Spacecraft Budgets

What Are Spacecraft Budgets?

Budgets allocate critical resources—mass, power, propellant, and data—across subsystems to ensure the spacecraft design remains feasible and aligned with mission objectives.

Preliminary Budgets

• Early estimates of essential spacecraft resources.
• Provide initial guidance to ensure the design remains within constraints.

Purpose:

• Keeps the design within feasible limits for mass, power, propellant, and data.
• Incorporates margins and contingencies to address uncertainties and potential changes.

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1. Mass Budget

Reason

Launch costs are tightly linked to spacecraft mass, and mass tends to grow during the design process.

• Key Components:
  • Dry Mass: Includes the structure, subsystems, and payload.
  • Wet Mass: Combines dry mass with propellant required for maneuvers.
• Design Considerations:
  • Add margins of 10–20% to account for unexpected mass increases.
  • Mass constraints directly influence launch vehicle selection and cost.

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2. Propellant Budget

Reason

The total propellant budget is driven by the mission’s ΔV requirements for orbit adjustments, station-keeping, and end-of-life maneuvers.

• Key Components:
  • ΔV needs for all mission phases.
  • Propellant mass based on propulsion system type and specific impulse (Isp).
• Design Considerations:
  • Use the rocket equation to calculate required propellant.
  • Propulsion types:
    • Chemical propulsion: High thrust but requires more propellant for rapid maneuvers.
    • Electric propulsion: Efficient, lower thrust but suitable for long-duration maneuvers.
  • Include reserves for uncertainties or additional maneuvers.

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3. Power Budget

Reason

Ensures all subsystems have sufficient power to function throughout the mission lifecycle, especially during critical phases.

• Key Components:
  • Peak and average power demands for payloads and subsystems.
  • Solar arrays or RTGs as the primary power sources.
• Design Considerations:
  • Include 20–30% margin for inefficiencies and potential growth in power usage.
  • Design must account for eclipse periods and other mission-specific conditions.
  • See Power Subsystem for details.

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4. Pointing and Alignment Budget

Reason

Allocates resources for maintaining the spacecraft’s attitude and meeting mission-specific pointing accuracy requirements.

• Key Components:
  • External disturbances like gravity gradients, solar pressure, and magnetic torques.
  • System capabilities for attitude determination and control.
• Design Considerations:
  • Include margins for real-world misalignments and unpredictable environmental forces.
  • Allocate resources to counteract external disturbances affecting alignment.

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5. Data System Budget

Reason

Manages onboard data storage and downlink capabilities to ensure mission data is transmitted and preserved efficiently.

• Key Components:
  • Data generation rates during mission phases.
  • Onboard storage capacity for data between downlink windows.
  • Downlink bandwidth and ground station availability.
• Design Considerations:
  • Estimate total data volume for all mission phases.
  • Design for compression and processing to mitigate bandwidth constraints.
  • Align data storage capabilities with the Telemetry Subsystem.

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Types of Margins in Spacecraft Budgeting

Margins Are Critical

Margins address uncertainties, subsystem-level growth, and unforeseen challenges.

Figure: Graphical Presentation of Margin (SMAD)

Component	Description
Allocation	- Initial resource budget per subsystem - Includes Current Best Estimate (CBE) and contingency - Based on preliminary design requirements
Current Best Estimate (CBE)	- Most accurate resource prediction per subsystem - Based on current design maturity - Updates throughout development
Contingency	- Extra resources within subsystem allocation - Handles minor uncertainties - Provides flexibility during design
System Margin	- Additional overall system reserves - Addresses unforeseen problems - Enables design changes when needed
Lien	- Reserved portion of system margin - Addresses specific high-risk issues - Released if risk is retired
Total Resource Margin	- Combines all contingencies and system margins - Provides overall performance buffer - Ensures mission requirements are met

Scaling of Spacecraft Budgets

Scaling Considerations

As spacecraft size increases, various budgets scale differently, impacting design choices and mission feasibility.

Feature	Scaling
Surface Area	Increases slower than volume (r² vs. r³)
Mass	Increases with volume
Power	Increases with volume (more components)
Thermal Control	Surface area for radiators becomes more scarce on larger spacecraft, since power goes with volume
Solar Arrays	Small spacecraft, with lower power, are more likely to have enough surface area for body-mounted arrays
Stability	Torque required to rotate a spacecraft goes up with moment of inertia. MOI ∝ (mass × r²) ⇒ torque ∝ r⁵. Larger spacecraft are much more stable.
Lateral Frequency	Larger structures tend to have lower frequencies, with an interplay among MOI of base, height, and mass—goes as 1/r for proportional growth, 1/(c.m. height)¹·⁵ for constant base and mass, as center of mass (c.m.) moves.

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Related Notes:

• Spacecraft Design Overview
• Design Drivers
• Spacecraft Subsystems