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>
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
