Combustion Ratios

[Essohbe]

Why are allot of people I read making cannons talking about trying for combustion ratios of 4:1 and 1.5:1, etc...

From everything I've read the optimal is less than 1:1 (I'm shooting for 0.7:1 for mine).

Am I right or are they wrong ?

Edited by jrrdw, descriptive topic title.

[PaperNinja]

I don't really know, but keep in mind that some of the ratios you saw might have been for hybrids, which tend to have much smaller chambers than 1x combustion.

[jackssmirkingrevenge]

The only real hard data I've seen available for chamber:barrel ratios is here:

http://www.burntlatke.com/launch.html

This seems to suggest the optimal ratio is in the region of 0.7-0.8:1

[ramses]

That data is for a constant chamber size, with variable barrel. If you have a fixed barrel length (like have a maximum overall length and an over/under design, I think the optimal ratio is closer to 2:1 (larger chamber). The general rule is that the bigger the chamber, the more powerful, but with quickly diminishing returns. That also assumes enough ignition points and turbulence to quickly burn through all of the chamber volume. If you have a lousy burn speed, it is conceivable that a small chamber would outperform an excessively large one.

Somewhere, there is data to back that up. I just don't know where.

[jimmy101]

This seems to suggest the optimal ratio is in the region of 0.7-0.8:1

That should read "most fuel efficient ratio". Whether fuel efficiency is important is up to the designer, though I suspect most spudders don't care about efficiency since fuel is basically free.

[Essohbe]

That should read "most fuel efficient ratio".

I saw a ballistic graph where they measured the muzzle velocity of a spud and all indications were that the .6-.8 to 1 ratio yielded that highest veolicty in FPS. Allot of it was marginal but that window there was allot better than the readings they were getting for something like 1.5:1, etc.... (bigger cannon theoughmind you, and my plan is a 4" chamber and 3" barrel).

[jimmy101]

That should read "most fuel efficient ratio".

I saw a ballistic graph where they measured the muzzle velocity of a spud and all indications were that the .6-.8 to 1 ratio yielded that highest veolicty in FPS. Allot of it was marginal but that window there was allot better than the readings they were getting for something like 1.5:1, etc.... (bigger cannon theoughmind you, and my plan is a 4" chamber and 3" barrel).

Not sure what data you are referring to but it is most likely Lake's data. That was acquired with a fixed chamber size and variable barrel size. For a fixed chamber size ~0.8 chamber to barrel gives the highest muzzle velocity.

However, if you take that optimal barrel and put it on a larger chamber the muzzle velocity will go up. The efficiency will go down but the performance will go up.