Structural Impact of Rocket Motor Sound and Vibration

With the technological advancements currently available, many applications where data acquisition systems are deemed essential today were never considered possible just a few years ago. This is the case at the National Aeronautics and Space Administration (NASA), where applications such as the launch of space vehicles are potentially hazardous to Ground Support Equipment (GSE) located within 300 ft of the launch pad and any nearby personnel.

Consequently, remote operation is a requirement. Today, it is possible to remotely collect high-fidelity data of a wide range of bandwidths up to 160 kHz for long periods with high-performance instrumentation data recorders connected to a network or the Internet.

One application that has graduated to the essential class is the measurement of the structural impact of rocket motor air- and structure-borne sound and vibration during launch of a space shuttle at Kennedy Space Center (KSC) with digital audio tape (DAT) recorders.

Vibroacoustics

The launch of a space shuttle exposes GSE and nearby structures to intense vibration due to acoustic pressure loads generated by rocket exhausts. This is a measure of vibroacoustics or vibroacoustic coupling, a structure’s affinity to vibrate when subjected to broadband acoustic loads.

This vibration can degrade structures and increase safety concerns and operational maintenance costs. As a result, continuous monitoring of launch-critical loads (acoustics) and structural response (vibration and strain) is vital for ensuring operational safety and long-term reliability of launch-pad structures.

The severe acoustic and random-vibration environment presented by space-system launch vehicles is difficult if not impossible to reproduce in a laboratory. Data gathered from field tests is the only practical avenue for accurate vibroacoustics analysis. This has resulted in decade-long research at KSC, which has focused on field measurements and the subsequent characterization of launch acoustic loads.

System Configuration

To make the required measurements, a unique test system has been configured. It consists of a specially made verification test article (VETA) mounted on the launch pad at approximately the main engine level at Pad 39A. The VETA structure resembles a cantilevered, steel I-beam approximately 10 ft long turned on end. Depending upon the particular tests, a variety of sensors is attached to the structure. Microphones also are deployed in the area around the pad.

The sensor wires are shielded from the intense liftoff heat and EMI/RFI as they pass through the pad base and into the blockhouse below. Sensor leads are either connected to signal conditioners whose output feeds a data recorder or sent directly to the data recorder, depending upon the sensor type.

The data recorder at the launch pad is connected to a host PC control system via the Sony PCIF250 I/F PC-Based Data Transfer System. The PC, which has a high-speed modem and Ethernet and intranet connections, runs a special version of Sony’s PcscanII recorder control and data acquisition software. All of the data acquisition instrumentation is located in an equipment-safe computer room 100 ft below the launch pad.

A second computer system with a modem and web and intranet connections and running PcscanII software for TCP/IP data exchange/analysis resides in the NASA laboratory located approximately 10 miles from the launch pad. This unique setup provides highly accurate monitoring of the vibration data from the VETA and total remote control of the data acquisition process at the launch-pad site.

Data Acquisition System

Composition of the data acquisition system was influenced by technical, cost, operation, and management considerations. The system requirements included the following:

  • Low initial/maintenance/operational costs.
  • Remote data acquisition, control, and data transfer.
  • Timer control for night launches.
  • Capability to control multiple recorders.
  • Capability for real-time data monitoring.
  • Instant data accessibility after launch.
  • Integration into and use of existing data.
  • Rugged, flexible, transportable, and an upgrade potential.

To accurately assess the vibroacous-tic environment of the space shuttle, a data acquisition system with a sampling rate of 12 kS/s for each of the 16 data channels, an 84-dB dynamic range, an amplitude sensitivity of 16-bit A/D and D/A, and 2 degrees or less phase shift between channels was essential. Sony’s PC216Ax DAT Instrumentation Recorders met these specifications. In addition to a frequency-resolution capability of direct current to 10 kHz, the recorders provided a 32-fold increase in amplitude sensitivity to the existing launch-pad system.

PcAnywhere, an off-the-shelf software program, aided in the point-to-point remote control capability and the remote computer restart options. PCscanII software enabled data transfer from the PC216Ax Recorder to the PC for display and analysis.

VETA

The VETA approach represented the first comprehensive effort to install a safe structure within the launch-pad perimeter and measure acoustic and vibration response data simultaneously. It was important to install the VETA in such a manner as to expose it to both direct and reflected acoustics.

Acoustic loads on the GSE were location-dependent to a large degree and varied when the GSE was partially or fully shielded from direct acoustics. Also, the loads were significantly affected by the vehicle trajectory as governed by the mission-dependent launch inclination. 
Depending upon the launch inclination, instantaneous acoustic loads during the roll maneuver were significantly higher than during liftoff. This necessitated instrumenting the VETA in both the front and back.

The accuracy of measurements associated with the test was extremely critical to the test/analysis correlation and eventual verification of the analytical method. Proper selection of transducers and transducer placement, mounting, and calibration were key to the success of the program.

Basic Test Procedure

Prior to shuttle launch, all system equipment is powered up, and Windows is started on PCs in the blockhouse and at the NASA laboratory. The PC scan software is started on the pad PC. All sensors, signal conditioners, data recorders, and PC systems are thoroughly checked and proper operation confirmed. Once the pad is closed for launch, there is no way to get back into the blockhouse to fix or change the setup. 
The laboratory computer connects to the pad PC using a modem and remote-control software. This connection allows personnel in the laboratory to monitor the PcscanII software and the data acquisition process on the pad PC.

As the shuttle launch begins, engineers can observe the real-time data from the sensors on the test structure and microphones nearby. Data acquisition to the pad PC hard drive can be controlled as needed.

Engineers also can see a good selection of live time and frequency domain data such as fast Fourier transform (FFT), 3rd octave, integration, or filtering for any channel. This capability is used to determine what data may be significant for more sophisticated analysis later.

Data collected on the pad PC can be transferred to the lab PC in semi-real time using the remote TCP/IP interface software. Alternatively, files can be transferred from the pad PC hard drive using the remote PC control software. The data collected and the resultant analysis are used to report on a wide range of environmental, structural damage, and other important aspects involving acoustics, shock, and vibration.

About the Authors

Ravi Margasahayam is the principal engineer at DYNACS Engineering, NASA John F. Kennedy Space Center.
Tim Halsey is regional sales manager at Sony Precision Technology America, 20381 Hermana Circle, Lake Forest, CA 92630, 949-770-8400, 
e-mail: [email protected].

Published by EE-Evaluation Engineering
All contents © 2001 Nelson Publishing Inc.
No reprint, distribution, or reuse in any medium is permitted
without the express written consent of the publisher.

January 2001

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