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tahanson43206,
AVCOAT is NOT a structural material. It's density is about 33lbs/ft^3 and thickness of material to protect Orion ranges between 1 inch (2.75lbs/ft^2) to 2 inches (5.5lbs/ft^2) areal density. 5.5lbs/ft^2 is already above the typical wing loading of an ultra-light, and so falls outside the range which keeps peak heating values low enough for practical reuse. To structurally support the AVCOAT heat shield, you must have a stiff and strong metallic or composite backing structure, as was/is the case for all AVCOAT heat shields from Gemini to Orion.
If the AVCOAT material itself is 2 inches thick, then it's 26.85kg/m^2, so already well above an ultra-light-like wing loading of 18.3kg/m^2. The point is that you're already beyond ultra-light-like wing loading BEFORE any structural backing material is added, never mind payload mass. Unfortunately, 2 inches of AVCOAT is only sufficient for 1 reentry from interplanetary velocities (moon, Mars, Venus). If you go any thicker or heavier to plausibly achieve multiple reuses without extensive refurbishment, then you're well above your mass per unit area limit necessary to knock-down those peak heating temperatures so that AVCOAT's ablation rate remains tolerable. In short, you must have a TPS with a much lower areal density, preferably structural in nature as is the case with HIAD and ADEPT, so you get some usable payload mass per unit area.
Optimum stiffener arrangement of hot structure HIAD rigid nose. The areal density of the ablative TPS design was 8.53 kg/m2 and the area density for the hot structure design was 10.07 kg/m2. This corresponds to a 15% increase in areal weight for the hot structure design as compared to the ablative TPS design.
HIAD and ADEPT are structural TPS materials / designs that are UNDER the areal density limit for achieving ultra-light-like wing loadings of 18.3kg/m^2. Anything heavier and non-structural in nature is going to weigh more than that per unit area, and thus increase peak heating rates to the point where ablation becomes significant. HIAD and ADEPT designs have been modeled that could withstand an interplanetary aerobraking maneuver, swiftly followed by EDL before the hot structure cools and becomes structurally unsound. My prior post in this thread included a paper on this, from NASA, from the group working on reusable / lightweight / deployable reentry heat shield technologies.
The reusable AVCOAT heat shield design doubled-up the thickness to achieve that outcome, and then they scraped away the charred material from the heat shield (after the protected vehicle was back on Earth) to retain its aerodynamic qualities for a subsequent reentry. That's one form of reusability that comes at the cost of additional mass per unit area.
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