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oapogee

Build guide

Step by step assembly of an oApogee payload, with an observable checkpoint at every stage and a link to the failure branch when a checkpoint does not pass.

draftintermediateupdated 2026-09-079 open markers

Written, not reviewed by anyone. Check the primary sources it links to.

Every step here ends with something you can see, hear, or measure. If you see the wrong thing, the step tells you where to go before you build anything on top of the mistake.

The structure is written and the numbers are not. Anything that depends on a physical board is marked and missing, because no board has been built.

Before you start

Read Safety and rules first, particularly the lithium cell section. The cell is the only part of this project whose failure mode is fire.

Have the bill of materials open. Check every part against it before you heat the iron, including the ones you already own, because a substitution you made months ago is not a substitution you will remember today.

Work somewhere you can leave things out. Putting a half-finished board in a drawer loses more than it saves.

Unverifiedsay how long this build actually takes, and whether it is realistically one sitting or two, once build_time_estimate in data/tiers.yaml is filled from timing people who have soldered before. The brief's "an evening" is a target, not a measurement.

Stage 0: Sort and inspect

  1. Lay every part out and identify it against the bill of materials.
  2. Check the battery connector polarity against the board silkscreen. Do not skip this. Lithium cells ship with both polarities on the same connector, and reversing one can destroy the board instantly.
  3. Inspect the cell for swelling, dents, and damaged wire insulation. Set it aside, in its bag, with the connector taped. It does not come out again until Stage 5.

Checkpoint 0. Every part on the list is present and identified, and you know which pin of your cell's connector is positive. If the polarity does not match the silkscreen, stop and read it runs on USB but dies immediately on battery before going further.

Photo neededthe full parts layout for a Track build on the Modules path, top down, every part labelled, on a plain background. This is the photo people screenshot to check their order arrived complete.

The path fork

The rest of this guide splits once, here, and rejoins at Stage 4.

  • Modules path: Stages 1M to 3M. Breakout boards wired to a carrier.
  • Board path: Stages 1B to 3B. The custom oApogee PCB.

Everything from Stage 4 onward is identical. Firmware, mounting, calibration, testing and flying do not care which way you built it.

If you are not sure, build Modules. The board is not fabricated yet.

Modules path

Stage 1M: The microcontroller carrier

  1. Fit the microcontroller module to the carrier board or protoboard.
  2. Solder the power and ground connections first, and nothing else yet.
  3. Inspect every joint under magnification before applying power.

Needs hardwarethe specific module has not been selected. This stage cannot be written concretely until it is. See the verify note on mcu_module in the bill of materials for the selection criteria.

Checkpoint 1M. With the multimeter in continuity mode and no power connected, there is no continuity between 3V3 and ground. If there is, you have a bridge, and finding it now costs a minute rather than a board.

  1. Connect USB. The module should enumerate.

Checkpoint 1M-b. The board appears to your computer. Holding the boot button while plugging in should present it as a USB drive. Releasing and replugging normally should present it as a serial device.

Nothing appears: it powers on but does not appear as a USB drive. Nothing at all happens: nothing happens when I plug it in. Try a different cable before anything else. Charge-only USB-C cables look identical to data cables and are the most common cause of both symptoms.

Stage 2M: Sensors onto their buses

The sensors do not share one bus. The barometer sits on I2C, the IMU and the high-g accelerometer share an SPI bus with a chip select each, and the GNSS receiver sits on a UART. Fit the barometer alone first and confirm it, so one part is proven before a second bus exists to complicate the picture.

  1. Fit the barometer breakout.
  2. Wire power, ground, SDA and SCL.
  3. Check the I2C pull-up resistors. The barometer is the only part on this bus, so there should be exactly one pull-up pair on SDA and SCL. Count any the carrier or the microcontroller module already provides, not just the pair the breakout brings. Most breakouts have a jumper or a solder bridge to remove theirs.

