Ultralight Tent Stove-for Alpine Camping Part 3 Safe and effective use

This post is the third in a four-part series on my efforts to design a truly ultralight tent stove. My aim was to efficiently and cleanly provide cooking and warmth from a small number of bush sticks to enhance the pleasure of skiing and camping in a winter wonderland. It is about safe and effective use of the stove.

This ultralight tent stove article part 3 should be read in context of the other three articles. Part 1 tells of my dream of an ultra light weight heat source for warmth and cooking when skiing and snow camping. It also tells how I was teased and spurred on by the amazing protracted and intense heat energy that could be released by the efficient combustion of a hand full of bush sticks. It is also about my lessons from failures. Part 2 is about my best stove so far, the ultralight tent stove or Micro Snow Stove Mk15 that largely met all my defined requirements. Part 3 is about the very important basics of safely and effectively using the ultralight tent stove under field conditions that are invariably difficult when heat from a stove is most needed. Pat 4 covers supplementary DIY tools to take and make in the bush when using the stove and safety, care and maintenance issues to get a long use form the little stove.

Imagine that your wonderful day of skiing, snowshoeing or walking has ended and you walk into your tent that is pitched in the snow gums (Eucalyptus pauciflora. the ubiquitous and iconic gum tree of the Australian snow country) and you are ready to retreat to your tent for some protection from the wind and snow and enjoy the warmth from your snow stove and the company of friends. This tent stove has very little thermal mass. I jokingly say that it is “a rocket mass heater without any mass.” The ultralight tent stove is a finely tuned stove that produces wood gas (smoke) and then burns it very efficiently and quickly, so it will not work well or at all unless the fuel for it is appropriately; prepared and fed into the burner. By contrast bigger stoves can easily burn wet wood once there is s big enough blaze and a good bed of hot coals. For the ultralight tent stove it is a bit more delicate, but a good burn can be achieved with damp dead snow gum but the strategy for preparing and maintaining a good burn is critical to success.

To demonstrate the distinct nature of the burning process in the ultralight tent stove I can establish an adequate burn that will give a cooktop temperature of 400 C. Then I interrupt the burn with the addition of one or two pieces of inappropriately wet wood or a piece of snow and the flame will go out and the stove becomes a smoke generator and it may just keep going like this for a long time and may not spontaneously ignite again. …”may be good for bee smoking but not much good for warmth…”  The simple solution to this problem is covered in the section title ‘flame-out recovery’ below.

What follows is pretty boring mundane stuff, but most of it is crucial to a safe, light and happy experience with a ultralight tent stove and I apologize to any of you experience bushmen if I am telling you how to suck eggs.

Keeping it light-tool List: what to take and what to make? I take DIY bow saw, wood splitting knife, modified anvil-secateurs, and spring-head nails (one/camp site) and fine chicken wire staples and a short length of hay-bale twine. With the above tools, I cut fire wood sticks, make the stove mounting pole, the split stick rails for the wood drying rack. I also cut a bush mallet for splitting wood with the knife, driving both the stove mounting pole and the wood splitting anvil into the snow inside the tent and the spring-head nail into a tree to make my fire wood sawing stop. The saw can also be used to cut a tent pole and a long handle for a snow shovel (see Part 4 for details).

Fuel supply and other bits. In wet and cold conditions preparing for successful stove burning requires a systematic approach. “…the good news is that a micro snow stove does not burn much wood so that it can be collected quickly, the bad news is that it needs to be selected good quality sticks to make the job efficient and easy…” This is my routine: If I have not already prepared my stove and a fire wood supply, I make it the first thing to do after putting up my tent. I leave on my ski boots and wet weather gear and take out my DIY bow saw and cut my bush pole for the stove mount 800-600 mm long depending on the snow depth/soil depth etc. The pole may be bent and this can be used to advantage to ‘angle it away from the edge of the snow pit to improve its hold in the snow.

Then I cut a solid piece 400*50-60 mm diameter section of branch to use as a bush mallet (for wood splitting etc) and another bigger piece to use as a wood splitting anvil inside the tent. I drive it into the snow inside the tent. In bad weather this is the place to prepare wood, yes by a warm fire while you wait for that first hot coffee to boil and cook dinner. Next, I use the saw to cut selected long lengths of standing dead snow gum branches that have little rot in them. I choose solid standing wood and use the biggest diameters that I can manage to safely and easily split. Generally, the thicker the wood the drier it will be inside. I cut ~2000 mm lengths of straight branches.

