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Space Systems Term 1 > Advanced Astronautics > Flashcards

Advanced Astronautics Flashcards

(103 cards)

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1
Q

What is the most suitable primary power supply from fuel cells, RTGs and solar arrays for a 12 day manned mission to the mood (power requirement 20 kW) and right justification of the selection

A

Fuel cell as it is a crewed short duration mission with high power requirement

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2
Q

What parameters affect the average temperature of a spherical micro-satellite operating at LEO?

A
  • orbit inclination and orbital altitude
  • IR emissivity of the satellites surface material
  • solar absorptivity of the satellites surface material
  • earth albedo
  • solar radiation intensity
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3
Q

Describe what happens in a skip re-entry trajectory. What is the main advantage of this type of trajectory?

A

Capsule pitches down, hits atmosphere and then pitches up again before re-entry.
Reduces entry speed which reduces heating and lowers the entry corridor boundary

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4
Q

Select the most suitable power supply from fuel cells, RTGs and solar arrays for an interplanetary science mission to Neptune. Justify the reason for selection

A

RTGs - large power supply needed for a long duration. Fuel cells would need too big tanks (heavy) and solar power would be limited for that distance

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5
Q

A flat plate is exposed to the sun in space. Which plate properties are primarily responsible for determining the equilibrium temperature it will reach?

A

? Solar absorptivity, specific heat, conductivity

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6
Q

Describe what spin-stabilisation is and why it is used and name two common types of spin-stabilised satellites

A

A way of achieving stability on a s/c without need for propulsion. In launch or separation, a satellite is rotated in the longitudinal axis. Keeps craft on course

Pioneer and Juno

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7
Q

Identify two passive thermal control subsystems that can be used for nano/micro satellites and describe the adv of passive subsystems (at least 3) over active thermal control subsystems

A
  • geometry: configuring satellite to provide required thermal radiating area - low temp objects in shadow etc
  • coatings
  • insulation blankets: multi layers of aluminised Mylar and other plastics, space with nylon/Dacron mesh

Requires no power requirement or moving parts. Simple, reliable and low cost

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8
Q

Describe the effects of average and peak electrical power requirements on the design of the power system

A

Peak requirements incorporates thermal energy storage into the system design to meet demand
The level of power requirements will affect what power/fuel system is used.

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9
Q

For the Apollo 13 mission, the radio blackout lasted around 6 minutes which was 87 seconds longer than expected. Assume that Apollo 13 had a similar re-entry speed to other Apollo missions. Give a potential reason why the Apollo 13 capsule encountered a longer radio blackout period than other Apollo missions.

A

A steeper re-entry angle which created a larger shock and larger plasma layer

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10
Q

Identify the condition/environment for the best scenario of a solar array operation during the mission

A

Sun facing mission with limited periods of dark (eclipse), low mass (no propellant) and doesn’t need quick attitude control manoeuvres

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11
Q

Identify the condition/environment for the worst scenario of a solar array operation

A

Long duration, high power with lots of time far from Sun/in dark

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12
Q

What are the standard voltages?

A

28V, 50V, 70V, 100V, 120V, 160V

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13
Q

What is a power systems primary function?

A

Supply continuous source of electric power to spacecraft loads over the whole mission
Support power requirements for average and peak electrical power

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14
Q

What are the components of a power system?

A

Primary energy source

Energy conversion

Power regulator - can connect both ways to rechargeable energy storage

Power distribution and protection

Power utilisation (loads)

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15
Q

What does an optimisation study of a power system give?

A

Best combination of energy source, storage technology and mechanism for conversion

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16
Q

What is the primary criteria for a power system?

A

Low mass, low cost

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17
Q

What are the standard bus voltages?

A

28, 50, 70, 100, 120 & 160V

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18
Q

Why is the maximum bus voltage 160V?

A

Any more increases chance of short circuit through plasma and extra shielding is needed. Also dangerous voltages that can kill any higher

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19
Q

Factors that influence bus voltage selections

A

Power level
Space environment and space plasma - at low alts more plasma therefore lower voltages to avoid shorts
Paschin min breakdown voltage between bare conductor
Human safety
Availablity of components

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20
Q

Guideline for optimum voltage

A

Opt V = 0.025*power requirement

Chose closest but cheaper to go lower

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21
Q

Scale equation for similar designs of power systems

A

Mass of new = mass of similar * (new power requirement/similar)^0.7

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22
Q

What makes up a battery cell?

