Upcycled Rocket Stove Griddle BBQ + Pizza Oven Attachment

Welcome Friends! In this Instructable I'll be showing you how to make a rocket stove grill/griddle, cheaply and easily(ish) from upcycled materials. This makes a great project for anyone learning to weld and work metal, so I'll give plenty of little technical tips as we go along.

Ever been at a classic barbecue and no matter where you seem to stand the smoke follows you, making your eyes water and your clothes smell? It might seem 'natural' to be breathing smoke, but it's not actually that healthy. There are few more things I don't love about barbecues: They take ages to get to the right temperature, they require I BUY or make charcoal (and how many of use actually make the charcoal?) and they leave loads of ash. The rocket griddle is my solution to most of those problems.

What is a 'rocket stove griddle' and why should I care?

Rocket stoves rock! They burn up wood that might otherwise be wasted, like twigs and smaller branches.They burn clean (no smoke), produce less ash, and use less fuel than traditional wood fires. They are used for space heating (rocket mass heaters), pot cooking, hot water heating, and camping (small portable ones to make tea).

They are not so often used for barbecuing or griddle cooking, hence I'm writing this instructable.

The rocket griddle we make here is designed to cook lots of food fast! In my first real test I cooked a feast for 20 people in about 40 minutes (from lighting cold). You could scale the design to suit. It would make a really cool addition to any outdoor kitchen, or as something you can take to events. It looks funky and exciting and gets lots of interest...

If you find this instructable interesting please consider voting for it in the metalworking contest :)

As always dear reader, I want to invigorate your inclination towards 'material based design'. That is, see what you have access to for free or cheap and roll with that - if you can reuse or upcycle along the way, even better! For that reason, treat this list as loose guidance and use it for inspiration rather than a rigid instruction manual.

The Burn Unit Materials:

I used one old gas bottle 7" (175mm) diameter and 67" (170cm) long (See the warning on gas bottle use in step 3 if you go this route). Just as good would be some some scrap pipe or box section steel. Box section material is much easier to make into tidy angled joints - but is harder to come across if you want to use upcycled-free materials.

Some steel channel ~4" wide and 1 1/2" deep. This was scrap that came from a posh pallet.

A scrap barrel.

Some steel bar or box off-cuts for the legs.

An old bolt and some mild steel pipe or bar for the door hinge.

A 4" square of flat plate (for the ash pipe door)

Some garden variety perlite for insulation - https://goo.gl/5Jox3c

The Griddle Materials:

An old double radiator, ask any scrappy. The radiator doesn't need to hold water ;)

Some scraps of angle iron for the sides.

Whatever you find to make the flue stabilisers - I used the carry handle of an old gas bottle and a few angle iron off-cuts.

The two flue pipes - these are 1.5m sections of 5" internal diameter double wall insulated flue. They are a bit bent up but work great - they came from our neighbours house, after a huge tree fell on it (video link of clearing and milling up that big tree). You can snag them second hand on e-bay if you're lucky but they aren't super expensive new, try- https://goo.gl/7tvMdg.

Two mild steel pipes to support the griddle.

The Side Table

A plank of wood, and some screw in supports.

Tools

There are so many different ways to achieve the same result that I won't list all the tools. In general I used this angle grinder and a mig welder.

Consumables you would do well to stock up on before you start:

4" angle grinder metal slitting discs (you probably want at least 10) - I find the thin slitting discs a lot quicker to cut with, as they are having to remove less metal. They are however more vulnerable to sideways loading.

40 grit flap disk for angle grinder - https://goo.gl/RxbPdV

Essential safety gear:

Protect your face, eyes, ears, hands and lungs! Here's how I do it:

I'm using a combined face shield and dust mask, it's a complete game changer! I originally got it for all kinds of other tasks like chainsaw milling, painting, epoxy use, and sanding. In the past, even when I wore a separate dust mask, I would get dry eyes - no more! It's the 3M 6800 full face equivalent which means it's a cheaper import (only about $30 here: https://goo.gl/kVGmyk ) but I use it with the genuine 3M Multi Gas/Vapor and dust filters. (ebay link https://goo.gl/YKyJRp ) which are great for stripping paint, whatever method you are using.

