According to Kevin Parkin, who is an Escape Dynamics adviser and who wrote his own Ph.D. thesis on microwave thermal propulsion, a beamed energy propulsion system is capable of producing 2.5 times as much thrust as a traditional chemical-based system. He said that the standard system tops out at an energetic reaction of 16 megajoules per kilogram, while the beamed energy approach can reach 40 megajoules per kilogram.
“Until five years ago,”  said Tseliakhovich, “we could not produce enough output of the microwave power. We did not have efficient enough gyrotrons.”
[In 2010] it’s possible to produce more than a megawatt of energy per gyrotron, Tseliakhovich said, speaking of devices that, according to Wikipedia, are “high-powered vacuum tubes which emit millimeter-wave beams by bunching electrons with cyclotron motion in a strong magnetic field.”
And Parkin said his own research demonstrated five years ago that it was possible to heat hydrogen to high enough temperatures using microwaves to create high-performance propulsion.
Tseliakhovich said he believes it will be technologically possible to build a prototype beamed microwave infrastructure and launch vehicle in as little as seven years , though he admits that the psychological shift required to back such an effort might take longer. That’s particularly true, he said, because sending such a rocket into space would require enough land to build a functional beamed microwave array and the support of a government interested in the technology.
But both Parkin and Diamandis–who, of course, have a stake in the technology’s success–think that Tseliakhovich’s time frame is realistic. The only question, said Diamandis, is how big a rocket built using the technology in that time frame would be.
According to Escape Dynamics, “the key operational components of the microwave beam power launch system are a ground-based microwave array and an engine based on the heat exchange between the hydrogen propellant and the incoming microwave radiation. Hydrogen heating is achieved with the heat exchanger, which heats the propellant to a temperature above 2,000 [degrees Kelvin], which is necessary for efficient operation of the engine.”
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.
Brian Wang is a Futurist Thought Leader and a popular Science blogger with 1 million readers per month. His blog Nextbigfuture.com is ranked #1 Science News Blog. It covers many disruptive technology and trends including Space, Robotics, Artificial Intelligence, Medicine, Anti-aging Biotechnology, and Nanotechnology.
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