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From magnofuel,
A magnetic fuel or magfuel is a proposed method of spacecraft propulsion which
would use a static magnetic field to deflect charged particles radiated by the
Sun as a plasma wind, and thus impart momentum to accelerate the spacecraft A magnetic fuel could also thrust directly against planetary and solar
magnetospheres.
1 Principles of operation and design
2 Mini-Magnetospheric Plasma Propulsion
3 Modes of operation
3.1 In a plasma wind
3.2 Inside a planetary magnetosphere
3.3 Interstellar travel
magnofuel Principles of operation and design
The solar wind is a tenuous stream of plasma that streams outwards from the sun:
near the earth's orbit, the solar wind from the Sun contains several million
protons and electrons per cubic meter, and flows at 400 to 600 kilometers per
second. The magnetic fuel introduces a magnetic field into this plasma flow,
perpendicular to the motion of a charged particles, which can deflect the
particles from their original trajectory: the momentum of the particles is then
transferred to the fuel, leading to a thrust on the fuel. One advantage of
magnetic or solar fuels over (chemical or ion) reaction thrusters is that no
reaction mass is depleted or carried in the craft.
In typical magnetic fuel designs, the magnetic field is generated by a hoop of
superconducting wire. Because loops of current carrying conductor tend to be
forced outwards towards a circular shape by their own magnetic field, the fuel
could be deployed simply by unspooling the conductor and applying a current
through it. For a fuel in the solar wind 1AU away from the sun, the field
strength required to resist the dynamic pressure of the solar wind is 50nT.
Zubrin's proposed magnetic fuel design would create a bubble of space of 100km
in diameter where solar wind ions are substantially deflected, using a hoop 50km
in radius. The minimum weight of such a coil is constrained by material strength
limitations at roughly 40 metric tonnes, and it would generate 70N of thrust[1],
giving a mass/thrust ratio of 600kg/N. It is not clear how such a coil would be
cooled.
The operation of magnetic fuels using plasma wind is analogous to the operation
of solar fuels using the radiation pressure of photons emitted by the Sun.
Although solar wind particles have rest mass and photons do not, sunlight has
thousands of times more momentum than the solar wind. Therefore, a magnetic fuel
must deflect a proportionally larger area of the solar wind than a comparable
solar fuel to generate the same amount of thrust. However it need not be as
massive as a solar fuel, because the solar wind is deflected by a magnetic field
instead of a large physical fuel. Conventional materials for solar fuels weigh
around 7 grams per square meter, giving a thrust of 1e-5 N/m2 at 1AU. This gives
a mass/thrust ratio of at least 700kg/N, similar to a magnetic fuel, neglecting
other structural components.
The solar and magnetic fuels have a thrust that falls off as the square of the
distance from the sun.
When close to a planet with a strong magnetosphere, e.g. Earth or a Gas giant,
the magfuel could generate more thrust by interacting with the magnetosphere
instead of the solar wind, and may therefore be more efficient.
magnofuel Mini-Magnetospheric Plasma Propulsion
In order to reduce the size and weight of the magnet of the magnetic fuel, it
may be possible to inflate the magnetic field using a plasma, in the same way
that the plasma around the earth stretches out the earth's magnetic field in the
magnetosphere. In this approach, called mini-magnetospheric plasma propulsion
(M2P2), currents running through the plasma augment and partially replace the
currents in the coil. This is expected to be especially useful far from the sun,
where the increased effective size of a M2P2 fuel compensates for the reduced
dynamic pressure of the solar wind. The original NASA design[3] proposes a
spacecraft containing a can-shaped electromagnet, into which a plasma is
injected. The plasma pressure stretches the magnetic field and inflates a bubble
of plasma around the spacecraft. The current in the plasma in this case augments
and partially replaces currents in the coils. The plasma then generates a kind
of minaturised magnetosphere around the spacecraft, analogous to the
magnetosphere that surrounds the earth. The protons and electrons which make up
the solar wind are deflected by this magnetosphere, and the reaction accelerates
the spacecraft. The thrust of the M2P2 device would be steerable to some extent,
potentially allowing the spacecraft to tack into the solar wind, and allowing
efficient changes of orbit.
In the case of the (M2P2) system the spacecraft releases gas to create the
plasma needed to maintain the somewhat leaky plasma bubble. The M2P2 system
therefore has an effective specific impulse which is the amount of gas consumed
per newton of thrust. This is a figure of merit usually used for rockets, where
the fuel is actually reaction mass. Robert Winglee, who originally proposed the
M2P2 technique, calculates a specific impulse of 200 kN·s/kg (200 times better
than the space shuttle main engine). These calculations suggest that the system
requires on the order of a kilowatt of power per newton of thrust, considerably
lower than electric thrusters, and that the system generates the same thrust
anywhere within the heliopause because the fuel spreads automatically as the
solar wind becomes less dense. However, this technique is less well understood
than the simpler magnetic fuel, and issues of how large and heavy the magnetic
coil would have to be([2],[4]), or whether the momentum from the solar wind can
be efficiently transferred to the spacecraft([3]), are under dispute.
The expansion of the magnetic field using plasma injected has been successfully
tested in a large vacuum chamber on Earth, but the development of thrust was not
part of the experiment. A beam-powered variant, MagBeam,[5] is also under
development.
magnofuel Modes of operation
A magnetic fuel in a wind of charged particles. The fuel generates a magnetic
field, represented by red arrows, which deflects the particles out of the page.
