SIRCA (silicone impregnated reusable ceramic ablator) SIRCA

spore: profile
id: sirca
spore_version: 0.2
family: ablators
name: SIRCA (silicone impregnated reusable ceramic ablator)
abbreviation: SIRCA
role: ablator
category: thermal-protection
type: ablators
summary: |
  SIRCA is a NASA Ames ablator made by impregnating a ceramic fibre tile with silicone resin. NASA lists three grades, SIRCA-14A, SIRCA-15F and SIRCA-25L, with virgin densities of 224, 240 and 400 kg/m^3. The SIRCA-15F grade was used on the aft plate heat shield of the Mars Pathfinder probe, which landed on Mars in July 1997, and was selected for the X-34 nose cap and leading edges. NASA's gas permeability study found virgin SIRCA about four orders of magnitude less permeable than virgin PICA.
best_for: High heating rate, low heat load and low mechanical load conditions, per NASA's description of SIRCA-14A
weakest_at: Lower thru-the-thickness tensile strength in the 14A and 15F grades than in 25L (TPSX), and gas permeability that changes as the silicone resin decomposes (NASA)
identity:
  formula: Ceramic fibre tile (FRCI-12 for 15F, AIM-10 for 14A, LI-2200 for 25L) infiltrated with silicone resin (RTV-655 for the 15F samples tested by NASA)
  source: tpsx-40
  where: 'Material page, composition field (SIRCA-15F: FRCI-12 tile infiltrated with silicone); NTRS report, Materials section (RTV-655 by General Electric). Substrate bulk densities for 14A (AIM-10, 0.16 g/cc) and 25L (LI-2200, 0.353 g/cc) are printed on their material pages, tpsx-41 and tpsx-42'
tags:
  stresses:
    - extreme-heat
  applications:
    - heat-shields
properties:
  - benchmark: density
    label: Density, SIRCA-14A virgin
    value: 224
    unit: kg/m^3
    scope: material
    conditions:
      grade: SIRCA-14A [virgin]
      uncertainty: 4.8 as printed
      basis: Properties at Standard Conditions, SI units
    method: Density row; printed as 2.24e+02 kg/m^3 and marked measured with Reference 1.
    status: ground
    source: tpsx-41
    where: Properties at Standard Conditions table, Density row
  - benchmark: density
    label: Nominal density of the FRCI-12 substrate before impregnation
    value: 192
    unit: kg/m^3
    scope: material
    conditions:
      material: FRCI-12 ceramic fibre tile, before silicone impregnation
      basis: nominal value as quoted in the NASA report
    method: Nominal value printed as 192 kg/m^3 (12 lb/ft3).
    status: ground
    source: ntrs-sirca-permeability
    where: Materials section, paragraph on SIRCA samples
  - benchmark: thermal_conductivity
    label: Thermal conductivity, isotropic, SIRCA-15F virgin
    value: 0.059
    unit: W/m-K
    scope: material
    conditions:
      grade: SIRCA-15F [virgin]
      direction: isotropic
      temperature: not stated on the page (STP column reads nonstp)
    method: Printed as 5.90e-02 W/m-K in the Thermal Conductivity (Isotropic) row; the page gives no temperature.
    status: ground
    source: tpsx-40
    where: Properties at Standard Conditions table, Thermal Conductivity (Isotropic) row
  - benchmark: thermal_conductivity
    label: Thermal conductivity, isotropic, SIRCA-14A virgin
    value: 0.0629
    unit: W/m-K
    scope: material
    conditions:
      grade: SIRCA-14A [virgin]
      direction: isotropic
      temperature: not stated on the page (STP column reads nonstp)
    method: Printed as 6.29e-02 W/m-K; the page lists the source as unknown and gives no reference.
