spacecraft sub-systems design
TRANSCRIPT
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SPACECRAFT SUB-SYSTEMS
DESIGN
D.J.P. Moura
SPACECRAFT ARCHITECTURE (system level)
SPACECRAFT
PAYLOAD PLATFORM
STRUCTURE
THERMAL CONTROL
PROPULSION POWER/ENERGY SUPPLY
POWER CONDITIONNING & DISTRIBUTION
ORBIT & ATTITUDE CONTROL
DATA MANAGEMENT
HARNESS
EXPERIMENTS(scientific satellite)
COMMUNICATIONSMECHANISMSRECEIVERS/AMPLIFIERS/ANTENNAS
(telecoms satellite)
TELESCOPE/DETECTOR/ELECTRONICS(observation satellite)
Fully mission dependant
~ Contant for a class of spacecraft
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SPACECRAFT
ARE COMPLEX
ITEMS
TO BUILD
Man
pow
er (
desi
gn, p
rocu
rem
ent,
inte
grat
ion,
con
trol
, tes
t, m
anag
emen
t…)
THERMAL CONTROL
MAIN FUNCTIONS
PROVIDE, DURING ALL THE PHASES AND MODES,
THE SPECIFIED TEMPERATURE RANGES
TO ALL EQUIPMENTS
Deep Space
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THERMAL CONTROL
PENDING THE TEMPERATURE NEEDS, SEVERAL TECHNIQUES ARE POSSIBLE
HERSCHEL
PLANCK
Classicalspacecraft
THERMAL CONTROL
PASSIVE RADIATOR CONCEPT
RADIATIVE COUPLING WITH (cold) DEEP SPACE
INSOLATION FROM EXTERNAL HEATING SOURCES
SELECT ADEQUATE ε (emissivity) AND
α (absorptivity) ACCORDING THE NEEDS
Q (internal heat to dissipate)
T (~ specified temperature)
External Heating Fluxes(EHF)
(1-α) EHF Radiated Flux
= ε σ T 4 Thermal equilibrium :
α EHF + Q = ε σ T 4 S
T en Kelvin,
σ = Boltzman constant = 5.67 10-8 W/m2/K4
Surface S
αab
sorp
tivity
SELECTIVE
BLACK
SANDBLASTED
METALS
UN
PO
LIS
HE
D
ME
TA
LS
METALLIC
PAINTS
BLACK PAINTS
GREY
&
PASTEL
PAINTS
WHITE PAINTS
SURFACE MIRRORS
PO
LIS
HE
D M
ET
ALS
0 1
1
ε emissivity
FOR TYPICAL TEMPS (~ -10/+40 °OPERATIONAL MODE, ~ - 20 /+50 °STORAGE),
PASSIVE RADIATORS ARE USED
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THERMAL CONTROL
TECHNOLOGIES
AVOID COUPLING WITH EXTERNAL SPACE :
MULTI-LAYER INSULATION (w # 5 10-2 W/m2/K)
RADIATIVE COUPLING WITH COLD SPACE :
SECOND SURFACE MIRROR (ε # 0.84, α # 0.06)
PAINTS (mainly black or white)
ELECTRICAL HEATERS
STRUCTURE
MAIN FUNCTIONS
COPE WITH DESIGN AND QUALIF LAUNCHER REQUIREMENTS (loads, stiffness, interface)
DIMENSIONAL STABILITY DURING LAUNCH AND SPACE ENVIRONMENTS
PROVIDE SURFACES FOR MOUNTING OF EQUIPMENTS & RADIATIVE COOLING
ELECTRICAL & THERMAL CONDUCTIVITY
PROTECTION AGAINST RADIATIONS
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STRUCTURE
CONCEPT
TECHNOLOGIES
BOX WALLS :
ALUMINIUM OR HONEYCOMB SANDWICH
CENTRAL TUBE :
ALUMINIUM OR CARBON FIBER
GENERALY PREFERED CONCEPT: BOX SHAPE WITH INTERNAL CENTRAL TUBE
WHERE TANKS ARE ATTACHED, SHEAR WALLS, STRUSTS …
ATTITUDE & ORBIT CONTROL SUB-SYSTEM
MAIN FUNCTION (1)
