Part 1: A Micro Wood Burning Stove For Alpine Tents – Dream or Virtual Reality?
This article is in four parts:
- Part 1: A Micro Wood Burning Stove For Alpine Tents – Dream or Virtual Reality?
- Part 2: Micro Snow Stove Mk 15 For Alpine Camping
- Part 3: Using Micro Snow Stove Mk 15
- Part 4: Getting The Best Out of a Micro Snow Stove Mk 15 – The Dream Becomes a Reality
Cold weather, snow, small tent – and a hot wood stove
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 your 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 3 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 it is warming you and your little tent with the power of a 890 watt radiator, there’s a gentle fire-light glow flickering around the tent, and your gloves and jackets are hanging up to dry. What’s more, a big hot soup or coffee for two will be ready to drink in 15 minutes time, and more water is coming 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 – 600 g weight penalty that can be discounted by 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 camp like this.
A Brief History of Micro Stoves for Tents
Oh what would I give for a nice warm tent stove tonight
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 once handed me a page ripped out of a Reader’s Digest article. It showed an orange, conical Reindeer Herders tent at sunset with a flue pipe protruding through the conical top. He 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.
North America seems to be at the centre of light-weight tent stove development and this is not surprising when you consider the temperatures that winter walkers an skiers must live and play in. Many portable stoves are available from companies such as Four Dog Stoves. However, at 8 lb these are not what one might call ‘ultralight’. On the other hand, one of the latest stoves by Titanium Goat is light (737 g) and seems, by my standards, to have reached the status of ultralight.
Titanium Goat WiFi stove
This elegantly designed stove is made of titanium. It appears to have a good flat and stable cook top but it is considerably larger than what I had in mind for my 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 feature with a clean burn appears to remain 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 estimate of the total join length is 1,600 mm). I can imagine how the rigors 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 exemplar 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 cook top and the stove assembly. For both stoves I would also raise the issue of how to support a hot stove on a deep snow surface. Snow melts.
My Stove Development Journey – Lessons from Failures
My goal was to design and build an utralight tent stove that gets a good balance over the following: light weight, packability, mechanical strength, chemical durability, cost, convenience, fuel efficiency, plus 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 had to be safe – in a tent. 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 process I also set a somewhat difficult goal of having all the stove components stored inside the stove body and to have absolutely minimal assembly of the stove when deploying it (with cold hands). Along the way I discovered that I also wanted my stove to be very small and compact so that it would produce intense radiant heat rather than just being an air heater.
A wheelbarrow full of not-quite-right stoves: success has many wannabe fathers and failure is a bit of an illegitimate offspring, but fortunately we learn most when we fail.
In the photo above we have the following
- Stanley thermos flask stove
- double dog food bowl stove
- Thermos stove with cook top
- inverted SS can stove with boiler cup insert
- toffee tin stoves (3) with pole mounts
- modular stacked tuna tin stove
- assorted (3) biscuit tin stoves
- micro bean can stove
- bean can boiler pot with flue pipe passing through the pot
- flat cake tin stove with cook top
- slender vertical pipe burner stove
- modular secondary air injector
- box stove with cook top and side burner attachment
- various (3) alternative side burner attachments
- box stove with boiler pot hole with cover
- box stove with side burner port and flue pipe port on end to increase usable area of cook top
- well used (2) box stoves
- stronger Bento Box stove with rounded pack friendly shape
- box stove showing partly opened sliding access door
- assorted stainless steel and titanium foil fittings for micro stoves (everything to the right of r and s)
Well if you look at my barrow load 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.
The Start of Things
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 at the time!) as you can see from the photo where the flue pipe doubled as the centre pole of my tent.
Stanley Flask stove, “a thing of beauty … but don’t try this in the bush”
The Stanley stove itself was pack-friendly 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. It 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, as the tent slowly sank.
It had another fault in that the burn was very unstable. It would burn too strongly and make much smoke, and then it would go weak before it would go strongly again. I added a secondary adjustable air port high up on the flask to both ‘burn excess smoke’ and to 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 Victorian 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.