    status: ground
    source: tpsx-41
    where: Properties at Standard Conditions table, Thermal Conductivity (Isotropic) row
  - benchmark: specific_heat
    value: 1200
    unit: J/kg-K
    scope: material
    conditions:
      grade: SIRCA-14A [virgin]
      temperature: not stated on the page (STP column reads TRUE)
    method: Printed as 1.20e+03 J/kg-K; the page lists the source as unknown.
    status: ground
    source: tpsx-41
    where: Properties at Standard Conditions table, Specific Heat row
  - benchmark: normal_emittance
    label: Emissivity
    value: 0.921
    unit: '-'
    scope: material
    conditions:
      grade: SIRCA-15F [virgin]
      uncertainty: 0.0461 as printed
      basis: dimensionless
    method: Printed as 9.21e-01 in the Emissivity row.
    status: ground
    source: tpsx-40
    where: Properties at Standard Conditions table, Emissivity row
  - benchmark: normal_emittance
    label: Emissivity
    value: 0.921
    unit: '-'
    scope: material
    conditions:
      grade: SIRCA-14A [virgin]
      basis: dimensionless
    method: Printed as 9.21e-01 in the Emissivity row, marked measured with Reference 1.
    status: ground
    source: tpsx-41
    where: Properties at Standard Conditions table, Emissivity row
  - benchmark: elongation
    label: Tensile strain to failure, thru-the-thickness, SIRCA-15F virgin
    value: 0.0093
    unit: m/m
    scope: material
    conditions:
      grade: SIRCA-15F [virgin]
      direction: thru-the-thickness
      temperature: room temperature
    method: Printed as 9.30e-03 m/m.
    status: ground
    source: tpsx-40
    where: Properties at Standard Conditions table, Tensile Strain to Failure (Thru-the-Thickness) row
  - benchmark: elongation
    label: Tensile strain to failure, in-plane, SIRCA-15F virgin
    value: 0.0066
    unit: m/m
    scope: material
    conditions:
      grade: SIRCA-15F [virgin]
      direction: in-plane
      temperature: room temperature
    method: Printed as 6.60e-03 m/m.
    status: ground
    source: tpsx-40
    where: Properties at Standard Conditions table, Tensile Strain to Failure (In-Plane) row
  - benchmark: cte
    label: Coefficient of thermal expansion, isotropic, SIRCA-15F virgin
    value: 0.00000142
    unit: 1/K
    scope: material
    conditions:
      grade: SIRCA-15F [virgin]
      direction: isotropic
      basis: interpolated with temperature, per the page
    method: Printed as 1.42e-06 1/K in the Coefficient of Thermal Expansion (Isotropic) row.
    status: ground
    source: tpsx-40
    where: Properties at Standard Conditions table, Coefficient of Thermal Expansion (Isotropic) row
  - benchmark: cte
    label: Coefficient of thermal expansion, isotropic, SIRCA-14A virgin
    value: 0.00000142
    unit: 1/K
    scope: material
    conditions:
      grade: SIRCA-14A [virgin]
      direction: isotropic
      basis: interpolated with temperature, per the page
    method: Printed as 1.42e-06 1/K in the Coefficient of Thermal Expansion (Isotropic) row.
    status: ground
    source: tpsx-41
    where: Properties at Standard Conditions table, Coefficient of Thermal Expansion (Isotropic) row
  - benchmark: density
    label: Final density after silicone impregnation, SIRCA-15F virgin (approximate)
    value: 0.25
    unit: g/cc
    scope: material
    conditions:
      grade: SIRCA-15F [virgin]
      basis: approximate final density after impregnation, as stated in the page description
    method: Printed as a final density of approximately 0.25 g/cc in the Description paragraph. The Density row of the table for the same grade prints 2.40e+02 kg/m^3 in scientific notation, so that row is not entered here.
    status: ground
    source: tpsx-40
    where: Description paragraph, SIRCA-15F material page
corrosion:
  - environment: Heating above the decomposition temperature of the silicone resin
    headline: Resin decomposition at high temperature changes the internal microstructure, so gas permeability depends on the heating environment.
