1. The ship that must not stop

Kim Stanley Robinson’s Aurora follows a generation ship whose propulsion and life support are finite, and whose survival depends on both working for longer than any one human lifetime. The desktop is not crossing interstellar space, but the same seriousness applies to a smaller scale: its propulsion budget is a hard limit. When the delta-v is gone, the mission is over.

Entries 253 through 257 defined the propulsion attachment. This entry closes that arc and decides what to define next.

2. What the propulsion arc settled

The desktop’s propulsion is now a coherent subsystem rather than a vague need to “move sometimes.”

The decisions are:

  • The propulsion attachment provides orbit maintenance, manoeuvring, and attitude assist.
  • It is a replaceable module with a visible propellant budget.
  • Total delta-v budget is roughly 300 m/s for a 5-year mission at 500 km.
  • The largest delta-v consumers are drag make-up and end-of-life disposal.
  • Green monopropellant is the baseline technology.
  • Cold gas is too inefficient for primary propulsion; electric propulsion is too slow for collision avoidance.
  • Propellant mass is roughly 18–20 kg in two 12-litre diaphragm tanks with a blowdown pressurisation system.
  • Six to eight thrusters are placed on the body ends and corners with clear plume paths.
  • Plume impingement must avoid solar arrays, radiators, antennas, optics, and MLI.
  • Total propulsion attachment mass is roughly 25–32 kg including propellant.

These decisions give the ADCS known force and torque capabilities. They also give the structure known loads from tank mounting and thruster reaction forces.

3. What remains outside the arc

Several integration questions were deliberately deferred:

  • How exactly does the ADCS command thrust magnitude and duration?
  • How are thruster firings coordinated with solar array pointing and power generation?
  • What happens during a long disposal burn when the spacecraft attitude cannot be maintained by reaction wheels alone?
  • How is propellant gauged and reported to the operator?

These will be answered when the ADCS, power, and operations attachments are defined.

4. Why ADCS is next

The next attachment to define is the attitude determination and control system: sensors, actuators, and software that point the desktop. There are three reasons for this order.

First, attitude control is the interface between propulsion and everything else. Thrusters provide force; ADCS decides when and how to use them. Reaction wheels and magnetorquers provide finer control without consuming propellant.

Second, the desktop’s pointing requirements are now known from the sensor, power, and thermal arcs. The imager needs nadir pointing with low jitter. The solar arrays need sun pointing. The radiators need cold-sky pointing. The star trackers need a clear view of the stars. ADCS must satisfy all of these simultaneously or in sequence.

Third, ADCS hardware is physically distributed across the desktop: reaction wheels inside the body, magnetorquers on the exterior, star trackers on the zenith face, sun sensors on the solar arrays, GPS antennas on the body. Its design cannot be done in isolation from the structure and the other attachments.

5. The ADCS questions to answer

The next few entries should answer:

  • What pointing accuracy and stability does the desktop need?
  • What sensors are needed: star trackers, sun sensors, Earth sensors, gyroscopes, magnetometers, GPS?
  • What actuators are needed: reaction wheels, magnetorquers, thrusters?
  • How is the control law partitioned between coarse and fine control?
  • How does ADCS handle safe mode and anomaly recovery?

These are ordinary engineering questions, but they deserve their own arc because pointing is what turns a collection of hardware into a working spacecraft.

What this changes

  • The propulsion attachment arc is closed with consistent specifications for delta-v, technology, tanks, feed system, and thruster placement.
  • The next arc will define the ADCS: sensors, actuators, and control.
  • Integration questions about propulsion are held until ADCS, power, and operations attachments are defined.
  • The desktop’s attachment grid now has a fifth proven pattern, following the sensor, power, thermal, and structural arcs.