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Meeting	ACGS Committee Meeting 108 - Gettysburg - October 2011
Agenda Location	10 SUBCOMMITTEE D – DYNAMICS, COMPUTATIONS, AND ANALYSIS 10.3 High Altitude Airship Research
Title	High Altitude Airship Research
Presenter	Thomas Myers
Affiliation	Systems Technology, Inc.
Available Downloads*	presentation
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Abstract	High Altitude Airship Research ACGS Gettysburg October 2011 Tom Myers, Systems Technology, Inc. Systems Technology, Inc. recently completed a Phase II SBIR for the Missile Defense Agency (MDA) dealing with control and guidance for the High Altitude Airship (HAATM) being developed by the Lockheed Martin Company. Airships differ from aircraft in that buoyancy (aerostatic) and apparent mass (unsteady aerodynamic) effects are significant only for airships. The mathematical representation of the apparent mass effects is embodied in Kirchhoff’s equations in which the apparent mass terms appear as tensors. One consequence is that it’s convenient to formulate the equations of motion in a body axis system with origin at the hull center as opposed to the center of mass. The possibility that maintaining the hull pressure difference at a fixed constant value during descent could reduce fatigue in the hull material led to a study of decent control laws to accomplish this. A simple analytical solution was compared to an optimal trajectory solution from direct collocation optimization. The analytical solution assumed a constant blower volumetric flow rate with airspeed variation as the control variable whereas the optimal (minimum time) solution used blower flow rate and airspeed as controls. It was found that the analytical solution could be extended to give a result comparable to the optimal solution without the need to run the trajectory optimization algorithm in flight. High-altitude stationkeeping is a major problem for the HAA due to limited maximum airspeed. Thus three control laws were considered for Post Thrust Limit (PTL) operation in which the wind speeds can exceed the airspeed. The first two control laws involve minimum displacement and minimum groundspeed respectively and the third is an extension of the minimum groundspeed law which returns the airship to station after the high wind event has passsed. Comparison of the three control laws shows that the third performs better than the first two. However, examination of actual wind data in the lower stratosphere revealed an annual high wind event called the “polar vortex breakdown” which can result in winds in excess of 40 m/s. In these events good stationkeeping is almost certainly impractical regardless of the stationkeeping control laws. However, these events can be predicted accurately with numerical weather prediction so that the vehicle can be moved to a safe haven before the event.