Assess if nominal Advanced Stirling Radioisotope Generator (ASRG)
vibration levels are sufficiently benign for unimpeded spacecraft
instrument operation

Lohman, Kurt T.

Schmidt, Elliot A.

Thelander, Steve D.

Williams, Craig H.

NASA Technical Reports Server

www.nasa.gov
National Aeronautics and
Space Administration
OVERVIEW
Problem Statement
Assess if nominal Advanced Stirling Radioisotope Generator (ASRG) 
vibration levels are sufficiently benign for unimpeded spacecraft 
instrument operation
• ASRG Vibration Spec review (Jet Propulsion Laboratory (JPL);
 Mar 09)—Concerns with existing criteria
• ASRG Vibration Review (JPL; May 10)—Concerns with     
 off-nominals; hardware test proposed
Approach
• Analysis performed using existing, dissimilar spacecraft with   
 validated finite element models (FEM)
 – Cassini Orbiter (by Glenn Research Center (GRC)/QinetiQ)
 – Mars Science Laboratory (MSL) (by JPL)
• Analytically mounted ASRGs (no modification of interface or   
 instruments to accommodate ASRG)
• ASRG forcing functions based on GRC measured data
• Performed NASTRAN Modal Frequency Response analysis
• Compared results with established spacecraft instrument vibration  
 limits
LIMITATIONS AND ASSUMPTIONS
• Initial results only, with engineering judgment applied to analysis ground rules
• Spacecraft, ASRG (Qualification Unit), and ASRG Adapter (Engineering Unit (EU)) FEMs
 – ASRG FEMs chosen for expediency (Lockheed Martin) and of varying maturities
 – Past accuracy testing exceed vibration analysis range (Cassini: 70 Hz; MSL 60 Hz)
 – Existing spacecraft adapters minimally modified (Cassini) or unmodified (MSL)
 – ASRG EU adapter
  • Not designed for Cassini or MSL
  • Not analyzed for launch environments
• Science instrument limiting criteria: Angular Displacement
 – Cassini (at Remote Sensing Pallet)
  • 4 µrad for NAC, WAC, and VIMS (Cassini Orbiter Functional Requirements Book—CAS–3–170)
  • 5 µrad for remaining instruments (ASRG Disturbance Specification—K. Lohman; JPL; Mar 09)
 – MSL (at top of Remote Sensing Mast; static)
  • 80 µrad (ASRG–MSL Vibration Analysis—K. Lohman; JPL; Feb 11)
• 1:1 RTG replacement with ASRG at existing hard points 
  • Cassini—analysis done at ½ power level
  • MSL—analysis done at full power level
• ASRG component angular displacements summed (worst case) rather than RSS’d for Cassini
• ASRG forcing functions based on measured vibration on rigid table (may not represent load on   
  compliant structure)
• Material properties may be inconsistent with mission environments
RECOMMENDATIONS FOR FUTURE ASRG USERS TO MITIGATE POTENTIAL VIBRATION ISSUES
 1. Proceed with standard launch vibration analysis. Design and modify the FEMs of ELV-to-spacecraft adapter and the ASRG-to-spacecraft  
  adapter until launch vehicle and spacecraft constraints are satisfied. 
 2. Review RPS Program-provided leading-edge analysis of frequency vs. response vs. spacecraft type for spacecraft reconfigured for   
  ASRG. Compare to other spacecraft vibration sources, determine if they are within a range of concern.
 3. Perform vibration analysis for in situ conditions due to other vibration sources on the spacecraft (i.e., reaction wheels, control rate gyros,  
  cryocoolers, etc.). Assess whether structural response (angular displacement) is within ranges tolerable for each instrument
  • Without the ASRG vibration contribution
  • With the ASRG vibration contribution (forcing functions provided)
 4. Identify whether ASRG or other vibration sources are major vibration contributors. If the major vibration contributor
  • Is NOT the ASRG (or ASRG is comparable to other vibration sources), then apply typical mitigation methods to other sources
  • IS the ASRG
   – And only a few sensors are of concern, isolate them individually with typical mitigation methods to avoid modal gain
   – Otherwise, if most/all sensors are of concern, apply typical mitigation techniques
      • Modify ASRG-to-spacecraft interface adapter design or materials
      • Redesign instrument pallet structure
      • Examine other integration orientations for the ASRGs, such as colinear to the spacecraft’s major axis
   – Reanalyze and redesign adapter until instrument vibration criteria are satisfied
 5. Reanalyze entire stack for Earth-to-orbit vibration environment
• MMRTG attached to MSL through   
 adapter fitting
• Stick model representation of MMRTG
• No modification of MMRTG or  
  ASRG adapters
• Attached with rigid elements
FOR FURTHER INFORMATION
Craig H. Williams
Craig.H.Williams@nasa.gov
216–977–7063
NASA Glenn Research Center
21000 Brookpark Road, Cleveland, Ohio 44135
Elliot A. Schmidt
Elliot.A.Schmidt@nasa.gov
216–433–5887
QinetiQ North America
NASA Glenn Research Center
21000 Brookpark Road, Cleveland, Ohio 44135
http://www.grc.nasa.gov
ONE ASRG-TO-MSL INTERFACE APPROACH
PS–00736–0312
CASSINI MAXIMUM SUM COMPONENT 
ANGULAR DISPLACEMENT 
• Softness of adapter combined with Cassini structure—
 ASRG has significant modes in 100 to 105 Hz range
• Not the case with RTG adapter only or 70 Hz EU isolation
