Satellite Systems Flashcards
Describe the (6) environmental factors acting on a satellite during launch. Which are the most severe?
1.Acceleration: Increases to the end of each working stage
2. Severe Acoustic/vibration environment
3. Mechanical Shocks: Explosive bolts, latches, stage ignition, etc
4. Thermal environment: Aerodynamic friction heats shroud
5. Ambinet atmospheric pressure: Permanently declines 4-5kPa/s
6. Electromagnetic interference. Can activate payload
Most severe are vibrations and shocks
What are the (9) principal effects of the space environment on a satellite?
- Vacuum
- EM radiation: Solar or Cosmic Rays, covers most of the spectrum
- Plasma (Solar wind, flare, Atmospheric Plasma in LEO)
- High energy particles/ions (cosmic/VA belts)
- Atmosphere (LEO experiences drag and erosion)
- Meteoroids/Micrometeoroids
- Man-made debris
- Gravitational Fields
- Magnetic Fields
Describe the effect of a vacuum on satellite materials and systems. Outline negative and positive effects
Negative:
Materials sublime and outgas
Condensation of gases on cold surfaces is more serious
Traditional lubricants dont work as they evaporate quickly, low volatility required
Contact surfaces tend to cold-weld.
Positive:
Metals do not corrode
Fatigue life is improved
Explain effect of “self-healing” of some metals in vacuum leading to improving material strength and fatigue life.
Microscopic cracks appear in a metals surfaces when it is stretched or bent. In atmosphere, this would be covered by oxides very fast, but in vacuum this can’t happen. Cracks opposite surfaces contact each other again and cold-weld restores materials integrity. On Earth, oxides form to prevent cold-weld, causing cracks to grow under alternating loads
Describe the sources of high energy particles and describe the effect of ionizing radiation on satellite systems.
- Solar Flares
- Van Allen Belts (High energy particles trapped by the terrestrial MF) Essential for MEO
- Cosmic Rays
Affects all materials, however semiconductors and biological tissue are especially sensitive to radiation
Describe the solar radiation mentioning all component of this radiation.
The Sun emits EM radiation across the spectrum, from high-energy x-rays to long-wavelength radio waves.
Peak of this emissions is visible portion of spectum, at 460nm.
Solar wind is plasma expelled at high velocity.
Sunspots emit X and Gamma rays
Describe the structure of terrestrial magnetic field. How the solar wind affects its structure?
Earths magnetosphere has two sources, the current in Earths core and currents caused by different motions of charged particles in magnetosphere. The field can be modelled as a dipole structure, however magnetic field is not aligned with rotational axis. The solar wind also distorts Earths simple dipole field, creating a shock front and a magnetotail
What hazard do meteoroids and material particles present to a satellite in orbit? What design is used to decrease the effect of meteoroid impact?
Impact of micrometeoroids causes a degradation of the surface thermal properties. Heavier particle can perforate and the component failure. Anexample of multilayer protect is a whipple bumper which uses a bumper to break-up particle and catch debris with Whipple bumper shield
Describe solar flares and their effect on a satellite/spacecraft.
Solar flares are sudden releases of magnetic energy. Flare ejects clouds of electrons, ions, atoms, superheated plasma and EM radiation. These pose a threat to spacecraft electronics and any biological objects, such as humans. High-energy protons pass through human body, causing biochemical damage, which presents hazard during interplanetary travel.
Which of the environment factors are the most dangerous for the crew travelling to Mars?
X-ray radiation is the main risk in manned mission to Mars, the moon, or other planets. Other risks include fragments ejected from Mars during collision impacts
Explain how you would calculate the area of a solar sail producing 1N thrust at Martian orbit . The sail is located perpendicular to the solar sail and reflect 100% of radiation.
SRP,e = 10^-5 Pa
SRP,m = SRP,e(R,e/R,m)^2
Area = F/(2SRP,m)
Describe Van Allen belts and their effects on Satellites
Layer of energetically charged particles trapped by terrestrial magnetic field. Electrons form outer belt whereas inner contains protons as well. Two belts go from 640-58000 km alt. Most particles originate from solar wind or cosmic rays. Magnetic field deflects or captures particles and protects atmospheres. Inner belt is more dangerous for satellites as protons cause more damage.
Describe the physical phenomenon lying in base of van Allen belt formation
The phenomenon of magnetic trapping of charged particles in the non-uniform magnetic field causes Van Allen belt formation. Particles spiral along MF lines, and as intensity of field increases, rotation velocity increases while translational velocity decreases. If MF intensity gets too high, translational component vanishes, particle cannot penetrate further into field, and is reflected. This point is known as mirror point.
Explain the South-Atlantic Anomaly.
An area where the VA belts come closest to Earths surface, to altitude of 200km. Causes increase flux of energetic particles in this region, exposes orbiting satellites to higher radiation levels.
