The Space Shuttle Main Engine is the most reliable and highly tested large rocket engine ever built. The SSMEs have achieved 100% flight success with a demonstrated reliability of over 0.999. The SSME is a reusable, staged-combustion cycle engine. Using a mixture of liquid oxygen and liquid hydrogen, the SSME can attain a maximum thrust level (in vacuum) of 512,950 pounds which is equivalent to greater than 12,000,000 horsepower. The regeneratively cooled engine also features high performance fuel and oxidizer turbopumps that develop 69,000 horsepower and 25,000 horsepower respectively. Ultra-high-pressure operation of the pumps and combustion chamber allows expansion of all hot gases through a high-area-ratio exhaust nozzle to achieve efficiencies never previously attained in a production rocket engine. These advantages allow a heavier payload to be carried without increasing the launch vehicle size.