Just as the development of aviation has been powered by advances in engine technology, spaceflight needs breakthroughs in propulsion systems if safe, efficient and routine space travel is to become anything other than a science fiction dream
As the newly appointed captain of the starship Enterprise, actor Patrick Stewart was keen to grasp the fundamental principles behind its famous warp drive. But the producers of the science fiction epic Star Trek, having explained the hypothetical concept, admitted no-one had any real idea how to make a starship go faster than the speed of light. Stewart replied: "Nonsense, all you have to do is say 'engage'."
"Star Trek is great as an inspirational tool, but at the same time it can be a handicap," says Marc Millis, head of the Breakthrough Propulsion Physics group at NASA's Glenn Research Center (GRC) in Cleveland, Ohio. Millis, who admits to having a model Star Trek shuttlecraft in his office, as well as an Enterprise technical manual at hand for inspiration, says science fiction can be a double-edged sword. "Sci-fi like Star Trek provides a visible icon, but it also raises expectations that these things are easily achievable, as well as narrowing people's focus on how to solve the problem."
As 2001 dawns, the problem of advancing space travel is as dependent on improvements in propulsion technology as air travel was throughout the 20th century. Propulsion continues to be the limiting factor in all aspects of spaceflight, be it access to Earth orbit, trips to the moon or other planets in the Solar System, or the far-off dream of deep space exploration and interstellar travel.
While aviation received a major boost with the development of the jet engine, spaceflight has yet to progress much beyond the rocket. Although today's rocket motors are more efficient than the pioneering engines of the 1930s, a breakthrough is needed if the cost of space access is to be reduced dramatically.
The state-of-the-art for the advanced propulsion concepts now under study is proportional to the distances for which they are designed. Therefore, propulsion techniques for leaving the Earth's atmosphere and for travel within the Solar System are better understood, or at least further along, than those being considered for interstellar travel.
Technologies being studied in the USA and elsewhere cover the full range from advanced chemical-based rockets, through nuclear propulsion to a wide variety of electromagnetic concepts. Beyond these, tentative research is beginning into futuristic fusion and antimatter propulsion concepts, gravity modification and even faster-than-light travel.
Combined cycles
One of the most promising chemical-based concepts on the near horizon is the rocket-based combined cycle (RBCC) engine, which mates a conventional rocket with other propulsion methods including air-breathing ramjets and scramjets (supersonic ramjets). "The idea of an air-breathing combined cycle has a lot of merit, mostly because it means not having to carry your own oxygen," says acting deputy GRC director Gerald Barna. "It seems an attractive option for the future because the technology is achievable, but it will take a substantial investment."
The RBCC has four operating modes. First the rocket fires, accelerating the vehicle while drawing air into the engine and compressing it. Fuel is fed into the compressed air, which burns like the afterburner of a jet engine. As velocity increases beyond Mach 2, the rocket is turned off, and ram pressure compresses the air, which continues to be mixed with fuel and burned in the exhaust. Around M5, fuel injection is moved forward to give more time for the air and fuel to mix as the RBCC moves to scramjet mode. By now, the vehicle is well into the upper atmosphere and, with oxygen depleting, the rocket is re-ignited to continue the climb to orbit.
Related research includes studies of the deeply cooled air rocket/ramjet engine (DCARE), in which performance is boosted by supercooling the incoming air. Work by Montana-based company MSE is expected to show several benefits, one being the burning of excess hydrogen in a ramjet engine mounted in combination with the rocket.
A related concept, the liquid air cycle engine (LACE), is the focus of studies in Japan. In this design liquid hydrogen is used to liquefy the air, which can then be used in the rocket engine. Studies show only about 20% of the required oxidiser can be produced this way but, as oxygen is six times as heavy as hydrogen, this is still a substantial saving and could be enough to drive the ejector rockets in an RBCC.
Although LACE has potential, there are drawbacks, chief among them the fact that oxygen must still be carried to operate the rockets when the vehicle leaves the atmosphere. Also the weight of the heat exchanger and de-icing systems required to liquefy the air could offset any oxidiser savings.
Another chemical-based rocket concept, being explored by the US Air Force and NASA, is the pulse-detonation rocket engine (PDRE). Basically, the PDRE is a straight pipe, closed at one end. A pulse of fuel and oxidiser is injected into the tube and detonated, and the expanding gas exits the open end, providing thrust. Unlike a normal liquid rocket, in which propellant is fed into the combustion chamber at high pressure, PDRE fuel is injected at low pressure. This means pumps can be lighter and cheaper. Another big difference is that, in the PDRE, a detonation wave travelling at 10 times the speed of sound completes the combustion before the gas has time to expand. This avoids losses and releases about 10% more energy from the fuel for thrust.
