The Vertical Static Test Structure is complete and passed the fit checks this weekend. It took a couple of tries to figure out the best way to install the strut and rocket without injury. The braces between the test stand and the 8020 strut definitely help to stiffen up the structure. I plan to remove all existing hardware from the test stand before the hot fire so I don't damage it in case something goes wrong with the vehicle static test. A closeup of the engine mount shows how the thrust loads are transferred directly from the engine to the test stand. The top of the vehicle is just held in a cradle made of 2 inch angles using hose clamps around the LOX tank. I ended up adding a 0.25 inch spacer between the top of the injector plate and the engine frame to fix an interference problem between the engine fuel fitting and the engine frame.
To allow better testing of the launch sequence in the state machine, I added a simulation class for the sensors that monitors the current state. That way I can fake out the sensor message to verify nominal and off-nominal paths in the state machine. I already found two bugs so this is definitely a worthwhile effort.
The test stand blast deflector for vertical testing is complete. It uses three 24 x 24 x 2 inch concrete pavers at 30 degree increments to deflect the exhaust horizontally down the flame trench in my test pit. The frame is made of 1.5 x 3/16 inch steel angle. A negative step of about 0.1 inches between pavers will hopefully minimize erosion on the leading edge of the downstream ones.
I had to re-spin the signal conditioning PCB used for the bridge sensors. The original design used an INA125 instrumentation amplifier in a single supply configuration which limits the bridge common mode voltage to a range of about 1 to 4 V with a +5 V supply. I didn't think that was a problem because the pressure sensors I used for checkout were all within that range. However, some new Kulite sensors I purchased for the vehicle had common mode voltages around 0.75 V which was out of the amplifier's range with a single-ended supply. The fix was add a TL7660 negative voltage converter to generate -5 V so the op amp can handle inputs closer to 0. Even with a fair amount of filtering, there is still about +/- 10 mV of ripple on the -5 V rail but I ran it through the full input and common mode range and didn't observe any adverse effects. I also took the opportunity to calibrate the three new transducers I recently purchased off eBay.
To minimize the chance of overheating the Digi XT-09 modem, I added a heatsink bracket that conducts the heat away from the back side of the XT-09 into the aluminum airframe longerons. Tests showed the internal temperature dropped about 23 deg C with the addition of the heatsink when running at 1 W. Since there won't be a lot of convective airflow in the avionics bay while sitting on the launch pad in the sun, I may end up adding a small 5 or 12 V fan as cheap insurance against the flight computer overheating. The CDI box and power panel have been mounted along with a battery holder. The electrical tasks remaining are to mount the GPS antenna, mount the TM antenna, and to fab the cable between the CDI sense lines and signal conditioning board.
On the engine, I replaced the aluminum Cv plugs that were used to close out the cross-drilled converging holes with threaded holes for use with 1/4-28 bolts and Parker Stat-O-Seals. I had wanted a way to clean out those passageways but couldn't find a suitable seal at the time. I plan to use the fluorosilicone Stat-O-Seals which should be good to about 400 deg F for a short time test. Opening up those holes allowed me to use a wire tube brush to clean out the gunk and mild corrosion left over from some water tests I ran last fall. All the plumbing for the vehicle has been re-cleaned and is ready for the upcoming hot-fire.
To hold down the rocket and test stand during the vertical test, I had originally planned on pouring a concrete pad in the test pit and bolting the test stand to the pad with concrete anchors. However, I discovered earth anchors and I believe they will be less work and should be just as effective for this application. There are many types but the ones I found are steel rods with a corkscrew plate on the end that you screw in the ground. They have >1000 lb of holding power each so I plan to use four of them with appropriate steel cables to tie down the test stand and keep the rocket from flying away during the test.
The next task is to build the support structure to attach the rocket vertically to the test stand.
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