# Presentation: 532988

## Analysis of Damping Treatments Applied to the MAP Spacecraft

**Scott Gordon**
**Code 542**
**Goddard Space Flight Center**
**Presented at**
**FEMCI Workshop**
**May 18, 2000**

## Outline

**Background/Problem Description**
**Modal Survey**
**Damping Treatments**
**Analysis Methodology**
**Analysis Results**
**Comparison with Test Data**
**Conclusions**

- FEMCI Workshop

- May 18, 2000 (SAG)

## Background

**MAP = Microwave Anisotropy Probe**
**MAP Spacecraft Level Acoustic Test - Conducted August, 1998**
- Flight spacecraft bus with mass mockups
- No thermal blanketing or electrical harnessing
- Instrument mass simulator
- ETU Solar Arrays
**Acoustic test performed to Delta II 7425-10 protoflight levels (142.9 OASPL)**

**M****AP Acoustic Test Configuration**

- FEMCI Workshop

- May 18, 2000 (SAG)

## Problem Description

**High acceleration response measured at thruster locations on top deck**
**Acceleration levels exceeded the qualification levels for the thrusters**
**Thruster Qual Levels**
- .2 G^2/Hz 20-2000 Hz
- 20 Grms
**Measured test levels**
- 44 Grms
- 116 G^2/Hz @ 140 Hz
**Problem addressed by adding damping treatments to spacecraft**

- FEMCI Workshop

- May 18, 2000 (SAG)

## Spacecraft Configuration

- FEMCI Workshop

- May 18, 2000 (SAG)

## Spacecraft Configuration - Cont.

**MAP Top Deck with Thruster Brackets**

**MAP Top Deck Configuration**
- 5/8” thick aluminum honeycomb panel
- .015” M46J/934 facesheets
- Hexagonal shape
- 94” across hexagon points
- 36”central cut-out
- Center supported at hex-hub
- Outer corners supported by truss members

- FEMCI Workshop

- May 18, 2000 (SAG)

## Spacecraft Configuration - Cont.

**Upper Deck Thrusters**
- 4 identical 1-lb thrusters mount to MAP upper deck
- 2 thrusters per thruster bracket (upper and lower)
- Each thruster mounts to small bracket which attaches to large bracket
- Large and small brackets built up from T800/EX1515 laminate flat stock
- Mounting faces are .072”, remaining faces are .036”

**Detail of MAP Upper Deck Thruster Bracket**

- FEMCI Workshop

- May 18, 2000 (SAG)

## Modal Survey

**A modal survey was performed to determine mode shapes contributing to high thruster response**
**5 x 5 mesh of single axis accelerometers used on the top deck**
**Triaxial accelerometers at each of the mounting bracket locations and at tip of large bracket**
**Results were correlated with FEM model**

- FEMCI Workshop

- May 18, 2000 (SAG)

## Modal Survey - Cont.

**Test data showed several candidate modes in the 120-200 Hz range which excited high thruster response**
**Candidate modes showed a combination of deck deflection and local bracket deformation**
**FEM results did not match test data exactly but had sufficient accuracy to capture contributing modes**

- FEMCI Workshop

- May 18, 2000 (SAG)

## Modal Survey - Cont.

**Acoustic test analytically simulated**
**Good correlation with X and Y response**
**Z response did not show same degree of correlation**
**Not as critical because Z response is significantly lower below 200 Hz**
**Conclusion:  Model and loading conditions could be used to define damping treatments**

- FEMCI Workshop

- May 18, 2000 (SAG)

## Damping Treatments - Thruster Brackets

**3M Scotchdamp ISD-242 applied to thruster brackets**
**GSFC Heritage:  Scotchdamp used by TRW on EOS-PM spacecraft**
**FEM analysis used to determine size and placement of damping treatments**
**.004” layer of scotchdamp with Gr/Ep constraint layer**
**Constraint layer material and thickness selected to match thruster bracket surface**

- FEMCI Workshop

- May 18, 2000 (SAG)

## Damping Treatments - Top Deck

**Lockheed-Martin SMRD strips applied to deck edges**
**GSFC Heritage:  Used on XTE spacecraft**
**.4” thick SMRD strip with honeycomb constraint layer**
**SMRD strips designed to target deck modes driving thruster response**
**Scotchdamp applied to top and bottom surfaces of top deck**
**Scotchdamp targeted at higher frequency response (300-500 Hz)**