My radio ear defenders I won in an Instructables contest- I was sceptical but they absolutely kick ass, and I now couldn't live without them, I bought even some for my dad for when he does the grass cutting- https://goo.gl/kVxMfn

Welding gloves - I use TIG ones even though I am MIG welding and grinding - they are soo much more comfy and flexible - https://goo.gl/dr7cbZ

The rocket stove principal has been around for a very long time. It usually involves a 'J' or 'L' shaped feed and combustion chamber, and features an insulated flue/riser that strongly draws the combustion gasses up, drawing fresh combustion air in behind it. When they work well the gasses are drawn in strongly enough that you can hear them roaring - hence the 'ROCKET stove'. Good ones give you very hot, efficient, complete combustion, and so don't make the kind of smoke that characterises a traditional wood burning fire.

If you check out the diagrams you get a good idea of how they work. The bean-can one I made while camping, is a very basic example of a 'J' shaped rocket stove (all be it one with a squashed burn tunnel) - the sticks are loaded in vertically, and burn sideways into the main riser. It's very crude, and doesn't have any insulation... In contrast the last pic is a diagram of my experimental workshop heater. It uses a similar principle but is lined with insulated fire bricks and refractory, for a much hotter burn. It has a separate heat exchanger to keep the burn hot, and an enclosed fuel magazine loading area. You can watch a video about this shop heater here.

The second and third pics show the difference between J and L shaped Rocket stoves, and the fourth is what I am going to call an 'LV' style one which is what I'm making (can you work out why LV?).

DANGER: Never cut into a pressure vessel (gas bottle) of unknown origin without taking precautions or you may die in a horrible fireball. If you don't know the back story, always be open to the possibility that someone has (miss)appropriated the vessel and filled it with a flammable gas. First open the valve fully and empty the contents (obviously if it is flammable, make sure you do this away from any source of sparks/ignition). If there is much of anything in it, it should really be used, not wasted! After that unscrew the brass valve from the bottle and if there was any chance it contained anything flammable fill it with water to displace any remaining gas. If in any doubt, don't use it. Note that most propane valves are left hand threaded.

A good general rule of thumb with this style of rocket stove is to have the riser length at least 2.5 times the length of the feed tube. With is in mind we can make the most of our scrap gas bottle and size it so the griddle comes to a comfortable working height.

Marking and cutting tip. To mark cylinders of all kinds up for straight cuts, wrap a square sheet round, line up edges, use some painters tape to fix in place while you draw your mark along it. See pic notes.

I'm using a 4" angle grinder with metal slitting discs to do the cutting. It works well.

The design here calls for the feed tube to join the riser at 45 degree angle. This kind of joint in pipes is known as a 'saddle' or 'fish mouth', it's difficult to eyeball and guesstimate this in such large diameter pipe.

Use one of the on-line calculators and print out a paper template. I used this one http://cq.cx/tubejoin.pl but I think there are a number of good ones. Don't be intimidated by this if you haven't done it before. You simply fill in the numbers for your pipe diameter and the angle you want them to join (45) and it does all the hard work for you. If you're using a tube the same size I was, it will take 2 sheets of A4, just stick them together, and you're ready to wrap them round the feed tube, mark, and cut with the angle grinder. Cutting the curves can be a challenge, just go slow and avoid twisting the cutting disk during the cut (which risks catastrophic disc failure - make sure to be wearing leather gloves and face shield).

Once you have cut the saddle on the feed tube, you can use that to mark up the hole you need to make in the riser.

I allowed a good bit of room below the junction to allow ash to fall down and not clog the combustion chamber.

One small safety note: whenever you're making angle grinder plunge holes like this, especially ones that have non-straight edges like this, keep your face behind your face shield AND ideally off line of the spinning abrasive disc. That way if (when) the disc 'catches' in the cut and transform to shrapnel, you won't be the first thing in its flight path.

To make a good weld joint it is important to abrade back to bare metal a good inch or so all around the joint. Any paint left in the area will vaporise and contaminate the weld.

To do this I use the angle grinder with a 40 grit (course) flap disk. You can also use this to make any last tweaks to your saddle joint.

Prep is of great importance. Remove paint on both the inside and outside faces all round the joint. Hold it in place, and first tack weld it. The gas bottle I was welding was 1/4" (6mm) thick steel, which requires quite high amperage to get good penetration.

With it tack welded up, I then stitch welded it. That's where you weld an inch or so bead, skip an inch weld another 1" bead and so on, until you have gone all round. Then you can go back and fill in the gaps to make a continuous weld. With this technique you can run slightly higher amperage setting, get more penetration, and still not overheat and warp the part you're welding. The welding torch too, can have a cool-down between 'stitches', so it's not so hard on your welding kit.

