*<<How do you know that all the products in the chamber are gaseous unless
you know for sure what the temperature is in the chamber? Have you measured
the temperature in the chamber during the burn?>>*
That is a good question. When designing a motor, you don't have the option
of measuring the actual combustion temperature since the motor has not yet
been built. As such, you simply make the assumption that GUIPEP prediction
of combustion products and temperature are right. Typically throw in a
knock-down factor for temperature of a few percent, as we don't expect to
get perfect combustion efficiency. We design and build the motor on this
basis, do a static firing taking thrust and perhaps pressure measurements.
If the results are in reasonable agreement, we're happy. If not, and you
suspect combustion may be a factor, we can then endeavour to measure
combustion temperature. Which, unfortunately, is not so straightforward to
do with great accuracy. And may simply lead you astray :-\
Richard
On Wed, Mar 7, 2018 at 2:36 PM, Bruce Beck <bbeck7@xxxxxxxxx> wrote:
How do you know that all the products in the chamber are gaseous unless
you know for sure what the temperature is in the chamber? Have you measured
the temperature in the chamber during the burn?
On Wed, Mar 7, 2018 at 11:08 AM, Richard Nakka <richard.rocketry@xxxxxxxxx
wrote:
Dave,
Interesting formulation. Quite cool burning, even cooler burning than
sugar propellants, which means you can use a steel nozzle, nice.
You ask a good question as to whether you'll have to consider 2-phase
flow. Approximately 9% of the exhaust mass is condensed phase consisting
mostly of Carbon with some Ferrous oxide, this explains the black smoke.
However, in the chamber you have all gaseous products. As the combustion
products flow through the nozzle, the temperature drops and the condensed
phase products form. This is different than with sugar propellants, where
the condensed phase products form in the combustion chamber. So your case
is a little trickier to analyze.
I would expect that two phase flow need not be considered since the
particles would form after the products have passed the nozzle throat. At
that point the particles would be travelling at the same velocity as the
gases (that they form from).
Richard
On Tue, Mar 6, 2018 at 9:57 PM, David Snyder <dvdsnyd@xxxxxxxxx> wrote:
Hi Richard,
Thank you for getting back to me.
I had thought that the answer could likely be as simple as you pointed
out.
A further inquiry:
I am working on a HTPB based propellant where I am trying to run through
the theoretical values as done in the technical notepads on Nakka's site.
The formulation is called "Pitch Black" It is a fairly low Isp composite
propellant that is easy to make and produces a dense black smoke.
I have to believe that 2 phase flow would have to be a factor with this
propellant due to the thick smoke it produces.
However, when running Guipep, there are no condensed combustion
products. However, in the exhaust flow results, there are a couple products
that are condensed.
Does anyone have any insight on how to treat the theoretical
calculations with this propellant. I understand it is not a sugar motor,
but I appreciate any insight and guidance you could provide.
I've included an attachment a sample guipep run.
Again, Thank you all for your help!
Dave
On Mon, Mar 5, 2018 at 12:35 PM, Richard Nakka <
richard.rocketry@xxxxxxxxx> wrote:
Dave,
Glad to hear about your interest in the science of rocket propellants.
Good question you asked, straightforward answer.
H3, KCN, CHF, CH2O, etc were left out of the calculation because these
are present in such tiny quanities. If one took the extra effort to include
these, you'd find that the final values for isentropic exponent, etc, would
be unchanged. Good exercise to try perhaps :-)
Richard
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On Sat, Mar 3, 2018 at 1:52 PM, David Snyder <dvdsnyd@xxxxxxxxx> wrote:
Hi Everyone,
I've been lurking for a while. There have been some very interesting
discussions so far. A little background on me - I have worked on Sugar
based propellants in the past, mostly KN-Sorbital, and KN-Erythritol. My
father and I have successfully flown a J powered KNER rocket to over a
mile
at our peak in sugar propellant. Our success wouldn't have been possible
without help derived from sites by Jimmy Yawn, Richard Nakka and Scott
Fintel. Thank you!
Now, We are looking at getting into composite propellant, but I am
trying to learn more about Guipep and the science behind propellant
development and formulation. For this, I am going back to sugar, since so
much of the theoretical side is documented by Nakka. I am trying to
replicate some of the theoretical calculations he has done.
So, I began going through the technical notepads on Nakka's site.
Specifically the KNSU notepad:
http://www.nakka-rocketry.net/techs.html
I understand the math behind most of it, and was able to very closely
replicate the numbers he came up with, but I am having trouble identifying
why certain combustion products were left out in some of the calculations.
Specifically in note 2:
Nearly all of the combustion products from the Guipep run are listed
in the first table where the total moles, gas moles and condensed moles
are
calculated.
However, I'm unsure of why some of the products are left out, notably
I'm unsure of why NH3, KCN, CHF, CH2O, and CNH were left out, but OH
was left in.
OH has a smaller mole fraction than NH3 and others as far as I can
tell when evaluating the complete list in excel.
Again, in the second table where the isentropic exponent for 2 phase
flow and others is calculated, I'm unsure of how the final list is
derived.
Further combustion products are left out in this list. Such as OH, KH, H,
K
and others.
Guipep also provides an exhaust products results. However, one of the
underlying assumptions is that chemical equilibrium is established in the
combustion chamber, and does not shift through the nozzle. Although, upon
inspection. The products in these results to not follow with that of the
table used to calculate the isentropic exponent either.
I can only assume that there is a reason for these decisions, but
don't have enough hand's on/gut-check style of knowledge to figure out why
yet. Before asking about some other questions I have, I'd like to gain
some
insight on this if possible Would anyone be able to help shed some light
on
why various combustion products were left out in each of the calculations?
Thank you for all of your help,
Dave
