JH

Power Systems and Attitude Control – Study Notes

Power Systems and Attitude Control – Study Notes

  • Exam and homework logistics (context for study):

    • Problem sets: ~30 questions per lecture covering concepts and learning objectives; responsibility on the student to use them for practice.
    • Practice exams: possible but exams can include up to 50 points drawn from the ~30-question sets.
    • Exam logistics: in the lab, with prescribed lab times; access to the same notes/time as homework and exam.
    • Exam length: about half the length of homework, but at the same level of technical detail.
    • Rubrics (homework 1 and 2) posted to guide the required level of detail.
    • General message: for studying, focus on the same depth of understanding as homework, but fewer problems.

  • Core topic: Power systems for spacecraft and attitude control.

    • Four core functions for power systems:
    • Generate power
    • Store power
    • Condition power
    • Distribute power
    • The distinction:
    • Generate and store are the primary energy creation and retention functions.
    • Conditioning ensures clean, stable electrical characteristics (voltage, noise, regulation).
    • Distribution delivers power to the right devices at the right voltage and timing, with noise suppression and protection.
    • Real-world design emphasis: you can replace components or refuel, but often it’s easier to build a new unit on the ground and launch it; rendezvous maneuvers are fuel-intensive and time-consuming.
    • Payload-driven philosophy: the payload (the mission’s primary objective) is the dominant driver of power system design.

  • Payload power demands and mission operation:

    • The payload is the “800 pound gorilla” in the room; power must be sufficient to operate the payload especially during eclipse.
    • Missions vary in how they handle eclipse:
    • Some keep payloads operating through eclipse with enough stored energy.
    • Others switch to power-saving modes (grouped power modes) to avoid oversized batteries/panels.
    • Power budget considerations depend on mission profile and required uptime for the payload.

  • Space environment and power sources:

    • Solar flux is the primary energy source for many spacecraft.
    • Photon flux from the sun drives solar panel energy generation.
    • Space radiation (protons and other particles) can damage hardware and affect batteries.
    • Batteries and energy storage basics:
    • Battery capacity is typically given in watt-hours (Wh): an energy quantity measuring how much energy a battery can deliver for a given period.
      • Example: a 100 W battery can supply 100 W for 1 hour, i.e. E = P t = 100 \text{ W} \times 1 \text{ h} = 100 \text{ Wh}.
    • Power × time equals energy: E = P t.
    • Not all batteries discharge at the same rate; discharge limits depend on chemistry and design (C-rate).
    • C-rate: the discharge rate relative to a battery’s capacity; e.g., a 1C rate would discharge a rated capacity in 1 hour.

  • Depth of discharge (DoD) and temperature effects:

    • DoD is the fraction of the battery’s energy that has been withdrawn relative to its total capacity.
    • Lithium-ion (Li-ion): typical recommended usable DoD up to about 80% (80% of rated capacity usable); this implies ~20% deadweight. Temperature strongly affects usable DoD.
    • Lead-acid batteries: often recommended usable DoD around ~50% (about half of capacity usable).
    • Li-ion advantages and caveats:
    • High energy density and good cycle life.
    • Temperature sensitivity affects performance and DoD limits.
    • Temperature example: cold environments (e.g., 10°C) degrade performance; you should plan for temperature effects in space (and in ground tests).
    • End-of-life considerations: margins remain important; end-of-life power must still exceed required operation power for mission success.

  • Power generation options beyond solar panels:

    • Solar panels are the common primary power source in many missions.
    • Nuclear options: Radioisotope Thermoelectric Generators (RTGs) convert heat from radioactive decay into electricity. They provide long-lived power when solar is insufficient (e.g., far from the Sun, or in shadowed regions).
    • Other generation methods are less common in typical small spacecraft but can be considered in niche missions.

  • Attitude control and power system interactions:

    • Attitude control focuses on pointing the spacecraft, not altitude.
    • Common confusion: attitude vs altitude. Altitude is height; attitude is orientation/pointing.
    • Attitude is critical for payload objectives and power generation (e.g., solar panel orientation matters for energy production).
    • Solar array orientation sensitivity: usually not extremely tight; about ±5° misalignment often still yields a large fraction of peak power.
    • For example, a misalignment of ~5° is discussed as resulting in modest power loss; the讲 discusses cos(5°) as the limiting factor (note: cos(5°) ≈ 0.996, i.e., ~0.4% loss in a realistic scenario; the speaker’s rough estimate suggested ~95% of peak).

  • Attitude sensing and sensing accuracy:

    • Sun sensors: coarse sensors (often a small number of pixels) providing rough sun direction; can yield ~1–2° accuracy with time and geometry known.
    • Star trackers: high-accuracy attitude sensors; capable of sub-degree accuracy (often ~0.1° or better; tens of arcseconds in some cases).
    • Rate gyros: measure angular rate; used with star trackers to provide continuous attitude and to determine drift.
    • Sensor fusion: star trackers + rate gyros yield accurate pointing and track angular drift; this improves control performance.

