The Squeezebox Files

Captain Paranoia lifts the foil lid on his ingenious self-made lightweight cooking system.


Posted: 27 October 2008
by Captain Paranoia

OM regulars may be familiar with Kevin Beeden or, more likely, his online alter ego, one Captain Paranoia. Well the Captain's been busy during those long summer evenings, designing and building his own lightweight cooking system, The Squeezebox Stove, and here it is. Kevin takes up the story...

[note: stoves are potentially hazardous, you follow Kevin's suggestions at your own risk]

Yes! It's here at last! the Squeezebox Stove is revealed!

What on Earth is the Squeezebox Stove?

The Squeezebox Stove is a lightweight, compact replacement for a Trangia stove base and windshield, weighing only 37g. It is intended to be used with a Red Bull meths burner replacing the Trangia burner.

Having dropped a number of hints about this over the past few months, and contemplated a patent application, I've decided not to bother, but to go public instead. A course of action exactly as I predicted months ago...

So, How Did it Start?

The idea started off as a way of making a self-supporting windshield, by adding folds to a piece of foil. This makes the sides of the windshield more stable, and stops them falling into the stove.

Now, the problem with this folding is that it means the windshield can no longer be stored flat, say, around a fuel bottle. However, it can be rolled up on itself, interlocking the folds, and stored inside the pan, provided it is cut low enough.

There are a couple of problems; we still need a pan support, and, because the windshield is stored inside the pan, its height must be less than the pan height, which means that the sides of the pan can't be protected. If we could add protection to the side of the pan, we could trap the hot gases against the pan, improving the heat transfer, and thus increasing the efficiency of the stove.

A Bright Spark

Then, lying in bed one night pondering these two problems, I thought of a way of modifying the folded windshield to make a pan support and burner windshield, by simply adding a second, half-width fold in the upper section of the windshield. This provides a step that the pan can rest on. I picked up my bedside ideas notepad and scribbled the following drawing:

It looked like this would solve two problems; to provide a pan support, and a partial side windshield. However, thinking about it a bit more the next morning, I realised that I needed to allow for the height of the burner, and for the gap between burner and pan. This meant that I couldn't make the second fold halfway up the windshield as envisaged in the sketch. 

An MSR Titan kettle has an internal depth of only 80mm, and allowing 32mm for a burner and 24mm for the burner gap, this left only 24mm for the 'upper windshield'. However, I went ahead, and built a partial paper prototype to test the design, and then a thick paper, full-size prototype to check that the idea really worked.



 

Note how the folds interlock to allow the windshield to fit inside the pan, with a big enough gap in the centre to accommodate a red bull burner. And that the pan has a good 700ml of water in it, and the paper prototype isn't at all bothered by this piffling weight.

When I got home, I dug out a flapjack tray, flattened it out, and made a partial prototype to see how robust it was. It seemed remarkably sturdy. The opposing folds proved rather tedious to make, though...

 

Another Bright Spark...

There was still the problem of how to protect the upper part of the pan, and keep hot gases close. The answer came in another scribble that night; add an upper section of folded foil that sits inside the double-folded part of the lower section, with even more folds to provide a chimney, whilst still keeping a defined separation between pan and windshield. I also came up with ways of getting air into the combustion chamber (in other words, some holes in the windshield...):

This idea was then tested in paper, and the result looked rather promising, and, to my eye anyway, strangely elegant:

 

It also looked surprisingly like my original sketch, which, given my poor artistic skills, and the fact that it was scribbled at night, in bed, is rather remarkable...

I tested one of the air intake ideas; yet another fold in the base, but this confirmed one of my worries; the reduced effective width of the folded sections made it less stable, and more prone to collapse. The handle cut-out and tabbed mating mechanisms were more successful, though.

 

What you can't see here is that the two sections pack away into the pan nicely; since they are based on the same fold dimensions, the two pieces stack snugly, and can be rolled up inside the pan, as seen in an earlier picture.

A Brief Interlude

I then got a bit side-tracked, playing with what little foil I had; the flapjack trays. I realised that I could make a pan support in the same way, using the folded foil to enclose and support a drinks can burner:

This works rather strangely; the burner starts as normal, but, as it heats up, the flame front moves from within the support, to burning at the openings, which is actually quite effective. The foil seemed remarkably unaffected by the heat, showing no signs of softening; something else I wanted to test.

