[AR] Re: Estimating Coefficient of Discharge (Cd)
- From: "Graham Sortino" <dmarc-noreply@xxxxxxxxxxxxx> (Redacted sender "gnsortino@xxxxxxxxx" for DMARC)
- To: "arocket@xxxxxxxxxxxxx" <arocket@xxxxxxxxxxxxx>
- Date: Mon, 15 Sep 2014 14:38:38 -0700
thanks again for this. It sounds like I need to do more research on flow
meters. I was hoping they'd be a tad cheaper but I think I'm at the point where
it may be a worthwhile investment. As an aside, I hadn't noticed before that
you posted electronics schematics on your site but I found it in the test stand
section (makes sense). As always I appreciate very much all the detail you put
on your site. It's a tremendously helpful resource.
Additionally, if anyone has any perspective on my other question regarding
why/how cavitation occurs in a liquid flow I'd really appreciate it. I suspect
this is a basic concept that I'm not getting but I had never consider this as a
factor before since I assumed my liquid flows would always be above the vapor
pressure (hence cavitation should not occur?).
On Monday, September 15, 2014 10:32 AM, Robert Watzlavick
<rocket@xxxxxxxxxxxxxx> wrote:
The 20% pressure drop is the minimum recommended value to improve combustion
stability by isolating feed pressure and chamber pressure. It certainly can be
larger but then you're carrying around extra pressuring gas and probably a
heavier structure for the gas tank. I'm not sure about the rule of thumb for
gaseous propellants. For my igniter, I had a much higher gas pressure drop than
the main engine because the igniter pressure was lower than the main chamber
but they both have the same feed supply.
I use Cox AN and FTI series turbine flowmeters for the test stand. Pretty much
all of them have a magnetic pickup and put out pulses proportional to the
volumetric flow rate. You can use a frequency to voltage converter to get an
analog signal out of it. There is a schematic on my test stand page for an
LM2917 based circuit. With low tempco resistors and caps, the output is very
repeatable and linear. Or you can just measure the frequency directly with a
counter, microcontroller input, etc. The main thing to remember is liquid
flowmeters don't like gas blowing through them so you have to shut off the flow
before you run out of propellant. With the tank pressures on a typical
pressure fed rocket engine, just gas blowing through can destroy the bearings.
Both Cox and FTI have good application notes full of useful info.
-Bob
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