    effect: The NASA report says the resins in PICA and SIRCA decompose at high temperature and alter the internal microstructure, so both permeability parameters depend on the heating environment, including its chemical composition. The authors note the relation holds only for fixed microstructures.
    rating: moderate
    conditions:
      material: virgin and charred SIRCA (silicone resin in ceramic tile)
      basis: statement in the Discussion of the NASA report; no separate numeric test of this effect is quoted
    source: ntrs-sirca-permeability
    where: Discussion, paragraph beginning 'The resins in PICA and SIRCA decompose at high temperature'
  - environment: Pyrolysis under argon, cycled to about 1250 K in a tube furnace
    headline: About 60 percent of the silicone resin mass remains as char after pyrolysis.
    effect: The NASA report gives the char yield of the silicone resin in SIRCA as about 0.60, against about 0.63 for the phenolic resin in PICA. Char yield is defined as the percentage of mass remaining after pyrolysis, so the rest of the resin mass leaves during pyrolysis.
    rating: moderate
    conditions:
      atmosphere: argon gas flow
      temperature: about 1250 K
      measure: char yield V, the percentage of mass remaining after pyrolysis
      material: silicone resin in SIRCA
    source: ntrs-sirca-permeability
    where: Experimental Methods, paragraph on charting (char yield)
  - environment: Specimen handling in the permeability test rig
    headline: Virgin SIRCA is soft, and the silicone resin can compact into the surface and block flow.
    effect: Initial tests on virgin SIRCA showed negligible gas flow. The authors thought soft silicone resin compacted into the end faces during machining or insertion. Specimens made with broken-off ends could not be removed and re-inserted without crushing the fracture surfaces, so SIRCA specimens could not be re-tested.
    rating: low
    conditions:
      basis: laboratory test rig, not an in-service exposure
    source: ntrs-sirca-permeability
    where: Experimental section, paragraphs on virgin SIRCA specimens
  - environment: Arc-jet heating at simulated Mars Pathfinder entry conditions, NASA Ames 60 MW Interaction Heating Facility
    headline: Arc-jet tests found no char erosion or mass loss up to a stagnation pressure of 0.25 atm.
    effect: 'The 1994 abstract by Tran, Rasky, Hsu and Turan says: Arc jet test results yielded no evidence of char erosion and mass loss at high stagnation pressures to 0.25 atm. The abstract gives no numeric mass loss or recession values for these tests.'
    rating: low
    conditions:
      material: SIRCA (silicone impregnated reusable ceramic ablator)
      basis: arc-jet test abstract; qualitative result, no numeric values printed
    source: ntrs-lca-1994
    where: Abstract, arc-jet test sentence (NTRS record 20010110781)
risks:
  hazards:
    - text: PubChem reports, from ECHA notifications, that poly(dimethylsiloxane) is classified H413 (May cause long lasting harmful effects to aquatic life) in 34.6 percent of company reports; 58.2 percent (212 of 364) of reports say it does not meet GHS hazard criteria.
      ghs:
        - H413
      source: pubchem-pdms
      where: Safety and Hazards, GHS Classification section
    - text: The ILO-WHO International Chemical Safety Card 0318 for poly(dimethylsiloxane) states that the substance is irritating to the eyes and recommends safety goggles, while the USCG CHRIS entry reads 'Contact of liquid with eyes may cause temporary discomfort.'
      ghs: []
      source: pubchem-pdms
      where: Sections 9.1.3, 9.6.2 and 9.6.3 of the PubChem page
  failure_risks:
    - mode: Change in gas permeability as the silicone resin decomposes
      trigger: Heating high enough to decompose the resin, which alters the internal microstructure
      consequence: Gas flow through the material depends on the heating environment, so permeability of a charred SIRCA cannot be taken from a virgin specimen. The report notes that this affects the modelling of hot gas penetration.