Rocketry's long heritage means no stone remains unturned in the search for better performance. Work is under way to derive more energy from conventional and exotic chemical fuels. These include atoms of carbon or boron, which when allowed to recombine into molecules release 10 times as much energy as can be obtained from combustion.
The initial breakthrough in stabilising tiny amounts of atoms by freezing them in solid hydrogen 'snow' was made by the US Air Force Research Laboratory (AFRL). GRC is now looking at ways of making larger quantities by using a liquid helium carrier for the solid hydrogen snow. NASA estimates the recombination energy of these atoms could produce a specific impulse (ISP) - the measure of rocket efficiency - of more than 550s, compared to 450s for the Space Shuttle main engines.
Stabilising such materials is the most difficult part. In the early 1990s, the US Department of Energy's Lawrence Livermore National Laboratories (LLNL) produced an exotic fuel from metallic hydrogen which generated a phenomenal ISP of more than 1,100s. The problem was it lasted only 50 nanoseconds.
More modest increases possible with advanced fuels called 'strained ring hydrocarbons' are being studied at NASA's Marshall Space Flight Center (MSFC) after development at AFRL. They hold the promise of a 10% increase in ISP.
Old ideas never die, and another series of studies, undertaken by the LLNL, University of Texas and US Army, is focused on a gun launch concept. Building on earlier work, the gun launch idea bridges the gap between chemical and electromagnetic propulsion concepts. Advocates say the low-cost appeal of the system is compelling because all the launch hardware remains on the ground. However, the required 8km/s orbital velocity is "just barely" achievable, according to NASA.
Concepts under study include a multi-stage light gas gun, a blast wave accelerator and a coil gun - all of which would probably be buried deep in a mountain. The chief problem, apart from reaching the required velocity, is the limited range of payloads that could survive launch accelerations of 30,000-50,000g. In addition, some way of circularising the orbit must be developed, to avoid re-entry.
Electromagnetic focus
Studies of electromagnetic propulsion concepts, meanwhile, continue to focus on magnetic levitation (maglev)-assisted, RBCC-powered launch vehicles. In a scene reminiscent of the sci-fi classic When World's Collide, the vehicle would hurtle along a maglev track on a sled until launching at a speed of around 650km/h. Such a ground-assisted launch could reduce vehicle size by 20% for the same payload, NASA estimates. Small test tracks have been built at MSFC and LLNL, and NASA plans a demonstration track at Kennedy Space Center following continued research.
Russian research into magnetohydrodynamics (MHD), aimed at a hypersonic aircraft concept called Ajax, has stimulated US studies of similar electromagnetic propulsion concepts at NASA's Ames Research Center and AFRL.
The MHD concept is based on the idea that high-velocity gases flowing through a strong magnetic field will be diverted in a direction perpendicular to the field. Electrons and negative ions are diverted in one direction, positive ions in the opposite direction. Electrodes placed to collect these charged particles would generate electricity while decreasing fluid velocity. This 'MHD generator' and a flow ioniser in the inlet would slow incoming gases to ideal ramjet conditions, while the electrical power generated would be sent to an 'MHD accelerator' in the exhaust to increase thrust. The concept would also be used to reduce drag and create a virtual inlet to double airflow into the ramjet.
A far more visible area of research, literally, is the Laser Lightcraft beamed-energy propulsion work under way at the US Army's White Sands test range in New Mexico. Under study by the Rensselaer Polytechnic Institute in New York, AFRL and MSFC, the concept is based on the idea that a pulse from a ground-based laser, tightly focused on a vehicle reaction chamber where it will detonate fuel, will be enough to sustain the vehicle.
In recent flight tests, a 30g spin-stabilised prototype was boosted to a height of 38m (115ft) using a 10.6 um CO2 laser putting out 10kW (13hp) of power at 20 pulses per second. This has led to estimates that 1MW of laser power will be required for each kilogram of payload pushed into orbit.
A similar concept, using microwave instead of laser energy, is being explored for propelling a lenticular-shaped helium balloon into space. As microwave energy is too weak to push an object into orbit, it will have to be either converted into electrical power or focused to detonate fuel in a similar way to the Laser Lightcraft. Dubbed the Microwave Lightcraft, the electrically powered concepts under study include an ion wind system or an MHD accelerator with a focused microwave airspike for ionisation.
