The Efficiency Paradox in Orbital Propulsion
Any spacecraft engineer will tell you that propulsion efficiency is measured by specific impulse (Isp) — the thrust delivered per unit of propellant mass consumed. Higher Isp means less propellant for the same velocity change, which means more payload capacity or longer operational life. Electric propulsion systems routinely achieve Isp values of 1,500–3,000 seconds. Chemical propulsion tops out at around 450 seconds for the best bipropellant systems. By this measure, electric propulsion wins decisively.
So why does anyone still use chemical propulsion for orbital transfers? Because Isp is only half the story. Electric propulsion systems produce very low thrust — typically measured in millinewtons, compared to newtons or kilonewtons for chemical systems. Low thrust means long transfer times. A transfer that a chemical system executes in hours takes weeks or months with electric propulsion. For time-sensitive missions, radiation-sensitive payloads that cannot linger in the Van Allen belts, or operators paying daily rates for a transfer vehicle, this delay carries real cost.
PAVE Space's hybrid architecture resolves this paradox by combining both propulsion modes in a single integrated system, allocating each to the mission phases where it performs best.
Phase 1: Chemical Burn for Orbit Raising
The most fuel-intensive phase of any orbital transfer is the initial orbit-raising manoeuvre — the delta-v required to escape the initial parking orbit and begin climbing toward the target altitude. For a typical LEO-to-MEO transfer, this phase requires 400–600 m/s of delta-v executed quickly to minimise time spent in radiation-intensive intermediate orbits.
PAVE Space uses a high-performance bipropellant thruster for this phase, achieving the required delta-v in a single burn lasting minutes rather than days. The thruster uses a custom propellant combination optimised for volumetric efficiency — maximising thrust within the constrained volume of a small transfer vehicle — rather than targeting maximum Isp. This trade-off is intentional: at this phase, speed matters more than efficiency.
Phase 2: Electric Propulsion for Circularisation and Fine-Tuning
Once the transfer vehicle has completed the initial orbit raise, the mission enters a different regime. Circularising the orbit — converting an elliptical transfer orbit into the circular target orbit — requires precise, low delta-v adjustments. These manoeuvres benefit enormously from the high Isp of electric propulsion, and the time penalty is acceptable because the spacecraft is already approaching its target altitude.
PAVE Space's electric propulsion module uses a gridded ion thruster developed in partnership with ETH Zurich's space propulsion group. The thruster achieves 2,200–2,800 seconds Isp at thrust levels of 50–180 mN, controllable across the full range to accommodate variable power availability from the solar array. Power processing electronics are integrated directly into the thruster module, minimising harness mass and improving system reliability.
The Numbers: Where the 40% Comes From
The efficiency comparison is against an all-chemical reference mission: a pure chemical transfer using bi-propellant propulsion for both orbit raising and circularisation, sized for the same payload and the same transfer time window as the PAVE Space hybrid system.
For a representative 300 kg payload transfer from 550 km LEO to 1,200 km MEO with a 72-hour transfer time constraint, the reference all-chemical system requires approximately 180 kg of propellant. The PAVE Space hybrid system, using chemical propulsion for orbit raising and electric for circularisation, requires approximately 108 kg — a 40% reduction. This translates directly to either increased payload capacity (72 kg more payload for the same transfer vehicle mass), extended vehicle lifetime (more propellant margin for additional missions), or lower transfer vehicle mass and therefore lower launch costs.
The 40% figure is conservative. For transfers with looser time constraints — 96 or 120 hours instead of 72 — the electric propulsion system can contribute more delta-v during the orbit-raising phase as well, pushing efficiency gains toward 50–55%.
Thermal and Power Management: The Hidden Challenge
Combining chemical and electric propulsion in a compact transfer vehicle creates thermal and power management challenges that are not obvious from the headline efficiency numbers. Chemical thrusters generate intense localised heat during burns; electric thrusters require sustained high power (200–400W) for extended periods. Managing both thermal loads without cross-contamination — ensuring the chemical thruster's heat does not degrade the ion thruster's performance, and ensuring the power draw for electric propulsion does not starve the attitude control system — required significant system-level engineering.
PAVE Space's thermal architecture uses dedicated radiator panels for each propulsion subsystem, coupled with a proprietary thermal isolation structure that prevents heat conduction between the chemical and electric thruster assemblies. The power management system uses a dual-bus architecture that prioritises attitude control and communication functions before allocating surplus power to the ion thruster, ensuring mission-critical operations are never compromised by propulsion demands.
Implications for the Transfer Services Market
The fuel efficiency advantage translates into competitive pricing. Transfer services are priced primarily on two factors: the cost of the transfer vehicle (which scales with mass and complexity) and the launch cost to deliver the transfer vehicle to its initial orbit. By reducing propellant mass by 40%, PAVE Space can either fly a smaller, cheaper transfer vehicle for the same capability, or offer longer operational life and more missions per vehicle — both of which reduce the per-mission cost passed on to customers.
Our pricing model targets $8,000–$12,000 per kilogram of payload delivered for standard LEO-to-MEO transfers, compared to $18,000–$25,000 for current dedicated chemical transfer stages. For operators moving multiple payloads per year, this difference represents millions of dollars in operational savings and a meaningful improvement in mission economics.