© 2015 Orbital ATK and UC Berkeley. All Rights Reserved.

Development of the Random Vibration Test Program for the ICON Instruments Josh Dunford, Orbital ATK, Space Systems Group Terry Scharton, UC Berkeley Consultant

Spacecraft and Launch Vehicle Dynamic Environments Workshop June 2 - 4, 2015

Outline

ICON is a NASA Goddard Space Flight Center Heliophysics Explorer Mission launching on Pegasus XL in 2017

The driving vibroacoustic requirements are observatory workmanship test levels Flight vibro-acoustic environments are relatively benign

The instrument-level tests must qualify the instruments for observatory tests without undue risk of over-test Acceleration spec based on BEM and SEA acoustic analysis and base-input

random vibration analysis Force specs have been defined based on semi-empirical force limits Overall force limit based on net CG test limit loads

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Ionospheric Connection Explorer

Mission goal: to understand how our space environment is controlled by terrestrial weather

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Project managementSystems engineeringS&MAUV instrumentsPayload electronicsMission/Science Ops

MIGHTI neutral wind interferometer (2)

Ion velocity meter

LEOStar-2 spacecraftObservatory I&T

Naval Research Laboratory

UT-Dallas

HQ- SMDGSFC – ExplorersKSC – LV Services

UC Berkeley

OATK- Space Systems Group

NASA

Payload structure

Centre Spatial de Liège

FUV Calibration

Space Dynamics Lab

Instrument supportCameras/electronics

Payload I&T

OATK-Magna

Payload

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The ICON payload assembly consists of four instruments taking simultaneous measurements of the neutral winds and plasma motion: MIGHTI: neutral wind profiler

FUV: neutral composition profiler and nighttime plasma density profiler

IVM: in-situ electric field/plasma motion

EUV: daytime plasma density profiler

and supporting hardware: Payload electronics Spacecraft AD sensors and nadir

antenna Aluminum honeycomb deck with

graphite-epoxy facesheets and bipods (Payload Interface Plate)

+Z View

-Z View

Acoustic and Random Vibration Environments

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Orbital and GSFC practice define minimum workmanship levels for spacecraft acoustics and base-input random vibration

Workmanship drives observatory acoustics across entire spectrum

Pegasus XL is a quiet ride – no lift-off event

Workmanship covers base-input random vibration above 80 Hz

Pegasus XL base-input random environment is defined on the hard side of the ICON 30-Hz loads isolation system – high-frequency is isolated

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105

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115

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135

140

10 100 1000 10000

Soun

d Pr

essu

re Le

vel (

dB re

f 2×1

0-5

Pa)

Frequency (Hz)

ICON Protoflight Test Level, 137.9dB OASPL

TO-EP0014 Min Workmanship, 137.9dB OASPL

Pegasus MPE +3dB, 127.6dB OASPL

0.001

0.01

0.1

1

10 100 1000

PSD

(g2 /

Hz)

Frequency (Hz)

TO-EP0014 Min Workmanship

Pegasus-XL Payload Interface Random Vibration - Protoflight

Composite ICON Protoflight Random Vibration

Instrument Random Vibration Spec

Instrument test spec checked against: Observatory FEM base-drive analysis

with loads isolation system for structure-borne random

Low-frequency results from payload BEM

High-frequency results from payload SEA

Component workmanship levels Force limits defined based on mount-

point apparent mass of payload structure C2 from two-DOF model (Fig. 8/7004C)

Instrument net CG responses are also limited to the test limit loads Determined from CLA with loads

isolation system – this covers CG responses from observatory-level vibro-acoustic analysis

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0.001

0.01

0.1

1

10 100 1000

Acce

lera

tion

Spec

tral

Den

sity

(g

2 /H

z)

Frequency (Hz)

Bump-up for structure-borne random

Workmanship

The instrument random vibration test should qualify the instruments for launch and prepare them for observatory workmanship

Structure-Borne Random Vibration

Detailed CDR-level observatory finite element model, including math model of 30-Hz loads isolation system