Needs hardwarethis guide does not rank the Modules-path failure modes by how often they happen, because ranking them needs reports from builds that have actually happened.

Checkpoint 2M. The serial console reports the barometer present and prints a pressure. Check it against the station pressure your local weather service publishes, not against a fixed band: standard sea level is 101325 Pa and the reading falls as elevation rises, so a correct sensor at a mile-high field reads far below any sea level figure. What tells you the part works is a value that is stable, plausible for where you are, and that rises slightly when you carry the board downstairs. A reading of zero, or a part that never appears at all, means the barometer never appears on the bus.

  1. Fit the IMU on the SPI bus: SCK, MOSI, MISO, and a chip select of its own. Confirm it before continuing.
  2. Fit the high-g accelerometer if you are building one, on the same SPI bus, with a second chip select that is not shared with anything. Confirm it.
  3. Fit the GNSS receiver if you are building Track, on a UART, with TX and RX crossed. Confirm it.

Checkpoint 2M-b. Every sensor you fitted reports present, and the values move sensibly when you move the board: tilting changes the accelerometer axes, rotating changes the gyroscope, and lifting it changes the pressure very slightly. Intermittent dropouts as you add parts mean sensors drop out intermittently.

Stage 3M: Radio, recovery aids, and power

  1. Fit the radio module if you are building Link or Track.
  2. Attach the antenna before you ever transmit. Transmitting into a disconnected antenna is bad for the radio.
  3. On Track, fit the GNSS antenna too, and give it a view of the sky. A receiver with no antenna does not report a poor position, it reports none at all, and the symptom on the bench looks the same as a receiver that is simply still searching.
  4. Fit the buzzer and the status LED.
  5. Fit the arming switch. The centre pin goes to the arming GPIO and one outer pin goes to ground. The third pin is deliberately left unconnected.
  6. Fit the battery connector, checking polarity one more time.

Checkpoint 3M. The buzzer sounds and the LED lights on command from the console. The console reports armed and disarmed as you move the switch, and the reported state matches the position you can see. The radio reports present. You have not transmitted yet.

Board path

Needs hardwarethis entire path depends on a fabricated PCB. Nothing below can be written concretely until boards exist, and none of it has been performed.

Stage 1B: Power section

  1. Solder the regulator, its passives, and the USB connector.
  2. Solder the charge controller and its programming resistor.
  3. Solder the core regulator inductor, L1. The microcontroller runs its core from an internal switching regulator and this is the one part of that regulator that lives outside the chip. Without it the core rail never comes up, which looks exactly like a dead microcontroller.

Checkpoint 1B. With no power applied, no continuity between 3V3 and ground. With USB applied and nothing else populated, 3V3 measures within tolerance at the regulator output.

Unverifiedstate the acceptable voltage range at this checkpoint, from the chosen regulator's datasheet.

Stage 2B: Microcontroller and flash

The microcontroller is a 60 pin QFN on a 0.4 mm pitch with a thermal pad on its underside, and the pad has to be soldered: it is the part's only ground connection. That means hot air, a hotplate or a reflow oven, and a stencil for the paste. An iron cannot reach under the part. If that is not equipment you have, the Modules path exists for exactly this reason, and having the board assembled by the fab is the other answer.

  1. Solder the microcontroller.
  2. Solder the QSPI flash.
  3. Solder the crystal, its two load capacitors, and the damping resistor next to it. Keep these joints tidy and keep the parts flat: this is the circuit the USB connection depends on, and a board with a crystal that does not start looks exactly like a board that is dead.

Checkpoint 2B. Hold SW2, tap SW3, release SW2, and the board appears to your computer as a USB drive. Copying a known-good test firmware onto it causes the status LED to light.

There is no dedicated bootloader pin on this microcontroller. Holding the flash chip select low as the chip leaves reset is what selects USB boot, which is what those two buttons do. A board with blank flash will also appear as a drive on its own, because the bootrom falls through to USB when it finds nothing to run, so the first flash works even if a button is unsoldered. The buttons are what let you do it a second time.

