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
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
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
didnt 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.
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 wont look very pretty, but it is fast to