Missile Aerodynamics Model Identification Using Flight Data

Accurate prediction of a missile's performance and trajectory in flight is crucial in the mission design process. This requires accurate models of all of the missile's characteristics, including inertial and aerodynamic properties. The development of high-fidelity models is very expensive in terms of both cost and time. The prediction of the missile's aerodynamic characteristics can be performed in various ways. The simplest method is to use semi-empirical models based purely on the missile's geometry, like the DATCOM software. This method is said to give the aerodynamic coefficients with an error in the range of 15 % to 20 % and is mostly used in the first stage of the design process. Another way is to use Computational Fluid Dynamics (CFD) methods. This usually requires very high computational power and many hours of calculations in order to determine every possible configuration of the flight conditions. The use of aerodynamic tunnel tests allows to get maximally close to the real flight conditions, but it requires very expensive equipment and possession of the complete missile to be placed inside the tunnel's measuring space. The last two methods are said to give rather accurate results. The most expensive, but also the most accurate method of determining the aerodynamic characteristics of the missile, are the flight tests. This requires a missile in the launch-ready state. To this point, the missile's characteristics are usually estimated using previously described methods and the flight tests' purpose is only to increase the fidelity of the model. The process of system identification requires the preparation of the input signals to be provided both to the missile's control system and the model which will be used for the identification. It is important to excite most of the object's frequencies in the best way possible. This paper will cover the aerodynamic model identification of the FOK rocket, developed by the Rocketry Division of the Students' Space Association (SSA) at the Warsaw University of Technology since 2017. It is an aerodynamically controlled rocket equipped with four forward control surfaces. The rocket is 75 millimeters in diameter and 1.3 meters in length, and was designed as a cheap testbed for control algorithms development. Its predicted apogee is 1500 meters and maximum velocity is about 200 meters per second. The paper will focus on the preparation of the flight tests in terms of input signals, and the process of system identification. Several methods, including the output error method, will be used for determining aerodynamic coefficients of the FOK rocket using flight data from the on-board inertial and satellite sensors and comparing it to the output of the SKA Rocket Flight Simulation software, also developed entirely by the SKA members.