During the Second World War, a great many rockets were made and used operationally by both sides. By the end of the conflict, a pattern had begun to emerge. For applications where burning times were short and performance unimportant when compared with the need for simplicity, solid propellant rockets were used exclusively. Where control of thrust and the ability to start and stop repeatedly were involved, as in aircraft propulsion, or where performance was vital, as in the V2 ballistic missile, liquid propellants were applied. Since the war, this general pattern has persisted. There has been a progressive improvement in the performance of both types. At the same time, the thrust and total impulse of rockets have risen, until engines producing over one million pounds of thrust are envisaged.