Approx btu/hr output of 5", 6" & 8" Rocket Systems

[woodrascal]

As per the subject line - does anyone know the approx btu/hr output of 5", 6" & 8" Rocket Systems?
I realise there's a lot of variables involved but it's just to give me some help with an idea.
Many thanks.

[Deleted]

Heat of combustion
Carbon MJ/kg 32.8 BTU/lb 14,100 kJ/mol 393.5

Completely dry wood consists of about 50% carbon.
With a moisture content of 20% about 40% carbon.
With a moisture content of 10% about 45% carbon.

If ten lb with 20% moisture are burned in one hour = 56,400 BTU/hour.
If ten lb with 20% moisture are burned in 40 minutes = 84,600 BTU/hour.

[grizbach]

This doesn't directly answer your question either, but I hope it helps.

Skin friction plays a big part in the smaller rockets.
I made a 5"er a while back. Later I expanded it to about a 5.75".
With only 32% more csa I got about twice the heat out of it.
That is why you see most builders recommend 6" minimum.

[woodrascal]

Thanks for the replies Gents!

Karl - I'm a bit confused by the figures. Are these based on 'optimum' combustion and how do these relate to my initial question about the btu/hr output based on the size of a rocket system? Are you saying it doesn't matter what size the system is?

Grizbach - Thanks for passing on your experiences. Do you think the heat output would continue to increase based on the CSA of the rocket system, or does it eventually peak at some point?

[cab]

If you figure roughly 6000 btu/lb of wood, and maybe an 80% efficiency, you should get roughly 5000 btu/lb per hour out of your stove. How many pounds of wood can you burn per hour? I would guess maybe 20 pounds per batch or so in a 6 inch system, which might take maybe 2-3 hours to burn, so maybe 30,000 btu/hr approximate heat output....

[Deleted]

Nov 15, 2013 10:13:02 GMT -8 woodrascal said:

Are these based on 'optimum' combustion and how do these relate to my initial question about the btu/hr output based on the size of a rocket system? Are you saying it doesn't matter what size the system is?

A rocket comes close to "optimum' combustion".
The size of the system determines how much wood can be burned at once.
In very small systems much of the heat will be lost in the mass of
burn chamber and raiser.
Larger systems are thus better.
If you know how much wood a system can eat then you can easily calculate BTUs/hour.

[Donkey]

This question comes up a lot. Would you guys mind posting the info (plus calcs) into the "Reference Library" section??
Thanks