Technology Evaluation Via Loss Management Models Formulated in Terms of Vehicle Weight

Abstract

Mass properties engineering is today an established field and an indispensable part of the design process. Detailed bookkeeping schemes have been developed to track constituent component weights in extreme detail, down to the last rib and rivet. Given this situation, it may be more accurate to refer to this field as empty weights engineering because the focus has always been primarily on management and tracking of vehicle empty weight. Meanwhile, one of the largest weight fractions, fuel weight, is bookkept in a single lump and largely ignored (except inasmuch as it impacts vehicle size and growth factor). It is intuitively obvious that the aerothermodynamic losses due to the engine, airframe systems, and aerodynamic drag of the vehicle are the fundamental drivers on fuel weight and should therefore be expressible as increments in fuel weight chargeable to each loss mechanism. The sum of all chargeable fuel weights is equal to the total fuel weight required to complete a prescribed mission. The intent of this paper is to formulate a method for quantifying thermodynamic performance in terms of mission fuel chargeable to each thermodynamic loss mechanism. This is then used in conjunction with known vehicle zero fuel weight groups to estimate the gross weight chargeable to each functional component of the vehicle. The results show that chargeable vehicle gross weight can be used as a common figure of merit linking mass properties and performance aspects of vehicle design. This method is then demonstrated for a Northrop F-5E aircraft, and the fuel weight breakdown is analytically calculated for the design mission. The results of this analysis show that 37.3% of the F-5E subsonic mission fuel requirement is due to propulsion system losses, 36.8% is chargeable to aerodynamic drag, and 24.3% is chargeable to vehicle empty weight. This translates into a chargeable fuel cost of roughly $173.90, $171.76, and $113.53 for each of these three loss mechanisms, respectively. Finally, the usefulness of this technique as a means of technology evaluation is considered. The strengths of this method are that it allows quantification of both weight and performance aspects of technology benefits in a single figure of merit, and also enables one to ascertain the benefits of individual technologies even when applied as part of a suite of technologies.