NASA's Ames Research Center has recently spent $2 million on a powerful microwave source to be used primarily for propulsion research. Kevin Parkin's team is collaborating with Escape Dynamics in Broomfield, Colorado, which is likewise dedicated to developing microwave-based rockets. High-power microwave sources called gyrotrons have been developed for nuclear fusion research, and the gyrotron bought by Ames Research Center can pump out a megawatt of microwave power. ($2 per watt)
Kevin Parkin's 2006 thesis proposed a new idea to achieve both high Isp and high T/W: The Microwave Thermal Thruster. This thruster covers the underside of a rocket aeroshell with a lightweight microwave absorbent heat exchange layer that may double as a re-entry heat shield. By illuminating the layer with microwaves directed from a ground-based phased array, an Isp of 700–900 seconds and T/W of 50–150 is possible using a hydrogen propellant. The single propellant simplifies vehicle design, and the high Isp increases payload fraction and structural margin
In the mid-90s Jordin Kare also had worked on a heat exchanger microwave powered rocket. It is fitted with a fuel tank containing a gas such as hydrogen, plus a set of pipes or channels into which the gas is pumped. An incoming laser beam heats the channels to a few thousand degrees and the gas expands and shoots out at high speed, pushing the rocket along.
A 120-meter-wide dish could keep a microwave beam focused to a few meters across at a range of 100 kilometers. The dish would combine the power from a few hundred gyrotrons, and to prevent too much of that power being absorbed by water vapour in the atmosphere, the beam facility would need to be in a high, dry location, such as Chile's Atacama desert.
That heat exchanger will be the crucial component, and Parkin has been experimenting with different materials and arrangements of channel. "At the moment we're making multichannel thrusters the size of a credit card by micro-machining them out of graphite," he says. These are hit with 20 kilowatts of microwave power, a fair advance on the 200 watts of his first experiment. Parkin says a full-scale vehicle might use carbon-fibre channels coated with silicon carbide, which absorbs microwaves well and can protect the channels during re-entry into the atmosphere as it is oxidation-resistant.
While real hardware is being developed at Carnegie Mellon University, Escape Dynamics is taking a virtual approach, designing and testing their prototype rockets using computer simulations. "By the end of this year we want to complete the proof-of-concept by taking the heat exchanger and beaming virtual energy to it, to see how efficient energy transfer is," says Tseliakhovich. If all goes well, they will construct a working prototype in 2012.
Parkin and Tseliakhovich calculate that a small microwave rocket should be able to carry up to 15 per cent of its weight as payload - compared with about 2 per cent for current launchers - and send cargo into space for less than $600 per kilogram.
The Spacex Falcon Heavy is expected to cost $1000 per kilogram, but it is not flying yet.
NASA's Glenn Research Center and the US Defense Advanced Research Projects Agency (DARPA) have spent the last six months studying beamed propulsion, looking at Parkin's plans along with two other schemes for beam-riding spacecraft.
Jordin Kare has a concept study for laser arrays and heat exchangers, 1000 of these units combine to launch a 100-kilogram payload, carried by an arrow-like rocket with a 29-metre-long propellant tank and a flat 4-metre heat exchanger mounted at the base. "I'm hoping to be able to do some heat exchanger tests very soon, since we have several kilowatts of lasers available," says Kare. In 2009, for example, US-based IPG Photonics announced a 10-kilowatt fibre laser for welding and cutting.
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