# Presentation: 344038

## Thermal Protection System (TPS) Return-to-Flight Activities

**Steve M. Poulos, Jr.**
**Orbiter Project Office**
**September, 2003**

## Eliminate/minimize debris sources

**Eliminate/minimize debris sources**
**Improve/develop inspection capability**
**Define TPS impact tolerance**
**Develop TPS repair capability**
**Evaluate ISS to keep the crew safe until they can be returned to Earth**

- 24 July 2000

## TPS RTF Activities Comprise Hardware Processing, Impact Testing, and Model Development

**Both Tile and Reinforced Carbon Carbon (RCC) for STS-114 Will Complete Their Normal Turnaround Flow Processing**
- RCC panels will also be subjected to non-destructive evaluation (NDE) to look for potential hidden damage such as cracks, voids, delaminations, or sub-surface oxidation.
**Analytical Models Will Be Developed to Predict Damage to Tile or RCC**
- Basic material properties testing will provide data necessary for model development
- Impact test results will verify the model predictions
**Impact Testing Will Be Performed on Both Tile and RCC to Determine the Damage Threshold**
- Tile impact tests will be performed on acreage tile, carrier panels, and door edge tile configurations
- RCC impact testing will be conducted on coupons and full-scale panels

- 24 July 2000

## Tile Turnaround Processing

**Tile RTF Preparation Will Meet All Turnaround Requirements and Include All Processing Verifications**
- Complete Visual TPS inspection for damages/discrepancies
- Nose Landing Gear Door (NLGD), Main Landing Gear Door (MLGD), External Tank Door (ETD) perimeter tile and Leading Edge Support Structure (LESS) Carrier Panel (C/P) tile/previous repair integrity inspection
- Elevon Cove Leak Check verification
- MLGD environmental seal contact verification
- Perform all lower surface flow path inspections
- ET Doors
- MLGD
- NLGD
- LESS C/P’s
- Chin Panel C/P’s

- 24 July 2000

## RCC Turnaround Processing

**RCC RTF Preparation Will Meet All Turnaround Requirements and Include All Processing Verifications**
- Step and gap evaluation (alignment)
- Spar fitting shimming to original build condition
- Panel/tee clevis fitting, shear fitting and spanner beam fitting shimming to per-print gap requirements
- Addressed all spar corrosion issues and hole thread mark issues
- Visual inspection of RCC for pin holes
**RCC Panels Are Also Undergoing NDE to Look for Potential Damage or Degradation Not Visible to the Naked Eye**
- Ultrasound to look for delaminations and voids
- Eddy Current to look for localized oxidation
- X-ray to look for cracks
- Thermography to look for delaminations or cracks

- 24 July 2000

## RCC Turnaround Processing

**RCC Metallic Attach Hardware Is Being Subjected to NDE**
- Visual to look for corrosion
- Dye-penetrant and eddy current on Selected Components to Look for cracks/embrittlement

- 24 July 2000

## Analytical Model Will Be Developed to Predict Damage to RCC

- RCC Characterization (material properties)

- _Level 2 - Subcomponent Tests_
- Damage Model Validation
- Flat Panel Impact Testing
- Combined Loading Evaluation

- _Level 3 – Flight Panel Tests_
- RCC leading edge panel attached to representative wing structure

**RCC Testing Plan Organized in a “Building-Block” Approach**

**2**

**Panels 9, 10, 16, and 17**

**Analytical Model Will Be Developed to Predict Damage to RCC**

- 24 July 2000

## Level 1 Coupon Tests Will Develop Basic RCC Material Properties

**Material characterization program (strength, stiffness, stress-strain curves, fracture) to evaluate the effects of several variables:**
- Silicon Carbide (SiC) Coating
- High strain-rate
- Mass loss (max value = 0.03 lb/ft3)
- Laminate thickness (19-ply and 38-ply)
**Test data will be used to update the material model input in the analytical tools used to predict impact damage (primarily LS-DYNA)**
**NDE scans required on all coupons prior to testing**

**3**

- 24 July 2000

## Level 1 Coupon Tests Will Develop Basic RCC Material Properties

**4-point toughness test – delamination mode**

**Compact Tension Toughness – through-thickness crack**

**9**

**Level 1 Coupon Tests Will Develop Basic RCC Material Properties**

**Fracture property coupons – Goal is to determine critical fracture properties of RCC material for use in damage tolerance analysis**

