Unit 8 • Waterfall FFT
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
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.
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.
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
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.
Waterfall FFT Vibration at S&A Location LS045V 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.
Solid rocket motor with “organ pipe-like” combustion cavities. Standing pressure waves set-up in the cavities, driven by vortex-shedding, etc.
Waterfall FFT Solid Motor Oscillation Suborbital Vehicle “B” Flight Accelerometer Data, Motor Adapter Bulkhead Longitudinal Axis
A different motor… 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.
Shepard Tones Ascending and Descending by Dutch artist M. C. Escher
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.
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.