The RB545, which was given the name "Swallow" by its manufacturer, British engine maker Rolls-Royce, was an air-breathing rocket engine. It would have functioned as an integrated dual-role powerplant, having been capable of air-breathing while operating within the atmosphere and operating in a similar manner to that of a rocket when having attained close to and within LEO. This engine would have also been capable of powering the spacecraft to hypersonic speeds. It was a crucial element of the programme, having been publicly attributed as "the heart of Hotol's very low launch costs".