Category: [TECHNOLOGY]
Type: [Starship Maneuvering System, Thermal Rocket]
Canon Tier: Tier 2 (Expanded Universe; pending Council review)
Tags: [NEW-TECH] [PLOT-DEVICE]
The hot-water RCS quad is the Terran Sphere’s standard attitude and docking thruster. Each quad is a sealed 0.25 t block carrying four 6 kN nozzles and a refractory heat core. Water is flashed to steam and superheated across the core at about 3,000 K, then expelled at about 4.4 km/s (Isp 450 s). The system is a thermal rocket, not a chemical one: it burns nothing, makes no fire, and is fuelled from any water tap. The main drive pushes a ship along one axis; the RCS is what lets it turn, translate sideways, brake in a berth, and live inside the 8 km torch line of a small port. Without RCS a ship is a dart: it goes where it points, and it cannot point.
| Symbol/Parameter | Meaning/Description | Value / Specification |
|---|---|---|
| Module mass | Dry, including heat core and plumbing | 0.25 t |
| Envelope | Stowed (x, y, z) | 0.8 × 0.6 × 0.8 m |
| Nozzles | Four per quad, one in each of +x, −x, +z, −z (tangential and axial) | 6 kN each |
| $I_{sp}$ | Specific impulse | 450 s |
| $v_e$ | Exhaust velocity, $I_{sp} g_0$ | $4.41 \times 10^3$ m s⁻¹ |
| $\dot m$ | Water flow per firing nozzle, $T / v_e$ | 1.36 kg s⁻¹ |
| $P_{jet}$ | Jet power per nozzle, $\tfrac{1}{2} T v_e$ | 13.2 MW |
| $h$ | Specific enthalpy delivered, $v_e^2 / 2$ | 9.7 MJ kg⁻¹ (about 2.6 for vaporizing, about 7.1 for superheating and part-dissociating the steam) |
| Core | Refractory carbide-graphite composite, 100 kg, 1,000 to 3,000 K | ≈ 300 MJ stored (heat capacity 1.5 kJ kg⁻¹ K⁻¹) |
| Core duty | Nozzle-seconds at full thrust per full core (assuming 80% heat-to-jet efficiency, 16.5 MW thermal) | ≈ 18 s per quad, using ≈ 25 kg of water |
| Standby draw | Holds the core hot against conduction losses | 0.5 kW (and 0.5 kW of heat) |
| Heater (coil) rating | In drive power mode (bus 150 MW, all eight quads) | 16.5 MW per quad |
| Heater recharge | Time to refill an empty core at the rating | ≈ 18 s |
| Heater, on battery only | Limited by the cab store’s 2 MW peak | ≈ 150 s per core, and ≈ 12% sustained duty |
| Water store (tank segment) | RCS water carried in the tank | 3 t |
| Water $\Delta v$ | $v_e \ln(m_0 / (m_0 - 3\ \text{t}))$ on a 46.2 t skiff | ≈ 300 m s⁻¹ |
Relevant Equations:
Placement and authority (utility skiff, 8 quads, 46.2 t wet). One cluster of four sits at the tank’s aft station (y = 7 m) and one at the cab’s forward station (y = 15 m), at azimuths 0, 90, 180 and 270°. The centre of mass sits at about y = 8.4 m.
| Axis | Nozzles used | Force | Acceleration (46.2 t) |
|---|---|---|---|
| Axial (±Y) | 8 axial nozzles | 48 kN | 1.04 m s⁻² |
| Lateral (±X or ±Z) | 4 tangential nozzles | 24 kN | 0.52 m s⁻² |
| Roll (about the spine) | 8 tangential nozzles at about 2.1 m | ≈ 100 kN m | ≈ 110° s⁻² |
| Pitch and yaw | 2 nozzles at each cluster, opposite senses (8 m between clusters) | ≈ 96 kN m | ≈ 8° s⁻² |
| Backup: drive gimbal (2°) | Main thrust 150 kN, arm 8.4 m | ≈ 44 kN m | ≈ 4° s⁻² |
Moments of inertia are estimated from the segment masses: about $6 \times 10^5$ kg m² in pitch or yaw and $5 \times 10^4$ kg m² in roll.
Water is the one propellant every hull already carries. It is dense (3 t is 3 m³), stores without cryogenics, is not toxic, doubles as emergency drinking water and radiation shielding, and every Belt ice-hauler delivers it by the kilotonne. The Isp is a poor 450 s next to the 10,000 s of the micro-torch, but at 6 kN a nozzle the thrust-to-mass is what a docking manoeuvre needs. The main reaction-mass tank is separate because the drive wants hydrogen-rich tamper and capsules, not steam.
Each quad keeps a hot refractory core that the water crosses on its way out. The core is trickle-heated from the ship bus at standby (0.5 kW) and, when the drive is in power mode, charged at megawatts: eight quads firing at once draw 8 × 16.5 = 132 MW, which fits inside the 150 MW the micro-torch delivers in power mode. On batteries alone the cab store (2 MW peak) can only refill a spent core in about 150 s, so a cold-iron ship has a burst, then a trickle. That is why captains light the drive’s power mode before a docking approach, and why a skiff with a dead drive creeps.
A single cluster gives translation but poor rotation; two clusters give a lever arm. The standard layout is stern quads at the tank, bow quads at the cab, four azimuths at each station, so that every axis has at least four nozzles working for it. The ship’s control allocation solves for whatever nozzles remain: lose a quad and the ship still flies, just asymmetrically. Lose both quads on one side of a cluster and the ship drifts when it translates, and must be flown by a human who has learnt the offset.
The plume is steam and fine ice, bright in the infrared for the first kilometre and gone within a few. A burst of quads is visible to a nearby sensor mast as a warm puff, and a ship that sneaks up on RCS alone is quieter than a ship that lights a torch, at the cost of time.
Story Seeds: