SUPERSONIC JET FUELS – QUALITY AND COST FACTORS 610498
The high Mach number supersonic bomber and its commercial transport version represent a challenge not only to designers but to fuel suppliers. Opinions differ on the relative importance of speed, structure and power plant but most designers are agreed that, compared to today's jet aircraft, the supersonic jet is far more fuel dependent in terms of both quality and cost. Fuel represents about 50% of gross take-off weight and accounts for over 50% of direct operating costs in the supersonic transport. The higher fuel consumption of supersonic jet engines means that the SST consumes more fuel to move the same traffic over a given route than the subsonic jet. As a consequence, a significantly greater demand for jet fuel will exist if some portion of world traffic moves supersonically rather than by conventional aircraft. Economic analysis indicates that fuel costs significantly greater than current fuel might wipe out the incentive for airlines to acquire a SST instead of a subsonic jet.
In a normal supersonic flight pattern, the fuel would be subject to increasing temperatures, not only in the wing and fuselage tanks but by heat absorption from the airframe and engine as it passed through the fuel system into the nozzle. Its temperature profile makes the volatility and thermal stability of the fuel prime quality factors. In addition, fuel should be high in density to accommodate the maximum energy in limited tank volume, exhibit good low temperature properties to insure satisfactory subsonic operation, include maximum BTU/pound to extend range, display good combustion characteristics in the engine and high cooling capacity to minimize insulation, heat exchange and refrigeration. Finally, supersonic fuel must be handled with pharmaceutical cleanliness to insure freedom from contamination that would adversely affect thermal stability. All of these features come at a price.
A low volatility fuel is preferable to minimize structural weight of pressure tankage but yields less from crude than a wide cut fuel like JP-4. However, since the future demand for supersonic fuel relative to the total demand for distillates is small, selection of a narrow cut, low volatility fraction is not likely to become a major cost factor.
Mechanism studies show that the thermal stability of most hydrocarbons is satisfactory, but the presence of trace quantities of sulfur, nitrogen and oxygen containing compounds is generally responsible for deposit forming reactions. As measured by the high temperature Research Coker, hydrogenation is a more effective process than extraction and certain additives are effective alternatives or supplements to processing steps to up-grade thermal stability. The cost of supersonic fuel rises sharply with severe processing but there is hope for a reasonable cost quality improvement program by a combination of feed stock selection, processing and additives.
Naphthenic fuels are the best compromise between the requirement for high energy content and the need for high density. High density fuels can be made either by selecting special crudes or hydrogenating aromatic extracts, but either route is expensive.
Paraffinic fuels exhibit about 20% greater cooling capacity than high density fuels and show the highest heat content per unit weight. Their generally superior combustion characteristics--as measured by the Luminometer rating--makes them preferred fuels for the engine. It is easier and cheaper to secure high Luminometer ratings by increasing volatility than by removing the nonparaffinic components from a kerosene cut.
Low temperature flow characteristics can be achieved either by blending or dewaxing to remove paraffins. Properties equivalent to current commercial kerosenes are sought in supersonic fuel. Costly processing may be required.
General considerations suggest that the best crude sources for supersonic fuels are likely to be paraffinic types found in the Middle East, North Africa and Indonesia. Nevertheless, since the demands for jet fuels relative to middle distillates and gasolines are small, availability of feed stocks appears adequate.
It is concluded that quality features needed in a supersonic fuel will tend to make it more expensive than current fuels, but it is likely that this cost can be kept within the desired economic boundaries by design compromises in the airframe such as insulation to reduce thermal stress, supplementary coolers to reduce the fuel heat sink load and the use of inerted and pressurized tanks.