Batch Rocket Heat Riser Base

I'm working on a batch box rocket with a sidewinder heat riser.

On Peters excellent resource page Sidewinder core it shows an exploded version of the cast core. As the port exits into the riser there is a small section of the riser base that is horizontal and then it slopes at an angle (45 degrees?) for a small distance before joining the vertical walls of the riser.

Rather than casting a core, I'm constructing my octagonal riser with hard firebrick splits and to incorporate this angled section could prove difficult... Would the performance of the system be compromised if I simply missed out this angled section and made the port exit directly into a flat based heat riser?

Thanks as always for any help.

Hi John,
A flat base of the riser has its own repercussions. A flat back corner of the riser will pose a drag factor as far as I can tell. Drag isn't a good thing in a batch box, since the thing is heavily dependent on high gas velocity. You could attempt to shape that 45 degrees or even hollow slope with clay and a large proportion of sand. The clay will be baked no doubt, it will shrink but it's easy to fix that once shrinkage is over.

Thanks Peter.

I'll perhaps try shaping a firebrick split to a close fit in the riser and patch around its edges with the clay/sand mix.

Thanks again.

The heat riser for my 6" batch is made from Insulating Fire Brick (IFB; 2800° F.), including the angled floor section. Very easy to cut and shape using standard hand tools. And a ceramic tile wet-saw cuts them like cutting butter, but I mostly used that when constructing the firebox from standard dense fire brick. Anyway, maybe pick up a couple of IFB to use for the heat riser's angled floor area?

Thanks for the advice Byron. I'm interested in you using Insulating Fire Brick as a heat riser and I presume it's working well.

I've got a large sheet of vermiculite fireboard (which may be a different composition to the type you use) Vermiculite Fireboard
and I've used it in the past for small heat risers and firebox insulation. In my experience I've found that it does seem to degrade in burn tunnels and heat risers during heavy use - maybe the stuff I've used is an inferior product?  I've gone down the hard firebrick route after Peters comments on the intense temps exhibited, especially at the base of the batchbox heat riser. My thinking is that by using hard wearing firebrick splits, I would avoid any possible problems later on. With my 'sidewinder' batchbox/riser configuration it would prove very difficult to repair any riser failure without having to dismantle the whole unit.

Many folks have constructed their RMH heat risers from IFB, which works very well. And as I recall, Matt Walker constructed his side-winder cook stove core, fire box and all, from IFB, despite it's tendency to wear easily from stuffing wood into the thing. But Matt is careful loading it, so it is holding up OK after a full season of use, last I heard.

For the heat riser I used the standard fire brick size IFB, placed on edge so the riser's wall thickness is 2.5 inches. The mortar used was made with a 1:1 mix of masonry sand and fire-clay (powdered / dry milled fire-clay).

Thanks again for the reply Byron.

The Vermiculite fireboard I have (and have used during the past) is only 25mm thick, which may explain why it has failed before.

The current plan is to use the splits for the riser and add an external layer of loose vermiculite insulation, mixed with a small amount of high alumina cement, to provide good insulation and help support the structure of the riser.

From: ../59026/rocket-mass-heaters-technology-availability

John Harrison wrote:  I was planning to build a 5" batch rocket using 25mm (1 inch) thick hard firebrick splits backed up by 20mm ceramic fibre insulation. I'm hoping that this combination of hard but thin and well insulated firebox components will provide the best of both worlds as regards durability and performance.
What do you think?  

Since your question is a little off topic for the other thread, I'll add my input in this thread:)

20mm ceramic fiber blanket (CFB) insulation is a little thin for that application, since it will be compressed a bit when used between the firebrick splits combustion unit and the encasing brick (or cob, stone, or etc.) Personally, I'd double the insulation thickness. I.e. my 6" batch is constructed of full size (2.5" thickness) firebrick, with 1" CFB between that and a solid brick surround of 7" thick (doubled common brick). The surround heats up nicely to being almost to hot to touch after firing the stove 4 to 5 hours straight.