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.
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
Ptarmigan
