As an extra cost, return an Engineer of yours from a battle to your berth.
Petard deals 2 damage to every crew in that battle.
Explained
Recon 2801.5
Look at the top 2 cards of your hold. Draw one; put the rest into your scrapyard.
As an extra cost, return an Engineer of yours from a battle to your berth.306.5e
After dispatch, choose a battle where you have an Engineer, then which of your Engineers there returns. It leaves the battle and stays exhausted, so the damage doesn't reach it. Without one, you can't deploy it (306.5e).
Petard deals 2 damage to every crew in that battle.705.7i
After the Recon. Every crew in the battle takes the damage, yours too, but not the crew that returned to your berth. Plating takes it first, the damage stays, and a crew left with no hull is destroyed at once, before the battle is fought (705.7i).