As you weld round, don't be afraid to flip the work piece over so that you are mainly welding flat from above. It's far easier and always desirable if possible.

This is made from a very rusty scrap of 4" box tubing (100mm). You could use any similar sized tube, square or round. The purpose of the ash tube is multiple:

1. It gives us a controllable supply of extra oxygen for combustion.
2. It allows access to the burn chamber and the area directly below, so we can pull the ashes out.
3. It provides a very convenient place for us to light the stove with a propane torch (or matches if you're that way inclined).

Using the angle grinder we can curve the edges of the ash tube to fit the riser. Notice that I offset the ash tube - that's deliberate. It helps the in-rushing air to swirl about the circumference of the burn chamber, increasing velocity and forming a vortex. This promotes better combustion as the combustion gasses and oxygen mix better, forming more complete combustion. It also blasts bits of char off the burning wood, revealing fresh surfaces for combustion.

Once the ash tube is shaped, we can present it to the riser and mark out the hole to be made. Then it's just a case of plunge cutting with the grinder.

I explained why we want a vortex in the previous step. Lets make it more extreme! You could probably skip this step and the rocket stove would function fine, but because my calculations suggested I wanted even more primary combustion air, I did this...

Essentially it's a piece of scrap 4" channel that I cut and bent round the riser. The idea is that the air gets pre-heated as it travels round the combustion chamber and enters at an angle that will promote the VORTEX.

For the rocket stove to work well the heat riser really needs to be hot and stay hot, so that the rising hot air has some oomph! Thus it is best to insulate the riser...

For this we can use an old barrel of suitable size. If your barrel's like mine, it is useful to cut it so it fits very tightly, as I did, rather than try and weld it. Why not weld? Not just because it requires a ton more prep work, but also it is very difficult, though not impossible to weld metals of such different thickness. You tend to burn a hole in the thin metal long before the thicker one even starts melting...

Anyway the method shown, works really well, just be sure not to slide it on 'til you're ready, because if you have done it right, it's not coming back off! It slides in one direction only. For that reason it's also super important to bend the tabs on the top and bottom of the barrel in the same upwards direction.

Once your barrel is firmly in place on the riser, you can fill it with perlite using a funnel. This gives your riser a lovely insulated sheath, meaning exhaust gasses will not cool quickly and will be drawn up strongly, creating the rocket effect.

To burn away any remaining combustible gasses, it helps to have a secondary supply of air, above the main burn chamber. Technically I think this should be called 'tertiary air' in this case, as there are two other air supplies coming from the two vortex forming intakes and the primary air rushing in through the fuel magazine. Whatever you call it (and I'm calling it secondary air, because it is associated with secondary combustion), there is not really a sharp distinction between primary combustion and secondary. I think of primary combustion being the initial breaking down of the fuel, and releasing combustible gasses - a lot of which are burnt there and then. Secondary combustion happens to any remaining un-burnt gasses that are heading up away from the flame. For these gasses to burn we need lots of heat (hence the air preheating), oxygen (more air), and a spark (hopefully delivered from the tail end of the primary burn flame).

So in this step we are just adding in directional (vortex assisting) preheated air.

To achieve this I used some more scrap steel channel which runs up past the burn chamber housing, where the gas can pre-heat, before being 'injected' through angled (vortex promoting) slits. We can improve the effect of pre-heating, by increasing the surface area of metal in contact with the incoming air. To do this simply weld on heat transfer 'fins' (aka whatever little scrappy off-cuts I had lying about) to the outside of the burn chamber. They don't have to look pretty as they will be covered up...

The channel gets clamped, tacked, and then stitch welded on. Nothing special going on here, just completing the air passage for preheated secondary air.

Because the main feed tube will not always be fully loaded with fuel (sticks), the air requirements for good combustion are likely to vary... For this reason it is essential to have a way to adjust the airflow through the ash tube. If the ash tube is fully open and there isn't much fuel restricting the flow of air in the feed tube, there will be too much air! In this instance the flames can reverse and burn up the feed tube...

We can make a nice functional hinge from an 5/16" (8mm) blot and a scrap of 1/2" (12mm) round. I drill a hole in the centre of the round on the lathe (you could use a hand drill or drill press (I happened to have the 3 jaw chuck mounted so it was quickest to use the lathe).

