Definition: Any means for launching or propelling spacecraft beyond the use of traditional chemical rocket engines.
Significance: If humanity is ever to explore the solar system and access resources beyond Earth, cheaper, faster, and more efficient propulsion systems must be developed.
The Fundamentals
If the space shuttle’s external fuel tank were placed on the pedestal of the Statue of Liberty, it would stand just taller than Lady Liberty’s torch. At launch, the mass of the space shuttle is about 2,040 metric tons (4.5 million pounds), but it can deliver only 6.5 percent of that mass to low-Earth orbit, and that costs $20,000 per kilogram ($9,100 per pound). For comparison, in 2001, gold sold for about $9,000 per kilogram. To achieve a stable low-Earth orbit, the payload must simultaneously be lifted about 300 kilometers above Earth’s surface and accelerated to a horizontal speed of nearly 8 kilometers per second (about Mach 23, or twenty-three times the speed of sound).
At 111 meters (364 feet), the Saturn V rocket that took the Apollo astronauts to the Moon stood over twice as tall as the space shuttle. In ascending to low-Earth orbit, the Saturn’s first three stages burned for a total of 11.5 minutes, using 75 terajoules (75 x 1012 joules) of chemical energy. That was about 1.5 percent of all of the energy in the world produced from fossil fuel during those 11.5 minutes. Only 6 percent of that energy went into lifting and accelerating the Apollo payload into orbit, while most of the remaining 94 percent was expended on lifting and accelerating the fuel used on the way up.
There are many plans for more efficient spacecraft. The Venture Star project featured the more efficient aerospike rocket engine. Solar sails, plasma bubbles, gravitational slingshots, the Pegasus spacecraft, and laser- or microwave-launched spacecraft are schemes that leave all or part of the main power source behind and thereby reduce the spacecraft’s mass. Scramjets gather part of their fuel, the oxidizer, in flight, and the hypothetical Bussard interstellar ramjet would gather fusion fuel in flight. Ion drives, plasma drives, and drives using nuclear fission and fusion are all schemes to increase the exhaust velocity of the propellant.
Aerospike Engines, Scramjets, and Pegasus
Worldwide, there are a number of projects under way to develop a fully reusable launch vehicle that will orbit pay-loads for one-tenth the cost of the space shuttle. The X-33 “Venture Star” is a sleek, wedge-shaped craft designed to take off vertically like a rocket and glide to a landing like an airplane. It pioneered the use of lightweight graphite composites in its structure and fuel tanks, and its efficient lifting body shape allowed it to fly with only stubby wings for stabilizers. Although many important technological advances were achieved, development problems led to the withdrawal of support by the National Aeronautics and Space Administration (NASA) in March, 2001, but work continued on the X-33′s Boeing Rocketdyne XRS-2200 aerospike engines.
A conventional rocket engine has a combustion chamber that opens into a bell-shaped nozzle. Fuel and oxidizer are mixed and burned in this chamber, and high-speed combustion products escape through the nozzle. For greater efficiency, the pressure of the exhaust plume should match the surrounding air pressure. The aerospike nozzle is V-shaped and is turned inside out: Fuel and oxidizer are mixed in ten combustion chambers (five on each side of the V), and the exhaust plume sprays down the outside of the V Since the outside of the exhaust plume is open to the atmosphere, it automatically blooms outward until it matches the ambient pressure, while the inside of the plume pushes against the V and provides thrust.
A scramjet is a supersonic combustion ramjet. Scramjets can be more efficient than rockets because they use oxygen from the air and must carry only the oxygen that they will use in space. A scramjet engine has no moving parts; it uses its supersonic speed (about Mach 10) and internal shape to compress air coming into its engine instead of using the rotating compressor of a normal jet engine. Hydrogen fuel is injected into the airstream in the engine, and the hot combustion gases (mostly water vapor) escape from the rear of the engine to provide thrust. A scramjet must be launched at supersonic speed before it can fly. In June, 2001, a B-52 aircraft lifted an X-43A scramjet mounted on a Pegasus-based rocket 6,000 meters (20,000 feet) into the atmosphere and launched the combination. The rocket was to accelerate to the scramjet’s cruising speed and then release it. Unfortunately, a structural failure occurred shortly after rocket ignition and the mission was terminated.