TO FULFILL ITS MISSION, A SPACECRAFT HAS TO PERFORM THE POINTING OF ITS
PAYLOAD TOWARDS THE “TARGET” (earth, other planet, stars...),
IN POSITION (max allowed angular error) AS WELL AS SOMETIMES (when imaging
instruments) AS WELL AS OFTEN IN STABILITY (max error during a given time)
KEEP THE CORRECT ATTITUDE/POINTING(attitude : movement around the center of mass)
Time
Pointing error
Max angular changeallowable during the exposure time
Exposure time
Spec of stability : angular error / exposure time
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Typical pointing/stability needs
Earth ObservationTelecommunications Astronomy
Typical pointing: 0.01°
No stability requirement
Typical pointing: 0.1 to 0.01°
+ good stability (~ 10 - 3 °/s)
Typical pointing: < 1 arc’’
+ excellent stability (~ 10 - 4 °/s)
ATTITUDE & ORBIT CONTROL SUB-SYSTEM
MAIN FUNCTION (2)
INDEED, A SPACECRAFT IS FACING MANY PERTURBATIONS CHANGING, ON THE
MEDIUM AND LONG TERMS, THE CHARACTERISTICS OF ITS ORBIT:
- Flatness of the Earth
- Irregularities of the Earth density
- Solar flux
- Solar Gravity
- Moon gravity
- Aerobracking (below 1000 km)
KEEP THE CORRECT ORBIT (orbit : trajectory of the center of mass)
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ATTITUDE & ORBIT CONTROL SUB-SYSTEM
STABILISATION CONCEPTS
PASSIVE : GRAVITY GRADIENT
- stable for favorable mechanical configuration
- for Low Earth Orbit < ~ 1500 km
- low pointing performance > 1 deg
SEMI PASSIVE : SPIN STABILISATION
- required stable configuration (main inertia axis)
- mainly for GEosynchronous Orbit (N/S direction fixed)
- medium pointing precision ~ 0.1 to 1 deg
ACTIVE : 3 AXIS STABILISATION
- unstable configuration
- control by exchange of kinetic momentum with internal wheels
- high pointing precision < 0.1 deg
100 rpm
ATTITUDE & ORBIT CONTROL SUB-SYSTEM
ON BOARD ARCHITECTURE FOR SEMI PASSIVE OR ACTIVE ST ABLISATION
FLIGHT
CONTROL SWACTUATORS
ATTITUDE
SENSORS
DYNAMICSPACECRAFTBEHAVIOUR
ALLOWABLE ERRORS(stored in memory)
ONBOARD COMPUTER
EXTERNALPERTURBATIONS
ATTITUDE
COMPUTATION SW
EXTERNAL
REFERENCES
- Kinetic momentum (mean angular speed >> 0)
- Reaction wheels (mean angular speed ~0)
- Magneto-couplers
- Propulsion thrusters (see propulsion s/s)
- Sun sensor (coarse, fine)
- Earth sensor
- Star tracker
- Gyrometers (angular changes and speed)
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ATTITUDE & ORBIT CONTROL SUB-SYSTEM
TECHNOLOGIES
Startracker Earth sensor
GyrometerReaction wheel
Magnetotorquer
PROPULSION
MAIN FUNCTION DELIVERS FORCES (orbit control) AND TORQUES (attitude control)
CONCEPT PROPULSION BY REACTION THANKS TO EJECTED MASS
t0M0V0
t1M1 = M0 – dMV1 = V0+dV
dM, Vejection(relative velocity)
