An Analytical Explanation for the X-43A Flush Air Data Sensing System Pressure Mismatch between Flight and Theory

Abstract

Following the successful Mach 7 flight test of the X-43A, unexpectedly low pressures were measured by the aft set of the onboard Flush Air Data Sensing System's pressure ports. These in-flight aft port readings were significantly lower below Mach 3.5 than was predicted by theory. The same lower readings were also seen in the Mach 10 flight of the X-43A and in wind-tunnel data. The pre-flight predictions were developed based on 2-dimensional wedge flow, which fails to predict some of the significant 3-dimensional flow features in this geometry at lower Mach numbers. Using Volterra's solution to the wave equation as a starting point, a three-dimensional finite wedge approximation to flow over the X-43A forebody is presented. The surface pressures from this approximation compare favorably with the measured wind tunnel and flight data at speeds of Mach 2.5 and 3. I Introductionhe Hyper-X program 1 was initially created to study various aspects of hypersonic air breathing propulsion and the initial series of flight vehicles were given an X-plane designation, X-43A. Three flight test vehicles were built and vehicles 2 and 3 flew successfully, 2,3 with top speeds of Mach 7 and Mach 10 respectively. Four flush mounted ports collected surface pressure data in flight for use in onboard angle of attack calculations. Pressures measured at speeds below Mach 3.5 varied significantly from the theoretically derived preflight database. This paper will present a three-dimensional finite * AerospaceEngineer,ControlsandDynamicsBranch,POBox273/MS4840D T https://ntrs.nasa.gov/search.jsp?R=20100037123 2018-05-12T10:42:24+00:00Z wedge approximation method that explains the observed pressure anomaly, and compares it with the wind tunnel data. II Vehicle DescriptionThe X-43A was designed as an approximately 12 foot long wedge-nosed vehicle, shown in Figure 1. The vehicle underside houses a gaseous hydrogen fueled scramjet engine. Due to the tight tolerance on acceptable angle of attack during the engine test, a Flush Air Data Sensing (FADS) system was installed to supplement the onboard Inertial Navigation System (INS). Further study showed the system would not be needed during the engine test, but was kept in order to collect air data to improve the parameter estimation results and as a research experiment. The X-43A FADS system was designed with an array of 9 pressure ports which are theoretically capable of producing a full wind relative aircraft state. The pressure port layout and forebody geometry are shown in Figure 2. Four ports were split between the upper and lower surfaces, to be used for angle of attack calculations; the discussion in this paper will be restricted to these four ports, numbers 2, 3, 4, and 5. FADS systems are typically designed for spherical ellipsoid nose caps, as flow over this type of surface is well understood, from subsonic to hypersonic speeds. However, for sustained hypersonic flight sharp leading edges are needed to minimize the vehicle's compression drag. Real time angle of attack information was cal...