Gelled Propellants

Gelled propellants have additives that make them thixotropic materials. They have the consistency of thick paint or jelly when at rest, but they liquify and flow through pipes, valves, pumps, or injectors when an adequate shear stress is applied. They offer these advantages.

Small aluminum particles can be suspended in the fuels where smoky exhaust is not objectionable. Inert solid particles can be suspended in oxidizer liquids. This increases propellant density, density impulse, and thus reduces the size of tanks and vehicles. Smaller vehicles have reduced drag and thus can allow an increase in the range or speed of tactical missiles.

There is no plugging of injector orifices or valve passages and good flow control has been demonstrated.

Individual gelled fuel propellants will be essentially nonflammable and will not usually sustain an open fire.

There is reduced susceptibility of leakage or spill, reduced sloshing of liquids in the tanks, and the boil-off rate is reduced.

Long-term storage without settling or separation is possible; more than 10 years has been demonstrated.

Explosions or detonations, which happen when a vehicle accident causes liquid propellants to become inadvertently premixed, are much less likely with gelled propellants, which are difficult to mix.

Many spilled gelled propellants can be diluted with water and disposed of safely.

Short-duration pulsing is possible.

Most storable oxidizers, a few cryogenic propellants, and most liquid stor-able fuels can be gelled.

Explosions are much less likely when a propellant tank is penetrated by a bullet or when a missile is exposed to an external fire or a nearby detonation.

These are some of the disadvantages:

There is a small decrease in specific impulse due to dilution with a gelling agent, and less efficient atomization or combustion. For example, the characteristic velocity c* of oxygen-kerosene propellant is decreased by 4 to 6% when the kerosene is gelled and aluminum is suspended in the fuel. When both the fuel and a nitric acid oxidizer are gelled, the performance loss (c*) can be as high as 8%. Clever injector design and the selection of good gelling agents can reduce this loss.

Loading or unloading of propellants is somewhat more complex.

Residual propellant quantity may be slightly higher, because the thixotropic fluid layer on the walls of the tanks and pipes may be slightly thicker.

Changes in ambient temperature will cause slight changes in propellant density and viscosity and therefore also in mixture ratio; this can result in more leftover or residual propellant and thus in a slight reduction of available total impulse. This can be minimized by careful selection of gelling agents so as to match the rheological property changes of oxidizer and fuel over a particular temperature range.

Suspended metals can make the plume smoky and visible.

Some gelling agents have resulted in unstable gelled propellants; that is, they separated or underwent chemical reactions.

Experimental rocket engines have shown these gelled propellants to be generally safer than ordinary liquid propellants and to have good performance and operational characteristics (see Refs 7-11 and 7-12). This makes them less susceptible to field accidents. A variety of different organic and inorganic gelling agents have been explored with a number of different liquid propellants.

Experimental thrust chambers and rocket engine systems have been satisfactorily demonstrated with several gelled propellant combinations. One experimental engine is shown in Fig. 6-8. As far as is known, no such rocket engine has yet been put into production or flight operation. An effort is underway to demonstrate this technology clearly and to qualify a rocket engine with gelled propellants for an actual flight application.

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