The paper discusses some major fuel-related issues which will influence the development of aviation over the next 50 years. Provided that global economic development is not halted by worldscale war or crisis, the demand for civil air transport is likely to continue to expand. The consequent rising requirement for aviation fuel is considered in relation to the projection that the total oil extraction rate from relatively accessible fields will reach a peak and then decline. The fuel options for aviation, against such a scenario, are explored. It is concluded that if the aviation demand cannot be met economically from conventional oilfield sources, supplementation by kerosene-like fuel synthesized from other feedstocks is much more likely, in the timeframe considered, than the radical step of a move to liquid hydrogen. The latter could be delayed until much later unless necessitated by constraints on carbon emissions. Other issues relating to aero-engine emissions are discussed, including the present uncertainties regarding the environmental significance of injection into the atmosphere at aircraft operating altitudes. The implications of restricting operations to the troposphere, to avoid the lower mixing rates of the stratosphere, are considered briefly.
When discarded from bottom trawl fisheries, survival of Nephrops norvegicus may be sufficiently high that this species can be exempted from the EU Landing Obligation. In three studies, Nephrops were sampled from trawlers in northern European waters, and the fate of individuals monitored for a minimum of 13 days in onshore tanks. Winter estimates of captive survival (means ± 95% confidence intervals), including immediate mortality during catch sorting, were 62 ± 2.8% for the West of Scotland, 57 ± 1.8% for the Farne Deeps (North Sea), and 67 ± 5.4% for the Skagerrak. The Farne Deeps fishery is not active in summer, but captive survival rates in summer in the other two areas were reduced to 47 ± 3.4% for West of Scotland and 40 ± 4.8% for the Skagerrak. Linear modelling of the West of Scotland and Skagerrak data suggested that higher survivals in winter were related to colder water or air temperatures, although temperatures during captive observation may also have had an impact. Net modifications in the Skagerrak study had an effect on survival, which was higher for Nephrops sampled from nets equipped with the more selective Swedish sorting grid compared to Seltra trawls.
This paper presents some preliminary results of twodimensional cutting tests of dry limestone samples at atmospheric pressure. Cutting tips having rake angles of + 30°, + 15°, 0°, -15° and -30° were used to make cuts on Leuders limestone samples at six depths of cut ranging from .005 to .060 in. at cutting speeds of 15, 50, 109 and 150 ft/min. The vertical and horizontal force components on the cutting tips were recorded with an oscilloscope equipped with a polaroid camera. Motion pictures of the cutting process at camera speeds of 5,000 to 8,000 frames/sec were taken at strategic points in the variable ranges.The movies provide considerable insight into the brittle failure mechanism in rocks. It appears that chipgenerating cracks usually have an initial orientation which is related to the resultant of the externally applied forces. The latter part of the crack curves upward toward the free surface being cut, this part being governed by some type of cantilever bending or prying. The linear and angular motion of the loosened chips also indicate the tensile nature of brittle failure. Analyses of the forces on the cutting tips indicate that: (1) relatively small increases in vertical loading result in large cut-depth increases for sharp tips (rake angles > 0°); (2) tool forces increase at an increasing rate as t~ rake angle decreases, particularly for rake angles < 0°; and (3), for the range of this study, rate of loading had little effect on the maximum forces. Both the movies and visual inspection of the cuttings indicated that the volume of rock removed by chipping was much larger than that by any grinding mechanism, even for tips having negative rake angles. Cutting size increases with increased cut depth and rake angles, and decreases slightly at high cutting speeds, the depth of cut having by far the most influence. The amount of contact between the rock and the cutting tip was always less than the depth of cut and rarely exceeded 0.010 in. even for cuts of 0.060 in.
The prospects for the longer-term evolution of aircraft and their propulsion systems are explored, taking a fifty year time-frame. It is considered that the most likely global scenario features continued economic development, aided by the introduction of fuel from alternative sources to offset a progressive shortfall in the availability of oil from relatively accessible fields. Pollution and other environmental problems are foreseen as exerting an increasing influence on decision making. Assuming such a scenario, the aviation fuel prospect is considered. It is concluded that the most likely alternative fuel for aviation in the next fifty years is some form of manufactured liquid hydrocarbon resembling current aviation kerosine, rather than liquid hydrogen, which is a candidate for later introduction. An assessment is offered of the classes of flight vehicle that will be required for civil and military purposes in the time-frame under review. The implications with regard to propulsion systems are then considered, with a discussion of design objectives, technological challenges and research requirements.
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