Cobra's Stove

Cobra Commander's personal camping stove, inspired by the popular Pepsi-can and Photon alcohol stoves.

"Had Destro possessed this stove, our roles may very well have been reversed."

--Cobra Commander

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   So you want to rule the world, do you..?  I hope you have a good stove..!

  This page details my construction of an ultra-lightweight camping stove, which burns denatured alcohol or other similar fuels.  With two soda cans and a pocket knife, I can build this stove, which boils a pint of water in around five minutes, using roughly two tablespoons of denatured alcohol.

   Cobra's stove attempts to bring together the best aspects  of Scott Henderson's Pepsi can stove, and Don Johnston's High Performance Alcohol Stove (AKA Photon stove).  Both designers do a fantastic job of detailing the construction of their stoves.

   In building and experimenting with several models of both of their stoves, I made some interesting observations in construction and performance.

If you would rather skip my comments on the Pepsi can and Photon stoves, you can jump straight to Cobra's stove construction notes.

Methodology

   I tasked each stove design with the challenge of boiling one pint of tap water using no more than two tablespoons of denatured alcohol, which I measured with a kitchen measuring spoon. I used a thin aluminum cook pot from a Boy Scout type mess kit and a small pot stand made out of hardware cloth. I experimented with and without a wind screen. I did not cover the cook pot during the boil tests.  Obviously, a covered pot will boil faster, but because I conducted the tests indoors, I felt that I should compensate for the testing environment in some way.  I reasoned that failure to achieve a full boil in an uncovered pot in this above-average environment would suggest that a stove would perform poorly on the trail.  I may not have conducted this experiment under the most objective environments, but since Serpentor usurped my authority, I have not had access to better scientific resources.

Comments on the Pepsi can stove

   Take a look at Scott Henderson's design.  His design employs a double wall inside a soda can housing with a large central hole and chamber through which one adds the fuel and lights the stove.  The inner wall has holes at the bottom allowing the fuel from the large central chamber to enter behind the inner wall and into the inner chamber, through which the alcohol vapors travel to exit through the jets on the rim of the stove.  Without this inner wall, most of the vapor would exit straight up and through the large central hole, making a large and inefficient flame.   However, even with the inner wall, the flames on the rim jets stop burning long before the stove consumes all of its fuel.  For the last two minutes or so of the stove's burn time, only the flame from the large center chamber burns.  I believe this happens because as the fuel volume drops, the alcohol vapor pressure decreases, and the vapor finds an easier exit through the large center chamber.  The stove does not burn as hot with just the single large flame from the center chamber.  The single large flame also burns much taller, and in a less than optimum position relative to the cooking pot than do the flames from the jets along the rim.   The stove does not burn at its optimum efficiency throughout the full burn time.  I experimented with making smaller center holes and different sizes and arrangements of jets on several models of this design. These changes did little to improve efficiency, and none of the models I built up to that point brought a pint of water to a full boil using two tablespoons of denatured alcohol.

   Scott proposed that adding some fiberglass insulation behind the inner wall may improve the stove's efficiency. I experimented with this, loosely packing some loose home insulation, which I harvested from my attic, into the bottom of the inner chamber, making sure it made contact with the bottom of the can. This does indeed improve the efficiency of the stove's burn. The absorbent insulation wicks most of the fuel from the main chamber into the inner chamber, making more vapor available to the jets along the rim. The jets along the rim burn for most of the stove's total burn time, improving the efficiency and performance of the stove. I did not test this, but I suppose one could replace the fiberglass insulation with just about any absorbent material, since it rests inside the inner chamber and does not contact any flame, though extremely flammable material may char. One should obviously avoid using any material which would melt. With the addition of the insulation, the stove did meet the challenge of my experiment, bringing a pint of water to a boil using two tablespoons of fuel.  I felt pleased with the performance of the Pepsi can stove with the addition of the fiberglass insulation, but the insulation added a layer of complexity, albeit a small one, to the design.  I consider it a "special material."  I felt more and more compelled to overcome construction time and complexity during my experiments with the Photon stove, which I will discuss later.

   After making several models, and learning some tricks with working with the thin soda can aluminum, I have no difficulty making a Pepsi can stove using only my trusty early model "Swiss Champ" style Swiss Army knife.  The scissors really help, and the file makes it easy to remove burs from the edges of the worked aluminum.  I really appreciated the fact that I could make the stove without special tools and materials-- just two cans and my pocket knife.

