Bussard ramjet

  The Bussard ramjet method of spacecraft propulsion was proposed in 1960 by the physicist Robert W. Bussard. It was popularized by Larry Niven in his Known Space series of books and referenced by Carl Sagan in the television series and book Cosmos. Bussard proposed a ramjet variant of a fusion rocket capable of fast interstellar spaceflight. It would use an enormous electro-magnetic field (ranging from kilometers to many thousands of kilometers in diameter) as a ram scoop to collect and compress hydrogen from the interstellar medium. High speed forces the reactive mass into a progressively constricted magnetic field, compressing it until thermonuclear fusion occurs. The magnetic field then directs the energy as rocket exhaust opposite to the intended direction of travel, thereby accelerating the vessel.

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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 exponential increase in mass. 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 velocity of light, and would be a very effective interstellar spacecraft. In practice, since the drag produced by collecting the interstellar medium increases with speed, 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 without significantly slowing the incident ions down (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).
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)
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 unionized and ionized hydrogen, with smaller amounts of helium, and no significant amounts of other gasses. This means that the ramjet scoop must sweep 10¹⁸ cubic meters of space to collect one gram 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 + 18 MeV, or ²H + ³H → ⁴He + ¹n⁰ + 20 MeV.

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.

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. By 1978 analyses indicated that Bussard ramjets were not feasible.^[1]

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.

These assumptions may have been valid for the specific version of the ramjet that was examined by Zubrin/Andrews.

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 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.

There may be other practical modifications of this concept. 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 (₁H¹ "Hydrogen-1", which does not easily partake in fusion reactions, instead of the easier-to-fuse deuterium and tritium isotopes proposed for use in fusion reactors. 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 possible 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 can be a net force in the direction of the ramjet's flight that will accelerate the spacecraft.

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 to 1/d² for distance d from the source of the ram scoop electric field.

In fiction

The ramjet has been used occasionally in science fiction:

• In Steven Baxter's novel Manifold: Space, the Gaijin 'flowerships' utilize a variant of Bussards ramjet for propulsion.

• 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 fusion ramjets play a key role. Niven's stories appear to have been the origin of the alternate term, often used in science fiction, of "ramscoop": first referred to as "ramscoopfusion" in the early story "Handicap", then abbreviated to the usual form in his later works.

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

• In Protector written by Larry Niven in 1973, the main protagonists use Bussard Ramjets.

• 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. Those are seemingly fitted on the forward end of the twin "warp nacelles", and have a "reverse" function that allows for spreading hydrogen as well as sucking it in.

• In the computer game series Marathon, Mars's moon Deimos was outfitted with a ramjet and converted into a generation ship, Deimos was then renamed as Marathon, the titular ship of the story.

• 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.

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

• Robert A. Heinlein also used 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.

• In Beelzebub's Tales to his Grandson by G. I. Gurdjieff published in 1950 there is a description of a Space Ship which uses a device involving an inner cylinder into which the ship gathers the materials of space - the radioactive particles, nebulous gases, is filled - which then are constricted. The regulated escape of these compressed energies forces the ship forward into the area of low pressure created by the engine's intake.

• In the Doom novels by Dafydd ab Hugh and Brad Linaweaver, the ship that Fly and Arlene use to travel from the Fred homeworld is powered by a hydrogen scoop.

• In Fiasco (novel) by Stanislaw Lem both Eurydice and Hermes starships use a form of Bussard ramjet based on interstellar hydrogen as a main (travelling) drive at close to c speeds, achieving the initial acceleration required using light sail supported by laser emissions. The description of the process in the book points to hydrogen->helium fusion as a primary thrust/power source.

References

• For more on ramjet math calculations see The Star Flight Handbook (ISBN 0-471-61912-4).
• Interstellar Medium
• Interstellar Hydrogen
• Images of Bussard Ramjet