NASTRAN SOL 111 based on modes through 1000 Hz 2% modal damping Random response post-processed with analytical

force-limiting and narrow-band notches below workmanship

Instrument test spec covers averaged interface acceleration predicts > 60 Hz

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1E-05

0.0001

0.001

0.01

0.1

1

10 100 1000

ASD

(g2 /

Hz)

Frequency (Hz)

Instrument Interface Random Predicts1/6-oct Frequency Resolution

Instrument Protoflight Test Spec, 7.0 g-RMS

Min Workmanship GSFC-STD-7000, 6.8 g-RMS

Low-Frequency Acoustic from BEM

Payload FEM modes through 750 Hz solved in NASTRAN Solid mesh “skinned” with zero-mass & -stiffness shell

elements – cracks and holes sealed, etc. Faces re-meshed with ~3in elements for BEM

Finer mesh than required for 600-Hz max analysis frequency (7+ elements/wavelength), but projection of structure modes onto FE Faces fails with coarser mesh

DAF modeled with 98 uniformly spaced plane waves Solved in VA One 2014 from 31.25 – 600 Hz (1/42-oct)

44 hours on 4 parallel CPUs

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Full Payload FEM

FE Structure

FE Fluid Faces

1E-05

0.0001

0.001

0.01

0.1

1

10 100 1000

ASD

(g2 /

Hz)

Frequency (Hz)

Instrument Interface Random Predicts1/6-oct Frequency Resolution

Instrument Protoflight Test Spec, 7.0 g-RMS

Min Workmanship GSFC-STD-7000, 6.8 g-RMS

High-Frequency Acoustic from SEA

Multiple SEA approaches produced results in good agreement VA One AutoSEA, VA One & spreadsheet PATH49 100% nonstructural mass – assume flexure-mounted instruments do not stiffen

panel Typically 1 mode in band > 550 Hz

5.2dB factor added to spatial average response to define maximum predicted response

High frequency responses are below workmanship Mid-frequency responses (low modal density) significantly higher than BEM

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1E-05

0.0001

0.001

0.01

0.1

1

10 100 1000

Acce

lera

tion

Spec

tral

Den

sity

(g2/

Hz)

Frequency (Hz)Instrument Protoflight Spec ERD Rev B, 7.0 g-RMS Min Workmanship GSFC-STD-7000, 6.8 g-RMS2014 VA One PATH 49 0/100 Proto Avg + 5.2dB, 1.7 g-RMS 2014 VA AutoSEA Proto Avg + 5.2dB, 0. g-RMS

High frequency (more accurate)

Low frequency (less accurate)

Two Methods of Limiting Force (CG Response) in Instrument Random Vibration Tests

First, total force limited to force specification Semi-empirical Force Limit Equation (Sect. 5.3/7004C) Simple Two-Degree-of-Freedom Method (Fig. 8/7004C) FEM calculations of flight mounting structure (the PIP) apparent masses Table of C2 values for different ICON instruments

Second, five sigma CG acceleration limited to TLL CG acceleration determined from Force PSD TLL determined from CLA with loads isolation system Uniform reduction of force or acceleration specification

Semi-empirical Force Limit Equation (Sect. 5.3/7004C)

Force limit (Sff) in random vibration test proportional to C2, total mass squared (Mo

2), and acceleration specification (Saa) Sff = C2 Mo

2 Saa , f < fo

Sff = C2 Mo

2 Saa (fo/f)n , f > fo

Mo = total mass of test item, n usually 2, fo usually mode with most effective

mass or highest peak

Methods of determining C2 in semi-empirical force limit Simple TDOF Method (test item to mounting structure mass ratios) FEM analysis of payload and mounting structure Heritage (flight experiments or previous tests) Norton-Thevenin analysis (difficult to apply correctly)

Simple Two-Degree-of-Freedom Method (Fig. 8/7004C)

It is conservative to use ordinate of Fig.8 as C2, because:

Mass used to normalize ordinate Fig. 8 is test item effective mass, not total mass as in semi-empirical method

Use of total instrument mass in mass ratio in abscissa of Fig. 8 somewhat reduces conservatism

For ICON, we used total instrument mass in ratio for random vibration, and effective mass for sine analysis to be more conservative

FEM Calculation of Flight Mounting Structure Apparent Masses

Mass ratio for C2 depends on the apparent mass of the flight mounting structure (Payload Interface Plate)

The drive-point impedance was calculated from the payload FEM at the mounting points of the three largest instruments Instrument FEM replaced with RBE2 Master node of RBE2 driven with unit load

Apparent mass read from least-squares fit

Calculated Values of C2 for Instrument Random Vibration Tests

C2 CALCULATIONS FOR ICON INSTRUMENT RANDOM VIBRATION TESTS -- Scharton 3/23

MIGHTI (50 lb)* f0 (Hz)** Mass part., *** PIP App. Mass (lb), **** m2/m1***** C2 (Q=10)****** Normal (y) 273 25% 69 lb @160 Hz 0.72 3 Lateral (x) 105 61% 303 lb @105 Hz 0.17 8 Lateral (z) 111 27% 297 lb @111 Hz 0.17 8

FUV (77 lb)* Normal (z) 89 39% 212 lb @ 89 Hz 0.35 5 Lateral (y) 126 26% 276 lb @126 Hz 0.27 6 Lateral (x) 186 22% 229 lb @160 Hz 0.32 5

EUV (17 lb)* Normal (?) ??? ??? 192 @ 100 Hz 0.09 14 Lateral (?) ??? ??? 310 @100 Hz 0.05 21

IVM (11lb)* Normal (?) ??? ??? 192 @ 100 Hz 0.06 18 Lateral (?) ??? ??? 310 @100 Hz 0.04 25

ICP (15 lb)* Normal (?) ??? ??? 192 @ 100 Hz 0.08 15 Lateral (?) ??? ??? 310 @100 Hz 0.05 21

Example calculations of C2 example based on preliminary data

Secondary Limiting – Instrument 5σ CG Acceleration Limited to TLL in Random Vibration Tests

RMS of acceleration of CG must be determined from PSD of measured force, not from accelerometers

Five sigma used for limiting unless limiting is so extreme as to compromise integrity of random vibration test (Note, five sigma was not always used to calculate maximum loads in pre-test analyses.)

Five sigma limiting implemented with uniform reduction of force specification, unless notching is too severe, e.g., non-resonance response reduced.

Some structural analysts prefer to stop the integration of force PSD after the fundamental modes to reduce the limiting, but this does not yield the true RMS nor limit the CG acceleration peaks to five sigma.

Test Limit Load Based on CLA

Test Limit Load (1.25 × DLL) determined from CLA with “worst-case” SoftRide Design isolated stack axial mode is 30 Hz with SoftRide CLA was run with isolated 34 Hz axial stack mode

Bounds worst-case stiffness uncertainty due to temperature and build tolerances Provides reduced isolation from Stage-1 Ignition transient

Low Pegasus XL flight and ascent vibro-acoustics do not drive instrument CG loads

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EUV FUVICPIVM A

IVM BMIGHTI A

MIGHTI BMIGHTI Cal Lamp MIGHTI Electronics

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10

100

1 10 100

Peak

Acc

eler

atio

n (g

)

Weight (lbm)

CLA Extrema Pegasus XL with SoftRide (MUF = 1.25)

Preliminary Instrument DLL from MAC

Revised Instrument DLL = 14.4g

Conclusions

Observatory-level workmanship test is the most severe vibro-acoustic event for ICON

Test specifications have been defined to qualify the instruments for flight and observatory workmanship, and to screen for workmanship

Risk of over-test is mitigated by limiting interface force (CG response) Force specification based on the ratio of the instrument mass to the flight

structure apparent mass Overall test limit load based on CLA

Questions?

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