How to stop players from trying to gauge success of meta-ideas via meta questions?

I am running a campaign in The Dark Eye, the Year of the Griffon to be exact.

Due to having quite an abundant downtime by having taken initiative quite early

the group had about 2 and a half months of downtime. That in itself is not a problem. Yet my group did spend the downtime in ways that start to make me think carefully: The siege-engineer did recruit all the carpenters and masons to fix up the walls

while the Maga did start to research a variant of Greek fire. That luckily took her most of the time… But exactly there lies the problem:

My players are all students, some of them with at least a good degree of chemistry and physics knowledge (including the Maga, who has 2 semesters of chemistry behind her). As a result, they tend to come up with ideas that make incredible sense in themselves… but the rules for TDE explicitly state that some stuff is simply not possible. For the most part, they did swallow that gunpowder simply can’t be made due to “it’s just like that” and that the recipe for fireworks is not only highly complex but also so arcane, that nobody knows it.

YET this makes them try to figure out how to use the little Greek fire they managed to produce up to now (4 flasks) much more efficiently – as in getting more area of effect. While they did not yet test it, they had ideas like

  • How about training a dog to carry a jar of greek fire into the orc camp and then shoot the jar from the walls with a flaming arrow?

On ideas like this I did say up to now “I did look into that topic in the books and I will have to make a ruling as there are no rules written yet. I have a vague idea how to rule on it, but would you mind stopping to speak in hypotheticals and bring this up in character through action or prompting that in an officer’s meeting?” Pretty much I try to go by In Character Action = In Character Consequences and Not Said = Not Done.

The idea to weaponize the results of a failure on the alchemy I could squat luckily: “You don’t exactly know how you made it create the poison gas it made… you could try to recreate the experiment though. Do you want that?”

Up to now, the only three things they did act on based on sentences like those was to build up some siege engines for defense (a trebuchet, 2 catapults, repairing one ballista), re-inventing greek fire (which did blow up the Maga twice already for it is so dangerous to work with as established in the rules) and digging ramparts1 around the city wall (ongoing, they are about 2/3rds done) and will be finished by the end of the months.

How can I discourage the players from directly adapting ideas from modern warfare methods (like gas, using chemical waste (dioxin) or trained dogs with bombs) and instead staying in the established canon, that puts greek fire at the pinnacle of chemical warfare and lobbing corpses as the pinnacle of biological warfare?

1 – Yes, they made Ramparts, not moats.