RESEARCH OBJECTIVE: Initial design studies for the HL-20 lifting body assumed that mission aborts during the launch phase would be terminated by splashdown in the Atlantic. Recent improvements in vehicle aerodynamic efficiency and launch escape adapter performance have raised the possibility of returning to a horizontal landing at the launch site following an emergency abort, including launch pad aborts prior to liftoff. The objective of this study was to investigate the feasibility of performing return-to-launch-site aborts for various launch phases, and to identify optimal strategies which maximize the window for return-to-launch-site aborts.
APPROACH: The analysis is based upon a preliminary version of the HL-20 lifting body with a Titan IV booster and was conducted using both analytical tools and piloted simulations. A model of the Launch Escape System, utilizing small vectorable solid rocket abort motors to provide an 8g acceleration away from the booster, was developed. Maximum performance steady-state gliding turns were identified both theoretically and numerically. Various on-pad abort maneuvers from two different launch sites to two different landing surfaces were explored in the piloted simulation and further refined using analytical tools. The effect of parametric variations in abort motor performance, vehicle weight, vehicle lift-to-drag ratio (L/D), and steady launch site winds were evaluated in the piloted as well as analytical simulations. Refinements to the automatic and manual control laws were made. Worst-case abort geometries (pad to runway) were identified.
ACCOMPLISHMENTS: Theoretical investigations of the kinematics of steep gliding turns were made, and a set of equations giving optimum bank angle as a function of drag polar, velocity, altitude, and wing loading were derived. Several candidate maneuvers for accomplishing a launch site abort were developed and investigated, both with and without steerable abort motor rockets. Sensitivity of the planar (no turn) abort maneuver to variations in abort motor performance, wing loading, and vehicle L/D was determined. An abort motor steering schedule was developed to maximize abort performance. Modifications to baseline control laws to assist in performing abort maneuvers were identified and implemented. Six different abort geometries (from three launch pads to two runways) were investigated; abort maneuvers for a final set of four geometries (launch pads 39A & 40 to Kennedy runway 33 and Cape Canaveral runway 13) were developed and abort motor thrust levels and steering inputs were identified. Both manual and automatic aborts for each of these geometries in worst-case wind conditions were demonstrated and documented (see attached figure). This study clearly establishes feasibility of HL-20 return-to-launch-site aborts through use of small, vectorable solid rocket motors.
SIGNIFICANCE: The capability of a "dry" abort makes the HL-20 a more attractive candidate for the Personnel Launch System. Initial studies of the lifting body assumed a "wet" abort in all launch phases; this study has demonstrated the feasibility of a "dry" abort option, with concomitant increases in safety and cost savings to the HL-20 concept.
