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 <mailto: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 hoseis 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 andbring 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 <mailto: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 tocompute thrust and I didn't have a good answer at the time. I was worried about leaks but after looking at the data andseeing 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.
