NASA Technical Reports Server Their documentation says "Over 500,000 aerospace-related citations, over 200,000 full-text online documents, and over 500,000 images and videos". Covers everything that has any connection to aerospace: major topics such as spacecraft, NASA missions, orbital dynamics, etc., but also combustion science, fracture mechanics, reliability modeling, engineering management, business and economic aspects of aerospace. (NACA reports are also available at the Cranfield mirror site )
SciTech Connect (DOE) Combines into one database the former "Information Bridge" full text reports (mostly 1994-present) and "Energy Citations Database" (1948-present). SciTech Connect covers anything at all related to energy, from coal and oil technology to alternative energy, government regulations, business, etc.
Defense Technical Information Center Covers a very wide range of subjects and is always worth a try if a topic appears to be something that would apply to military technology, weapons, reliability, management, electronics, computers and control. THE place to try for military standards – not a great chance the one you want will be full text, but it seems to find more online MIL STDS than does Google. Most standards require a trip to Library of Congress Technical Reports Section or purchasing the standard from a commercial source. DTIC has a big portion of what is in NTIS, but most of that is NOT full text online – at least not yet.
Science.gov Joins many databases into one technical reports source. Search is much slower and it's often faster to use the above 3 sites to narrow a subject search (if it’s at all related to aerospace try NTRS, if it’s anything related to energy try SciTech Connect, if it’s anything of interest to the military try DTIC, etc.). But items frequently turn up in science.gov that are not in those 3 databases. Its main drawback is that it takes a while to grind through 50+ databases.
TRAIL (Technical Report Archive and Image Library) is an initiative led by the University of Arizona in collaboration with CRL and other interested agencies to identify, digitize, archive, and provide access to federal technical reports issued prior to 1975
USA.gov (the main US Gov’t portal). Signficant source for elusive, or smaller regional reports. It even gets many state government reports; such as highway engineering manuals, etc.
NOTES ON Searching: try to search 2 or maybe 3 unusual names and title words. The more words you put in, the bigger the chance that one of them is NOT the actual name or title word. (this principle applies in most databases). NTRS and SciTech Connect especially have fairly simple search engines – fancy Boolean seems not to work consistently. USA.gov seems to work the same as Google - use double quotes for phrases, don't use * truncation, etc.