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Tank burst test, Engine scale-up, Plasma cutter

November 1, 2003 notes

November 1, 2003 notes

 

Tank burst test

 

We hydrostatically tested one of the Structural fiberglass tanks this weekend.  The 24” x 50” tanks we use on the subscale vehicle have plastic screw threads, which we were confident would be the first thing to go, so I ordered a somewhat larger tank that had a bolted flange.  I had hoped that the single flanged tank would have a complete fiberglass overwrap on the non-flanged end, but it turned out that it has a cast aluminum integral piece that was probably used to hold the tank during the winding process.  To my surprise, it had a few pockets of exposed liner on this bottom flange, which I expected to be the first part to let go.

 

We filled the tank with water, then filled our trailer test tank with water, and connected that through a very small orifice line to the top of the tank, then began pressurizing it.  As always, these tanks make a distressing amount of snapping and popping sounds as the pressure is increased.  We took it up 50 psi at a time to 600 psi, let it sit there for a while, then relieved the pressure.  We took it back up slowly again, which it did without much popping, and let it sit at 600 psi for a little while.  When we took it up 50 psi more, the bottom flange burst.  When we looked at the bottom, it appears that the aluminum flange broke before the liner could extrude through the holes, although there were significant visible stress lines on the exposed liner.

 

We will probably try this again with a double-flanged tank.  We have heard an anecdotal reference from a guy that tested one of the big tanks at Rotary Rocket to over 1000 psi, at which point it blew the gasket, rather than bursting.

 

After bursting the tank, we cut a dome off with an angle grinder to examine the liner.  The polyethylene liner is about three times thicker than the actual fiberglass overwrap, and accounts for half the weight of the tank.  We had entertained some thoughts of trying to remove the thick liners and replacing them with a thin thermal spray coating of polyethylene to save weight, but the liner is stuck to the fiberglass extremely tightly.

 

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Engine Scale-Up

 

Scaling from the 1.25” throat nozzles to the 2” throat nozzles has proven a bit more difficult than expected, but we are almost there.  We believe that the top three-pass catalyst is still fine for the greater flow, but there are still several variables that we are juggling: amount of catalyst bale, compression on the catalyst bale, support for the catalyst bale, and space between the three-pass and the first catalyst bale.

 

We made 12 engine tests on Saturday alone, bolting, welding, and packing together a whole lot of combinations.  A few things we have learned for sure:

 

Compressing the bale to 1000 psi gauge (about 1500 pounds over 5.5” diameter) causes a few problems – the restriction before the nozzle causes quite loud bangs on the initial preheat pulse, and the hard packed bales have a tendency to have holes burned through them, likely at the locations where there were somewhat easier paths through the otherwise nearly solid pack.  660 grams of catalyst at 1000 psi pressing gave far too much back pressure, to the point that the big nozzle didn’t flow much more than the small one.

 

Incremental compression is a significant issue.  The long chamber was always just hand filled with bale because the glow plugs protruded into the chamber, preventing us from using the press, but the bottom couple pieces of catalyst were compressed at least as hard as the ones we pressed to 1000 psi in the shorter chamber.  Some incremental support is necessary, as we started to do with our 90% peroxide screen packs.

 

The flow from the center 4” diameter outlet of the three-pass catalyst does not spread out evenly to the full 5.5” or 6” diameter of the chamber as rapidly as we would like.  The runs that only had mildly compressed catalyst in short chambers would have a clearly visible dark center where the mass flow was higher, even when they had bright red outer areas.  The same amount of catalyst in the long chamber did not show the dark center, and ran better.  We had hoped that 2” or so of depth would allow an even spread, but it looks like it may take more like 4” or more.  We may try forcing this with an intermediate spreading plate of only 10% or so open area after the 3-pass pack.

 

A mixture ratio of 2.5 : 1 by volume does work fine, and runs a bit cooler than our normal 5 : 1 ratio, but is a little harder to preheat.  A 3 : 1 ratio burns noticeably hotter than our normal ratio, based on the color of the chamber glow.  A 4 : 1 ratio would be max performance, but would almost certainly melt the engine on a long run.

 

We really thought our final test would do it – we made a brand new chamber section with 6.5” of tube between flanges, and we used three 330 gram batches of catalyst, each supported individually by a perforated plate, and each pressed down to 1.5” of height, which left 3” between the top of the catalyst and the 3-pass pack.  This worked fairly well, but there was still a central flow problem, and, interestingly, in the video and pictures you can clearly see that only two bands of catalyst pack are heating up.  The top pack section did not have a plate welded down on top of it, so it had probably relaxed into a much looser configuration, which may not be very effective.  On Tuesday we will rig something to press that down consistently, and possibly add a central deflector blank to divert some of the central mass flow.  In hindsight, we probably should have pressed the individual layers down to 1” thick instead of 1.5” thick.

 

Once we get the 2” nozzle with 6” chamber working right, we will be ready to scale to the 4” nozzle with 12” chamber.

 

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http://media.armadilloaerospace.com/2003_11_01/finalEngineFire.mpg

 

Plasma Cutter

 

We finally got around to buying a plasma cutter.  It is the perfect tool for cutting out discs of thick perforated metal, which we have had to do quite a bit lately.  We wanted something that could cleanly cut at least ½” thick aluminum plate, but a good deal showed up on one that can cleanly cut 1” aluminum or steel plate, so we went ahead and got it.  We need to buy some jigs for cutting nice circles with it.

 

Making precision parts on the lathe and mill is nice in its own way, but I often feel that if you have a plasma cutter, a TIG welder, and a big enough pile of metal, that you can build just about anything…  It won’t look very pretty, but it is fast to put together.

 

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