      severity: moderate
      source: ntrs-sirca-permeability
      where: Discussion, paragraph on resin decomposition; Introduction, paragraph on permeability data
  side_effects: []
  handling:
    - text: Virgin SIRCA specimens could not be re-tested after permeability measurement, because removing them crushed the fracture surfaces.
      source: ntrs-sirca-permeability
      where: Experimental section, paragraphs on virgin SIRCA specimens
    - text: To avoid clogged end faces, the NASA team turned longer cylinders and broke the ends off in situ, then cut two specimens from one cylinder to test the virgin and charred states.
      source: ntrs-sirca-permeability
      where: Experimental section, specimen preparation paragraphs
  mitigations: []
uses:
  - application: heat-shields
    device: Mars Pathfinder aft plate heat shield and part of the conical aft section (SIRCA-15F)
    example: NASA's TPSX entry says SIRCA was used for the aft plate heat shield and part of the conical aft section of the Mars Pathfinder probe, which landed on Mars in July 1997. The NASA report says the same.
    source: tpsx-40
    where: Applications section of the SIRCA-15F material page; NTRS report, Introduction
  - application: heat-shields
    device: X-34 nose cap and wing and rudder leading edges (reusable thermal protection, SIRCA-15F)
    example: NASA's TPSX entry says SIRCA-15F was selected for the X-34 nose cap and wing and rudder leading edges as reusable thermal protection. The page does not say the X-34 flew with it.
    source: tpsx-40
    where: Applications section of the SIRCA-15F material page; NTRS report, Introduction (selected for the leading edges and nose cap of the X-34)
  - application: heat-shields
    device: Backshell Interface Plate for the Mars 2003 mission (manufacture)
    example: NASA's TPSX entry lists manufacture of SIRCA in support of the Backshell Interface Plate for the Mars 2003 mission. The page gives no flight result.
    source: tpsx-40
    where: Applications section of the SIRCA-15F material page
supply:
  - subject: Silicon metal (feedstock for the silicone resin in SIRCA)
    abundance: 'USGS: ''World and domestic resources for making silicon metal and alloys are abundant'' and the source is silica, such as quartzite'
    producers:
      - country: China
        share_percent: 87
        year: 2025
      - country: Brazil
        share_percent: 3.9
        year: 2025
      - country: Norway
        share_percent: 2.8
        year: 2025
      - country: France
        share_percent: 1.5
        year: 2025
    source: usgs-silicon-mcs2026
    where: World Production table, silicon metal, 2025 (world total 4,600 thousand metric tons, silicon content; China 4,000; shares calculated by Space Mushroom against that total). USGS also says 'China accounted for almost 80% of total global estimated production of silicon materials in 2025.' The chapter names silicones as a use of silicon.
documents:
  - title: 'NASA Ames TPS database: SIRCA-15F [virgin] (material page id 40)'
    type: database
    url: https://tpsx.arc.nasa.gov/Material?id=40
    year: 2006
  - title: 'NASA Ames TPS database: SIRCA-14A [virgin] (material page id 41)'
    type: database
    url: https://tpsx.arc.nasa.gov/Material?id=41
    year: 2006
  - title: 'NASA Ames TPS database: SIRCA-25L [virgin] (material page id 42)'
    type: database
    url: https://tpsx.arc.nasa.gov/Material?id=42
    year: 2006
  - title: 'NASA Ames TPS database: PICA [virgin] (material page id 43), comparison only'
    type: database
    url: https://tpsx.arc.nasa.gov/Material?id=43
    year: 2006
  - title: 'NASA CR-208331: Gas Permeability of the Lightweight Ceramic Ablators (PICA and SIRCA), 1997-1998 annual report'
    type: report
    url: https://ntrs.nasa.gov/api/citations/19980235586/downloads/19980235586.pdf
    year: 1998
  - title: 'USGS Mineral Commodity Summaries 2026: Silicon (E. K. Schnebele)'
    type: report
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-silicon.pdf
    year: 2026
  - title: 'PubChem compound record: Poly(dimethylsiloxane), GHS Classification and Safety and Hazards sections'
    type: database
    url: https://pubchem.ncbi.nlm.nih.gov/compound/Polydimethylsiloxane
  - title: 'NASA NTRS record 20010110781: Light Weight Ceramic Ablators for Mars Follow-on Mission Vehicle Thermal Protection System (Tran, Rasky, Hsu, Turan, 1994 abstract)'
    type: paper
    url: https://ntrs.nasa.gov/citations/20010110781
    year: 1994
gaps:
  - No commercial producer of SIRCA was found; the TPSX pages name NASA Ames as the developer. The supply section gives the silicon metal feedstock for the silicone resin (USGS). No producer is cited for the FRCI-12 or LI-2200 ceramic fibre tiles.