Thrust = dM/dt * Vejection M1 = M0 * exp (- dV/Vejection)
Conservation of the total quantity of movement:
M0*V0 = (M0-dM)*(V0+dV) + dM*(V0+dV-Ve) => M0*dV = - dM*Ve => dM/M0 = - dV/Ve
t1dM
V0+dV-Ve
Vejection / g = Specific Impulse (Isp)
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PROPULSION
TECHNOLOGIES THRUSTERS (1)
- COLD GAZ PROPULSION (Isp # 150 s)
- CHEMICAL PROPULSION
(solid, mono or bipropellant)
PROPULSION
TECHNOLOGIES THRUSTERS (2)
- ELECTRICAL PROPULSION
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PROPULSION
CONCEPT TANK
FREE FALL CONDITION EXCLUDES EMPTYING BY GRAVITY
SURFACE TENSION BECOMES DOMINANT
POWER/ENERGY SUPPLY
MAIN FUNCTIONS PROVIDE POWER/ENERGY DURING ALL PHASES AND MODES
CONCEPTS FOR POWER SUPPLY
PHOTOVOLTAIC EFFECT (η # 25 %)
Incoming solar flux
SEEBECK EFFECT (η # 5 %)
On board nuclear source (Pu238)
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POWER/ENERGY SUPPLY
TECHNOLOGIES FOR POWER SUPPLY
SOLAR ARRAY (100 W to 25 kW) Radio Isotopic Thermal Generator (~150 W)
POWER/ENERGY SUPPLY
CONCEPTS / TECHNOLOGIES FOR ENERGY SUPPLY
CHEMICAL REACTION
Li Ion SECONDARY CELLS (# 150 W.h/kg)
STACKED IN BATTERIES
(at 100 % depth of discharge)
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CONDITIONNING &
DISTRIBUTION
POWER/ENERGY CONDITIONNING & DISTRIBUTION
MAIN FUNCTIONS REGULATE THE POWER SUPPLIED AND DELIVER IT TO USERS
INSURE SYSTEM LEVEL ELECTRICAL PROTECTION
CONCEPT POWER IS DISTRIBUTED UNDER REGULATED (50 V)
OR SEMI REGULATED VOLTAGE (24/36 V), USING ELECTRONIC FUSES
(for avoiding failure propagation)
ENERGY
STORAGE
POWER
SOURCE
USERS
Power line (redunded)
Charge
Discharge
ON BOARD DATA HANDLING
MAIN FUNCTIONS COLLECT & PROCESS TELEMETRY DATA
PROCESS & DELIVER TELECOMMAND DATA
ON BOARD PROCESSING (reconfiguration, autonomy…)
DATA STORAGE
CONCEPT
User
Main
Computer
Group of Users
Data bus
Com
s/s TC decoder
TM coder
Direct TC for critical functions
ON BOARD DIGITAL DATA BUS WITH USERS
User User
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ON BOARD DATA HANDLING
TECHNOLOGY CORE IS THE PROCESSOR (current cap.: 16 bits, 20 MHz, 20 Mips)
AND THE SOFTWARE (often ADA) - STANDARD INTERFACES (OBDH)
COMMUNICATIONS
MAIN FUNCTIONS RECEIVE & DECODE TELECOMMANDS FROM THE EARTH
CODE & SEND TELEMETRY TO THE EARTH
SUPPORT SPACECRAFT LOCALISATION
CONCEPT
Receiver
Transmitter Modulator
Ultra StableOscillator
Demodulator
PHASE MODULATION OF A RADIOFREQUENCY CARRIER
High Gain Antenna(high focussing)
Operational mode
Low Gain Antenna(large coverage)
Emergency mode
On board data handling s/s
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COMMUNICATIONS
TECHNOLOGIES USED BANDS : S (2 GHz), C (4/6 GHz), X (7/8 GHz), Ku (12/14 GHz) ...