The Next Stage
After this lesson I only used roll-up stainless steel or titanium flue pipes, and I pitched my tent with a separate tent pole (or no pole at all, which is my favorite setup, but that is another story). This meant that the flue pipe could be very light, long and compact as it did not need to be strong.
Toffee Tin Stove mounted on a wood pole stuck in the ground
This change was very significant: it allowed me to mount the stove on a single wood pole to keep it up off the ground or snow. That is, the pole was only required to support the stove, the cooking pot and the wood drying rack (to be described later), and not the tent. With this new design a lovely area became available for a suspended wood drying and 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 other wise fall into the snow.
I also learned that if this drying rack was made big enough and the 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 for a while with many conventional stove designs (with generally upward burning flames), but the poor 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 cook top for thirsty and hungry skiers would be paramount.
Wood Burning Theory and Clean Efficient Combustion
Efficient clean wood combustion takes three sequential steps. The first involves the 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 flame-less combustion of the charcoal at around ~1300 C, (TC Forensic and Scientific Services which produces intense heat at the site of combustion, much like the coals in a blacksmith’s forge.
In a good stove design enough of the heat from the flame and the charcoal burning is fed back into the incoming fuel to sustain the pyrolysis (gasification) and the flame. This can be done variously with direct radiant heating, conduction of heat through metal stove parts and pre-heating of the air supply. Furthermore, the pyrolitic smoke should pass through/over the hot charcoal bed as it burns to raise the temperature of combustion and help with 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 and the transfer of the heat to the stove body.
In an ideal design these processes would be in balance with a steady heating of wood for pyrolitic production of wood gas, combustion of gas and maintenance of a hot coal bed to aid complete combustion. These issues are described in detail by Aprovecho Research Centre.
Many of these burning conditions can be achieved in inverted gas burner stoves such as the Stickman Stove. These 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. Nice, but not what I wanted.
Some people will claim that such clean burning occurs in rocket stoves as describe in Wikipedia. In these stoves the fuel is usually hand fed and they do not have fully inverted burners. I disagree with this claim as such stoves can shoot a flame into the air: this demonstrates that the combustion is not completed in the stove. The same criticism 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. See for instance the article at Instructables
Typical rocket mass heater, illustration by www.richsoil.com
They generally use a small inverted self regulating pyrolysis/burning chamber without the self-feeding of fuel as described in exemplar stove designs from the Aprovecho Research Centre cited above. 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 might be welcome in a walker’s small tent, but probably not in their backpack.
However, I have used these designs as an inspiration for my developments. I just made my stoves very small and disproportionately reduced the mass. ‘I just made a rocket mass heater without significant mass’.
Heat Sources for Comfort in a Small Tent
You are in a small tent in the snow. Do you want a slow source of heat for the air in the tent or an instant source of radiant heat for your body and wet equipment? Probably you will want both. You see, while warm air is nice, it will rapidly rise to the top of the tent and be constantly cooled by the tent canopy and by the very 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 fire place 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 have been designed to have a small surface area, prioritized for cooking and giving off intense radiant heat, rather than having a big surface area acting as an air heater.
It is timely to mention that, with such intense heat, any metal containing such a fierce burn (such as titanium or stainless steel with or without vitreous enamel coating) will be oxidized, so some form of refractory protection (a possible subject for another article) will be required if the stove is to have a long service life. The evidence for this statement is in some of the eroded components within the ‘barrow load of not quite right stoves’ shown above. Yes, fire combined with oxygen can absolutely eat titanium or stainless steel, much like aluminium, once the protective oxide film is broken.
The next Part 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’.
Bio for Tim Clark
Tim chasing freshies in Japan
Tim is a retired research scientist who just can’t stop tinkering with technology, and is a serial inventor. He is not a pyromaniac (he claims), but he loves fire. His house has 4.2 kw of grid-connected solar panels, a 30 evacuated-tube off-grid hot water service and a 2.8 L solar kettle on his house and another on his yacht.
He has a passion for wild places, back country/telemark skiing, bushwalking, camping, sailing and diving. He considers that as a concession to his ‘mature’ age he has earned the privilege of warmth and unlimited hot cocoa in the wilderness.