Spacecraft Power Systems Flashcards
What are the four main power generation technologies used in spacecraft?
Fuel cells, RTGs, fission reactors, and solar cells.
How do temperature and radiation exposure affect solar cells?
They reduce efficiency by increasing defect concentration, reducing carrier mobility, and decreasing current gain.
What is the maximum power point on a solar cell’s I-V curve?
It is the point where the product of current and voltage is maximized.
Compare the relative performance of Si and GaAs solar cells.
GaAs cells have higher efficiency and better radiation hardness compared to Si cells.
What are the common battery technologies used in space?
Lithium-Ion (Li-Ion), Nickel-Hydrogen (NiH2), Nickel-Cadmium (NiCd), Silver-Zinc, and Lithium-Polymer.
What is Depth of Discharge (DoD) in battery terminology?
It is the percentage of the battery’s total capacity that has been used.
Why is DoD important for battery cycle lifetime?
Higher DoD generally reduces the cycle lifetime of batteries.
What are the typical eclipse durations in LEO and GEO?
LEO: ~30 minutes per eclipse; GEO: ~72 minutes per eclipse during equinox.
What are the main types of power regulation methods in spacecraft?
Switch-mode and linear regulation.
What is the advantage of switch-mode power regulators over linear regulators?
They are more efficient (90%-95%) and can step up or step down voltage.
Name three power system topologies used in spacecraft.
Centralized, distributed, and hybrid.
What is a simple power budget analysis for a spacecraft?
Balancing power input with demand while accounting for subsystem efficiencies.
What are typical efficiencies of power system components?
Solar array: 16%-28%; Regulators: 75%-90%; Batteries: 80%-95%.
How do fuel cells generate power in spacecraft?
Fuel cells use a chemical reaction between hydrogen (H₂) and oxygen (O₂) to produce energy and water as a byproduct.
What are the advantages of using fuel cells in spacecraft?
High power-to-mass ratio, water byproduct, and independence from sunlight.
What are the disadvantages of fuel cells in spacecraft?
Limited supply of hydrogen and oxygen, making them suitable only for short-duration missions.
On which missions have fuel cells been prominently used?
They were used on Apollo missions for short-duration spaceflights.
What is an RTG, and how does it work?
A Radioisotope Thermoelectric Generator (RTG) converts heat from the natural radioactive decay of isotopes (e.g., uranium or plutonium) into electricity using thermocouples.
Why are RTGs used in deep space missions?
They provide long-lasting power and are not dependent on sunlight, making them ideal for missions beyond Jupiter.
What are the safety measures for RTGs in spacecraft?
RTGs are mounted on booms to keep them away from the spacecraft and are designed to survive launch explosions without spreading radioactive debris.
What are the disadvantages of RTGs?
High cost, danger of radiation, and low conversion efficiency.
How do fission reactors generate power in space?
Fission reactors use nuclear fission to produce heat, which is then converted into electricity through thermionic systems or a thermal cycle with turbines.
What are the potential applications of fission reactors in space?
They are considered promising for deep-space manned missions requiring high power levels.
What are the challenges of using fission reactors in spacecraft?
They are still in the research phase and face issues related to safety, cost, and thermal management.
What type of fluid is often used in thermal cycles for fission reactors?
Liquid sodium metal, which expands when heated to drive turbines.
How do solar cells generate power?
Solar cells use photovoltaic conversion to transform sunlight into electrical energy through semiconductor p-n junctions.
What materials are commonly used in solar cells for spacecraft?
Silicon, gallium arsenide (GaAs), and advanced triple-junction cells.
What factors influence the efficiency of solar cells in space?
Incident photon energy, material properties, temperature, and radiation exposure.
What is the solar constant, and how does it relate to solar cells?
The solar constant is 1353 W/m² (Air Mass Zero), which represents the energy received from the Sun in space and is a baseline for designing solar arrays.
What are the advantages of solar cells in spacecraft?
Renewable energy source, relatively lightweight, and reliable in sunlight-rich environments.
What are the limitations of solar cells in space?
Dependency on sunlight, reduced efficiency beyond Jupiter, and susceptibility to radiation damage.