Several nations are working on scramjets. In August, 2001, India announced plans to develop the Avatar, a 25-metric-ton craft believed to be able to cheaply carry a 1-metric-ton payload into a 100-kilometer-high orbit. The Avatar will take off and land like a conventional airliner using a combination of turbofan, ramjet, and scramjet engines fueled with hydrogen. A unique feature is that it is to cruise at Mach 8 for an hour at an altitude of 10 kilometers while it takes in and liquefies 21 metric tons of oxygen before it uses a hydrogen-and-oxygen-fueled rocket to push into space.
The Pegasus rocket has placed dozens of satellites into orbit and is the most successful small commercial launch vehicle in the world. The”Stargazer” Lockheed L-1101 aircraft carries Pegasus to a launch point 12 kilometers high, above the densest and most turbulent part of the atmosphere. The three-stage rocket is then released and ignited. It can carry a 450-kilogram payload into low-Earth orbit.
Solar Sails and Plasma Bubbles
The surface of the Sun is a fearsome place—a seething, turbulent ocean of blinding incandescent gases, incessantly rocked by sonic booms as gigantic gouts of matter race upward through the photosphere. The flood of energy from the Sun tears particles from its outermost part, the solar corona, and constantly drives this sun-stuff into space. This is the solar wind: electrons along with ionized hydrogen and helium atoms streaming outward at an average speed of 400 kilometers per second.
Earth’s magnetosphere is the region surrounding the planet that is dominated by its own magnetic field, not the Sun’s. Geophysicist Robert Winglee of the University of Washington realized that the solar wind pushing against Earth’s magnetosphere pushes Earth away from the Sun, except that Earth is far to heavy for this to produce any measurable effect. However, Winglee proposed that if a light spacecraft could generate a large magnetic field, the solar wind would propel the spacecraft. He calls this hypothesis Mini-Magnetospheric Plasma Propulsion, or M2P2.
Winglee and his colleagues suggest that a 200-kilogram spacecraft (including 50 kilograms of helium) might be built around an electromagnet coil powered by solar cells. Winglee’s group demonstrated that injecting ionized helium into a coil’s magnetic field forces the field to expand like a bubble, becoming a mini-magnetosphere. They calculate that in space this magnetosphere would be 15 to 20 kilometers in radius, and with the solar wind pushing on it, the craft should reach speeds of 50 to 80 kilometers per second after three months. This is ten times faster than the speeds previously reached by chemical rockets. Such a craft could reach Saturn in six months instead of the seven years required for the Cassini mission.
The mini-magnetosphere is not really spherical. Its shape depends upon its interaction with the solar wind and on the parameters of the coil. To oversimplify, a mini-magnetosphere may be pictured as a flat sheet of paper orbiting the Sun. If the sheet is tilted so that its leading edge is closer to the Sun than its trailing edge, solar wind particles bouncing off of it will push it forward in its orbit and make it go faster and spiral outward from the Sun. Conversely, if the trailing edge is closer to the Sun, solar wind particles will slow it in its orbit, and the Sun’s gravity will pull it inward. Since the magnetosphere is practically without mass, it should be easy to maneuver it by simply rotating the field-generating coil.
The concept of propelling a craft with solar sails is similar to M2P2, but these sails are propelled by sunlight, not the solar wind. At the orbit of Earth, the pressure of sunlight is about 9 newtons (2 pounds) per square kilometer. An 820-meter square-rigged sail, named “the clipper” by its designers, is expected to have a mass of about 2,000 kilograms. Carrying a 2,000-kilogram payload, it could travel from Earth to Mars or to the outer planets in about the same time, or less, than a chemical rocket would require. Once solar sail technology is achieved, its use would be cheaper than the use of chemical rockets because it requires much less mass to be lofted into Earth orbit for each mission. Solar sails are also reusable. They can be returned to Earth orbit, but their sunward speed is limited by the relatively weak pull of solar gravity. Energy Science Laboratories of San Diego, California, has developed a novel sail fabric, a very porous mesh of carbon fibers. They have demonstrated that the fabric is light enough to be pushed by laser light, and that it can withstand temperatures of 2,500 degrees Celsius. This is important because, someday, solar sails may be given a push by shining a battery of high-intensity lasers on them.
