Mk 77 Incendiary Bomb, Area of Effect

[Onkl]

Hi guys

I've been searching for information on how a bomb like the mk 77 would spread out its load of flammable liquid ("napalm") - what the area of effect would look like in game terms.

The only thing I found was this:

A 2,500-square-yard area could be engulfed in flame by a single bomb.

From what footage I've seen and logically it would spread out according to the vector of the dropped bomb. Does anybody have any ballpark how an area of effect in GURPS terms would look like for such an aircraft dropped incendiary bomb. I'm especially interested in the Mk 77 Mod 5 but any information will do.

Cheers

Onkl

[Bruno]

If I recall, while normal explosions normally divide damage by (range from center in yards * 3), fuel-air explosions only divide damage by (range from center in yards).

This isn't really the way explosions fall off, either way. It certainly only results in circular areas, not an elipse.

I personally think that fuel-air explosions might be better represented with a flat AOE with a relatively large area, and then a drop off from the edge. But I'd use a more complex equation for regular explosions too, if I were bothering to do that.

Both cases would be annoyingly fiddly. In actual play, I'd draw a roughly 2500 square yard area template of approximately the right shape for the aircraft speed, and be done with it.

[acrosome]

Originally Posted by Bruno

If I recall, while normal explosions normally divide damage by (range from center in yards * 3), fuel-air explosions only divide damage by (range from center in yards).

Napalm isn't a fuel-air explosion, though. It's just flaming liquid.

I suppose that it wouldn't be too inaccurate to model it with the 2500 sqyd as a cone- starting at the point of impact, and spreading out in the direction of the impact. See the diagram B385- the arrow is the point of impact and the flame spreads through all front hexes until you reach your 2500 sqyd. Maybe you'd add some in the side and back hexes for splash, too, but not much.

[Onkl]

Originally Posted by acrosome

Napalm isn't a fuel-air explosion, though. It's just flaming liquid.

I suppose that it wouldn't be too inaccurate to model it with the 2500 sqyd as a cone- starting at the point of impact, and spreading out in the direction of the impact. See the diagram B385- the arrow is the point of impact and the flame spreads through all front hexes until you reach your 2500 sqyd. Maybe you'd add some in the side and back hexes for splash, too, but not much.

Thanks, I'll have to give it a shot to see where that leads me but sounds accurate enough for my purposes.

Thanks Bruno and acrosome!

Cheers

Onkl

[Tomsdad]

I think the ellipse shape found in real life is a by product of the bomb being dropped at speed (the contents spew out while the canister is still moving forward from the initial impact. Modern bombs are self propelled as well now (I think the M77 is)

Personally I'd work out various area of effect shapes going from a circle to more and more extreme ellipse, to go with faster impact speeds

The M77 is 750lbs making it half again as heavy as the heaviest bomb in Hightech.

As to 2500 sq yards since it doesn't actually site what bomb gave that it's tough to say (the Vietnam era napalm bombs were the M47 but I think the M77 is just a modernised version not a significantly different sized version).

But anyway a circle with an area of 2500 sq yards has a diameter of 56 yards

given the largest non FAE explosive bombs in HT (which are lighter than the M77) are still dangerous out to that, that might be reasonable.

Eitherway as per the rules in HT it's 1d-1 per sec for "at least a minute", needed total submersion or suffocation to quench (and that's napalm, the more modern stuff might be harder still to quench)

[The Colonel]

The chatter page on Wikipedia includes a statement about Korean War era napalm canisters:
"napalm was used as an area weapon that covered an area 275-feet long by 80-feet wide "
Which would fit the cinematic depiction of the stuff going in, but may not be how the newer bombs work...

[acrosome]

Originally Posted by Tomsdad

I think the ellipse shape found in real life is a byproduct of the bomb being dropped at speed (the contents spew out while the canister is still moving forward from the initial impact. Modern bombs are self propelled as well now (I think m77 is)

Personally I'd work out various area of effect shapes going from a circle to more and more extreme ellipse, to go with faster impact speeds

Hmm. Yes, working out a cone for half of 2500 sqyd, and then stacking an identical "reversed" cone at the far end might be a better model, if a bit complex. Working out various impact speeds is a bit much to ask of any non-physicist, though. Imagine the extreme example- a dead drop straight down with no forward velocity. What the heck would you do for that? How far would the thickened fuel spread? How quickly? What is it's viscosity (ρ)?

