Hybrid Rocket Propulsion Overview

Rocket engines are a class of jet propulsion systems in which two (or more) propellants are reacted in an open volume. The heat evolved causes rapid expansion of gases through a [typically] convergent-divergent nozzle and the momentum of the resulting supersonic jet imparts a force on the rocket chamber. All rockets, by definition, carry both the fuel and oxidizer required for the combustion reaction differentiating them from air-breathing jet engines. There exist three general classes of chemical rocket systems liquids, solids and hybrids. In a liquid system, both the fuel and oxidizer consist of liquids which are pumped separately into the combustion chamber and reacted. In a solid system the fuel and oxidizer both exist in solid form and are either molecularly bonded or intimately mixed physically. A hybrid system, as its name suggests, consists of one solid propellant and one liquid propellant. The fuel can be either the liquid or the solid and the same goes for the oxidizer. However, useful oxidizers tend to be liquids and so the typical configuration of a hybrid rocket consists of a liquid oxidizer reacted with a solid fuel.

The figure displayed above illustrates a typical configuration for a hybrid rocket. In this diagram, the fuel is contained within the combustion chamber in the form of a ported cylinder. The oxidizer is stored separately in a tank and when thrust is desired the valve is opened and vaporized oxidizer flows down the port where combustion takes place. A turbulent diffusion flame is established over the fuel surface as shown in the figure above. Heat transfer from the flame vaporizes the fuel sustaining combustion and the fuel surface regresses in the radial direction as it is consumed. Combustion occurs in the port near the surface of the fuel and the fuel regresses in the radial direction as it is consumed. See second figure above.

Hybrids combine some of the advantages of liquids and solids and also exhibit unique advantages. The most important is probably the inherent safety associated with storing the fuel in the solid phase. A general summary of the other advantages is given in the table below:

Compared to:

  Solid Rockets Liquid Rockets
Simplicity Chemically Simpler
Tolerant of processing errors
Mechanically Simpler
Tolerant of fabrication errors
Safety Reduced chemical explosion hazard Reduced fire hazard
Less prone to hard-starts
Operability Throttling, Start/Stop/Restart Capability Operation requires only a single liquid
Performance Higher Specific Impulse (Isp) Higher fuel density
Easy inclusion of high-energy additives (Al, Be, etc.)
Environmental No perchlorates required
Non-toxic exhaust products
Solid fuel presents reduced contamination hazard

Reduced development costs
Reduced recurring costs