Some of the key challenges of planetary entry are to dissipate the large kinetic energy of the entry vehicle and to land with precision. Past missions to Mars were based on unguided entry, where entry vehicles carried payloads of less than 0.6 T and landed within 100 km of the designated target. The Mars Science Laboratory (MSL) is expected to carry a mass of almost 1 T to within 20 km of the target site. Guided lifting entry is needed to meet these higher deceleration and targeting demands. If the aerodynamic characteristics of the decelerator are variable during flight, more trajectory options are possible, and can be tailored to specific mission requirements. In addition to the entry trajectory modulation, having variable aerodynamic properties will also favor maneuvering of the vehicle prior to descent. For proper supersonic parachute deployment, the vehicle needs to turn to a lower angle of attack.

One approach to entry trajectory improvement and angle of attack control is to embed a variable geometry decelerator in the design of the vehicle. Variation in geometry enables the vehicle to adjust its aerodynamic performance continuously without additional fuel cost because only electric power is needed for actuating the mechanisms that control the shape change. Novel structural and control concepts have been developed that enable the decelerator to undergo variation in geometry.

Changing the aerodynamic characteristics of a flight vehicle by active means can potentially provide a mechanically simple, affordable, and enabling solution for entry, descent, and landing across a wide range of mission types, sample capture and return, and reentry to Earth, Titan, Venus, or Mars. Unguided ballistic entry is not sufficient to meet this more stringent deceleration, heating, and targeting demands.

Two structural concepts for implementing the cone angle variation, a segmented shell, and a corrugated shell, have been presented. It is possible that a multiparameter optimization approach will be necessary to fully explore the potential of the proposed solution. Since the shape of corrugated shell deviates from the conventional sphere-cone decelerator, the variation of aerodynamic characteristics with cone angle obtained is an approximation to that of the corrugated shell decelerator. A more precise numerical computation of the pressure distribution on the corrugated shell surface using panel method is currently underway. This numerical procedure will be incorporated into the trajectory simulation and the structural analysis. Further work will include tuning the current corrugated shell geometry using an energy-based optimization approach to minimize stress and actuation force, and exploring trajectory modulation with decelerators undergoing asymmetric variation in geometry.

Variations in cone angle for a decelerator with sphere-cone geometry have the effect of altering the trim angle of attack and the corresponding lift-to-drag ratio and ballistic coefficients during flight. This capability enables trajectory optimization with fewer aerodynamic constraints. A trajectory simulation with variable aerodynamic characteristics demonstrated a reduced deceleration peak and improved landing accuracy. The analytic expressions of the longitudinal aerodynamic coefficients were derived, and guidance laws that track reference heat flux, drag, and aerodynamic acceleration loads are also proposed. These guidance laws, based on dynamic inversion, have been tested in an integrated simulation environment, and the results indicate that use of variable geometry is feasible to track specific profiles of dynamic and heat load conditions during reentry.

The proposed concept of a decelerator system that is first deployed and then is able to adaptively change its geometry during operation is novel and is expected to lead to reductions of drag up to 20 percent and peak temperature by 20 percent, thus obviating the need for both expensive thermal protection systems and heavy expelled mass ballast to change the aerodynamic configuration of the vehicle.

This work was done by Marco B. Quadrelli, Sergio Pellegrino, and Kawai Kwok of Caltech for NASA’s Jet Propulsion Laboratory. NPO-47102

This Brief includes a Technical Support Package (TSP).
Combined Structural and Trajectory Control of Variable-Geometry Planetary Entry Systems

(reference NPO-47102) is currently available for download from the TSP library.

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