Question bank — Ablative materials — phenolic-impregnated carbon ablator (PICA), AVCOAT, SLA

False — a blanket only slows heat; given enough time heat soaks through. An ablator actively removes energy by consuming its own mass, so it can survive fluxes a passive TPS blanket cannot.

False — the dominant processes are endothermic pyrolysis and transpiration cooling, which absorb heat. Combustion (oxidation) is a secondary, often unwanted process that eats the protective char.

False — what matters is the effective heat of ablation $Q^{\ast }$ (J/kg, energy carried off per kilogram) and the recession depth $s=\int ({\dot{m}}^{\prime \prime }/\rho )dt$. Low-density PICA (~270 kg/m³) beats many denser materials because its $Q^{\ast }$ is high and its skeleton stays put.

False — the char is the most valuable part: it re-radiates heat as $\epsilon \sigma T_w^4$ (emissivity × Stefan–Boltzmann constant × wall temperature to the fourth power) and shields the virgin layer. We want it to recede slowly, not fall off. See the Stefan–Boltzmann Radiation Law.

False — outward gas flow (blowing/transpiration) thickens the boundary layer and pushes back the hot shock gas, so less heat is conducted inward. The gases also absorbed energy when the bonds broke.

False — each is tuned to a heating regime: PICA for high flux / low pressure (deep-space return), SLA for low flux over large area (Mars), AVCOAT for high integral load on big curved shields (Apollo/Orion). Wrong pick → spallation or excess mass.

False — spallation is mechanical char loss that removes the char before it has finished radiating and insulating. It wastes protective mass and is a failure mode, not a feature (this capped SLA-561V's use on MSL).

False — phenolic is prized precisely because it does not melt and drip; it chars, leaving a high-carbon skeleton. High char yield (~50–60%) means most of the resin becomes protective carbon. See Phenol-Formaldehyde (Phenolic) Resins.

False — reusable tiles are designed for the milder heating of low-Earth-orbit return. Deep-space return (~11 km/s, MW/m² fluxes) overwhelms them; that regime demands a single-use ablator.

False — the $\epsilon \sigma T_w^4$ term is a genuine energy sink and grows with the fourth power of surface temperature $T_w$, so at char temperatures of a few thousand kelvin it carries away a large share of the incoming flux ${\dot{q}}_{in}$. See Re-entry Aerodynamics & Shock Heating.

Error: $Q^{\ast }$ is in J/kg, energy per kilogram of material lost. Total capacity is $Q^{\ast }$ times the mass you're willing to ablate, not $Q^{\ast }$ alone.

Error: ${\dot{m}}^{\prime \prime }/\rho$ (kg/m²·s ÷ kg/m³) has units of m/s — it's the instantaneous retreat speed, not a distance. The depth is its time integral $s=\int ({\dot{m}}^{\prime \prime }/\rho )dt$, because you must sum the retreat speed over the whole entry to get how far the surface has moved.

Error: ${\dot{q}}_{cond}$ is heat conducted into the cold structure — the very thing we're trying to prevent. A good design drives it toward zero, letting the mass-loss term ${\dot{m}}^{\prime \prime }{Q}^{\ast }$ and re-radiation $\epsilon \sigma T_w^4$ take the load.

Error: density alone decides nothing. AVCOAT (~500 kg/m³) is chosen for mechanical robustness over large curved shields, not for lightness; PICA is chosen where every kilogram counts. Different jobs, not better/worse.

Error: char oxidation consumes the protective char, which is undesirable — you lose your radiator/insulator. Even where reactions release some heat, the net effect of removing char is harmful; ablation's cooling comes from endothermic pyrolysis and blowing.

Error: Mars entry is moderate flux but with large aeroshell area and long duration; SLA's ultralight, flexible silicone matrix wins on mass efficiency, not on surviving comet-grade fluxes.

Error: higher $Q^{\ast }$ means each kilogram absorbs more energy, so less mass is lost (${\dot{m}}^{\prime \prime }=({\dot{q}}_{in}-\epsilon \sigma T_w^4-{\dot{q}}_{cond})/Q^{\ast }$ shrinks) for the same heat load — it says nothing directly about surface temperature, which is set mostly by the radiative balance.

Because destroying mass carries energy off the vehicle. Turning surface material into hot gas that flows away removes far more heat than any passive material could simply store or block over a long re-entry.

Because launch cost scales with mass, and recession depth $s=\int ({\dot{m}}^{\prime \prime }/\rho )dt$ can stay acceptable when $Q^{\ast }$ is high. A light, high-$Q^{\ast }$ ablator like PICA gives protection at minimal mass penalty.

The char reaches thousands of kelvin, and radiation scales as $T_w^4$, so at those temperatures re-radiation dumps enormous power back to space (Stefan–Boltzmann). Its porous refractory structure also conducts poorly, protecting the layer beneath.

The honeycomb gives mechanical robustness to filled/gunned ablators spread over large curved shields, keeping the material in place under aerodynamic shear. PICA's rigid carbon-fiber preform already holds its own shape, so it can be machined into tiles without a honeycomb.

The outward-flowing gas thickens the boundary layer and physically holds the hot shock gas away from the wall (transpiration cooling), cutting the convective heat ${\dot{q}}_{in}$ that would otherwise conduct inward as ${\dot{q}}_{cond}$.

Breaking chemical bonds costs energy, and that energy is drawn from the incoming heat instead of reaching the structure. So the "falling apart" is itself a heat sink, not just a side effect.

A purely thick insulator would eventually let heat soak through and would be crushingly heavy for MW/m² deep-space returns. Ablation removes energy actively, achieving protection at far lower mass than brute-force thickness.

The rigid carbon-fiber preform survives and holds the shield's shape, so aerodynamics stay predictable even after the resin is gone — a key reason PICA is trusted at high flux.

The solid carbon char itself begins to sublime, $\text{C}(s)\to \text{C}(g)$, a strongly endothermic phase change that soaks up additional energy at the very hottest conditions.

The numerator ${\dot{q}}_{in}-\epsilon \sigma T_w^4-{\dot{q}}_{cond}$ can go to zero (or negative), so ${\dot{m}}^{\prime \prime }$ drops to zero — ablation pauses and the surface radiates faster than heat arrives, cooling rather than receding.

Little free O₂ means the oxidation channel is weak, so protection leans on pyrolysis and blowing rather than $\text{C}+\frac{1}{2}{\text{O}}_2\to \text{CO}$. This is part of why low-flux, CO₂-friendly SLA suits Mars. See Mars Entry Descent Landing (EDL).

Mechanical spallation, where chunks of char are torn away before radiating. This is exactly the shear limit that constrained SLA-561V usage on MSL.

Not necessarily — if re-radiation and the char's insulation carried the load, essentially no mass need be lost (${\dot{m}}^{\prime \prime }\approx 0$), which is the ideal outcome, though real high-flux returns almost always recede somewhat.

Then ${\dot{m}}^{\prime \prime }=\frac{{\dot{q}}_{in}-\epsilon \sigma T_w^4-{\dot{q}}_{cond}}{Q^{\ast }}\to 0$ — no mass is lost at all, meaning the ideal ablator loses nothing while still shielding. Real materials only approach this.