Ultralight tent Stove Part 1- Lessons through failure
This post is the first 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 the background trials and lessons from failures.
Dream or Virtual Reality? Imagine skiing all day, having had six second last runs with your mates as the sun goes down. You are happy, but a little cool, damp, tired, dehydrated and hungry and the snow is gently covering up you tracks so that tomorrow will be all about more fresh tracks. Just think of the joy of walking into your tent, heaving off your ski boots, sitting down and within 5 minutes having a glowing, gently growling slow combustion stove, fueled by fire damaged snow gum sticks (a truly renewable fuel with no environmental cost for its creation, transport or containment), instantly warming you and your little tent with the power of a 890 watt radiator, a gentle fire light glow flickering around the tent, gloves and jackets hanging up to dry, a big hot soup or coffee for two ready to drink in 20 minutes time and more water being made from snow to save that dreaded trip to the creek. “…ahhh he exaggerates a little ….” That was my dream of heaven on earth. Now it is a reality and it comes with only a 500-600g weight penalty that can be discounted buy the weight of an alternative conventional stove and fuel load. It also delights me that this comfort and fun comes in unlimited quantity with little cost to the planet (or to me) and what I consider to be a manageable risk. Follow my story if you think you might like to backpack camp like this.
A brief history of micro stoves for tents. Portable wood burning tent stoves are not new. They have been and still are used by many nomadic people around the world where life might be death without a stove. A skiing friend handed me a page ripped out of a Reader’sDigest article with an orange, conical Sami Reindeer Herders tent at sunset with a flue pipe protruding through the conical top and said with a big grin “….Tim, look at the similarity of this setup to yours…” We agreed that there is seldom anything that is strictly new and all that we really do is improve things with the wonderful technologies and materials that our age has blessed us with. A photo of the scrappy remains of the ripped pages is below (sorry about the quality).
North America seems to be at the centre of light weigh tent stove development and this is s not surprising when you consider the temperatures that they must live and play in. Many portable stoves are available from companies such as Four Dog Stoves . However, at 8 lb these are not utltralight. One of the latest stoves by Titanium Goat is light (737 g) and seems, by my standards, to have reached the status of ultralight. The elegantly designed stove is made of titanium for backpacking. It appears to have a good flat and stable cooktop but is considerably larger than what I had in mind for use in a small tent. It can easily be loaded up with a considerable amount of wood for an extended burn time, but the issue of balancing this with a clean burn appears to remains a problem. I also have concerns about uncontrolled air leakage from the extensive butt joins between the fine (0.12 mm) titanium foil side wall and the base and top plate (my estimated total join length is, 1,600 mm). I can imagine how the rigours of high-temperature heat distortion and packing and unpacking would diminish the quality of the seal. The need for ‘flue pipe dampers’ and ‘spark arrestor’ screens appears to me to be addressing a stove design deficiency rather than an improvement to stove design. The Kifaru Oval stove is another exemplary ultralight stove and has been compared by an owner (Muelman) who has both WiFi and Oval stoves. He indicates that the Oval is even lighter but in his carefully considered opinion, the WiFi is the leader in many design aspects including the leg support for the stove and the cooktop and the stove assembly (Kifaruforums). For both stoves, I would also raise the issue of how to support a hot stove on a deep snow surface.
My stove development journey- Lessons from failures. My purpose was to design and build an ultralight tent stove that gets a good balance regarding the following: lightweight, packability, mechanical strength, chemical durability, cost, convenience, fuel efficiency, and use of damp bush sticks for fuel. It also needed to provide heat to: warm bodies, dry clothes, melt snow, and prepare hot drinks and food. Light from the stove to illuminate the tent would be a bonus and of course, it needed to be safe. I also wanted the stove to be able to be conveniently used on the ground or in deep snow in cold and windy conditions and to be able to have the comfort of a tent snow pit while still using the stove.
During the development journey, I also set a difficult goal of having all the stove components stored inside the stove body and to have an absolutely minimal assembly of the stove when deploying it (with cold hands). I also discovered that I wanted my stove to be very small and compact so that it would produce intense radiant heat rather than just being an air heater.
Well if you look at my groundsheet of not-quite-right-stoves I should be an expert, but it is not so. I still have a lot to learn and I keep them as a reminder of what does and does not work and they are a great resource to have on hand when I wish to quickly plug something together to try out a new idea.
My first stove was made from a big Stanley thermos flask. It sat in a small metal dish in the middle of the circular tent on an insulation pad on the top of my ground sheet. It was a very elegant design (so I thought!) as you can see from the photo where the flue pipe doubled as the centre pole of my tent.
The Stanley stove was packfriendly when slipped into an old bushwalking sock. In contrast, the flue pipe/tent pole (aluminium vacuum cleaner tubes) was dirty, smelly and the joints would get locked together with tar and was consequently carried in a separate fabric tube on the outside of my pack. Encouragingly, the stove made lots of radiant heat but it required fiddly small, sawn and split wood fuel blocks to sustain a good burn. Too much heat would come out of the base which would melt through objects placed below it. I have a ground sheet with a perfectly round dish-shaped hole in it to remind me of this. This heat leakage would make the stove unusable on a deep snow surface. It had another fault in that the burn was very unstable. It would burn too strong and make much smoke and then it go weak before it would go strong again. I added a secondary adjustable air port up high on the flask to both ‘burn the excess smoke’, and reduce the primary air flow that caused the excess smoke. Having both air ports with flow restrictors on them made a big improvement. However it was a constant fiddle keeping the burn rate under control, the flue would be smelly with tar and be horrible to backpack. One cold night up on the High Plains by Ropers Hut the quality of the wood must have been too good and the smoke burner must have worked too well! “… Yes you got it, the flue pipe/tent pole melted! At least there was no tar to worry about and the silnylon tent lives on.”
