FL-T400 Fuel Tank
From Kerbal Space Program Wiki
The FL-T500 Fuel Tank provides fuel to an LV-T30 Liquid Fuel Engine attached beneath it.
usage=The FL-T500 Fuel Tank is currently the only Liquid Fuel tank and the only way to power the LV-T30 Liquid Fuel Engine. It provides thrust for a short duration that may be extended by stacking multiple fuel tanks on top of each other. It can not be attached to radial decouplers.