Hybrid Rockets

A flame in the boundary layer

A hybrid rocket keeps its oxidizer — liquid oxygen, nitrous oxide, hydrogen peroxide — in a tank, and its fuel — rubber, plastic or wax — as a solid grain with a port down the middle. The oxidizer is injected at the head end and flows down the port.

Crucially, the fuel does not contain any oxidizer, so it cannot burn on its own. Combustion happens in a diffusion flame standing in the boundary layer above the fuel surface: fuel vapour rises from the surface, oxidizer diffuses in from the core flow, and they burn where they meet. Heat conducted and radiated back from that flame vaporizes more fuel. That is a very different mechanism from a solid's surface flame, and it leads to a different law for how fast the fuel surface recedes — its regression rateRegression rateIn a hybrid, how fast the solid fuel surface recedes: ṙ = a Gₒₓⁿ, set by heat transfer through the boundary layer.:

$\dot r = a\,G_{ox}^{\,n}$

where $G_{ox}$ is the oxidizer mass flux through the port. The regression rate depends on the flow, not on the chamber pressure — which is why hybrids cannot run away the way solids can.

The O/F shift

This law has an awkward consequence. As the burn proceeds, the port widens: the oxidizer flux falls (so each square metre of surface regresses more slowly), while the burning area grows. The fuel flow and the oxidizer flow evolve differently, so the mixture ratio drifts during the burn — the O/F shiftO/F shiftAs a hybrid’s port opens up, oxidizer flux falls while burning area grows, so the mixture ratio drifts during the burn..

The explorer shows the second, more fundamental problem: regression rates are small. Classic polymeric fuels like HTPB regress at around a millimetre per second, so producing enough fuel for a large thrust demands a long grain with a lot of surface — a long thin rocket, or a grain with many ports, which leaves slivers of unburned fuel and weakens the grain.

Liquefying fuels

In the late 1990s, work at Stanford by Karabeyoglu, Cantwell and others found that some fuels — notably paraffin wax — regress roughly three times faster than polymers (three to four times in tests; the course's regression fits give about 2.7× at the same flux). Paraffin forms a thin, low-viscosity liquid layer on its surface, and the fast oxidizer flow tears droplets off it (like wind blowing spray off a wave), carrying fuel into the flame without having to vaporize it first. These liquefying fuelsLiquefying fuelParaffin-based hybrid fuels form a thin melt layer whose droplets entrain into the flow, regressing 3–4× faster than HTPB. make single-port hybrids of practical size possible.

A family portrait

Sutton's summary of hybrid advantages is compelling:

• Safety. Neither component is explosive on its own, so the motor can be manufactured, transported and stored like inert materials. A crack in the fuel grain is harmless, because it cannot burn without oxidizer.
• Control. Throttle, shut off and restart simply by controlling the oxidizer valve.
• Cost and simplicity. Only one liquid, and often no turbopump.
• Environment. No perchlorates, no HCl.

The disadvantages are just as real: low regression rates, O/F shift, sluggish combustion efficiency, fuel slivers, and low-frequency combustion instabilities. SpaceShipOne's and SpaceShipTwo's motors, and many sounding rockets and student rockets, use hybrids; no orbital launcher yet has.

Next: propulsion without combustion — electric thrusters that push with the force of a sheet of paper, for years.