I tested how much this little support would take, by putting it on my digital kitchen scales, putting a pan on top, and then pressing down on the pan. At 3600g, I stopped pressing, as this seemed enough, and I didn't want to take it to destructive testing. The pan support weighs 6g. A 600:1 weight/support ratio seems pretty good...

Anyway, back to the Squeezebox Stove...

It had already crossed my mind that the folds could be used to adjust the diameter of the stove to accommodate pans of different diameters (admittedly, they'd have to be tall enough to fit the support inside), but then I realised that, if necessary, the Squeezebox stove could be folded right down to a very compact form, possibly for delivery.

 


squeezebox stove

Foiled At Last

I finally managed to get hold of some foil that was the same thickness as the flapjack foil that seemed so suitable. It turned out to be a little softer, though, so must be a different alloy. However, it did allow me to build a full foil version at last:

Note that, in my enthusiasm to test the folding aspect of the design, I still hadn't punched the air holes in the base, so I had to test it on the base of a Trangia to allow adequate airflow. So I'd just better stress: the Trangia base isn't needed for a Squeezebox Stove with punched airholes. You can also see other experiments in the background; a Caldera Clone, a Thermawrap cosy, and a whole pile of camping pans. This is a pretty typical shot of my kitchen... The discolouration of the foil is nothing to do with the action of the stove; that's water damage on the foil as I received it.

I'd have included a shot of the Squeezebox Stove in action, but, even with the kitchen lights out, there's little evidence that the thing is working; all the photos came out rather black, with just a tiny zig-zag pattern of light at the base. This shows that the flame is contained entirely within the stove. Oh, go on, then, just for a laugh:

The entire Squeezebox Stove in this form weighed 29g, compared with 315g for a Trangia 27 base and windshield. Combined with a Red Bull burner, weighing 8g (vs 88g for the Trangia burner), this made a very lightweight Trangia replacement.

Playing with these prototypes showed that it's possible to assemble the Squeezebox Stove, put a pan in place, and then pick up the entire assembly and move it about; you simply hold the base with your fingers under the pan, and put your thumbs on top of the pan. This allows you to light the burner, and then lift the Squeezebox and pan and place it over the burner.

I tried moving the stove whilst the burner was running, but burnt my fingers... The sidewalls get too hot to touch, but cool within about ten seconds after the burner goes out. The temperature of exhaust gases coming out at the top was about 200C.

Two Steps Forward, One Step Back

Initial burn tests looked promising, but I noticed it was a bit smelly, and a bit sooty. Analysis of this problem made me realise that there was an issue with the cross-sectional area of the 'chimney'; because of the folds in the base, and the fact that the pan sits directly on each of the inner folds, the area available for gas flow is rather too small. You can see this in the plan view (yes you can):

So, although looking very pretty, being lightweight and compact, it didn't burn very well. Which, for a stove, isn't so good...

(For the mathematically inclined, calculating the angles of the inner and outer vertices of the stove to produce the above drawing is an interesting problem, and I eventually abandoned an analytical approach, and used Excel Solver to find a numerical solution.)

A Return to the Drawing Board

This airflow problem stumped me for a while. I liked the idea of the close fit of the sidewall of the Squeezebox Stove to the pan, as this kept the hot gases close, and ensured that the assembly remained stable, with the upper windshield held firmly in place against the pan, and at a fixed offset. How could I maintain this close fit, and yet improve the cross-section area?

In the end, I decided that the only way to increase the chimney area was to increase the diameter of the stove, and stop the upper folds of the base touching the pan, sacrificing some stability. I also lowered the position of the cut of every other upper fold, so that they don't touch the base of the pan, and thus allow gas to pass. The increased area can be seen in this plan view:

The inner circle is the pan diameter, and the outer circle is the increased support diameter, showing the air gap, and the fact that the upper folds no longer touch the pan.

The increased diameter of the stove means that the pan might slop about within this diameter, and potentially fall off the inner fold supports. To solve this, I increased the size of the fold sections and added one more 'point' (from 11 to 12), which reduced the diameter of the inner vertex ring, and added an angled cut when making the pan support folds, to encourage the pan to sit in the centre. Since the number of pan supports is now reduced, I folded the cut section down to strengthen the edge supporting the pan. I did the same with the angled cut, and will also use this edge folding to reinforce the top and bottom edges.

