The dewar was at 20 psi in this case. Now I have to clean out the LOX
tank and all the plumbing since some crumbs may have made it
downstream. I have a LOX filter but it was upstream of this particular
line. Lesson learned!
-Bob
On 09/11/2016 11:53 PM, Andrew Burns wrote:
Re. the mud daubers, from experience spiders can hold about 5 psi quite comfortably...
Andrew
On Mon, Sep 12, 2016 at 2:49 PM, Robert Watzlavick <rocket@xxxxxxxxxxxxxx <mailto:rocket@xxxxxxxxxxxxxx>> wrote:
Oh, and I forgot the real excitement that happened. The one 3/8
inch tube I forgot to cap off that was sitting in my garage the
past two months got clogged by mud daubers. I opened the dewar
and the tank wouldn't fill at all. It took a lot more effort than
I expected to clean it out.
-Bob
On 09/11/2016 09:45 PM, Robert Watzlavick wrote:
I finished up the last of the water and LN2 cold tests I
planned for my 250 lbf LOX/kerosene rocket. The goals for the
tests were:
1. Test an alternate LOX loading scheme to reduce the tank
ullage - Ben's suggestion worked great. After filling, I
closed the LOX vent valve first, waited a few seconds, then
closed the LOX fill valve. This improved the pressurization
time of that tank from 2 seconds to around 0.6 seconds. Also,
the helium pressure only dropped about 200 psi instead of 400
psi when pressurizing the tanks.
2. Adjust the pressure regulator setpoints - The Aqua 1247
regulators I'm using have quite a bit of droop under the flow
rates I'm using so I was able to tweak them statically to give
the desired pressures under full flow. During the run, the
pressures drop a bit which will change the effective mixture
ratio from around 1.87 at the start of the run down to 1.81 at
the end. The LOX pressure falls off faster during the run
even though it is a lower pressure (340 psi) than the fuel
(500 psi). We previously discussed this and I believe it is
due to the cooling of the helium as it enters the LOX tank,
probably not due to absorption in the liquid.
3. LOX insulation on tank and fill lines - I used 1/2 inch
rubber pipe insulation on the LOX tank (in 4 sections) and
fill lines which reduced the fill time from 8 minutes down to
about 4 minutes. The Aerogel/Pyrogel option looks like a
better alternative for the tanks since it is thinner. An
alternative would be to use 1/4 or 3/8 inch thick pipe insulation.
4. Helium pressure vs. run time - In my previous tests using
2200 psi helium, I wasn't even able to get 10 seconds of run
time. By using a 3500 psi supply cylinder along with the tank
ullage technique above, I was able to load the onboard tank to
about 2500 psi and that was enough for about 17 seconds of
runtime, more than enough for a first flight. I'm planning on
10 seconds or less but I want enough helium in the tank to
account for slow leaks during pad operations.
5. Sticky LOX vent valve - I wrapped the LOX vent valve in a
4x6 inch poly bag with appropriate cutouts and ran a slow
stream of nitrogen through it to displace all the ambient
air. The idea was to prevent ice from forming on the valve
body and then dripping back into the bearing on a subsequent
fill operation. This worked as expected so I'll probably do
the same thing to the LOX fill valve and main valves. It also
keeps condensation from dripping on to the servos when the LOX
tanks warms up.
Thanks everyone for your suggestions and advice. The next
steps are do a hot fire with the rocket strapped down so I'm
looking for ideas on how to hold down a lightweight
stringer-based vehicle (no skin yet) for a vertical test. I
don't have a launch rail here so I was thinking of gripping
two of the four stringers and clamping them to my test stand. Another idea was to tie into the engine/frame bracket somehow
to handle the thrust loads and use guy wires to keep the top
of the vehicle pointing upwards. An unplanned launch would
definitely not be a good thing! I could also use some
suggestions on how to safely redirect the exhaust so I can
still take video and pictures during the firing. I've seen
curved or angled plates used.
-Bob