Caused by non-concentricity of Earth and its dipole. Anomoly appears where MF is weakest
Describe the problem with lubrication in space.
Usual lubrication as liquid substances usually evaporate fast in vacuum. Also large/fast change of temperatures and radiation can change the properties of standard lubricants.
Special solid material withstanding vacuum, fast change of temperature and radiation have to be
used, eg ceramic, special alloys: MoS2, Stellite
Describe origin of atomic oxygen (O) and its effect on a satellite.
Intensive ultraviolet radiation lead to dissociation of neutral O2 molecules into atomic oxygen, LEO atmospheric 96% atomic Oxygen.
Atomic Oxygen is a extremely reactive, and travels at high speed in space. Causes erosion, formation of oxides, chemoluminiscent glow, general material degradation. Silver is especially vulnerable, along with lube Molybdenum Disulfide
Explain how you would calculate expected time to collide the satellite of 5 m diameter by a meteoroid in the mass range 0.01-0.5 g, given equation of approximation of number of meteoroids in mass range, eg:
log10(𝑁 ≥ 𝑚) = –14.37–1.213*log10(𝑚) , 0.01<𝑚<0.5g
Calculate Area, modelled as a circle. Sub in min and max meteoroid mass, calculate number, N, within range by finding difference. Then use equation t = 1/(dN*A) to find time.
Describe the evolution of an elliptic orbit effected by atmospheric drag at its perigee.
Orbit evolution caused by the atmospheric drag: elliptic orbit → circular → spiral → burned.
Draw Diagram
Describe the problem of zero weight in a spacecraft and methods of its solution.
Due to zero gravity, small effects can disturb satellite attitude normally not accounted for on Earth, causing an extremely small torque which leads to rotation, such as:
GF non-uniformity
MF non-uniformity
Outgassing
Solar and antenna radiation pressure
LEO can use GF to control attitude but any hgiher orbits the GF is too weak
Describe the (10) existing satellite missions. Indicate types of orbit most preferable for the given mission.
- Communications: GEO for low latitudes; Molniya-Tundra orbits for high latitudes; Constellations of polar LEO satellites for global coverage
- Navigation (positioning): Inclined MEO for global coverage
- Earth observation and imaging: Polar LEO for global coverage
- Weather observation (meteorology sat): Polar LEO, or GEO
- Space observation including astronomy: LEO, HEO, GEO and L-points
- Scientific experiments: LEO, MEO, GEO with small inclination
- Space stations: LEO (below Van Allen belts)
- New technology testing and displaying: Various
- Military: Polar LEO for global coverage, but various
- Amateur radio: small satellite (LEO, but not only)
Derive equation of the gravitational potential per unit mass: E,p = − 𝜇/𝑟
W = int(F dr, r2, r1) = m * int(mu/r^2 dr, r2, r1)
W = [-m * (mu/r)]r2,r1 = (mu/r1 - mu/r2) * m
W = PE(r2) - PE(r1)
Therefore PE(r) = -mu/r * m +const
Per unit mass, pe(r) = -mu/r
Given hp, ha, find time to move from perigee to furthest point from line of apiside if TA>180 degrees. (t - tp)
a = (hp+ha/2)+RE
T = 2pi*sqrt(a^3/mu)
T/4
Given ha and hp, find a,b,e,T and then True Anomoly and distance to centre of Earth at t-tp = 3
a = ((hp+RE)+(ha+RE))/2
e = (ra-rp)/(ra+rp)
b = asqrt(1-e^2)
T = 2pisqrt(a^3/mu)
n = 2pi/T
M = n(t-tp)
Use graph to find E
x = acos(E) - ae
y = bsin(E)
r = sqrt(x^2 + y^2)
TA = tan-1(y/x)
Given satellite coordinates x and y, find r and theta (TA)
r = sqrt(x^2 + y^2)
TA = tan-1(y/x)
Describe Geocentric Equatorial Inertial (GEI) coordinates
A frame of reference fixed relative to the stars. X¡ - and Y-axes lie in the Earth’s equatorial plane and the Earth spins about the Z-axis. The X¡ -axis is in a direction from Earth to the Sun at the vernal equinox (21 March). The direction thus indicated is termed the first point of Aries ¡.
It did point towards the constellation of Aries some 2000 years ago, but at present it points to Aquarius. The Z-axis is along the Earth’s spin axis, in the northerly direction, i.e. at an angle ~23 degrees to the normal of the ecliptic plane.
Find position of satellite in (Cartesian) Geographic coordinates (CGeo) having GEI coordinates [x,y,z] km if the sidereal time is ST hour.
