Within the atmosphere, air is taken in through two vertically mounted intake ramps, then the flow would be split, passing the correct amount to the pre-coolers, and the excess to spill ducts. Hydrogen from the fuel tanks would be passed through two heat exchangers to pre-cool the air prior to entering a high overall pressure-ratio turbojet-like engine cycle — the heated hydrogen driving a turbine to compress and feed the cooled air into the rocket engine, where it was combusted with some of the hydrogen used to cool the air. The majority of the remaining hot hydrogen was released from the back of the engine, with a small amount drawn off to reheat the air in the spill ducts in a ramjet arrangement to produce "negative intake momentum drag." These ramjets were typically depicted as two glowing red circles below the rocket engines in pictures of HOTOL.