After this lesson, I only used roll-up stainless steel or titanium flue pipes with a separate tent pole (or no pole at all is my favourite setup, but that is another story Duo Tents). This meant that the flue pipe could be very light, long and compact as it did not need to be strong. This change also allowed me to easily mount the stove on a single bush pole to keep it up off the ground or snow, as the pole was only required to support the stove, cooking pot and the later-described wood drying rack (and not the tent). With this new arrangement, a lovely area became available for a suspended fuel stick drying/storing rack below the hot stove. Part of the rack, or all of it, can be covered with aluminium cooking foil. It reflects heat from the stove back into the wood and also provides a convenient surface for storing and drying little pieces of wood that are very valuable for fire management but would otherwise fall into the snow. I also learned that if this drying rack was made big enough and the fuel sticks on it were long enough it formed a large natural barrier to prevent accidental contact of clothing and sleeping bags with the stove. The stove’s fine metal foil components even ‘sound a warning’ if the rack is touched.
I persisted with many conventional stove designs (with a generally upward burning flame), but control of burn rate and associated messy tar deposits and very limited cooking capacity did not match up well to my dream stove. So it was back to the drawing board for a radically better design where easy fueling under wet cold conditions, burn stability and a good camp cooktop for thirsty and hungry skiers would be paramount.
Good wood burning theory. Efficient clean wood combustion takes three sequential steps.
The first involves heated pyrolysis of smoke (gases, tar and soot) from the wood.
The second step is the combustion of the gas by a moving flame (~1000 C).
The third step is the direct flameless combustion of the charcoal (~1300 C) Forensic and Scientific Services. This produces intense heat at the site of combustion, much like a blacksmith forge.
In good stove designs, enough of the heat from the flame and the charcoal burning is fed back into the incoming fuel to sustain the pyrolysis and the flame. This can be done with direct radiant heating, conduction of heat through metal stove parts and pre-heating of the air supply. Furthermore, the pyrolitic smoke flame should pass through/over the hot charcoal bed to raise the temperature of combustion and help the combustion of the less combustible smoke components such as tar and soot. As the flame and hot reaction gases flow toward the flue pipe, they should be turbulently mixed with air and slowed down in a large heat exchanger void to complete combustion at high temperature and the transfer of the heat to the stove body.
In an ideal burn, these processes should be in balance with steady heating of wood for, pyrolitic production of wood gas, combustion of gas and maintenance of a small residual hot coal bed to aid complete combustion. These issues are described in detail by Aprovecho Research Centre in a PDF.Many of these burning conditions can be achieved in invertedwood gas burner stoves such as the Stickman Stoves that are portable devices which have no flue pipe, are limited to batch loading, and no longer work properly when ‘topped-up’ with fuel while burning. Some people will argue that such clean burning occurs in rocket stoves as described in Wikipedia. In these stoves, the fuel is usually hand fed and they do not have fully inverted burners. The fact they can shoot a flame into the air also demonstrates that the combustion is not completed in the stove. Similarly, the same argument applies to boastful tent-stovers who claim that they made a cone of fire 2 feet high at the top of their flue pipe.
Rocket mass heaters are clean and efficient burners and are very popular for DIY home heating/cooking projects Build a Rocket Mass Heater. They generally use small inverted self-regulating pyrolysis/burning chamber without the self-feeding of fuel as described in exemplary stove designs from the Aprovecho Research Centre. They require a large (oil drum size) and heavy insulated chamber for completion of the combustion at high temperatures and large surface area and volume for heat exchange. Regardless of the theory, these stoves can burn wood completely and cleanly at very high temperatures and in a steady manner. A miniature one of these stoves would be welcome in a backpackers tent, but unfortunately not in their backpack. However, I have used these designs as inspiration for my developments, but I just made my stoves very small and disproportionately reduce the mass. “ …I just made a rocket mass heater without significant mass…”
Considerations of heat sources for comfort. In a small tent do you want a steady source of heat for the air in the tent or an instant source of radiant heat for your body and wet equipment? Probably you want both. Warmed air will rapidly rise to the top of the tent and be constantly cooled by the tent canopy and by necessary tent ventilation. I find that the direct radiant heat from the surface of the stove gives the most comfort. “… that is why we back our bums up to a fireplace…in an otherwise warm room…and friends who share my tent on mountain trips agree with this choice and gleefully pop more sticks down the fuel tube….”. For this reason, my stoves would be designed to have a small surface area, prioritized for cooking, giving off intense radiant heat, rather than having a big surface acting as an air heater.
In this regard, I consider stainless steel and titanium are both superior to mild steel they reach higher surface temperatures on account of their poor thermal conductivitiy.
It is timely to mention that, with such intense heat, any metal that contains such a fierce burn (titanium or stainless steel with even with vitreous enamel coating will be oxidized. Consequently, some form of refractory protection such as DIY refractories will be required if the stove is to have a long service life.
The evidence for this oxidation is in some of the eroded components within the ‘barrow load of not quite right stove above’ “….yes fire combined with oxygen can absolutely eat titanium or stainless steel, much like aluminium, once the protective oxide film is broken…”.
Part 2 will describe my best stove so far that builds upon these ideas and the achievement of others. While probably understating the true number of not-quite-right predecessors I will be calling this stove Micro Snow Stove Mk15.
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.
