The advantages, limitations and applications of hybrid motors that combine a solid fuel with a liquid or gaseous oxidiser.
Rocket motors are traditionally divided into two classes: solid motors, where fuel and oxidiser are together in solid form, and liquid motors, where both are liquid. A hybrid motor sits between these two worlds: the fuel is solid and the oxidiser is liquid or gas.
Operating principle
The fuel (for example paraffin wax or HTPB polymer) sits in the combustion chamber as a solid block. The oxidiser (liquid oxygen, nitrous oxide or gaseous oxygen) is pressure-fed from a tank into the chamber. Material vaporising from the fuel surface mixes with the oxidiser and burns. Thrust is adjusted by controlling oxidiser flow.
Advantages
- Safety: Because fuel and oxidiser are stored separately, the risk of accidental ignition is very low.
- Controllability: It can be throttled, stopped and restarted with a valve.
- Cost: Fuel is cheap and simple to make and transport.
- Environment: Paraffin-based fuels produce few toxic combustion products.
Limitations
In hybrid motors combustion efficiency depends on flow at the fuel surface, so high thrust needs a multi-port fuel grain. Specific impulse (Isp) is lower than liquid motors and similar to solid motors. Combustion instability is the main issue requiring attention in design.
Applications
Suited to sounding rockets, first stages of small launchers and commercial space tourism. Gökbilge's SR-8 sounding rocket uses a 28 kN hybrid motor, and the first stage of the ÜF-250 launcher has reached 140 kN.
Test process
Each motor design is verified first with a small-scale laboratory motor and then with full-scale static firings. In tests thrust, pressure, temperature and vibration are recorded at 20 kHz. Data from the 18 tests we ran in 2026 brought our simulation models to a 4% error margin.
The future
We are working on 3D-printed fuel grains and next-generation oxidiser feed systems for hybrid motors; the aim is to raise combustion efficiency by 8–10% and shrink motor size.
Parts of a hybrid motor
A typical hybrid motor has five main parts: oxidiser tank, feed valve, injector, combustion chamber containing the fuel grain, and nozzle. There are one or more ports in the middle of the fuel grain; the oxidiser touches the fuel as it flows through them. At the rear of the chamber is a post-combustion zone where mixing is completed.
Design calculations
Key calculations in motor sizing are fuel regression rate, oxidiser mass flow and oxidiser-to-fuel ratio. Paraffin-based fuels regress 3–4 times faster than classical polymers, which gives higher thrust with fewer ports. Nozzle throat diameter is chosen from chamber pressure and target thrust, with graphite or composite liners for temperature resistance.