Heat shield ablation rate seems wrong

[OHara]

There is feedback on the bug tracker with this same title, marked 'needs clarification'.

The ablation rate uses an activation-energy model but with a strangely low activation energy, so that the rate depends rather weakly on temperature.  Ablator goes away during mining or being near the sun, but often remains when the heat shield reaches its structural failure temperature of 3300K on reentry.  The heat being carried away by ablation is not much more than that being radiated away.

Ablator seems to have a narrow range of usefulness.  Using what seems to be a standard use-case of a Mk1-2 pod on a 2.5-m heatshield entering Eve's atmosphere at 5600m/s, I found

• with no ablator, I need a periapsis 59km (3 Gs) or lower to aerocapture;
  a periapsis of 48km (10 Gs) reaches the 3300K limit of the shield and explodes.
• with 800kg ablator, I need a periapsis of 58km to aerocapture;
  a periapsis of 45km reaches the temperature limit of the shield and explodes when 540kg of ablator remain

I suggested a module-manager patch that increases the temperature dependence of the ablator.

Now the MK1-2 pod can survive any periapsis, although the G-loading can be extreme, because there is enough ablator to carry away the orbital energy.

If I add 10 tonnes of ore below the pod,  for a total of 14 tonnes behind the heat shield, then I do use up all the ablator.  In this case, 

• with no ablator, a 56km periapsis barely aerocaptures with heatshield-temperature right at its limit
• stock ablator lets me lower the periapsis to 50km, but at 49km (7 Gs) it explodes with 500kg left
• modified ablator lets me lower the periapsis to essentially the same level, but all the modified ablator is used up before I explode.

I like the modified ablator because it behaves more predictably, without my wondering when I might get 'a spot of burn through'.  If you all agree, change the status on the bug tracker, or suggest different test cases for ablative heat shields.

Edited March 12, 2017 by OHara