So I was thinking about some options to run my players through that didn't involve simply ordering them to "Blow up target X." I was thinking of some other things that soldiers have to deal with, and came up with the idea of the character's platoon taking part in trying to put out a massive fire.

My idea is that they're dropped off on a planet that's had a battle going on for quite some time. So far the enemy xenos (minor unnamed, read "made up" species). Because of how well the battle was going, Segmentum Command ordered 80% of the ground troops into the next theater of war approximately a month before the players arrive, leaving the entire system being fought for by a small handful of well coordinated (but now hugely understaffed) force against an almost beaten enemy who's regrouping.

So far there's a single Sword class frigate in orbit assisting as possible. It's going to get ambushed by a pair of Eldar raiders and crash over the targets that it's playing fire support for (READ: The players).

It still has its thrusters going when it crash lands, but the commander wants to evacuate as much crew as possible, but I know of absolutely nothing in the 40k universe that would be effective at fighting fires. And this thing is going to be burning for a long time otherwise.

So far a friend of mine and I have discussed repurposing tanker vessels, and possibly using Marauders refitted to carry water (there's still a significant navy presence on the ground; Command didn't give them time to fully evacuate. Hooray orders). But as far as I know, there isn't anything specialized for dealing with a fire of that magnitude and capacity.

TLDR - How would you put out a major fire in the 40k universe?

In general terms, look to OTL solutions and put a 40K spin on them. And since this is a game, there’s no need to be too technical or accurate.

In case of large fires, firemen create fire breaks to channel and ideally halt moving fires, sometimes in combination with smaller, controllable fires which they create to starve the main fire of combustible materials. At the same time, the main fire is targeted by fire fighting aircraft dropping water or chemicals or even seeding clouds to create rains.

In 40K, I’d expect AdMech produced chemicals which can either starve fires or cause heavy rains. And because its 40K, they damage the environment with dangerous chemicals or poisonous gasses. These can be delivered by aircraft or missiles.

In your scenario, I’d focus on what the players can do. Possible actions are

- Rescuing saviour pods with Imperial Navy staff (who then became a pain in the ass)

- Locating bits of the frigate and either preventing the enemy from looting it or salvaging it themselves. Perhaps throw in some additonal scavenging jawas xenos like the stryxis as an added complication, e.g. threeway fight. Perhaps the players must broker a temporary alliance with the xenos they are already fighting to fight the addtional xenos. And then betray them before they are betrayed themselves. Or send in the Eldar to pick up the navy charts or something equally important and have the players encounter the joys of splinter guns.

- Dig a fire break (multiple extended tests with survival, navigation, etc.) and upset some vicious animals (or perhaps a single large animal).

- The players need to mark a certain area with flares or coloured smoke to direct the aircraft but their smoke attracts hungry, vicious flying animals (blood ravens or pterosaurs).

- Once the AdMech swings into action, the players have to travel through blasted, fire-blackened terrain, without sufficient supplies and or rebreathers while slowly dying from the chemicals in the air….Perhaps the only way to survive is raid the enemy xenos or find surviving parts of the frigate in the landscape and fight against surviving voidsmen for the limited resources. Adds a bit of morality to the game. Do the players take everything for themselves and survive while the voidsmen die? Or do they share and take the characteristic penalty (each player has his toughness lowered by 5 points)?

A retrofitted Hellhound full of fire-suppressing foam (albeit with some sort of convoluted Mechanicus designation) could be very effective as a firefighting vehicle. Bonus points for the irony!

Mars-pattern Kimberly-Clark foam dispensor?

Ryza-pattern Heavencat fire engine?

Gryphonne-pattern Ice Hound?

Or perhaps just the Mars-pattern Hellhound Phoenix....

Just spitballing here but since fire is such an expression of holiness in the Imperium, would they actually fight fire? Perhaps it is seen as an expression of the Emperor's wrath, sent to cleanse sinners in his holy fire?

It would be very 'inquistory/Imperial creed religiously' to let fire run its course and feel that any people burnt to death were obviously sinners while those that merely suffocated were thus exonerated (much like potential witches having the choice between drowning or burning at the stake if they floated/could swim)....

If they truly cared about fires, redemptionist crazies and other assorted (religious) madmen wouldn't be allowed to run around with flamers.