Can I get some opinions on this drawing plan I made please?

Hello everyone, my first post here.

Can I get some opinions on this drawing plan I made please? The purpose of this would be produce heat like a camp fire and to be able to cook on it camping style, and a bit of entertainment factor. My son is in Scouts and this will double as science project for daughter. This rocket is not meant to last ages. I'm using scrap pieces of 5" exhaust stack pipes from a semi, so the thickness of the metal isn't much and why I know it isn't expected to live long. One of the pipes I have has the angled top end of a stack so I thought I would use that angle to try making a vortex in the heat riser.

About the part I drew in red: I'm essentially attempting to replicate the process of how a Solo Stove Bonfire pit works, which I believe is considered gasification, I've seen it referred to as afterburn too. I don't know how it would turn out in this application though, nor do I know where best to build the dimensions of it.

Thank you in advance.

Hi Ace;
Welcome to Permies!
A very cool idea to have both kids utilize the same project!
I love inspiring kids to do fun stuff!

Your drawing shows a basic L tube rocket.
It would need cutting and welding on it.  
I see no need to offset to create a vortex, it will rocket all on its own!
As an L tube, it will need tending throughout the burn.
It will certainly cook a meal.

Here are some things I would do that you might consider.
I would use clay bricks rather than metal. The kids can build it themselves!
Use clay mud to seal the air gaps, kids love mud!  
I would stick with the rocket stove dimensions using the same size from the feed tube to the riser.
I would build a J tube design rocket as it would self-feed and burn hot,
This would be fun for you and the kids to play with at home and then build one in minutes for the scouts.

Fire and Mud should be right up a kid's alley!
Maybe offer hot dogs roasted (carefully) over the riser as a reward!

Thank you Thomas, I do appreciate the suggestion.

I'm ok with and planned to do cutting and welding. The vortex is more for cool effect to spark interest better. My plan is to make about half of the feed tube shelf ventilated, like mesh, and the burn grate the rest of shelf length. So hopefully there will be some tolerance for tending frequency.

I chose the exhaust pipe bcuz that's what I have readily available and I'm more experienced working with metal. I do not have clay brick or clay to make mud. I have structural brick but it is my understanding that they're not very heat tolerant. Plus I want this to be at least somewhat portable without throwing out my back. I do realize that the suggestion you've made would produce a superior rocket, but that plan won't work for me at this time.

I've been reading a lot all over the web, even that donkey32.proboard site. I considered a j-tube or even k-tube, unfortunately i cannot remember at the moment why I decided L-tube. It may have been for simplicity with measuring/cutting all the angles on round tubing; square tube would be a bit easier to do a J or K set up.

I think an L-tube rocket would be fine for your purposes. Kids will have fun tending the fire (what kid doesn't like to play with fire?), especially if you can raise the system so the feed is a foot off the ground. Depending on the size of the kids, the major consideration would be the final riser top elevation from the ground.

I have built a few L-tube rockets (an oven and two maple syrup evaporators, all larger than your idea) and they all work great. The 5" diameter will take a fair amount of fuel and will not run out in just a few minutes. The under-fuel air supply is already your secondary air, preheated by the burning fuel above, and I think the inner sleeve and especially the release only 6-12" below riser top is not going to be beneficial; rather, they would dilute the concentration of heat in the riser where it needs to be hottest.

My experience with L-tubes with strong draft (5' to 13' stacks) is that the fire goes back to the feed mouth and is not at all confined to the base of the riser. The longer the feed is, the more total combustion flow length there is and the more complete your combustion will be. The unburned gases released in the early to mid feed will be assisted by the fresh air supply at the riser base. Your stove will not have such strong draft with only up to 30" of stack and I expect the fire to fill the feed pretty quickly even if you start it at the back. I doubt the offset feed-to-riser connection will make a real difference in combustion, but I do not think it will hurt and the angled end of your piece makes it easy to do.

What will make a difference in combustion would be insulating the feed and riser, even wrapping some fiberglass around them, and ideally adding a noncombustible wrapper to keep fibers from getting loose in kids' breathing room. That would shorten the system life, but not by enough to cause an issue with a kids' demo/experimentation tool.

Finally, I think you are underestimating the value of having the kids build their own simple L-tube from bricks and mud, even if you are more comfortable working in steel. You don't need good clay for a temporary structure like this, nor do you have to have just the right kind of bricks. The fire will not get hot enough for long enough to significantly degrade them in a few afternoons. The bricks can easily be taken down, mud shaken off, and packed in a box for transport. You could do a nice one with exactly 41 bricks and a gallon of mud.

