An adapter is a part which allows to place multiple stacks of parts below a single stack.
With version 0.21 there are twelve different types of adapters:
Six one-to-one adapters, four connecting a tiny size to a small size, and two connecting a small size to a large size.
- FL-A5 Adapter: This is a steep sloping adapter connecting tiny to small
- FL-A10 Adapter: This is a smooth sloping adapter connecting tiny to small
- NCS Adapter: This is a tall adapter connecting tiny to small
- Modular Girder Adapter: This it a strut adapter connecting tiny to small
- Rockomax Brand Adapter: A larger version of the FL-A10 Adapter, connects small to large
- Rockomax Brand Adapter 02: A larger version of the FL-A5 Adapter, connects large to small
Three one-to multiple adaptors, connecting one small to several small parts.
- TVR-200 Stack Bi-Coupler: Connects one-to-two.
- TVR-1180C Mk1 Stack Tri-Coupler: Connects one-to-three, this was the first adapter ever implemented into the game.
- TVR-2160C Mk2 Stack Quad-Coupler: Connects one-to-four.
And three equivalents of the previous ones, each connecting small to large.
- TVR-200L Stack Bi-Adapter: The equivalent of the Bi-Coupler.
- TVR-300L Stack Tri-Adapter: The equivalent of the Tri-Coupler.
- TVR-400L Stack Quad-Adapter: The equivalent of the Quad-Coupler.
All of these parts are Fuel Crossfeed Capable, meaning that when there is a fuel tank and an engine both connected to the adaptor on different sides, propellents, liquid fuel and oxidizer, will flow through the adapter on the way to the engine.
The branches are unsupported and prone to bending or breaking if the weight and thrust are not properly distributed. Rockets incorporating an adapter are also likely to wobble, unless the stacks are opportunely bound together with struts.
The main uses of these adaptors include connecting a small payload, such as a space probe, to a large launcher, or using several small engines on the bottom of a large fuel tank to increase efficiency. If a one-to-multible adaptor is on an upper stage which has engines underneath it, adding decouplers at the bottom of the engines and an upside-down one-to multiple adapter can allow for a large launcher underneath a powerful interplanetary vehicle.
Another, unconventional use of those parts is as a landing 'foot' when landing very large craft (such as bases). The stock landing legs aren't big or powerful, even though in version 0.22.0 they were re-modeled to include pistons for making a landed craft more stable. Using the strut parts as 'legs' and an adaptor such as the Tri-Coupler as a 'foot', it can artificially make its own landing leg. Although, it will be mostly useless on sloped terrain, since it, unlike the landing legs, wouldn't have any pistons for keeping it from falling over on a slope.