[Vehicles] Gunpowder Engines and Stirling Engines

[Fred Brackin]

Originally Posted by malloyd

In order to get an explosion your fuel needs to contain (or be very thoroughly mixed with) the oxidizer - if you have to wait for an oxidizer to penetrate from the outside of the "explosive" it can't do so fast enough for the thing to actually explode.

I believe Fuel Air Explosives to be an exception to this though they do have a "mix" time. You're also not that far away from an internal combustion engine with a carburator or fuel injector.

Thermobarics might also be an exception but a thermobarically-propelled vehicle is another thing I would only watch from a safe distance.

[Varyon]

Originally Posted by Anthony

I came across the same figure, but the reaction is not identified and no known reactions used for smokeless powder even reach half that value.

Yeah, it looks like everywhere that lists that 11.3 MJ/kg upper value for gunpowder references that paper, which is stuck behind a paywall. I managed to find the table that value came from, wherein the formula is named "SF-3," but I'm not finding anything relevant on Google for that.

[Ulzgoroth]

From the paper intro I wonder whether that might be the thermite-spiked gunpowder they mention...

Originally Posted by Fred Brackin

I believe Fuel Air Explosives to be an exception to this though they do have a "mix" time. You're also not that far away from an internal combustion engine with a carburator or fuel injector.

Thermobarics might also be an exception but a thermobarically-propelled vehicle is another thing I would only watch from a safe distance.

But you repeat yourself. Fuel air explosives are thermobarics, and there's really no difference between a fuel-air bomb and an internal combustion engine cylinder ignition except for confinement and perhaps compression. Which, if the confinement failed, would only make the whole thing more dangerous.

I wouldn't want anything to do with an internal combustion engine that tried to run on powdered magnesium/oxygen burning, granted...

Fuel air explosives, of course, do work by having the fuel very thoroughly mixed with the oxidizer. Typically achieved quickly by using a small explosive to blast the fuel out into the surrounding air before igniting it.

[malloyd]

Originally Posted by Anthony

I came across the same figure, but the reaction is not identified and no known reactions used for smokeless powder even reach half that value.

If this is from an actual comparison, I suspect reading the wrong line. 11.3 MJ/kg sounds like a figure for the gasoline. Higher heating value is usually given as 44-46 MJ/kg for gasoline, and stochiometric air is typically 14.7, so at 21% oxygen that's 3.08 g oxygen. 44-46/(1+3.08) = 10.8-11.3 MJ/kg

For an explosive, it's way too high. For gunpowder the usual stoichiometry assumed is 4 KNO3 + 7C + S -> 3 CO2 + 3 CO + 2 N2 + K2CO3 + K2S. Plugging in the standard enthalpies of formation that's 1065.3 kJ, divided by the 520.53 g on the reactants gives 2.05 MJ/kg

For calculating kiloton equivalent of nuclear weapons TNT is normally taken as 1 kcal/g = 4.18 MJ/kg.

It's not remotely plausible a smokeless powder is 5.5 times as energetic as black powder and 2.7 times as powerful as TNT.

[Fred Brackin]

Originally Posted by Ulzgoroth

F

But you repeat yourself. Fuel air explosives are thermobarics,

Gurps HT does not use the terms interchangeably. In fact they are 2 separate lines (and even 2 separate TLs) on the explosives table on p.183.

I believe the explosive mixing you describe is more characteristic of what HT names "thermobarics". The experimental FAE bombs used by the USAF in vietnam (thus making them TL7) first released pressurized ethylene oxide gas and then igniting it when it was supposed to have achieved an optimum stochiometric ratio.

To my knowledge these weapons proved to be finicky about detonating as is the case with accidentally occurring fuel air explosions seen in grain siols, coal bunkers and other places.

The more modern thermobarics as seen in the MOAB don't seem to be finicky and I think most of the energy comes from powdered aluminum though I could see the easier to ignite magnesium being used in the detonator.

[Varyon]

Originally Posted by Ulzgoroth

From the paper intro I wonder whether that might be the thermite-spiked gunpowder they mention...

I considered that, yeah. Based on the third available table, what they name "171-25" (9.8 MJ/kg) appears to be 5% aluminum mixed into... something (that at 11% Al or higher exceeds the energy density of "SF-3," where that 11.3 MJ/kg figure appears to have come from). Without the full article, we're really just blind men describing an elephant, so hard to say, really.

[Ulzgoroth]

Originally Posted by Fred Brackin

Gurps HT does not use the terms interchangeably. In fact they are 2 separate lines (and even 2 separate TLs) on the explosives table on p.183.

I believe the explosive mixing you describe is more characteristic of what HT names "thermobarics". The experimental FAE bombs used by the USAF in vietnam (thus making them TL7) first released pressurized ethylene oxide gas and then igniting it when it was supposed to have achieved an optimum stochiometric ratio.

To my knowledge these weapons proved to be finicky about detonating as is the case with accidentally occurring fuel air explosions seen in grain siols, coal bunkers and other places.

The more modern thermobarics as seen in the MOAB don't seem to be finicky and I think most of the energy comes from powdered aluminum though I could see the easier to ignite magnesium being used in the detonator.

Black powder has three lines and three different tech levels. There's no descriptive section for thermobarics, and the section for FAE seems a bit confused: one sentence it's describing a proposed WWII coal dust warhead as an FAE, the next it's asserting that the theory specifies a "volatile mixture of pressurized fuels" (emphasis mine).

I don't use GURPS High Tech as my primary source for explosive terminology, so I'm not terribly concerned by that.

[Fred Brackin]

Originally Posted by Ulzgoroth

B

I don't use GURPS High Tech as my primary source for explosive terminology, so I'm not terribly concerned by that.

<shrug> This is a Gurps board and HT is excellently researched by people whose efforts I trust.