Air Force

Burn rate control for solid fuel rockets

Electrically operable thrust control promotes burn surface area increase or coning action in the rocket fuel charge

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Solid fuel rockets are used for military and long mission applications because of their inherent simplicity, avoidance of the complex plumbing, mixing and control elements required in liquid fuel rockets, and are easily preserved for future use.

One downside of solid fuel rockets is the difficulty involved in modulation of the thrust-producing reaction once grain burn has commenced.

The need for thrust modulation is readily apparent in practical applications of a rocket in commercial, military, or scientific uses. The functions of threat avoidance, multiple purpose missions, and atmospheric re-entry each present an opportunity for some form of thrust change.

Thrust modulation might also be desirable in tailoring the placement velocity and the orbit of a spacecraft, in trading thrust magnitude for thrust duration in a rocket, or in balancing the thrust applied to a multiple rocket vehicle, especially during the initial liftoff, low air velocity, flight portion.

Practical thrust modulation in a solid fuel rocket has met a significant degree of difficulty, however, and in most arrangements has required compromises to the rocket’s payload, reliability, attained degree-of-control, and added rocket complexity. Implemented or proposed systems including variable exit nozzles, pulsed modulators, and moving filament throttle arrangements each have their shortcomings.

Placement of increased energy sources applied to areas of the rocket grain in order to produce coning action and increased burn face surface area is an approach which has shown initial benefits and Air Force researchers have made further improvements to this.

The extension of the method uses electrical loss heating and other heat energy developed in an electrical battery for grain preheating and for controlling the thrust. Preheating battery cells are placed across the grain cross-section in order to provide a moving preheat zone as the grain burn progresses. The preheat zone of the battery cell is triggered into activity by the approaching burn face and its battery actuating heat.

There are several benefits derived from increased burn rate from local fuel preheating along an embedded control element. These include the formation of cone-shaped distortions of the fuel burn area with a resulting increase in the burning fuel surface area. Such increase in burning surface area occurs even though only a small portion of the fuel is heated by a control element—and therefore requires relatively small amounts of energy from the control elements in achieving meaningful thrust control. Increased burn surface area also provides an increased rate of gas generation from a rocket motor and this additionally results in higher pressure in the burn chamber. This increased pressure can additionally increase the grain burn rate in a closed circle fashion.

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