What are common sources of electromagnetic interference (EMI) in spacecraft?
Radiated emissions, conducted emissions, and ground loops.
How does grounding policy help minimize EMI?
By ensuring consistent reference potentials and avoiding ground loops.
What is the function of a star-point ground?
It minimizes ground loops by connecting all subsystems to a single grounding point.
Why is it important to use separate ground planes in spacecraft design?
To reduce interference between digital, analog, and power signals.
What is the role of shielding in EMI reduction?
It blocks electromagnetic fields from interfering with sensitive components.
How do coaxial cables and twisted pairs help in EMI control?
They reduce magnetic field interference and improve signal integrity.
What is Single Event Latch-up (SEL) in CMOS structures?
A destructive high-current condition caused by cosmic-ray hits activating parasitic transistors.
How can circuits be protected from SEL?
Using fast over-current tripping power switches.
Why is input/output protection important in spacecraft interfaces?
To prevent damage from harnessing errors or incorrect connections.
What is the purpose of matched drivers or opto-couplers?
To minimize ground loop issues and improve signal fidelity.
What are good practices for shielding wires?
Using individual screened wires or twisted pairs to minimize interference.
What is differential signaling, and why is it used?
A method where two complementary signals are sent to reduce noise susceptibility.
Why is harness design critical in spacecraft?
To ensure reliable connections and minimize EMI.
What materials are suitable for spacecraft connectors?
Space-compatible materials like gold pins and PTFE plastics.
How should digital outputs be protected?
With current limiting resistors and diodes to prevent overvoltage and undervoltage.
What is the impact of a ground loop in spacecraft systems?
It introduces unwanted noise and interference between subsystems.
Why are optical links used in spacecraft?
For excellent isolation, though they degrade with radiation exposure.
What is the function of a Pi-filter in spacecraft systems?
It reduces conducted emissions in power lines.
What happens when harnesses are not partitioned?
Increased cross-talk and interference among power, digital, and analog lines.
How can EMI from high-frequency systems be minimized?
By choosing slower digital techniques and using shielding.
Why should power supply lines have power planes?
To minimize inductance and reduce AC noise.
What is the purpose of grounding schemes?
To ensure all subsystems operate with a consistent reference potential.
Why is under-voltage protection important?
To switch off systems before damage occurs due to low voltage.
What is the impact of cosmic rays on CMOS logic?
They can cause transient errors or SEL.
Why should fast data busses avoid certain protection measures?
Because added resistance or capacitance can slow down data rates.
What is the purpose of anti-reflective coatings on solar cells?
To improve efficiency by reducing photon losses.
Why are EMI screening shells used for connectors?
To reduce external interference and ensure signal integrity.
Explain difference between primary cell and secondary cell battery technologies
a primary cell is non-rechargeable, a secondary cell is rechargeable
briefly explain how a solar PV cell works
-These are planar P-N diode junctions
- Solar radiation creates electron-hole pairs in the depletion region.
- This EMF drives a photocurrent which can be passed through a load such as a spacecraft power system
briefly explain how an RTG works
- Thesscomprise a series of Si:Ge junctions held at high and low temperatures to form a thermopile, thus generating a flow of current via the Seebeck effect
- The cold junctions areheld cold via radiators radiating to cold space, whilst the hot junctions are embedded in a hot highly radioactive material (e.g. plutonium, or uranium
- . The conversion efficiency is very low, but as the radioactive decay constant of these materialsis very long, the power level decays only very slowly lasting many decade
- The RTG requires substantial material shielding and protection against breaking up in the event of a launch acciden
briefly explain how a fuel cell works
- These work by reverse electrolysis –i.e. hydrogen and oxygen gas are combined in the fuel cell to create liquid water and electricity via a proton-exchange membrane.
- Electrical power is available until the reactants are exhausted –and so this mechanism is suitable for space missions lasting a few days up to a week or so.
- Water is a waste product that can then be turned into drinking water.
State three ways to improve performance of PV cells
- Better materials: GaAs more efficient. Extra junctions
- Cover glass
- Back-surface reflectors
