Creative Commons-Licentie Batch Box Rocket by Peter van den Berg 

is available under the Creative Commons Attribution-ShareAlike 4.0 International license

All heater cores are designed by Peter van den Berg, unless stated otherwise.

1: Brick core

A masonry firebox and riser is the simplest to produce, although fire bricks need to be cut. Using an optimized design this cutting could be minimal.
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This duct supplies the secondary air in the original design. Simply constructed, it works solely due to the law of physics known as the Bernoulli principle.
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2: Brick core

The problem of the non-round riser of the brick core above is solved in this second design, the riser is now octagonal which is a better approximation to round. The layers are applied alternately on edge and flat in a reasonable running bond.
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3: Cast core

This is a firebox assembly together with the lower third of the heat riser in two separate parts. This one is built already quite often and works well but is vulnerable to cracking.
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4: Cast core

This is a recent design (2015) with the firebox divided into three parts. Instead of a p-channel there's a floor channel present.
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Floor channel

Also a secondary air supply but this one applies the air halfway in the height of the port. This duct lies on the floor of the firebox and recieves its air from the main air inlet.
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5: Cast core (sidewinder)

A design where the riser isn't situated behind the firebox but at the side instead, allowing a shallower depth.
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6: Brick sidewinder

A simpler way to build a one-off sidewinder as compared to making molds and cast the parts is building from firebricks. Unfortunately the number of bricks to be cut is quite substantial.
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1: Straight brick core

Using a good layout and a suitable clay/sand mix this masonry version is an easy way to test these principles for yourself. This can be done outdoors, in a barn or shed, anywhere that is convenient. A word of warning, the riser will likely have flames shooting out of the top, so use chimney pipe to vent when tested in enclosed spaces. It is not 'only' a convenient test bed, it is more than suitable for permanent use as the core for a space heater (the clay sand mix allows for easy disassembly after testing). As will be described later, space heaters take advantage of the ultra clean burn provided by these combustion units and are able to capture and store the heat produced.

Simple brick batch rocket core

Of course it's sensible to build on stable footing, preferably insulating or using an intermediate insulation layer. The drawing in SketchUp 16 format of this design is available to download via this link. The internal shape of the riser is square, this is not as good as round, it will work but the shape isn't optimal. This version is suitable to build out of hard fire bricks. The layout of these bricks may need adjustment, this design is based on a brick size common in the Netherlands. Naturally, at least the riser should be surrounded by heat resistent insulation material. This isn't the case when this core is built using light weight insulating fire bricks. On the other hand, these are probably just a tad too vulnerable to abrasion to be used in the firebox.


The steel rectangular duct on top of the firebox provides the secondary air supply. This part is commonly called the p-channel, short for Peter channel after its designer. Common mild steel or stainless steel is suitable material to manufacture this part. In this 150 mm (6") system a rectangular duct of 60x20x2 mm (2.36"x 0.8 " x 0.8") is being used.

P-channel layout

The downward hanging end stops a little lower than the top of the port, providing a small overhang. To keep things simple, this overhang is the same as the depth of the channel. At the back which is facing the riser a small piece is cut out to promote the suction of air. The same principle that causes the p-channel to work, Bernoulli's principle, means the pressure within the system is lower than that outside. Hence no smoke will escape the p-channel, indeed any other small cracks or the main air inlet itself. Conversely, if smoke does come out of the p-channel or main air inlet, then it means there is something wrong in the system.

This secondary air needs to be added to the stream in advance of the strong turbulence in the port and behind. Supplying air in the riser itself seems obvious but it won't work, mixing isn't powerful enough this way.

The duct is being cooled by the incoming air so the life expectancy of common steel is surprisingly long. Damage by corrosion in a space heater which has been used for two seasons is barely visible. So, it would be wise never to shut off the p-channel during operation. It is suspected a larger wall thickness of the duct could promote quicker corrosion because the steel can't shed heat as fast as the thinner material.

2: Straight brick core, octagon riser

The problem of the non round riser is solved in this next design, the riser is octagonal now which is a better approximation to round. The layers are applied alternately on edge and flat in a reasonable running bond. The smoother the riser inside the better, so it's rewarding to be very careful when building this part.

The octagon is a favorable shape because this way the double vortex will initiate earlier in the burn. The drawback of this shape is the higher mass. Such a disadvantage won't be in play when insulating fire bricks are used for this build. Even insulating material around the riser isn't necessary anymore, the bricks are insulating enough by themselves.