Pros

Cons
  • One only needs a pocket knife to construct
  • Ease of adding fuel
  • Wind resistant
  • Does not require a priming dish to light
  • Does not burn consistently during the full burn time
  • Does not burn fuel efficiently
  • Takes some practice to construct the double wall

Comments on the Photon Stove

   Take a look at Don Johnston's design. The Photon stove has just one large chamber, a ring of jets along the rim, and small hole in the center of the stove through which one pours the fuel. One inserts a screw into the hole before starting the stove to better seal off the stove and increase vapor pressure, thereby making a double wall, as in the Pepsi can design, unnecessary.  Because one cannot light the stove from this small hole, it requires a priming dish.  The stove sits on a priming dish which burns just a few drops of fuel, just enough to heat the fuel in the stove to the point where vapor begins exiting through the jets along the stove's rim. Once the alcohol vapor begins moving out of the jets, the flame from the priming dish ignites the jets. Using too much fuel in the priming dish causes the stove to burn extra hot in the beginning, and wastes the fuel. Using just the right amount of fuel in the priming dish to get the stove lit takes a bit of practice.

   Following Don's detailed instructions, I built three models of the Photon stove, enduring the fifteen hour epoxy drying times, the critical jet hole sizes, and striving for greater accuracy in constructing each model.  Yet none of the models came even close to boiling a pint of water with two tablespoons of denatured alcohol following my experiment model.  Had I kept working at it, I believe I would have had more success, but I lost patience with this design because of how long it takes to build one of these stoves, and the requirement of special tools and materials.   However, I did appreciate the fact that the flame jets along the rim make for a relatively steady and consistent burn throughout the stove's full burn time, distributing the heat efficiently.

Pros

Cons
  • Consistent heat throughout the total burn time
  • Ease of adding fuel
  • Burns fuel efficiently
  • Simple design concept-- no double wall
  • Time consuming construction
  • Requires special tools and materials
  • Lighting the stove with the priming dish takes practice

Discussion

   Even though I felt that the Pepsi can stove generally out-performed the Photon stove, I thought that the Photon stove design concept had the most potential.  It does not require a double wall to direct the alcohol vapor through the jets along the rim, and it has no tall center flame which inefficiently licks up the sides of the cooking pot instead of burning at an optimal height like the jets along the rim.  In re-evaluating the Photon stove design concept, I decided to abandon Don's construction guidelines, and opted for a simpler design that I could construct with just a pocket knife and that did not require any gluing, extra-precise jet hole sizes, or materials other than two soda cans.  My simpler design comes at the cost of a smaller hole through which to fuel the stove, and the necessity of a priming dish to start the stove.  

   For your consideration, I offer Cobra's stove......

 

Constructing the Cobra's stove

   To build Cobra's stove, I simply cut the ends off of two soda cans, and fit them together, seating one inside the other, open ends facing each other.   One of the ends has jet holes punched along the rim, and a single jet hole in the center, through which I add the fuel.  I found it much easier to punch the jet holes on the bottom of one of the cans before I cut the end off.  I used the awl/reamer tool on my trusty Swiss Champ to make very tiny initial holes, then used the cork-screw to finish the job, sizing them roughly one-sixteenth of an inch.  As you can see from the photo, I did not pay precise attention to spacing, number or size of the holes.  One of my models has eighteen jets along the rim, another has seventeen, and another sixteen.  If you make very large holes, the stove may burn too hot and consume all of its fuel too soon, before it can do it's job of boiling the water.
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   I learned a few tricks that make working with the thin soda can aluminum much easier than just cutting it with a knife or scissors.  To cut the ends off of the cans with nice straight edges, I first cut a score line.  I rest my knife on an object so that the blade touches the can about an inch and a quarter from the bottom, then rotate the can, cutting the score line.
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   After I cut score lines on both cans, I cut the ends off of the cans, cutting above the score lines.  I then cut a tab into the can wall up to the score line and bend the tab back and forth along the score line until it snaps off.  I continue cutting and snapping off tabs until I have a nice straight edge on both cans

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   Now I assemble the stove by seating one can end into the other.  I push the end without the jet holes,  which will obviously serve as the bottom of the stove, into the top part of the stove.  Though both the Pepsi can and Photon stove instructions suggest doing so, I do not cut slits in the can walls of either end to help with the seating one can into the other.  Since my design does not use glue or any other way of sealing the stove other than the friction of a tight fit, cutting slits may compromise the seal, and may allow alcohol vapor to exit at the edges.  I slightly bend the top of the wall on the bottom can inward a bit to help coax it into the top part.  I just try to remain patient and gently work the cans together.  If you make several small inward bends at the top of the can wall of the bottom can (creating an accordion effect at the top part of the can), this may help you seat the two together. The smaller the better.  You want to avoid long sharp bends.  I make these bends with my thumb and index finger, going around the circumference of the can.  You may have to go around the can making bends several times before the bottom can slides easily into the top.

