my hippie engineering question--granny cart as frame for rocket stove?

What could go wrong?

I'm going for semi-portability, so light weight but OK if it takes a minute to set it up and take it down.  It's going to have to come with us when we move someday.  And I want to be able to say, "Oh, that? that's just a different kind of grill I like to use."  No further questions your honor.

(Putting the granny cart on its side, so the handle touches the ground, and then putting the heavy (firebrick) end of the core on toward the wheel, the riser toward the back and hanging off a bit).

The granny cart I have is about 23 inches long, the core (from Walker stoves) is 26 inches long.  So I'd need a bit more metal of some kind to support it on that end.  I think I can scrounge up a baking tray that can be sacrificed for the cause.

And then there's protecting the riser from getting knocked over, some kind of cage around that could be made from thick chicken wire I just got from a neighbor (not hte hexagonal kind,  the big rectangles, but it's not completely stiff, so I'd want to give a wide berth around the riser).  That doesn't seem quite strong enough, but I can wrap around several times.  Maybe if I score a second granny cart.

I'm thinking that granny carts are often free.

Maybe a propane grill will also appear on my street again and I can scavenge it for some more parts, extra wheels, struts.

Plan b--use bed frame angle iron.

I have no soldering iron or skills, no sheet metal, nothing that can really cut serious metal just tinsnips.  

I have enough cinder blocks I think I can build a tower on either side of the granny cart and then throw a grill rack over that and call it a stove top.

(As for why I think granny carts may be free--although they're $50 at a large big box store, there are three of them in the basement of my landlord's house here, and I have not seen even one of them get used even when his mom was still living here.)

How hot does the outside of the ceramic fiberboard core get generally, ballpark?  
Thanks, team!

Hi Joshua;
I have a question...  Just what is a "granny" cart??? Got a picture? Would you be referring to a metal grocery cart?

thomas rubino wrote:Hi Joshua;
I have a question...  Just what is a "granny" cart??? Got a picture? Would you be referring to a metal grocery cart?

Yes, it's a rolly grocery cart, supposed to be able to carry 200 lbs of groceries.  The bricks are 18 lbs, the ceramic fiberboard core maybe another 2 lbs.  

It might be a bit too wobbly if I roll it places, I might want to have a handle to be able to pick it up like the suitcase "aluminum series" Matt Walker makes, and just not use the wheels.

The question is what temperature the outside of the ceramic fiberboard reaches when you burn?  

I bought a batt of mineral wool and gave it to someone but i might be able to get some of it back.  

And I have some chicken wire I could wrap around that to make a sturdy-ish frame.

This is the kind of thing I'm talking about.  Thanks, team!

Hi Joshua;  I don't have a exact answer but I know red clay brick can go outside of CF board. So 300F or less.
Your currently using an open core?  No covering at all?
Matt's aluminum series would be perfect for a rolling rocket stove!
A full size grocery store cart would fit your stove better than that granny cart.
I believe there are legal ways to buy one used.

Thanks Thomas, a shopping cart would be perfect, especially a bigwheel shopping cart with really great suspension.  I realized it's too bumpy around here.  I moved the thing to a flatter concrete patio area, put it on a glass window I had and under that the chicken wire that can be used to help pick up the window from the sides without placing too much strain on the glass.  And fired it up with just aluminum foil around the riser.  I wanted to harden the ceramic fiber board (and, well, basically, play with fire).  It works great!  And the outside of the ceramic fiberboard was room temperature, actually.  Thanks for the 300 F figure, that's super helpful.  Now to me that sounds like I could definitely line it with cardboard, since paper burns at 451F, and that leaves a wide margin.  And aluminum foil between would make that even more safe and give extra insurance against leakage.  And then that wood would also be OK.  I know this sounds super risky and dangerous but if it really won't go above 300F there's no way wood could possibly catch fire.

Wood is lightweight, workable, stable, somewhat insulative...

And the last quality makes it somewhat dangerous since if you have something at 300F for a long time with insulation around it, then it can go way higher.

