[AR] Re: Latest test results
- From: Paul Mueller <paul.mueller.iii@xxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Thu, 3 Oct 2013 12:59:40 -0600
What o-ring material will you use for the LOX tank?
Paul M
On Thu, Oct 3, 2013 at 6:35 AM, Robert Watzlavick <rocket@xxxxxxxxxxxxxx>wrote:
> The other problem is that the linear slide has some static friction, on
> the order of a couple of pounds. However, when the engine is running,
> there probably is enough vibration to take that out.
>
> In other news, I started prototyping propellant tanks last night, using 5
> inch OD x 0.125 wall 6061 tubing. I had to make a plug to grip it in the
> lathe headstock without collapsing the tubing. I need to make some more
> fixtures to hold everything steady but I was able to true up one end
> without extraordinary effort. I will have to make a new large OD steady
> rest however. I'm going to try with an all machined designed since I don't
> have a TIG welder. Plus I like the idea of being able to take everything
> apart for inspection, repair, etc. The end caps will act as tank bulkheads
> as well as tank section coupler. They will use o-rings for seals, similar
> to hydraulic cylinders and 12 radial #10-32 pan head fasteners to hold it
> all together. I know shear load on threads isn't the best (hi-loks would
> be better with the nut on the back side) but the load on each fastener is
> so low it should work, especially if I use thread inserts. I found some
> NAS bolts that have a partial shoulder so the thin wall tank portion won't
> be touching the threads. Some leakage will occur past the o-ring for the
> LOX tank which may be tolerable if it's low enough but cold tests with LN2
> will show for sure. Spring loaded shaft seals would fix that but there are
> no standard off-the-shelf parts and it would be ~$500 to get some made
> (which is still cheaper than a TIG welder...)
>
> -Bob
>
>
>
>
> On 10/03/2013 01:28 AM, Ben Brockert wrote:
>
> 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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