Jason Wallin reports that he has good success with fitting them together when he puts the top part in the freezer and slightly heats the bottom part in the oven.

Once the bottom can begins to move easily into the top, I turn the stove over and gently tap the bottom can into the top with my pocket knife.  I go around the circumference gently tapping until the bottom can stops moving into the top.  If I have trouble sliding the top can over a bend in the bottom can and they do not want to come together easily, I just pull them apart, rotate the cans a tad and try again.  Trying to force the top part over a sharp bend can result in tearing the can wall.  I do not recommend gluing or taping the edge in an attempt to make a better seal between the two cans. 

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   To fine tune your stove, first test it and time how long it takes to bring 2 cups to a full boil.  If you can achieve a boil in 5 minutes, then you have made a great stove..!  If it takes over 5 minutes, then slightly enlarge the jets along the rim.  I use the corkscrew attachment on my trusty early model "Swiss Champ" Swiss Army Knife.  Try to avoid radical changes in their size-- usually just a barely perceptible change in the size will improve performance.  I do not bother to enlarge the center jet / fueling-hole while fine tuning. 
   Depending upon what kind of soda cans you used to build your stove, the concave center of the top of the stove most likely has a volume of at least a tablespoon-- probably a bit more.  To fuel my stove for boiling a pint of water, I fill the well in the center and let the fuel drain into the stove through the center jet, then repeat. 

   I use a 20oz soda bottle to carry my fuel.  I punched a very small hole in a second bottle cap to make a "squeeze bottle" cap.  Squeezing the fuel into the center well really helps with fueling the stove, as I always seem to splash/dribble when I try to pour fuel.   Keep the hole in the squeeze cap small-- smaller than the size of the jet holes on the stove so that the fuel doesn't come out until you actually squeeze the bottle.  I used the awl/reamer and corkscrew attachment on my trusty early model "Swiss Champ" Swiss Army knife for this procedure.

   Though I prefer to fuel my stove with the squeeze bottle, I have also experimented with an infant medicine syringe, bought at a drug store, to pump the fuel into the center jet.  Infant medicine syringes typically come with a short length of soft tubing to fit over the business end of the syringe.  This makes an easy job of drawing the fuel into the syringe, and fits comfortably over the center jet hole.  As an added bonus, these syringes have graduated markings in ounces/millimeters and even teaspoons, however these graduations did not jive with my kitchen tablespoon.  Experiment to find the right volume.

   You need a priming dish to light the stove.  I made a great priming dish from the bottom of a can of Frito-Lay bean dip.  I Cut the bottom off, leaving about 1/4 of an inch of the can wall.  I also experimented with the metal end of a can of Slim Jims that Destro had lying around-- the only thing I could find at the time :).  Don Johnston suggests cutting out and using the bottom of a 10oz tuna can.  I observed that the priming dish works best with no more than 3/4 of an inch of space between the edge of dish and the side of the stove when the stove sits in the middle of it.  The bigger the priming dish, the further away from the stove the priming flame burns, thus not heating the can as efficiently as a smaller dish.  

   To prime the stove, I dribble a several drops (5-7) of fuel near the edge of the priming dish, swirl the fuel around the inner edge of the dish, place the stove in the center, and light priming dish.  Don Johnston actually has a video clip of how he does it on his web site.  Within a matter of seconds, I can hear the fuel in the stove start to bubble, then the flame from the priming dish lights the jets on the stove when the vapor begins to exit.  If you're priming flame burns out before your stove gets lit, you can use a soda straw as a cheapo eye-dropper to put a few more drops in the priming dish.
   I took Don Johnston's advice and made a pot stand out of some wiry material called "hardware cloth" that I found in the garden section of Home Depot.  The smallest amount of hardware cloth I could buy will probably make about 100 pot stands, but it only cost about $6.00.  I used the stuff with half-inch squares.  I cut a piece five squares high and 20 squares long, and wrap it around a can of Cobra Industries roach spray to give it a curved form.  Then I use a small piece of wire, or a staple and tie the top part of the stand together.  I can then fit the stand into the priming dish.
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Windscreen

   Using a windscreen helps the stove by reflecting back heat in cold conditions, and protects the flames in windy conditions.  I shelter my stove with my sleeping pad in extremely windy weather.

