The NASA Standard Spacecraft Computer-1 (NSSC-1) was a computer developed as a standard component for the MultiMission Modular Spacecraft at the Goddard Space Flight Center (GSFC) in 1974. The basic spacecraft was built of standardized components and modules, for cost reduction. The computer had 18 bits of core memory or plated wire memory; up to 64 k. 18 bits was chosen because it gave more accuracy (x4) for data over a 16 bit machine. Floating point was not supported.
The NSSC-1 was used on the
and other missions that were mostly limited to the solar system.
The hardware was developed by Westinghouse and GSFC. The machine used diode-transistor logic, the lowest power parts available at the time on the preferred parts list; initially fabricated from 1700 SSI (NOR gate) packages, it was later moved to 69 MSI (medium-scale integration) chips. 
Programming and support
The NSSC-1 had an assembler/loader/simulator toolset hosted on Xerox XDS 930 (24- bit) mainframe. An associated simulator ran at 1/1000 of real time. The Xerox computer was interfaced to a breadboard OBP in a rack. (Which, of course, operated at room temperature ambient conditions). Later, the Software Development and Validation Facility (SDVF) added a flight dynamics simulator hosted on a PDP-11/70 minicomputer. 
A purpose-built NSSC-1 Flight Executive was developed and used on the Solar Maximum Mission (SMM) and subsequent flights. It time sliced tasks at 25 ms. It included a stored command processor that handled both absolute time and relative time commands. It included a status buffer that could be telemetered back to the ground. It required a lot of memory, typically more than half of that available, leaving the rest for applications and spare.
In the 1980s the RCA 1802 was used for many missions -- like Galileo. This mission and other missions started the trend away from custom built NASA CPUs in spacecraft. The exploration of the inner and outer parts of the solar system would have to be done with existing (civilian and military-aerospace) CPUs.
Before the RAD family of 32 bit CPUs were used in space missions, the MIL-STD-1750A (a CPU that could run modern applications) saw substantial use.
Since the arrival of the IBM RAD6000 in the 2000s and the RAD750 in the 2010s, using the NSSC-1 has become unthinkable. Its computing power was not great, and most modern space missions require flight computers to have substantial and substantive computing power.
- Trevathan, Charles E., Taylor, Thomas D., Hartenstein, Raymond G., Merwarth, Ann C., and Stewart, William N. "Development and Application of NASA’s First Standard Spacecraft Computer," CACM V27 n9, Sept 1984, pp. 902–913
- Styles, F., Taylor, T., Tharpe, M. and Trevathan, C. “A General-Purpose On-Board Processor for Scientific Spacecraft,” NASA/GSFC, X-562-67-202, July 1967.
- Stakem, Patrick H. The History of Spacecraft Computers from the V-2 to the Space Station, 2010, PRB Publishing, ASIN B004L626U6