Hypersonic Scramjets, Spaceplanes, and Air Launched Orbitals

The Air Force's desired "High Speed Strike Weapon" would travel at five times the speed of sound or faster, theoretically launching from a stealthy F-22 Raptor jet or a future F-35 Joint Strike Fighter, and travelling so fast and at such long distances as to render an enemy's anti-aircraft systems defunct. The Air Force's Research Laboratory Munitions Directorate is gathering possible design partners later this month at Elgin Air Force Base in Florida before any solicitation. According to an Air Force notice, whatever prototype gets built will ultimately need to strike "time-critical" targets -- on the move, possibly -- from " tactically relevant standoff distances." _Wired

The hypersonic stealth missile would be powered by a scramjet air-breathing engine, traveling at speeds of Mach 5 or greater. While such a speed is not sufficient to achieve Earth orbit, it would allow jet pilots to launch from a considerable distance, in a stealthy manner.

Hypersonic Stealth Rocket
Meanwhile, the US Air Force X-37B space plane is due to land soon, after spending more than a year orbiting the Earth.

The X-37B looks much like NASA's now-retired space shuttles, only much smaller. The space plane is about 29 feet long by 15 feet wide, with a payload bay the size of a pickup truck bed. A solar array packed in the payload bay powers the spacecraft. For comparison, two entire X-37Bs could fit inside the payload bay of a space shuttle.

OTV-2's flight represents a big jump for the X-37B space plane. The vehicle has been aloft for 462 days as of June 8, more than doubling the on-orbit time of the first space-flown X-37B, known as OTV-1. _X37B Due to Land

US Air Force

The X-37B space plane is unmanned, and is launched vertically on a large booster. But the technology for air-launching larger space vehicles and space planes is improving, thanks to Microsoft billionaire Paul Allen, and aerospace engineer Burt Rutan.

The Stratolaunch system would super-size the arrangement used for the SpaceShipOne launches: Scaled Composites has been tapped to build a carrier airplane that weighs more than 1.2 million pounds, with a wingspan of more than 380 feet. That tonnage rivals the weight of the Antonov An-225, which is recognized as the world's heaviest aircraft. Stratolaunch's dual-fuselage plane would be powered by six 747 engines, and would require a 12,000-foot runway for landing.

...The plane would be capable of flying up to 1,300 nautical miles to reach its launch point. SpaceX would provide a shortened version of its Falcon 9 rocket for the next phase of Stratolaunch's route to orbit. Wentz described it as a "Falcon 4 or 5." The multistage booster would be attached to the plane using a mating and integration system developed by Dynetics, and released during the mothership's flight at 30,000 feet. After release, the 490,000-pound rocket would light up to send commercial and government payloads weighing up to 13,500 pounds into low Earth orbit. _Cosmic Log

Stratolaunch
The advantages of an air launch are several, perhaps the largest being the much wider range of orbits and launch windows achievable using much less fuel.

Combining the approaches discussed above, would give one an air-launched stealth plane which used hypersonic scramjet engines to save rocket fuel in the "second stage." The "first stage" would be the airplane that launched the spaceplane. Airplanes are highly economical and reliable, and completely reusable.

Hypersonic scramjets as a second stage should also allow complete recovery and re-usability, depending on configuration and design. Eventually, the first stage ground launched airplanes might incorporate both air breathing turbines and hypersonic scramjet engines, but such designs have a ways to go in testing. Clearly the huge Stratolaunch design could not tolerate such speeds.

Wikipedia: Scramjet and Specific Impulse
The third stage would be the spaceplane itself, which can grow larger as the first stage is safely enlarged. The spaceplane should be completely reusable, and should be able to land almost anytime and anywhere on Earth with a suitable runway.

As for stealth capability of the ground launcher and space plane, that would be a matter of design and mission requirement. If you wanted to secretly send a spaceplane and crew into orbit, the best way to do so would be using stealth air-launched spaceplane technology, unannounced, taking off from a secret runway in the Southern Hemisphere, as far away from observers as possible.

