1. The Heechee cockpit
Frederik Pohl’s Gateway is full of alien spacecraft with interfaces that humans barely understand. Pilots learn which levers to pull without knowing why they work. The desktop’s attachment interface must be the opposite: every feature has a visible purpose, and a robot or astronaut can operate it without guessing.
Entry 249 picked materials and manufacturing. This entry designs the interface where attachments meet the desktop.
2. What the interface must do
The attachment interface has three simultaneous jobs:
- Mechanical: locate and retain the attachment against launch loads, thermal distortion, and robotic handling.
- Thermal: conduct or isolate heat between the attachment and the desktop’s thermal bus.
- Electrical/data: connect power and data without manual alignment.
These three must happen in one motion when an attachment is inserted. A design that requires three separate operations is not a real interface; it is an assembly procedure.
3. Mechanical interface
The mechanical part of the interface has four elements:
- Alignment features: guide pins or tapered cones that position the attachment within a fraction of a millimetre before any connector engages.
- Slide rails: allow the attachment to be pushed in along a single axis with low friction and no jamming.
- Latch or lock: a motor-driven or manually operated mechanism that holds the attachment against the docking face with a known preload.
- Hard stops: define the final position so the alignment features and connectors see the right compression.
For the desktop, a practical interface is a pair of parallel rails with a lead-in taper, plus two alignment pins and a central motor-driven latch. This is similar to an equipment rack in a server room, but with stronger latches and tighter tolerances.
4. Thermal interface
Attachments fall into two thermal categories:
- Hot attachments that need cooling, such as compute modules, power electronics, and some payloads. They need a conduction path to the desktop’s heat pipes or radiator panels.
- Isolated attachments that must stay warm or cold independently, such as precision optics or cryogenic payloads. They need thermal isolation from the desktop body.
For hot attachments, the interface includes a flat metal contact surface with a thermal interface material, such as a graphite sheet or soft metal foil, between the attachment and the structure. For isolated attachments, the interface uses low-conductivity mounts, such as titanium stand-offs or glass-fibre brackets.
The thermal design must be part of the attachment specification. A customer payload that produces 20 W of heat cannot simply be bolted anywhere; it must mate with a cooled surface.
5. Electrical and data interface
The electrical interface is a set of blind-mate connectors mounted on the desktop side of the interface. When the attachment is pushed home, the connectors engage automatically.
A reasonable first interface provides:
- Primary power: 28 V bus, current-limited per attachment.
- Secondary power: 12 V and 5 V for low-voltage electronics.
- Data: Ethernet or SpaceWire for high-rate data, plus a CAN or I²C management bus.
- Safety interlock: a circuit that confirms the attachment is latched before power is applied.
The connector pinout must be standard across attachment types. A compute module and a storage module may use the same connector even though they draw different currents and send different data.
6. Form factor and envelope
Attachments should conform to a small set of standard sizes. A standard size makes rails, latches, and connectors reusable and lets attachments be designed without knowing the exact desktop they will ride on.
A possible set of form factors:
| Form factor | Volume | Typical use |
|---|---|---|
| Half-slot | ~100 × 150 × 200 mm | Small compute boards, power converters, sensors |
| Full-slot | ~200 × 150 × 200 mm | Compute modules, storage arrays, communications units |
| Double-slot | ~400 × 150 × 200 mm | Larger payloads, battery packs, instruments |
| External panel | ~300 × 300 × 50 mm | Radiators, antennas, solar array segments |
These are starting points. The actual dimensions should be set once the internal bay volume and the attachment catalog are better known.
7. Robotic compatibility
The interface must work when operated by a robot arm with limited dexterity. This means:
- Tapers and guides that forgive small misalignments.
- Latches that can be driven by a single rotary or linear actuator.
- Visual targets, such as fiducial markers, for the robot’s cameras.
- Access clearances for the gripper and arm links.
- Force and torque limits that prevent the robot from damaging the interface.
If an astronaut can operate the interface with gloved hands, the robot probably can too. Designing for both is safer than designing for either alone.
8. Interface standards
Several standards exist for spacecraft interfaces:
- NASA Docking System / International Docking System Standard: for vehicle-level docking, not internal attachments.
- OSAM interfaces: various on-orbit servicing and assembly standards from NASA and DARPA.
- CubeSat form factors: too small and rigid for the desktop’s modular payloads.
- VITA and CompactPCI: terrestrial standards that could inspire connector and card-edge designs.
The desktop will likely need its own interface standard because it sits between CubeSat simplicity and crewed spacecraft docking. The standard should be documented with mechanical drawings, connector pinouts, and thermal interface requirements.
What this changes
- The attachment interface combines mechanical alignment, thermal conduction or isolation, and blind-mate electrical/data connection.
- Rails, alignment pins, hard stops, and a motor-driven latch form the mechanical interface.
- Hot attachments conduct heat to the desktop thermal bus; isolated attachments use low-conductivity mounts.
- A standard set of form factors lets attachments be designed independently of the desktop.
- The interface is designed for robotic operation with force limits and visual fiducials.
- The next entry can design robotic access and the servicer-friendly details of the structure.