So... it turns out there's a 40mm grenade called the XM1060 that's a thermobaric weapon. In my head canon at least, 40mm grenades are class C ordnance. I can't find any published statistics with regard to lethal radius, but the videos I've seen make it look fairly large. Assumptions:
1) 5 meter lethal radius for Class C thermobaric ordnance (40mm)
2) "Lethal" in AToW terms would be 4 points of damage past the armor of a typical trooper (3 vs. X damage, based on Flak Armor)
3) AP for Thermobaric weapons should be below HE, but above Frag
Class C HE is 6X/10A, and Frag is 4X/12A. This would put thermobaric AP at 5 (how convenient; at 5 meters, this is 0).
BD has to be equal to at least 4, after soaking the entire 3 points of Flak armor vs. X damage at 5 meters. As BD also decreases 1 point per meter from detonation, this would simply yield 12A damage. The difference from Frag however, is that Interstellar Operations describes the kill mechanism for fuel air munitions as also removing all the oxygen from the air. The suffocation rules are on page 181, and would thus imply a "Continuous" damage code that inflicts 2 points of Fatigue per turn until a character can get out of the affected area, which would extend 12 meters from the point of detonation. So, for a Class C thermobaric munition, it would be 5X/12AC* (*being, "see the suffocation rules on page 181"). IO also notes "soft" targets take 1.5 times the base damage. I think a note for "if AP exceeds BAR, multiply damage by 1.5" will do. For Classes D and E, I'm increasing the damage a bit, as those two classes are supposed to be the bridge to "real" ordnance, which is FAR more deadly. All told:
ORDNANCE
ITEM RATINGS AP/BD COST AFF MASS NOTES
ORDNANCE, THERMOBARIC
CLASS A C/E-F-E/E 3X/7AC*† 6 — 200 g 7m radius
CLASS B C/E-F-E/E 4X/9AC*† 30 — 450 g 9m radius
CLASS C C/E-F-E/E 5X/12AC*† 60 — 600 g 12m radius
CLASS D C/E-F-E/E 5X/14AC*† 90 — 2 kg 14m radius
CLASS E C/E-F-E/E 5X/16AC*† 120 — 4 kg 16m radius
* see suffocation rules on p. 181 within the indicated radius
† if AP exceeds target BAR, multiply damage by 1.5