The United States is unlikely to pull something like that off without multiple leaks to the press and interested foreign powers. Russia is unlikely to be able to put all the technology and logistics together without disastrous failure. China might be able to put something similar together within 10 years, but unless China's international projection of power and coordinated activity expands significantly, it is unlikely to be able to do so secretly.

Guide to spaceplanes of the past

More 14 June: The scramjet needs to be traveling at supersonic speeds before it will work, so any second stage of an air launch system which uses scramjets will need to have some way to reach a speed fast enough for the scramjet to burn properly.

NASA: Rocket - Scramjet Hybrid Second Stage (PDF) (highly redacted)

Air Breathing Space Vehicles

Scramspace: An Australian Project

US Air Force Targets Energy Technology Advances in Space

A recent report released by the USAF, Energy Horizons USAF 2011-2026, looks at a number of new energy technologies which might advance the USAF's mission in space -- including small modular reactors for space-based systems.

In terms of nuclear power in space, several satellite systems have been energized by Radioisotope Thermoelectric Generators (RTG). This source provides consistent power, and at a much higher energy and power density than current technologies.

Work on small modular nuclear reactors on Earth is highlighted in the Air Force report: "While the implementation of such a technology should be weighed heavily against potential catastrophic outcomes, many investments into small modular reactors can be leveraged for space-based systems. As these nuclear power plants decrease in size, their utility on board space-based assets increases."

The report explains that the Air Force space systems portfolio should consider piloting small modular nuclear systems, a view previously recommended by the Air Force Scientific Advisory Board. _Space

Space.com Orbital Concentrator Solar Array

In the sweeping report a number of desirable high-tech advances are mentioned.

For example, the Air Force is currently limited to 27 kilowatt (kW) arrays for satellite power. But more power is required for some future space missions, the report states, such as flights currently being eyed by the Air Force, national security organizations and NASA. "Employing larger and more efficient arrays will enable missions that require very high power, such as space-based radar or space-based laser missions," the report states.

In the long term, the report says, increased solar cell efficiencies and revolutionary materials foreshadow the potential of 500 kW on-orbit power generation technologies, "which would be transformational for performing missions from space-based systems."

Furthermore, there are other breakthrough space energy technologies that have the potential of achieving up to 70 percent efficiency, the report adds. Examples include quantum dots and dilute nitrides in solar cells. But there are also totally new technologies such as space tethers that could harvest energy from the Earth's geomagnetic field.

...The Air Force report also delves into the wireless transfer of power, a technology that continues to offer big promises despite the daunting challenges involved in making it a reality.

While there are many challenges in "space-to-earth" power beaming, "space-to-space power beaming" could be transformational, the report stresses.

An energy-beaming benefit for the military is powering sets of fractionated, distributed satellite systems, the report explains. Doing so would enable spacecraft to be smaller, more survivable, and more capable than current systems.

A power paradigm change

In orbit, many spacecraft systems — sensors, communications equipment and on-board processing — can require intense amounts of power.

Like all computing architectures, these systems are currently composed exclusively of silicon- based technology. However, decades of work has begun to change this paradigm, the report points out. Newer systems require less energy and offer a reduced thermal load in comparison to their silicon counterparts, the report adds.

Advances in satellite propulsion are also spotlighted in the newly issued report. Today, the ability of space-based systems to alter their orbits is based on blasts of on-board fuel. The possibility of on-orbit refueling for these systems is now being studied.

In the mid- and far-term, the report suggests, other propulsion technologies will provide exceptionally efficient propulsion. That will allow the fuel onboard orbiting systems to be utilized for longer periods of time. Hall and electric thrusters, for instance, promise extended utility of limited onboard propellants.

Whatever the technology, new methods of generating power in space hold great promise for the Air Force's plans for new satellites and other space missions. _Space.com

Cross-posted from Al Fin Energy

Brian Wang has more, excerpted from the AF PDF document

More USAF technology horizons from Brian Wang

Can Aneutronic Fusion Space Thrusters Open the Solar System?