Natural death of snow gum branches and our alpine fires give us lots of material to pick from. I take the branches over to a suitable bent-over live snow gum and drive in a ‘spring-head’ roofing nail with the mallet to make a simple sawing stop to allow me to safely and quickly cut the branches into billets of ~400 mm long, with no risk of cutting my fingers into pieces. To make splitting easier I usually cut through the middle of branch junctions. I pop the billets into a supermarket bag to take back to the tent. When splitting I split symmetrically through the middle of the stick and the branch junction as this is the plane that will be easiest to split.

Stove/stove pipe location in tent. This should be determined at home when you modify your tent to accommodate a ultralight tent stove, keeping in mind the location of you snow pit (if you have one). It is not a simple choice and there is much debate and diverse opinions about the best location. In fact there is probably no ‘best location’ even for my favorite simple conical tent [see photo]. This tent has five experimental flue penetration points and all had a good theoretical reason for their placement.

For my conical tent the quality area for multiple occupants is in a radial distribution around center, leaving a wedge area adjacent to the slit doorway for standing entry and wet or snowy things and this area can include a narrow snow pit and steps for comfortable entry, sitting, standing, boot changing and attending to cooking duties while leaving the remainder of the tent for bedding benches and equipment storage.

Briefly my preference is for a near centre stove location, behind the protection of the centre pole (if there is one, but that is another story on my Duo Tents Page), with the roof penetration as close as practicable to the peak of the tent. This configuration gives the best distribution of radiant heat (as explained before this stove is designed more to be a radiator rather than an air heater), the safest location of the hot stove from trip hazards and door flap contact. It also makes the roof penetration in the most stable portion of the roof surface for stability in high winds and minimizes any issue of rain water entry to the tent/flue gland at the penetration point (whether the stove is deployed or not). The flexibility of the angle of the flue pipe opening over the flue pipe connector allows the flue pipe to be tilted at a considerable angle to the cook top while maintaining an adequate connection with the stove. This means that the stove can be conveniently located in a range of position that only need to be somewhat near the vertical alignment with the flue/tent gland. Lastly, having the tent/flue gland at the highest point in the tent has two more distinct functional/safety advantage. It puts the tent penetration point as far away as possible from the stove and this allow more heat exchange to the tent air, but more importantly makes the flue temperature as low as possible where it passes through the tent fabric. Safety temperature test;  periodically I cautiously check the temperature of the flue pipe just below the point where it passes through the tent canopy via a tent flue gland using the zzzt finger test (all discussed in Part 4). The temperature at this point should be below 100 C. I feel most in control when I can comfortably grasp the flue pipe for a moment and feel no discomfort. This temperature should be very safe and leaves a considerable margin for the unexpected as described later. Also consider my later comments about the flue pipe not being hot enough (Part 4).

Stove Container for Backpacking. I now pack the stove in a simple bag made from a low-tech cloth flour bag fabric. It is light and water wicking, breathing, washable and can be used as a mitt for handling hot things around the stove. I think it is worth explaining that my early stove designs were very focused on reducing stove body weight (Article 1. stoves; m, o, p, q and s). For these stove bodies I used very light gauge titanium or stainless steel foil and they only weighed ~100 g or less. However the penalties for such lightweight construction were many; ugliness, intricate and tricky welding, sharp corners, need for internal bracing, shorter stove life, sub optimal usability of the internal stove body volume for packing stove components and need for a bulky strong carry box. This box was required to protect the stove from other packed items and also to protect other items from the stove while backpacking (125-150 g extra weight). The sweet irony of my ‘heavier current stove design’ is that it eliminates all the above problems and does not weigh significantly more than the ‘light stove in a box’ and its rounded corners allows it to pack in smoothly and efficiently amongst the many soft items that we carry in our backpacks. “… the lightest stove does not necessarily equate to the most convenient backpacking load… shape and packability matters…”

Stove unpacking and set-up. I unpack the ultralight tent stove on a groundsheet or similar flat soft surface as there are many small parts that can easily be lost and the fused quartz glass can be broken if dropped onto a hard surface.

Immediately after closing the empty stove body I use the ash shovel to sprinkle a small amount of sifted ash (or fine soil) into the joint between the two body parts. I also use the shovel to spread the ash evenly around the whole joint and pack it in a little. This ash greatly reduces the small amount of air leakage into the stove body via this joint and improves the stove draw and efficiency. It is not much trouble to do, and if I forget to do it before setting up the stove I can still do it while the stove is burning (with a little more difficulty) and the improvement in the stove performance is noticeable. Just imagine the air leakage that would be occurring in rollup stoves with poorer quality butt joints that I estimate are 1600mm in length.

