Advanced Astronautics Flashcards
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
Fuel cell as it is a crewed short duration mission with high power requirement
What parameters affect the average temperature of a spherical micro-satellite operating at LEO?
- orbit inclination and orbital altitude
- IR emissivity of the satellites surface material
- solar absorptivity of the satellites surface material
- earth albedo
- solar radiation intensity
Describe what happens in a skip re-entry trajectory. What is the main advantage of this type of trajectory?
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
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
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
A flat plate is exposed to the sun in space. Which plate properties are primarily responsible for determining the equilibrium temperature it will reach?
? Solar absorptivity, specific heat, conductivity
Describe what spin-stabilisation is and why it is used and name two common types of spin-stabilised satellites
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
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
- 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
Describe the effects of average and peak electrical power requirements on the design of the power system
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.
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 steeper re-entry angle which created a larger shock and larger plasma layer
Identify the condition/environment for the best scenario of a solar array operation during the mission
Sun facing mission with limited periods of dark (eclipse), low mass (no propellant) and doesn’t need quick attitude control manoeuvres
Identify the condition/environment for the worst scenario of a solar array operation
Long duration, high power with lots of time far from Sun/in dark
What are the standard voltages?
28V, 50V, 70V, 100V, 120V, 160V
What is a power systems primary function?
Supply continuous source of electric power to spacecraft loads over the whole mission
Support power requirements for average and peak electrical power
What are the components of a power system?
Primary energy source
Energy conversion
Power regulator - can connect both ways to rechargeable energy storage
Power distribution and protection
Power utilisation (loads)
What does an optimisation study of a power system give?
Best combination of energy source, storage technology and mechanism for conversion
What is the primary criteria for a power system?
Low mass, low cost
What are the standard bus voltages?
28, 50, 70, 100, 120 & 160V
Why is the maximum bus voltage 160V?
Any more increases chance of short circuit through plasma and extra shielding is needed. Also dangerous voltages that can kill any higher
Factors that influence bus voltage selections
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
Guideline for optimum voltage
Opt V = 0.025*power requirement
Chose closest but cheaper to go lower
Scale equation for similar designs of power systems
Mass of new = mass of similar * (new power requirement/similar)^0.7
What makes up a battery cell?
2 electrode plates submersed in an electrolyte. Converts stored chemical energy into direct electricity current
Non reversible electrochemistry
What is a fuel cell?
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
What do solar panels always come with?
A battery - needs support in shadow or eclipse
How long do solar panels last?
Few months to 20 years
What is an issue with solar panels?
Performance is not constant
What is the efficiency of a battery, solar cell and fuel cell?
Battery 70-80%
Solar 20-30%
Fuel 10%
At what temperatures do solar panels work better?
Cold therefore need insulation and heat transfer to take away suns heat
What is a solar concentrator-dynamic power system?
Uses heat to generate steam and drive rotating turbo- generator or reciprocating alternator thermodynamic energy convertor
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
What mission is nuclear-thermoelectric suitable for?
Interplanetary and deep space
What are the power levels of RTGs and reactors?
RTG several 100s W
Reactor 30-300kW
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
Power of a nuclear/chemical-dynamic system
100s kW to multi mega Ws
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
How does charging affect a battery life?
Charge more frequently extends the life
Avoid state of charge of 0
How is power regulated?
Battery charge and discharge convertor
Shunt dissipator - controls bus in sunlight
Mode controller - responds to bus volt error signal
What is a shunt dissipator?
Heat sink to dump energy to get more reliable current
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
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
What are the power system architectures?
Direct Energy Transfer DET
Peak Power Tracker PPT
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
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
True or false, the photo conversion efficiency of a cell is insensitive to solar radiation in practical working range
True
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
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)
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 + adeltaT
Voc = Vo+ bdeltaT
P = VI = Po + (aVo+bIo)deltaT
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
What is the shadow effect?
A large array is partially shadowed. Should be considered in design to lower string voltage or use bypass diodes
What is the design process?
- Analyse orbit parameters, load power requirement and heritage data on similar satellites
- Top level trade analysis
- Select power system architecture for optimised design
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
Driving factors for bus voltage, power generation and storage technology?
- payload power level
- orbit parameter
- mission life
- # sats in program
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
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
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
Equations for number of strings and series
N series cells per string = array V / Vmp
N strings per array = Array I / Imp
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
Design process for batteries
- Determine # series cells to meet voltage required
- Determine ampere hour discharge to meet load current demand
- For required # cycles, determine max allowable DoD
- Find total ampere-hour capacity = ampere-hour disc req / allowable DoD
- # of battery pack needed in parallel
- Temperature rise and thermal control requirements
- Required charge/discharge rate controls
Ah_b = PeTe / (NBefficiency_discharge((Nc-1)Vo_dis-Vd-Vhdis)DoD)
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
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
What does the thermal system do?
Maintains all elements of the s/c system within the temperature limits for all mission phases.
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
What % of the overall s/c is the thermal system in terms of cost and weight?
2-3 %
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)
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
What are some examples of passive temperature control?
Geometry
Insulation blanket
Sun shield
Fin
Heat pipe
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
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
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
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
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
What are active methods for controlling thermal system?
Heater and cooler, shutters/Louvers, active pump
What heaters are used for thermal control?
A wire wound resistance heater or deposited resistance strip heater
(light mass)
what coolers are used for thermal control?
thermoelectric or Peltier coolers to cool detectors in IR instruments
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
How do shutters/louvers affect thermal system
used like blinds with one side having a highly reflective material
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
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
What are the design principles of a thermal system?
- temp limits of all s/c components categorised by thermal control group with inputs
- establish thermal boundary conditions
- altitude and orientation for all phases
- power group - electrical dissipation in all electrical components
- design for worst case - define temp limits and understand other requirements
- 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
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
What thermal system tests can be done?
solar balance test and thermal vacuum test
what is the dominant heat transfer in solids
diffusion on a microscopic level and particles collide
what is radiation
energy radiates by photons - travels at speed of light with zero mass
what does uniform temperature imply
a surface will absorb a surface with the same level of emissivity - Kirchoffs law
why will a white painted surface have a low temp?
very low emissivity, low down in the wavelengths 5 mui metres
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
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
What is radiosity
total radiant energy to the surface includes emitting, reflect, and re reflect
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 sigmaT_x^4 and sigmaT_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)
How are conduction heat transfers modelled
thermal networks with T^1
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
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
What is the primary objective of the EDL phase?
What is the EDL phase? When does it start?
What information do we need for the EDL phase?
How could we describe the motion of reentry vehicles
What type of reentries are there for EDL missions
What are the common issues in EDL missions
How can we protect a vehicle in re entry
How can we test a reentry vehicle
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
How can we improve the accuracy of reentry
Reduce time spent in atmosphere to reduce affects of lift and drag
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