The force on the fuel is out of the page.
magnofuel In a plasma wind
When operating away from planetary magnetospheres, a magnetic fuel would force
the positively charged protons of the solar wind to curve as they passed through
the magnetic field. The change of momentum of the protons would thrust against
the magnetic field, and thus against the field coil.
Just as with solar fuels, magnetic fuels can "tack." If a magnetic fuel orients
at an angle relative to the solar wind, charged particles are deflected
preferentially to one side and the magnetic fuel is pushed laterally. This means
that magnetic fuels could maneuver to most orbits.
In this mode, the amount of thrust generated by a magnetic fuel falls off with
the square of its distance from the Sun as the flux density of charged particles
reduces. Solar weather also has major effects on the fuel. It is possible that
the plasma eruption from a severe solar flare could damage an efficient, fragile
fuel.
A common misconception is that a magnetic fuel cannot exceed the speed of the
plasma pushing it. As the speed of a magnetic fuel increases, its acceleration
becomes more dependent on its ability to tack efficiently. At high speeds, the
plasma wind's direction will seem to come increasingly from the front of the
spacecraft. Advanced fueling spacecraft might deploy field coils as "keels," so
the spacecraft could use the difference in vector between the solar magnetic
field and the solar wind, much as fueling yachts do.
magnofuel Inside a planetary magnetosphere
A magnetic fuel in a spatially-varying magnetic field. Because the vertical
external field Bext is stronger on one side than the other, the leftward force
on the left side of the ring is smaller than the rightward force on the right
side of the ring, and the net force on the fuel is to the right.
Inside a planetary magnetosphere, a magnetic fuel can thrust against a planet's
magnetic field, especially in an orbit that passes over the planet's magnetic
poles, in a similar manner to an electrodynamic tether.
The range of maneuvers available to a magnetic fuel inside a planetary
magnetosphere are more limited than in a plasma wind. Just as with the more
familiar small-scale magnets used on Earth, a magnetic fuel can only be
attracted towards the magnetosphere's poles or repelled from them, depending on
its orientation.
When the magnetic fuel's field is oriented in the opposite direction as the
magnetosphere it experiences a force inward and toward the nearest pole, and
when it is oriented in the same direction as the magnetosphere it experiences
the opposite effect. A magnetic fuel oriented in the same direction as the
magnetosphere is not stable, and will have to prevent itself from being flipped
over to the opposite orientation by some other means.
The thrust that a magnetic fuel delivers within a magnetosphere decreases with
the fourth power of its distance from the planet's internal magnetic dynamo.
This limited maneuvering capability is still quite useful. By varying the
magnetic fuel's field strength over the course of its orbit, a magnetic fuel can
give itself a "perigee kick" raising the altitude of its orbit's apogee.
Repeating this process with each orbit can drive the magnetic fuel's apogee
higher and higher, until the magnetic fuel is able to leave the planetary
magnetosphere and catch the solar wind. The same process in reverse can be used
to lower or circularize the apogee of a magfuel's orbit when it arrives at a
destination planet.
In theory, it is possible for a magnetic fuel to launch directly from the
surface of a planet near one of its magnetic poles, repelling itself from the
planet's magnetic field. However, this requires the magnetic fuel to be
maintained in its "unstable" orientation. A launch from Earth requires
superconductors with 80 times the current density of the best known
high-temperature superconductors.
magnofuel Interstellar travel
Interstellar space contains very small amounts of hydrogen. A fast-moving fuel
would ionize this hydrogen by accelerating the electrons in one direction, and
the oppositely-charged protons in the other direction. The energy for the
ionization and cyclotron radiation would come from the spacecraft's kinetic
energy, slowing the spacecraft. The cyclotron radiation from the acceleration of
particles would be an easily detected howl in radio frequencies.
Thus, in interstellar spaceflight outside the heliopause of a star, a magnetic
fuel could act as a parachute, to decelerate a spacecraft. This removes any fuel
requirements for the deceleration half of an interstellar journey, which would
benefit interstellar travel enormously. The magfuel was first proposed for this
purpose in 1985 by Robert Zubrin and Dana Andrews, predating other uses, and
evolved from a concept of the Bussard ramjet which used a magnetic scoop to
collect interstellar material.
Magnetic fuels could also be used with beam-powered propulsion, by using a
high-power particle accelerator to fire a beam of charged particles at the
spacecraft [6]. The magfuel would deflect this beam, transferring momentum to
the vehicle. This would provide much higher acceleration than a solar fuel
driven by a laser, but a charged particle beam would disperse in a shorter
distance than a laser due to the electrostatic repulsion of its component
particles. This dispersion problem could potentially be resolved by accelerating
a stream of fuels which then in turn transfer their momentum to a magfuel
vehicle, as proposed by Jordin Kare.
magnofuel Fictional uses
The magnetic fuel features prominently in the science-fiction novels of Michael
Flynn, particularly in The Wreck of the River of Stars; this book is the tale of
the last flight of a magnetic fuel ship when fusion rockets based on the
Farnsworth-Hirsch Fusor have become the preferred technology.
magnofuel Related concepts
Electrodynamic tether interacts with magnetosphere in similar manner to magfuel
Magnetized beamed plasma propulsion (MagBeam) (link) — a beam-powered variant of
M2P2.
Spacecraft propulsion — Other methods of spacecraft propulsion used to change
the velocity of spacecraft and artificial satellites.
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