Dave Barr wrote:
I've been kicking around ideas for upping the initial burning surface area
for a cored nozzleless motor
shaped expansion molded into the propellant, while less efficient than a
cone, should increase the surface area a bit without substantially
reducing
the propellant volume.
Further in that vein, intentionally disinhibiting the exterior of this
expansion tetrahedron should increase the initial thrust spike, as well.
Can anyone point to research / reading in this area?
On Thursday, October 19, 2017, <spas@xxxxxxxxx> wrote:
Richard wrote:(regardless
Cool. Serge, you 'forced' me to do some quick calcs :-)
With core diameter 1/3 outside diameter and a grain length of 10x
outside
diameter, you have a kn range starting at 121 decreasing to 40
perhapsof OD).
For nozzleless motor Kn can be calculated by simple formula:
Kn = 4 * L / D
where L and D are core dimentions.
Using my KNSB design chart:
http://www.nakka-rocketry.net/design1.html#Charts
your initial chamber pressure is around 215 psi or 15 atmospheres,
consistent with what you reported. Clearly you have 'choked flow' with
your
core acting as a nozzle with sonic exhaust velocity.
That got me thinking. Perhaps a silly idea but what if you tapered your
core at the exit? To act as a divergent section, that should initially
give
you a boost in exhaust velocity (supersonic) and therefore thrust,
as much as 30% or so. What do you think? See attached graphic.
Yes, most industrial nozzleless motors have such divergent section. But
at
15 atm its effectiveness will be low and as pressure quikly drops,
effectiveness will quikly decrease even more. I plan to test motors with
much longer core and I hope KNSB can withstand much bigger pressure. If
these tests will be sucsessfull, than I will try to make divergent.
Serge Pipko
Kiev, Ukraine
http://serge77-rocketry.net
Richard
On Wed, Oct 18, 2017 at 11:29 AM, <spas@xxxxxxxxx> wrote:
Hi Trevor, the short ansver is - for a given propellant OD you need
core
diameter about 1/3 OD and core length 10 OD. This is for KNSB or other
candy propellant with similar burn rate.
But give me a day or two, I'll collect all my data for more
informative
ansver.
I'd love to read up about your nozzleless motors. They sound likethey
would be very cool. What size/shape core? How long of a casing? I'mpropellant.
thinking of trying nozzleless in 1.5" pvc pipe with 6" of
Just
have to figure out the core to actually produce thrust.
Trevor Mushung
