Airships offer unique design challenges compared to conventional aerospace vehicles. During cruise flight modes, Airships can obtain dynamic lift by either envelope design or by adjusting pitch to compensate for either negative or positive buoyancy. However, when hovering the vehicle must have a propulsion system that can compensate for changes in buoyancy, such as consumed fuel. Release of ballast or lifting gas are options to adjust vehicle buoyancy, but often not desirable. To provide additional vertical thrust, the propulsion system must have additional engines not used for cruise propulsion, or the ability to redirect existing thrust. Both options add expense and complexity to the design. The use of a "hybrid" power generation system comprised of multiple power conversion processes and fuels is treated in a generalized approach. An example system using gaseous hydrogen stored in a gas bag within a helium-filled airship envelope, with pressurized liquid ammonia in tanks, is examined. Expressions are derived for the ratio of ammonia internal combustion engine to hydrogen fuel cell power to maintain neutral buoyancy of the airship considering the consumption of fuel, and the collection of by-product water. Two other cases, analyzing jet-fuel supplied turbo-propulsion engines with gaseous hydrogen and ammonia were also studied. This paper develops and applies a generalized parametric graph-theory-based energy model for the conversion between chemical to mechanical or electrical energy domains. This methodology is suitable to other applications as a tool for system-level conceptual design and requirements definition. Additionally, the relationships for power generation and distribution to vehicle systems are suitable for optimization methods. This analysis was performed in the context of a large commercial cargo airship concept, which was required to hover while loading and unloading 160 tons of cargo, and could vary in buoyancy up to 50 tons due to fuel burn alone.
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