Ok, so you can’t really fire a vacuum optimized engine at sea level… and this, really, is where aerospikes come in. The idea behind aerospikes is you allow the ambient air pressure to actually form the walls containing the flow of the exhaust so it’s always in nearly ideal conditions at any altitude. And not only will you not destroy your engine by lighting it at sea level, it’ll actually perform pretty darn well!
(...) Here is the biggest problem with aerospikes. By design, the throat of their combustion chamber has substantially more surface area to cool. For a toroidal aerospike, it’s combustion chamber basically has a giant plug in the middle of it which greatly increases the surface area of what needs to be cooled.