single-sideband radio transmission. A type of radio transmission in which only one of the sidebands of the signal is transmitted. The other sideband and the carrier wave are suppressed.
single-spool turbine engine (gas turbine engine). A gas turbine engine that has only one rotating mass.
The compressor is directly connected to the turbine, and everything rotates at the same speed. A twin-spool turbine engine has a low-pressure compressor driven by one turbine and a high-pressure compressor driven by another. The two compressors are linked only by the air that flows through them, and they turn at different speeds.
single spread (adhesive application). A method of adhesive application in which the adhesive is spread on only one of the two surfaces being joined.
single-throw switch. An electrical switch that has only two conditions, ON or OFF. In one direction of the operating control, the contacts are open (OFF). Moving the control to the opposite position closes the contacts (ON).
 A single-pole, single-throw (SPST) switch can select one of the two conditions in one circuit, and a double-pole, single-throw (DPST) switch can select one of the two conditions in two circuits with the movement of a single control.

sintered metal. A type of molded metal in which powdered metal is pressed together and molded into the desired shape while it is cold. Then the molded metal is heated to form it into a strong cohesive material.
sintering. A process in which powdered metal is heated but not quite melted and then squeezed together.

sinusoidal curve. See sine wave.
siphon. A device used to move a liquid from a container, over an edge higher than the level of the liquid, down to a lower level. A curved tube is put into the liquid, and some form of mechanical force is applied to start the liquid flowing through the tube. A suction is often put on the end of the tube to start the liquid flowing.
When the flow is started, atmospheric pressure forces the liquid from the higher container into the tube, and the weight of the liquid flowing through the tube to the lower level keeps the liquid flowing.
Sitka spruce. A tall, straight spruce tree that grows on the northern Pacific coast of North America. Sitka spruce wood is strong, lightweight, and straight grained, and is used as the reference to which all other aircraft structural woods are compared.
sizing (textile materials). A filler put into some types of fabric or paper to fill the pores and make the material smooth and stiff. Starch is often put into cloth as a sizing so it will stay smooth when it is ironed.
sketch. A rough drawing made to illustrate a part or principle. Sketches are made according to regular mechanical drawing practices, but they are usually made without the use of drawing instruments, and they show only a minimum of detail.