Advanced Propulsion

Space Settlement relevance

Even though modern chemical rockets have sent us to the Moon, to build large space settlements we will need something more. Today, the high cost of launching into space limits the number of people who go there. Advanced propulsion, however, offers a chance to dramatically lower launch costs. Futhermore, many of the ideas being proposed would allow us to move around the solar system much quicker than we do today. Instead of months to Mars, it would be weeks. Years to Jupiter and Saturn would be months. And centuries to the stars would become years.

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Nuclear Fission Rockets

Nuclear rockets have, in one way or another, been studied for the past fifty years. Some of the earliest ideas for nuclear rockets came from the nuclear physicists struggling to build the bomb at Los Alamos. These desgins evolved into what are called solid core nuclear rockets. Solid core nuclear rockets work by taking liquid hydrogen and pumping it to the reactor through a jacket surrounding the rocket engine. This pumping process helps cool the rocket, and it also preheats the liquid hydrogen. Hundreds of narrow channels pass through the nuclear reactor. As the liquid hydrogen flows through these channels, heat from the reactor changes the fuel into rapidly expanding gas. The gas flows through the exhaust nozzle at speeds up to 35,400 kilometers per hour. A diagram of a solid core rocket. Several tests were carried out during the 1960s with nuclear rockets. The most well known of these are the Nuclear Engine for Rocket Vehicle Applications (NERVA) tests. NERVA involved the Los Alamos laboratory, Westinghouse, Aerojet, and other industrial partners. Together, they were able to build and test a solid core rocket with a thrust of 250,000 pounds, and a specific impulse (Isp) of 850 seconds, nearly twice the best Isp of chemical rockets (specific impulse is a "miles per gallon" for rockets). Unfortunately, the public's fear of anything nuclear led to the cancellation of NERVA in 1971. The field of nuclear rocketry would lie nearly dormant for the next 10 years.

In 1982 there was an upsurge of support for nuclear rockets. The old NERVA designs were dusted off just in time for Ronald Reagan's Strategic Defense Initiative (SDI, or "Star Wars"). This time though, a new design emerged: the gas core nuclear rocket. Gas core rockets use a gaseous nuclear fuel instead of the solid graphite core used in NERVA. A gaseous fuel could attain tempertures of several tens of thousands of degrees which would provide an Isp of 3000 to 5000 seconds. A gas core engine would allow manned missions to Mars to be accomplished in half the time that is being considered now. Originally, nuclear rockets were going to move and power large SDI space-based lasers. When it was decided that conventional means would work just as well, nuclear rockets were largely dropped from SDI. Only the small Timberwolf Project continued looking into their uses.

Today, research into nuclear rockets proceeds at a slow but steady pace. Most of the work is done at Los Alamos, where nuclear rockets originated. Most of the problems that the scientists are trying to solve now are how to build a nozzle and combustion chamber that can withstand the high tmeperatures of a gas core rocket, and how to contain the uranium plasma that forms. If viable nuclear rockets are ever developed, they would have the possibility of opening the solar system to manned space flight. A trip to Mars would easily be within our grasp. We will have to wait and see.

Ion Rockets

The ion rocket does away with the hot, fiery blast of chemical and nuclear rocket engines. Instead, it uses a small electrostatic grid to accelerate ions to very high exhaust velocities. By doing so, it can achieve an Isp of thousands of seconds. Below is a diagram of an ion engine.

To operate the engine, xenon gas is supplied into the discharge chamber where the xenon atoms are ionized by bombardment with electrons emitted from the main hollow cathode. The resulting ions are accelerated electrostatically with an ion accelerating grid system to produce thrust. Magnetic rings on the walls of the discharge chamber act to contain the ions as they are accelerated. The ion accelerating system consists of two grids with many small holes. A positive voltage, usually 1000 V, is applied to the inner grid (the screen grid) and a negative voltage to the outer grid (the accelerator grid). The neutralizer emits electrons into the ion exhaust to prevent the spacecraft from aquiring an electric charge. This allows very high exhaust velocities and specific impulses. Unfortunately, the amount of thrust that an ion engine produces is very small. The engine exerts about as much force as a sheet of paper does. However, because ion engines have a high specific impulse they can thrust for very long periods and slowly build up substantial velocity.

