94 Dossier | Aant Farm TPR72 Heat recuperator As mentioned, Aant Farm’s recuperator design has been inspired largely by a recuperator that was designed and developed for a previous engine aimed at the IARPA Great Horned Owl programme. The system has a multi-pass configuration, in which the outgoing air from the compressor exit makes two passes through the exhaust flow before entering the burner, first from front to back through the outer tube bank, then from back to front through the inner bank. “To house the tubes, we’ve used our in-house 100-ton press for compoundforming the tank pressure heads, mainly via rubber pad and matched die forming,” Seegers explains. “Those parts are then TIG-welded to the core header plates to yield a complete assembly.” Throughout the recuperator’s design process, thermal efficiency has been prioritised over strength-to-weight ratio, as the design is already intrinsically stiff. Seegers notes however that a more applicable parameter than strengthto-weight might be “ratio of weight to effectiveness”. In this context, ‘effectiveness’ is used to refer to the ratio of actual heat transferred from the hot exhaust gases flowing about the recuperator microtubes to the intake air inside of them, to what is theoretically possible with the given high and low temperatures. High effectiveness hence means more heat transferred and therefore better efficiency. However, as a heat recuperator becomes more effective, its mass tends to increase exponentially. For example, increasing a plate-fin recuperator’s effectiveness from 85% to 90% could double or triple its total mass. “There’s a calculation known as the Breguet Range Equation, which essentially uses an aircraft’s mass, its lift-to-drag ratio, its fuel load and the efficiency of its powerplant to calculate range,” Seegers explains. “The equation can therefore be used to evaluate the merit of additional tube mass in a recuperator versus the resultant reduced fuel burn. We looked at several mission profiles and chose our effectiveness based on lower tube mass. “So if we make the recuperator a little less effective, maybe it consumes 7- 8 kg more fuel over a given mission, but surplus fuel weight burns off during the mission. Excess recuperator mass on the other hand does not – in fact it compounds the weight problem by August/September 2023 | Uncrewed Systems Technology After exiting the gas generator turbine rotor, the hot gas then enters the power turbine nozzle and rotor, where it expands further to power the gearbox, or in the case of the TA65-1, the main alternator To house the tubes we’ve used our inhouse 100-ton press for compounding the tank pressure heads, mainly via rubber pad and matched die forming
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