Well for my next subject post I'll try a simple one. When mixing fire clay with sand for refractory mortar
1- can I use silica sand? I have a big bag left over from my pool filter
2- Can I just add straw to this mix for cobing over burn tube/barrel, and making the transition area out of barrel in to the mass (gravel floor)
I've already had a system running for a couple days (only because of absolute necessity) but need to finish with a few more mods to get it done right. More on that soon.

1- can I use silica sand? I have a big bag left over from my pool filter

It should work OK. Sand reacts chemically with the lime in the mix to form calcium alumino-silicates, which are the strong molecules that bond the whole thing together. That reaction occurs on the surface of the sand grains, so physical properties like the fineness of the sand and whether it is rough or smooth make a difference. This is why bricklayers ask for "sharp" sand when they order a load so they can mix up mortar. "Sharp" sand has more surface area than smooth, spherical sand grains, and more surface to bond with. If the filter sand is extremely fine, then it is going to have lots more surface area, and hence more lime will be required in the mix to react with the increased surface area of the sand.

If you ever look at floor leveler concrete mixes, they use this extremely fine sand, and a consequence of it is that they end up with about twice the strength of regular concrete mixes.

2- Can I just add straw to this mix for cobing over burn tube/barrel, and making the transition area out of barrel in to the mass (gravel floor)
I've already had a system running for a couple days (only because of absolute necessity) but need to finish with a few more mods to get it done right. More on that soon.

Again, it should work OK. You may want to screen it so that you don't have any long pieces in it; anything longer than 2" is not adding strength to the mix, it is making it weaker. You can get away with a few long pieces of straw if you are making a cob wall or bench, but for your transition area, you need to take a little more care.

pool filter sand has large particles. makes a great sand for fish tank substrate though...

John Adamz : Question #1) is a clear yes ! Question #2) is a little confusing as it has several parts to be addressed, and may also confuse future readers (and myself ? ) !

You can build with layers of carefully compacted perlite within a Cob bathtub shape that will give support and then receive your combustion core which then gets more
insulation wrapped around it, alternately you can make a perlite and clay sip mix and use that under and around your Core, also bricks made out of clay slip and sawdust
and pre fired can be used in this location if properly supported by a cob bathtub shape that supports these materials, There is no advantage to using Structural Cob, cob
with straw in it, in this location as the straw will burn up, of all of the other combinations of materials I have listed are more insulating I Think !

While there are many ways of covering the outside of the barrel, insulative materials are a bad idea for the exterior of the barrel, we want free radiation and conduction
of heat energy off of the barrel, the straw in the structural cob will slowly pyrolyze and you will get free hydrocarbons and participates that will smell like smoke even if
you do not see 'the smoke' !

The transitional area which receives the vertically falling gases, and using a large funnel shape both direct the gases through a right angle turn to flow into the horizontal
chimney and form an ash pit and also contains the first clean out 'T' and cap.

This is an area where the highest degree of care must be shown, not only do you want an Over Sized Cross Sectional Area , you want the curves to be gradual and sweeping
and the walls to be as smooth as sheetrock, making an under frame at this location with hardware cloth, and then adding a rough cob and then applying a finish coat of cob
with your smoothest Cob ever, made up with your finest sand means screening the sand to get enough Fines to make a smooth finish coat, but is sure to pay you back for
the time and care you show here ! As you actually have a Working Rocket you tell us, this is not extremely timely, but a slightly fuller answer dealing with your planned
usages should remove error from the path of future fellow members traveling behind you ! For the good of the Craft! Big AL

The sand I have is very fine, I did use some in my salt water aquarium. For a given volume it would have a lot of surface area. Do I have to add the lime (and cement?) to the cob mix? I thought I read it is possible to just use the fire clay and sand for the cob. I also read that I can use wood ash and fire clay wood ash mortar. I have a bucket of refractory cement for setting the firebrick. However since I only have about $20 left to finish this project I might take it back and get the fire clay and other ingredients.
Alternatively can I mix the fire clay with quickcrete if I remove the gravel aggregate. I have 3 bags of that laying around.

John Adamz : I find that while I have used buckets of furnace cement, it was for making and sealing up of new smoke pipe openings in old chimneys and to reset
old hard fire brick in old furnaces that have long since been retired. Ceramists tell me that the portland cement in furnace cement is 10% or less, and has been
left there because even tho the lime in it will continue to fail in high heats, amounts less than 10% are not weakening and the portland makes a stiffer paste easier
to work with !

I cannot recommend any use of portland cement in the mortar mix, For my own Combustion chamber brick building i wet the fire brick in water and then in clay slip
if i have new bricks, for old red bricks and for a brick Heat Riser Tower i use a clay and sand mortar, and find i need 4 Xs the sand as my local clay! Big AL

I saw this cob recipe on a very nicely done youtube vid. It's 1 can clay, 3 cans sand, a handful of straw. I'm using the powder fire clay mix in a bag.
Sounds like a good enough recipe to go with. Anybody say different?
Lucky for me it's going to be in the high 60's for a couple of days. Should be good to help set up the cob to use when it gets cold again in 3-4 days.