  - No safety data sheet or GHS classification for SIRCA itself was found. The hazard entries use PubChem data for poly(dimethylsiloxane), the base polymer of silicone resins, which is not the RTV-655 resin NASA used.
  - No mitigation or handling-control source was found for SIRCA in service, so risks.mitigations is empty.
  - No maximum use temperature is listed on any of the three SIRCA grade pages. The 15F page says so explicitly.
  - Temperature for thermal conductivity, specific heat and emissivity is not stated on the grade pages (the STP column reads nonstp, TRUE or unknown).
  - The numeric continuum permeability values in the NASA report are garbled in the PDF text (the exponents are unreadable), so no permeability number is used. Its Tables 1 to 4 (density and permeability per specimen) do not extract as text.
  - The NASA report states that PICA permeability is comparable to un-impregnated LI-900 and FRCI-12 tiles, and that SIRCA is four orders of magnitude less permeable. No SIRCA permeability value was taken from the garbled table text.
  - The 15F density is printed as 2.40e+02 kg/m^3 in the table and as approximately 0.25 g/cc in the description. Only the description value is entered as a property, because the table value is in scientific notation. The difference is not resolved by any source we hold.
  - The 14A and 15F compressive and tensile-modulus rows print the same values on the grade pages, so 14A compressive and modulus values were left out as not independently sourced.
  - No SIRCA-specific ageing, humidity, oxidation or long-duration exposure data were found. The NASA executive summary says the branch tests thermal shock and hostile-environment tolerance but gives no SIRCA results.
  - The 14A grade page cites Reference 1 (SIRCA-15F mechanical properties report) for its density, emissivity and tensile values; the page does not explain how that applies to 14A.
  - Flight environment tags are not set. The aft plate flew on Mars Pathfinder, but the sources do not name the entry environment in a form we could tag.
  - No image or media was sourced; no image file is included.
  - The AIAA 96-1819 paper that the TPSX pages cite (Tran et al.) was not found online. The 1994 NTRS abstract (record 20010110781) gives qualitative arc-jet results only; its density range covers light weight ceramic ablators in general, so it is not used as a SIRCA value.