   I use a windscreen made out of an oven-liner, which looks like a large thin baking tray and feels like thick aluminum foil.  I first trim off the raised edges, then cut a length long enough to wrap around my pot with about a 1/2 inch gap between the pot and the windscreen.   Depending upon the size of your pot, you may need to cut 2 lengths and fold/crimp them together to make a length long enough to give you that 1/2 inch gap.  I trim the height so that my windscreen stands about 1 inch taller than the bottom of my pot when the pot sits on the pot stand.  I then make make an 1/8th inch fold on the bottom and top of the windscreen to remove the sharp edges and help stiffen it.  It helps to start the fold at one end then just continue folding along the entire length.  I then punch holes along the bottom so the stove can breath.  I also cut a small notch to accommodate the handles of my pot.  I then make 2 opposite inter-locking folds at each end so the ends can connect.   My windscreen weighs about 1/2 of an ounce.
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Cozy Cooking

    To further improve the efficiency of my backcountry kitchen, I use a pot cozy.  Most grocery store-bought food suitable for the trail, like macaroni and cheese, and other noodle/rice dishes require simmering, unlike dehydrated "camping food".  Instead of using fuel to simmer, I place the pot into an insulated pouch I made out of Reflectix™.   Other companies make this stuff under different brands for insulating heating/cooling duct-work, etc..  This stuff looks like bubble-wrap sandwiched in-between 2 layers of aluminum foil.  It weighs very little, and boasts that it reflects back 97% heat.  One could use other insulation, like closed-cell foam used for sleeping pads, though it has a great deal more bulk than Reflectix™ type insulation.   cozy.jpg (69840 bytes)
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   To make a cozy, I cut a length of insulation long enough to wrap around the pot, then tape it together with thin reflective insulation tape, though thin brown packing tape will work just as well.  Duct tape will work too, but weights more.  You do not need to use the heavy-duty aluminum furnace tape.  I then cut a circle out for the top and tape it on.  I also cut out a circle on which to set my pot.

   To cozy simmer, I bring my water and food to a boil, then take it off the stove and place the cozy over it, and wait.  I follow the general rule of thumb for cozy cooking and add an additional 5 minutes for every 10 minutes of required simmering.  I resist the temptation to check my food, as this will release a great deal of the heat I want to keep in.  Obviously do not use the cozy over a burning stove, as it will indeed melt/burn.  Cozy cooking really works, and my cozy weighs about 3/4 of an ounce.  Cozy cooking allows me to cook just about anything that requires simmering using just enough fuel to boil.  My entire cook set, pot, stove, priming-dish/stand and windscreen nests nicely into the cozy for packing.

 

One final note on safety

  One wouldn't need to complete the Cobra forces basic training program to safely construct and use this stove, but please remember that this stove burns fuel under pressure.  The fuel burns under low pressure during normal operation, and the jets provide more than enough room for the vapor to exit.  I tested the stove with a ridiculously hot priming fire, which created large flames from the jets, so much so that most blew themselves out, and the stove did not show the slightest indication of stress or danger.  However, I do urge you to inspect the jets for obstructions before using the stove.  Please use common sense i.e., don't do something ridiculous like making a stove based on this design with only one tiny jet hole and then trying to use it.  

Disclaimer (also inspired by Don Johnston)

   If you construct this stove you do so entirely at your own risk.  Regard the stove YOU construct as unproven and experimental.  I strongly urge you to test your stove in a safe test environment, wearing safety glasses and clothing, and taking all precautions to protect life and property, such as having a fire extinguisher handy should something go wrong.  If you do not heed the warnings of Cobra Commander and get yourself hurt or dead and want to blame me, please consider the fact that my sister, Cobra Counselor, practices law and will aggressively protect the interests of the Cobra cause...

   I welcome any questions or comments about Cobra's stove.  You can reach me at ukulele@yahoo.com