IEEE Spectrum Getty Images
If humans are ever to travel beyond the inner solar system, they will need to devise space propulsion methods beyond conventional chemical rockets. Nuclear reactions are orders of magnitude more powerful than chemical reactions, and seem the natural approach for space propulsion to the asteroid belt and the outer solar system. But working out the best form of nuclear space propulsion is apt to take time and a lot of work. NASA physicist and engineer John Chapman thinks that fusion -- aneutronic fusion -- is the way to go.

Instead of using deuterium and tritium as the fuel stocks, the new motor extracts energy from boron fuel. Using boron, an "aneutronic" fuel, yields several advantages over conventional nuclear fusion. Aneutronic fusion, in which neutrons represent less than 1 percent of the energy-carrying particles that are the result of a reaction, is easier to manage.


"Neutrons are problematic, because for one thing they’re difficult to harness," says John J. Chapman, the concept’s inventor and a physicist and electronics engineer at NASA’s Langley Research Center, in Virginia. To make use of neutrons, "you need an absorbing wall that converts the kinetic energy of the particles to thermal energy," he says. "In effect, all you’ve got is a fancy heat engine, with all its resultant losses and limitations."


In Chapman’s aneutronic fusion reactor scheme, a commercially available benchtop laser starts the reaction. A beam with energy on the order of 2 x 1018 watts per square centimeter, pulse frequencies up to 75 megahertz, and wavelengths between 1 and 10 micrometers is aimed at a two-layer, 20-centimeter-diameter target.


The first layer is a 5- to 10-µm-thick sheet of conductive metal foil. It responds to the teravolt-per-meter electric field created by the laser pulse by "acting as a de facto proton accelerator," says Chapman. The electric field releases a shower of highly energetic electrons from the foil, leaving behind a tremendous net positive charge. The result is a massive self-repulsive force between the protons that causes the metal material to explode. The explosion accelerates protons in the direction of the target’s second layer, a film of boron-11.


...There, a complicated nuclear dance begins. The protons (which carry energy on the order of roughly 163 kiloelectron volts) strike boron nuclei to form excited carbon nuclei. The carbons immediately decay, each into a helium-4 nucleus (an alpha particle) and a beryllium nucleus. Almost instantaneously, the beryllium nuclei decay, with each one breaking into two more alpha particles. So for each proton-boron pair that reacts, you get three alpha particles, each with a kinetic energy of 2.9 megaelectron volts.

...Electromagnetic forces push the target and the alpha particles in the opposite directions, and the particles exit the spacecraft through a nozzle, providing the vehicle’s thrust. Each pulse of the laser should generate roughly 100 000 particles, making the method tremendously efficient, says Chapman. And according to his calculations, improvements in short-pulse laser systems could make this form of thruster more than 40 times as efficient as even the best of today’s ionic propulsion systems that push spacecraft around. Even at 50 percent efficiency, burning off 40 milligrams of the boron fuel would deliver a gigajoule of energy. The amount of power depends on the laser pulse rate. The motor could generate 1 megawatt per second if the pulses are frequent enough to start reactions that consume that amount of boron in 1000 seconds. (According to Chapman, using this aneutronic fusion technique with helium-3 isotopes would yield roughly 60 percent more energy per unit mass. But boron is a more attractive fuel source because it is abundant on Earth and helium-3 is scarce.)

Another big advantage of fusion space propulsion, Chapman claims, is that some of the energy can be converted into electricity to power a spacecraft’s onboard control systems. "A traveling wave tube—basically an inverse klystron—captures most of the particles’ flux kinetic energy and efficiently converts it into electrical energy," says Chapman. The process, he says, is 60 to 70 percent efficient. _IEEESpectrum

The more powerful and efficient your propulsion, the more payload you can carry, the less fuel, and the greater your choices for timely flight path and orbital selection.