Brick batch rocket core, octagon riser

The small yellow triangles are in reality the same material, the different color is only there for easy visualisation. Construction of this core is different from the first brick design, the connection between firebox and riser is dissimilar. This drawing is available for download too, use this link.

Below are two other examples of how to make the riser octagonal out of normal brick sizes. Both use bricks that have been cut with a 45 degree angle to create equal sized "halves" to form an octagon shape. The lower one is simple but allows only one size of riser while the other allows the bricks to be adjusted to a slightly wider riser. A wider riser would mean the firebox and port should be wider as well, the scaling method still applies.

Octagon riser cross section, 45 degrees cut

3: Straight cast core, 4 parts

This a tried and proven design with quite a few successful examples around the world. One conspicuous although not fatal flaw is the fact that this version is prone to cracks left and right in the firebox. The firebox will stay whole, after the cracks have appeared nothing much will happen later on. Probably the cracks themselves act as expansion joints, tension is released and the cracks won't grow anymore. Thickness of the walls are as thin as 30 mm (1.18") and where applicable fill pieces are used to economize on material. This isn't done to save money but to reduce the mass of the core. The less material in there, the sooner the thing is up to working temperature.

The part of the riser above the firebox is also split up in two parts which are identical so those could both be cast in the same mold.

Complete cast rocket core

The cross on the sides indicates which side is on top during casting. That side will remain relatively rough but that's of no consequence. Not only is the octagon a preferred shape (a very good approximation to round) it is also easily formed when building a mold. A table saw set to 45 degrees will cut all of the pieces required. Careful use of a hand held electric saw with a fence/guide will also serve if a table saw is not available.

This drawing is also free to use, downloadable via this link.

4: Straight cast core, 5 parts

This design is quite recent (2015) and has been in daily use for one winter without visible cracks. In my implementation the bottom rests on a support frame and the upper side is held together by cams and corresponding recesses in the top part. The picture below shows a slightly adapted drawing but is essentially the same as the one in my own heater.

5 part cast batch rocket core

Naturally, the riser of this one need to be insulated from top to bottom as well. Construction is fairly simple, shown by the picture below.

Floor channel placement and layout

Floor channel

This secondary air supply duct is an alternative to the p-channel, not to be used in the same heater. The construction and placement is quite different, it lies at the floor of the firebox in a trench and it is fed through the same inlet as the main air supply. It is called a floor channel and as such it is a variant of Matt Walker's pre-port tube but the vertical part is shorter, square and less wide. As a consequence, the tube is less of an obstruction inside the firebox.

On top of that the air is supplied halfway up the port which in turn keeps the double vortex low in the riser on average. This version is somewhat easier to produce than Matt's, the only tools used to produce this thing is a small grinder equipped with a cutting wheel and a welder. This floor channel is very easy to replace, an advantage as compared to the p-channel which can't be taken out unless the heater is opened up.

floor channel assembly

The channel is made out of normal 2 mm thick steel rectangular duct and has very little corrosion or deterioration after a year of daily use in a temperate marine winter. The explanation for the lack of corrosion is that the inside of the duct is cooled by the incoming air. And the duct itself is in an oxygen poor corner of the firebox while the burn is going on, steel won't corrode in the absence of oxygen.

So, this is one of the few steel parts inside the firebox which isn't destroyed in ten runs, durability is reasonable. My next channel will be made out of stainless steel 304, life time expectancy should be even better.

The burns during winter of 2015/2016 were entirely dedicated to get this air supply right, about 12 variants have been tested. The drawing shows a horizontal duct of 60x30x2 mm (2.36" x 1.18" x 0.08") , the vertical piece measures 35x35x2 mm (1.38"x 1.38" x 0.08"). The internal cross section area of the larger duct is about 1.5 times as large as the smaller one. This is not a coincidence, three sizes of the horizontal pipe are tested in several combinations with three vertical sizes. The duct of 60x30x2 is just as effective as a 60x40x2 item, as long as the inlet is larger than the exhaust of the channel. Using the steel ducts as drawn, the vertical part is 5.4% of the riser CSA and the horizontal part 8.25%. By keeping close to these percentages it is possible to calculate the dimensions for a larger or smaller batchrocket according to their larger or smaller riser CSA.