Electric engines
Microwave power is just the starting point when it comes to concepts for interplanetary and deep space propulsion. Some of the more promising ideas for high-power electric engines for journeys beyond Mars are being investigated by NASA's Jet Propulsion Laboratory (JPL), Princeton University and GRC. They include a magnetoplasmadynamic (MPD) thruster that is theoretically capable of providing ISPs beyond 5,000s. The MPD concept is based on the creation of an intense electric arc between coaxial electrodes. The resulting high magnetic field thrusts a small amount of ionised propellant out of the open end of the engine.
Another high-power electric engine being studied by NASA's Johnson Space Center (JSC), is a plasma rocket, in which microwave energy is used to heat plasma in a magnetic bottle with mirrors at each end. The most energetic ions close to the bottle's axis 'leak' through the mirrors and produce thrust with an ISP exceeding 5,000s. Unlike the MPD thruster, which depends on electrodes that wear out, the plasma rocket needs none. But current magnet technology makes the concept very heavy.
Lighter electromagnetic propulsion concepts include giant solar sails made of carbonfibre fabric only 2mm thick. JPL is studying missions in which photon pressure from the Sun, or a laser, on a large area of this ultra-lightweight film would produce a small, but significant, acceleration. Just like a boat sail, thrust could be varied by altering the angle of the film to the light source.
Re-entering the realm of 2001: A Space Odyssey, focus is once more turning to nuclear propulsion concepts. Clarke and Kubrick's Discovery spacecraft heads towards Jupiter powered by nuclear rockets not unlike those now being revisited by NASA in its search for propulsion concepts for a manned Mars mission. While safety and cost concerns continue to militate against nuclear engine concepts, the technical arguments remain in their favour. Two main themes are being explored: nuclear thermal rockets (NTR) and nuclear electric propulsion (NEP).
In an NTR, a gas, say hydrogen, flows through the reactor core and generates high thrust. A hydrogen-fuelled NTR concept being studied by the USA's Los Alamos National Laboratory could provide an ISP of 3,000s or more, and take a six-person crew to Mars in three months - faster if some fuel was pre-positioned in Mars orbit. Unlike solid core nuclear reactors, which are limited by the melting point of nuclear fuel, the NTR develops a vortex of hydrogen gas to shield chamber walls from temperatures up to 55,000°C (99,000°F).
NEP concepts, which are similar to nuclear submarine powerplants in that the reactor is used to power an electric propulsion system, are suitable for slower deep space missions. The USA's Sandia National Laboratories is exploring a nuclear-driven ion thruster that could deliver a 1,000kg (2,200lb) science payload to Saturn in four years, Neptune in nine or the Kuiper asteroid belt in 13. The NEP fission-powered craft would rely on a small reactor to power a 100kW electric propulsion system.
Nuclear propulsion research is drawing heavily on a range of studies conducted in the 1960s, including a proof-of-concept nuclear-powered ramjet called Pluto (which used a heat exchanger developed by the Coors brewery in Colorado). Another 1960s project of interest is Orion, which generated thrust by exploding small nuclear bombs behind a blast shield. Orion, which fell victim to the Nuclear Test Ban treaty, is being revisited in the form of concepts like Medusa, which uses a sail to capture the nuclear blast wind.
Fusion - the energy which powers the Sun - is also being studied for space propulsion, though it remains a far-off possibility. "Fusion propulsion promises an ISP of over 100,000s, enabling human missions anywhere in the solar system at almost any time. To follow our dreams we must pursue this," says John Cole, space transportation research manager at MSFC.
Working with the Department of Energy, (which has identified around six potential propulsion candidates from current fusion concepts), NASA is monitoring research at Princeton University, where the National Spherical Torus Experiment is attempting to demonstrate breakeven fusion.
One space propulsion concept being considered is similar to the Orion project. Small magnetised targets containing fusionable material are ejected out of the back of the spacecraft. These are compressed with lasers or particle beams until fusion occurs, when the target detonates providing pressure on a blast shield and producing thrust.
Engage warp drive
While fusion propulsion promises to bring the rest of the Solar System within reach, the vast gulf of interstellar space still remains out of bounds to all but science fiction authors. Recognising the need for revolutionary thinking, NASA established the Breakthrough Propulsion Physics (BPP) programme in 1996. Its threefold charter is to: discover new propulsion methods that eliminate or dramatically reduce the need for propellant; find how to attain the ultimate achievable transit speeds to reduce dramatically deep space travel time; and discover fundamentally new on-board energy production methods for propulsion systems.
