Learn Portuguese (Portugal): O foguetão voltará assim que tiver lançado o satélite. - The rocket will return as soon as it has launched the satellite.

The time phrase assim que (as soon as) triggers the subjunctive when talking about an uncompleted future event. Because the launch must be fully completed before the rocket can return, we use the compound form (tiver lançado) rather than the simple future subjunctive to emphasize that the action will be finished.

How exactly is the Future Perfect Subjunctive formed here?

You take the simple Future Subjunctive of the auxiliary verb ter (which is tiver for the third-person singular) and add the invariable past participle of your main verb (lançado).

Could I use the simple Future Subjunctive (assim que lançar) instead?

Yes, assim que lançar is grammatically correct and very common. Using the compound form tiver lançado just adds a stronger emphasis that the action of launching the satellite will be completely done and dusted before the return happens.

Why is voltará in the indicative mood while the second half of the sentence uses the subjunctive?

The main clause (O foguetão voltará) is a direct, factual declaration about what will happen, which requires the indicative mood. The subjunctive is only triggered in the second clause because assim que introduces a future condition that has not yet occurred.