Next I put the stabilizing strut twitch ring over the mounting post and then fit the insulated stove mount to the body of the ultralight tent stove with the mounting pin that is conveniently located in a micro tube that is welded on to the mount. I fit this mount firmly on the bush pole which has been driven into the snow or soil (or both). The top of the pole should be slightly oversized and it should be paired down with the splitting knife to tightly fit the stove mount. Normally the pole (or at least the pole top) will be perpendicular, but other variations are possible when dealing with a tent with a snow pit as the stove mount is articulated and the pole might be better if curved. I assemble and fit the flue pipe and fit and adjust; the stabilizing strut, flue pipe tie down ribbon and wood drying rack hangers and the bearing sticks of the rack, described in detail later.

I set the fire with a little saved dry wood. I put three or four ~50-75 mm long sticks on the bottom of the stove body directly below the burner assembly. These ‘foundation sticks’ insulate the starting flame from the cold metal and soon become the initial coal bed. Then with the secateurs, I make very small split sticks, splinters and chips that are no longer than 40 mm (about enough to fill the burner glass).

Now before I set the fire going, I take some of the wood billets and split them in half and then quarters on the anvil. I set the billet on the anvil, set the knife on the billet and hit the back of the knife with the mallet and splitting could not be easier. I split this way until I can use my anvil- secateurs to complete the splitting. The damper the wood the more I split it to speed drying. With this mode of splitting, the secateurs blade should be used more as a splitting wedge rather than a cutting device and it works with the least effort if shallow splits are started and are followed up with more splitting. By contrast one big deep cut may just jamb the blade because it is not following the grain. This all may sound fiddly, but it is not. It is very efficient and even satisfying to see the wood rack fill with very little effort and not having to dressing up in your snow gear to go out into the snow to get more fuel. It is much better than swinging an axe or such like in a tent. After splitting, I do not cut the sticks shorter, but I lay the long sticks across the fuel rack and store up at least two layers before starting the fire. This way, as soon as I light the fire, the critical drying will start immediately and this will maximize the opportunity to get plenty of fuel ready by the time I use up my original precious dry wood. The long split sticks can be docked to the required length on demand using the secateurs and I put these on the top of the drying rack to dry quickly. The maximum length should be such that when placed in the fuel tube the ends of the longest stick can still be covered by the snuffer can with a little overlap with the fuel tube.

Bent sticks can be straightened, to better pack into the fuel tube, by nearly cutting the stick through from the inside of the curve with the secateurs and straightening with finger pressure. Such straightened sticks will usually feed downwards much better than two separate shorter sticks and it helps to maintain fuel density in the burner tube and a stronger burn in the burner glass.

Burner start-up. Starting with the burner glass in place (without the fuel tube connected, I hang the fuel tube in the fuel tube stabilizing ring in readiness for deployment), I put a 1-2 g piece of firestarter on top of the ‘foundation sticks’ opposite the primary air port on the ultralight tent stove. I light the fire starter with a waxie (described later), leave the port fully open and gently sprinkle the prepared splinters onto the flame and follow up with some small chips. I then cover the burner glass opening with the ash shovel and quickly and gently blow into the primary air port to provide oxygen and help to start the draft of warm gas through the stove labyrinth and up the flue pipe. If I blow too hard the initial flame will go out. In this case I use a couple of waxies to re-start the flame. If I delay the blowing a little smoke will come into the tent before the flue pipe warms up and the flame may go out because of lack of oxygen. In earlier stove designs putting an extra piece of burning fire starter inside the stove labyrinth under the flue pipe ensured that the stove would start quickly, but this process is not simple as the flame will extinguish if the fire starter is dropped into the stove. Now I also use a safety tie down on the flue pipe and this makes this method impractical. Either way the stove will end up with a strong draw that will suck in air strongly. Before the stove gets uncomfortably hot, I quickly put the fuel tube in place in the burner glass, restrict the tertiary air port opening by rotating the fuel tube and sprinkle in more small chips, a little at a time, plus one or two more substantial pieces of fuel stick and quickly fit the snuffer can to maintain a strong primary air flow. When a strong burn is established, I fill the fuel tube with fine split sticks and start to restrict the primary air port to approximately half open, keeping the snuffer can in place and partly open the tertiary air port and adjust the primary air to get a ‘hint of smoke’ (as described in Part 2).

