Rocket Engines

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Rocket Engines

From the first rockets built by the Chinese over a millennium ago to the precision engines used by modern missiles, rocket engines all work in accordance with Isaac Newton's Third Law of Motion: For every action there is an opposite and equal reaction. In a rocket engine, hot gas expelled at high velocity generates thrust in the opposite direction. The most common means of doing so uses chemical reactions to produce the hot gas. The first rockets used solid propellants, such as black powder, but they were very inefficient. Liquid-propellant rocket engines, first developed in 1926 by Robert H. Goddard, are much more powerful and opened the way to spaceflight.

The Origins of Modern Engines

Atlas and Delta launch vehicles were originally U.S. Air Force (USAF) rockets developed in the 1950s. To power these missiles, Rocketdyne developed a family of rocket engines that burned kerosene and liquid oxygen (LOX) based on German V-2 rocket technology obtained after World War II. As these rockets were adapted to their new role as launch vehicles in the 1960s, still larger versions of their engines (such as Rocketdyne's 1.5 million-pound thrust F-1) were built for the Saturn rockets that sent Apollo missions to the Moon.

Delta II and III use the 200,000-pound thrust RS-27A, which is an updated descendant of the MB-3 used in the original Delta. The Rocketdyne-built MA-5 power plant in the Atlas 2A, in use since 1961, has also been upgraded. The Atlas has a distinctive stage-and-a-half design, which allows it to jettison a pair of booster engines when they are no longer needed, leaving a smaller sustainer engine to power the stage. The booster engines of the new MA-5A are a pair of RS-27 thrust chambers, giving the core of the new Atlas 2AS a liftoff thrust of 490,000 pounds. The Russians have also incrementally improved their long-used launch vehicles' engines over the decades. Energomash, the corporate descendant of the Soviet design bureau that developed many Russian rocket engines, worked with the American firm Pratt & Whitney to build the 585,000-pound thrust RD-180 to power the American Atlas 3 and 5.

Boosting Performance

While modern solid rockets are less efficient than liquid-propellant engines, their simplicity and relatively low cost make them ideal for certain roles. For decades many American launch vehicles used solid rocket upper stages. The Delta II, with its Star 48B motor built by Thiokol, continues this series's use of solid rocket third stages. Some small launch vehicles, such as the American Pegasus, Taurus, and Athena, as well as the Japanese J-1 and M-5, use solid rockets in all their stages to reduce costs.

Solid rocket motors strapped to the first stage of a launch vehicle have also proved to be an economical means of increasing a rocket's payload capability. Since 1964 the Delta has used increasingly larger clusters of Castor solid rocket motors built by Thiokol to help enhance the design's performance. The new Delta II and III use as many as nine GEM-40 motors built by Alliant Techsystems. Even the Atlas 2AS uses four Castor 4A rockets to increase liftoff thrust, a first for this series.

The use of solid rocket boosters is most apparent in the Titan family of launch vehicles. A pair of 120-inch-in-diameter solid rocket motors made by Thiokol were attached to the USAF Titan II missile core in 1965 to produce the Titan IIIC, which had over four times the payload capability. The Titan uses Aerojet-General LR87 and LR91 engines burning liquid hypergolic propellants that ignite spontaneously on contact. Successive Titans have used more powerful solid boosters attached to upgraded cores to further increase the payload. The Titan 4B uses a pair of solid rocket motor units built by Alliant Techsystems to produce 3.4 million pounds of thrust at liftoff.

Another means of boosting rocket performance is by using cryogenic propellants, such as liquid hydrogen and LOX, which have twice the efficiency of most other propellants. The first engine to use these cryogenic propellants was the 15,000-pound thrust RL-10 engine built by Pratt & Whitney and used in the high-performance Centaur upper stage since 1960. The Centaur, with improved versions of the RL-10, has been used in combination with the Atlas and Titan. The RL-10B-2 has been used in the second stage of the Delta III and IV.

The most efficient engines have been nuclear ones. While other engines use chemical reactions to produce heat, in nuclear engines a compact nuclear reactor heats liquid hydrogen or other fluid to generate thrust with more than twice the efficiency of conventional chemical rocket engines. During the 1960s the National Aeronautics and Space Administration (NASA) developed the Nuclear Rocket for Rocket Vehicle Applications (NERVA) with a reactor built by Westinghouse Electric and the engine itself built by Aerojet-General. Before work stopped in 1972, in part due to post-Apollo budget cuts, NERVA was intended for use in advanced lunar and interplanetary missions.

A New Generation

The space shuttle makes the ultimate use of solid rocket motor technology and high-efficiency cryogenic rocket engines. A pair of solid rocket motors built by Thiokol generate 5.3 million pounds of thrust for liftoff while a trio of Rocketdyne-built space shuttle main engines (SSMEs), generating 375,000 pounds of thrust each, supply most of the energy needed to reach orbit. Other launch vehicles use similar arrangements of solid boosters and cryogenic engines, such as the European Ariane 5 and Japanese H-2. The Delta IV uses the cryogenic RS-68 engine with various boosters. Built by Rocketdyne, the RS-68^* was the first totally new American rocket engine design since the SSME was designed in 1971.

More innovations are in store for launch vehicles. One of the more novel designs is the XRS-2200 linear aerospike developed by Rocketdyne for the X-33. Here the engine's nozzle is replaced with an exhaust ramp, allowingthe engine to work efficiently at all altitudes, unlike conventional engines. A larger version of the linear aerospike would power the Venture Star single-stage-to-orbit vehicle.

see also Launch Industry (volume 1); Launch Services (volume 1); Launch Vehicles, Expendable (volume 1); Launch Vehicles, Reusable (volume 1); Reusable Launch Vehicles (volume 4); Rockets (volume 3).

Andrew J. LePage