How to design and make a Rocket Stove Griddle
Friends! We are going to be using these gas
bottles to make a rocket stove barbecue. This is where the idea for this rocket stove
griddle barbecue came from. It was making a rocket stove from bean cans at a camp out.
That was really great but I wanted something a bit more epic in scale. I already had the
experience of making a rocket stove heater for the workshop which I talk all about in
this video and assess it five years into it’s life.
If you are interested in the principals and design of rocket stoves do check that out
in the link below (video description). That’s the background, but in this video we
are going to be making a rocket stove bbq griddle from this gas bottle. A great way
to mark up any cylindrical object for cutting is to use a flexible, flat, square sheet.
Line up the edges when you wrap it around and your’e all set.
That way I can slide it to where I want it. Which is… about there.
A bit of the old, black on black… So I haven’t seen any other rocket stoves
on-line whether cooking or heating that use quite such a large diameter tube for the main
body of the object, so this will be a bit of an experiment and I’m not sure if it’s
going to work but I am quite hopeful. Ok time for some cutting.
So, if you know a better way of cutting these old gas bottles I’m all ears, please do leave
a comment below. I am using 1mm thick slitting discs on the angle grinder. Obviously it is
important to make sure the gas bottle is empty of any gas before you start! Especially if
that gas was flammable. In this case it was just CO2 so it’s a little bit less crucial
as long as there is no pressure there I’m OK.
So the material making up the gas cylinders I have cut up in the past, the propane ones
I used to make the workshop stove, were all 3mm thick. This is actually 6mm thick steel.
It’s really quite impressive stuff. As far as I can tell from the spark pattern, and
it is normally just a guestimate, it IS mild steel.
You can see here how quickly the discs get used up., So I have read that the ideal size
for the fee chamber is approximately 2.5 times shorter than the main riser on rocket stoves.
So that’s the kind of dimensions I am going for.
So it turns out , this is quite a complicated cut – to get this pipe to line up exactly
with this one, we need to cut what’s called a ‘fish mouth’ or a ‘saddle cut’. And to do
that I have printed off from one of there on-line templates just a wraparound thing.
I was really surprised at how ell these on-line template things work, you just enter the diameter
of the pipe, the angle it’s meeting the other pipe, and so on, and it prints you out a nice
template thing. It makes it so simple: I’m just marking round,
using a Stanley knife to cut away, and then the mighty angle grinder to get stuck in there…
So with all this cutting I thought I’d experiment with this alligator saw thing and a metal
cutting blade, and it was woefully disappointing and didn’t really get anywhere.
With the gas bottles in the past I have had quite good luck using my jigsaw, which seems
kinda similar to the alligator saw, but that was no good. Now friends, when you watch these videos I
hope you appreciate how long it takes me to wiggle-jiggle things around and the thinking
time involved, and just how much gets cut out of these videos. In this case this was
really impressive! It was just straight out of the gate, the saddle worked perfectly.
So much so I actually just felt like I had to sit and think for a minute anyway, Let
me know in the comments, do you want to hear more musing or do you want more action. Ok this is good. So we are basically there
with this. So what I am going to do now is reach through there and mark the hole I need
to make in this one. There is definitely a technique to doing these
plunge holes with an angle grinder and making the edges smooth. And I think I have gotten
a little bit better but I’m not sure what the secret is really other than practice.
Tilting the blades slightly away from the cut will tend to round the edge or cut more
of a curve, but there is only so much you can do that really in 6mm thick steel. That’s
especially true when you are using thin 1mm slitting discs. I could have used another
one of those cool on-line templates to make the hole in the riser that this is fitting
to, but is seemed prudent just to use the shape that I had and draw round that. Which
is what I did. Of course, the edge of all this hole wants to be nice and smooth so that
sticks and whatever you feed in there is not going to snag up.
The gravity feeding element of these rocket stoves is one of the main benefits of them,
so that’s quite important. Quite a rusty bit! Rusting away in my hand… The cool thing about the power hacksaw is
that it means you can get on with something else while it’s cutting. In this case this
is some “freehand straight-line” cutting with the angle grinder. So this hole is quite multi-purpose, it provides
a good amount of air for combustion. It’s also where I start the fire from in the rocket
stove, and it is also where you can clean the ashes out.
My initial measuring-stroke-guessing wasn’t quite what it should be so I will definitely
need to cut this rusty bad boy down a bit so that we can reach in and the propane torch
will fit in there to light it up. My feeling is that a good blast from a MAPP gas or propane
plumbing style torch is THE way to light these rocket stoves because it gets to clean burning
temperatures much quicker which means you are not polluting with partly combusted gasses
and it’s a much more efficient burn. It’s not ideal but… a little bit of work
on that. A good number of these discs got small. Lets have an interlude to go through a small
bit of theory with the design of the rocket stove. So ideally you want the air flow of
all the combustion gasses to come in to be about equal to the flow of exhaust gasses
and that keeps a really efficient burn. So this is a 7 inch tube. (that’s a diameter
symbol). That’s approximately 38.5 inches squared
of area available for air to rush through. This one is the same – this is going to be
the gasses coming in to the combustion area here. The exhaust gasses going out here. We
want to unify this. So you might think yeah great same amount of air going is as going
out equals good. But this area is the fuel magazine, so when that’s loaded with lots
of sticks that available space for air to rush through is much reduced.
