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Space Systems > Spacecraft Power Systems > Flashcards

Spacecraft Power Systems Flashcards

1
Q

What are the four main power generation technologies used in spacecraft?

A

Fuel cells, RTGs, fission reactors, and solar cells.

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2
Q

How do temperature and radiation exposure affect solar cells?

A

They reduce efficiency by increasing defect concentration, reducing carrier mobility, and decreasing current gain.

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3
Q

What is the maximum power point on a solar cell’s I-V curve?

A

It is the point where the product of current and voltage is maximized.

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4
Q

Compare the relative performance of Si and GaAs solar cells.

A

GaAs cells have higher efficiency and better radiation hardness compared to Si cells.

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5
Q

What are the common battery technologies used in space?

A

Lithium-Ion (Li-Ion), Nickel-Hydrogen (NiH2), Nickel-Cadmium (NiCd), Silver-Zinc, and Lithium-Polymer.

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6
Q

What is Depth of Discharge (DoD) in battery terminology?

A

It is the percentage of the battery’s total capacity that has been used.

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7
Q

Why is DoD important for battery cycle lifetime?

A

Higher DoD generally reduces the cycle lifetime of batteries.

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8
Q

What are the typical eclipse durations in LEO and GEO?

A

LEO: ~30 minutes per eclipse; GEO: ~72 minutes per eclipse during equinox.

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9
Q

What are the main types of power regulation methods in spacecraft?

A

Switch-mode and linear regulation.

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10
Q

What is the advantage of switch-mode power regulators over linear regulators?

A

They are more efficient (90%-95%) and can step up or step down voltage.

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11
Q

Name three power system topologies used in spacecraft.

A

Centralized, distributed, and hybrid.

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12
Q

What is a simple power budget analysis for a spacecraft?

A

Balancing power input with demand while accounting for subsystem efficiencies.

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13
Q

What are typical efficiencies of power system components?

A

Solar array: 16%-28%; Regulators: 75%-90%; Batteries: 80%-95%.

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14
Q

How do fuel cells generate power in spacecraft?

A

Fuel cells use a chemical reaction between hydrogen (H₂) and oxygen (O₂) to produce energy and water as a byproduct.

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15
Q

What are the advantages of using fuel cells in spacecraft?

A

High power-to-mass ratio, water byproduct, and independence from sunlight.

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16
Q

What are the disadvantages of fuel cells in spacecraft?

A

Limited supply of hydrogen and oxygen, making them suitable only for short-duration missions.

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17
Q

On which missions have fuel cells been prominently used?

A

They were used on Apollo missions for short-duration spaceflights.

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18
Q

What is an RTG, and how does it work?

A

A Radioisotope Thermoelectric Generator (RTG) converts heat from the natural radioactive decay of isotopes (e.g., uranium or plutonium) into electricity using thermocouples.

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19
Q

Why are RTGs used in deep space missions?

A

They provide long-lasting power and are not dependent on sunlight, making them ideal for missions beyond Jupiter.

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20
Q

What are the safety measures for RTGs in spacecraft?

A

RTGs are mounted on booms to keep them away from the spacecraft and are designed to survive launch explosions without spreading radioactive debris.

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21
Q

What are the disadvantages of RTGs?

A

High cost, danger of radiation, and low conversion efficiency.

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22
Q

How do fission reactors generate power in space?

A

Fission reactors use nuclear fission to produce heat, which is then converted into electricity through thermionic systems or a thermal cycle with turbines.

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23
Q

What are the potential applications of fission reactors in space?

A

They are considered promising for deep-space manned missions requiring high power levels.

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24
Q

What are the challenges of using fission reactors in spacecraft?

A

They are still in the research phase and face issues related to safety, cost, and thermal management.

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25
Q

What type of fluid is often used in thermal cycles for fission reactors?

A

Liquid sodium metal, which expands when heated to drive turbines.

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26
Q

How do solar cells generate power?

A

Solar cells use photovoltaic conversion to transform sunlight into electrical energy through semiconductor p-n junctions.

27
Q

What materials are commonly used in solar cells for spacecraft?

A

Silicon, gallium arsenide (GaAs), and advanced triple-junction cells.

