[AR] Re: Latest test results
- From: Ben Brockert <wikkit@xxxxxxxxx>
- To: Robert Watzlavick <rocket@xxxxxxxxxxxxxx>
- Date: Thu, 3 Oct 2013 00:28:04 -0600
Thanks for putting up the photos, Robert. I can see how the hoses would put
forces on the loadcell when pressurized.
With some additional hard line and a shorter fuel line it looks like it
could be made to fit, but as long as the pressure transducer is working
well it may be unnecessary effort.
On Fri, Sep 27, 2013 at 10:21 PM, Robert Watzlavick
<rocket@xxxxxxxxxxxxxx>wrote:
> The last 3 photos on this page show the hose arrangement:
> http://www.watzlavick.com/robert/rocket/testStand/index.html
>
> Let me know if you want higher resolution photos. The fuel hose is a
> Swagelok SS-6BHT-12 and the LOX hose is a SS-FL8TA6TA6-12. Originally, I
> went to flex hoses instead of hard tubing to try and bring in the hoses
> from the side to cancel out the forces. But I ended up needing the
> flexibility just to make it all fit and still have some room left for
> adjustments.
>
> -Bob
>
>
> On 09/27/2013 03:22 PM, Ben Brockert wrote:
>
> Bob, is there a good photo or diagram of your current hose layout on the
> stand? I've done it a few different ways in different designs. The force of
> hoses on the stand is an interesting engineering problem, and it comes up
> in gimbaling the engine as well.
>
> Ben
>
>
> On Wed, Sep 18, 2013 at 6:30 AM, Robert Watzlavick
> <rocket@xxxxxxxxxxxxxx>wrote:
>
>> Well, that's part of the problem as it's difficult to correct for the
>> force of the hoses. From what I understand, in wind tunnel testing of
>> blown ducts where force measurement is important, the professionals go to
>> great lengths to bring the hoses in the side at 90 degree angles with
>> flexures, splitting the mass flow equally between each side. In theory, if
>> the hose is very flexible compared to the test setup (usually not the
>> case), it is straightforward to calculate the error since it is just the
>> pressure in the line acting over the cross sectional area in the direction
>> of the line. Just imagine a free body diagram of the engine. But even the
>> flex hoses are pretty stiff so that doesn't really work. For my setup, I
>> ran some cold water tests at full pressure and observed apparent force
>> readings anywhere from 0 to 10 lbf depending on how the hoses were
>> oriented. I even noticed that when my tanks were pressurized (but before
>> the hot fire run), I saw a few lb of force exerted just due to things
>> moving around on the test stand under pressure.
>>
>> Somebody asked me a while back why I didn't just use Pc to compute thrust
>> and I didn't have a good answer at the time. I was worried about leaks but
>> after looking at the data and seeing it line up well, I'm convinced that if
>> you have a good chamber pressure transducer, know the throat diameter, and
>> have good estimates of thrust coefficient, it's probably more accurate than
>> directly measuring thrust from a load cell.
>>
>> -Bob
>>
>>
>> On 09/18/2013 06:47 AM, Graham Sortino wrote:
>>
>> If you have a moment could you explain how you calculated you the
>> force impact of the flex hoses, etc on the load cell? I'm in the process of
>> designing a new test stand and I've been thinking about how best to
>> compensate for this.
>>
>> For example, If I have an engine that measures 40lbs of force on a load
>> cell and I measure there are 10 lbs of force from hoses and things keeping
>> it in place I'd assume the engine thrust is actually 50? What I'm not sure
>> of is how to calculate the counteracting force of things trying to keep the
>> engine in place.
>>
>>
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