PICA

PICA (Phenolic-Impregnated Carbon Ablator) is an ablative material used in heat shields and thermal protection systems in many spacecraft systems. The PICA name and production process is under NASA patent (which is loaned to, among others, SpaceX for use on the Dragon heat shield). Luckily, we have access to the myriad Chinese researchers attempting to duplicate the material for domestic use.

This replicates the PICA production process and measured properties from the paper "Lightweight, flexible, and heat-insulated phenolic impregnated carbon ablator (PICA) with adjustable flexibility and high compressive resilience property" by Liu et al. doi/full/10.1002/app.51712.

Production process

The base material for PICA production is flexible carbon felt. In this paper, the material had a density around 0.16g/cm3.

The phenolic mixture is formed from resorcinol (or resorcin, benzene-1,3-diol), formaldehyde in water (around 37-40%wt in H2O) and anhydrous sodium carbonate catalyst. These may be able to be procured as a two-part mix from a supplier.

The two organic components of the phenolic mixture are combined with an amount of de-ionised water and mixed well with a magnetic stir bar. The catalyst is added to complete the RF "sol" liquid.

The flexible carbon felt is then submerged in the sol and a vacuum pulled - this helps draw water off the sol. The whole mixture is heated to 90°C under vacuum for two days, to allow the sol to gel. Once this is complete, the PICA is cured at 90°C and ambient pressure for another 12 hours.

The paper included an additional degassing step, where the PICA was heated to 120°C for 6 hours, but is it unclear if this is a necessary part of the process.

Material properties

Sample name	Raw felt	5RF	10RF	15RF	20RF
Resorcinol %wt in sol	-	3.23%	6.47%	9.71%	12.94%
Formaldehyde (37-40%wt water) %wt in sol	-	4.64%	9.29%	13.93%	18.57%
Sodim carbonate catalyst %wt in sol	-	6.2e-3%	12.5e-3%	18.7e-3%	24.9e-3%
Deionised water %wt in sol	-	92.11%	84.23%	76.34%	68.46%
Density (g/cm3)	0.16 ± 0.01	0.20 ± 0.01	0.24 ± 0.02	0.27 ± 0.02	0.31 ± 0.02
Thermal conductivity (W/(m.K))	0.110 ± 0.005	0.104 ± 0.004	0.107 ± 0.003	0.113 ± 0.005	0.133 ± 0.003
Compressive strength @ 60% strain (MPa)	0.25 ± 0.05	1.37 ± 0.08	2.62 ± 0.06	8.44 ± 0.07	N/A (failed before 60% yield)
Number of folds (180° fold and relaxation)	-	>500	>500	>500	1
Time for heat penetration through material 1000C back face temperature Time to reach 600C measured (s)	118s	-	280 (char noticed, reduction in aerogel fraction after test)	252 (char noticed, reduction in aerogel fraction after test)

Recommendations

10RF or 15RF flexible PICA is likely the ideal material for usage on the nosecone of Aquila, provided the production process can be carried out with sufficient reliability. 10RF is more insulating and appears to be a marginally better insulator, at the expense of lower strength. If possible, it may be wise to form the PICA in a Nomex honeycomb structure to provide additional in-plane strength. It would also be advisable to determine the effective surface roughness of PICA under supersonic loading (CUED supersonic tunnel may be suitable for a small sample) - the felt structure may deflect and cause ripples under shear drag forces.

Approximate material costs

Flexible carbon felt: non-aerospace grade available for furnace lining. Extremely cheap samples can be found on Amazon/AliExpress (pennies per cubic centimetre), furnace grade is closer to pounds per cubic centimetre for odd-size offcuts and thicknesses.

Resorcinol can be obtained from Sigma Aldrich for around £100/kg.

Formaldehyde is a nasty chemical so may present transport/handling issues but can also be obtained from Fischer Sci (in the identical concentration to the paper - likely the same supply chain) for around $20/L.

So little sodium carbonate is used that we can probably pinch some from chemistry.

Safety

The main safety concerns come from the resorcinol and formaldehyde. The latter hits basically every hazard diamond - flammable, corrosive, toxic, carcinogenic. SDS recommends working in a fume hood with good airflow, eyewash stations nearby etc. Basically we would need a full chemical lab for the initial layup process. Full respirators are recommended for poor ventilation, which includes our likely use case. Waste is classified as hazardous so a proper waste handling stream would be required.