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Spy drone with no fuel

Mix the fuel in the atmosphere
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This would be used in a conflict area.

If a spy drone (aircaft, whatever) did not need to carry its own fuel, then it could be made smaller, and fly for longer. So I had an idea to distribute a certain kind of fuel in the air, and let the engine suck in fuel and air together. This would probably work best with a diesel cycle where the compression is higher, and I assume that the concentration for flammability is lower.

Various sources show that n-Pentane is dense and flammable at about 1.4% concentration in air. I couldn't easily find the concentration under pressure, but I assume it is lower.

So, if the atmosphere in an area is slightly enriched with n-pentane, it wouldn't ordinarily become flammable, but only when under pressure. Maybe some other gases are better (not Hydrogen: it needs to be 4%).

Note, it seems that breathing apparatus ought to be worn for continuous exposure above 600ppm. n-pentane is not a greenhouse gas.

Of course, even with low concentrations, it's a huge quantity of gas.

Ling, Mar 24 2007

Solar Drone New Endurance Champ http://www.defenset...rchives/001647.html
Bah. Fossil fuels is so last millenium. [jutta, Mar 24 2007]

Bermuda Triangle http://en.wikipedia...ki/Bermuda_Triangle
Natural phenomina that might allow for limited usage. [MercuryNotMars, Mar 29 2007]

Methane Clathrate companion article http://en.wikipedia...i/Methane_clathrate
spy on swamps and the Bermuda triangle? [MercuryNotMars, Mar 29 2007]

Lower explosive limit of common gases http://www.engineer...n-limits-d_423.html
With no compression [Ling, Mar 30 2007]

Temperature & LEL http://www.bankseng...tors_and_detona.htm
How temperature affects flammability [Ling, Mar 31 2007]

Diesel thermodynamics http://hyperphysics.../thermo/diesel.html
Typical compression temp = 500-600C [Ling, Mar 31 2007]

[link]






       Air fuel bombs.
normzone, Mar 24 2007
  

       How about filling the air with electrons and using a drone with an electric motor?
MaxwellBuchanan, Mar 24 2007
  

       Ok, I just have to do the math on this.   

       Let's say you want your drone to work over a 100 x 100 km area. And let say you can keep your drone in a 100m band of altitude. Ok, that is 10^12 cubic meters of air. Even at 1% concentration you are going to need 10^10 cubic meters of fuel. or 10^13 liters. The Exxon Valdez had a capacity of 4x10^7 liters. So you are going to need a half million tankers that size.   

       Did I drop a decimal point in there somewhere? Maybe. Ok, lets assume that I am within two orders of magnitude. So it is going to take between 5,000 and 50 million tankers of fuel.   

       And that is assuming no dissipation.   

       And the first time someone lights a match, the whole country goes up in flames.
Galbinus_Caeli, Mar 25 2007
  

       Liquid volume does not equate to gaseous volume. Even so, I agree that large quantities are required - I mentioned that already. The trick would be to find something better than n-pentane, and reduce the lower flammable limit by compression as well.   

       If the lower flammable limit is only possible by compression, the whole country will not blow up.   

       Jutta, apparently it is the minature drones that have the fuel and endurance problems.
Ling, Mar 25 2007
  

       Ok, fine. Divide the amount by 270 for liquid to gaseous. So instead of a half million tankers, you only need 2,000.   

       Might be a better idea just to have a crop duster aircraft preceding the drone spraying a lane of fuel for the drone to fly down.
Galbinus_Caeli, Mar 25 2007
  

       Nobody mentioned yet that the country will be covered in oil. Mmm, no-bid baby seal-scrubbing contracts...
Smurfsahoy, Mar 25 2007
  

       This is a bizarre one, [Ling]. I was hoping you would show that some extant atmospheric component could be used for fuel.
bungston, Mar 26 2007
  

       This is interesting, yet requires a huge bunch of thinking out. I know that's the point of halfbakery, I skulked around here for quite a while before I finally got an account, ignored it for a while, and finally started using it, heh, but it requires much more than usual.   

       For one, you need to create a non- flammable fuel, like Galbinus_Caeli said, secondly, you need to figure out how to put it in the air without polluting it permanently, and finally, how are you going to get all that gas? I'm far too lazy right now to do the math, but GC said it again. You're going to need a HUGE thing of gas.   

