Reaction engines are propulsion systems that generate thrust by expelling mass in the opposite direction. This principle is based on Newton's third law of motion, which states that for every action, there is an equal and opposite reaction. These engines are commonly used in various applications, including rockets and jet engines, to propel vehicles through air and space. One notable example of a reaction engine is the SABRE engine, developed by Reaction Engines Limited. This innovative engine combines elements of both jet and rocket technology, allowing it to operate efficiently at different altitudes and speeds. The SABRE engine aims to revolutionize air and space travel by enabling faster and more cost-effective access to orbit.