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Robotic probe using
Microfusion (AIM) propulsion,
where pellets of deuterium-tritium (DT) and uranium 238 are liquified
and injected into an antiproton plasma chamber and irradiated until
the U238 absorbs the antiprotons and fissions to compress the DT to
fuse making charged-particle rocket exhaust, to explore the Oort Cloud.
(For more information, see AIMStar proposal -- in pdf.)
Interplanetary Propulsion : at NASA/ASTP.
Rocket propulsion, which relies on Newton's law of action and reaction, is the fastest means of interplanetary travel today. In general, a rocket propels itself by expelling material (exhaust) in the direction opposite to its desired motion. In today's chemical rockets, the exhaust is often a gas heated (and perhaps created) by a reaction between a fuel and an oxidizer. In other rocket-type propulsion systems, propulsive thrust is imparted to exhaust by a propellant heater or accelerator using a power source located elsewhere on the space vehicle (or "ship"). Non-chemical power sources in an advanced stage of development today, or in recent decades, range from solar power collectors for small probes to nuclear reactors for fast but massive manned vehicles to Mars and beyond, where sunlight becomes inefficiently feeble for quick acceleration.
NASA -- larger
Exploration ships to Jupiter and its
moons (e.g., Io) and beyond could
someday be propelled by lasers
generated by Solar power satellites
located among the inner planets.
Propulsive thrust is measured in newtons. It is the product of the velocity of exhaust (relative to the ship) and the rate of propellant flow. Hence, the same thrust can be generated by ejecting more propellant at low velocity or less propellant at high velocity, where a high exhaust temperature compensates for using less propellant (or "fuel").
To compare the relative performance of different propulsion systems, engineers ("rocket scientists") calculate the exhaust velocity and divide by the acceleration of gravity at Earth sea level (9.8 meters or 32 feet per second per second) -- the "specific impulse" of each system. Thus, high fuel efficiency is equivalent to a high specific impulse. While propulsive thrust is directly proportional to specific impulse, the power required to generate thrust is proportional to the square of specific impulse. Today, the great amount of power needed for producing the very high specific impulse needed to move large masses (such as manned space vehicles) quickly appears to require energy sources such as nuclear fusion and matter-antimatter annihilation.
|Propulsion Type||Specific Impulse [sec]||Thrust-to-Weight Ratio|
|Chemical Bipropellant||200 - 410||.1 - 10|
|Electromagnetic||1200 - 5000||10-4 - 10-3|
|Nuclear Fission||500 - 3000||.01 - 10|
|Nuclear Fusion||10+4 - 10+5||10-5 - 10-2|
|Matter-Antimatter Annihilation||10+3 - 10+6||10-3 - 1|
A hybrid propulsion concept called the Variable Specific Impulse Magnetoplasma Rocket (VASIMR) has been in development since the 1980s. In order to achieve a higher fuel efficiency than a chemical rocket, VASIMR ionizes a light element gas such as hydrogen or helium and accelerates the resulting "plasma" (positively charged atomic nuclei and free negatively charged electrons) with electric and magnetic fields. To strip neutral gas atoms of some or all of their electrons in creating a plasma, a very high temperature is needed. While plasmas begin forming at about 10,000 Kelvin, laboratory experiments have achieved 10 million Kelvin. Unlike traditional rocket-type propulsive systems, VASIMR can adjust its thrust (through specific impulse) by varying the power used to heat the plasma propellant at different stages of its flow, before it is expelled as exhaust. This "throttle" capability improves maneuverability, which also enhances safety.
Space Flight Center
View of ship leaving Earth orbit
Advanced plasma engines that produce
high-power jets of ionized gas are being
considered for interplanetary travel.
See a short Voyage to Mars animation
(with music) of an automated cargo ship
that travels from Earth orbit to Mars.
A 10-kilowatt spacecraft relying on Solar power has been proposed for demonstration in mid-2004. Manned spaceships to Mars and beyond, however, would probably require a fusion reactor that produces 10 to 100 gigawatts of power. For manned missions, the magnetic field and the proposed hydrogen propellant used in VASIMR could be used to provide radiation shielding. (For further details, see Franklin R. Chang Diaz, Scientific American, November 2000.)
Advanced Chemical Propulsion Concepts (in pdf) : at NASA/ASTP.
Advanced Space Transportation
Two residential modules
linked by a long tether,
rotating about a spacecraft,
for long voyages.
Electromagnetic Propulsion Concepts (in pdf): at NASA/ASTP.
Nuclear Propulsion Concepts (in pdf) : at NASA/ASTP.
Fusion / AntiMatter Propulsion (in pdf) : at NASA/ASTP.
