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When a large rocket takes off, it starts burning fuel at an incredibly fast rate. This means that every second, a few hundred kilos are being used up. Rockets like the space shuttle only keep getting lighter as they climb, as they shed the excess fuel tanks the second they are empty. The shuttle (with its boosters) weighed over 2 million kg (4.4 million pounds)  before takeoff, and only 80,000 kg (165,000 pounds) when it was empty.