AMPLIFIER: SSPA (low frequency, low power), TWT (high power)
RECEIVER: LOW NOISE (FET)
ANTENNA: OPTIMIZED ACCORDING GAIN/COVERAGE
Traveling Wave Tube (S band, 200 W, η = 62 %, 1.8 kg) Antenna : focussing energy
Feedhorn
Low gain antenna
Main reflector
Sub reflector
COMMUNICATIONS : LINK BUDGET
Antenna axis
Antenna parameters
Aperture θ = 21/ F / D (F in GHz, D in m)
Gain in axis ~ 28,000 / θ2
(in dB 10 log G)
Simple link budget
The quality of a link is defined by the Carrier power to Noise
density ratio (C/No) given by the following relation (in dB):
C/No = EIRP + Fsl + G/T – kEIRP = Emitted Isotropic Radiated Power representing the
effective emitted power (in dBW, includes the amplifier output Pe,
internal emitting losses and the antenna gain Ge)
Fsl = Free space loss = 20 log (wavelength/(4 * π * path length))
G/T = figure of merit of the receiver (in dB/K), which includes the
receiving antenna gain and the temperature noise of the receiver
system (# 1300 K for receiving at spacecraft level)
k = Boltzmann constant = - 228,6 dBW/Hz/K
Emission : Pe Ge(EIRP)
Reception : G/T(figure of merit)Free space loss
Multiple link budget Link 1 Link 2
Internal
Global link
(No/C)g = (No/C)1 + (No/C)i + (No/C)2
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MECHANISMS
MAIN FUNCTIONS IN ORBIT DEPLOYMENT (single shot activation)
FINE POINTING (limited movment)
CONTINUOUS ACTIVATION (solar array pointing, wheels...)
CONFIGURATION CHANGE (filter wheel, shutter ...)
CONCEPT MOST IMPORTANT SOURCE OF FAILURE (difficult to test in
representative conditions, hard to have redundancy)
SIMPLICITY IS A MUST
CAREFULL CHOICE OF MATERIALS
HIGH DESIGN MARGIN (particularly torques)
A LOT OF VALIDATION/TESTS
SINGLE ACTUATION : SPRING, SHAPE MEMORY ALLOY, PYRO
MULTIPLE ACTUATION : ELECTROMAGNETIC MOTORS
MECHANISMS
TECHNOLOGIES
LUBRIFICATION: DRY (MoS2, soft metal, PTFE) FLUID (Perfluoropolyether PFPE)
TRANSMISSION: GEARS
HARMONIC DRIVES
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MECHANISMS
TECHNOLOGIES
MOTORS: STEPPER DC BRUSHLESS
GUIDANCE: CONTACT (rolling bearings) DISTANT (magnetic bearings)
=> Needs closed loop control
PYROTECHNICS
MAIN FUNCTION SINGLE USE ACTUATOR (valve opening, launch lock release...)
CONCEPT & TECHNOLOGIES SOLID POWDER BLOCK INITIATED ELECTRICALY
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HARNESS - WIRING
MAIN FUNCTIONS INTERCONNECT EQUIPMENTS FOR DATA EXCHANGE OR
POWER SUPPLY
CONNECTORS ARE PART OF THE HARNESS
CONCEPT & TECHNOLOGIES MATERIAL SELECTION (avoid outgassing)
CAREFULL ROUTING FOR:
- AVOIDING ELECTRO-MAGNETIC EFFECTS
(EMI/EMC, coupling with Earth magnetic field)
- EASE INTEGRATION AND TEST
FLIGHT HARNESS REALISED BY SPECIALIZED COMPANIES
HOW ASSESSING TECHNICAL MATURITY ?
Instruments and spacecraft sub-systems are classified according to a
"Technology Readiness Level" (TRL) on a scale of 1 to 9. Levels 1 to 4 relate to
creative, innovative technologies before or during mission assessment phase.
Levels 5 to 9 relate to existing technologies and to missions in definition phase.
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SYNTHESIS
THE DESIGN OF A SUB-SYSTEMS CANNOT BE INDEPENDANTLY FROM THE
OTHERS SINCE THERE IS A LOT OF INTERACTIONS
IN FACT, THE OPTIMUM AT SYSTEM/ARCHITECTURE LEVEL IS QUITE
OFTEN NOT THE SUM OF THE OPTIMAL SOLUTIONS AT SUBSYSTEM
LEVELS BUT MUCH MORE THE OPTIMAL SOLUTION IN TERM OF
INTERFACES
THIS IS WHY SYSTEM/ARCHITECTURE ENGINEERING IS ESSENTIAL
DEGREE OF MATURITY OF A TECHNOLOGY OR EQUIMENT IS MEASURED
BY TRL
Thanks for your attention
Questions ?