Thank you for playing. When you are slowing down a lot, you probably need to unlock the rocket and upgrade it. It should provide enough speed. I will try to fix the bugs you mentioned. Perhaps it is because you have every layer as a heat or pressure layer. The health is caclated by the following:
if(temperature > resistance) health -= (temperature - resistance) / resistance * Time.fixedDeltaTime
So if you resistance is 0, health will be subtracted by infinity, which is why you die at 0 km.
Thanks again for playing