VASIMR Plasma Rocket Puts Solar System In Reach

Once we have this capability, Mars isn’t really the only place that we can go. With a megawatt-class VASIMR, basically we will have access to the entire solar system. Mars is an interesting place, but so are Europa and Ganymede and Enceladus and Titan. _NextBigFuture

The plasma source cell involves the main injection of neutral gas (typically hydrogen, or other light gases) to be turned into plasma and the ionization subsystem. The RF booster cell acts as an amplifier to further energize the plasma to the desired temperature using electromagnetic waves. The magnetic nozzle cell converts the energy of the plasma into directed motion and ultimately useful thrust._NASA
As long as humans are limited to chemical rocket propulsion with specific impulse around 500, we are stuck in this small part of the solar system. But with new rockets that have specific impulses in the tens of thousands, suddenly humans can think about exploring Jupiter's moons, and beyond. The VASIMR plasma rocket is an example of a high specific impulse rocket engine, with very good prospects, particularly when paired with a small nuclear reactor power source.

Seed: Working with plasma sounds difficult. Why would you ever want to use it in a rocket?
FCD: There is a term in rocketry, “specific impulse,” which measures how efficiently a rocket obtains thrust from its propellant. The higher the specific impulse, the more efficient the rocket, and the less fuel it requires. In general, specific impulse increases as a rocket’s exhaust gets hotter. A good chemical rocket’s specific impulse is on the order of about 500. And the specific impulse of the VASIMR and most other plasma-based rockets is in the thousands, even the tens of thousands. So we’re talking about an orders-of-magnitude performance improvement of the rocket. That’s why we go to all the trouble of working with plasma, because there’s a huge payoff in terms of how much fuel you use to get any given payload from point A to point B in outer space.

Seed: Aren’t there other kinds of plasma engines already? How are they different from VASIMR?
FCD: There are other kinds, yes. In all plasma rockets, you have to produce thrust by accelerating the plasma. Other plasma rockets do this with electric current from metallic grids that are immersed in the plasma. Too much plasma flowing past these grids will make them essentially melt, so you can’t go to extremely high power. You can somewhat get around this by making the grids very large, or making arrays of them, but you’re still limited by grid erosion and damage. This means most plasma rockets are inherently low-power devices.

In VASIMR, however, there are no grids. Its plasma is contained by magnetic fields and heated and accelerated by electromagnetic waves. Since no parts of the rocket are immersed in the plasma flow, you can make the plasma very dense and hot and get much better performance.

Seed: How did you come up with this idea?
FCD: VASIMR is an example of the need for cross-pollination between disciplines to spark new ideas and new technologies. It came from research in controlled thermonuclear fusion, and in particular from a device called a “magnetic diverter,” which was the subject of my PhD thesis when I was at MIT in 1977. I’d always been interested in propulsion, and realized that this technology was suited for rocketry, but there wasn’t much work being done anywhere else. Back then I was always surprised to find that people who were working on fusion and plasma physics weren’t paying attention to what was going on in propulsion research, and vice-versa. It almost looked to me like time had stood still for these folks. They were pursuing old ideas, they weren’t communicating. Things have changed now, of course.

Seed: Have they? We’ve been sending people and machines into space for more than half a century, but we’re still mostly using chemical rockets.
FCD: Well, part of the problem with electric propulsion back then, and to a lesser degree today, is that it’s hard to get enough electricity to power the rocket. Typically, electricity in space comes from sunlight, solar power. That works okay in Earth orbit and other places close to the Sun. But people have to realize sooner or later that, if we’re ever going to explore Mars and beyond, we have to make a commitment to developing high-power electricity sources for space. What we really need is nuclear power to generate electricity in space. If we don’t develop it, we might as well quit, because we’re not going to go very far. Nuclear power is central to any robust and realistic human exploration of space. People don’t really talk about this at NASA. Everybody is still avoiding facing this because of widespread anti-nuclear sentiment.

