| abstract
| - A Bussard Ramjet is a fusion reaction drive that uses electromagnetic fields to funnel hydrogen into a reactor, where the energy released by the fusion is used as propellant for locomotion. Whilst the technology is in use in the Known Space stories, the actual technology of this name is theoretical. See Wikipedia:Bussard Ramjet
- 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 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 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. 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 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.
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