32 replies to this topic

[BrightStar]

Passfire has an excellent article 'Clay Rocket Nozzles', complete with post-burn cutaway analyses of motors - well worth reading.

In summary, amongst many other things, they confirm that a flat inner surface on the nozzle leads to a quick burn-through of the cardboard casing adjacent to it, while with a conical inner surface it survives much better. Their explanation wouldn't score highly in a fluid dynamics exam but the photographic evidence seems convincing.

The secondary advantage claimed is that given a certain total plug length, the exhaust gas path through the narrowest part is shorter with an internal conical indent, hence less exhaust friction and more thrust is possible before blowout.

Edited by BrightStar, 05 July 2007 - 09:42 PM.

[dr thrust]

hi, is nozzle plug length critical?

[Pyromaster2003]

The nozzle is always 1 x the ID of the tube so if you have a 20mm ID tube you have a 20mm long nozzle. The choke iv found the best is 1/5th the ID of the tube.

If you notice about the passfire article the tube that burnt through had a longer nozzle which will make more pressure and force on the tube, the one that never burnt through had a thin nozzle, and a cone shaped outer (aswel as inner) clay end making the actual ID even smaller.

I have done more tests and they burnt through even with a cone shaped ram, its strage coz i made loads of end burners before and non of them did this, the bp im using now it faster so the extra bit of pressure in the tube is obviously too much for even commercial thick walled tubes. From the sound of these (my camera doesnt pick up the sound as it is) the pressure is IMMENSE believe me! Quite scary sound like its about to go BOOOOOOOM. Anyway iv come up with another idea i will try when i have time:

http://www.pyrotubes...rnerdesign.html