Seed: What has to happen to make that change?
FCD: In 1958, the first nuclear submarine, the USS Nautilus, was able to actually navigate under the north polar cap and surface on the other side. No other submarine had ever been able to do that before. It was an eye-opener, a game-changer, a paradigm shift. The idea was that nuclear power enabled a completely different class of missions for these types of ships. Now, nuclear submarines are common. Something similar has to happen in space.

In fact, with the power close to what a nuclear submarine generates, you could use VASIMR to fly humans to Mars in 39 days. A chemical rocket makes the trip in eight months. That’s eight months of exposing your astronauts to debilitating cosmic radiation and weightlessness. By the time they get to where they’re supposed to work, they’re gonna be in bad shape—almost invalids! They’ll have to spend a big chunk of their time just recovering from the trip. That’s simply not a smart way to conduct an exploration program. By not addressing the key problems of limited power and propulsion, NASA is forced to work with extremely complicated and expensive mission architectures that are very limited in capability.

Seed: So you believe that in the long run it would be more cost-effective to develop nuclear-electric capabilities in space, even given potential regulatory difficulties?
FCD: Absolutely. People have fears of nuclear power in space, but it’s a fear that isn’t really based on any organized and clear assessment of the true risks and costs. When you send these missions based on chemical propulsion to Mars, they aren’t only going to be extremely expensive, but also extremely fragile. Imagine being on Earth, watching astronauts on an eight-month death trip from which there is no return, all because they made a small mistake or something failed. It would be an agonizing process, and there would be a lot of questions asked if you lost a crew. Well, in space, power is life. You can plan against a lot of contingencies by simply having more power available for a crew to use. _SEED_via_BrianWang

Scramjet: New York to Sydney in 2 Hours at Hypersonic Speeds

Rocket engine performance from a scramjet engine? That is what the X-51A Flight Test Program project is hoping to create. DARPA and the Air Force Research Laboratory are funding the collaborative effort to achieve mach 6 speeds initially. Eventually they hope to reach mach 15.

Ordinary jets have a major limitation: They can't go faster than Mach 3 without their turbine blades melting. Rocket ships can reach Mach 25, but they have to carry tremendous amounts of liquid oxygen to burn their fuel. The space shuttle, for example, weighs only 165,000 pounds empty, but it must carry 226,000 pounds of liquid hydrogen and 1.4 million pounds of liquid oxygen to reach orbit.

An air-breathing jet engine with no moving, meltable parts, such as a scramjet, can solve these problems. A scramjet is an advanced form of a "ramjet," an engine that takes the air rushing into the engine and "rams" it into the combustion chamber, creating intense pressures that can sustain combustion at the furious rate that Mach-3-plus speeds demand. But ramjets have limits too. The air entering the engine has to be slowed to subsonic speeds for it to run efficiently. And that air is so hot that no matter what measures are taken to cool it, a ramjet-powered craft must stay under Mach 5 to keep from disintegrating.

But a scramjet—a "supersonic combustion ramjet"—changes things. A scramjet does away with the diffuser that a ramjet uses to slow down incoming air, allowing the air to move through the engine at supersonic speeds so it can fly above Mach 5. The tradeoff: A scramjet engine in flight is a delicate system. Achieving balanced combustion at those speeds is an engineering challenge often compared to keeping a match lit in a hurricane.

PopSci
The impetus for rapid development of such a craft comes from the fact that the Chinese military has its own project to develop scramjet technology.

Last July, engineers from China showed up at the American Institute of Aeronautics and Astronautics Joint Propulsion Conference in Cincinnati and revealed a growing scramjet research program of their own, including a new hypersonic wind tunnel in Beijing and work on rocket-powered combined-cycle scramjets. None of the American scramjet experts we talked to would discuss their reactions to the Chinese revelations. But Craig Covault, an editor at Aviation Week & Space Technology who reported on the conference, believes one of the main reasons the Chinese attended was to glean all available intel on Western scramjet research. "I would bet that they have a serious research program under way that has a lot more going on than just the few papers that they issued at this forum," Covault says. "The reason that they issued them was just kind of a message to the rest of the world that they are engaged in these high-tech things. It also allowed them to get the 500 or more other papers in propulsion technology of all kinds delivered at the conference."

