop.

In fiction

The ramjet has been used occasionally in science fiction:

Poul Anderson's 1970 novel Tau Zero is about a journey through the universe in a Bussard ramjet

Larry Niven's Known Space fictional universe has several stories in which a ramjet plays a key role.

Niven's novel A World Out of Time, based on his short story "Rammer", also concerns a traveller using a Bussard Ramjet.

In the novel Footfall by Larry Niven and Jerry Pournelle, the alien invaders use a Bussard ramjet for interstellar travel.

In the Star Trek fictional universe vessels commonly have magnetic hydrogen collectors, referred to as Bussard collectors or Bussard ramscoops.

In the computer game series Marathon, Mars's moon Deimos was outfitted with a ramjet and converted into a generation ship.

The massive Seedships Calypso and Tantalus, from the PC game Alien Legacy, are equipped with Bussard ramjets.

The Argo from Robert J. Sawyer's book Golden Fleece uses a Bussard ramjet for both locomotion and murder. In the latter case, the ship's deranged central computer, JASON, directs an occupied shuttle into the collection field. At near light-speed relative to the Argo, the hydrogen acts as hard radiation and instantly kills the shuttle's unfortunate occupant.

Interstellar travel is accomplished with ramjets in the Slow Zone in Vernor Vinge's books A Deepness in the Sky and A Fire Upon the Deep.

Robert A. Heinlein also used the ramjet propelled ships in several of his books.

In Peter F. Hamilton's Night's Dawn an alien race, inhabiting massive "cities" orbiting a red giant, uses a Bussard ramjet with an electromagnetic scoop. It does not only gather hydrogen for propulsion, but also heavier atoms as an alternative to mining.

In Red Dwarf, the titular craft has a large 'Hydrogen Scoop' on the front and the books make reference to it gathering fuel directly from space- presumably a Bussard Ramjet of some sort.

In Coyote by Allen Steele the Starship Alabama uses a Bussard ramjet for propulsion on the journey to 47 Ursae Majoris.

References

For more on ramjet math calculations see The Star Flight Handbook.