Rocket stove design and efficiency

I’d like to make a small steel rocket stove for cooking in place of a barbecue or open fire pit when I am out camping. I’ve seen so many designs and have built a few myself including a j tube. I love my j tube and it burns very clean with no smoke, however it’s not ideal for cooking as id like to get flame licking the bottom of the pan or pot to speed up the cooking process.

All that being said the two other designs I’ve seen is the L and what I would call the K with separate angles feed tube and air intake at the bottom. I’d like something self feeding so it doesn’t have to be babysat at all times.

I’ve drawn a crude picture of my options, and with regards to number 4, would this design be cleaner burning than an L or a K type? It is basically a J tube without the long horizontal burn chamber, in hopes that I can get some flames reaching outside the riser to cook with.

Also would adding a P channel to any of these setups clean up the burn possibly?

Thanks for any replies!

Just drew up with what I am thinking may work as a P channel.

Part of the J-tube design is the horizontal burn tunnel and a sharp angle into the heat riser, which creates beneficial turbulence and promotes mixing of gases for full combustion. The short version with no horizontal flow eliminates that turbulence generator and would probably be less effective at clean burning. It would require testing instruments to quantify that; visuals would probably not be precise enough.

You could make a short burn tunnel, say just one diameter long, which would be enough to start the fire rushing horizontally before it turns vertical.

A principal function of the P-channel is to make sure that there is always a certain amount of fresh air delivered to the fire even if the feed tube is crowded with wood.

Hi glen thanks for the reply!

I was thinking that possibly, even without the horizontal run, that it might at least be cleaner than the L design, and there wouldn’t be any wood/fuel directly in the flame path. In part I guess I’m assuming that a lot of the flame would be some of the volatile gases burning off. I do realize that without the horizontal burn chamber it won’t be near as clean as it could be, but maybe cleaner than the conventional L style perhaps? I’ve watched many videos of people’s L and K style stoves, and although they are neat they do seem a bit smoky, I’m assuming this is because in both these styles the wood is directly in the flame path using up most of the oxygen.

I also love the self feeding feature of the J style, the L you constantly have to push the fuel farther in as it burns, and the K style that seems popular I don’t like for two reasons, one being that the Exhaust may try to use the feed tube as a chimney, and two the feed tube on an angle doesn’t really work and the wood gets hung up from friction, and you end up constantly pushing the fuel in just like the L version.

One other reason for possibly trying to make a stove without the horizontal burn chamber is to be more compact for traveling with. At the end of the day it is just a simple cook stove so maximum burn efficiency is not a priority, but I had this idea in my head that possibly it may be a slightly more efficient/cleaner burn than the conventional cook stoves you see online, plus the added bonus of a self feed that actually works.

Any thoughts on some of those points? On my days off coming up I am planning on giving it a go, I may even end up making one of each style with similar dimensions and see a side by side comparison.

I have built two L-tube rocket fired maple syrup evaporators, both with 8" diameter chimneys, and a 6" L-tube cob oven. All burn very clean with zero smoke, though they do have the chimney draft to help with flow. I find that the fire works its way all the way back to the mouth of the L (30" long), so the "pushing sticks in" is really just adding fuel as the fire needs it. A small 3" or 4" metal L may have a very different behavior especially if uninsulated. Little stoves in general are simply going to be less likely to burn efficiently due to physics and scale factors, with a greater proportion of surface area to lose heat and less space and time for combustion to complete.

At any rate, the fire will always be among the sticks, not just beyond them, even if it can be kept to the ends of the sticks (not likely in my experience).

"a lot of the flame would be some of the volatile gases burning off."

The flame is always the volatile gases burning off. Wood doesn't burn, its volatiles vaporize when hot enough and the vapors burn. You need the fire in among the sticks or they won't get hot enough to vaporize. Having the wood in the airflow path contributes to good combustion, unless you are trying to make charcoal.

For a small metal cookstove, I think some length of burn tunnel would be beneficial. The fire needs a certain length of travel to have time to combust before hitting the pot, and having some of that length horizontal makes the unit less tall and more stable. If you have a small stove and the flame is not reaching the pot, you must have a really tiny fire. What kind of dimensions are you working with?