In theory, yes. = placing APCP grains with higher metal content furthest away
from nozzle provides longer resonance time in chamber and can provide higher
combustion efficiency.
BUT:
- APCP motor equations assume uniform APCP composition inside motor. If using
propellant grains with different formulas, things get very complex in a hurry.
To name a few things:
: Combustion chamber has length. Need to understand that nozzle restriction
creates a internal feedback (or wave reflection). Hence non-uniformity in
combustion elements will create non-uniform pressure waves inside chamber.
This can increase time required to reach steady state combustion pressure/temp.
Depending on grain design (faces, boundaries, ends, etc) and frequency of
pressure waves created, can result in significant additive or subtractive
localized pressure values (boom or chuff results). Even high end finite
element modeling has challenges simulating these complex conditions.
Hint - Must design space BETWEEN grains to avoid localized pressure spikes.
: Metal is uncompressible element in combustion reaction. Formulas with high
levels of uncompressible elements create additional erosion inside chamber and
on nozzle throat; both of which are highly variable based in mass flow rates,
pressure and temperature. If you only have one erosive area/grain in a motor,
non-uniform chemistry can be tolerated; but if several lower grains
experiencing erosion, plan on loads of trial and error.
That said:
One practical application for "spice up the topmost grain with higher metal
content" is done for opposite reason(s), I.E. using different bottom grain
formulas; to reduce erosivity of high metal loading, or improving plume
color/smoke generation.
Example: For commercial reloads on motors with very high L/D ratios (like a
3"OD, 60" long 10KNS motor), it was common to use a special bottom grain (or
two).
The bottom grain was designed to reactive favorably to erosivity by using a
formula with different burn rate exponent, lower combustion temperature, and
smaller particles.
Since APCP placed nearest nozzle typically exits nozzle with least amount of
combustion. It occasionally was helpful for the bottom grain to have extra
rubber or zinc to generate more smoke in plume (white smoke amplifies color),
and/or to provide higher levels of color generating chemicals. These surplus
chemicals would react in flame plume to enhance color and/or reduce washout
created by high plume temperatures due metal loading. This does almost nothing
for ISP, but makes for better looking flame plume if you care about such
things. :)
Thanks for reading.
My apologies to experts who already know about the above APCP design
challenges...
Jim
-----Original Message-----
From: arocket-bounce@xxxxxxxxxxxxx [mailto:arocket-bounce@xxxxxxxxxxxxx] On ;
Behalf Of Uwe Klein
Sent: Tuesday, April 10, 2018 12:35 AM
To: arocket@xxxxxxxxxxxxx
Subject: [AR] Re: Solid Motor Length & Volume (Was Re: Aluminium vs Magnalium)
Am 09.04.2018 um 20:37 schrieb Henry Vanderbilt:
This came up earlier, with several mentioning motor length as
determining how high a metal particle fraction could usefully be
included in an APCP solid motor.