Payload Interface
The S2 is designed for custom, field swappable payloads. Below are specifications to be used for creating a custom payload.
Design Guidelines
- The payload must not exceed 2.3kg (5lbs)
- Rail launch is around 8G, payload should be desiged to withstand these forces
- Payload must not protrude beyond the main bulkhead (highlighted in figure below). The rest of the nose cone may be fully filled so long as it meets the weight and CG requirements
Electrical and Power Interface
The primary interface to the payload is a 9 pin connector from the aircraft. The payload side requires a 9 position D-Sub Plug with Male pins. BST strongly recommend the following connector from Digikey:
The pinouts for the payload connector are listed in the table below (figure below from the solder cup side of the connector):
| Pin | Function | Notes |
|---|---|---|
| 1 | V+ (Raw Battery, 19.2V - 25.2V) | 3A Max |
| 2 | GND | 3A Max |
| 3 | UART Tx | Tx from Aircraft |
| 4 | UART Rx | Rx to Aircraft |
| 5 | CAN HI | |
| 6 | CAN LO | |
| 7 | CAN GND | 500mA Max |
| 8 | CAN PWR | 500mA Max |
| 9 | UART GND |
Danger
Never use a null modem cable with the S2, it can severely damage the avionics or payload.
Mechanical Interface
Below are the dimensions for the nosecone and a typical length for the scaffolding used to build payloads on.
And an image of the scaffold itself. This consists of four carbon fiber or fiberglass rods glued into a 3d printed ring used to house the two orange buttons that secure the nosecone to the fuselage and the DB9 for the electrical and power interface, and a rear carbon fiber bulkhead.
The scaffold can be seen inside the nosecone itself below, which serves mostly to keep air, precipitation, and dust out of the payload. The nosecone is generally just secured to the rear ring, but can also be secured around hatches used to house cameras, or tubing attachments for instruments to extract air from the flow around the vehicle.
Finally, a dust cover can be placed on the rear of the compartment. While this doesn't completely seal the enclosure, it does help to keep debris out of the payload area.
3d CAD of all of these compoents can be found here
A worksheet for calculating the allowable CG for a payload can be found here
Software Interface
The payload can either receive telemetry from the autopilot, or send / receive packets from the ground station. This is set up over the "Serial" configuration menu in the Aircraft Control Window. Interaction with either the aircraft or the payload is performed using an API that has been implmented in python and c, and combined with examples in the BST SDK.