Actually the heat shield was drained and had its front and rear node occupied. This make the HS almost dragless, especially at supersonic, as front and rear face of drag cube had been removed. During Eve reentry, it was fuel empty so it slow down fast, won't explode due to heat. During Eve ascent, the KSP's shock cone mesh-based heat system (node-attachment-unrelated) will form a cone-shape(probably) region behind the HS and will shield heat for the sharp-nose fairing (sharp-nose fairing means much less drag, but is far easier to overheat), and other parts are not that vulnerable. btw the trajectory is very fine-tuned (rocket phase at least 30 hours of tests, more than 100 quicksaves), by using canard+stock SAS control laws mechanism to be able to affect AoA a little bit, I control the maximum ascent angle (the highest value of angle of velocity vector and horizon) to about 15deg (<14.5 deg result in explosion. the maximum thermal percentage seems to be almost exactly a function of max ascent angle, so it will be safe and won't explode at all (despite, it will go to 90%+, but indeed it's under design so no problem).
By the way, in the world of Eve SSTO, actually this is a small design. One of Eve SSTO's locking properties is KS25, we need 85-100t per 1000kN (85t per KS25 is best for small, clipping-less Eve SSTOs), so any Eve SSTO design <85t don't seem to be good, as small engines don't have comprehensive performance as good as KS25), Eve SSTO mass seem to be 85N-100N tonnes (N is an integer). mine is only 170t + 2xKS25, already minimal design (less than a single Mammoth). And, Eve SSTO also have size effect, at a region of, at least, 170-3000t, with top-tier design technique, the larger, the more effective (mainly increase in aero effiency, allow of use for mammoth (reduce 25% engine mass), smaller portion of truly dead mass like landing gear and mining equipments, etc.).
