So last week my group was camped out in a shipping yard on a planet we are trying to escape. Our sniper saw a patrol coming our way so we booby trapped the area, including tripwires to grenades next to a shipping container full of promethium.

Well, we were fleeing when they triggered the explosion. Upon looking to see how much damage was done (because we wanted to know how badly we damaged the shipping yard) we realized that we set off an explosion that would have covered well over 10km. This is because, according to the rules, the blast radius is 4x kg used in meters. A shipping container on earth can hold 24 metric tons, that's a 24km blast zone if the container is full. That seems a bit excessive.

Luckily the GM stated that there was an upper limit on radii of explosions, and we were still alive. Does anyone else see this as a potential issue? You could use demo charges and/or promethium to make a conventional bomb with nukelike damage zones.

theshadowduke said:

A bit of a fringe case, but yes, it could benefit from some homeruling.

While 24km may be a bit over-the-top (mainly due to a linear formula being used to calculate a cubic volume of space), the amount should probably have destroyed the shipping yard, especially if the explosion reaches other containers. In one of the Cain novels, a hijacked promethium transport very nearly obliterates a space port.

Being a physics major I point out the inverse square law. Something like that may well be too complicated to include in the rules, but it should give an idea about maximum ranges.

I have to say though, that even at small levels it scales quite badly. 5kg of promethium covers a 1256 square meter area. That's a 40m diameter circle of destruction. I think they may have went a little overboard with blast ranges on what is effectively gasoline.

Physics-wise, you're definitely right.

On the other hand, Cifer is right in pointing out the source material. 40k is directed by Michael Bay with a skull fetish, after all. So it stands to reason all explosions are bigger than in reality.

Basically, there's no good answer here. Go with what makes you feel better.

I have to say though, that even at small levels it scales quite badly. 5kg of promethium covers a 1256 square meter area. That's a 40m diameter circle of destruction. I think they may have went a little overboard with blast ranges on what is effectively gasoline.

Promethium is not gasoline. It's gasoline after thirty-eight thousand years of evolution (give or take a few thousand for stagnance). Further, the gel-form is AFAIK the concentrate. Without diluting it, you're more likely to shred rather than power your engine when using it.