       With gas prices at their highest, this may be a terrible idea. :) (just kidding)
Aet Lindling, Mar 26 2007
  

       Maybe a fart-homing drone is the answer.
AbsintheWithoutLeave, Mar 26 2007
  

       I know observing any event changes its outcome, but how much more true if the observation process asphyxiates all the participants?
GutPunchLullabies, Mar 26 2007
  

       how about using a "tanker drone" to refuel the lesser drones? it could carry enough to fill a whole whack of 'em at one time.   

       mid air refueling should be a pretty easy task for the computer-pilots. i reckon it could probably fuel 2-3 simultaneously (wingtip and tail hoses).
TIB, Mar 29 2007
  

       //Maybe a fart-homing drone is the answer.//
Is that why flies fly around one particular end of cows?
Ling, Mar 29 2007
  

       Solar Power.
Noexit, Mar 29 2007
  

       Maybe you aren't looking hard enough for fuel sources that already exist in the air. Something dense (all solids and liquids), with a high energy content... Bugs! Flying insects should give you enough power to keep an efficient flying machine in the sky. Of course, you'll have to add technology that allows your device to chase and catch these things.   

       I think you know where I'm going with this. Now what's the mean airspeed of a fully laden african swallow again?
Worldgineer, Mar 29 2007
  

       Perhaps we could train bats to carry tiny cameras.   

       Or hire vampires as spies.
Galbinus_Caeli, Mar 29 2007
  

       //Or hire vampires as spies//
Genius - presumably they wouldn't show up on CCTV.
coprocephalous, Mar 29 2007
  

       It seems to me that one of the explainations for Bermuda triangle is bubbeling methane sinking ships and changing the chemical composition of the air so that it does not combust correctly in airplane engines.   

       Unevenly and unpredictably this would ocasionally be usable I am guessing. Though the article does suggest that this effect kicks in at 1% concentration, probably not an efficient burn.   

       links posted
MercuryNotMars, Mar 29 2007
  

       Robert Zubrin has plans to mix hydrogen with the CO2 in the martian atmosphere to produce methane. His idea involves a 100 KWE nuclear generator and a mini Sabatier engine and electrolysis plant, which would be a bit hefty for a spy plane...   

       Of course, earth has a lot less CO2, but there's also a lot more O2, and Nitrogen, both of which are good for making unstable chemicals. How about a nitro burning, nitro making vehicle?
ye_river_xiv, Mar 30 2007
  

       There are gases which burn at even lower concentrations than n-pentane, I think as low as 0.4%. Unfortunately, the one I saw was extremely toxic. The trick would be to find something that can combust at lower concentrations, but only at high compression. It also needs to be non-toxic. A very tall order, I suppose.
Ling, Mar 30 2007
  

       fundamentally I am guessing there is a difference between simply burning/exploding and expanding enough to push a piston. To mix fuel and air you probably want a fuel that is going to use most of the O2. Forget toxic if you find a fuel that burns at low concentrations that just means that it can set off a chain reaction that continues. That is only one aspect of what makes a good fuel.   

       If you get a fuel that works at low concentrations It will then contain high energy concentrations per partial pressure. then it will be just as energy inefficient in the part of the fuel not used.   

       I am reminded however of my brother in law, a mechanic, who went in for some training. He told me about one problem with deisel engines that if they ever suck gasoline vapors they can and will rev up without any ability to shut them down with bad results. I believe this was a problem with cummings deisels, I am not sure if it is fixed. I imagine if you can crimp or cut the fuel line this would shut it down but I was not told that by anyone.   

       What do you have in mind ling? Ramjet, Jet, Piston? You talk about compression I am wondering if the idea is just a fundamental of engines or dealing with the unique situation of having a flammable atmosphere?
MercuryNotMars, Mar 31 2007
  

       I mentioned diesel because I had large compression ratios in mind. But a diesel engine would have no control of power in the normal way.   

       The LEL and UEL of gases widens with temperature. So my reasoning is that with a higher compression, the temperature is increa ed. A standard diesel engine can get the temperature above the auto ignition temperature of diesel.   

       The idea is to allow minature vehicles to fly, without the burden of having to carry the fuel along with them. I have recently read that this is the biggest obstacle for minature drones.   

       Using the links, I calculate that the typical compression temperature is about 1000F, and this changes LEL to around 22% of standard figures. That means butane would burn in a standard diesel engine at only 0.4% concentration.
Ling, Mar 31 2007
  
      
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