http://www.boeing.com/defense-space/space/propul/images/spot_ssme.jpg  14 feet tall and almost four tons.

The SpaceShuttleMainEngine 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.

http://www.boeing.com/defense-space/space/propul/SSME.html
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The SpaceShuttleMainEngine controller SoftwareProcess is '''very''' heavyweight -- HighCeremonyMethod, BigDesignUpFront, YaddaYaddaYadda (kind of funny because the part number, which is more or less the version number, for the DSP software has been YADA\d\d lately).  We sometimes joke that software changes require more paperwork than the SSME can lift.  HaHaOnlySerious.

The good news: there's serious talk of using TestDrivenDevelopment for future phases and for different engine controllers!

-- GregBacon

''See also TheyWriteTheRightStuff.'' A different group, they write software that commands the engine controller software.  BoeingHuntsville (which includes the group that develops the SSMEC software) is rated at CapabilityMaturityModel level 3, but the team discussed in TheyWriteTheRightStuff is CMM level 5.

''Hey, Greg? Where do plug the ROMs into that thing?''

Why, the frobnicator port, of course!