Whether or not the Chinese are actually as far along as they say, the US is determined to proceed with the hypersonic engine development. Being able to deliver a large weapons payload, or a squad of well-equipped special forces troops anywhere in the world in under 2 hours, is a temptation the US military cannot resist.

In terms of space launch capability, using a scramjet first stage to boost a rocket powered orbital second stage might reduce launch costs to orbit by more than half. Using such a system to launch sensitive payloads such as humans into orbit, and launching non-sensitive materials by laser launch, electromagnetic launch, or other cheap and high-g launch method, may work out to a combination that gets more people into earth orbit, lunar space, and la grange orbits.

The Man Who Sold the Moon, Part II

The title of this post comes from a book by SF author Robert Heinlein. It details the quest of a private businessman to conquer cis-lunar space, for profit and the spirit of mankind.

Something similar is happening with internet tycoon Elon Musk, and his private company SpaceX. Musk is betting the farm that he can build a better launch vehicle--one that can compete with the tax-supported dinosaurs from the US, Europe, Russia, and other large governments.

Five years ago, Musk was just another lucky young Internet lion starting a commercial space company. But he was more audacious than his peers — he wouldn't be satisfied with a quick, touristy trip to the edge of Earth's atmosphere, like the X-Prize-winning SpaceShipOne. That rocket, and the passenger version that will make up Richard Branson's Virgin Galactic fleet, goes just over 60 miles high. And still it took the aeronautical genius of Burt Rutan and $20 million from Microsoft cofounder Paul Allen to get SpaceShipOne up and down. Musk wants to fly resupply missions — with astronauts! — to the International Space Station, at 250 miles up in low Earth orbit.

...But first he's got to get the thing off the ground. At the dawn of the space age, between 1957 and 1966, the US sent 429 rockets into orbit; a quarter of them failed. Musk is 36 years old and has spent a fortune to build the world's first privately funded, orbit-capable vehicle to take passengers into space. And now it's sitting on its launchpad, going nowhere. Worse, it's already failed once.

....Before he founded SpaceX in 2002, Musk created two Internet companies: Zip2, which he sold to Compaq in 1999 for $307 million in cash, and PayPal, which went public shortly before being sold to eBay. Musk, the largest shareholder, was 30 years old, crazy rich, and "tired of the Internet."

Sitting in traffic on the Long Island Expressway in 2001, mulling the problems of the world, Musk started wondering about NASA's plans to send people to Mars. Which, he discovered when he finally reached a computer, didn't exist. Musk was horrified. A native of South Africa, he had earned physics and business degrees from the University of Pennsylvania and dropped out of a graduate program in physics at Stanford. He had always been interested in space, convinced that humans were destined to be a multiplanet species. But where were the Columbuses and da Gamas of the 21st century?

...The list of companies that have tried and failed to go orbital is long enough to have spawned a hackneyed joke: What's the fastest way to become a commercial space millionaire? Start as a commercial space billionaire. "Moore's law does not apply to rockets," says John Pike, a space analyst at GlobalSecurity.org. "Humanity has spent hundreds of billions of dollars on space exploration in the past half century, and the numbers have not changed: about $10,000 per pound to put something in low Earth orbit. Elon Musk is asserting that his future is going to be remarkably different, and that's a tall claim."

So how will Musk charge half that? "I thought it would be hard, and it's harder than I thought," he admits. "But I want to make rockets 100 times, if not 1,000 times, better. The ultimate objective is to make humanity a multiplanet species. Thirty years from now, there'll be a base on the moon and on Mars, and people will be going back and forth on SpaceX rockets."

Source

Musk has the right attitude to make it work. In many ways, he seems like one of the heroes from a Heinlein or Ayn Rand novel. And he is spending his own money to try to reach his goal, unlike most of the space evangelists on the stump.

