A bootstrap is a self-generating or self-sustaining process. The term comes from the notion of a self-made person pulling themselves up by their own bootstraps. It is used widely in a number of fields of science and technology. The term "booting a computer" (i.e., starting up the system by loading some initial instructions) goes back to the early days of electronic computing when a short length of punched paper tape was generated, which was used to load an instruction into the computer to load, in turn, the operating system (also usually from a paper tape reader). The tape resembled the fabric or leather straps on boots which were used to "pull one's boots on". It followed that a computer that used a strip of paper tape to initiate the reading of a reel of paper tape was pulling itself up by its own bootstraps. In modern computer parlance, a bootstrap is a short computer program that is permanently resident or easily loaded into a computer and whose execution brings a larger program, such an operating system or its loader, into memory.
In rocketry, the term bootstrap is used in connection with liquid-fueled rocket engines in which, during main-stage operation, the gas generator is fed by the main propellants pumped by the turbopump. The turbopump in turn is driven by hot gases from the gas generator system. Such a system must be initiated by a starting system which supplies outside power or propellants. When rocket-engine operation is no longer dependent on outside power or propellants it is said to be in bootstrap operation.
[Thanks to Charles Watson for contributions to this entry.]
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