Electric Propulsion
The thrust produced by a rocket depends upon how much reaction mass is ejected per second, and how fast it is ejected. Chemical rockets can deliver a large amount of thrust because they can push out a great deal of mass per second, but ejection speed is limited by the amount of energy released by the chemical reaction of fuel and oxidizer. Electric propulsion engines typically deliver a small thrust with high efficiency since they can handle only a small amount of mass per second, but they can eject it at very high velocities. With a few exceptions, electric propulsion has been commonly employed only in the thrusters used by satellites for station keeping (staying where they are supposed to be).
Resistojets use electric resistance to heat propellent gases and thereby increase their ejection speed. They have operated with ammonia, biowastes, hydrazine, and hydrogen. Arcjets ignite an electric arc in the propellant flowing through a rocket nozzle. Arcjets are twice as fuel efficient as chemical thrusters, but ion engines are more efficient yet. The 480-kilogram spacecraft Deep Space 1 (DS-1) is propelled by an ion engine and powered by 2,400 watts from solar arrays. Launched on October 24,1998, its mission was to test twelve new technologies, including the ion drive and a relatively autonomous navigation system. While DS-1 came within 26 kilometers of asteroid Braille on July 28, 1999, problems kept it from obtaining any closeup images. Its extended mission was to fly through the coma (head) of comet Borrelly on September 22,2001, when it sampled the materials of the coma and photographed the comet’s nucleus.
DS-1′s ion engine uses xenon, a gas 4.5 times heavier than air, for a propellent. Xenon in the engine chamber is bombarded by electrons that ionize the xenon. The rear of the chamber is fitted with two wire mesh screens; the first is positively charged, while the second is negatively charged with up to 1,280 volts. Positive xenon atoms passing through the first screen are accelerated to 28 kilometers per second by the voltage on the second screen. The ejection of these ions into space propels the craft forward. Electrons sprayed into the exhaust stream keep the craft from building up a static charge. Although the engine exerts no more force than the weight of a sheet or two of paper, its 82 kilograms of xenon is enough for 6,000 hours of operation and can increase DS-1′s speed by 4 kilometers per second. It is ten times more efficient than a chemical engine with the same weight of fuel.
Nuclear Power
The great attraction for using nuclear power in space is that nuclear reactions pack millions of times more energy than chemical reactions. While the United States placed a single nuclear reactor in space in 1965, the former Soviet Union has used small nuclear reactors to provide electrical power on dozens of satellites. Both nations have used radioisotope thermoelectric generators (RTGs) that convert the heat from radioactive decay directly into electricity, but neither nation has used nuclear power for propulsion. Since they have no moving parts and are well constructed, RTGs are considered to be relatively safe, but they are not very efficient. However, using electricity from an RTG to power an ion engine in the regions beyond Mars, where solar power is weak, is an attractive possibility.
The Nuclear Engine for Rocket Vehicle Application (NERVA) was almost ready for flight testing when the project was canceled in 1972. Under development for a manned mission to Mars, the NERVA engine heated hydrogen by passing it through the reactor core and then expelled it from a rocket nozzle. Uranium carbide fuel elements were coated with carbon and niobium to protect them from corrosion by the hydrogen propellant. The Mars craft would be assembled in Earth orbit and, using nuclear engines, it could travel to Mars, stay for two months, and return to Earth in the space of about one year. A program to develop a nuclear engine code-named Timber wind began in the 1980′s and continues under the Space Nuclear Thermal Propulsion (SNTP) Program. Fluidized bed reactors and other advanced reactors that can operate at higher temperatures are being studied since they should be more efficient than the NERVA engine.