sources:
  tpsx-40:
    title: 'NASA Ames TPS database: SIRCA-15F [virgin], Properties at Standard Conditions'
    id: TPSX material id 40
    url: https://tpsx.arc.nasa.gov/Material?id=40
    year: 2006
    kind: database
    license: NASA public web page (cited, not reproduced)
  tpsx-41:
    title: 'NASA Ames TPS database: SIRCA-14A [virgin], Properties at Standard Conditions'
    id: TPSX material id 41
    url: https://tpsx.arc.nasa.gov/Material?id=41
    year: 2006
    kind: database
    license: NASA public web page (cited, not reproduced)
  tpsx-42:
    title: 'NASA Ames TPS database: SIRCA-25L [virgin], Properties at Standard Conditions'
    id: TPSX material id 42
    url: https://tpsx.arc.nasa.gov/Material?id=42
    year: 2006
    kind: database
    license: NASA public web page (cited, not reproduced)
  tpsx-43:
    title: 'NASA Ames TPS database: PICA [virgin] (comparison grade)'
    id: TPSX material id 43
    url: https://tpsx.arc.nasa.gov/Material?id=43
    year: 2006
    kind: database
    license: NASA public web page (cited, not reproduced)
  ntrs-sirca-permeability:
    title: 'Thermal Protection Materials Development, 1997-1998 Annual Report: Gas Permeability of the Lightweight Ceramic Ablators (PICA and SIRCA), NASA/CR-208331 (Selvaduray and Cox, San Jose State University)'
    id: NTRS 19980235586; NASA/CR-208331
    url: https://ntrs.nasa.gov/api/citations/19980235586/downloads/19980235586.pdf
    year: 1998
    kind: report
    license: public domain (NASA)
  usgs-silicon-mcs2026:
    title: 'USGS Mineral Commodity Summaries 2026: Silicon'
    id: USGS MCS 2026, February 2026
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-silicon.pdf
    year: 2026
    kind: report
    license: public domain (USGS)
  pubchem-pdms:
    title: 'PubChem: Poly(dimethylsiloxane), GHS Classification and Safety and Hazards'
    id: PubChem Reference Collection SID 516575524
    url: https://pubchem.ncbi.nlm.nih.gov/compound/Polydimethylsiloxane
    kind: database
    license: PubChem public database (NCBI); GHS data from ECHA C&L notifications
  ntrs-lca-1994:
    title: 'Tran, Rasky, Hsu and Turan: Light Weight Ceramic Ablators for Mars Follow-on Mission Vehicle Thermal Protection System (NASA NTRS abstract, 1994)'
    id: NTRS 20010110781
    url: https://ntrs.nasa.gov/citations/20010110781
    year: 1994
    kind: paper
    license: public domain (NASA)
license: CC-BY-4.0
updated: '2026-11-01'
review:
  status: draft
  method: assistant
  checked_values: 12
  unreachable: 0
  caveats: 2
  note: Written by an AI assistant on 2026-11-01 from three NASA TPSX grade pages, the NASA Ames PICA page and one NASA report. Values were read from fetched pages in this run; the PDF's permeability tables did not extract cleanly and were left out.
story:
  recipe:
    line: SIRCA is a ceramic fibre tile that is impregnated with a silicone resin. NASA made it by partially impregnating fibrous tile substrates with a resin, which is also how PICA is made with phenolic resin. The NASA report does not give the atom-level composition of the silicone resin.
    parts:
      - name: Ceramic fibre tile substrate
        share: FRCI-12 nominal 192 kg/m^3 for SIRCA-15F; AIM-10 at 0.16 g/cc for SIRCA-14A; LI-2200 at 0.353 g/cc for SIRCA-25L
        role: Porous, low-density skeleton that holds the resin and sets the open structure NASA measured for gas flow.
      - name: Silicone resin, RTV-655 (General Electric)
        share: impregnated to a final density of approximately 0.25 g/cc for SIRCA-15F, per the TPSX description
        role: Fills part of the tile pore space. On heating it decomposes and leaves about 60 percent of its mass as char, which changes the material's gas permeability.
  risk_groups:
    - title: For people
      level: 0
      verdict: No hazard classification found
      lead: We found no safety data sheet or GHS classification for SIRCA itself. Its silicone resin is closest to poly(dimethylsiloxane), for which PubChem lists one GHS hazard statement, for aquatic life.
      points:
        - label: Hazard data
          line: PubChem lists H413 (long lasting harmful effects to aquatic life) for poly(dimethylsiloxane), and reports that most company notifications do not meet GHS hazard criteria. Check the supplier's data for any SIRCA part you handle.
