Fact sheet number: FS-2000-07-159-MSFC
When a NASA Space Shuttle lifts off the launch pad, it does so with the help of three reusable, high-performance rocket engines. Each of these powerful main engines is 14 feet (4.2 meters) long, weighs approximately 7,000 pounds (3,150 kilograms) and is 7.5 feet (2.25 meters) in diameter at the end of its nozzle.
Developed in the 1970s by NASA's Marshall Space Flight Center in Huntsville, Ala., the Space Shuttle Main Engine is the world's most sophisticated reusable rocket engine.
The engines operate for about eight-and-one-half minutes during liftoff and ascent -- long enough to burn more than 500,000 gallons (1.9 million liters) of super-cold liquid hydrogen and liquid oxygen propellants stored in the huge external tank attached to the underside of the Shuttle. Liquid oxygen is stored at -298 degrees Fahrenheit (-183 degrees Celsius) and liquid hydrogen at -423 degrees Fahrenheit (-250 degrees Celsius). The engines shut down just before the Shuttle, traveling at about 17,000 mph, reaches orbit.
NASA continues to increase the reliability and safety of Shuttle flights through a series of enhancements to the Space Shuttle Main Engines. The engines were modified in 1988, again in 1995, and more improvements are being developed in 2000.
The newest modifications include new high-pressure fuel and oxidizer turbopumps, a two-duct powerhead, a single-coil heat exchanger and a large-throat main combustion chamber.
Each engine has two powerful high-pressure turbopumps that supply up to 970 pounds (440 kilograms) of liquid oxygen per second and up to 162 pounds (73 kilograms) of liquid hydrogen fuel per second to the engine's main combustion chamber. In this chamber, the hydrogen propellant and oxygen oxidizer mix and burn at high pressures and at temperatures exceeding 6,000 degrees Fahrenheit (3,315 degrees Celsius) to produce thrust.
The current hydrogen turbopump design, with 20-year-old technology, requires pump removal and maintenance between flights. It features welded construction, which requires meticulous inspections.
In early 2001, the first flight is expected of a redesigned hydrogen turbopump. The new design uses a unique casting process to eliminate welds, significantly increasing the number of missions between major overhauls.
The current design requires special coatings for thermal protection to the turbine blade airfoils. In the new turbopump design, these coatings are no longer necessary because the hardware is constructed of materials that are more heat tolerant and less sensitive to the hydrogen environment.
Also incorporated into the improved pumps is a new bearing. Bearings separate high-speed rotating parts from stationary parts and prevent friction between the two. The new bearings are made of silicon nitride -- ceramic material 30 percent harder and 40 percent lighter than steel. They have an ultra-smooth finish that produces less friction during pump operation.
In July 1995, a redesigned oxygen turbopump first flew on a Shuttle.
Considered the backbone of the engine, the powerhead consists of the main injector and two preburners, or small combustion chambers. Liquid oxygen and hydrogen are partially burned in the preburners, generating hot gases. The liquids continue to move through ducts into the main combustion chamber, while the gases created in these chambers drive the high-pressure turbopumps, which give the Shuttle thrust.
The two-duct hot gas manifold is a new powerhead design that first flew on the Shuttle in July 1995. It significantly improves fluid flows in the system by decreasing pressure and turbulence, thus reducing maintenance and enhancing the overall performance of the engine.
The previous powerhead featured five tube-like ducts -- three on one side of the engine where hot gases flow from the fuel turbine, and two on the side where hot gases flow from the oxidizer turbine. The two-duct hot gas manifold replaced the three small fuel ducts with two enlarged ducts -- smoothing the fuel flow, reducing pressure and turbulence, and lowering temperatures in the engine during operation. This design reduces stress on the main injector and requires fewer welds, eliminating potential weak spots in the powerhead.
Single-Coil Heat Exchanger
The Shuttle's engines supply pressure to the external tank, which in turn provides propellants to the engines. This pressure is produced by the engine's heat exchanger, a 40-foot-long (12-meter) piece of coiled stainless steel alloy tubing.
To pressurize the external tank, liquid oxygen is routed through the tubing, which passes through the engine's hot gas manifold. Hot exhaust from the high-pressure oxidizer turbopump turbine heats the alloy tubing. As the tubing gets hot, so does the liquid oxygen. The oxygen reaches about 500 degrees Fahrenheit (260 degrees Celsius) and supplies pressure to the external tank. The external tank delivers liquid oxygen at the correct pressures to the Shuttle main engines, and the oxidizer ultimately mixes with the liquid hydrogen at engine ignition.
Until mid-1995, the heat exchanger had seven welds in the 40-foot (12-meter) tube. Welding can change the properties of a metal and have flaws. The newly designed exchanger is a continuous piece of stainless steel alloy with no welds. The increased thickness of the redesigned heat exchanger reduces wear on the tube and lessens the chances of damage. It also reduces maintenance and post-flight inspections. Beginning with the STS-70 mission in July 1995, a new enhanced single-coil heat exchanger has flown on each Shuttle.
Large Throat Main Combustion Chamber
A Shuttle engine’s main combustion chamber is where the liquid hydrogen and liquid oxygen are mixed and burned to provide thrust. In January 1998, the first large throat main combustion chamber flew on the STS-89 Shuttle mission. The throat of the new chamber is about 10 percent larger than the previous design -- improving the engine's reliability by reducing pressure and temperature in the chamber and throughout the engine.
This allows the high-pressure pumps to operate at lower turbine temperatures and pressures. It improves chamber cooling and extends the life of the hardware. The new large throat main combustion chamber is cast from large pieces of metal, rather than constructed of smaller pieces welded together. In addition to reducing the number of welds, casting also reduces the assembly time and labor required to build and to maintain the hardware.