"It really is like taking a trip to the Monolith," says Millis, referring to the mysterious object in 2001 that imparted knowledge to man's early ancestors then stood watch over his progress towards the stars.
Breakthroughs are needed to combat the three critical challenges facing would-be starship designers. Most obvious is the need for speed. The nearest neighbouring star (Alpha Centauri) is 4.3 light years away. The Apollo spacecraft, which took three days to reach the Moon, would take over 900,000 years to get there. Even Voyager, which reached 59,600km/h (37,000mph) as it left the Solar System, would take 80,000 years.
The second challenge is mass. Rockets use too much propellant to make such a journey, and to send a Shuttle-sized payload to the nearest star using a nuclear fission rocket would require roughly 100 million supertanker-sized propellant tanks. Even a fusion rocket would require 1,000 such tanks.
The third challenge is energy. Even if there was a drive that could convert energy directly into motion without propellant, it would still need a lot of energy. A Shuttle-sized craft on one-way trip to Alpha Centauri at subrelativistic speed, would require over 7x10-19 Joules of energy, or about the same as the Shuttle's engines would use if they ran continuously for 50 years. According to Millis, this final problem can only be resolved by a breakthrough "where we can take advantage of the energy in the space vacuum, a breakthrough in energy production physics, or a breakthrough where the laws of kinetic energy don't apply."
Hitch-hiker's guide
Now using Star Trek as a sort of hitch-hiker's guide to astrophysics, Millis paints a picture of where the BPP programme is heading. "We don't know if these goals are achievable at this stage, but we are taking the small steps and selecting a suite of different approaches touching a variety of issues." Some of these address "warp drive", or speed; some "matter-antimatter", or energy and mass.
"Warp speed is faster-than-light travel, and it could be like using a moving sidewalk at an airport. Imagine there was a 2mph speed limit, and you jumped on the sidewalk instead. You'd move faster but, theoretically you would not be breaking the speed limit. Instead, you'd be in a bubble of space and time moving relatively faster than people walking alongside, but you'd still not be breaking the limit."
Although Einstein's Special Relativity forbids objects to move faster than light within space-time, it is known that space-time itself can be warped and distorted, as in a wormhole. To create this effect, scientists believe a ring of negative energy would need to be wrapped around the ship. Unfortunately classical physics says negative energy cannot exist, but quantum physics leans to a "maybe".
When it comes to antimatter, the magic fuel for Enterprise's warp engines, Millis is equally open. "Antimatter is real stuff, not just science fiction. It is matter with its electrical charge reversed. Positrons, antiprotons and other antiparticles can be routinely created at particle accelerator labs, such as CERN in Europe, and can even be trapped and stored for days or weeks at a time." The problem is largely one of cost. Millis estimates it would today cost around $100 billion to create one milligram of antimatter. That would be enough for large-scale applications, but the price would have to drop by a factor of 10,000 before it could be commercially viable, says Millis.
Five tasks are under way through the BPP. One experiment is testing whether the transient inertia effect, in which mass could theoretically be "coupled" to the surrounding space, is a genuine physical effect. The transient inertia effect is one theoretical interpretation of Mach's Principle, which states that inertial reaction forces result from gravitational attraction of all matter in the universe to an accelerating object.
If, as has been suggested, inertial mass varies, a spacecraft with acceleration in phase with these variations might be subjected to a net unidirectional force relative to the surrounding mass of the universe. Such a spacecraft would not require propellant, and would essentially hitch a free ride on waves rolling through the fabric of space-time.
Another experiment is focused on the existence of, magnitude of, and ability to interact with quantum vacuum energy. Sometimes called zero point energy, this is the random electromagnetic oscillations left in a vacuum after all other energy has been removed. Evidence for vacuum energy comes from the Casimir Effect, in which two metal plates close enough together to prevent light waves fitting between them are pushed together by light pressure on the outer sides of the plates.
A third experiment is trying to determine if electromagnetism can dynamically couple to space, time and gravity. If so, the creation of an imbalance, or asymmetry, could lead to potential propulsive devices. A fourth is focused on tests to verify, or refute, claimed 'gravity manipulation' effects seen by Finnish and Russian researchers using magnetised superconductors. Lastly, an experiment is under way to see if "quantum tunnelling", the apparent observation of photons travelling through a vacuum faster than light, is a real phenomenon. "It might not give a propulsive effect, but it could provide the keys to the door," says Millis. Or perhaps, he says, the keys to the engine of a future starship.
Source: Flight International
