6-DEGREE-OF-FREEDOM GUIDANCE AND CONTROL-ENTRY ANALYSIS OF THE HL-20

The ability of the HL-20 lifting body to fly has been evaluated for an automated (no pilot inputs) entry from atmospheric interface to landi ng. This evaluation was required to demonstrate that not only successf ul touchdown conditions would be possible for this low lift-to-drag-ra tio vehicle (congruent-to 4), but also the vehicle would not exceed it s design dynamic pressure limit of 400 psf during entry. This dynamic pressure constraint limit, coupled with limited available pitch-contro l authority at low supersonic speeds, restricts the available maneuver ing capability for the HL-20 to acquire the runway. One result of this analysis was that this restrictive maneuvering capability does not al low the use of a model-following atmospheric entry-guidance algorithm, such as that used by the Space Shuttle, but instead requires a more a daptable guidance algorithm. Therefore, for this analysis, a predictor -corrector guidance algorithm was developed that would provide success ful touchdown conditions while not violating the dynamic pressure cons traint. A flight-control system was designed and incorporated, along w ith the predictor-corrector guidance algorithm, into a six-degree-of-f reedom simulation. This simulation was tested under many combinations of off-nominal atmospheric density profiles and winds and showed that the HL-20 remained controllable. This simulation also indicated that t he HL-20 could reach the landing site and execute a successful landing under all off-nominal conditions simulated.