The LR-87 was a twin-chamber liquid-propellant rocket engine developed to power the Titan I intercontinental ballistic missile. The engine developed a total thrust of 136,078 kilograms (300,000 pounds) at sea level. It burned liquid oxygen and a grade of aviation kerosene. The combustion chambers were gimbal mounted to allow them to swivel, controlling the missile trajectory during the powered phase of flight. The engine was developed by Aerojet General Corporation. [Illustration: 55. LR-87 engine just after suspension in the museum.] The LR-87 on exhibit is from the U.S. Air Force. Toward 2076: The Future of Rocket Propulsion [Illustration: 56. A 21st-century space colony in orbit between Earth and the Moon, as suggested by Dr. Gerard O’Neill of Princeton University. This colony could accommodate 200,000 persons, using solar energy for power and lunar or asteroid materials for construction. The teacup-shaped containers ringing the cylinder are agricultural stations, and the mirrors would direct sunlight into the interior, regulate the seasons, and control the day-night cycle.] During the first twenty years of the space age, all launch vehicles were propelled by solid or liquid chemical rockets; however, nuclear and electric rocket motors are needed to provide the higher thrusts and velocities required for possible future manned journeys to other planets. Robert H. Goddard, the American rocket pioneer, was the first to suggest the possibility of electric rocket motors, but it was not until 1964 that electric rockets were actually tested in space. Two types of ion engines represent the most fully developed electric propulsion systems. In contact ion engines, a propellant gas (mercury or cesium, for example) is ionized, or given an electrical charge, by passage through a hot porous metal. The resulting ions are accelerated out of the engine by an electrical field. The charged ions are neutralized as they approach the nozzle to form an exhaust beam that imparts the thrust. Bombardment ion engines rely on the bombardment of the propellant gas by electrons from a cathode, or negative electrode, to create ions. The ions are accelerated from the engine in the same manner as in the contact ion engine. A Cesium Ion Rocket Engine This small contact ion engine produces .0009 kilogram (.002 pound) of thrust by passing vaporized cesium through hot tungsten. On Earth this amount of power is scarcely enough to lift a one-carat jewel an inch off a table, but in the frictionless vacuum of space, it is sufficient to provide attitude control for satellites. It can also accelerate a spacecraft to high interplanetary velocities by operating continuously for thousands of hours.