Rocket mass heater as cook stove in outdoor kitchen?

I’m helping finish a community outdoor kitchen.  We have a large 8" rocket mass heater with a 19” diameter metal plate on top as a cook surface, that was meant to be the primary cook stove in the kitchen.  It takes a very long time to heat the cook surface; we’ve never managed to boil our large 16” pot of water on it even after running the stove all day.  

It’s possible there are slight issues with some of the dimensions that could be improved to make it more efficient, and we intend to figure that out.

But my larger question is why we are trying to use rocket mass heaters for cooking outdoors at all.  Even if we made it maximally efficient, so much heat goes into the mass, and the fire itself is so far from the cook surface, that the time it takes to get the cook surface hot and the wood consumption might be more than a typical wood cookstove, and you can't fit as many pots on this either.  However, this thing is already built, so we’re trying to decide what to do with it.  

Has anyone else use a rocket mass heater as a cook stove outdoors, where you weren't getting much or any benefit from heating the mass?  Or any thoughts on what we should do here?

The original plan was to have a larger metal cooktop that you could fit six pots on, but I don't think there's any way that would have made sense to do; the extra large cooktop acts as a radiator and doesn't get as hot.
Because it's taking up a lot of space inside the kitchen, if it's not going to be useful to cook on, we might want to tear it apart and just put a wood cook stove there instead.  However, so much work went into this thing that we are reluctant to take it apart.

Hi Philip;
You need what we call a bypass.
A shortcut from the riser to the final chimney without  heating the mass first.
Then your barrel top should start boiling water.

Your 8" J tube is built to specification's?
I can't tell from your photo where the chimney is in relation to your transition area.

Tell us more about what you have ,with pictures or drawings.

Bypass gates can be be home built or store bought.

thomas rubino wrote:Hi Philip;
You need what we call a bypass.
A shortcut from the riser to the final chimney without  heating the mass first.
Then your barrel top should start boiling water.

I'm interested to understand how this would heat the cooktop faster?  Wouldn't the bypass be after the cooktop?

thomas rubino wrote:Your 8" J tube is built to specification's?
I can't tell from your photo where the chimney is in relation to your transition area.

Tell us more about what you have ,with pictures or drawings.

Bypass gates can be be home built or store bought.

I believe it is built to specs, I didn't build it but they were following the book.  I'll see if I can get photos from construction.  The exhaust goes out to the right of the photo under that big mass and then up a chimney to the roof.  

We did make the gap between the riser and the tube tighter today and it made it heat faster.  But it still seems like a lot of wood burnt for the heat we're getting on the cooktop.

Hi Philip, is your metal cooktop welded to the barrel or is it just sitting on top of the drum that was installed? There is a slight lip on the barrels so if that cook top metal would have a gap between itself and the barrel surface which would greatly reduce the temperature.

I think Thomas was suggesting a bypass as the heat is going to be absorbed by the cooktop, the ambient air, the water to boil, then by the cob mass, then out the chimney. All this conduction will lower the exhaust temp and slow the stove down resulting in a lower temp burn. If you have it bypass the mass, your draft and temp will increase. Generally rockets are best if given a single job (i.e. heat mass or boil water)

sounds like you hardly need the mass at all: seems like a rocket cooktop shouldn’t need to be very massy.

Daniel Ray wrote:Hi Philip, is your metal cooktop welded to the barrel or is it just sitting on top of the drum that was installed? There is a slight lip on the barrels so if that cook top metal would have a gap between itself and the barrel surface which would greatly reduce the temperature.

It's not welded on but has a fiberglass rope gasket and sits on pretty tight.

I think Thomas was suggesting a bypass as the heat is going to be absorbed by the cooktop, the ambient air, the water to boil, then by the cob mass, then out the chimney. All this conduction will lower the exhaust temp and slow the stove down resulting in a lower temp burn. If you have it bypass the mass, your draft and temp will increase. Generally rockets are best if given a single job (i.e. heat mass or boil water)

Ah, that makes sense.  So the longer chimney with mass all the way slows the draft which makes the burn less efficient.  Makes sense.  It would be a pain to retrofit a bypass into this and the mass is already all there, but maybe carving away as much of the mass as possible from near the barrel, and stuffing that area with insulation, would be a way to make it a bit more efficient.  

There is this quite long thread about a hot plate, it seems to get over 1000f and gets red hot
https://permies.com/t/86886/Rocket-stove-hot-plate

So the 'plate' is not welded on and it has a rope gasket. Got that.

Does the heat hit the bottom of the plate directly after rocketing out the top of the riser? Or does the thinner drum underneath the 'plate' still have its top, creating an air gap between the drum top and the plate?    

david pittaway wrote:
Does the heat hit the bottom of the plate directly after rocketing out the top of the riser? Or does the thinner drum underneath the 'plate' still have its top, creating an air gap between the drum top and the plate?    

The heat hits the plate directly -- we're thinking of cutting a hole in it so the heat can hit the bottom of a pot directly.  But maybe we should figure out why it's not getting hot enough already before we do that.

Philip, I was looking a little closer at the photo and it may just be the angle of the camera, but it looks as if the barrel top is not much higher than the feed tube. A reliable measurement is 1:2:4 (1 for the feed tube height, 2 for the burn tunnel length, and 4 for the riser). I suspect that the riser in this rocket is closer to 1:2:2 which would explain low temps. A simple problem to solve in this case, just increase the height of the riser and attach a portion of a barrel to the one you already have and set your cooktop back on.

Philip McGarvey wrote:The heat hits the plate directly -- we're thinking of cutting a hole in it so the heat can hit the bottom of a pot directly.

Side note; old time cook stoves had exactly that, for the same reason. I have seen some with burner plates that were a nested set; the smallest is less than 2". Drilling a few bolt-size holes can achieve the same effect, and you just drop in bolts afterward for a smoke seal. I have done this with sheet metal tent stoves. If the stove surface is not perfectly flat, a ring of gasket material reduces smoke leakage.