The time has come to jettison the traditional chemical rocket propulsion system and move to one powered by beamed microwaves, say a group of researchers.
For decades, even as rockets have gotten lighter and more powerful, the basic system for putting them in space hasn't changed. A combustion chamber is loaded with propellants, which are put through a chemical reaction, causing hot gases to accelerate and be ejected through a nozzle at very high velocity, which in turn, provides momentum to the rocket's engine.
But a team led by 25-year-old CalTech Ph.D. student Dmitriy Tseliakhovich thinks that the time has come for a new rocket propulsion paradigm, one that requires no chemical explosions, which could cut the cost of putting payloads in space by a factor of ten or more, and which could dramatically reduce the environmental impact of a launch.
The technology is being looked at by some as a major key to making affordable private space flights--particularly those geared towards getting cargo outside the Earth's atmosphere--for small- and medium-sized businesses that would like to explore the resources of space, but which are today unable to get off the planet.
And while there is clearly a long way to go before a propulsion system powered by microwave beams or any other external source is ready for prime time, those involved in the research believe that all the required technology is already here.
To Tseliakhovich, the major rationale for pursuing external propulsion is the fact that the price for putting payloads in space hasn't changed in decades. He explains that in 2005 dollars, the cost has stayed constant for at least 50 years at around $10,000 per kilogram, mainly because so much of a rocket's space and weight is devoted to the fuel that gets it off the ground.
In order to overcome that threshold, he argued, it is necessary to develop a fundamentally new system capable of delivering the 7 kilometers per second velocity required to get a rocket out of the atmosphere.
"What we propose," said Tseliakhovich, who is the founder of Escape Dynamics, a start-up devoted to solving the problem, "is, let's get rid of producing energy on board the launch vehicle and delivery energy by way of microwave beams."
To many, that idea may sound like pure science fiction, but there are some very accomplished people in the space industry who believe that beamed microwaves could very well become the propulsion system of the future.
"What's been really evident to me [is] that today's rockets really are the same line of evolution as Chinese rockets a thousand years ago, [where you have] a hot tube, and hot gases come out the other end," said Peter Diamandis, the chairman and CEO of the X Prize Foundation and co-vice chairman of Space Adventures. "There's been no leap from the propeller to the jet, as we had in aviation. I had read about beamed power, and as I looked at it...what became evident to me was that the technology to implement it was here today. Nothing magical needed to be created."
Indeed, Diamandis had already been thinking about creating an X Prize or possibly an X Challenge dedicated to coming up with a next-generation propulsion system, perhaps, he said, using some new system to get something like 10 kilograms of cargo up to 30 kilometers in space.
So when Tseliakhovich came to him at a conference full of ideas about future versions of propulsion, it was Diamandis who first turned the young Ph.D. student onto the idea of beamed power.
How it would work
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."
Essentially, the idea is that microwaves beamed from the ground would heat hydrogen, which would then flow through a heat exchanger and out through the rocket's nozzle. This system would allow for a single-stage launch vehicle that would be both reusable, and highly reliable, Tseliakhovich said.
One of the major advantages of a beamed microwave external propulsion system, said Kevin Parkin, the deputy director of the Mission Design Center at NASA's Ames Research Center in Mountain View, Calif., is that it can bypass some of the typical constraints of a traditional propulsion engine.
According to 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.
"So you get a higher performance out of the rocket by sending the same amount of mass out the back," Parkin said. "So that translates to a rocket with a bigger payload."
And what that means, Parkin added, is that a rocket launched under this paradigm could have more of its mass devoted to structural integrity, a key component in getting to a reusable launch vehicle that requires being inspected only once over 100 flights or so. "It's more akin to an airliner than a rocket," he explained.
Not a new concept
To be sure, the idea of using beamed energy to get a rocket off the ground is nothing new. Parkin said that Russian rocket scientist Konstantin Tsiolkovsky first proposed the idea in 1924. But back then, Parkin added, there was no such thing as beams or lasers, let alone those powerful enough to launch a vehicle into space.
Today, that's no longer the case, argued Tseliakhovich. "Until five years ago," said Tseliakhovich, "we could not produce enough output of the microwave power. We did not have efficient enough gyrotrons."
Today, however, 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.
So why has no one come along since then to try to take the technology where Tseliakhovich now wants to?
In part, said Parkin, it's because not many people know about the technology, and because it often takes the scientific community 10 years or more to cotton to a radical change in philosophy.
Further, he said, there are those who are critical of the very concept of beamed energy propulsion, mainly because they worry that the technology is not really capable of producing enough of an increase in propulsion efficiency to make investing time and money in it worthwhile.
But to Diamandis, bringing high technology into the equation means that for the first time, Moore's Law could be applied to the science of propulsion, and that could mean that the cost of putting payloads in space could very well drop rapidly as does the price of computer components.
Diamandis acknowledges that there are those who don't believe in the idea of beamed energy-based propulsion, and said there are others who oppose investing in it because they fear it threatens current infrastructure.
But NASA is already looking into the technology, Parkin said, pointing to a research project under way at the U.S. space agency's Glenn Research Center in Cleveland.
For his part, 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.