Lets say that approximately a quarter, or something like that. So if we say that’s 10
inches squared available there, we ant to come as close as possible to 38.5 inches of
available air for combustion to occur. Which is where this comes in. It’s coming here – it’s
at a slightly offset angle to try and create a vortex in the burn chamber which makes for
a more efficient burn. Anyway this is a four inch squared tube which means it’s 16 inch
squared available area, a little but less when you take into account the wall thickness,
but lets just say 16 for now. Which gives us 26, umm, so we still want to find quite
a bit of extra air coming in! Which is where this thing comes in – it is going to pre-heat
air, and then we will have, hopefully some air at the back as well. Right! Enough with the chit chat. Lets get
welding! This is how the table converts to welding table. Normally I’ve got that wooden
ply sheet on for woodworking assembly, and other misc. tasks. So I have spent a long
time wizzing off all the paint with a flap disc on the angle grinder. Like an inch round
all these welds before tacking it up. For these welds I am using the highest setting
on my MIG welder. Along with a high wire feet, and that’s to try and get really good penetration
and weld strength, It is easy to get welds that are pretty on
thick material using lower settings but they can be very superficial. Not melting the base
material enough to get a proper join. In fact for 6mm thick steel you do actually need quite
a powerful welder to really get a good joint. Following the tack welded assembly I am using
a fairly standard seam welding method where I am kinda going an inch at a time That helps
prevent excessive warpage, but also in this case, it stops the paint from melting off
and filling your life with fumes. I was actually a little bit weary of this
internal weld I would have to do, but it actually turned out really nicely – I was a little
bit surprised. Now looking down the tube – not really any
light coming in. and then external inspection: it all looks great.
It’s been a little bit tricky to get this, clamped on how I want it. So here we go. The idea is the air is going
to come in along here, round the outside getting nicely preheated against this edge, before
coming in here at a nice tangent angle. Well that was epic and ridiculous, don’t do
it that way! It’s just this snagging here. That’s the only
problem. It’s just so close up the top here. You know what I am doing with this barrel
over the riser anyway right? Well this method of getting it on, I really
do like. It makes a really tight joint and without having to weld the really thin material
of the barrel onto the super thick material of the riser, which would be very difficult.
The slight miscalculation down here where I am having to adjust it was, err, unfortunate,
because it is very much a one way process, this thing. It slides in one direction, but
if you try and shift the barrel in the other direction it totally grips on and is incredibly
strong, so you can actually lift the whole thing now using the handles on the barrel.
It is also a very tight fitting seal you dont need to weld with and doesn’t need to be super
super accurate when you are cutting it. I am now using some more of this channel material
to fabricate another pre-heated air inlet, this time it’s going to be very much for secondary
combustion. So it is going to be higher up the flue. The idea with this is that even
if all the oxygen is been used in the primary burn down in the main burn chamber there will
still be some supplied so the flame will continue on up. It’s taking shape. It’s become quite heavy! So I calculated how many 8mm holes I would
need to drill in the steel to equal the amount of air coming through that channel. It ended
up being way too many, so the holes ended up being the beginning and end of some slots
that I desperately tried to cut at a really like steep tangential angle. Again, vortex
forming! And then before putting that channel on I
think we can improve the pre-heating somewhat by getting some fins welded to the body there.
That’s just going to increase the contact the air has with the surface of the hot metal.
I continued with the upcycling theme and used odd bits and bobs, so it looked pretty awful,
but it’s all behind the air channel anyway. So it is really nice on experimental things
like this where your’e not 100% sure and it’s prototyping, to control the airflow is really
quite nice. Right now I’m just welding on some scrap steel,
just to stabilise the base a little bit. Ooop, and there goes the trip switch, which
does happen occasionally when I am welding at maximum amps.
Come on, it stands up, lets give this baby a try! Started going now. Back here we can see the
paint’s bubbling off. That’s it! That is the power unit!
Friend, if you made it this far I salute you ! There was some geeky details in there and
like me you are obviously rocket stove interested, so lets do connect in the comments below.
To turn this into the rocket stove griddle / BBQ that it just want’s to be you are going
to have to join me for the part two video, where I take some random household items and
upcycle them into a preposterously efficient barbeque griddle. Apart from that, check out
my adventures in stove top fan upcycling. Or my five year review of the experimental
rocket stove style workshop heater.