Propulsion Options for the AdMerk Rocket-Free Spaceplane

Precooled air-breathing engines like SABRE ingest atmospheric oxygen at lower altitudes and speeds, then switch to onboard oxidizer in thin air. A precooler heat exchanger chills incoming air from roughly 1000°C down to -150°C in about 0.01 seconds, allowing a lightweight compressor and high-pressure combustion with liquid hydrogen — yielding an effective specific impulse far above conventional rockets while in air-breathing mode.

SABRE is designed to propel a single-stage vehicle to roughly Mach 5+ at ~28 km on air-breathing thrust, then transition to closed-cycle rocket mode for orbital insertion. This lets a SABRE-powered vehicle reach orbit with a higher payload fraction than a pure rocket, though it still needs a rocket phase to close the remaining velocity gap to orbit.

No combined-cycle air-breathing engine has yet flown to hypersonic speed, but key components are proven on the ground: a full-scale precooler was tested at Mach 3.3+ intake conditions in 2019, and a SABRE core demonstrator passed ground firing tests through 2022. NASA and ESA studies confirm the concept's feasibility while noting the complexity of integrating heavy heat exchangers and cryogenic handling into an orbiter.

Nuclear thermal propulsion uses a fission reactor to superheat a propellant (typically liquid hydrogen) and expel it through a nozzle — replacing chemical combustion with fission heat. Running the reactor at very high temperatures roughly doubles specific impulse versus a chemical engine of the same propellant.

The U.S. NERVA program (1960s) ground-tested more than a dozen reactors and proved that nuclear engines can restart and throttle reliably, reaching a technology readiness level of roughly 6–7 before cancellation in 1973. NASA and DARPA's DRACO program aimed to flight-demonstrate a nuclear thermal stage in Earth orbit by 2027.

For a spaceplane, a nuclear engine would realistically ignite only above the atmosphere — for example, after an air-breathing first stage reaches Mach 5 and ~30 km — both for safety and because solid-core engines can't generate enough thrust-to-weight to lift off the ground unassisted.

Beamed-energy propulsion delivers power to the vehicle externally via laser or microwave beams, so the vehicle doesn't carry a heavy onboard power source or oxidizer — for example, a laser thermal rocket could focus a beam to heat hydrogen propellant for thrust.

In 2000, the 'Lightcraft' experiments used a pulsed CO₂ laser to launch a 50-gram craft to 71 meters altitude. A NASA-funded 2012 study concluded beamed-energy launch is technically feasible in principle but requires new technologies and large investment over the long term.

Suborbital demonstrations are more plausible near-term than full orbital insertion, since a beam must continuously track and power the craft — orbital use would need multiple ground stations or an in-space relay as the vehicle curves out of direct line-of-sight.

Plasma thrusters accelerate a propellant — xenon, argon, and similar — via electric and magnetic fields, producing a high-velocity plasma exhaust. They offer very high specific impulse but at the cost of low thrust and a substantial power source requirement, so they're not viable for launch from Earth's surface.

The VASIMR prototype has demonstrated tunable specific impulse up to roughly 5,000 seconds using RF energy to heat plasma in a magnetic nozzle, but its VX-200 test article produces only about 5 N of thrust at 200 kW input — roughly the weight of a ream of paper.

For a spaceplane, plasma thrusters are best suited to orbital maneuvering — fine-tuning orbits or long-term station-keeping — rather than primary ascent propulsion, at least within a 5–10 year horizon.

Ion and Hall-effect thrusters are the smaller, well-proven electric propulsion devices used on satellites and probes for decades — for example, Deep Space 1 and the Dawn probe used ion propulsion for large delta-V maneuvers over time, and NASA's NEXT ion thruster ran for over 48,000 hours in ground tests.

Despite very high fuel efficiency, thrust is tiny — even the most powerful electric thruster tested to date (a 100 kW-class nested Hall thruster) produced only about 5.4 N in the lab, a record for the field but still trivial for launch purposes.

For a spaceplane, electric thrusters are best treated as a complementary system for orbital trim once already in space — not a primary ascent method. Their technology readiness is the highest of any option reviewed here, but their applicability to launch is the lowest.