VibrationdataVibrationdata

1

Unit 8

Waterfall FFT

VibrationdataVibrationdata

2

Terrier Black Brant

The Terrier-Black Brant is a two-stage, solid propellant, rail launched, guided, fin-stabilized sounding rocket.

This spin-stabilization is clearly visible in the video posted at: http://www.youtube.com/watch?v=ULlUfSoGIPw

VibrationdataVibrationdata

3

Free-Free Beam, Fundamental Bending Mode

Launch vehicles behave somewhat as free-free beams in flight.

Excitation sources include high-altitude winds, aerodynamic buffeting, thrust, control systems, etc.

VibrationdataVibrationdata

Waterfall FFT Terrier-Black Brant

The accelerometer data reveals the fundamental bending mode frequency. The frequency begins at about 8 Hz and then sweeps up to 13.2 Hz as propellant mass is expelled.

The spectral peaks at 5 Hz are due to vehicle imbalance at the induced roll rate. This is a 2X of the true roll rate at 2.5 Hz.

VibrationdataVibrationdata

5

Orion Launch Abort System PA-1 Test

VibrationdataVibrationdata

6

Pad Abort 1 Test Configuration

BoilerplateCrew Module

Abort MotorJettison Motor

Forward Interstage

Attitude Control Motor (ACM)

Nose Cone

Aero Closeout

Adapter Cone

Raceway

Aft Interstage

VibrationdataVibrationdata

Waterfall FFT

LS051V Fwd LAS (Nose cone) Lo Freq Accelerometer, Tangential axis # 2

The fundamental body-bending frequency begins near 8 Hz and gradually increases over time due to the decrease in propellant mass.

VibrationdataVibrationdata

The abort motor had an oscillation which swept downward in frequency from 685 to 670 Hz during the tail-off. This resonance is considered to be a phenomenon of the low pressure end of burn gas dynamics of the reverse flow motor.

Waterfall FFT Vibration at S&A Location LS045V

VibrationdataVibrationdata

Solid rocket motor with “organ pipe-like” combustion cavities.Standing pressure waves set-up in the cavities, driven by vortex-shedding, etc.

VibrationdataVibrationdata

Waterfall FFT Solid Motor Oscillation Suborbital Vehicle “B” Flight Accelerometer Data, Motor Adapter Bulkhead Longitudinal Axis

VibrationdataVibrationdata

Waterfall FFT Solid Motor Oscillation Static Test File Data, Forward Motor Dome

The pressure oscillation sweeps downward from 700 to 550 Hz, immediately after motor ignition.

A different motor…

VibrationdataVibrationdata

Shepard Tones

Ascending and Descending by Dutch artist M. C. Escher

VibrationdataVibrationdata

Waterfall FFT Shepard Tone

A Shepard tone, named after Roger Shepard, is a series of sweeping sine tones, with frequencies spaced at one octave intervals.

VibrationdataVibrationdata

14

Exercises

Plot a Waterfall FFT for each of the following files:

terrier_black_brant_2KHz_lpf_x.txt

solid_motor_flight.txt

Fwd_Dome_ignition.txt

Orion_PA1_LS51v.txt

Generate a Shepard Tone using the parameters of your choice. Then plot the Waterfall FFT.