That crap gets complex.

Originally Posted by Tomsdad

As to 2500 sq yards since it doesn't actually site what bomb gave that it's tough to say (the Vietnam era napalm bombs were the M47 but I think the M77 is just a modernised version not a significantly different sized version).

Yeah, I can't find other sources, either. It seems like all but Mk77 Mod 0 are 75 gallons rather than 110. (Your quote of 750 pounds is the Mod 0- the later mods are more like 500 pounds, according to Wikipedia.) I can't find anything on the Mark 47, other than an older WWII-era incendiary called M-47.

Frankly, after observing some very disturbing footage, I think 2500 sqyds is one hell of an over-estimate. (Not surprising considering the wretched syntax. One has to wonder about the quality of it's scholarship, or perhaps the author is just ESL?) Anyway, 2500 sqft might be a better estimate- a radius of 56 feet- and confusing units is a common type of misquote. Of course, I have no idea what weapon system is being used in that footage. And bear in mind that most of the more spectacular Vietnam-era napalm fireballs that you can see in YouTube footage are the result of multiple munitions being dropped in rapid succession. The footage I linked appears to be single munitions- or, at least I never see more than one flaming expended bomb carcass rolling along.

[Bruno]

I have no idea how I missed we were talking about napalm. No excuses.

Originally Posted by acrosome

the xtreme example- a dead drop straight down with no forward velocity. What the heck would you do for that?

A big ol' circle. :)

[acrosome]

Originally Posted by Bruno

I have no idea how I missed we were talking about napalm. No excuses.



A big ol' circle. :)

Ok, Smartgirl... how big? :)

And since when do you live in Canada? Aren't you a Kiwi? Or am I confusing you with someone else?

[Tomsdad]

Originally Posted by acrosome

Hmm. Yes, working out a cone for half of 2500 sqyd, and then stacking an identical "reversed" cone at the far end might be a better model, if a bit complex. Working out various impact speeds is a bit much to ask of any non-physicist, though. Imagine the extreme example- a dead drop straight down with no forward velocity. What the heck would you do for that? How far would the thickened fuel spread? How quickly? What is it's viscosity (ρ)?

That crap gets complex.

Well I was literately just thinking of stretching a circle out into different ovals and maintaining total area!

But you're not wrong!

Originally Posted by acrosome

Yeah, I can't find other sources, either. It seems like all but Mk77 Mod 0 are 75 gallons rather than 110. (Your quote of 750 pounds is the Mod 0- the later mods are more like 500 pounds, according to Wikipedia.) I can't find anything on the Mark 47, other than an older WWII-era incendiary called M-47.

Frankly, after observing some very disturbing footage, I think 2500 sqyds is one hell of an over-estimate. (Not surprising considering the wretched syntax. One has to wonder about the quality of it's scholarship, or perhaps the author is just ESL?) Anyway, 2500 sqft might be a better estimate- a radius of 56 feet- and confusing units is a common type of misquote. Of course, I have no idea what weapon system is being used in that footage. And bear in mind that most of the more spectacular Vietnam-era napalm fireballs that you can see in YouTube footage are the result of multiple munitions being dropped in rapid succession. The footage I linked appears to be single munitions- or, at least I never see more than one flaming expended bomb carcass rolling along.

True (actually the Mod 0 is the heavier 750lb one, the Mod 1 is the lighter 500lb one, but since we still don't know what is being used to create the cited 2500 sq yrds its doesn't really matter)

I have to say my assessment on the 2500sq yards was really just to take that circle and compare it to the bombs in HT! (I.e not very scientific).

That footage is hard to judge as scale by

But that said 2500 sq ft would be circle 56ft across so that would be roughly half a basket ball court. Which too me seems small for even the smaller 500lb cannister?