A

2 electrode plates submersed in an electrolyte. Converts stored chemical energy into direct electricity current

Non reversible electrochemistry

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23
Q

What is a fuel cell?

A

A battery that works for longer
Powers loads of several watts for a few days/weeks
Two electrode plates in electrolyte that converts stored chemical energy in fuel to electricity

Lasts as long as there’s feed supply

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24
Q

What do solar panels always come with?

A

A battery - needs support in shadow or eclipse

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25
How long do solar panels last?
Few months to 20 years
26
What is an issue with solar panels?
Performance is not constant
27
What is the efficiency of a battery, solar cell and fuel cell?
Battery 70-80% Solar 20-30% Fuel 10%
28
At what temperatures do solar panels work better?
Cold therefore need insulation and heat transfer to take away suns heat
29
What is a solar concentrator-dynamic power system?
Uses heat to generate steam and drive rotating turbo- generator or reciprocating alternator thermodynamic energy convertor
30
Pros and cons to a solar concentrator dynamic power system
More efficient to solar arrays by minimising the deployed collection area and aerodynamic drag. Hours with no performance degradation But heavy
31
What mission is nuclear-thermoelectric suitable for?
Interplanetary and deep space
32
What are the power levels of RTGs and reactors?
RTG several 100s W Reactor 30-300kW
33
Pros and cons of nuclear thermoelectric
Supplies power continuously - no need for battery Heavy radiation shielding needed around electronics and humans Safe and easy to handle nuclear fuels are expensive Power reduces proportionally with remaining fuel
34
Power of a nuclear/chemical-dynamic system
100s kW to multi mega Ws
35
What is a solar array?
Numerous PV cells stacked in series-parallel connections to get desired voltage and current Makes more of a constant source over normal operating range
36
How does charging affect a battery life?
Charge more frequently extends the life Avoid state of charge of 0
37
How is power regulated?
Battery charge and discharge convertor Shunt dissipator - controls bus in sunlight Mode controller - responds to bus volt error signal
38
What is a shunt dissipator?
Heat sink to dump energy to get more reliable current
39
What is a mode controller?
Programmable controller that sets modes in response to error signal where there is a difference in actual bus voltage to reference voltage Sends control signal to shunt regulator, battery charge regulator and battery discharge regulator
40
What is a mode controller?
Sets modes in response to error signal where there is a difference in actual bus voltage to reference voltage Sends control signal to shunt regulator, battery charge regulator and battery discharge regulator
41
What are the power system architectures?
Direct Energy Transfer DET Peak Power Tracker PPT
42
What is Direct Energy Transfer?
Solar energy transferred to loads with no series component in between Exceptions - slip rings to provide rotary join between s/c and solar array - power distribution unit with load switching relays and fuses to protect power system from faults in load circuit Unregulated bus - cheaper, less parts, less mass, short times that battery is needed Fully regulated bus - big missions, charge needed over longer time Sunlight regulated bus
43
What is Peak Power Tracker?
Solar array output is always set at value for max power transfer from array to load The power loss in PPT converter must be less than gain in operating system at peak power point at all times Used in missions with wide temperature extremes and/or solar illumination intensities
44
True or false, the photo conversion efficiency of a cell is insensitive to solar radiation in practical working range
True
45
How is an array performance affected by the sun?
The more sun the more energy. The efficiency will stay the same but with part of the array shaded less photons, less energy
46
When should Is=Iocos(theta) be used? And what else is used?
Sun angles 0-50 degrees Kelly cosine is more accurate and applies for all angles (no power over 85 Deg)
47
What is the temperature effect?
Increase the temperature and the short circuit current of a cell increases and the open circuit voltage decreases. Equations: Isc = Io + a*deltaT Voc = Vo+ b*deltaT P = VI = Po + (aVo+bIo)deltaT
48
How does location in orbit affect array performance?
In LEO, the eclipse time is longer but there is a smaller temperature variation GEO, shorter eclipse time but bigger temp variation
49
What is the shadow effect?
A large array is partially shadowed. Should be considered in design to lower string voltage or use bypass diodes
50
What is the design process?
1. Analyse orbit parameters, load power requirement and heritage data on similar satellites 2. Top level trade analysis 3. Select power system architecture for optimised design
51
Trade offs in design process
Primary - minimise mass and cost Secondary - altitude - increase alt, longer period, longer charging time, shorter discharge - micrometeoroids/debris - angle between E-S line and orbit plane, some angles have strong radiation or heavy debris so might need stronger cell - array angle to sun - sun line tracking method - # wings - symmetry for rotation, stability and drag - 1.5% eccentricity in E orbits about Sun Saving mass allows for more in payload
