So, my players managed to cajole a Rogue Trader into giving them an Argus Lighter, to serve as their own (permanent) flyer/orbital transport. I'm cool with that- less fiddly transportation stuff for me to deal with when they go on missions. Except... fuel.

I'm way too o.c.d. to just 'hand wave' fuel range, and, since this is First Edition, there is the matter of cost. Keeping track of the party's fuel consumption on a previous mission where they had an Outrider and had to cover a lot of ground was a pain in the butt; a flying transport will be astronomically worse in that regard...

Then it occurred to me: would it be plausible for an Argus Lighter to have a reactor instead of requiring promethium? Then I could just say "The fuel rods have about 2 years of life left" and not have to worry about it for the remainder of the campaign.

Would that be reasonable, or an immersion-breaking oversimplification?

From my experience (m y last group used Aquila ) it will be the best solution .

Well, nuclear reactors powering airplanes has been a thing since the 60's. Granted, they've never been put into use because of various problems, mostly their impracticality compared to ICBM's and a few design issues that were never adequately resolved. So that could work.

Anyway, fitting a fission reactor onto a lighter means you'll probably be taking up a lot of space with the reactor itself and the necessary shielding to prevent acute radiation syndrome for the crew. But do note, the fuel rods will probably need replacing fairly often, depending on use, too.

Then again, 40k has the technology for man-portable fusion reactors (Astartes power armour backpacks), so the shielding probably doesn't need to be as big. As well, those reactors seem to be capable of functioning indefinitely without maintenance or refueling.

So, yeah. You can probably decide how you want it to function.

Lead them into thinking they will need promethium to refuel it, so they struggle to find a place to do so. Once there, the technologically-ignorant Acolytes learn that their shuttle doesn't need fuel, that it is powered by an ancient reactor. Turns out their shuttle is of an older, more advanced STC and is thus much more rare.

Which begs the question: Why did the Rogue Trader give it to lowly Inquisitorial Acolytes?

Perhaps the shuttle didn't actually belong to him, and was stolen from the Adeptus Mechanicum or some other powerful cartel/sect/dynasty/organization - and they want it back. Maybe it has plans or data logs that could incriminate someone important to the Sector government. The shuttle could have been connected to a high-profile crime, which the Rogue Trader thought prudent to dump on the PCs.

My point is, make the shuttle advanced, give it some nifty bells and whistles so the PCs really like having it. Then make it central to a plot hook or two that means trouble for the PCs.