A novel airdata system based on flush-mounted pressure sensors has been developed for the X-33 aerospace vehicle. Denoted the "X-33 flush airdata sensing" (FADS) system, it was designed to overcome limitations of pitot-static probes that were used previously, as explained below.
The X-33 is an autonomous, lifting-body-type demonstrator aerospace vehicle designed for use in validating those items of technology necessary for development of a single-stage-to-orbit launch vehicle. The sensors in the original X-33 airdata system were a pair of pitot-static probes (like those used on the Space Shuttle Orbiter) that were deployed on a roll-over mechanism after completion of the high-heat atmospheric-reentry phase of flight. These probes were undesirable for several reasons, including difficulty of integration into the X-33 structure, lack of way to achieve accurate calibration for the first few flights, and lack of a method for measuring sideslip.
The foregoing considerations, along with the success of several recent FADS-system flight-test programs at NASA Dryden Flight Research Center, led to the decision to use a FADS on the X-33 vehicle. The X-33 FADS system was required to provide valid airdata below mach 4 during the launch and landing phases of flight, and to remain operational with a single failure anywhere in the system.
An airdata sensing system is needed because the X-33 flight-control and guidance software require the airdata state of the vehicle during the launch, terminal-area energy-management (TAEM), approach, and landing portions of the flight trajectory. Inertial systems are not acceptable because they do not account for wind conditions. Minimizing the angle of attack and angle of sideslip of the X-33 vehicle during the launch portion reduces the loads on the airframe. During the TAEM, approach, and landing portions, the mach number, velocity, and angle of sideslip are used to improve flying qualities and compensate for the wind conditions.
The hardware of the X-33 FADS system measures pressures at six locations on the nose cap of the vehicle. At each location, there are two ports, which are plumbed to individual absolute-pressure transducers to create a dual redundant system with no moving parts. The FADS algorithm, which was developed by Dryden Flight Research Center, includes a calibrateable pressure-distribution mathematical model, through which the measured pressures are related to the vehicle airdata. This model is a splice of (1) the closed-form potential-flow solution for a blunt body, applicable at low subsonic speeds; and (2) the modified Newtonian flow model, applicable at hypersonic speeds. Data from wind-tunnel tests have been used to calibrate the X-33 FADS model for the effects of flow compression, body shape, and such other systematic effects as shock-wave compression or Prandtl-Meyer expansion on the forebody. Once calibrated, the model can be inverted in real time to calculate the airdata state, to the required accuracy, as a function of the measured pressures.
Some innovations have been made to improve upon the previous FADS design for the X-33 application:
- An improved solution subalgorithm for the FADS pressure model makes it possible to decouple (1) the computation of flow-incidence angles, angle of attack, and sideslip from (2) the solution for the mach number, static pressure, and total pressure. This decoupling offers several software and redundancy-management benefits.
- A measure of the error in the pressure-model is used to select the better of the dual-redundant airdata subsystems. This feature makes it possible for both soft and hard failures, including undetected failures, to occur in one subsystem without degrading the airdata computation. This level of redundancy, referred to as "fail-operational," typically requires a mid-value selection of a triple-redundant system.
- A methodology for analyzing the stability of the FADS algorithm was also developed.
This work was done by Stephen A. Whitmore, Brent Cobleigh, and Ed Haering, Jr., of Dryden Flight Research Center. DRC-98-55