ST,d = (ST/24)×360
Z = Z,gei
𝑋 = cos ST × 𝑋,gei + sin ST × Y,gei
𝑌 = − sin ST × 𝑋,gei + cos ST × 𝑌,gei
List the (7) parameters which are necessary to determine special position of a satellite in the Cartesian Geographic (CGeo)
- Longitude of ascending node
- Orbit inclination
- Argument of periapsis
- Perigee altitude
- Apogee altitude
- Time since last passing perigee
- Sidereal time
Explain the meaning of terms: (i) the true anomaly, (ii) the eccentric anomaly, (iii) the mean anomaly, (iv) the mean motion. Draw a sketch of an elliptic orbit clarifying the terms.
True anomaly - Angle between line of apsides and direction of satellite from focus
Eccentric Anomaly - Angle between direction to perigee and direction to auxillary point r’
Mean motion - The average angular velocity relative measured from centre of ellipsis.
Mean Anomaly - Angle made by the mean motion multiplied by the time since last passing the perigee.
Explain the meaning of terms: (i) line of nodes, (ii) ascending node, (iii) right ascension of this node, (iv) inclination angle, (v) argument of periapsis. Draw a sketch of an elliptic orbit in space clarifying the terms.
(i) Line of nodes is where the orbital plane intersects the equatorial plane
(ii) the ascending node is the point which the satellite intersects the equator moving from the southern into the northern hemisphere.
(iii) The right ascension of this node is the angle of the direction to the ascending node referenced from the X,gam-axis
(iv) The inclination angle 𝑖 is the angle between the orbital plane and the equatorial plane
(v) The orientation within the plane is defined by the angle w, called the argument of periapsis, which is the angle measured in the direction of motion between the line of nodes and the vector pointing to the periapsis.
Explain the difference between the solar day and the sidereal day
A solar day is the time it takes for Earth to complete one rotation relative to the Sun, approximately 24 hours. A sidereal day is the time for one complete rotation relative to a distant star, about 23 hours, 56 minutes, and 4 seconds. The difference is due to Earth’s orbital motion around the Sun.
Describe which forces should be balanced in a Lagrange point.
For a two-body system, the graviational components of force of both bodies and the centrifugal force from its angular velocity whilst orbiting
Describe how the Earth’s oblateness disturbs satellite orbit motion and explain why the inclination angle of 63.4o is the most appropriate for high elliptic orbits.
Earth’s oblateness results in rotation of the line of nodes. It leads to a rotation of the orbit within the orbital plane, the direction of which depends on the orbit inclination.
The procession is zero at the inclination at 63.4 deg. This`frozen apogee’ condition gives a good effect for high-latitude communication satellites using a highly eccentric orbit.
Calculate the distance from earth’s centre to centre of mass in the system earth-moon given mass of Earth, Moon, and average moon orbit radius
rC = (mE0 + mMrM)/(mE+mM)
Ans should be within Earths crust if its Earth-Moon system
Describe factors disturbing the satellite orbit from that described in the framework of two body motion.
- Earth’s GF not being uniform
- Atmosphere drag (LEO)
- The Moon’s gravitational field
- The Sun’s gravitational field
- Sun radiation pressure (SRP)
Calculate propellant required to transfer from a GSO with incline orbit to a GEO orbit, given inclination angle, final mass and specific impulse.
w = 2pi/T where T is 86400s
r = cuberoot(mu/w^2)
v = rw
dv = 2 * v * sin(i/2)
mp = m * e^(dv/v -1)
Describe the terms energy consumption and propellant mass consumption of a thrustor. How they depend on the exhaust speed?
Energy consumption in a thruster refers to the power needed to operate, influenced by design efficiency. Propellant mass consumption is the rate at which propellant is expelled to generate thrust, affected by the exhaust speed. Higher exhaust speeds generally increase energy consumption but decrease propellant mass consumption, optimizing spacecraft efficiency.
Explain the term specific impulse and why it is an important characteristic for a thruster?
It quantifies how effectively a propulsion system utilizes propellant to generate thrust. Defined as the ratio of thrust produced to the rate of propellant consumption. Thrusters with a higher Isp require less propellant to achieve a given momentum change. Units are seconds
Describe cold gas thruster design
Engines based on releasing a gas stored under pressure in a reservoir through a nozzle, such as Freon, Propane or Nitrogen. They’re simple, reliable, but have low thrust and Isp.
Describe Field-emission electric propulsion Thrusters (FEEP)
A propellant of liquid metal such as mercury, caesium, etc emit ions when under influence of electric field. Ions are accelerated by field and expelled at high velocity. Good for micronewton attitude control.
Describe design and operation of the Electrostatic (gridded ion) thruster
An electron gun is used to ionise neutral molecules in chamber, which is surronded by magnets. These positive ions accelerate between by high differential voltage grid, exiting the back of chamber and meeting an electron gun to neutralise the flow
What is the advantage of the Hall Effect thruster by comparing with the Gridded Ion thruster?
Hall effect thrusters generally have higher thrust and efficiency as it allows for the direct acceleration of the ions, and no grid is required