Small amounts of antimatter would be useful for initiating and maintaining fission or fusion reactions in hybrid rockets, where matter-antimatter annihilation would be used to cause atomic nuclei to undergo nuclear fission or to form heavier nuclei through nuclear fusion and release tremendous amounts of energy. Although current nuclear fission can only transfer heat energy from a uranium core to surrounding chemical propellant, antiproton catalyzed microfission (ACMF) permits all energy from fission reactions to be used for propulsive purposes that creates a more efficient engine (Isp = 13,500 sec) that can be used for interplanetary manned missions. Antiproton Initiated Microfission/fusion (AIM) uses smaller amounts of antimatter and fissionable material to spark a microfusion reaction with lightweight atomic nuclei for a much higher specific impulse (61,000 sec).
propulsion, where pellets of deuterium-tritium (DT) and uranium 238
are compressed by particle beams and then irradiated with a beam of
antiprotons absorbed by U238 which fissions and releases neutrons that
compress the mixture until the DT fuses making charged-particle rocket
exhaust, to explore Jupiter and other Solar System objects beyond Mars.
(For more information, see AMCF/AIM propulsion paper -- in pdf.)
Solar power satellites can be built to convert direct Solar radiation received in the full, unobstructed intensity possible in space to direct current (DC), electrical power. This DC power is then converted to microwaves which is transmitted through space as an electronically steerable microwave beam -- "wireless power transmission" (WPT)). The microwave beam is then captured by a receiver(s) on electrically propelled spaceships for interplanetary (within Solar System) as well as interstellar transport (at sublight speeds) by providing beamed power for space propulsion systems, such as those using Space Sails.
NASA's Marshall Space Center is
developing Space Sail technology
for a 2010 deep space probe that
would travel at least 250 times the
Earth-Sun distance (astronomical
units or AUs) and explore the
region of interstellar space
surrounding the Solar System.
Although past spacecraft development has focused on propellants which are expelled by the energy generated on board the vessel, some proposed spacecraft designs would use energy that is not carried on board. This energy would be transmitted to the spacecraft in the form of a radio frequency or laser beam, thus greatly reducing the mass of the spacecraft. (For now, further details can be found in Paul Woodmansee's discussion of Light Sails and at a NASA web link page on Solar Sails. Another illustration of an interplanetary application can be found at the Center for Space Power, a NASA "Commercial Space Center" hosted by Texas A&M University.)
Mini-Magnetosphere Plasma Propulsion (M2P2)
Most planets in the Solar System have magnetic fields that extend into space like a giant bubble. For example, Earth lies at the heart of such a magnetic bubble, one that occupies a volume at least a thousand times greater than the planet itself. This magnetosphere protects life on Earth from the Solar wind and from potentially deadly solar flares, unlike Mars and the Moon which lack their own magnetospheres.
and U. Washington
A spacecraft with Mini-
Propulsion (M2P2) could
attain a specific impulse
(a measure of propulsive
efficiency) of tens of
thousands of seconds
-- 10 to 20 times
better than the Space
Shuttle's main engine.
In addition to providing shielding from Solar radiation, a magnetosphere can work like a space sail because the Solar wind pushes on it constantly. While planets like Earth are too massive to blow away, however, magnetospheres around much smaller space vehicles would be more easily moved. Calculations suggest that a 9.3-mile-wide (15 km) miniature magnetosphere at the Earth's distance from the Sun would feel enough Solar pressure (1 to 3 Newtons of force) to accelerate a 440-pound (200 kg) spacecraft from a dead stop to 180,000 mph (290,000 kph) in only three months.
The force exerted on a magnetosphere increases with its size, as the Solar wind has more to push against. To make a magnetosphere big enough to move a spacecraft, NASA are injecting ionized gas (plasma) near a magnetic coil, giving the project its name, Mini-Magnetospheric Plasma Propulsion (M2P2). NASA engineers are currently experimenting with a helicon plasma generator that ionizes gaseous argon and helium with high-power radio waves, a method routinely used to etch commercial semiconductors. A test in a large vacuum chamber has been succesfully completed at NASA's Marshall Space Flight Center.
Magnetospheres that move further away from the Sun naturally expand as the Solar wind pressure plummets, for the same reason that a balloon inflated at sea level will expand in the less dense air of higher altitudes. Luckily, the cross section of magnetospheres increases by the same factor that the solar wind pressure declines. Hence, the propulsive thrust of an M2P2-powered craft would remain the same whether the spacecraft is near the Sun or in the outer reaches of the Solar System.
Orbital Transfer : at NASA/ASTP.
While slower, orbital transfer concepts for interplanetary travel may be more cost-effective for moving large amounts of cargo.
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