Update on Microwave Beam-Heated Propulsion

Kevin Parkin described the history and status of microwave beam-heated propulsion at Space Access 2019.

Technical summary is at Parkin Research. Kevin showed more pictures of the test systems.

Kevin gave a previous update on beam propulsion in 2018 at Breakthrough Propulsion.

Kevin wants to get heat reflectors and better coatings tested in a $1 million program that would get the technology ready for an orbital launch system. Currently, the technology is at TRL 5 (Technology Risk Level).

A $100 million project would then implement a pilot system.

The beam director cost and performance is a critical driver of capabilities and costs.

Traditional chemical rockets are a marginal technology for the purpose of launch to orbit. Microwave and laser thermal rocket engines bypass the fundamental energy density limits of chemical propellants by swapping the combustion chamber for a heat exchanger. The energy that would be released by chemical reactions is instead transmitted from the ground and heats an inert monopropellant as it passes through the heat exchanger. The resulting rocket is safer and cooler, yet so energetic that it lifts payloads to orbit in a single stage. It can lift so much more payload that, for expendable rockets, the payload cost falls from $10,000 per kilogram delivered to low earth orbit to less than $1,000/kg. For reusable rockets, the payload cost falls from $3,000/kg to less than $300/kg. At high launch rates, the fundamental limits are the cost of the energy, which is less than $100 per kilogram of payload, and the cost of the propellant, which is less than $10 per kilogram of payload.

Thermal rockets bypass key limitations of traditional chemical rockets, and in so doing are easily able to reach orbit using a single stage, as opposed to the traditional two to four stages. A microwave or laser thermal propulsion system combines the specific impulse of a nuclear thermal rocket engine with the thrust to weight ratio of a conventional rocket engine, and the result on system performance is profound: For a given rocket, the payload mass is 3-12 times heavier. In addition to this direct effect, which can be shown using the rocket equation, it saves additional money to use one propellant instead of two, and one stage instead of two to four stages. The combined effect is that the overall cost is 6-144 times cheaper than a conventional rocket, depending on the particular assumptions made.

A single foil balloon tank holds a slush methane propellant. This propellant is then pumped through the heat exchanger, reaching close to the temperature of an incandescent light bulb filament just prior to being expanded through a plug nozzle to produce thrust. The beam tracks the heat exchanger, which faces the general direction of the beam throughout the ascent to orbit. There is only a single propellant, single tank, single turbopump, and single stage all the way from the ground to orbit.

In the baseline case above, there continues to be no directed energy thermal launch system, and 100% of the market is served by families of traditional chemical rockets, the largest of which is capable of launching a 20 metric ton satellite to LEO. The cost of launch to continues to be $125M/week for the U.S. Government, excluding R&D. This equates to a $130Bn expenditure over the 20-year period.

In the pessimistic case, a directed energy thermal rocket with a 200 kg payload capacity to LEO captures 1% of the total revenue for satellite launch. It has a relatively poor jet power per unit payload mass (though not the worst), and a relatively poor cost reduction factor. Consequently, its initial infrastructure needs to cost less than $2/Watt in order to save money over the period. Current estimates of beam director cost are $1-5/Watt at this scale, and further research is expected to lower this value and its associated uncertainty.

In the optimistic case, a family of directed energy thermal rockets capable of launching up to 20 tons to LEO captures 100% of the revenue for satellite launch, replacing traditional rockets altogether. They have a relatively good jet power per unit payload mass (though not the best) and a relatively good cost reduction factor. Consequently, the initial infrastructure needs to cost less than $22/Watt in order to save money over the period. Current estimates of beam director cost are one to two orders of magnitude below this value, depending in part on the choice of frequency.

The business case does not yet exist for a 200 kg directed energy thermal launch system that fits within the cost, risk and schedule of private capital. More work is needed to lower technical risks. Beam director cost and performance improvement is key.