In contrast with the normal implementation this version should be lit on top of the fuel pile at the back, known as "upside down firing". This method will deliver the best results.

BAtch rocket results diagram

This diagram represents a testrun using larger pieces of bone dry birch, lit on top. The start temperature of the heater itself happened to be moderately warm, start temperature in the chimney 30 ºC (86 ºF). The CO dropped at 18 minutes into the burn below 500 ppm and stayed down there for 58 minutes. The averages of this run: O2 13%, eff. 95.2%, CO 282 ppm, Tr 66.4 ºC (152 ºF). Mark the end level of CO is unusually low.

To conclude: the drawing of this core is available via this link.

5: Cast sidewinder, 7 parts

This is a variant of a batch box rocket where the riser is placed either left or right of the firebox. The design is developed by Adiel Shnior and Shilo Kinarty, two guys building rocket mass heaters in Israël. Max Edleson, a heater builder in the USA named it the "sidewinder". This is the name of a desert snake which moves by winding sideways over the sand.

As the riser is no longer directly in line with the combustion chamber, it is naturally no longer symmetrical. As a consequence, we need an extra mold to cast the combustion chamber, see below.

Cast sidewinder batch box rocket core

This design is similar to the earlier with a separate top piece, the most complicated mold is the back wall which has half of the lower part of the riser incorporated. It is possible to get away without making a second dedicated mold to cast the combustion chamber sides if a "fill piece" can be placed and secured into the mold prior to casting at the spot where the port should be. In this manner the one mold can cast both sides, one with an exit to the port due to the fill piece put into place before casting, the other without an exit. The top part of the firebox is the same as in the straight design and the other lower half of the riser is a relatively simple mold. For better understanding of the layout there's an exploded view below.

Exploded view cast batch box rocket combustion unit

It looks very complicated, but with some careful inspection and thought it is entirely within a competent home handyman's ability. Although seemingly more complicated, these molds are assembled no differently than described in the "How to build" section. A coated plywood box with suitably formed pieces of extruded polystyrene (or other) to form the final shape.

To many this might be the first attempt to think with 'negative spaces' and how to demold the casting. I do believe it is within the capabilities of a good 'backyard tinkerer' so fear not. It might be a very good idea to test your molds with much cheaper materials, such as plaster of Paris, or even a weak cement sand mix, before (potentially) wasting the very expensive refractory castables on a mold that might need modification before the proper end result can be obtained.

As mentioned in the introduction, this is an open source project hence is available for commercial purposes (please read any attached conditions to that in the introduction section). If it is planned to make more than a few of these it is sensible to first make positive 'master molds' (that is, the actual shape itself, the 'positive') and from them make the rubber molds (ie these are now the 'negatives') to be used for the actual production run.

These second molds will need to be replaced periodically by recasting from the master mold. It seems to me a viable business opportunity to make and sell these castings along with good instructions on assembly and also the 'DIY actions' required to reach the final product. The total number of molds including the heat riser is five, possible to cast 7 parts in there. The drawing of this design is available through this link.

6: Brick sidewinder core

This design isn't difficult to build using fire bricks and a wet saw. Not more difficult than the straight batch rocket design, anyway. In order to make things a bit simpler the design size is a little bit enlarged, from 150 mm (6") to 160 mm (6.3"). By doing this, the firebox becomes also slightly larger so somewhat larger and thicker fuel could be loaded.

Brick sidewinder batch box rocket, simple build

It would be sensible to keep the same depth as in the straight brick core version. Together with the riser no longer being at the back this would result in an installation depth of 486 mm using bricks this size. This would be a saving of 216 mm as compared to 702 mm of the straight core. The differences in firing behaviour are fairly small so this will be a good alternative. These sizes are all nominal, no thickness of the mortar between the bricks is taken into account.

The floor channel's vertical part is somewhat lengthened to accomodate the larger size of the riser and the port. Besides, this channel is constructed simpler as compared to the one in the casted sidewinder version, no 90 degree bend in it and the vertical part is placed a-symmetrical on the horizontal part in order to create sufficient distance to the port. On both sides of the vertical part there should be the same space as half the port width, measured perpendicular to the sides of the vertical channel, between this and the corners of the port.

Of course it is possible and also better to use the octagonal riser of the design 2:Brick core which will yield better firing results. Use this link to download the SketchUp drawing.

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