mini rocket

i made a mini rocket stove, to scale  from one inch pipe . i was burning splinters and it just would up draft,
whats the deal ?

Do you have an insulated combustion chamber?

yeah all insulated

How did you do that?

Can you upload a pic?

i used ash in side a coffee can and then a tomato can for riser
it didn't work so i took it apart

I know that a 4 inch system is supposed to be horribly problematic.  I would, therefore, expect that a one inch system would be worse.

laminar flow. your effective area is the size of a soda stray.

so smaller fuel ?

no, bigger system. at this point you are running into the laws of fluid physics and gas flow dynamics. I suppose with enough time money and a huge lab I could figure out a micro channeling to absorb most of the laminar flow  in that small a pipe but the system would be little more costly.

If you could get it working then you would get to run into the walls of heat physics. basically at that scale the rules change drastically Small effects have large results.

I did something similar. Cut cans apart and hammered as flat as possible (cheap-a[size=8pt]$$[/size] sheet metal), made a 1.5"sq elbow, about 4" tall. Put it inside a 4"can with fiberglass. I was partly influenced by the "rocket lantern": how small can it be? and I want to try the continuous feed of a long stick... Want to adapt it to have a globe from a railroad lantern.

Didn't use any fasteners, except "Insert tab A into slot B". Soon adapted extra "fasteners" of little rectangles of metal, cut & bent into "staples" on the same scale as the tab/slot. Worked fine, except you've got to be careful to not let the slots become permanent open gaps as you bend & hammer them flat (design must include means of sticking something like a chisel or nail extension of some kind in to tight spots to hammer these staples into a tight fit).

Didn't get the fiberglass to catch fire but it out-gassed tremendously as the plastic/pink burned off, leaving white fiber fluff at least .75" out from the center tube. Will try perlite or ash, but that'll mean closer tolerances to keep it from spilling out.
Little thing drew and fired just fine! Up to 6" tall strong flame, tiny rocket sound, very little ash for firing it for half hour continuous.
I had a little grate of wire strung through holes poked in the sides, so maybe that and the square shape disrupted any laminar flow?

Don't know how much difference any insulation makes on this scale, unless you put a lot bigger body around even this tiny draft tunnel. The entire thing got very hot, so no question about long-term holding any heat in the combustion area.
Body was almost hot enough to light a cigarette, outside of the 2" feed/intake chute easily charred/ignited wood.
Perhaps the design premise must include the fact that the entire thing will get hot, so that's how you keep the combustion chimney hot to burn the gasses, rather than trying to insulate it!
It soon became impossible to keep the fire from creeping out into the feed shelf, even if it then drafted in/up through the elbow! The brick I set it on soon became more than hot enough to be an excellent camping bed warmer...

Basic design premise for a rocket stove on this small scale (up to coffee-can body) : A wood fire this small will get hot enough to ignite wood at least 2" away. You must design with this in mind. Continuous-feed fuel stocks will all go up.

Next iteration I have a couple of choices: a slanted fuel feed chute to drop chunks in. Can't use any form of self-feed, since it'd catch fire all up the fuel magazine. (also better material for the inner chimney. Can sheet metal won't hold up for long.)
Quandary: Should I just use this as the air inlet, from above the fire, drawing down? How to direct air under the fuel? Use a wire grid to bounce fuel up onto the grate, while allowing air to preferentially go down under the grate/combustion zone?
It'll obviously require continuous feeding, but it'll be great for making tea, ramen noodles, heating a tent/room/RV.
Alternative would be the standard horizontal fuel/air inlet tube, and risk having all the horizontal shelf becoming like the big mass heater's horizontal burn tunnel going into the vertical updraft chimney.

I considered making it from small pipe, but decided the metal walls of the pipe would be too much thermal mass. Maybe this is where the OP test rig failed? Also more work than cutting & snipping thin metal.

About the notorious failure of the 4" system, I have to wonder if the rest of it was scaled down appropriately? I wouldn't expect it to push a column of hot air as far as a 7" diameter. Small camping woodstoves use a 2" uptake, so I'd expect a rocket's added speed of the updraft to work as well.

cool story got a pic

[quote author=carbonout]
cool story got a pic

Not of that one. Tinkering about, getting ready for another one.
One reason I used the sheet metal & square shape construction, is that I pre-built everything of paper first.
It didn't take long to make & test it, so I'll quit procrastinating one day (and have access to a camera).

I'm thinking of a replacement for my Sierra Zip stove & its batteries, and a proper room air heat exchanger as well as cooking surface, and flue pipe for a tent/RV application.

the problem with 4 inch systems is not that they have not been scaled appropriately its that they require fine tuning and you must feed them constantly. this is the rocket mass heater version. the little J tube rockets are another thing again. they work fine cause the system length is so small.

carbonout wrote:
i made a mini rocket stove, to scale  from one inch pipe . i was burning splinters and it just would up draft,
whats the deal ?

You're talking about an L shaped rocket stove with a side load right? I don't know why you want one that small>>? did a toothpick factory go out of business and give you a huge amount of toothpicks?

I don't understand what you mean that you have a tomato can for a riser? The exhaust should be the same size as the burn chamber? is the burn chamber 1"?

sorry that was to be L tube

I would like to see a pic of the thing... I built a small L shaped rocket stove from a one pound coffee can and used a small 14 oz corn can for the burn chamber...so about 2.75 inch ID.  It works (not as good as my 4 inch ID one) and i decided not to build a smaller one...

