Do you have a reason to think it’s not just the same as when you miss the exhaust port – i.e. you lose a shield?
Then it starts over at the beginning of stage 3 (trench run).
When you lose all shields, GAME OVER.
I have a book somewhere that has most such details for nearly all 80’s arcade games . . . I’m not sure where it is right now, but it can probably be found on the Internet somewhere.
Or, asking on arcade stack exchange will no doubt turn up an answer.