SO--I'm thinking of making a frame with wood but leaving plenty of spaces.  Sort of like a garden planter.  

I realize I lose a fair amount in efficiency by not insulating it, but I a) don't really have the bricks for the whole thing b) don't have a shopping cart at the moment and c) don't really have an inconspicuous place for that.  I kinda feel I can get away with the granny cart but not a shopping cart.  And d) if it were insulate with bricks, that's more of a thermal mass than an insulator, yes?

How about wool?

" the temperature must reach 570–600°C before wool will ignite;" from learnaboutwool.com.  (1112 F).  

I think there's a really safe margin of error for that.  And I love the idea of wrapping the whole of it in wool, and then making wood around that.

For the pot holder, I am thinking of scavenging a propane grill (they have a nice big opening at the bottom) and then putting that up on some cinder blocks (it's a bit too short otherwise).

OK, am I still crazy to consider using wood?

The aluminum suitcase thing still looks like a much better idea overall, I just don't happen to have any and I like the idea of doing this with materials anyone in a lot of places can get for free--pallet wood, I mean.

Here are pictures.  Please feel free to point out flaming death points, I won't be offended.

Also, yes, that is a Lodge.  I wouldn't dare use my partner's Griswolds in this project until I've got a solid track record and no points on my insurance.

(Also, I don't know if my wood is really dry.  A picture of my woodpile would show you that it is a few pallets leaned up against a wall, and I picked the "HT" (heat treated) ones from the sideways bars, ie the part that doesn't touch the wet ground when it rains).  It doesn't have any roof over it.  I suspect it's fairly wet wood compared to what the pros use.  I don't have a moisture meter.  I like the idea of heat treating my wood with the rocket stove when I'm done cooking, maybe.

Josh;  As long as it is near water so any unplanned fires do not get away.  Your wool idea is worth a try.
I will tell you that wood changes with repeated heating and the ignition point drops much lower.
Not sure what you are thinking about the cardboard.  Wood ignites at  456 F  Paper igniting at 451 F ?  I think a sheet of paper would reach ignition temp very fast whatever the flame point might be.

The outside surface of the ceramic fiber board may be room temperature when you have only one fire, but with repeated loading, it will get quite hot.
As Thomas mentioned, woods combustion point lowers with each cycle of heating and because of that would be very risky to have it line the core (other than burning in the feed tube).
I remember the pebble style rocket mass heater made at Wheaton labs many years back incorporated a lot of wood to encase the firebox and upon inspection, they found that under the core, the wood was all charred.
No experience with wool but could be worth a try.
I've used perlite, vermiculite, rock wool and ceramic fiber blanket all with good success to line my cores.

Not sure if you know, but Matt hosts a live broadcast on his youtube channel once a week where he answers all kinds of rocket stove related questions. Well worth checking out. I haven't missed one yet they are so good!
Stove Chat

Yes, thanks for those good points!  I have my water handy in a big highly pressurized tank right over the flame...just kidding.

I am not going the wood route, unless I can get some magic %100 non-flammable wood.

I think it's all good enough for the time being, and soon I hope I'll get the granny cart and be able to add a bit of mobility (especially having a handle of some sort to grip will help with that).  

So here's what went wrong...so far...

It rained REALLY HARD.  Which we really needed but I swear the forecast had not said we had more than a 3% chance.  And that concrete patio area pools water like a Sepp Holzer pond in monsoon season.  And the fiberboard does NOT completely harden from just being fired a few times--it absorbed 93% of the not forecast rain.  (Note to self--altrnate use fir ceramic fiberboard, the quicker picker upper.)...and so it was much heavier trying to lift it after the rain had been going a while and the glass broke and I got a little piece in my foot while trying to sweep the water away from that area.

I managed to get it up on a few boards to be above sea level, but definitely would do it a bit differently next time.  

The granny cart idea would have worked better, I just haven't yet heard back that I can use the granny cart.

Joshua Myrvaagnes wrote:The granny cart idea would have worked better, I just haven't yet heard back that I can use the granny cart.

