Proper noun
SSME
(US, space science, aviation) Initialism of Space Shuttle Main Engine.
This allowed it to achieve a mass fraction of about 0.74, comparing with 0.86 for the SSME, one of the best conventional engines. Source: Internet
SSME upgrades were denoted as "block numbers", such as block I, block II, and block IIA. Source: Internet
The three second delay after confirmation of SSME operation was to allow the stack to return to nearly vertical. Source: Internet