ROCKET SCIENCERecipe For Deep Space - 'Microwave At 1,100 Degree C For X years'
It isn't an interplanetary corn popper, but in the not-to-distant future a rocket propulsion system using microwave generators may help propel deep space probes across the solar system and beyond.
Under development in UAH's Propulsion Research Center (PRC), the new propulsion system uses magnets and electrical fields to accelerate plasma ionized in a microwave "oven," creating a low-thrust engine that might be three to five times more efficient than traditional fuel-and-oxidizer rocket motors.
To anyone well versed in advanced magnetohydrodynamics, the proposed system's concept is simplicity itself: An easily ionized gas (the UAH team uses nitrogen for preliminary tests) and powerful microwave radiation are pumped into a cavity, where the gas is heated to more than 725 degree C (about 1,340 degree Fahrenheit).
Some of the gas becomes ionized, creating a cloud of ions sheathed in free electrons.
As this plasma rushes out of the chamber it passes through a channel across which positive and negative electrodes create a powerful electrical potential, said Zhongmin Li, the UAH Ph.D. candidate who is working on the prototype test engine.
That electrical field rips more electrons away from nitrogen atoms, creating more ions and adding energy to the plasma. By the time it leaves the chamber, the plasma's temperature is up to as much as 1,100 degree C -- more than 2,000 degree Fahrenheit.
At that point, the escaping plasma generates an impressive plume inside the UAH team's Pyrex vacuum chamber. But there is one more piece of the puzzle waiting to be built, says Dr. Clark Hawk, the PRC's director.
In the finished engine, the ionized plasma will rush through an exit tube around which the team will generate perpendicular electrical and magnetic fields.
Those fields will generate a Lorenz effect, a force that is perpendicular to both fields. As the plasma passes through the tube, the Lorenz effect will act as an accelerator to increase the engine's thrust.
While the Lorenz force pushes on all of the particles in the plasma, "the electrons have a very small mass so it's easy to accelerate them very quickly," said Li. "And when the electrons move they create an electrical field that pulls the ions along behind them, giving the ions an additional pull."
The end result is a highly efficient engine: One pound of fuel and oxidizer in a typical rocket engine will generate up to 450 pounds of thrust for one second.
The microwave plasma engine needs no oxidizer, and one pound of the fuel material might generate between 1,400 and 2,000 pounds of thrust for one second -- or more likely 10 pounds of thrust for 140 to 200 seconds.
Low thrust is one of the tradeoffs for efficiency, said Hawk. "This system uses a lot of electrical power, so you also trade off the weight of the oxidizer for the weight of the power system. The power system might be a nuclear power plant or maybe solar panels."
One intriguing option still to be investigated is the concept of "harvesting" fuel from space, Hawk said. This may be possible because the engine might operate on a wide range of easily-ionized materials, including carbon dioxide, water vapor, argon, helium and nitrogen.
Propulsion Research Center at UAH
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