52
Driving factors for bus voltage, power generation and storage technology?
- payload power level - orbit parameter - mission life - # sats in program
53
Trades for PV cell
Requirement - generate required EP at EOL Trade - degradation of array power output under charged particles Depends on mission environment and life time
54
Array trades
Mass decrease - affects drag, inertia, propellant mass Area proportional to required power generation Influences propulsion system and attitude control system Affects natural frequency - lower nat freq - more stable but larger array
55
Battery trades
GEO - high launch cost, fewer charge/discharge cycles LEO - low launch cost, high ionised radiation, more charge cycles Battery influences thermal control system
56
Equations for number of strings and series
N series cells per string = array V / Vmp N strings per array = Array I / Imp
57
Factors that affect battery selection
Specific energy and energy density Cycle life, stability of capacity and voltage Round trip energy efficiency Mass and volume constraints Temperature effects on performance Ampere-hour capacity ratings available Ease and speed of recharge Self discharge rate Safety issues
58
Design process for batteries
1. Determine # series cells to meet voltage required 2. Determine ampere hour discharge to meet load current demand 3. For required # cycles, determine max allowable DoD 4. Find total ampere-hour capacity = ampere-hour disc req / allowable DoD 5. # of battery pack needed in parallel 6. Temperature rise and thermal control requirements 7. Required charge/discharge rate controls Ah_b = PeTe / (NB*efficiency_discharge*((Nc-1)*Vo_dis-Vd-Vhdis)*DoD)
59
Identify the condition/environment for the best scenario of a solar array operation during the mission.
No thermal and radiation losses 100% packing efficiency But there will be unavoidable losses
60
What are the steps for finding the number of cells for a battery?
Design for EOL Check against the battery tolerances Pick one within tolerance If both are, choose the lightest option
61
What does the thermal system do?
Maintains all elements of the s/c system within the temperature limits for all mission phases.
62
What are some heat input sources that affect the thermal system?
The sun, Earth, dissipation from internal electronics. These can vary over time & with geometry
63
What % of the overall s/c is the thermal system in terms of cost and weight?
2-3 %
64
What are the thermal system requirement categories?
top level - temp margins, testing requirements, environmental definitions Derived requirements - subsystem weight allocation, cost goal Temp limit - various subsystem groups for components based on supplier data (impossible to control every component individually)
65
what are some issues to account for within the thermal control
biggest interaction is dissipating electrical energy batteries have very narrow temp range IR instruments need operation at cryogenic temps Thermal inputs from the sun/earth can affect ACS (photons) variations in an orbit (eclipse, solar intensity with seasonal distance change) operational activities - v low alt - heat from free molecular flow, thruster firing and onboard equipment surfaces can change characteristics with UV exposure, atomic oxygen and from impacts anomalous events - failure in wiring harness, sun shield failing
66
What are some examples of passive temperature control?
Geometry Insulation blanket Sun shield Fin Heat pipe
67
How does geometry affect thermal system?
Configure the s/c to provide required thermal radiating area low temp in shadows, high temp components in sun
68
How does an insulation blanket affect the thermal system?
Has a multilayer design of several layers of aluminised Mylar spaced with nylon/Dacron to reflect heat energy from sun/planet. Protected with a layer of fibreglass to avoid scratches and damage from UV, atomic oxygen and impacts which lower efficiency Fairly cheap, low mass option but the properties will change with exposure
69
How does a sun shield affect the thermal system?
A polished Al or Au plate reflects visible light energy but absorbs in the IR range Can use silvered teflon with a glass cover more rigid but heavier
70
How does a fin affect the thermal system?
dissipates a large amount of heat over a large surface area which alters the emissivity. Can be challenging to design - a circle of small ones means hard to get a adequate view factor but a big long one risks cracking if too thick (temp differences across sides). Fin has to go to outerspace
71
How does a heat pipe affect the thermal system?
Heat pipe is a tubular pipe partially filled with working fluid with a wick running through to act as capillary effect. Connect from hot to dump or to a component that needs heating. Must be below boiling temps but above freezing Need reservoirs and gas valves to control - becomes active
72
What are active methods for controlling thermal system?
Heater and cooler, shutters/Louvers, active pump
73
What heaters are used for thermal control?