Not sure if your asking our permission or not but hey, its your experiment, we're just along for the ride!  :)
So if you burn the paint off the metal or melt the wheels on the cart, it just means you need some more insulation. No problemo. That's what testing is all about.

Also, I remember Matt Walker has said that he left his ceramic fibre board core outside all winter with snow and many freeze/thaw cycles and said it held up great.

So update--i couldn't get hold of a hacksaw so easily so I used a black office shelving thingamabob I call Thoor Ballylee 3.0 (apologies to Yeats and Jeffries).  It is ugly.  It is a monster.  It is an assault on aesthetics. But it is safer than my old "design" and I hope to weather-proof it soon.  Thoor Ballylee 3.0 is probably full of toxic gick but I don't burn long enough for the outside of the core to get above room temperature.  So far I think this is safe-ish, but dont try this at home kids.

This looks like calculated  madness, very inspiring!

That utterly looks like a mad rocket scientist made it....I love it!

hahaha, well, it's just what I happened to have lying around.  So far it works fine, no explosions or accidental monsters.

I wanted to point out that the string holding the metal in place is a risk, and I put wood between the core and the metal where I could, feeling secure that even if it charred it would be inside a big metal case so the fire couldn't get far.  Same with the string.  But again, I've never felt it get hotter than slightly warm/room temperature on the outside of the core.

I may stand it upright, so that the roof can go over the whole thing and keep the stove, wood, and the cook out of the rain.  But then the "bottom" (currently the back side) of Thoor Ballylee 3.0 is the part that looks most likely to melt or catch fire.  It's got plastic wheels.  Also a little less stable since one wheel is broken.  The wheels are useless for lawn or garden terrain, I may have to get them off.  Other problem--now the core would stick out the end and you could trip over it, which I wanted especially to prevent.  

My next idea was to put Thoor upside down.  That might be unstable too--it narrows toward the (original) top.  But still quite sturdy.

Bricks would be a lot easier, but this needs to be pretty easy to dismantle for the winter.

All the shelving unit-y hardware stuff came off Thoor, by the way.

So, in sum, I'm not really advocating for this design in general, just that some of its aspects may be useful in some cases, and just sharing the lessons learned.  And it seems to work well enough for my purposes at the moment, which are:

--keep the core from falling apart/instability from use
--keep riser (chimney) from any possibility of falling over when in use
--be a little closer to being able to rest something on it that protects from rain--today I put a plastic Rubbermaid tub lid and trash can lid over the chimney and the slanty part, covering it all from the forecast rain.  (Then, of course, it didn't rain after all.) (So I guess it works as a rain-maker device by being in danger of getting destroyed by moisture).
--let me get my money out of the gas company's coffers as much as possible ASAP
--let me play with fire ASAP

Regarding the third item, rain damage--even though Matt Walker had a good experience with his core that he'd left out all winter, I found that some small bits of mine had come off the bottom for some reason, probably from joints or places where movement stressed it when I used it.  One of the little nails sort of migrated over, and the whole bottom piece cracked in the middle, because the glass-window-under-it idea for the Mark 1 was a bad idea.  Don't use glass, it's gonna break, it's just a question of exactly how.  So I think leaving it out in the rain unused is probably less hard on the ceramic fiberboard than using and letting it get wet sometimes.  

Oh, in case I failed to mention, the tape on the aluminum foil did burn, that was a bad idea.  Matt Walker managed to wrap it in foil in the video, but that was a) not on a windy day b) narrower foil than what I had and c) a much less heavy roll than I had, because my partner shops at Costco.  Whatever burnt out of the tape has now finished off-gassing, so I hope I didn't unbalance the ecosystem with that burn.  I guess it contradicts the sense of "it never gets hotter than room temperature outside the core," but for some reason the tape wrapped inside the aluminum foil did get hot enough to make a weird smell, and later when some of the foil came off I saw the tape looked burned.

I think that's it, I made all the mistakes, and yet it still really works!