A typical chemical rocket is 90 percent fuel, with the remaining 10 percent split between payload and structure, so an externally powered launch vehicle — one powered by on-the-ground microwave transmitters — has certain advantages:
EDI’s external propulsion launch system will operate at a specific impulse above 750 s and this breakthrough increase in efficiency reduces the fraction of mass dedicated to propellant to less than 72%. The increase of 3x in the mass fraction dedicated to structure and payload for the first time opens doors for reusability and single-stage-to-orbit flight. Our first generation vehicle is optimized for 100-200 kg payloads and is designed to operate like an airplane: the vehicle flies into orbit, delivers the payload, re-enters the atmosphere after completing one or more orbits around Earth and lands back at the spaceport.
Key benefits of external propulsion:
- Space launch vehicles become fully and rapidly reusable.
- Cost per launch can eventually be reduced to $150 per kg.
- The need for combustion is eliminated, leading to safer and simpler launch vehicles.
- Useful payload fraction goes up from 1.5-3% to 8-12% and the structural mass is increased by 1.5-2x.
- Small satellites can be launched as primary payloads allowing higher degree of flexibility for customers.
- Space launch is effectively powered with electricity from the grid through a battery-storage system pioneered by the company, and in the long term can rely on renewable sources of energy.
Their white paper goes into more detail:
Escape Dynamics’ baseline technology uses a wireless energy transfer system based on millimeter-wave high power microwave sources. The baseline frequency is 92 GHz; however, other mm-wave frequencies (90-170GHz) are also considered. The energy is delivered to the moving vehicle via a phased array of antennas enclosed in proprietary side-lobe suppressing radomes, which ensure safety of the energy transfer.
Our baseline propulsion approach is a thermal thruster which uses hydrogen as a working fluid and a heat exchanger for coupling external microwave energy into the thermal energy of the hydrogen. External microwave energy is absorbed in a ceramic matrix composite (CMC) heat exchanger with dimensions of approximately 3m by 5m. The hydrogen is initially stored as a liquid in a cryogenic tank and is supplied to the heat exchanger via a turbopump designed to raise the hydrogen’s pressure to approximately 150atm. The hydrogen is heated to above 2000C as it flows through the heat exchanger and is exhausted through an aerospike nozzle optimized for a SSTO flight. The heat exchanger also serves as a primary component of the thermal protection system (TPS) during the return from orbit.