2017-06-10

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.