Air Rocket Thrust Experiment Involving Computerized Data Acquisition, Calibration, And Uncertainty Analysis

The development and modification of a laboratory experiment to determine the thrust characteristics of an air propelled rocket is described. The experiment is used in the junior level Instrumentations and Measurements course in the Department of Mechanical Engineering at the University of Tulsa. It involves elements of instrument calibration, computerized data acquisition, and uncertainty propagation. The experimental details of the laboratory are described along with the goals of the experiment and a description of the data analysis procedure. Student opinions of the lab experience as determined by a survey about the lab are presented and ways to improve the lab based on student comments are discussed. Introduction Three important concepts that are covered in most experimental methods courses are instrument calibration, computerized data acquisition, and propagation of uncertainty into a calculated result. While these concepts are often taught in the classroom, students benefit more by actively learning about these concepts in the laboratory. This paper describes an experiment that has been developed for the Instrumentation and Measurements course in the University of Tulsa’s Department of Mechanical Engineering. This junior level lecture-laboratory course contains biweekly laboratory exercises where student teams perform hands on experiments and then write up their lab experience and results in a formal technical lab report. The lively laboratory is particularly well received by the students because it involves the firing of an instrumented ballistic pendulum. The excitement of firing a ballistic air rocket in the laboratory adds to student motivation and helps to retain their attention. An initial version of the experiment was developed by Dr. Denis Zigrang at the University of Tulsa and subsequently modified [1]. The modified experiment involves acquiring data using the computer and data acquisition software and calibrating several standard measurement transducers, e.g. accelerometer, rotary variable differential transformer (RVDT), and diaphragm pressure transducer. In the subsequent lab write up, the students are required to calculate the experimental uncertainty in the calculated impulse based on the elemental uncertainties in the measured signals. Experiment Description The purpose of the experiment is to determine the thrust characteristics of an air rocket. This experiment should familiarize students with typical instrument calibration procedures, computerized data acquisition, and the operation of a rotary variable differential transformer (RVDT), unbonded strain gage accelerometer, and pressure transducer. After completing the P ge 10141.1 “Proceedings of the 2005 American Society for Engineering Education Annual Conference & Exposition Copyright 2005, American Society for Engineering Education experiment, the student should be able to calibrate linear instruments such as pressure transducers and accelerometers. The student should also have gained enough knowledge through the experiment and discussion to explain the operating principles of RVDTs, accelerometers, and diaphragm pressure transducers and the components of a data acquisition system. In the experiment, a modified fire extinguisher (air rocket) is mounted to the end of a pendulum. An electric solenoid valve is attached at the head of the air rocket. On the rocket is also mounted a 1-D unbonded strain gage accelerometer and a diaphragm pressure transducer. A RVDT is mounted on the pivot of the pendulum to measure the angular position of the pendulum arm. A PC based data acquisition system, consisting of a PCI Bus Data Acquisition Board, screw terminal connector, signal conditioning modules, and LabVIEW software is used to record the signals from the transducers and to fire the rocket by opening the electric valve at the head of rocket. The details of the equipment used in the experiment are listed in Table 1 below. Table 1. Equipment List. Item Manufacturer Model/Version 1. Modified fire extinguisher Walter Kidde and Company F-240 2. Electric solenoid valve Automatic Switch Company 8215C2 3. 1-D unbonded strain gage accelerometer Statham 652 (A3) 4. Diaphragm pressure transducer Bourns 2900 5. RVDT Schaevitz Sensors R30D 6. Dial output analog pressure gage Marsh 34693-3 6. PCI Bus Data Acquisition Board Keithley Instruments, Inc. KPCI-3108 7. Screw terminal connector Keithley Instruments, Inc. STP-36 8. Signal conditioning modules Omega Engineering, Inc. OM3 9. LabVIEW software National Instruments Version 6.1 The pendulum assembly is shown in Figure 1. The air rocket (shown in Figure 2) is mounted to the end of the pendulum arm with the firing nozzle oriented in the plane of the pendulum rotation. The pendulum rotates on a pair of ball bearings to minimize frictional losses in the system. The RVDT is connected to the rotating axel and sits in a circular mount on the top of the assembly. The pendulum arm can be locked in an arrangement exactly 45 degrees from the vertical position by sliding a pin in the 45 degree positioning slot.