Science: Control by Sound

When U.S. rocket engineers talk about the bright possibilities of solid-fuel rockets, they always have to pause over one big requirement: how to control the fuel’s burning rate. A current system is to shape the charge, measure the ingredients—and hope. This week Acoustica Associates, Inc. of Plainview, N.Y. announced an initial $85,188 contract with the National Aeronautics and Space Administration to explore a radical new control that is as exciting as it is simple. The company thinks that it can handle solid fuels by filling the rocket with sound—plain, ordinary noise.

Acoustica engineers explain that when a solid fuel burns in a high-pressure combustion chamber, the components, e.g., ammonium perchlorate and polystyrene, turn to gases that mix in a thin layer on its surface. Part of the heat generated strikes back to the fuel, gasifies more of it, and so keeps the flame burning. When this characteristic was discovered by Dr. Martin Summerfield of Princeton, the next step was to look for something that would control the gas mixture. A faster mixing would increase the burning rate, while slower mixing would decrease it. If the control were precise enough, scientists would then have what amounts to a throttle for solid fuels.

Hole in the Middle. Sound waves can perform the trick. When they pass through the reaction layer, they jostle the gases, make them mix and react either faster or slower, depending on the level of sound.

When they reach the hardware stage, Acoustica’s engineers will build an experimental rocket engine with a cylindrical cavity running through the mass of fuel (see diagram). A “grain”‘ of this shape is simple and strong, but if left alone it burns at an uneven rate: as the fuel is consumed, the cavity gets bigger and exposes more surface to the heat. Since the amount of hot gas generated is proportional to the area of burning fuel, the gas pressure keeps rising until just before burnout. The effect is that solid-fuel rockets of this type must have superstrong casings to hold the peak pressure, and must thus pay a penalty in weight.

Acoustica’s idea is to control the fuel by blowing high-pressure gas through a heat-resistant whistle at the forward end of the cylinder’s cavity. While the cavity is still small, the whistle will screech at full power, increasing the burning rate of the fuel. As the cavity grows bigger, a valve will reduce the amount of gas passing through the whistle. The volume of sound will decrease, and so will the fuel’s burning rate. If the valve is manipulated efficiently by some pressure-sensing instrument, it will keep the hot gas inside the rocket at constant pressure from take-off to burnout. Rocket walls can be made lighter, and the bird itself will fly higher.

And a Fiddle on Top. Building a lightweight, sound-controlled rocket will not be easy. But Acoustica’s engineers think that it is certainly possible. For a sound-making device, they intend to use a Levavasseur whistle that has no moving parts and can be made of heat-resistant material. The rocketeers figure that the best frequency to use is 10,000 cycles, about the pitch of a very high violin note. Yet the volume of sound must be well above the loudest fiddle; an ear-shattering 170 decibels, which is 100 times the sound pressure of a supersonic boom from a jet aircraft.

Sonic control has many possibilities besides smoothing the pressure curve of burning fuel. It may develop into a way to program the complete flight of a solid-propellant missile. Shifting sound levels could vary the thrust to give the rocket better maneuvering capabilities; fuel might also be compounded that does not burn at all unless the rocket’s cavity is filled with powerful sound, thus accomplishing total cutoff with the whistle. The big spaceships that NASA plans to toss into space will use clusters of rocket engines. If they are solid fueled and equipped with whistles, they could be used to steer the ship; a slight reduction of sound level in one of the outside engines would make the whole ship veer in its direction.

NASA and several engine makers have high hopes for Acoustica’s experiment. But why talk about it? One good reason is that as early as 1952 Russian Physicist P. M. Kubanski started publishing scientific papers about the effect of sound waves on heat transfer. There is at least a chance that the Russians are already using sonic controls in some of their rockets—and that in turn might explain how they got those giant Sputniks in orbit.