The gradual evolution of a constrained upward-tending knowledge search and acquisition process over decades has been described as a technological trajectory (Perez-Perez 1983; Freeman 1989). We would modify the definition slightly. For us, a technological trajectory is a sequence of developments starting from a distinct functional configuration utilizing a basic principle. For instance, the 'atmospheric' reciprocating steam engine beginning with Newcomen can be regarded as the starting point of a trajectory. The trajectory changed direction and was accelerated by James Watt's condensing engine. This was followed by his double-acting valve system, the 'sun and planet' gearing, and the crank and flywheel scheme for converting reciprocating motion into rotary motion. Trevithick's and Evans' high pressure engines (circa 1800), the double and triple compound engines and other later innovations, such as the monotube boiler, continued the same basic trajectory by making reciprocating steam engines bigger, more efficient and more powerful.
A new trajectory arguably started with Charles Parson's steam turbine (1884). This was followed by de Laval's innovation (high speed helical gear, 1890), which facilitated applications at low speeds, and Curtiss' velocity compounding (1898), which permitted still smaller sizes (Forbes and Dijksterhuis 1963, p. 462). The internal combustion piston engine of Nikolaus Otto started a different trajectory, as did the gas turbine.
Similarly, the whale oil lamp and the kerosine lamp were arguably a continuation of the prior trajectory (open flames) that went back to torches in pre-Roman times. The gas light started a new trajectory early in the 19th century. The electric arc light began another new trajectory, replacing gas light. That trajectory was accelerated by the advent of incandescent lamps, fluorescent lights, and so on. The newer light-emitting diodes (LEDs) appear to be the natural end of the sequence.
In this case, and many others, the performance and efficiency of the new technology increased dramatically along the trajectory, from first introduction to maturity - and presumably to eventual phase-out and replacement. In the case of electric power generation by steam turbines the efficiency gain from 1900 to 1970 was a factor of ten (from 3.5 to 35 percent). In fact, the rate of progress along an established trajectory is relatively predictable, at least for some time. As noted already in Chapter 1, Section 1.5, performance improvement along a trajectory is partly the result of learning (or experience) and partly due to the level of continuing R&D investment. The latter (in the private sector, at least) is likely to be dependent on the recent rates of return on R&D (Foster 1986; Mansfield 1965; Mansfield et al. 1977).
A pattern of crisis-driven innovation has recurred a number of times. The crisis may arise because of war, blockade, resource scarcity or even from the extraordinary success of a new technology. The latter may result in an imbalance between supply of some essential component and demand for the service or 'functionality' of the technology. Or a crisis may arise when increasing demand for a product or service cannot be met by the older technology due to the approach of a physical limit. Any of these cases can be regarded as a barrier. It is worthwhile giving examples of each as a way of introducing a general pattern.
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