The internet, telecom, computer hardware/software, and consumer electronics have all launched billionaires and mega-millionaires into the financial stratosphere. Clearly, for many of these free spirits, the next big springboard to even greater things is outer space--and the potential to become the world's first trillionaire, even after adjusting for inflation.

Probing the System at 800 km/s

Finnish scientists have invented an electric solar sail that could carry a space probe out of the solar system in 15 years.

The solar wind is a very tenuous but high speed (300-800 km/s) plasma stream blowing radially outward from the Sun. The solar wind powers the aurora and governs space weather. The average dynamic pressure (force per unit area) of the solar wind is 2 nanopascal, corresponding to 0.2 grams weight per square kilometre.

Using such a weak dynamic pressure for pushing a spacecraft requires a very large area sail, much larger than what can be provided by a solid surface. In the electric sail, the sail is formed by an electric field existing around a thin, charged tether whose voltage is maintained by an onboard solar-powered electron gun. A 20-km long tether made of wire which is thinner than human hair fits in a small reel, but gives a square kilometre effective area when stretched out in space and charged. In the paper published today in Annales Geophysicae , two-dimensional first-principles plasma simulations run on a supercomputer were used to compute the thrust per unit tether length in different solar wind conditions and tether voltages to check the feasibility of the method. Theoretical analysis and one-dimensional simulations were used to validate the results.

The results indicate that ~50 nN/m force per unit length of the tether can be achieved in average solar wind, which could enable final speeds in the range 50-100 km/s (10-20 AU/year) for a lightweight spacecraft. At such high speed one could reach e.g. Pluto in less than four years and fly out of the heliosphere into interstellar space in less than 15 years.

Source

The concept of the electric sail is similar to the magnetic sail, or the mini-magnetospheric plasma sail. The trick is to devise a large electromagnetic sail that would interact with the solar wind without being destroyed by it.

Understand, this is strictly a one-way ride to the outer system and beyond. Of course if you can find reaction mass or rocket fuel out there, you may be able to devise a method of return. Buena suerte!

Nano-Electric Impulse Engine: Specific Impulse Adjustable from 100s to 10,000s

This nanotech electric impulse thruster is scalable for use with energy ranges from watts to megawatts power.

Termed the nanoparticle field extraction thruster – nanoFET – this highly integrated propulsion concept is a high efficiency, variable specific impulse engine type that can be readily scalable for a large range of future space science and exploration missions ("Nanoparticle Electric Propulsion for Space Exploration"; pdf download, 460 KB).
The nanoFET utilizes highly scalable MEMS/NEMS structures to feed, extract and accelerate nanoparticles through micron-sized thrusters. The nanoparticles to be used as propellant can be of various geometries and materials.

....Here is how it works: Conductive nanoparticles would be transported to a small liquid-filled reservoir by a micro-fluidic flow transport system. Particles that come into contact with the bottom conducting plate would become charged and pulled to the liquid surface by the imposed electric field. If the electrostatic force near the surface can cause charged nanoparticles to break through the surface tension, field focusing would quickly accelerate the particles through the surface. Once extracted, the charged nanoparticles would be accelerated by the vacuum electric field and ejected, thus generating thrust.

....Another advantage of this system is that it affords a much broader set of missions with a single engine type – nanoFETs have an unprecedented thrust-to-power ratio for electric propulsion systems; they can adjust specific impulse over a large range from 100s to 10,000s; they show a high efficiency range of over 90% over the entire specific impulse range; they do not have the life-limiting factors common in ion thrusters.

The system is also very flexible with regard to the size and type of particles that can be used. Almost any conductive nanoparticle, such as carbon nanotubes, fullerenes, as well as metal nanospheres and nanowires could be used. Currently, the researchers are experimenting with silver, nickel and copper nanoparticles ranging in size from 5 nm to 70 nm.

Source

This thruster lacks the thrust needed to launch a craft from Earth to orbit. But once in orbit, its high efficiency would enable a wider range of missions per payload weight.