Fuel Transfer Software

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Description

The Fuel Transfer Software manages propellant transfer between tanks by coordinating valves, pumps, and destination tank demand. It implements a state machine that handles the complete transfer sequence from path initialization through completion or abort. The software supports both same-spacecraft transfers and cross-spacecraft transfers via a Fuel Interconnect.

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Example Use Cases

• On-Orbit Refueling: Transfer propellant from a tanker spacecraft to a client spacecraft through a docked interconnect.
• Tank Balancing: Equalize propellant mass between tanks to maintain spacecraft center of mass.
• Depot Operations: Automated filling of destination tanks to a target level.

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Module Implementation

State Machine

The transfer sequence follows a state machine with the following states:

State	Description
Idle	Ready to begin a new transfer
Opening	Opening valves and enabling pumps
Transferring	Actively moving propellant
Closing	Shutting down path components
Complete	Transfer finished successfully
Aborted	Transfer stopped due to error or abort command

When a transfer is started, the software transitions from Idle to Opening. Once all path components are ready (valves open, pumps enabled), it transitions to Transferring. The transfer continues until a completion condition is met or a stop is requested, at which point it transitions through Closing to either Complete or Aborted.

Transfer Modes

The software supports multiple operating modes that determine when a transfer completes:

Mode	Completion Condition
Manual	Runs until explicitly stopped
Transfer Amount	Stops after transferring target mass $M_{\text{target}}$
Transfer To Percent	Stops when destination reaches target fill percentage
Balance Tanks	Stops when tanks reach equal mass
Continuous	Runs until stopped or source depleted

Demand Calculation

During transfer, the software sets the destination tank’s desired ingoing flow rate. For most modes, this equals the configured desired flow rate ${\dot{m}}_{\text{desired}}$, clamped to the remaining tank capacity:

$${\dot{m}}_{\text{demand}}=\min ({\dot{m}}_{\text{desired}},C_{\text{dest}}-M_{\text{dest}})$$

where $C_{\text{dest}}$ is the destination tank capacity and $M_{\text{dest}}$ is its current mass.

In Balance Tanks mode, the target is the average mass of both tanks:

$$M_{\text{avg}}=\frac{M_{\text{source}}+M_{\text{dest}}}{2}$$

Flow demand is set to zero once the destination tank reaches or exceeds this average.

Progress Tracking

The software computes percent complete based on the transfer mode. For Transfer Amount mode:

$$P=\frac{M_{\text{transferred}}}{M_{\text{target}}}\times 100$$

For Balance Tanks mode, progress is computed from the reduction in tank mass difference:

$$P=\left(1-\frac{|M_{\text{source}}-M_{\text{dest}}|}{|M_{\text{source,0}}-M_{\text{dest,0}}|}\right)\times 100$$

where the subscript 0 denotes initial values at transfer start.

Estimated time remaining is computed when flow rate is non-zero:

$$t_{\text{remaining}}=\frac{M_{\text{remaining}}}{{\dot{m}}_{\text{actual}}}$$

Abort Conditions

The transfer is automatically aborted if any of the following conditions occur:

• Source or destination tank becomes unavailable
• Destination tank enters thermal shutoff (if enabled)
• Cross-spacecraft interconnect link breaks (if enabled)
• Source tank reaches minimum reserve level

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Assumptions/Limitations

• The software coordinates existing fuel network components; it does not model fluid dynamics directly.
• Path readiness requires valves to be at least 99% open before transitioning to the Transferring state.
• Cross-spacecraft transfers require a physical dock to be established before the transfer can begin.
• The software assumes instantaneous command execution for valve and pump state changes.
• Tank balancing targets exact equality; it does not account for different tank geometries or ullage effects.