- 24 July 2000

## Level 2 Flat Panel Tests Will Validate RCC Damage Model

**Level 2 Flat Panel Tests Will Validate the RCC Damage Model (LS-DYNA) and Determine the Threshold Between Acceptable and Unacceptable damage.**
- Flat panel impact tests used to determine threshold of damage initiation
- Follow-on structural tests used to determine threshold of acceptable damage (damage tolerance program to evaluate residual strength and damage propagation)
**Phase A (RTF Critical):  Initial Flat Panel Impact Tests Examine the Effects of Different Projectile Materials Corresponding to Most Likely Vehicle-Generated Debris Types:**
- Foam (BX-265)
- Ablator (select from Super Lightweight Ablator (SLA), Marshall Convergent Coating (MCC)-1, Booster Trowelable Assembly (BTA), others)
- Ice
- Metal (steel or aluminum)
**Subsequent Tests (Phases B and C) Will Examine Variable Impact Angles, Projectile Sizes, and Velocities**
- These tests are required for full model validation but not RTF
**NDE Scans Required Prior to and After Testing (Ultrasound, Thermography, etc.)**

**10**

- 24 July 2000

## Level 3 Full-Scale Tests Provide System Validation of the Analytical Model

**Level 3 Full-Scale Tests Provide System Validation of the Analytical Model**

**Level 3 structural testing approach**
- Supports model validation
- Follows completion of Level 1 & 2 testing
- Maximizes model validation developed in a building block approach
- Uses RCC assets: Panels #9, 10, 16, & 17
- 2 foam impacts
- 1 at low damage condition (below survivable damage threshold)
- 1 at high damage condition (above survivable damage threshold)
- 1 ablator impact at survivable damage threshold
- A two panel test configuration required
- One target RCC panel/T-seal, & one real or fiberglass downstream panel
- Wing spar structure may be present, but not required
- Current analysis shows support structure is not critical
- Compare model predictions to test results

- 24 July 2000

## RTF Tile Impact Testing  

**Objectives:**
- Generate test data to support the development of a refined analytical impact model
- Characterize threshold velocity and total damage
- Available debris sources and tile types
- Characterize damage scatter
- Characterize benefit of densified layer
- Characterize effect of projectile orientation
- Compare damage tolerance of new and aged tile types
- Characterize tile damage levels which have a potential for on-orbit repair
- Evaluate tile configurations sensitivity to available impact debris (MLGD, Carrier Panel)

- 24 July 2000

## Multiple Tile Impact Variables Need to Be Understood

**Approach:**
- Refined tile damage models will be established for most prevalent debris sources
- Foam and ice constitute the spectrum of debris hardnesses
- Lockheed Insulation (LI)-900 tile most sensitive to impact
- Remaining variables will be inserted at various points in the testing to support model correlation
- Additional foam types
- Ablators including MCC1,SLA,Cork
- Metal
- Existing and proposed tile types will be evaluated
- LI-2200, Fiber Reinforced Composite Insulation (FRCI)-12, Boeing Reuseable Insulation (BRI)-20 and BRI-8
**Total number of shots required expected to be ~1000 for the program**

- 24 July 2000

## Phase I Tile Impact Testing

**Phase I Objectives:**
- Characterize LI-900 with foam impactors on main landing gear doors and and wing acreage test articles at Southwest Research in San Antonio
- Square and rectangular foam cross sections will be utilized
- Will characterize damage caused by foams (North Carolina Foam, Inc., BX-265, Polymer Development Lab)
- Impact aged tiles versus new
- Determine if damage is greater for aged tiles
- Determine if densified layer remains for through-the-thickness damage
**Phase I Parameters:**
- Particle sizes chosen to bound expected debris
- Angles chosen to bound impacts on lower surface
**Initial Velocities Chosen to Create Damage**
- Velocities will vary and be defined by test results

- 24 July 2000

## Phase II & Phase III Tile Impact Testing 

**Phase II **
- Testing evaluates ice, metals and ablators on LI-900 substrate
- LI-900 Tile Repairs
**Phase III**
- Test alternate tile configurations
- MLGD edge tile
- Carrier panels
- Test alternate tile types (BRI 20, FRCI)
- Impactors include foam, ice, metal and ablators

- 24 July 2000