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.
For breakaway connectors on my rocket, I had originally planned to use DE-9 connectors and RJ-45 jacks w/o the tab. But I was having trouble figuring out to keep the protuberances to a minimum and I didn't like the the off-axis loads on the pins and shells. I remembered the MagSafe connectors for Apple laptops but I couldn't find anything off the shelf that was: 1) orderable, 2) had enough pins, and 3) was easy to integrate into the skin of a rocket.
It doesn't look too complicated so I decided to try making a custom version. I found these Mill-Max spring loaded pins which are a ready-made solution. Connectors with multiple pin arrangements are orderable from Digikey and Mouser. Some round magnets with a countersink are the last piece needed to make the connector. Some photos of the prototype:
The white acetal blocks are 0.5 by 2.0 inches and the pin spacing is 0.1 inches. The parts cost for the prototype is around $15. I went with the concave target pins instead of the flat ones for the vehicle side because I thought they would be better for the battery charging connections. The contacts are rated at 2 A continuous, 3 A peak. The spring loaded pins have a travel of 0.055 inches with a spring force of .15 lbf at mid-deflection so a connector with 8 of them needs at least 1.1 lbf of compression to hold it together. The magnets have a spec of 4.2 lbf but it's not clear if that's with another magnet or a steel plate.
To register the two assemblies together, I recessed the magnets by 0.02 inches on the vehicle connector and then let the magnets protrude an equivalent amount on the removable connector. The magnets are strong enough that if you get them anywhere close to each other, they just want to snap in place. Opposite polarity magnets on each end of each connector allow it to be assembled in only one orientation. I drilled two holes in each end of the removable portion to attach a lanyard so when the vehicle lifts off, it won't pull on the wires. The two halves separate cleanly as expected. To test the signal integrity of the connector, I cut a network cable in two and wired it to each side. I'm only going to use it at 100Base-T speeds (and you only need 2 pair for that) but at GB speeds, iperf shows 894 Mb/s in one direction and 938 Mb/s in the other. The original unmodified CAT-5E cable ran at 938 Mb/s.
Here's the production version with 18 pins:
The GSE (Ground Service Equipment) box will be located about 25 feet away from the launch pad (inside, rear). The enclosure is a Ridgid toolbox from Home Depot and contains the following:
Battery charger and ground power supply
CDI power supply for the igniter
Solenoid power supply
Beaglebone Black for remote reset and serial console debug of the onboard flight computer
WiFi router (cabled to the flight computer for ground operation) with extra ports on the rear
AC switch and receptacle for powering test equipment on the pad
To minimize onboard complexity, the flight battery connects directly to the ground power without a dedicated onboard charging circuit. This is not optimal from a battery life perspective but is commonly used in low-cost applications and is certainly acceptable here. An external li-ion charger won't work since it can't distinguish between the load and battery so I made an external current-limited power supply using an LT1185 which limits the charge to 2 A (safe charge rate for the battery). Trickle charging at 4.1 V/cell does not fully charge the battery but significantly increases the lifespan when using this scheme. The voltage drop in the cables, input diode, etc. are significant but adjusting the offboard regulator to approach 16.4 V (4.1 V/cell) as the charge current tapers off to 0 works well.
The vehicle power panel is finished and will be mounted in a bay near the flight computer. It has a 5 A circuit breaker for the onboard battery, an external power switch, and an ARM/SAFE switch.
To minimize interference between the TM transmitter and GPS receiver, I'm using a PCTEL 3961D-HR high-rejection active GPS antenna. With the TM transmitter at 1 W on 900 MHz, most of the other GPS antennas I tried lost all satellites, even with the antennas 1-2 feet apart. I really wanted a active quadrifilar helix antenna with an appropriate filter but was unable to find one readily orderable. I also discovered that I need to send the UBX-AID-HUI message to the GPS receiver at startup or the UTC time is off by 2 seconds for up to 12.5 minutes until it receives the full navigation message.
The CDI electronics box for the igniter is complete. It houses the RC-EXL module plus a small PCB containing the opto isolated current signal conditioning, a solid state relay, and a 555 timer to generate the pulses for the CDI module.