Propulsion Technologies Roundup

An Impulse Engine Scotty Would Love

NASA Tech Briefs, Sept. 2001, Machinery/Automation: "Variable-Specific-Impulse Magnetoplasma Rocket"

NASA's Johnson Space Center has developed a high-power, electric, thermal plasma engine of such originality that a prototype is being developed in collaboration with the Department of Energy and Oak Ridge National Laboratory. Dubbed the variable-specific-impulse magnetoplasma rocket (VASIMR), it would employ commonly available hydrogen propellant and continuous thrust to achieve high-speed during long space missions. While not designed to lift payloads from the ground, high-energy possibilities for this technology include an orbital transfer rocket and interplanetary booster for robotic or manned payloads. The prospect of reduce mission times to Mars is a boon for manned exploration of the red planet.

VASIMR contains three magnetic cells. The forward cell is where hydrogen is injected and ionized into plasma; the central cell heats plasma with radio-frequency excitation (like a microwave oven) and acceleration through magnetic fields; the aft cell is a magnetic nozzle that converts thermal energy of the plasma into a directed flow, while protecting the nozzle walls. The total system is a low-mass engine that achieves high vehicle speed through continuous thrust -- not the short "burns" of several minutes or less used by conventional liquid propulsion.

Unlike related ion and plasma propulsion systems, there are no electrodes to erode and no need to neutralize the ions to preserve particle momentum on engine exit. The VASIMR concept has residual benefits for interplanetary manned missions: the engine's magnetic field would deflect harmful radiation from the Sun, and the continuous acceleration provides a small amount of artificial gravity and reduces the physiological effects of weightlessness.

If successful, VASIMR may be the first of a line engines that will allow humans "Star Trek"-like access to the planets.

Put a Tornado in Your Rocket!

Aerospace America, Sept. 2001, Engineering Notebook: "Cooling off a hot new engine"

Imagine the cost of riding a conveyance (train, plane, or automobile) several hundred miles and having to replace the vehicle at the end of every trip. This certainly would make travel an expensive proposition. That's one reason most space launches are so expensive: the vehicle is discarded as the payload gets to orbit!

1 of 3 Shuttle Main Engines - NASA

One of the original "selling points" of the Space Shuttle was the reusability of its large and expensive liquid hydrogen/oxygen engines; yet, the two-decade Shuttle-era has not achieved "cheap" access to space. Perhaps the concept of the Shuttle design was good but in need of technical refinement. For instance, the 3,000º-C temperature generated in the Shuttle's combustion chamber -- above the boiling point of iron -- shortens operational lifetime and safety margins. So a new-generation, reduced life-cycle cost, reusable, liquid-fueled engine would be most welcome in the quest for cheap access to space.

And now a small company in Madison, Wisconsin may have developed such an engine. Orbital Technologies has built liquid-fueled test engines with combustion chamber walls protected by a vortex of cooler gasses -- thus increasing engine life, operational safety margins, and decreasing material costs. This technology can be used in any size engine -- from maneuvering thrusters to launch engines.

In the innovative, new engine cold liquid oxygen is pumped into the base of the combustion chamber where temperatures cause it to turn into a gas. A swirling vortex action sweeps the gas up the sides of the chamber where it is ignited by a propellant -- such as hydrogen. The cool gas keeps the walls of the combustion chamber remarkably cool and alleviates the requirement for high-heat resistant combustion chambers. So cool, that a small, early test-version of the engine had a core gas temperature of 3,000º-C and wall temperature at a comparative balmy 60º-C. Larger, all-metal versions of the engine have wall temperatures reaching only 200º-C.

A 1,000-lb thrust version of the engine is now in testing and the company may begin developing a 250,000-lb version of the vortex engine.

Vortex rocket engine technology is a low-cost, simple design breakthrough that may also be employed in combined-cycle engines -- like air-breathing ramjets. NASA has sponsored the engine development through its Small Business Innovative Research program.

Pocket Plasma Pusher

NASA Tech Briefs, Oct. 2001, Books and Reports: "Miniature Electrothermal Thruster"

NASA's Jet Propulsion Laboratory is proposing the development of a miniature spacecraft engine that uses electrical heat and magnetic fields to produce high-thrust plasma. The engine chamber would be on the order of 2.5-mm in diameter by 15-mm long and lined with a non-conducting material such as diamond. One end of the chamber would contain an electrode and the opposite side would lead to a metal-coated expansion nozzle.

Plasma jet from a high-power engine - NASA

In operation, the liquid ammonia (NH₃) propellant would be vaporized into the chamber and subject to a rapidly varying magnetic field -- 25 GHz, the resonance frequency of the chamber. This causes the ammonia to heat up and dissociate into a plasma of nitrogen, hydrogen, and free electrons. Expansion of this plasma out the nozzle would generate thrust.

High operating temperature and pressure should be possible by setting up an electromagnetic field configuration that prevents the plasma from contacting the chamber walls, using propellant liquid to cool the camber walls, and employing a heat-resistant electrode.