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
