Waterfall FFT

1
Unit 8

• Waterfall FFT

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/watc
h?vULlUfSoGIPw
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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.
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Waterfall FFT Terrier-Black Brant
Time (sec)
Frequency (Hz)
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.
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Orion Launch Abort System PA-1 Test
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Pad Abort 1 Test Configuration
Attitude Control Motor (ACM)
Forward Interstage
Jettison Motor
Abort Motor
BoilerplateCrew Module
Nose Cone
Aft Interstage
Raceway
Adapter Cone
Aero Closeout
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Waterfall FFT LS051V Fwd LAS (Nose cone) Lo
Freq Accelerometer, Tangential axis 2
Time (sec)
Frequency (Hz)
The fundamental body-bending frequency begins
near 8 Hz and gradually increases over time due
to the decrease in propellant mass.
8
Waterfall FFT Vibration at SA Location
LS045V
Time (sec)
Frequency (Hz)
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.
9
Solid rocket motor with organ pipe-like
combustion cavities. Standing pressure waves
set-up in the cavities, driven by
vortex-shedding, etc.
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Waterfall FFT Solid Motor Oscillation
Suborbital Vehicle A  Flight Accelerometer
Data, Motor Adapter Bulkhead Longitudinal Axis
Time (sec)
Frequency (Hz)
The pressure oscillation sweeps downward from 520
to 450 Hz.
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Waterfall FFT Solid Motor Oscillation
Suborbital Vehicle B  Flight Accelerometer
Data, Motor Adapter Bulkhead Longitudinal Axis
Time (sec)
Frequency (Hz)
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A different motor
Waterfall FFT Solid Motor Oscillation Static
Test File Data, Forward Motor Dome
Time (sec)
Frequency (Hz)
The pressure oscillation sweeps downward from 700
to 550 Hz, immediately after motor ignition.
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Shepard Tones
Ascending and Descending by Dutch artist M. C.
Escher
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Waterfall FFT Shepard Tone
Time (sec)
Frequency (Hz)
A Shepard tone, named after Roger Shepard, is a
series of sweeping sine tones, with frequencies
spaced at one octave intervals.
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Exercises
Plot a Waterfall FFT for each of the following
files terrier_black_brant_2KHz_lpf_x.txt soli
d_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.