Cut the 1/2" (12mm) round in half with a hacksaw and we are basically there...

Line everything up - a magnetic clamp is very handy! With the bolt inside weld one half of the round to the ash tube, and the other to a square of steel, that will act as a door.

After some fiddling you will have a functional door. Hooray!

At this point the structure is almost ready to try out. Lets make it a tad more stable first by welding on something to act as legs / stabilisers.

I would have liked to have slightly longer legs on this to make a wider, more stable base, but these scraps were the lengths I had so...

Time to enjoy the excitement of a test burn!!

Shove a bunch of very dry paper and small twigs in first, follow that up with with dry sticks. I fully endorse lighting it in style with a propane or MAPP gas torch (which if you do any kind of plumbing is a must have item! - I use and recommend the TS4000, I've been using that torch for well over 10 years for everything from plumbing to brazing to releasing stuck bolts and heating metal bars for bending. It self ignites, works with propane or MAPP and it's never had a hiccup).

The reasons I like lighting it this way are:

1. There's no mucking about with matches blowing out in the wind.
2. You have to have done something quite wrong to get a lighting fail.
3. Most importantly you get to complete combustion temperatures really quickly with the gas boost. That means much less time creating polluting smoke!

On the first light be ready for the smell and fumes as the paint burns off the outside of the gas bottles.

That went great, right?

Time to make the actual cooking surface - the griddle.

When I was scrounging round for metal to make a hotplate I had the thought to use an old radiator. When I was doing the calculations to see what kind of gap we wanted between the top of the burn unit and the underside of the hotplate, it worked out to be exactly the gap between a double radiator. In other words the whole thing is almost completely pre-fabricated for us :)

In designing the flow of air round the rocket griddle system, we want as little changes of pressure as possible. the area that the gasses flow through in the 7 inch rocket stove body is 38.4 square inches. All those hot combustion gasses need to flow out into the space below the hotplate without restriction. The double radiator I had had about a 1.75" (45mm) gap between the leaves. Thus the area the gasses had to move through from the 7" rocket riser to the gap between the leaves was the (circumference ≈ 22") x (the gap ≈ 1.75") = 38.5 square inches. Very close. Great, lets proceed!

Challenge one is to remove the paint. There are a number of options here. During my test burn of the rocket, I thought I would burn the paint off the radiator no problem. Not so! Turns out radiator paint is quite heat resistant (who knew?). So much so that I had the radiator sat atop the rocket stove for some time, basically glowing red hot. I poured paraffin (I don't recommend this by the way!) over the surface and lit it up - you can see this in the first pic or the video in the last step ...

I discovered later that the paint was mainly unscathed!! Remarkable.

If you are test firing the burn unit anyway, doing this probably helps loosen the paint ready for scraping. It's not THE solution I hoped it would be though.

If you watched the video, you will know that removing the paint wasn't super easy, or fun. We only really need to remove ALL the paint from the top surface - the one we will be cooking food on. I ended up using a combination of the random orbit sander, together with flap discs on the angle grinder and a wire wheel in the drill.

You can use chemical paint strippers, but I shy away from brutal chemicals by default. They probably would have saved quite a bit of dusty work, though I'm not sure.

Whatever method you use, good PPE is really important - revisit step 2 for my recommendations.

We want one hole in the middle bottom leaf of the rad, and two smaller ones in either corner of the top.

To make the bottom one mate up with the top of the burn unit and barrel well, you can cut away the outer ribs so the barrel nestles in there nicely (see pics).

So the smoke doesn't just fly out the sides, we want to make some cover strips. I use old angle iron from a bed because it is what I had, and because angle iron covers a good variety of gap sizes, if you orient it the way I did.

We don't need to provide an air tight seal here with the welding - some stitches here and there will do, and a little gap or two round the corners shouldn't matter if you have insulated flue pipes. Save yourself much prep time, welding wire and gas, and don't seam weld these parts.

The flues to be relatively easy to take on and off this beast, so that we can move it about. The flue holders simply locate the pipes over their corresponding holes.

To weld them I located the pipe dead over the hole (look with a torch from above to be sure), stuck the supports to the pipe with painters tape, and then welded them to the radiator. One side I used an old gas bottle handle, the other, 4 scraps of angle iron. Both methods worked well.

With all the paint off, the sides and flue supports on, we can try out and season the grilling surface.