If by "structural brick" you mean concrete brick, you are right, those might not be stable in a fire. If you started with them thoroughly dry, no moisture inside, they would be safe but might spall or crumble in the hottest parts of the core after some use. Concrete block firepits do disintegrate... after seasons' worth of bonfires.

A quick sketch...

Thank you Glenn. Sorry for the delay response.

So I think the pipe I have already welded to a base plate is 37" long. You're suggesting to have it long instead of cutting shorter to 24-30? There are bunches of pre-made portable rockets for sale online that are typically around 24" tall so that and portability are why I said 24-30.

Can fiberglass insulation meant for an attic be used? The fluffy yellow stuff that makes skin itch after contact? Can the fluffy yellow be wrapped directly against the metal pipe or will it melt and/or ignite? I have some of that I can wrap the pipes with and some wires to tie it in place. Then hopefully some aluminum foil tape to wrap the fluffy, or will the heat defeat the adhesive? Not sure what else I could use to seal up the fluffy. I suppose household kitchen grade heavy duty aluminum foil with more wire tied but that wouldn't be durable with transportation.

On the inner sleeve thing: I've heard the possibility of it not working in other words, and I understand the reasoning theory. My opposition to saying it won't work is that it's how the Solo Stove Bonfire works and many similarly designed units including DIY as big as a 55 gal drum. The Bonfire is only 14" overall height, albeit much wider than a rocket. Solo makes smaller versions now too. I attached a diagram with this post just in case you're not familiar with them. I believe there should also be a venturi effect with the inner sleeve. During the initial testing of the sleeve insert it will still be removable so if it does not work all I'll have to do is cover up the drilled holes of outer pipe. If it does work I can weld it in place. I drew the bottom of the sleeve directly above the L vertex bcuz it would only add complexity to start the sleeve near the bottom from having to cut to match the profile of feed tube and that could cause irregular flow at the top of sleeve. I really wasn't sure what measurement to use on the top side. Inner sleeve pipe outlets of Solo Stoves are just below the very top. If this were to work similar, then in theory more flame should be produced at this outlet, which would scorch a cooking pan that would be atop the heat riser; which I why I thought 6-12" below. But again this could be tested with a longer pipe first.

I guess "structural" brick was not the right term. I have an assorted pile of mostly red and some tan bricks that would typically be used for building a brick house. I guess the original owner of my house was a brick layer, and it's a brick house. Some have 3 bigger holes through them, some have 12 small holes, some are solid, very few have any branding imprints. They've just been sitting in a pile behind the shed for decades so we will have a go at building a rocket anyway.

For the burn grate, I haven't quite decided what to use yet. I have some old gas range oven racks that I could cut to fit, but spacing is big so would probably have to double layer. How well does bug screen hold up to heat? I have a square yard of stainless steel screen that I could trim then fab a frame for to fit burn tube (another salvaged semi part). If it's not high heat tolerant, would that be bcuz of how thin the wire is? That in mind, how thick should the grate media be? Wire coat hanger diameter plus? Also would screen hold onto coals too long? If I recall correctly the mesh squares are either 1/16" or 1/32", I could snug fit a drill bit that size into a square. I have a few other odd possibilities in mind. Trying to use what I have already.

Even some some wood stoves and ovens use preheated fresh air channels to achieve secondary combustion.

Hi Ace,
I have little experience with RMH, beyond reading and dreaming about them right now. However, I felt the need to chime in on the fiberglass insulation. That stuff should not be anywhere near fire. It will melt and burn and give off nasty stuff. If you don't get specific insulation meant for woodstoves and fires, you could probably get away with rockwool insulation.

Thank you Matt, I had a feeling that what you're saying was the case. I read people talk about fiberglass insulation and the fluffy stuff is what I have in mind. I've seen mention of rockwool but no idea what it is and certainly don't have any.

Glenn Herbert wrote:A quick sketch...

I forgot to mention, thank you for the diagram, we'll refer to this when we go for the brick method.

Fiberglass is made from, well, glass.
Rockwool is made the same way fiberglass is, but it is spun from molten basalt.  Thus it has a higher temperature rating.

Hi Ace,
Thomas has it correct. Some rockwool is also made from the leftover slag of metal production, but either way, you are talking about things that will only burn at extremely high temperatures. Ever since seeing this video a while back on youtube about fire testing insulation... I have never looked at insulation the same again. Some things might be cheap and hold heat in, but in case of a fire, you are might die from the chemical inhalation before the fire gets you.

***Edit*** I call it rockwool, but I think that might actually be a product name. I think the generic term would be mineral wool.