Stage 3B: Sensors, radio, and connectors

  1. Solder the barometer, the IMU, and the high-g accelerometer if fitting one.
  2. Solder the radio and the GNSS receiver if fitting them, and the coaxial socket that goes with each. They are the same part in two places: J3 feeds the radio antenna, J5 feeds the GNSS antenna.
  3. Solder the battery connector, the buzzer, the LED, and the arming switch.
  4. Solder the USB connector last. Its legs pass through the board and it sits over other work, so fitting it early makes everything under it harder to reach and harder to inspect.

Checkpoint 3B. Every populated part reports present over the serial console, and the sensor values move sensibly when you move the board. The console reports armed and disarmed as you move the switch, and the reported state matches the position you can see.

Photo neededa completed board, top and bottom, at high enough resolution to check joint quality against.

Stage 4: First real firmware

Both paths rejoin here.

  1. Follow Firmware and flashing to load the current release.
  2. Run the self-test from the serial console.

Checkpoint 4. Self-test passes for every part you fitted, and reports absent for every part you did not. A part you fitted that reports absent is a soldering problem, not a firmware problem. A part you did not fit reporting present means the firmware is misconfigured for your tier.

  1. Run the pad calibration procedure.

Checkpoint 4-b. Reported altitude sits near zero and stays there, drifting only slowly. Large or fast drift at this point means the pressure reference did not settle. A large negative starting value means it was taken somewhere else: see my altitude reads negative.

Stage 5: Battery

  1. Connect the cell, having checked the polarity for the third time.
  2. Confirm the board runs from the cell with USB disconnected.
  3. Confirm charging works with USB connected.

Checkpoint 5. It runs on the cell alone and charges when plugged in. If it runs on USB and dies on the cell, stop and read it runs on USB but dies immediately on battery.

Needs hardwaredocument the LED behaviour for charging, charge complete, and charge fault, once that behaviour is implemented.

Stage 6: Into the enclosure

Follow Mounting for the sled or the pod, whichever form factor you are using. That page also covers the static ports, which are not optional and are the most commonly botched part of the whole build.

  1. Print the enclosure for your form factor and airframe diameter.
  2. Secure the board so it cannot move.
  3. Secure the cell so it cannot move, and route the wires so the enclosure closing cannot pinch them.
  4. Drill and deburr the static ports.

Checkpoint 6. Shake the closed assembly hard and listen. Nothing rattles, nothing buzzes, nothing shifts. Anything you can hear moving in your hand will move much harder under boost. Open it and confirm the cell is still where you put it.

Stage 7: Ground testing

Do all four tests on the safety page, in order, before this goes anywhere near a rocket: the bench dry run, the shake test, the drop test, and the full arm-to-landing rehearsal. A payload that is going to fail should fail on your bench, where it costs an evening.

Checkpoint 7. A complete log file, on the flash, that exports to CSV and opens. If you can read a bench run end to end, you can read a flight.

Stage 8: The rocket

  1. Re-simulate your rocket in OpenRocket with the payload mass and position included. This is not optional and it is the step with a consequence for people other than you.
  2. Fit the payload.
  3. Confirm the rocket still slides freely on the rail, with the pod fitted. An external pod is the most common thing to catch on a rail button.

Checkpoint 8. You have re-simulated with the payload's measured mass at the position you actually mounted it, the margin has not gone backwards, and the rocket moves freely on the rail with the pod fitted.

Unverifiedthis checkpoint asks whether the margin moved rather than whether it clears a threshold, because the caliber margin to aim for is still open on the Mounting page. Until it closes, use the margin your rocket flew well at as the reference and treat a large move as a reason to stop.

Then work through the preflight checklist on the day.

Ground station

If you built Link or Track, you also need a receiver. That is a separate build: see Ground station.