The most audacious nuclear engine is the nuclear pulse rocket that was the basis of the ORION project, which ended in 1965. The mass of the ORION vehicle was a grandiose 585 metric tons. The rear of the vehicle was connected by shock absorbers to a massive pusher plate. Every few seconds, a small fission bomb with a ten-ton yield was to be dropped out the back end and exploded about 100 meters behind ORION, so that the blast wave would drive ORION forward. About 2,000 bombs would be required for a 250-day round trip to Mars. To prove the concept, a small prototype was successfully launched from the ground with tiny chemical bombs, but international treaties now prohibit nuclear explosions in space, and therefore the ORION project is unlikely to be revived.
None of the proposed nuclear engines are very efficient at converting nuclear energy into a means of propulsion, but they are still attractive because of the large amount of energy in nuclear fuel. If the rare artificial element americium-242m could be produced in significant quantities, a much more efficient engine might be constructed. The key is that a thin film of americium-242m can sustain a chain reaction. High-energy fission fragments escape from a thin film and can be directed by magnets out the rear of the craft to provide propulsion. A spacecraft with such an engine might travel to Mars in two weeks instead of the eight to ten months required by chemical rockets.
Tethers and Bolos
Tethers up to 20 kilometers long have already been tested in space. A tether is a cable that can be unreeled from an orbiting craft such as the space shuttle. A mass on the far end of the tether will help keep it stable. The tether may be deployed upward by letting centrifugal force carry it farther from Earth, or it may be deployed downward by letting Earth’s gravity carry it down. If the tether includes a conducting cable, it can be used to convert a satellite’s momentum into electrical energy, since a conductor moving in Earth’s magnetic field will act like a generator. To keep a current flowing in the cable, electron guns will expel electrons into space and prevent the buildup of a static charge. If used long enough, this system will bring down a satellite from low-Earth orbit, and thereby save the roughly 20 percent of rocket fuel that is reserved to de-orbit spent satellites. If solar cells are used to produce a current in the tether, the generator becomes a motor, and the spacecraft’s orbit will be raised. Because of the air resistance that exists in low-Earth orbit, the International Space Station needs a boost from time to time. If it were boosted with tethers powered by the station’s solar panels, up to two billion dollars in fuel costs might be saved over ten years.
Abolo consists of two masses connected by a tether and set spinning. The end masses are equipped with grapples and thrusters to adjust position. Long tethers will probably be Hoytethers, a loosely woven Kevlar web. Their open structure makes Hoytethers less likely to be severed by meteoroids. If a bolo station (at the center of mass of the bolo) is in low-Earth orbit and therefore has a speed of 7.7 kilometers per second and the rotating tether’s tip speed is 2.4 kilometers per second with respect to the station, the bolo’s rotation direction is such that the tip speed subtracts from the orbital speed for the tip closest to Earth. A spacecraft launched from Earth need only be traveling at 5.3 kilometers per second when it rendezvouses with, and is seized by, the lower grapple. If the bolo is much more massive than the spacecraft, the spacecraft will be lifted and accelerated by the tether so that the spacecraft is traveling at 10.1 kilometers per second when the tip is farthest from Earth.
At the appropriate time, the bolo will release the spacecraft to travel to its next destination, perhaps a second bolo in geosynchronous orbit, which in turn might pass it on to a bolo in lunar orbit, which might set it on the Moon. The great efficiency of such a system is that it minimizes the fuel that must be lifted and accelerated from Earth. However, the bolos will slow down or fall into lower orbits as they give energy to the spacecraft. The bolo in low-Earth orbit could be boosted by using solar panels and a conducting tether. Other bolos might be boosted with solar-powered ion engines. Only steering energy would be required if the amount of mass going from the Moon to low-Earth orbit were the same as that going from low-Earth orbit to the Moon. (The falling mass would provide the energy to lift the rising mass.) A nearly constant flow of traffic would be required to make a bolo system cost-effective.