28
Q

What factors influence the efficiency of solar cells in space?

A

Incident photon energy, material properties, temperature, and radiation exposure.

29
Q

What is the solar constant, and how does it relate to solar cells?

A

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.

30
Q

What are the advantages of solar cells in spacecraft?

A

Renewable energy source, relatively lightweight, and reliable in sunlight-rich environments.

31
Q

What are the limitations of solar cells in space?

A

Dependency on sunlight, reduced efficiency beyond Jupiter, and susceptibility to radiation damage.

32
Q

What are common sources of electromagnetic interference (EMI) in spacecraft?

A

Radiated emissions, conducted emissions, and ground loops.

33
Q

How does grounding policy help minimize EMI?

A

By ensuring consistent reference potentials and avoiding ground loops.

34
Q

What is the function of a star-point ground?

A

It minimizes ground loops by connecting all subsystems to a single grounding point.

35
Q

Why is it important to use separate ground planes in spacecraft design?

A

To reduce interference between digital, analog, and power signals.

36
Q

What is the role of shielding in EMI reduction?

A

It blocks electromagnetic fields from interfering with sensitive components.

37
Q

How do coaxial cables and twisted pairs help in EMI control?

A

They reduce magnetic field interference and improve signal integrity.

38
Q

What is Single Event Latch-up (SEL) in CMOS structures?

A

A destructive high-current condition caused by cosmic-ray hits activating parasitic transistors.

39
Q

How can circuits be protected from SEL?

A

Using fast over-current tripping power switches.

40
Q

Why is input/output protection important in spacecraft interfaces?

A

To prevent damage from harnessing errors or incorrect connections.

41
Q

What is the purpose of matched drivers or opto-couplers?

A

To minimize ground loop issues and improve signal fidelity.

42
Q

What are good practices for shielding wires?

A

Using individual screened wires or twisted pairs to minimize interference.

43
Q

What is differential signaling, and why is it used?

A

A method where two complementary signals are sent to reduce noise susceptibility.

44
Q

Why is harness design critical in spacecraft?

A

To ensure reliable connections and minimize EMI.

45
Q

What materials are suitable for spacecraft connectors?

A

Space-compatible materials like gold pins and PTFE plastics.

46
Q

How should digital outputs be protected?

A

With current limiting resistors and diodes to prevent overvoltage and undervoltage.

47
Q

What is the impact of a ground loop in spacecraft systems?

A

It introduces unwanted noise and interference between subsystems.

48
Q

Why are optical links used in spacecraft?

A

For excellent isolation, though they degrade with radiation exposure.

49
Q

What is the function of a Pi-filter in spacecraft systems?

A

It reduces conducted emissions in power lines.

50
Q

What happens when harnesses are not partitioned?

A

Increased cross-talk and interference among power, digital, and analog lines.

51
Q

How can EMI from high-frequency systems be minimized?

A

By choosing slower digital techniques and using shielding.

52
Q

Why should power supply lines have power planes?

A

To minimize inductance and reduce AC noise.

53
Q

What is the purpose of grounding schemes?

A

To ensure all subsystems operate with a consistent reference potential.

54
Q

Why is under-voltage protection important?

A

To switch off systems before damage occurs due to low voltage.

55
Q

What is the impact of cosmic rays on CMOS logic?

A

They can cause transient errors or SEL.

56
Q

Why should fast data busses avoid certain protection measures?

A

Because added resistance or capacitance can slow down data rates.

57
Q

What is the purpose of anti-reflective coatings on solar cells?

A

To improve efficiency by reducing photon losses.

58
Q

Why are EMI screening shells used for connectors?

A

To reduce external interference and ensure signal integrity.

59
Q

Explain difference between primary cell and secondary cell battery technologies

A

a primary cell is non-rechargeable, a secondary cell is rechargeable

60
Q

briefly explain how a solar PV cell works

A

-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

61
Q

briefly explain how an RTG works

A
  • 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
62
Q

briefly explain how a fuel cell works

A
  • 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.
63
Q

State three ways to improve performance of PV cells

A
  • Better materials: GaAs more efficient. Extra junctions
  • Cover glass
  • Back-surface reflectors

Space Systems (40 decks)

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