At the moment the griddle is a little precarious, and requires some side supporting struts. If you notice in the second video, we have some old pole things that we hammer into the ground, level with the underside of the griddle - For our rocket stove grill that's a very secure way of doing it. Contrast that with the first test photo above where we are propping it up with a stool and half an old toilet! Sometimes you just really need to test a concept!

Seasoning the steel is very similar to how you would season cast iron, the idea is to bake on a thin layer of oil, creating a non-stick surface to cook on. For us this basically involved firing it up, and using a paper towel to wipe some vegetable oil (other oils like flaxseed work too) over the surface, wait a couple of mins and repeat, until you are getting a nice dark colour.

You don't need to overdo this, If you wipe it occasionally with paper towel as you are actually cooking things on it, the fat from the food will help season it too... In my experience, there is little to no problem with food sticking to the surface of the griddle - YAY!

To add a finishing flourish to the griddle, I wanted to make a little wooden side table to hold plates and other accoutrements. To support this 'table' (and essentially it's just a board of nice oak wood), I was going to weld a small scrap of pipe I had to the threaded blanking nuts on the radiator. I was getting ready to weld it when I realised the pipe actually had a 1/2" BSP thread on the end! It needed cleaning up with a small file, but it actually fit straight in the female thread of the radiator. Jackpot!

So I used that one side and the actual radiator valve the other. This particular valve had two 8mm holes that would perfectly fit some dowels...

After running this rough-sawn chunk of oak through the thicknesser, I cut a flat edge on one side using the tracksaw. With that established I cut it to length and ripped a 45 degree notch in the straight edge on the tablesaw. This mates with the outward facing angle iron.

After that I just added two little dowels to meet up with the holes in the radiator valve, and called it good!

As a table it is very basic, but it serves, and is easy to take on and off for moving and storage.

Unlike many of my experiments this worked really well, without much extra tinkering! You get an excellent and predictable range of heats on the surface of the griddle, very soon after lighting (especially if you light it how I recommend with one of these torches). It seems to use very little wood, and that's all sticks and scrap little bits that are easy to find for free. There is no visible smoke very shortly after lighting, so I know combustion is efficient and complete. Most importantly it cooks food for a LOT of people really well.

I can well imagine the principle being scaled up for really large events. A longer double radiator could be used with the rocket burn unit placed at one end, rather than in the middle.

Future improvements:

I intended to insulate the lower portion of the burn unit with another barrel and perlite 'skin'. This would raise the temperatures in the primary combustion chamber further, and therefore increase efficiency. The only reason I hadn't done that was that the griddle was pressed into action early for an event. It worked so well I started to think it may not be worth the bother, considering the complicated 'LV' shape the barrel needs to be fabricated around. I should either go ahead and do that or paint it with high temp paint...

Limitations

If I were making a permanent and fixed outdoor kitchen area, that's going to be used regularly, I would construct the inside of the rocket stove with refractory - not steel. This might improve the rocket effect (better insulated burn chamber) and last better, assuming it was covered from the rain. Steel will eventually burn through because of the super hot temps in the combustion chamber of a rocket stove. While this isn't a problem here, because we only use it occasionally for events (and it's 6mm thick!), if you used the rocket griddle daily it might be an issue.

This step is a bit of a mad prototype thing and I am just including it at the end for fun. It is basically an attempt to make the rocket stove grill into a pizza oven, or just a normal oven I guess...

We have had this half round freakish thing for years now, since we built the workshops straw bale, clay plastered walls. It is basically a load of clay plaster that was left over, shaped around, and embedded with, a scaffold of old wire. If your curious about the ratio of ingredients (clay, sand, fibres, etc) we used in the clay plaster mix, check out the full detailed guide we wrote here:

I had forgotten all about it, as it was stowed up in the rafters, but then at the first test of the rocket stove griddle someone at the BBQ suggested the crazy idea of cooking a pizza on it (OK it was me). It was just a frozen pizza and I put it on the less hot part of the griddle and It worked, sort of! Anyway, it got me thinking that a little enclosure would make for a reasonable oven. And it did!

The clay plaster construction means it is high in thermal mass, which means it takes a while to heat up, but once it does it radiates a nice consistent heat from all angles, into the oven space.

If you watch the video I go into details about what is good and bad about the 'oven attachment'. If you try something similar I'd be really keen to know how you get on.

Peace and happy making friends!

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