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Space Propulsion Group, Inc. 2019


BUTTE, MT 59701

Tel: (406) 533-6870

Hybrid Motor Development

SPG has a variety of hybrid motors to offer to its customers at thrust levels ranging from 10 lbf to 30,000 lbf. Oxidizers that are available with these systems are liquid oxygen, nitrous oxide, Nytrox, and Di-nitrogen Tetroxide (MON). Motor case options are carbon composite or metal (aluminum titanium or steel).

SPG is the world leader in the formulation, casting, and processing of high regression rate hybrid rocket fuels. Our fuels technology allows us to design formulations for specific attributes such as regression rate, structural strength, glass transition temperature and toughness among others. In addition, our base fuel matrix is highly hydrophobic which enables formulations using high-energy additives such as aluminum, lithium hydride, magnesium or boron.

We have substantial experience in composite structural wrapping of fuel grains. Flight weight combustion chambers can be built with composite winding directly bonded to the fuel grain. Our composite overwrap technology works as both a stiffener and a liner allowing close to 100% fuel utilization.

SPG now has capability to cast grains up to 36-inches in diameter and 14 feet long. Two 22-inch OD paraffin fuel grains (700 kg each), which are believed to be the world’s largest monolithic pieces made out of paraffin wax, have been successfully fabricated using the existing facilities. These fuel grains were burned in the 24-inch hybrid motor.

Our processing techniques can easily be scaled to larger sizes as applications require.

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 to the left 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.

Shown in the video below is a test fire of the Peregrine Hybrid Rocket. The Peregrine Hybrid developed as a partnership between Greg Zilliac at NASA Ames, Stanford University, and Space Propulsion Group to develop a large, wax-based hybrid that uses nitrous oxide in place of liquid oxygen. The Peregrine hybrid rocket is a simpler system, in that it has roughly half the number of components of a liquid rocket motor, and is less explosive than its counterparts using all-solid or liquid propulsion. This is because, with Peregrine’s solid fuel and liquid oxidizer stored separately in the engine, they are unlikely to mix inadvertently and react.

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: