1. The moon that moved

Alastair Reynolds’ Pushing Ice features a moon-sized object that turns out to be a spacecraft, its propulsion systems vast and ancient. The desktop’s tanks are smaller, but the same principle applies: propellant must be stored safely, moved when needed, and accounted for precisely. A leak, a bubble, or a frozen line is not a small problem in space.

Entry 255 chose green monopropellant as the baseline. This entry sizes the tanks and routes the feed system.

2. Propellant mass recap

From entry 254, the desktop needs roughly 15 kg of green monopropellant for 300 m/s of delta-v, assuming a 120 kg dry mass and an Isp of 220 s. Add a small amount for attitude assist and margins, and a practical loaded propellant mass is 18–20 kg.

Green monopropellants have densities around 1.2 g/cm³, so 20 kg occupies about 16–17 litres of tank volume. Add ullage space for pressurant and expansion, and the total tank volume should be roughly 20–25 litres.

3. Tank options

Two common architectures:

Architecture Description Best for
Diaphragm tank A flexible diaphragm separates propellant from pressurant Monopropellant, predictable expulsion
Surface tension tank Propellant is managed by capillary devices and vanes Bipropellant, zero-g settling
Piston tank A piston separates propellant from pressurant Precise expulsion, higher cost

For a green monopropellant system, diaphragm tanks are the simplest choice. The diaphragm prevents gas from entering the feed line and provides positive expulsion without slosh concerns.

4. Tank sizing and number

A single 25-litre tank is roughly 300 mm in diameter and 400 mm long, depending on the aspect ratio. That is large but fits inside a 1 m desktop body if placed along one axis.

An alternative is two smaller tanks, each 12–15 litres, mounted symmetrically. This improves the centre of gravity and provides redundancy, but adds plumbing and mass.

For the first desktop, two tanks of roughly 12 litres each is a reasonable choice. They can be placed on opposite sides of the central electronics bay, balancing the mass and leaving room for the robot arm.

5. Pressurisation

Monopropellant thrusters need a feed pressure, typically 0.5–2 MPa. The pressure can come from:

  • Stored high-pressure gas, such as helium, regulated down to the thruster inlet pressure.
  • Propellant vapour pressure, for some propellants that have sufficient vapour pressure at operating temperature.
  • Blowdown system, where the tank is pressurised at launch and the pressure decreases as propellant is used.

A blowdown system with helium pressurant is the simplest for a small spacecraft. The tank is filled with propellant and pressurised to a few MPa. As propellant is expelled, the pressure drops, and the thruster must operate across a wider pressure range.

6. Feed lines and routing

The feed system must move propellant from the tanks to the thrusters with minimal pressure drop and no trapped gas. Key elements:

  • Feed lines: stainless steel or titanium tubing, sized for the mass flow rate and pressure drop.
  • Isolation valves: allow thrusters or tank branches to be closed off.
  • Filters: protect thruster catalyst beds from particles.
  • Pressure sensors: monitor tank and feed pressures.
  • Temperature sensors and heaters: keep propellant within its operating range.

Lines should be routed away from hot radiators and solar arrays. They should be supported at intervals to avoid vibration and thermal expansion issues.

7. Thruster manifold

A manifold distributes propellant from the feed line to multiple thrusters. For the desktop, a simple arrangement is:

  • One feed line from each tank.
  • A common manifold near the thruster cluster.
  • Individual isolation valves for each thruster.
  • Filters and pressure sensors at the manifold.

The manifold should be located so that all thrusters see similar inlet pressures. Avoid long branches that create uneven pressure drops.

8. Thermal control

Green monopropellants have minimum operating temperatures. If the propellant gets too cold, it may not decompose reliably in the catalyst bed. Heaters and thermostats are needed on tanks, lines, and thruster valves.

The thermal control attachment from earlier entries must include the propulsion hardware in its heater budget. The propulsion attachment is not thermally independent.

9. Mass estimate

For the propulsion attachment:

  • Propellant: 18–20 kg.
  • Two diaphragm tanks and fittings: 4–6 kg.
  • Thrusters, valves, filters, sensors: 2–4 kg.
  • Feed lines, heaters, and harness: 1–2 kg.
  • Total propulsion attachment mass: roughly 25–32 kg.

This is comparable to the structural and power attachments and reflects the fact that propulsion is one of the heaviest subsystems on a manoeuvring spacecraft.

What this changes

  • The desktop carries roughly 18–20 kg of green monopropellant in two 12-litre diaphragm tanks.
  • A blowdown pressurisation system with helium is the baseline.
  • Feed lines, filters, valves, and sensors route propellant to the thrusters.
  • Heaters keep tanks, lines, and thrusters at operating temperature.
  • The propulsion attachment mass is estimated at 25–32 kg including propellant.
  • The next entry can place the thrusters and check plume impingement.