The development process occurs in a stress field. It is expensive, and ultimate success is made uncertain by risks. The process of technological development encompasses everything from the initial idea to the successful serial implementation. The considerations discussed here deal primarily with construction methods, material technologies, and production technologies. Experience has shown that these are related to the greatest risks (Chapter 14.2).
The point of departure should be the state of technology and the known development tendencies (Ill. 14-1). In order to better estimate the investment of time and money, it is helpful to closely examine previous experiences. If technological developments are considered over longer periods, certain laws seem to appear. The customer`s priorities are the driving force behind a development approach. For example, in fighter aircraft engines, thrust/weight ratios are the goal (Ill. 14-4), while civilian applications demand low specific fuel consumption.
The first step is to do foundational work and to estimate feasibility. Then the step must be made to serial implementability. Usually, new “revolutionary” technologies require decades of periodic “run-ups” (Ill.14-5). The development in serial implementation meets the demands of the customer after a typical, time-dependent process. At first, the development steps are relatively large. As technologies mature, the development curve flattens out.
Illustration 14-1: This diagram corresponds to a subjective estimate. It is primarily based on observation of technological developments in professional journals, patents, and conference papers (Refs. 14-1 bis 14-3). The wide variety of developments is impressive, even though there is no claim to completeness. Therefore, one cannot claim that everything has already been developed, and no more major improvements can be expected.
On the contrary, new technologies create new tasks. One example is blisks, which require effective, serially implementable damping measures that must be developed in parallel. The developments
are arranged concentrically in the diagram in a way that represents their time of realization. “Being introduced” means that military serial implementation is underway or planned for the near future. “Medium term” means a probable introduction in about 5 to 10 years after the year 2000. “Long term” refers to 20+ years until serial implementation.
Illustration 14-2: An indicator of the potential of a material technology for engine construction should contain the parameters strength, temperature, and weight. Under this assumption, the progress of the specific strength at the operating temperatures is especially informative. Unfortunately, comparative indications based on dynamic strengths, such as LCF and HCF, are not available.
Specific strength is strength, such as fracture strength, thermal strength, or a creep limit, relative to the specific weight. The result is a value with a length dimension. For fractures, this is called the tension length. This value can be seen as the length at which a wire made from the specific material breaks under its own weight. This characteristic value is especially relevant for parts that are stressed by their own weight. These are usually rotor parts such as disks and blades.
The provided material strengths should only be seen as potentials, and require ideal conditions. Therefore, they are hardly usable in parts.
In fiber-reinforced materials, this usually means a unidirectional bundle with loads in the direction of the fibers. If the values refer to the strength of individual fibers, then these are physical limit values that cannot even be remotely realized in an actual part. Complex structures with cross-section jumps, which are common in engine construction (e.g. blisks), demand multi-layered assemblages with different fiber orientations. In these parts, the unidirectional strength is not even remotely attainable. Constructions with unidirectionally wrapped fiber rings come closer to the ideal, but lose effectiveness if they have to be combined with lower-strength materials (e.g. for introducing force in hybrid constructions).
Ceramics demand suitably low flaw size if they are to attain the specified values. Because of the dependence of strength on the surfaces and volumes (Ill. 14-33), there is a major difference between the strength that is valid in a small specimen volume and that which is valid in a much larger volume of highly stressed engine parts (Ill. 12.3-7).
If the material is damaged by the surrounding conditions that are typical of the specific application, then the given values are only hypothetical, as long as a suitable protection is not implemented. Examples of this include oxidation of carbon fibers or salt corrosion on titanium alloys at temperatures above 500°C (Volume 1, Ill. 22.214.171.124-8).
Illustration 14-3: Experience seems to indicate certain general trends over the temporal progression of a development. Indicators include the complexity (top diagram) and maturity of the technology (bottom diagram). Time intervals, not specific dates, are plotted on the abscissa.
The relative technological effort is understood to be the immediate effort, relative to the total effort from the initial idea to serial implementation.
One can see a variously pronounced wave movement. The time interval between the waves seems to be between 10 and 20 years. One might speculate that there is a connection to the generational changes of the technical decision makers. It is possible that the problems that prevented serial implementation faded and/or new technologies promised a solution.
Selected examples are powder-metallurgical disks, rotor blades made from fiber-reinforced synthetics, and turbine blades made from high-strength monolithic ceramics. Powder-
metallurgically manufactured disks are currently in serial use in various re-formed versions (HIP and forge). The intermediate maximum should be marked by the development of the “As HIP” version (pure isostatic sealing without re-forging), which was viewed very skeptically after a disk failed in an aircraft (Ill. 14-10). The renewed increase in effort is related to the successful implementation of re-forming. This is primarily intended to minimize the probability of dangerous material flaws.
Fan rotor blades made from fiber-reinforced synthetics seemed to be due for serial application in the early 1970s. However, during acceptance runs in the first generation of large fan engines, the bird strike behavior was deemed insufficient. As a result, titnaium blades were reverted to (Ill. 14-14). It was not until the 1990s that serial application of fiber-reinforced synthetic blades was successful. The limiting conditions of the bird strike requirements (e.g. circumferential speed) were were determined (Ill. 14-26), and serial implementation was successful with a corresponding configuration of the blades and optimization of the technology (cross-braces, protective edges).
The development of monolithic ceramic turbine blades is especially impressive. In the early 1940s, a first high point was reached, although it was still far short of serial implementation. The problem was unresolved issues regarding brittleness, which were related to the force introduction and the low minimum strength of the ceramic materials available at that time. The steep learning curve of the simultaneously serially implemented heat-resistant metallic alloys also meant that the new technology did not have any sufficient advantages.
The next intermediate high point was in the early 1960s, when it was hoped that these materials could be used in industrial gas turbines. However, serial implementation was not attained for similar reasons as before. Another intermediate high occurred in the early 1980s, but this time, considerably improved cooling of serially implemented Ni-based blades prevented introduction of the ceramic technology. At this point, the available ceramics were too sensitive to oxidation above
1400 °C, which damaged them so much that they were unable to guarantee the necessary strength levels over the required long life spans. Now, in the first decade of the new century, there seems to be a renewed upswing in development activities. These should be seen in connection with positive long-term experiences in industrial testing applications and the development of short-term miltary applications (unmanned aerial vehicles).
Relative technological maturity is understood to be the state of a technology relative to the demands of serial implementation. This term is only very subjectively definable. Therefore, the first thing to be determined is which measured value(s) is/are to be used. For example, in a turbine blade, should the values be the high-temperature strength or the maximum allowable temperature determined by oxidation and structural stability?
The selected examples are the same as in the case of technological effort. One can see that the maturity and effort progress in phase. While PM disks and fiber-reinforced synthetic fan blades have reached serial implementation, this goal is not yet in sight for ceramic turbine blades.
Illustration 14-4: The evolution of a serial technology follows a typical course. This is also true for development trends that result from the sum and interaction of individual technological developments. At first, in the introduction phase, one can see a fairly flat curve progress, which is determined by observation of operating behavior and gathering of experience. Progress then accelerates. This can be explained by a general acceptance of the technology and a correspondingly greater effort towards further improvements. At the end of the trend, the curve flattens out again. The technologies are largely exhausted, and further improvements demand a steeply increasing effort.
In this case, the example is the trend of fighter engines, which is marked by the desire for high thrust/weight ratios (Ill. 11.1-6).