Burning damp wood. Damp wood is a reality for those of us who venture out into the bush in cold and wet weather. The ultralight tent stove is designed to run on damp fuel, but drying the spilt fuel sticks in bulk below the stove on the drying rack is essential for good burning. I select thick dead branches of sound wood as they will have the driest wood inside. It also helps optimize the air port settings for wet wood burning by opening the primary, restricting the secondary and tertiary air ports. This will reduce the ‘flame pull down’ in the burner (as described in Part 2- How Does the ultralight tent stove work) and it will increase the zone of pyrolysis in the burner assembly.  In my experience when using wet fuel sticks they will often burn to black pencil points and the heating from the pyrolysis zone can not effectively dry the inner core of the sticks to make them collapse in time, and the rest of the stick is left stranded out of reach of the heat from the flame and pyrolysis. However, there are techniques that can help to get enough heat out of such damp wood to get this drying process under way. If a little dry wood is available it can be mixed in with the wet fuel in the front of the fuel tube nearest the primary air port to spread flame over the other less combustible sticks. Alternatively the dry stick can be poked in through the primary air port with the valve assembly temporarily remove. CAUTION removal of the valve assembly takes away the spark arrestor function of the valve assembly, so it should be replace as soon as possible. Luckily sparks are not usually an issue with the stove that is not burning strongly. Waxi strips (described later) can be slipped down in the front of the fuel tube so that the melting wax will dribble on to the pyrolysis zone.

Simply splitting the wet sticks into finer strips will make them much more combustible and quicker to dry both on the drying rack and inside the burner assembly and also quicker to collapse as charcoal and allow fresh fuel to move down the fuel tube. If fine splitting does not work, then cutting such incombustible sticks into short fuel pellets (~20 mm long) will usually work. “..yes it is a bit tedious and becomes a full time job for one person… but the heat generated will solve the problem soon enough… if you string it out for long enough you tent companion/s may end up doing all the cooking..”  I demonstrated this method by burning green fallen eucalyptus branches still with decaying leaves attached. The next desperation method involves ‘toasting small fuel sticks on a toaster rack (The toaster rack is the stainless steel band that hold the stove box together while backpacking. It can also be used a bread toaster (but smoke from falling crumbs is a problem, see frying alternative later) and a simmer ring for pots when placed on the stove cook top. This will dry wood to char point very quickly. CAUTION  It is easy to avoid being burnt by the visible char on the toasted fuel sticks, but watch out for the invisible steam spurting from the ends of the sticks when handling them. It gives no warning!

The floppy fuel tube. At first sight the fuel tube that is vaguely supported by linked rings attached to the flue pipe is a rather sloppy affair, and you would be correct! However, there is method in such madness. The fused quartz burner glass on the ultralight tent stove amazingly has almost no coefficient of thermal expansion, while in contrast the stainless steel burner tube and the burner glass mounting ring have a very considerable expansion coefficient (about the worst metal in this regard). Where there is a temperature gradient of some 500+ C in normal operation, keeping a close fit between the glass and stainless steel is not possible. This results in a design that is sloppy when cold or destructively tight when hot or even destructively tight when cooled down again but not perfectly aligned.  I have a bag full of broken burner glasses to demonstrate this. My current design accommodates this expansion differential by being sloppy and this also has the following advantages: the fuel tube can be jiggled (or gently wacked with a light portion of fuel stick) to help to move fuel sticks downwards if they become ‘hung-up’ in the fuel or burner tubes and the jiggling can help to fit more fuel into the tubes. It also means that the loose fitting tube can be tilted in the burner glass away from the alignment of the flue pipe and give the operator more ‘knuckle clearance’ from the hot flue pipe while fueling the stove. The tilting also allows for adjustment of the tertiary air port slit width by a combination of tilt and rotation of the fuel tube relative to the burner glass.

Snuffer can uses. The snuffer can could be a lot shorter and still function as a flow restrictor for the secondary air port (the ash shovel or a piece of cooking foil will suffice), but I prefer to use a longer can as this will allow: quick and ‘fiddle free’ restriction of secondary air supply in case of a reverse burn situations in the burner assembly (as described in Article 2). It also stays on when jiggling the fuel tube or the stove is bumped. Also it can effectively increase the usable fuel stick length (and the fuel load) that can be safely loaded into the burner assembly, and free your hands for cooking duties (if that is indeed an advantage). The longer length allows it to act as an air warmer for the secondary air supply as air moves as a thin layer upwards between the hot fuel tube and the inside of the close fitting snuffer can. It also makes it more useful as an ash collecting pan and a rain cover for the flue pipe when the stove is not in use.