  • Attitude control actuators and concepts:

    • Reaction wheels: a spinning flywheel that stores angular momentum; by speeding up or slowing down the wheel, the spacecraft rotates in the opposite direction to conserve angular momentum.
    • Analogy: spinning bicycle wheels and maintaining stability; tilting the spinning wheel on a chair causes a change in orientation due to angular momentum transfer.
    • Practical use: three-axis stabilization often relies on a set of reaction wheels.
    • Thrusters: provide torque via short, discrete impulses; good for larger reorientations but not precise for fine attitude control due to thrust variability and on/off timing.
    • Gravity-gradient stabilization (boom-and-mass): a long boom with a mass at the end can passively stabilize the spacecraft by aligning the boom with Earth’s gravity gradient (useful for long-duration stability with minimal active control).
    • Actuators summary:
    • Reaction wheels store angular momentum and provide fine pointing control.
    • Thrusters provide impulse-based control (less precise and more energy-intensive per small maneuver).
    • Gravity-gradient devices provide passive stabilization for certain configurations.
    • Practical note: attitude control and propulsion are intertwined with energy budgets; electric and propulsion systems must be coordinated with power generation and consumption.

  • Propulsion and propellant types (attitude and propulsion context):

    • Propulsion types affect how you move through space:
    • Chemical propulsion (storable propellants): often used for major orbital changes and rendezvous; propellants include:
      • Nasty, toxic options used in some stages: kerosene (RP-1) with LOX (liquid oxygen); hydrogen with oxygen; and methane with oxygen have been used or proposed (historically German V2 used kerosene).
      • Bi-propellant combos are common for expendable stages and attitude-control thrusters.
    • Electric propulsion (electrothermal/electric propulsion): high-efficiency, low-thrust propulsion that uses electricity to ionize and accelerate propellants (e.g., xenon in ion thrusters); extremely efficient for in-space propulsion but requires substantial electrical power supply.
      • Mechanism: ions are accelerated by electric fields to produce thrust; excellent specific impulse but low thrust.
    • Nuclear and radioisotope options (RTG): provide a steady heat source converted to electricity; useful for missions with limited sunlight or long durations beyond solar reach.

  • Mission design considerations tied to power and attitude:

    • End-of-life margins: as spacecraft ages, the power system must still meet the mission’s energy demands; margins are critical to ensure continued operation until retirement.
    • Mass considerations: the EPS mass can be nontrivial; harnessing and copper wiring contribute to mass; a notable fraction of spacecraft mass can be devoted to the power system (often around 10% of the overall system or the spacecraft, depending on design).
    • Ground testing and launch realities:
    • Testing on the ground is essential to verify energy storage and distribution under realistic thermal conditions.
    • Rendezvous and docking maneuvers are energy-intensive, requiring precise planning of propulsion and attitude control; fuel costs for these maneuvers can be substantial and must be accounted for in design.

  • Quick reference: key equations and concepts to remember (LaTeX-ready):

    • Energy from power over time: E = P t
    • Battery capacity in watt-hours (example): a battery rated at P = 100\,\text{W} for t = 1\,\text{h} stores E = 100 \times 1 = 100\,\text{Wh}
    • Attitude control dynamics (conceptual): the angular momentum of a rigid body is \mathbf{L} = \mathbf{I} \boldsymbol{\omega}; reaction wheels change the spacecraft attitude by exchanging angular momentum with the spacecraft body, preserving total angular momentum.
    • Solar power and panel orientation: with a sun-angle offset, the effective power is modulated by the cosine of the incidence angle: P{out} = P{max} \cos \theta (theta is the angle between sun direction and panel normal). Small misalignment (a few degrees) typically yields only modest losses; exact numbers depend on panel design and shading.
    • Gravity-gradient torque (conceptual): due to Earth’s gravity gradient across a spacecraft, a torque can be generated that tends to align the spacecraft with the local vertical; a long boom with a mass at the end can exploit this for passive stability.

  • Practical takeaways for exam prep:

    • Be able to distinguish and describe the four power system functions (generate, store, condition, distribute) and why each is necessary.
    • Understand the payload-centric design principle and how eclipse and mission mode decisions affect power sizing.
    • Recall typical DoD values and their dependence on chemistry and temperature:
    • Li-ion: DoD up to ~80% (usable energy ~80% of rated capacity) with temperature sensitivity.
    • Lead-acid: DoD up to ~50% (usable energy ~50% of rated capacity).
    • Know basic energy and power relationships (E = Pt) and how to interpret battery capacity in practice.
    • Recognize the role of different sensors (sun sensors, star trackers) and actuators (reaction wheels, thrusters, gravity-gradient devices) in attitude control.
    • Be able to compare and contrast energy-generation options (solar vs RTG) and understand the trade-offs in different mission contexts.
    • Understand practical implications of propulsion choices for mission design (chemical vs electric propulsion) and their impact on power budgets and mission timelines.

  • Contextual and ethical/practical considerations:

    • Handling of hazardous materials (chemicals, fuels) and nuclear sources requires careful safety, regulatory, and environmental considerations in mission design and operations.
    • In-space operations involve trade-offs among reliability, mass, energy efficiency, and propulsion system efficiency, all of which influence mission success and safety.
    • The balance between high-precision attitude control and energy consumption is a core design constraint; fine-pointing sensors and actuators must be matched to available power.

  • Summary takeaway for the exam:

    • Expect questions that test the ability to describe the four power system functions, explain how payload needs drive power design, and demonstrate understanding of energy storage concepts (DoD, C-rate, temperature effects).
    • Be prepared to discuss attitude sensing/actuation options, including the practical pros/cons of solar orientation, star trackers, sun sensors, rate gyros, reaction wheels, thrusters, and gravity-gradient stabilization.
    • Be able to articulate the rough trade-offs between solar power and RTGs, and why certain missions rely on one or the other depending on distance from the Sun and mission lifetime.
    • You may be asked to connect these concepts to real-world examples (e.g., eclipse operations, ground test considerations, and rendezvous fuel costs).