System

Autonomous Underwater Vehicle, continuing first-pass decomposition. Three of eight subsystems are now decomposed: {{entity:Navigation and Guidance Subsystem}} (6 components, sessions 313-314), {{entity:Power Subsystem}} (4 components, session 315), and now {{entity:Propulsion Subsystem}} (5 components, this session). Five subsystems remain: Communications, Sensor Payload, Emergency and Safety, Pressure Hull and Structure, and Vehicle Management Computer. Project stands at 67 requirements across all documents with 48 trace links after baseline BL-003.

Decomposition

The {{entity:Propulsion Subsystem}} was decomposed into five components driven by two dominant engineering constraints: the 600 bar depth rating ({{sys:SYS-FUNC-010}}) and the 130 dB acoustic noise limit ({{sys:SYS-FUNC-009}}).

The primary thrust chain comprises {{entity:Motor Drive Electronics}} ({{hex:D1F53018}}), a field-oriented control drive executing sinusoidal commutation above 20 kHz to keep switching harmonics outside the survey band; {{entity:Brushless DC Propulsion Motor}} ({{hex:D7C51018}}), an oil-filled pressure-compensated BLDC motor delivering 250W continuous at 88% efficiency; and {{entity:Propeller and Shaft Assembly}} ({{hex:CEC51008}}), a 5-blade fixed-pitch propeller with magnetic coupling that eliminates the rotary shaft seal entirely — the critical design decision for 6000m operation.

Maneuvering is handled by {{entity:Control Surface Actuator Assembly}} ({{hex:D7F51018}}), servo-driven rudder and elevator fins with 0.1-degree resolution for survey-grade trajectory tracking, and {{entity:Buoyancy Trim System}} ({{hex:53F53208}}), an oil-hydraulic variable buoyancy system that decouples depth control from propulsive power consumption.

flowchart TB
  n0["Motor Drive Electronics"]
  n1["Brushless DC Propulsion Motor"]
  n2["Propeller and Shaft Assembly"]
  n3["Control Surface Actuator Assembly"]
  n4["Buoyancy Trim System"]
  n5["Vehicle Management Computer"]
  n6["Power Distribution Unit"]
  n5 -->|CAN: speed/torque cmds| n0
  n0 -->|3-phase commutated power| n1
  n1 -->|torque via magnetic coupling| n2
  n5 -->|CAN: fin deflection cmds| n3
  n5 -->|CAN: buoyancy offset cmds| n4
  n6 -->|48V DC power| n0
  n6 -->|24V DC power| n3
  n6 -->|48V DC power| n4

Analysis

The acoustic noise budget decomposition is the most engineering-significant aspect of this subsystem. {{sys:SYS-FUNC-009}} allocates 130 dB total to propulsion. The motor receives 120 dB ({{sub:SUB-FUNC-017}}), leaving 10 dB headroom for propeller and mechanical sources. This is tight but achievable with the magnetic coupling (no shaft seal friction noise) and FOC commutation (minimal torque ripple). The propeller cavitation inception requirement ({{sub:SUB-FUNC-018}}) sets inception above 4.5 knots — well above the 3-knot cruise — ensuring cavitation never contributes broadband noise during survey operations.

The magnetic coupling air gap specification ({{ifc:IFC-INTERFACEDEFINITIONS-013}}, 8 mm maximum) is directly constrained by the titanium pressure boundary wall thickness required by {{sys:SYS-FUNC-010}}. This is a genuine case where the structural requirement cascades into the propulsion interface specification — every millimetre of additional wall thickness for pressure margin reduces coupling torque.

Lint found one pre-existing high-severity finding (BMS lacks Physical Object trait) and two acceptable low findings (ARC/VER documents lacking SHALL keyword, which is correct for those document types). A duplicate architecture decision ({{stk:ARC-ARCHITECTUREDECISIONS-003}}) from a failed CLI parse was identified and deleted.

Requirements

Ten subsystem requirements created ({{sub:SUB-FUNC-014}} through {{sub:SUB-FUNC-023}}), covering FOC switching frequency, speed regulation, motor power/efficiency, acoustic noise allocation, propeller thrust and cavitation, magnetic coupling torque, control surface deflection, buoyancy displacement range, motor fault protection, and actuator pressure rating.

Six interface requirements ({{ifc:IFC-INTERFACEDEFINITIONS-008}} through {{ifc:IFC-INTERFACEDEFINITIONS-013}}) define the MDE-to-motor power interface, three CAN bus command interfaces (VMC to MDE, CSAA, and BTS), the PDU power delivery interface, and the mechanical magnetic coupling interface.

Five verification entries ({{sub:VER-TEST-009}} through {{sub:VER-TEST-013}}) cover the four interface tests plus one system-level integration demonstration of the complete propulsion chain at 3-knot cruise. Thirteen trace links connect system requirements to subsystem and interface requirements with derivation rationale.

Next

Five subsystems remain undecomposed. Priority for the next session should be the {{entity:Communications Subsystem}} — it already has a CONNECTS fact (USBL transponder) and is architecturally significant for mission recovery and operator awareness. Alternatively, the {{entity:Emergency and Safety Subsystem}} has the highest safety criticality and should be addressed before the project reaches first-pass-complete status. The entity graph similarity endpoints returned 404 errors for newly classified entities; this should resolve once entities are indexed.