A wire wound resistance heater or deposited resistance strip heater (light mass)
74
what coolers are used for thermal control?
thermoelectric or Peltier coolers to cool detectors in IR instruments
75
how is cryosat cooling done
expansion of high pressure gas through a small hole using N2 and H2 For short missions, gas doesn't need to be properly contained For manned missions, ice forms with use of cryogenics from human water -not good
76
How do shutters/louvers affect thermal system
used like blinds with one side having a highly reflective material
77
How does an active pump fluid loop affect thermal system
Like a car cooling system - heavy pipe with working fluid routed to heat exchanger in area to be cooled/heated heat transfer by forced convection into fluid
77
What are the pros and cons of passive and active systems?
Passive Pros - no power requirement, no moving parts, simple and therefore reliable Cons - inflexible, low heat transfer rate, performance variability (coatings) Active Pros - flexible/adaptable, high transfer rate Con - power, moving parts (reliablity eh), mass, higher costs Use only when have to
78
What are the design principles of a thermal system?
1. temp limits of all s/c components categorised by thermal control group with inputs 2. establish thermal boundary conditions - altitude and orientation for all phases - power group - electrical dissipation in all electrical components - design for worst case 3. define temp limits and understand other requirements 4. start design with analytical tools - radiation program - define absorbed energy on s/c external faces - generalised thermal analyser Do hand calcs as a rough estimate before computation
79
What is the worst case to design for in thermal system?
upper limit - everything in use, full sun illumination and activity, closest point of orbit to sun lower - eclipse, full shadows, furthest from sun
80
What thermal system tests can be done?
solar balance test and thermal vacuum test
81
what is the dominant heat transfer in solids
diffusion on a microscopic level and particles collide
82
what is radiation
energy radiates by photons - travels at speed of light with zero mass
83
what does uniform temperature imply
a surface will absorb a surface with the same level of emissivity - Kirchoffs law
84
why will a white painted surface have a low temp?
very low emissivity, low down in the wavelengths 5 mui metres
85
What is a view factor
express net radiation exchange Q between 2 surfaces that are perfect emitters and absorbers. fraction of radiant energy directly incident on receiving surface relative to total radiant energy leaving sending surface
86
Assumptions for the Oppenheim Radiation Network
- all surfaces 'grey' - all surface emissivity constant over wavelength bands applicable to operating T - all surfaces emissivity and reflect diffusely (except shiny metallic) - all surfaces are isothermal - no temp gradient - radiosity is uniform on each surface
87
What is radiosity
total radiant energy to the surface includes emitting, reflect, and re reflect
88
What are the characteristics of an Oppenheim Radiation Network
- each given surface (x), has an adjacent conductor in for e_x/(1-e_x) * A_x - conductor is connected between 2 nodes and a dummy node Jx - potential at surface node is sigma*T_x^4 and sigma*T_j^4 at J node - network is completed by connecting each J node to every other J node - conductors connecting J nodes are in form AxFx->y - if a surface is perfectly insulated, conductor is eliminated. surface potential moves to J node (sigma*Tx^4)
89
How are conduction heat transfers modelled
thermal networks with T^1
90
What are the methods for thermal network solving
explicit - step by step application of equation at each node till all are updated and time step complete Limited to critical time step. Quick but can miss detail Implicit - inversion of matrix of all nodes at each time step - no time step limit but usually not grater than a factor of 2 or 3 for accuracy Analysed by transient eqs until temp does not change
91
Identify the condition/environment for the worst scenario of a solar array operation during the mission.
With thermal and radiation losses (efficiency not equal to 100%) Not 100% packing efficiency and unavoidable losses
92
What is the primary objective of the EDL phase?
93
What is the EDL phase? When does it start?
94
What information do we need for the EDL phase?
95
How could we describe the motion of reentry vehicles
96
What type of reentries are there for EDL missions
97
What are the common issues in EDL missions
98
How can we protect a vehicle in re entry
99
How can we test a reentry vehicle
100
What does the atmosphere do to a reentry vehicle
Generates drag and lift - generates disturbance so vehicle can move from initial trajectory Can reduce accuracy of trajectory - less time in atmo - less error
101
How can we improve the accuracy of reentry
Reduce time spent in atmosphere to reduce affects of lift and drag
102
The accuracy of icbm is approx less than 1m but the accuracy of a reentry capsule is approx 100 km why?
ICBM is unmanned - can go through the atmosphere faster without affecting payload and therefore less affected by atmo Capsule is manned - have to go slower

Space Systems Term 1 (4 decks)

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