- FEMCI Workshop

- May 18, 2000 (SAG)

## Damping Treatments - Cont.

- FEMCI Workshop

- May 18, 2000 (SAG)

## Analysis Methodology

**Methodology outlined in “Finite Element Prediction of Damping in Structures with Constrained Viscoelastic Layers”, C.D. Johnson and D.A. Kienholz, AIAA Journal, Vol. 20, No. 9, Sept 1982, pp. 1284-1290**
**Approach uses standard NASTRAN elements to model VEM damping treatments**
- Solid elements (HEXA and PENTA) for the VEM Layer
- Thin shell elements (QUAD4) for the constraint layer
**Equivalent modal damping developed based on % strain energy in VEM for a particular mode**
**Equivalent modal damping can then be used in standard NASTRAN dynamic solutions to calculate damped response.**

- FEMCI Workshop

- May 18, 2000 (SAG)

## Add solid elements representing VEM and shell elements representing constraint layer to FEM structural model
Run normal modes solution and recover %strain energy in the solid elements representing the VEM
Calculate modal damping associated with the VEM for each mode by applying the following equation

**Run normal modes solution and recover %strain energy in the solid elements representing the VEM**
**Calculate modal damping associated with the VEM for each mode by applying the following equation**

- Analysis Procedure

- FEMCI Workshop

- May 18, 2000 (SAG)

## Analysis Procedure - Cont.

**The VEM modal damping (********v****) is added to the nominal modal damping to get the total damping for that mode**
**For the MAP dynamic analysis, nominal modal damping was 1.6% of critical based on spacecraft acoustic test**
**The VEM material properties used in the analysis are shown in the table below:**

- FEMCI Workshop

- May 18, 2000 (SAG)

## Analysis Verification*

**Beam coupons with and without scotchdamp were tested to verify methodology**
**Analytical predictions showed good correlation with test data**

- *Data from Steve Hendricks at Swales Aerospace

- FEMCI Workshop

- May 18, 2000 (SAG)

## Analysis Results

**Total reduction of 17 dB predicted due to Scotchdamp on bracket and SMRD on deck**
**This still does not meet manufacturers thruster qual levels**
**Several additional factors**
- Blanketing & harnesses (10dB)
- Rubber shims at small bracket interface (3-9 dB)
- Scotchdamp on top deck (3 dB)

- FEMCI Workshop

- May 18, 2000 (SAG)

## Intermediate Acoustic Test

**Acoustic test performed July 1999 to assess effectiveness of damping treatments**
**Flight MAP spacecraft bus, most spacecraft electronics, electrical harnesses and blanketing as close to flight as possible, ETU solar arrays, no instrument or simulator**
**Measured reductions sufficient to show thrusters qualified for flight environment**

- FEMCI Workshop

- May 18, 2000 (SAG)

## Damping Prediction - Test vs Analysis

**Analytical prediction within 3 dB of peak test response at 120 Hz**
**Overpredicts response above and below target frequency**
**Analysis does not account for reduction in input or other factors**
**Analysis shows poor correlation with data from spacecraft acoustic test**

- FEMCI Workshop

- May 18, 2000 (SAG)

## Damping Predictions - Test vs Analysis

**Several factors may have accounted for poor correlation between analytical predictions and test data**
- NASTRAN model may not have sufficient resolution to accurately predict damping for the complicated mode shapes driving the thruster response
- Analytical technique for predicting modal damping was not verified for SMRD
- Low level (-7 dB) acoustic data was scaled to full level.  Damping may not be fully effective until higher levels of input
- Expected acoustic reductions may not be cumulative.
- Expected acoustic reductions may not be fully effective for localized thruster response.

- FEMCI Workshop

- May 18, 2000 (SAG)

## Conclusion

**Addition of damping treatments successfully reduced acceleration response at thruster mounting locations to acceptable levels**
**Methodology used was straightforward to implement and could be used with existing NASTRAN models**
**Modal damping technique used to optimize damping treatments as well as predict response**
**Technique did not accurately predict peak acceleration response**
**Predictions of dynamic response should be verified by testing the structure under representative loading conditions.**

- FEMCI Workshop

- May 18, 2000 (SAG)