Garry Fisher's batch rocket.

Hello,  A buddy of mine and myself are planning on making one of these, and we had a few questions.  

1.  Because our bricks here in the USA are most likely of different dimensions than yours were, we get a firebox that is an inch or two higher than the spreadsheet says it should be for our proposed 8 inch system.  Is that enough difference,  that we should consider ripping the top row, to get our height exactly 17-14/16?

2.  My buddy has purchased some concrete fire brick chimney liners.  They are 7x7 inside .   First,  are these a good enough material to handle the temps we will get?  He was hoping to use them for the riser.  I understand, that we would need to go down to the 7 in system.   Could this "riser" be set right on the floor of the stove,  and cut a hole in it,  matching the port,  or should it be set up higher, like you did the round riser, in this design?

3.  If I chose to make my riser all of firebricks,  would I simply make an 8 x 8 inch riser directly behind the port,  without any other brick around it,  or do I need another brick " wall". To support the barrels,  and also fill with perlite,  to the level of the top of the firebox,  where the riser enters the barrels?   Did that question make sense?  

4.  I understand that the riser goes to about three inches below the level of the first barrel, and then another barrel is on top of that.   My buddy questioned why there is a whole Barrel more of gap over the riser, as opposed to the typical rocket stove,  where you have a two to three inch gap.  Has it to do with the amount of heat that is going up the riser?

I will include some pics of what we quickly stacked up to give an idea where we are headed.  

Thank you for your time.  I'm sure I forgot half the questions I had.  

Two more questions please.

1.  In this design,  the p channel doesn't run down the length of the firebox,  but seems to start right in front of the barrel.  Didn't you need it to go the full length to get cooler air?  This would be sucking in pretty hot air at this position.  Does either way work as well?

2.  In the following picture,  does this piece of round riser go down to the floor of the stove,  and have a hole cut in it to match the port?  

Garry, i've used flue elements approximately the same as yours. They crack after a while. What i do now, is split them with a diamond blade, from top to bottom. So they're able to move. But they still crack. So next step for me,  May be gluing superwool around, with a refractory mix, which would hold the pieces together. I lost a piece of my workshop heater at the end of last winter. And make a window or door for inspection, and future rebuild of the heat riser is a good idea.


http://donkey32.proboards.com/thread/1817/starting-build-220mm-rocket-double

The round ceramic felt tube showed in the pic you posted, has been cut to match the back end of the firebox, and it's port. Remember, port shouldn't be deeper than 3 inches.

The p channel can be taken prety much anywhere. As long as it draws fresh air, and that this air is pre heated a smidge.

One inch tolerance is fine. 2 inches, that's the thickness of a brick, so adjustment could be made. But even 2 inches in height is no big deal.

About your number four. Barrel gap, in a batch rocket is bigger, because we try to keep gas velocity at it's highest speed. Usually, 8 inch riser = 8 inch gap.

HTH.

Max.

Thank you Max for your answers.  I appreciate it.  Thanks for the link as well.  

Just to clarify your answer on barrel gap,  for an 8 in riser,  you meant the 8 inch gap is the gap between the riser and the edge of the barrel,  right?  
The other "gap",  is the space from the top of the riser to the top of the second barrel.  I understood that the riser should go up to about three inches below the top of the first barrel,  and then place another barrel on top of that.    Did I get that wrong?  

Sure enjoying looking at the link you sent.  Thanks again


Gary

US standard firebricks are 4 1/2" x 9" x 2 1/2" or sometimes 3", so they should almost precisely make up the 17 7/8" height from the chart. I have seen off-brand firebricks with smaller dimensions, maybe 4 1/4" x 8 1/2". Be aware of the possibility if buying firebricks from different sources.

Gary Fisher wrote:Thank you Max for your answers.  I appreciate it.  Thanks for the link as well.  

Just to clarify your answer on barrel gap,  for an 8 in riser,  you meant the 8 inch gap is the gap between the riser and the edge of the barrel,  right?  
The other "gap",  is the space from the top of the riser to the top of the second barrel.  I understood that the riser should go up to about three inches below the top of the first barrel,  and then place another barrel on top of that.    Did I get that wrong?  

