To provide stable and efficient thrust, a liquid rocket must maintain high pressure in its combustion chamber, requiring a great deal of force to keep propellants flowing in against the pressure. At the same time, the entire system, from fuel tanks to exhaust nozzle, must obey a “pressure budget” in which flow is always from higher to lower pressure. Even a momentary lapse of pressure can cause hot combustion products to flow the wrong way through the system, which is bad. Over the years, a wide variety of engine power cycles have been used to meet these challenges, each adapted to the specific needs of the rockets they power.