Hybrid Rocket Propulsion Overview

Dr Arif Karabeyoglu

 

 

 

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
Cost	Reduced development costs Reduced recurring costs

 

SPG’s Advanced Hybrid Rocket Technology

The significant technological advances made by SPG have resulted in the development of high performance hybrid rockets without compromising their simplicity, cost and safety advantages over the solid and liquid propulsion systems. SPG's LOX/paraffin-based hybrid motor has a delivered Isp performance of 340 seconds (for an area ratio of 70). The structural coefficients are comparable to the levels that exist in the modern solid and liquid rockets.

 

Some of the key virtues of SPGs hybrid rocket technology can be listed as (see Table for a summary):

• Development of a portfolio of fast burning liquefying fuels (mostly paraffin-based) has enabled the use of a single circular port design approach which significantly simplifies the grain design/processing and improves the fuel utilization to 98-99% level.
• Use of cost effective propellants and motor fabrication technologies.
• Controlling the regression rate by adjusting the concentration of additives. This feature allows for the use of a single motor for a wide range of missions.
• Elimination of the low frequency instabilities in the LOX/paraffin-based hybrids by the use of a novel injector/fore-end configuration as opposed to the conventional methods of addition of heat or injection of a pyrophoric substance (such as TEA) at the fore of the motor. The conception of this passive method used to mitigate the low frequency instabilities substantially simplifies the propulsion system and eliminates the use of additional subsystems that increases the weight, complexity and the cost of the vehicle.
• Development and implementation of methods to attenuate the acoustic instabilities.
• Realization of high combustion efficiencies, in the 95-97% range, in the LOX/paraffin-based motors. Similar efficiencies have been measured in nitrous oxide and Nytrox systems which are somewhat easier to achieve due to the noncryogenic nature of the oxidizer.
• Use of gaseous oxygen as the pressurant eliminates the need for the expensive helium gas. Efficient combustion of the oxygen gas, which has been demonstrated in motor testing, results in substantial improvement in the structural coefficient.
• Use of environmentally friendly propellants and materials in the propulsion system.
• No exotic materials are used in the construction of the motors.
• Use of carbon composite case material along with the high fuel utilization lead to low burn out weight for the hybrid motors.
• A pintle throttling valve has been developed and tested. Throttling ratios of 3:1 has been successfully demonstrated.
• SPG hybrids are inherently safe and suitable for manned missions.