Sure enjoying looking at the link you sent.  Thanks again


Gary

In a batch, the top gap should be as big as the heat riser diameter.  The side gap should be calculated as 2 or three time the csa of the heat riser. For a downdraft channel. For a bell, it's the isa which is important.

Glenn Herbert wrote:US standard firebricks are 4 1/2" x 9" x 2 1/2" or sometimes 3", so they should almost precisely make up the 17 7/8" height from the chart. I have seen off-brand firebricks with smaller dimensions, maybe 4 1/4" x 8 1/2". Be aware of the possibility if buying firebricks from different sources.

Yes,  those are my dimensions for my bricks.  Thanks
Gary

I merged your stuff with the following thread. I hope that is okay by you.

Peter van den Berg wrote:

Robert Dearborn wrote:One feature im unclear on from the sketches and the podcast - is there a heat riser  inside the barrel above the heater ? What is its proportion ?  Im assuming it doesnt reach to within 2"  of the underside of THAT monster barrel !

Your assumption is spot on, there is a riser inside the barrels. It's reaching up to about 3 to 4 inches below the top of the first one, so the top gap is one barrel plus 3". The riser is an 8" ceramic fiber tube, it could be shorter but it is being used as a whole.



The space around the riser and the firebricks is filled in with a perlite/clay slip in order to fix it in place.



The barrels happened to be open-top types. Ernie adviced to fit the lid around the riser and cut open the bottom end of both the stacked drums. It is even possible to extend the tower with yet another  barrel. The gap between the riser and the hole in the lid is stuffed with superwool, of course.



When cutting, one inch of the bottom has been left around the perimeter, this way there's a better chance the barrel would stay circular.

This question is for Max or Peter.  I asked Max earlier about this top gap,  and here it appears that the gap is three inches plus a whole barrel above the riser.  I seemed to understand from Max,  that the top gap is 8 inches from the top of an 8 in riser.   What did I miss in my interpretation?  

Gary Fisher wrote:

This question is for Max or Peter.  I asked Max earlier about this top gap,  and here it appears that the gap is three inches plus a whole barrel above the riser.  I seemed to understand from Max,  that the top gap is 8 inches from the top of an 8 in riser.   What did I miss in my interpretation?  

Gary.

Top gap"should" be 8 inches minimum, for an 8 inch riser.

This is Peter's "rule". Peter has often broken it, increasing the "top gap" Like in his barrel towers.

I on the other end, have broken it the other way. Getting closer to what is usual in J rockets. Tho, anything less than 3 times CSA in ring projection http://donkey32.proboards.com/thread/1406/calculating-ring-circumference-projection-gap

Between the top of the heat riser's edges and it's container, would it be bell, barrel or whatnot. I would never advise. Tho, there's tricks. Like making a "trumpet bell end" on top of the heat riser, to increase it's circumference, in order to reduce the gap.

Calculations is the key. Not rules of thumb. Until you really understand what you are doing. And even then, they seem to get even more important.

Gary Fisher wrote:This question is for Max or Peter.  I asked Max earlier about this top gap,  and here it appears that the gap is three inches plus a whole barrel above the riser.  I seemed to understand from Max,  that the top gap is 8 inches from the top of an 8 in riser.   What did I miss in my interpretation?  

During experimentation I tried to find out what the minimum and maximum values would be, about practically everything. A figure of 8" for the top gap minimum is correct, although it's done with a 6" riser. I think that the thickness of the riser wall also plays a role here, so the riser diameter plus twice the wall thickness of the same seems to be the correct way to calculate the minimum value. Although in a permanent build I prefered to stay well above minimum, more like one foot for a 6" system.

A maximum value is hard to pin down, I once tried a top "gap" of over 1.5 meters (5') and didn't find any adverse effects. Rather the contrary, the maximum heat at the top went down slightly, probably due to the fact the flames couldn't reach the top of the bell anymore. I'd regard it as a good thing to design a heater with as little friction as possible, which is good for letting the core "breath" freely and it is very good for efficiency as well. That's because such a heater can function properly with a quite low chimney temperature and at the same time being capable of starting from cold without any fuss.

Thanks Max and Peter for your replies.  I understand better now.
Gary