Ignition doesn't have to be net positive. It just needs to enter into state of self sustained reaction that is net positive. Ignition itself can be horribly i

Ignition doesn't have to be net positive. It just needs to enter into state of self sustained reaction that is net positive. Ignition itself can be horribly inefficient. Nobody is designing fussion where ignition's output is directly used for the whole system to reignite again. It would be equivalent to moving hand with match that fires a match, where energy from ignition is used to trigger another hand to move another match.