The adapter is an obsolete part. It has a trapezoid shape. The original purpose is to connect Small Fuel Tanks to Large Fuel Tanks. Adapters weren't required to connect them, but reduced aerodynamic drag.
The adapter was removed from SFS in version 1.4, which instead made fuel tanks automatically adapt to a slightly wider or thinner width fuel tank. Here is a list of fuel tanks that can adapt to one another.
- 4 wide from 4 to 8 wide
- 6 wide from 4 to 10 wide
- 8 wide from 4 to 12 wide
- 10 wide from 6 to 12 wide
- 12 wide from 8 to 12 wide
Parachute bases are 1.6 squares wide, but fuel tanks adapt to them as if they're 4. Tanks can be made effectively 1.6-wide by putting parachutes on both ends, and docking ports part-clipped inside the tanks can allow them to be detached.
Fuel tanks adapt to separators they're adjacent to, if their centers are aligned, as if the separator is a fuel tank, but separators create fairings to center-aligned fuel tanks above them (or otherwise in the direction they're pointing in) with gaps between them, according to the table listed in Separator.
Trivia
- Before version 1.35, the adapter could not transfer or hold any fuel, effectively separating the fuel tanks adjacent to it.