Since the stove sides are no longer in contact with the pan, there's no need for the complex folding of the upper section, as this was only done to provide a continuous chimney opening with a fixed offset.

The resulting stove burns a lot cleaner, and has a gas flow area comparable with the air intake area of a Trangia 27 base, but it doesn't look as pretty as the original design, and is heavier, at 37g. The upper section is much easier to make, though, so that's one advantage.

As you can see, I was again in too much of a rush to punch holes... However, if we punch four holes in each folded section, using a 6.35mm (1/4") punch, we end up with a total of 96 holes, giving a total air intake area of about 3000mm2, which is about the same as provided by the Trangia 27 base unit. The picture is also taken almost as soon as I'd finished the rough folding operation, so the folds are still a bit ill-defined; I've since sharpened these folds up to make them stronger and neater.

The Good Points

The advantages the Squeezebox Stove offers over conventional pan supports are as follows:

  • Combines pan support and windshield.
  • Lightweight.
  • Compact, fitting inside the pan it supports.
  • Can be picked up and moved around with the pan in place.
  • Can be made with simple tools (ruler, straight edge & craft knife) and patience.

The Not So Good Points...

The possible disadvantages are:

  • Foil is thin, so vulnerable to damage. Alternative materials might address this (e.g. Ti foil, or a different grade of aluminium alloy). It's also mitigated by being protected by being stored inside the pan.
  • Being lightweight, it might blow away, unlike the heavier Trangia it replaces. However, since the entire assembly with pan can be lifted as one, the pan can be used to stabilise the stove.
  • Takes a bit of practice to mate the lower and upper sections.
  • Whilst there's some leeway in the diameter, it's best targetted at a specific pan, so you'd need one for each pan design you use. For lightweight backpacking, this probably isn't an issue, as normal people only have one pan, and not a vast collection...
  • It works best with handle-free, or shallow handle pans, as this allows the upper section to be locked.
  • It works best with a sharp-radiused pan, as that maximises the supported area on the pan. Rounded-bottomed pans may slip off the supports, but do provide more gas flow area.

A Complete, Compact Cooking System

The Squeezebox Stove is best used as part of a nesting cooking system:

  • Thermawrap cosy
  • Plastic cosy cooking pot
  • Pan
  • Squeezebox Stove
  • red bull burner
  • meths measure

To protect the pan from contamination by any dirt stuck on the stove, the pan can be lined with a J-cloth, which also comes in handy as a washcloth and insulating pan-grab. A second J-cloth can be used to protect the plastic pot from soot or scratches from the pan.

Finally, a Real-Life Test

A recent trip to Dartmoor provided the opportunity to field test the Squeezebox Stove. I punched some holes in the base, and packed it all away, taking no reserve stove. It passed the test with flying colours, and even survived a little bending when I clouted it with a stray foot coming back from the pub one night... It worked perfectly well in the porch, heating the fly to no more than body heat.

Over To You

What do you think? Do you fancy having a go at making one? A PostScript or PDF template can be provided if you wish.

Do you have any suggestions, comments, or criticisms? Glowing praise and adulation would be the preferred option...

I have some ideas on how the thing might be mass produced using a roller method, but I'd welcome suggestions from anyone with experience in the packaging industry who is familiar with complex folding operations.

[note: stoves are potentially hazardous, you follow Kevin's suggestions at your own risk]

© 2008 Kevin Beeden aka captain paranoia


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Discuss this story

Bravo! A Tour de Force. Seriously. I you make them, I will buy. If you don't make them, I will try.

Posted: 28/10/2008 at 11:56

Glad you like it.  I'l try to put up some pictures of the latest version, with improvements such as folded edges, etc.  All of which make it more robust.  Oh, and with holes punched in the right place before I folded it up...

If I could make them, I'd sell them.  But it's the manufacture that's the problem...  All those folds are rather tedious to form by hand, but the end result is certainly worth it.

Unlike the Caldera Clone, I can't provide an automatic design tool, as the design requires a numerical solution, which I can't do in PostScript.


Posted: 28/10/2008 at 12:47

Include me to the list with potential buyers. This thing seems to offer everything you need from a windscreen: windprotection, heat exchanger function, easily storable, it's stable, ... And while you intended tit for a meth stove, I see no reason why it couldn't be adapted to any other type of stove.

I am pretty clumsy but if you're not able to massproduce them, I want to try it myself.


Posted: 28/10/2008 at 13:11

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