You might get the 1 inch rocket to work if you just want to see it work.  Just heat the exhaust manifold with a torch to get air flowing through it. Then load toothpicks in and keep pushing them in.

i did 1" so i could practice making rocket stove on the cheap. 3 tons of thermal mass is a lot to have wrong

slightly off topic but could a small turbine be incorporated in the flue to generate power to charge batteries or run low voltage lighting?

Bird wrote:
slightly off topic but could a small turbine be incorporated in the flue to generate power to charge batteries or run low voltage lighting?

You could probably make that work, but it really doesn't seem like a good way to get electricity.

If you are going to build a wind turbine, stick it out in the wind.

If you want to generate power/electricity from burning wood you might check out wood gasification.  Knowledgepublications on youtube has done a lot of work in bringing back awareness of this old technology.

curiosity saited was just thinking of the energy going up the flue

if you have any links for a power efficient air conditioner thou

heres the problem as i see it: if you want to generate energy from the stove flue you are getting into high tech stuff. bias metals can generate electricity if they have a hot and cold side. this is the type of stuff that makes a rocket stove get expensive. the basic rocket mass heater can be built for as little as 50 bucks if you are willing to scrounge the materials. however if you include a bunch of high tech materials the price to build goes up exponentially.

now as to the tiny rocket mass heater you are running into a scale problem and that is not fixable because we live in an oxygen atmosphere. laminar flow in such a small system makes it so it will only work A: in a hydrocarbon atmosphere. or B: in a very very cold atmosphere.

in other words your going to have to build it in 40 below for the stove to overcome the laminar flow issue because the cold can force the stove to push the hot exhaust gases hard enough to work.

I would build a 4 inch or 6 inch system with no mass and try it out. remember the the whole system must have the same cross sectional diameter as the burn tunnel. that means the same area for the fire and gases to flow through. A stove build does not have to have all the mass to test it.

ok i am rambling; this would be easy to explain if we had pictures, if you need help send an email and we can exchange phone numbers and do this real time. 

Yeah it is hard to discuss something without a clear picture of what it is we are talking about.  I was thinking of a simple rocket stove, not a mass heater..

Here is the large coffee can stove that lasted 2 years... i am going to have to build another in the spring.



The little coffee can about 2.75 ID:

ronie wrote:
You're talking about an L shaped rocket stove with a side load right? I don't know why you want one that small>>? did a toothpick factory go out of business and give you a huge amount of toothpicks?

Really small fuel, yes. The kind of stuff you get out of the way before you set up your tent, and don't need any tools to break it up to make it small enough to fit in.

For a tiny scale "rocket stove", we're not talking about a mass heater, just a burner. As I said above, it seems pointless to try to insulate the burn tube on something this small. I'm going to try just making enough of a "body" to hold it together, and support a 1.5L pot & skirt.

As for the laminar flow problem, and fuel/air not mixing enough, I fail to see it as a show-stopper (from experience with a 1.5" square X 4" riser I made. Make it rough, put in a grate to keep ash/coals from blocking the air inlet under the shelf, and it seems to break up the air flow well enough).

From my experience, seeking to make a small camp-size single stove, the 2.75" coffee-can job is actually too big for this kind of application. Way too much energy output for a single-burner campstove you set up in the vestibule.

I've camped with and gone on week-long trips with my zipstove, among other camp stoves. The zip stove is handy and cooks really fast. But it's tippy for setting a cook pot on, and you need to remove the pot & skirt and everything to re-fuel it. It also almost continually smokes, if there's actually any flame, as opposed to just coals. I'd like to see if a small-scale rocket with enough of a vertical burn tube could eliminate much of that. My litttle experiment seemed promising.

As for electricity, tegpower.com has a little generator that's air-cooled, and should put out enough to charge small appliances and run some LEDs, if you have the circuits and outputs right. From experience with my zip stove, it'd be a challenge to not burn the thermoelectrics out from too much heat. Probably enough to set it on the ground nearby.

found this:

Specs: Dual Power Output 4.5 VDC / 1.2 amps or 10 VDC / 125+ ma /. Can be ganged and wired together in multiples to achieve higher voltages and amperage. Life expectancy - 200,000 hrs. All aluminum construction, dimensions - 5" X 3" X 3".

Applications: Experiment with and demonstrate the practical applications of thermoelectric generators such as; charging batteries, cellphones and other electronic devices. All that is required to generate electricity is a source of heat (180-F to 300-F) Attach to side off wood stove, clamp to stove pipe, oil or alcohol lamp, candle flame, campfire, solar, etc.

(Oh, approx $99, if you want to experiment...)

My goal was to make a thermal mass heater so i could prototype ideas for my house fast.

It seems to me that we have too many subjects and not any meetings of minds here... the mass heaters have a subject thread under rocket mass heaters.. Electricity from turbines or heat or wood gassification could stand to go elsewhere too.

The L shaped rocket 'cook' stoves is what i thought of under this thread called mini rocket...

John's camping stove has different parameters than my semi-stationary cook stove. 

Got some great info here, just seems we are all on different pages of similar books.

If you have a digital camera, it wud be very helpful to get pics uploaded so we can get a better idea of what we are discussing...

i'll have to build it another one to show you

charles johnson "carbonout" wrote:
i'll have to build it another one to show you

please do.
i would like to get your system to work for you. Also please document the whole thing so i can make it on my test bed and see where the problems are.

i will this time im going to use a two inch system