h a l f b a k e r yPoint of hors d'oevre
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After a bit of a read through the history of naval warfare,
it’s clear that any warship taking a hit to the magazine is
doomed. To counteract this, vast amounts of armor are
used to protect these vulnerable areas. This is a shame
because it could be used to protect the room in which the
rum
is stored. The problem here is that all those big shells
constitute a ready-made ship-killing charge. This is
because both fuel and oxidizer are carefully combined at
their optimum ratios all the time. So, you have to make
the oxidizer, carefully mix it, and assemble the shells
without coughing too loudly…. Then, they’re stored in one
or several armored magazines… waiting. Tedious and
dangerous.
Let’s get rid of the oxidizer component, and those boring
old explosives. They’re heavy and not-particularly-energy-
dense (3-5 MJ/kg). Let’s use gasoline (47 MJ/kg… much
better). We’re going to need a projectile… and behind
this a chamber in which to detonate the gasoline…. Using
compression ignition. Like a diesel engine, only diesel is
slow. So, we open the breach, insert the projectile, close
the breach, flood the chamber with compressed air, inject
gasoline into the incoming compressed air stream, for nice
mixing… then, set off a small charge, this raises the
pressure inside the combustion chamber to the point
where the gasoline/air mix detonates. Then BOOM! Off
goes the projectile in the general direction of the enemy.
The compressed air is used (200 Bar, nothing special) to
bring the volume of the combustion chamber down.. and I
believe that 200 Bar is well below the spontaneous
detonation temp/pressure threshold for gasoline-air. Once
the projectile has gone the chamber is depressurized, and
a new projectile can be inserted…. No casing to worry
about.
So, you can increase the ammo carried (or decrease
weight), increase survivability in case of a direct hit AND
you can tune the amount of gasoline. So you can go all out
and have maximum velocity-ship piercing death-ensuring
level, or you can dial it back and lob a projectile so it lands
intact on their deck… with a note attached.
Petrol mortar
http://www.youtube....r_detailpage#t=162s
Interesting comparison of petrol vs. gunpowder [mitxela, Aug 04 2012]
[link]
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Energy density is the problem, indeed. |
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You're right about the difference in energy
density between propellants and volatile
hydrocarbons… but there's a snag. |
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A kilo of propellant occupies about half a
litre. Allowing a factor of 10 for efficiency
(less than quoted), that kilo of propellant
equates to 100g of octane. |
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However, that octane needs to be not only
atomised, but in a stoic ratio with air. |
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Octane has a molecular weight of 114. So 114
g of octane (one mole) occupies 22 litres at
STP. Close enough to 100g for this discussion. |
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Each mole of octane requires 13 moles of
oxygen for combustion. That's 22 x 14 litres,
except air is only 20% oxygen, so 13 x 5 x 22
litres of air; 1430 litres. |
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Compress that to 200 Bar and it's about 7
litres, a manageable volume. |
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That's for a stoic mixture, which will give you
detonation; what you actually want is a nice
steady "push", so actually your mixture needs
to be on the weak side to give slower
burning. |
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The difficult bit is going to be achieving the
perfect dispersion of the fuel throught the
chamber, which suggests that a gas such as
ethane would be a better proposition. |
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//a stoic ratio with air// Is this the ratio at which
the octane just resigns itself to the fact that it will
burn? |
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Natural gas/propane/butane all have better energy
densities, and they're already on board in the
galley.... I chose gasoline because there are baked
solutions for safe storage... self-sealing tanks/bag
etc. |
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Diesel would be ideal in many ways... fitting nicely
with the whole single-fuel policy and not so
dangerous. Perhaps the diesel detonation will give
you a better 'push'? I thought it might be too
slow. |
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Combustion chamber volume is fine I think, in the
order of a metre or so.... volume goes up pretty
quickly if you expand any dimension... I suspect
the volume/shape would need careful
optimization. |
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//Is this the ratio at which the octane just resigns
itself to the fact that it will burn?// |
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deep breath, shoulders back.... in the manner
befitting a gentleman. |
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//Tuneable Naval Guns// I was hoping for something
more musical... |
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well, if you like, there can be a very small bleed-off
from the combustion chamber... leading to a whistle.
naval guns tend to be in threes. You could have each
turret toot out a full chord. A musical captain may
be able to create pleasing tunes while visiting
destruction upon whatever it is... |
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//each turret toot out a full chord// |
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That would be cool. I'd like to hear the heavy
artillery doing a walking bass while the smaller stuff
runs through some nifty blues riffs. |
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That would be the "riff of grapeshot", would
it ? |
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"Also Sprach Zarathustra" as scored for battleship quartet. |
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"Right, boys, let's give 'em two verses of 'House of
the Rising Sun' then seven bars of 'Blackberry Boogie'
- and make it staccato." |
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3 guns per turret, 4 turrets in total... so a Battleship,
a cruiser and a couple of Destroyers should give you a
very useful 4 octave range... The Admiral could fire
the guns with a specially linked keyboard... |
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'they're not responding to the radio Captain...
