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Spacecraft Systems Engineering - Coggle Diagram
Spacecraft Systems Engineering
Orbital Mechanics
Types of trajectories
Hyperbolic
Parabolic
Circular
Elliptical
GM or µ is the standard gravitational parameter
Each planet has a specific escape velocity based on their size and std gravitational parameter
Orbital period is how long an object takes to orbit a planet
Hohmann transfer
is the minimum energy, two manoeuvre transfer from one circular orbit to another coplanar circular orbit
The
perihelion
is the point in the orbit of a planet, asteroid or comet that is nearest to the sun. It is the opposite of
aphelion
, which is the point farthest from the sun.
Missions
Inclination angle
dictates how far north/south you go on the Earth. It is the angle between the s/c orbit and the equatorial plane
7 parameters define a spacecraft orbit:
Velocity (U,W,V)
Position (X,Y,Z)
Time (T)
Most spacecraft are placed into
sun-synchronous orbits
which means that as the Earth revolves around the sun, the angle between the spacecraft and the sun doesn't change. This means, solar panels can be made to constantly face the sun without being moved. Satellites in this orbit aren't good for Earth observation
Molniya orbits are highly elliptical and have an inclination angle of 63.3°. They are used by comms in the northern hemisphere as the satellite stays high in the sky for longer.
Can use multiple satellites to create composite images of the Earth. 3 GEO satellites can do this.
Constellations are defined by: inclination angle: number of satellites/ number of planes/ spacing between planes
Reliability. Each satellite has an individual reliability, so use the binomial distribution to work out the reliability of a constellation.
Mission Analysis
Pre Launch
Launch phase
Orbit Transfer
On Station Operations
Decommissioning
. At the end of it's useful life a satellite must be removed from its orbit. Below 500km, a satellite will gradually fall down and burn up so thrusters are used to control the descent and guide it to unpopulated areas or the sea. For higher satellites the opposite happens, they are placed into a higher orbit.
The ground station is responsible for:
Tracking
Telemetry
Command operation
control
data processing
voice and data links (ISS)
Thermal control
Electrical and mechanical equipment works best within a narrow range of temperatures
Most materials expand and contract with temperature changes which implies thermal distortion
A spacecraft receives heating from three sources:
Solar (~1370 W/m^2)
Albedo (~480 W/m^2) (35% of the sun's energy is reflected off the Earth)
Planetary (~273 W/m^2)
Heat pipes are used to pull heat away from sensitive components
Louvres are fitted to radiators in order to regulate the heat loss
Cryocoolers can cool sensors to ~20K
Rockets
Propulsion systems
Jet propulsion
Duct propulsion
: Some or all of the expelled mass is first captured from the surrounding environment.
rocket propulsion
: Only mass stored in the vehicle is expelled.
The
Tsiolkovsky
equation gives the resultant ▲V from a rocket burn in terms of a) the effective exhaust gas velocity (Ve*) and b) the fraction of all mass that is used as propellant in the rocket engine burn.
Energy sources
Chemical reaction
Ion drives
Nuclear pulse
Solar sails
Three basic elements of a launch vehicle are: the
payload(s)
, the
propellant tanks
and the
engine
The
impulse
(I) of a rocket is the change in momentum of an object when it is acted upon by a force F for time t. It is useful because it doesn't depend on the mass of the payload.
Impulse alone doesn't tell us anything about the efficiency of the rocket so we define the
specific impulse
. Which is the change in momentum of the rocket per unit weight of propellant expelled.
The units of specific impulse are seconds which removes the risk of error due to seconds being a global unit.
Liquid fuelled rockets
fuel stored in tanks and moved by pumps
Often used for main engines on launch vehicles
Components
Fuel and oxidiser tanks
gas generator
turbo pumps
injector
combustion chamber
Solid fuelled rockets
Often used for boosters on launch vehicles, for sounding rockets or for short range military missiles.
fuel stored as a solid at environmental temp. must be burned where it is.
"Propellant grain" contains solid fuel and oxidiser
fuel burns from the inside out
Grain profiling
allows control of the trust-time curve
The grain must be strong enough to line the case without cracking or slumping from it's weight, resist thermal expansion, vibration or pressure.
Nozzles
The nozzle is a factor that can control the exhaust gas velocity
essentially a pinched tube
various shapes have been used that result in different expansion of the exhaust gasses
Vanes can be inserted to add steering
nozzles get extremely hot so need cooling via either radiation cooling using black body radiation or regenerative cooling where fuel is circulated around the nozzle before being combusted.
Other types of rocket engines
Hybrids
: use a solid propellant and a liquid oxidiser, they combine many of the advantages of both liquid and solid fuel but aren't easily scalable due to their complexity
Resistojets
: use electrical energy to expand the propellant. They have a high specific impulse but low thrust and are often used for orbital manourving.
Arcjets
: similar to resistojets, propellant passed through an electrical arc
Solar thermal rockets
Nuclear thermal rockets
Ion thrusters
Solar sails
Electrical power systems
All systems will include: A primary and secondary energy source and a power control and distribution network
Solar arrays
are the most common power source on spacecraft
Fuel cells
are used to produce energy with low heat, can deliver a lot of power over a short time
RTG
(radioisotope thermal generator): a radioactive material is used to produce hear. A thermocouple is used to turn the heat into electrical energy. These are commonly used for deep space missions