OneEyedPyro
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Preheated liquid FAE?
I heard a tale from a retired soldier down at the bar a few years back. He said in Kuwait that the retreating Iraqis had rigged train car sized
liquified natural gas tanks to blow by simply disabling the pressure relief valve and igniting a fire underneath them, they found tires and such
doused in fuel oil piled underneath some tanks that were for whatever reason never lit.
He said the shockwaves and fireballs of these could be felt and seen from over the horizon illuminating the sky. Pieces of work trucks parked at the
sites were found utterly destroyed hundreds of meters away and the fuel tanks would fly for miles!
Point is. I wonder if instead of a BLEVE type explosion the concept could be used in a proper detonating FAE to add energy to the system and aid in
fuel dispersion? TATB and some other suitable burster HEs can comfortably survive temps beyond 200C which with some fuels like propane would probably
equate to well over 1,000PSI.
Maybe pointless and impractical for the most part but I can imagine an air dropped device using an onboard turbine to power a heater and circulating
pump to achieve the desired temperature mid air.
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dettoo456
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Pressurized gases like propane could perform well, but only in the gas phase since ignition of liquid propane will draw too much energy into boiling
and therefore drop the total energy of the system.
Gasoline or other hydrocarbon systems could be boiled in a sealed vessel beyond their flash points and then rapidly expelled by an IHE like HNS, but
itd just add complexity to the system. More commonly, a pyrophoric substance like BrCl3 or BrF3 will just be mixed with Pentane or Hexane to allow
easy initiation without a secondary ignition source needed. In that case, only a dispersal charge is required.
A different approach to ‘preheating’ could be something like a sealed styrene vessel with a frangible polymerization catalyst encased inside. If
the styrene could be polymerized fast enough by release of the catalyst, rapid dispersal and ignition could be achieved.
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OneEyedPyro
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Quote: Originally posted by dettoo456  | Pressurized gases like propane could perform well, but only in the gas phase since ignition of liquid propane will draw too much energy into boiling
and therefore drop the total energy of the system.
Gasoline or other hydrocarbon systems could be boiled in a sealed vessel beyond their flash points and then rapidly expelled by an IHE like HNS, but
itd just add complexity to the system. More commonly, a pyrophoric substance like BrCl3 or BrF3 will just be mixed with Pentane or Hexane to allow
easy initiation without a secondary ignition source needed. In that case, only a dispersal charge is required.
A different approach to ‘preheating’ could be something like a sealed styrene vessel with a frangible polymerization catalyst encased inside. If
the styrene could be polymerized fast enough by release of the catalyst, rapid dispersal and ignition could be achieved. |
I'd think with any liquid or even solid fuel that preheating could result in a more powerful and reliable FAE, as in more likely to have a successful
detonation under less than ideal conditions regarding timing and fuel/air ratios.
Imagine a few thousand pounds of aluminum powder soaked in diethyl ether that's heated to 200C then dispersed and detonated in a 2 stage FAE compared
to same one that's well below 0C from the temperatures of high altitude flight.
Even in the event it just deflagrated there should be a great deal more violent of a deflagration, no?
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dettoo456
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The energy release during detonation (or deflagration) is only equivalent to the oxidation of the fuel by atmospheric oxygen - nothing more. During
heating of the fuel in a hermetic environment, assuming you wouldn’t want any reactions occurring during heating, there won’t be any additional
energy put into the system to be released. It’s in fact the opposite, especially since such energy as heat would be almost immediately dumped due to
the high surface area and reactivity of the fuel when dispersed - it’ll ignite anyways when subjected to rapid release and a small initiating
charge; its high temperature would just aid in sensitivity but not necessarily useful energy.
For example, a well known safety concern when milling Al powder is that it can self-ignite when rapidly exposed to the atmosphere - especially when
the powder is hot after the milling process. But, if you rapidly expose the Al powder to the atmosphere when cold, and do it right above a flame
source, it’ll still self ignite with lots of energy. In this case, the Al sensitivity IS increased as to not warrant the use of a secondary ignition
source, but the energy from the combustion itself isn’t much different.
That’s primarily because the energy density of the fuel, as in the HOFs and bond energies, is so much higher than just the energy used to heat them.
That heat applied to the system is minuscule compared to the combustion of the bulk material.
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OneEyedPyro
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| Quote: Originally posted by dettoo456 | The energy release during detonation (or deflagration) is only equivalent to the oxidation of the fuel by atmospheric oxygen - nothing more. During
heating of the fuel in a hermetic environment, assuming you wouldn’t want any reactions occurring during heating, there won’t be any additional
energy put into the system to be released. It’s in fact the opposite, especially since such energy as heat would be almost immediately dumped due to
the high surface area and reactivity of the fuel when dispersed - it’ll ignite anyways when subjected to rapid release and a small initiating
charge; its high temperature would just aid in sensitivity but not necessarily useful energy.
For example, a well known safety concern when milling Al powder is that it can self-ignite when rapidly exposed to the atmosphere - especially when
the powder is hot after the milling process. But, if you rapidly expose the Al powder to the atmosphere when cold, and do it right above a flame
source, it’ll still self ignite with lots of energy. In this case, the Al sensitivity IS increased as to not warrant the use of a secondary ignition
source, but the energy from the combustion itself isn’t much different.
That’s primarily because the energy density of the fuel, as in the HOFs and bond energies, is so much higher than just the energy used to heat them.
That heat applied to the system is minuscule compared to the combustion of the bulk material. |
I'm not really saying that the heat itself is directly adding to the output but rather bolstering the power and reliability of the subsequent
detonation.
What I'm posing is more of the concept that it would likely cause better dispersion and atomiztion of the fuel due to it independently expanding from
the epicenter into a cloud rather than only physically being ejected by a burster.
More important is the fact that an FAEs detonation is entirely the result of a shockwave compressing the fuel/air mixture to autoignition temperatures
thus continuing to drive the shockwave. If the cloud of fuel is already at a higher than normal temperature one can only assume that it would take
less energy to initiate the detonation, that the acceptable fuel/air ratios and timing should be more forgiving, the resulting VoD would likely be
higher.
I'm well aware of the pyrophoric nature of Al. Heating it could be an issue as far as preignition, then again maybe not.
One of the biggest issues with thermobarics in actual use hasn't been preignition but rather no detonation or ignition at all. Perhaps at high fuel
temps the aluminum/ether would autoignite after cloud formation and undergo DDT on its own even if the detonating charge failed.
[Edited on 9-3-2024 by OneEyedPyro]
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