### Post by satamax on Nov 5, 2014 3:16:07 GMT -8

Here's a copy of what i explaned to Allen Lumley at permies. And thought it could be usefull to keep it here for newbs.

Hope this can be of some use.

Ring projection, the projection of the tube's wall into the barrel gap.

I mean, the theoretical surface of the wall of the exhaust tube as if it was completely inserted in the barrel up to the heat riser, comprised between outer shell of the heat riser, and the barrel.

For example, you have a flue or exhaust tube of 6"

That's a perimeter of 6"x 3.1415926 or so that's 18.84 inches.

Multiplied by the gap let say 2" = 37.69²

Heat riser CSA of a 6 incher 28.27

28.27² x 1.5 =42.41² So a gap of 2 inches with a 6 inch exhaust tube isn't good enough.

42, 41/ 18.84= 2" 1/4 so the gap needs to be at least 2.1/4 to have a ring projection of 42,41 square inches necessary for good performance.

If you aim for 3 times the CSA,

You could reverse it this way

2 inch gap

28,27²x3 = 84.82² /2 = 42,41/3.1415926 = 13.49 inches in diameter for the "transition area"

Allen, that's it, the theoretical surface of the projection of a perimeter into a gap, = ring projection to me.

And that has to be at the absolute minimum 1.5 times the CSA. Would it be on top of the riser or at the side of the barrel for the transition area. Up to 3times the csa for the transition area which seems to be the "safe" number. For the top of the riser, even several feet can be done no prob, so it's a minimum we're looking at, not maximum. That 3 times figure arose, i think because it was a convenient number for newbs.

At 4 times CSA, you get to the lower limit for bell's . Well, let me explain, a bell has to have a horizontal cross sectional area of at least 4 times the CSA to work at all. There's no upper limit, exept the ISA.

So again, that brings us back to that 3 times the CSA for that transition area, otherwise it might start acting as a bell if bigger.

A little side note; for example, heat riser's gap can't be smaller than that 1.5 times CSA, because of the laminar flow effect. The gases at the inside corner of the "elbow" where they exit the heat riser and turn; are completely stalling, and each layer goes a little faster as they're more distant form a surface. Thus creating turbulences and reduction in pression, which in turn slow down the flow. The top of the barrel stalls the gases too, due again to friction. The top of a heat riser can be made better, being rounded outwards, , like a rounded funnel. (trumpet bell) That means that the ring projection is bigger too (and helps the transition of the flow from one direction to another) . The absolute contrary of what everybody does

I suspect that the whole sheebang of a rocket could be tuned to a certain note, due to the hemoltz resonances. (at the top of the heat riser, enlarging it as i have described, would drop the note down)

Well, i'm going astray, due to previous life experience

I mean, the theoretical surface of the wall of the exhaust tube as if it was completely inserted in the barrel up to the heat riser, comprised between outer shell of the heat riser, and the barrel.

For example, you have a flue or exhaust tube of 6"

That's a perimeter of 6"x 3.1415926 or so that's 18.84 inches.

Multiplied by the gap let say 2" = 37.69²

Heat riser CSA of a 6 incher 28.27

28.27² x 1.5 =42.41² So a gap of 2 inches with a 6 inch exhaust tube isn't good enough.

42, 41/ 18.84= 2" 1/4 so the gap needs to be at least 2.1/4 to have a ring projection of 42,41 square inches necessary for good performance.

If you aim for 3 times the CSA,

You could reverse it this way

2 inch gap

28,27²x3 = 84.82² /2 = 42,41/3.1415926 = 13.49 inches in diameter for the "transition area"

Allen, that's it, the theoretical surface of the projection of a perimeter into a gap, = ring projection to me.

And that has to be at the absolute minimum 1.5 times the CSA. Would it be on top of the riser or at the side of the barrel for the transition area. Up to 3times the csa for the transition area which seems to be the "safe" number. For the top of the riser, even several feet can be done no prob, so it's a minimum we're looking at, not maximum. That 3 times figure arose, i think because it was a convenient number for newbs.

At 4 times CSA, you get to the lower limit for bell's . Well, let me explain, a bell has to have a horizontal cross sectional area of at least 4 times the CSA to work at all. There's no upper limit, exept the ISA.

So again, that brings us back to that 3 times the CSA for that transition area, otherwise it might start acting as a bell if bigger.

A little side note; for example, heat riser's gap can't be smaller than that 1.5 times CSA, because of the laminar flow effect. The gases at the inside corner of the "elbow" where they exit the heat riser and turn; are completely stalling, and each layer goes a little faster as they're more distant form a surface. Thus creating turbulences and reduction in pression, which in turn slow down the flow. The top of the barrel stalls the gases too, due again to friction. The top of a heat riser can be made better, being rounded outwards, , like a rounded funnel. (trumpet bell) That means that the ring projection is bigger too (and helps the transition of the flow from one direction to another) . The absolute contrary of what everybody does

I suspect that the whole sheebang of a rocket could be tuned to a certain note, due to the hemoltz resonances. (at the top of the heat riser, enlarging it as i have described, would drop the note down)

Well, i'm going astray, due to previous life experience

Hope this can be of some use.