 adapter (can be seen when comparing mode shapes)
• 39 Hz adapter amplifies forcing function
• 70 Hz adapter reduces modal gain
Adapter Subadapter
ASRG/Adapter Model
CONCLUSIONS FROM INTERIM RESULTS
• Vibration source characteristics, spacecraft structure stiffness, instrument    
 location, and instrument attachment have profound impact on vibration
 response
 – Cassini—generally flexible structure with vibration-sensitive instruments
 – MSL—generally rigid structure with vibration-tolerant instruments
• Angular displacement criteria generally satisfied
• ASRG-to-spacecraft adapter stiffness can greatly influence responses
 (~3× compared to no-ASRG adapter)
• Comparison of ASRG operation to traditional spacecraft vibration sources
 (i.e., reaction wheels, control rate gyros, cryocoolers, etc.) deserves more    
 scrutiny
Satisfied
Instrument
Criteria 
Without
adapter
With 39 Hz
adapter
With 70 Hz
adapter
10
out of
10
7
out of
10
10
out of
10
Without
adapter
With 39 Hz
adapter
Cassini (Summed 3 ASRGs) MSL (1 ASRG)
With 70 Hz
adapter
1
out of
1
1
out of
1
N/A
ANALYSIS RESULTS—ANGULAR DISPLACEMENT
MSL ChemCam Camera Plate Response—RSS Response
20
0.01
0.1
1
10
100
Range of instrument sensitivity
ChemCam requirement—80 µrad
Cassini requirement— 5 µrad
39 Hz EU isolation adapter
With RTG adapter
200
Frequency, Hz—1 Hz bandwidth
An
gu
la
r d
is
pl
ac
em
en
t, 
µr
ad
THREE ASRG-TO-CASSINI INTERFACE APPROACHES
RTG adapter only
 • ASRGs mounted on existing RTG adapters at four attach points
 • RTG adapter moved to corner nodes to fit ASRG SVIF adapter
Integrated RTG adapter + ASRG EU Isolation adapter
 • Modulus of elasticity scaled by 0.64 to match 39 Hz
  bending modes from GRC test 
 • Not designed for Cassini launch loads or stiffness
Integrated RTG adapter + stiffened ASRG EU Isolation adapter
 • Increased modulus of elasticity to achieve
  70 Hz first bending mode
 • Drove axial mode above 102 Hz and avoided
  modal gain
EU Adapter CAD
SUM COMPONENT ANGULAR DISPLACEMENT
WITH RTG ADAPTER ONLY
CASSINI ANGULAR DISPLACEMENT WITH
39 HZ EU ISOLATION ADAPTER
Instrument Limits Versus Predicted Angular Displacement
Three instrument limits violated
Maximum predicted angular
  displacement
Instrument limit
CASSINI ANGULAR DISPLACEMENT WITH
70 HZ EU ISOLATION ADAPTER
Instrument Limits Versus Predicted Angular Displacement
No instrument limits violated
Maximum predicted angular
  displacement
Instrument limit
PROCESSED ASRG VIBRATION LOAD DATA
• Vibration load data from ASRG EU testing at GRC (force versus frequency from 
 0 to 1600 Hz)
• Forcing Function Application
 – ASRG force applied to center of RBE3 connect to four attach points
 – Forcing functions aligned with each ASRG
 – Dynamic load applied in the same manner as it was measured
X
Y
Z
Dynamic load
Star tracker
UVIS and UVIS
mounting plate
WAC
Se
ct
io
n 
35
2 S
pa
ce
cr
af
t M
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ha
ni
ca
l E
ng
in
ee
rin
g 
Se
ct
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n
Se
ct
io
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35
2 S
pa
ce
cr
af
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ha
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ca
l E
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in
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Se
ct
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Adapter
MMRTG
CASSINI FINITE ELEMENT MODEL
• Modified launch FEM for verification of coupled loads (test verified by modal
 survey up to ~70 Hz) into a FEM for probe release configuration
  – Off-loaded propellant to probe release levels
  – Removed Huygens probe and launch vehicle adapter
  – Removed Radioisotope Thermoelectric Generators
  (RTGs), replaced with ASRGs 1:1 (no power match)
• Remote Sensing Pallet grid locations used
 for analysis
Instrument location grid numbers————————————————————
• VIMS Pallet and Instrument (#17401) • UVIS, UVIS Mounting Plate (#17410)
• Star Trackers (#17402-17403) • NAC (#17411)
• CIRs Instrument (#17407) • WAC (#17412)
• VIMs Electronics (#17408) • cg of Pallet + Instruments (#17413)
• CIRs Electronics (#17409)
High-gain
antenna
Launch vehicle adapter
Remote
sensing
platform
Thruster
cluster (×4)
RTG (×3)
Huygens
probe
MARS SCIENCE LAB DEPLOYED CONFIGURATION
AND FINITE ELEMENT MODELS
Response computed
at top of remote
sensing mast (RSM)
Camera plate
(ChemCam
location)
Response
node 78820
Multi-Mission RTG
(MMRTG) used for MSL mission 
Initial MSL FEM in
Deployed Configuration 
Mast
RSM shown with camera plate on topSe
ct
io
n 
35
2 S
pa
ce
cr
af
t M
ec
ha
ni
ca
l E
ng
in
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rin
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Se
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ChemCam
https://ntrs.nasa.gov/search.jsp?R=20150010206 2019-08-31T07:50:16+00:00Z


Initial Vibration Analysis of the Advanced Stirling Radioisotope Generator's (ASRG's) Affect on Spacecraft Instruments

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