The theory behind ion engines was first developed by Dr. Wernher von Braun in the 1930s. However, because the German Army was more concerned with weapons of war than interplanetary rockets, he was not able to test any of his ideas. After World War II, when von Braun and hundreds of other German rocket scientists were brought to the US, he was able to begin making headway with his ideas. By 1958, the Army Ballistic Missile Agency was sufficently interested by ion propulsion that they intitiated a contract with Electro-Optical Systems to study ion drives. This was followed by more contracts with different companies that culminated in the test firing of a 0.1 pound-thrust engine by Hughes Research Laboratory. Shortly thereafter, the needs of the Apollo program took away the people involved in ion rockets, and development quietly stopped.

In the early 1990s, interest in ion engines slowly revived. JPL and NASA/Lewis teamed up for the NASA Solar Electric Power Technology Applications Readiness (NSTAR) project. This project looked at an ion engine using xenon as a propellent. In 1996, such an engine was built and fired for over 8,000 hours before the test was complete. Building of this success, the Deep Space 1 mission was the first space probe to use an ion thruster. After a bumpy start, the ion engine performed perfectly, bringing DS1 to within 16 miles of the asteroid Braille. Currently (August 1999), the world's only ion propelled space probe is headed out to fly by two comets in 2001.

Solar Sails

All of the schemes to move around the solar system that we've looked at use hot gas or ions from onboard fuel tanks to propell themselves. Solar sails, however, do away with onboard fuel and the mass that goes along with it. Instead, solar sails use the Sun's light to move from one orbit to another. Odd as it may seem, sunlight actually exerts a small amount of pressure when it bounces off a reflective surface. This was first demonstrated by James Clerk Maxwell in 1873. The principle was  used in 1974 when Mariner 10 swung by Mercury. For a short time before the encounter, mission controllers used Mariner's solar arrays to reflect the Sun's light and rotate the spacecraft. The pressure, though, was vanishingly small. This is the only drawback to solar sails. They need to be huge to get just a little acceleration. For example, the light on a football field-sized solar sail near Earth would only push with as much force as a pebble. By making the sail bigger, you can increase the pressure on it. Alternatively, a laser could be fired at the sail to push it along. The laser would have to have a tremendous power output: around 2 gigawatts to move a moderately sized sail. This has led to some far-flung ideas using 1,000 km sails and lasers with thousands of gigawatts of power to send probes to the stars. Here are some images of proposed solar sails.

More realistically, several schemes have surfaced that use 100 meter diameter sails to conduct interesting missions. GEOSTORM proposes using a solar sail to hold a probe between the Earth and the Sun. GEOSTORM could provide advance warning of solar flares that could be dangerous to orbiting satellites. Another possible mission for solar sail powered craft would be in geosynchronous orbit. Geosynchronous orbit is the orbit 35,000 km directly above the equator. At that altitude a satellite goes around one orbit in the same amount of time it takes the Earth to make one revolution. This means that a satellite will stay over one spot on the equator at all times. This is very useful for communications satellites, and practically all communication satellites are in geosynchronous orbit. Each satellite is assigned a "slot" in the orbit. Recently, it has been getting very crowded up there and the price per slot is rising; up to about $250 million. By using a solar sail, a satellite could be pulled 2-3 degrees off the equator, but still have the same orbital period. The satellite would still be able to maintain a stationary position in relation to the ground, but it would be able to avoid the expensive natural geosynchronous zone. This is possible because the solar sail would be continually pushing the satellite up into an "artificial orbit" that would not be possible otherwise.

A third mission for solar sails would be to capture small asteroids. A solar sail would fly to a candidate asteroid and latch on. Slowly the asteroid's orbit would be adjusted so that it could be captured by the Earth. Once in Earth orbit, the asteroid could be mined for its materials. Certain types of asteroids could have water in them, which could then be broken down into rocket fuel. An asteroid would also provide large amounts of iron, silicates, and rare metals like platinum. Looking further into the future, solar sails could be used to move cargo to Mars bases cheaply, or to send probes far into interstellar space. All riding on winds of light.

Author: Thomas Beatty

Curator: Al Globus
If you find any errors on this page contact Al Globus.
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