Question: How did you first get interested in the concept of microwave propulsion?
Answer: As an undergraduate physics major, I became interested in how to reduce the costs of getting payloads into orbit. I examined a number of possibilities and concluded that the microwave concept was the optimal approach for the near term.
Question: How exactly does this microwave propulsion technology work?
Answer: A laser or microwave beam fired from the ground hits the underside of the spacecraft. The heat from the beam ends up in the fuel, either liquid hydrogen or methane, and the expanding gases exiting the nozzle convert that heat to thrust. It is actually a surprisingly simple concept, since it eschews the complicated plumbing required by conventional rockets. Moreover, a conventional rocket provides about 16 megajoules per kilogram of propellant. Microwave beams can provide up to 40 megajoules per kilogram. Microwave propulsion should allow specific impulse, a measure of a fuel's efficiency, in the 800 seconds range. By contrast, the Space Shuttle Main Engines, which are probably the most efficient chemical engines in the world, have a specific impulse of 450.
Question: Could a microwave propulsion system get a spacecraft into orbit without any supplemental rockets?
Answer: Yes, but at the moment we’re looking at the minimum cost and scale for an initial rocket, and that may be two-stage-to-orbit. It is definitely feasible to launch a single-stage rocket and get it into orbit using only microwave propulsion.
Question: How does the cost of microwave propulsion compare with that of conventional rockets?
Answer: There are different ways of measuring propulsion efficiency, but by just about any measurement, microwave propulsion should compare favorably with conventional propulsion. Microwave propulsion should be at least a factor of 3 cheaper than conventional disposable rockets, but for reusable spacecraft a 20 fold cost/performance increase is attainable.
Question: How does laser propulsion compare to microwave propulsion? What are the efficiencies of these devices?
Answer: Both microwave and lasers will lose some efficiency to atmospheric absorption. Microwave is currently more cost-effective than laser - one can buy a one megawatt millimeter wave source for $2 million dollars. The price for both laser and millimeter sources are steadily dropping, and should continue to fall for the foreseeable future. But at this point millimeter wave sources are cheaper. The lasers would be better suited to longer range activities, such as putting a payload into geosynchronous orbit, or putting a payload on the moon.
Question: How much would it cost to build the necessary infrastructure? How much would it cost to maintain it?
Answer: We have done studies indicating that a facility for launching a small satellite (less than 100 kilograms) could be constructed for less than a billion. We believe that there is a market for satellites in the 40-50 kilo range. For larger scale the cost would be in the billions, but still highly competitive compared to conventional rocket infrastructures.
Question: Could this technology be used to put 100 ton payloads into orbit?
Answer: Yes, this technology scales at worst linearly, meaning that twice the size should at worst be twice the cost. We should be able to achieve economies of scale, and we don't foresee any fundamental obstacles to scaling this technology to large payloads.
Question: Could microwave propulsion be used to launch payloads directly to the moon?
Answer: We have done studies indicating that direct ascent to the moon or mars is feasible. This technology compares quite favorably against conventional rockets with regard to high delta-V trajectories such as these. Single stage to the moon is feasible with microwave propulsion. It isn't with conventional rockets.
Question: Wouldn't a nuclear-thermal propulsion system provide the highest specific impulse (efficiency)?
Answer: Yes, nuclear fission propulsion has the potential to be even more efficient than microwave propulsion. In a nuclear spacecraft the fuel would flow through a nuclear core operating at 3,000 Kelvin. The Russians actually did 10,000 hours of testing on nuclear propulsion, and may even have reached operational status. The concern was always radioactivity, but with proper coatings no nuclear fuel is mixed in with the propellant. Despite being able to achieve a specific impulse of 950 seconds, nuclear propulsion for putting spacecraft into earth orbit is a non-starter.
Question: What is the efficiency of the beams involved? How large can the beams be made?
Answer: Microwaves are produced by gyrotrons, which today are 56% efficient. About 10% efficiency is lost to the atmosphere, and another 20% is lost to spillage. So the total efficiency of the system results in a third of the initial energy going out the nozzle of the propulsion system. We are currently looking at a 500 megawatt heat-exchanger. But a future system could be scaled to gigawatts, so there are no showstoppers to creating multi-gigawatt systems.
Question: How would a spaceship powered by microwaves differ from a conventional spacecraft?
Answer: A microwave spaceship would be simpler, and wouldn't require an oxidant. So the plumbing would be more straightforward. A single-stage-to-orbit vehicle powered by microwaves could have payload fractions of up to 15%. By comparison, the best conventional multi-stage rockets have payload fractions of less than 5%.
Question: Could microwave propulsion be used to put people into space?
Answer: Definitely. All that would be required for a human-rated system is additional beam facilities downrange or to switch to lasers, which are longer range. The first systems will be used to put cargo into orbit, and will have higher g’s. But once the technology is proven I have no doubt that it will be used to put humans into space.
Question: How optimistic are you about garnering funding?
Answer: At this point my primary concern is getting talented people involved in this project. At the moment there are only 3 people working on it. But all of the successful aerospace projects, such as Goddard's research and the skunkworks, had small, tightly focused teams. So we don't need huge numbers of people to make this concept viable.
Question: What is the estimated cost per pound to get into low earth orbit (LEO)?
Answer: We are confident that we can get the price down to about $600 per kilogram to LEO. Current costs are about $10000 per kilo to get into LEO. But halving the cost would not cause demand to double. So we need to get the cost down to $600 per kilogram in order to be economically viable.
Question: When is the earliest that a microwave power scheme could become operational?
Answer: We are scheduled to take delivery of the gyrotron in April 2012. We hope to have a subscale demonstration by 2018, and to prove suborbital capability within a decade. Within two decades, we should have true orbital, single-stage-to-orbit capacity. Of course, if we had higher funding, we could reduce those timeframes substantially. We are convinced that a microwave/laser propulsion system will be the enabling technology that opens up space to exploration and development.
Kevin Parkin thesis (2006) on microwave propulsion (261 pages, 29 Megabytes)
Microwave rocket patent
The military is developing larger gyrotrons for microwave weapons
The Airborne Active Denial System would require a beam generator of unprecedented size, says Diana Loree, manager of the program at the Air Force Research Lab. Megawatt microwave generators (called gyrotrons) already exist, producing intense heat in plasma-research laboratories and factories that need to melt glass or composite materials, but the military program requires a generator twice as large as any existing model. AFRL staff hope to demonstrate a giant gyrotron during ground tests in 2014
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