perhaps a quick 'frere jacques' across the bows?' |
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The aircraft carriers are going from steam to electromagnets to propel the deck aircraft catapult. Could you do something with water and electricity? |
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Throw in the new naval laser weapons, and a smoke screen, you get a light show. Like, to die for, I guess. |
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I read this as Tunable Gavel Nuns and was thinking, maybe it's the cobbles... |
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I remember reading about using an arc of current to flash water to steam to propel naval artillery, but on a recent google was unable to find out what had come of the work. IIRC it had the advantage of being both tunable and highly efficient through regulation of the current throughout the cycle. Anyone know any more on that? |
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Likewise I remember reading about experiments to "plasma boost" the propelant used for high velocity APFSDS rounds, but once again came up bubkis on Google. Like many things, you need to know the silly acronym they were using in order to make any headway. |
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[8th] - got any pointers for further reading? |
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We could tell you, but then we'd have to kill you. Not that we have a problem with that, mind ... |
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Try Googling "AMRTRR" andor "TASPOM". |
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I'd like to see someone try to compress 1400 litres of
octane fuel-air mixture down to 7 litres without it
spontaneously combusting. Once you go past about
13:1 compression it detonates, causing "pinging" in
gasoline engines. |
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Diesel will go to about 40:1 before it touches off. |
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// I'd like to see someone try to compress 1400
litres of octane fuel-air mixture down to 7 litres
without it spontaneously combusting. Once you go
past about 13:1 compression it detonates, causing
"pinging" in gasoline engines. |
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Diesel will go to about 40:1 before it touches
off// |
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No, it doesn't. Diesel at 40:1 in an IC engine is at
40:1 AND it's been compressed to that point very
rapidly and as a consequence is at several hundred
celcius.... even so, you need a glow plug to get
the whole process started. |
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Gasoline/air needs MUCH more pressure... which is
why there are no compression-ignition gasoline
engines... they'd have to be VERY well built. |
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'pinging' is caused by hot spots in an engine...
chunks of still-burning carbon and the like.
Gasoline engines don't detonate... they'd be more
efficient if they did... they feature propagating
conflagration originating at the spark. |
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The compression and temperature have been
thought through. |
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Would also be good for psy-ops, playing "Chopsticks" repeatedly would demoralise onshore populations, or playing "Stairway to Heaven", getting the same note wrong every time and then going back to the beginning as my neighbour used to do on his guitar... |
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Also, the idea of petrol powered guns is slightly baked in a tedious, naff short story some idiot wrote. I should know, it was me. |
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A bit more on the energy density problem: |
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For a gasoline+air mixture to be combustable, it must be about 1.4 and 7.6% gas vapor by mass. The density of air at 1 atmosphere is about 1.2 kg/m^3, so there’s about .0168 to .106 kg gas/m^3. The specific energy of gas is about 47.2 MJ/Kg, so the energy density of the gas+air is about 0.793 to 5.02 MJ/m^3. |
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The maximum charge on a 16” (0.406 m) naval gun (might as well run the numbers for the biggest still around, no?) is around 250 kg smokeless powder. It’s specific energy varies, but 4.6 MJ/kg is a good one for a rough approximation like this, so its energy is about 1150 MJ. |
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Typical smokeless powder density is about the same as water (it’s milled into grains, so has a lot of air in it), so this pack into about 0.25 m^3, which stacks inside a 0.203 m radius cylinder about a manageable 2 m long. |
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The same about 1150 MJ of energy would take about from about 230 to 1500 m^3 of 1 atmosphere pressure gas+air mixture, about 900 to 6000 times the volume of an equivalent powder charge, which fills a 0.203 m radius cylinder about 1900 to 46000 m long, much too long to fit in a bun breach, even a gigantic naval one. You can compress the mixture, of course, but even if you manage something like 20 atmospheres (which you’d need to do with much care and refrigeration to keep it from igniting) you still need 95+ m long gun breech. |
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Gas+air mixtures work well for beer can and spud guns, but I don’t see how you could up their energy to what’s needed for a naval gun. |
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Hot spots maybe but "pinging" or "knocking" is
caused by local pockets of fuel/air burning outside
of the flame front. It is more often due to lower
octane rating fuel or to inconsistent heating in
the combustion chamber. |
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Interestingly for this discussion, it can be readily
reduced by adding a little water vapour into the
chamber, to reduce the temperature of remianing
gases in the cylinder and minimise hot spot
propagation. |
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Just a thought - what about Oxygen enriched liquid Air instead of compressed air? |
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I believe a mix of liquid air and fuel will not spontaneously ignite. |
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//believe//? I'd prefer a little more certainty in such
matters. |
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