Recipe for deep space voyagers — ‘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. (Ask your physics teacher.) 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.