    - title: In the test rig
      lead: These points come from laboratory permeability work, not from service.
      points:
        - label: Virgin specimens
          line: Virgin SIRCA showed negligible gas flow at first, probably because soft silicone resin compacted into the specimen ends. Specimens could not be re-tested once removed.
        - label: Charring
          line: On heating to about 1250 K under argon, the silicone resin leaves about 0.60 of its mass as char, and the char changes gas flow.
    - title: In service
      lead: The sources do not report failures of SIRCA in flight, so these are design points only.
      points:
        - label: Permeability change
          line: NASA states that the resins in PICA and SIRCA decompose at high temperature and change the internal microstructure, so gas permeability depends on the heating environment.
  why: SIRCA is a silicone-based ablator, a lightweight ceramic tile that NASA Ames impregnated with silicone resin. Its density is low, at 224 to 400 kg/m^3 depending on the grade, and it was designed to take a heat load and then shed it by ablating. NASA used it for the aft plate of the Mars Pathfinder probe, which landed on Mars in July 1997, and selected it for the leading edges and nose cap of the X-34 vehicle. Its grade choice depends on heating rate, heat load and mechanical load.
  uses:
    - where: Mars Pathfinder aft heat shield
      line: NASA's TPSX entry for SIRCA-15F says the material was used on the aft plate heat shield and part of the conical aft section of the Mars Pathfinder probe, which landed on Mars in July 1997. The NASA report says the same.
    - where: X-34 nose cap and leading edges
      line: NASA's TPSX entry says SIRCA-15F was selected for the X-34 nose cap and wing and rudder leading edges as reusable thermal protection. The page does not say the vehicle flew with it.
    - where: Mars 2003 backshell interface plate
      line: NASA's TPSX entry lists manufacture of SIRCA in support of the Backshell Interface Plate for the Mars 2003 mission. The page gives no flight result.
  can_do:
    - label: Density range
      benchmark: density
      value: 224 to 400 kg/m^3
      line: Virgin SIRCA densities listed by NASA are 224 kg/m^3 for SIRCA-14A, 240 kg/m^3 for SIRCA-15F and 400 kg/m^3 for SIRCA-25L.
    - label: Heat-rate envelope
      value: High heating rate, low heat load
      line: NASA describes SIRCA-14A as best suited to high heating rate, low heat load and low mechanical load conditions. The SIRCA-25L page says it tolerates higher heating rates than the 15F and 14A grades.
  compare: SIRCA is a silicone-based ablator, so its closest comparison is PICA, which uses a phenolic resin in a carbon-fibre tile. NASA's permeability study found virgin SIRCA about four orders of magnitude less permeable to gas than virgin PICA. The TPSX page for PICA (id 43) lists its own property table, which is not repeated here. Among the three SIRCA grades, 25L has the highest density (400 kg/m^3) and 14A the lowest (224 kg/m^3).
  compare_rows:
    - label: Density
      benchmark: density
    - label: Thermal conductivity
      benchmark: thermal_conductivity
    - label: Emittance
      benchmark: normal_emittance
  handling:
    - Virgin SIRCA is soft, and silicone resin can compact into the surface and block gas flow during specimen preparation.
    - A SIRCA specimen cannot be taken out of a test rig and put back without crushing its fracture surfaces.
    - The NASA report gives no service handling or safety guidance for SIRCA.
  good_for:
    - High heating rates at low heat load and low mechanical load, per NASA's description of SIRCA-14A.
    - Reusable leading edges and nose caps, as NASA's X-34 selection for SIRCA-15F shows.
  watch_out:
    - Gas permeability changes as the silicone resin decomposes, so charred and virgin values differ.
    - The 14A and 15F density and tensile values rest on one NASA reference that the 14A page does not explain.
    - No maximum use temperature is listed on any SIRCA grade page.
  verdict: SIRCA is a lightweight silicone-based ablator with flight heritage on Mars Pathfinder. Its published values are sparse, and several rows are marked as unknown or predicted, so check the grade and the page before you use a number.