[AR] Re: Latest test results
- From: Robert Watzlavick <rocket@xxxxxxxxxxxxxx>
- To: "arocket@xxxxxxxxxxxxx" <arocket@xxxxxxxxxxxxx>
- Date: Thu, 3 Oct 2013 14:50:58 -0500
Viton or Teflon. I've been able to use SAE o-ring fittings with viton seals on
the LOX manifold and haven't noticed any leaks (had a camera pointing right at
it during the run). I also didn't see any leaks there with my LN2 testing. But
that port design is different than the typical rod end seal arrangement I was
going to use on the tanks.
-Bob
On Oct 3, 2013, at 13:59, Paul Mueller <paul.mueller.iii@xxxxxxxxx> wrote:
> 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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