If the suffer can is sealed closely across the fuel tube opening (as would be the case when using a soft drink can with a dimple bottom), no air will enter the secondary air port, but this will mean that when the can is removed for refueling, invariably a burner tube full of smoke will be released into the tent. To eliminate this unpleasant problem I started by put a small precision punched air bleed hole in the center of the end of the can to continuously purge the smoke from the can. This worked perfectly and was very elegant. “…. but beware of elegant designs…” However the modification negated the beneficial air heat exchange as described above and it ruined the can for other important uses. The solution to both problems was to dispense with the neat bleed hole and instead crudely beat the ‘energy drink can’ bottom into an ugly, crinkly flat shape. The crinkles prevent a good seal with the top of the fuel tube, allowing a gentle purging air flow. The can is fitted with a nylon cord handle that is attached with silicone rubber and can be used to safely remove the snuffer when it gets quite hot. With a can with no bleeder hole in it can also be used as an ash pan when removing excess ash from the stove. It also makes a convenient flue pipe ‘rain cover’ when the stove is deployed but not in use. Lastly, I found that this modified ‘flat-ended-can’ made packing this component into the stove box for backpacking more efficient than it would have been with the original ‘dimple-end’.

Flame-out recovery. As described earlier the ultralight tent stove can become a smoke generator (without flame) and it may just keep going like this for a long time and may not spontaneously ignite again. However you can instantly re- establish the flame with a suitable ignition source such as a long stem gas lighter. I have damage too many lighters after flame-outs by putting the lighter up the primary air port for just a little too long! I think the intense heat from the burner damages the lighter.

To carry the flame along the long primary air port I use a folded ‘waxie’ paper (described in Part 4). I fold it into a long ‘v’ shape (to give it rigidity and minimize contact with metal surfaces) and pop it into the primary air port, without removing the valve assembly, and quickly light the tail end of it and the draft of the stove will pull the flame in and with a ‘whoomph’ the burner will start happily burning again, sometimes blowing a lovely smoke ring that a Hobbit would be proud of. So I always keep my lighter and waxies handy.

Stove ash management. In my early not quite right stoves I often spread ash or soil on the base of the stove to deflect heat away from where the stove was sitting. These materials proved to be a very abundant and effective insulation. When I discovered stove pole mounting for the ultralight tent stove, I welcomed high temperatures under my stove for the purpose of drying wood and no longer wished for a layer of ash. However ash is an unavoidable consequence of clean burning of wood when the draft is well controlled and the ash is not simply blasted out of the flue pipe. The build up of ash is slow and it accumulates in the burner zone and a little beyond and the insulation factor is not a major concern, especially after 3 or 4 h of burning. However a ‘wedge of ash does build up under the burner tube and it has the effect of making the burn zone progressively thinner and seriously impairs the power of the micro snow stove. All is not lost, because there are a couple of strategies that can help to recover the stove power (short of stopping the stove and removing it from the post and tipping the ash out). A short term solution is to poke the ash away down the labyrinth to restore the burner depth to get a couple more hours of burn time. The other option is to use a little (1.5 g) stainless steel rake to scrape most of the ash out through the primary air port and into the snuffer can.

Cooking on the ultralight tent stove. The stove has a small (130*130mm) cooktop for melting snow, boiling water and cooking meals. While the cooktop is flat it is not perfectly flat due to heat distortion and camping pots are usually not perfectly flat either, so heat transfer to cooking pots is mainly by radiant heat rather than by conduction. Nevertheless, the stove can boil 500 ml of water in about 15 min in a small aluminium pot that can cover most of the cooktop. Cooking of stews and porridge is good as the gentle heat transfer does not easily cause the food to stick or burn to the pot. My preferred meals to ‘cook’ on the stove are those that are pre cooked before dehydration, so that the stove is only really required to heat the meal to assist rehydration and raising the meal to the required eating temperature rather than providing cooking per se. Shallow frying in a trace of oil in an aluminium pan works well for items like pancakes and falafels and stir-fry vegetables. CAUTION frying with more than a few drops of oil is not advisable because of the risk of an oil fire starting from oil spillage onto the hot cook top. Toasting is also possible, but fallen crumbs can make a lot of smoke, so I find light frying in olive oil to be a better option and it tastes great and I expect that the extra energy does no harm when so physically active.

Part 4, the last article of this series is about DIY tools, maintenance, safety and getting the best out of the ultralight tent stove.

Tim.

Tim

Addendum

This post dates back some time now and could be considered redundant. However, it contains some fresh ideas and lessons that may be helpful to others. In my opinion, it has been largely supplanted by my very light, compact, and hot KISS tent stove.