If this true http://www.ahanw.org/forum/topic.asp?TOPIC_ID=602 (that hydrogen burns at 4000F) then wouldn't thermolysis occur when the hydrogen combines when oxygen, since at 4000F water would have the hydrogen separate from the oxygen(it seems impossible since there would be continuous energy being produced if the hydrogen were burned again)?

It does.

 

That means oxygen+hydrogen combustion is always incomplete where it occurs. As heat is used - for instance to accelerate the produced gas in a nozzle - and the temperature drops, the reaction completes.

 

In a rocket, you would add an excess of lighter hydrogen in order to use all the heavier oxygen - and even a bit more hydrogen, because the rigid diatomic hydrogen molecule expands more efficiently in the nozzle than the triatomic water. (Nothing to do with bearable temperature as some books allege).

 

Similarly, a hydrocarbon+oxygen flame can't produce only CO2. In a decent rocket, you get 50-50 CO. This is an excellent reason to increase the chamber pressure of a kerosene engine, as CO2 produces much more heat than CO. This excellent reason adds to the more efficient nozzle with the bigger pressure ratio.

So, you're saying that if thermolysis occurred, the heat would be used up in the thermolysis reaction, right?