Pure Theory Flashcards
Draw the essential function blocks of an EPS for a satellite
[Primary Power source] -> [Power conditioning]
[Energy Storage] <-> [Charge and discharge control]
Both lead into
[Power distribution] -> [power consumer]
If the conversion of primary energy into electrical energy is performed by using the
photovoltaic effect of solar cells, describe briefly how the photoelectric current is
generated.
Essentially, two processes are responsible for the conversion of sunlight into electrical energy
within a solar cell:
1) The absorption of solar (electromagnetic) radiation within a light-absorbing semiconductor and
the associated generation of charge carriers; and
2) The separation of those electrons and holes under the influence of the electric field across a
semiconductor junction, thus generating an electromotive force and photocurrent.
A solar cell is characterised by the following five parameters. Describe briefly what
each of the parameters stand for and show how parameter 5) can be represented in
terms of the preceding four parameters.
1) Isc
2) Voc
3) Imp
4) Vmp
5) Fill factor
1) Isc: Short circuit current (voltage V = 0; load resistance R = 0)
2) Voc: Open circuit voltage (current I = 0 at infinite load resistance R = )
3) Imp: Maximum power point current (current at maximum solar cell output power)
4) Vmp: Maximum power point voltage (voltage at maximum solar cell output power)
5) Fill factor: (Isc x Voc)/(Imp x Vmp)
By referring to the Equation 3-1 and the given parameters, suggest four
possible improvements to increase the array’s operating point current (Iop). This
effectively increases the array’s electrical power.
a) Increase the number of series-connected cells per string (currently 25).
b) Increase the number of parallel solar cell strings (currently 100).
c) Increase the active area of the solar cell (currently 26 cm2).
d) Increase the cell efficiency by increasing the short circuit current
(currently 0.0168 A/cm2); short circuit current increase proportionally to
the photon intensity.
What is a solar sail and explain briefly its physical principle for generating sailing
propulsion.
- Solar sail comprises large ultra-thin mirrors and are a form of spacecraft propulsion utilising
the solar radiation pressure. [1] - Solar radiation exerts a pressure on the sail due to reflection. [1]
- The momentum of a photon is given by p = E/c (E: photon, c: speed of light). An actual sail
will have an overall specular reflection efficiency of about 90%. [1]
Describe the abovementioned tasks i) – iv), stating what type of propulsion
subsystems are generally used for the task.
i) Apogee Injection
ii) Orbit Control
iii) Attitude Control
iv) Additional Tasks
i) Apogee Injection:
- Apogee engine with a thrust level of 400 to 600N (Bipropellant Propulsion).
- Engine is activated in the apogee to move the satellite into the required circular orbit.
ii) Orbit Control:
- Thrust levels of 10 to 22N (Monopropellant Propulsion).
- Maintenance of the required position (north-south and east-west station keeping)
- Injection into a graveyard orbit
iii) Attitude Control:
- Thrust levels between 1 and 22N (Monopropellant/Bipropellant Propulsion).
- Orientation of the satellite for pointing antennas toward the Earth or the solar arrays toward
the Sun.
iv) Additional Tasks:
- Thrust levels in the order of some millinewtons up to about 1N (Electric propulsion).
- Providing large v, precise course corrections during flights lasting several years. [1]
What are the three major categories of electric propulsion? Give one example for
each of the three categories.
i) Electrostatic propulsion
(e.g. Ion Bombardment, Hall effect thrusters)
ii) Electromagnetic propulsion
(e.g. MPD, Variable specific-impulse plasma thrusters)
iii) Electrothermal propulsion
(e.g. Resistojet, Arcjet thrusters)
What are the major advantages and disadvantages of an electric propulsion system
as compared to conventional chemical propulsion system?
Advantages:
- High specific impulse (Isp) and velocity increment v
- High payload ratio (60%)
- Less propellants
Disadvantages:
- A very low thrust (millinewtons to 1N)
- Must be qualified for much higher thrust durations
- High amount of thrust-level-dependent electrical energy
Electrothermal thrusters use ionisable gases as the propellant. Arcjet thruster, for
example, uses Hydrazine (N2H4). What is the advantage and the disadvantage of
using N2H4 as compared to Hydrogen (H2)?
[Hydrazine (N2H4)]
Advantage
- Can be dual used for a combination propulsion system on satellite. Can be easily
stored.
Disadvantage
- Chemical erosion problems are intensified at higher specific impulses. Heat
transfer problems at the nozzle and chamber.
What is plasma?
Plasma: it is designated as the fourth state of matter (the others are solid, liquid,
and gas). It has some of the properties of a gas but is affected by electric and
magnetic fields.
What is Lorentz force?
Lorentz force: it is the combination of electric and magnetic force on a point
charge due to electromagnetic fields, F = q(E + v x B)
What are the propulsion requirements for the three key functions: a) orbit
achievement; b) orbit adjustments; and c) disposal?
a) orbit achievement: high thrust, high efficiency, low precision.
b) orbit adjustments: high precision, efficiency not an issue.
c) disposal: high efficiency, high dormant reliability, low precision.
In comparison to chemical propulsion, what are the advantages of using electric
propulsion systems?
- Chemical propulsion has high thrust but limited exhaust velocity (usually in the range of
2.5 to 4.5 km/s). High exhaust velocity can be achieved by electric propulsion systems
(typically 20 km/s). - Attractive mass saving in terms of propellant (payload ratio 6% for chemical propulsion,
60% for electric propulsion systems). - Electric propulsion can provide a small thrust for a long time (ideal for space exploration).
State the four different types of electrothermal thrusters.
- Resistojet.
- Arcjet (DC, AC, Pulsed).
- Electrothermal hydrazine.
- Microwave electrothermal.
- Pulsed electrothermal.
Draw a diagram and explain the working principle of a solar sail.
Solar sails use photo pressure of force on thin, lightweight reflective sheet to produce
thrust; ideal reflection of sunlight from surface (produces 9 N/km2 at 1 AU)
Reflected momentum on right
Incident momentum on left
Draw a diagram and explain the relationship of solar array power versus dry mass of
geostationary earth orbit (GEO) satellites by considering the ‘scaling for power level’.
mn = ms*(Pn/Ps)^0.7
y = array power from 0 to 20,000
x = dry mass, 0 to 4000kg
Figure 4-1 shows a schematic of regulated bus (BR) with direct energy transfer (DET)
electrical power system architecture. Figure 4-2 shows the solar array current versus voltage
characteristic. With reference to these figures, what are the advantages and disadvantages
of BR with DET?
DET electronics are simple (as compared to regulated bus with MPPT) and save weight
and therefore saves launch cost.
[Disadvantages]
Solar array voltage and the power operating point must be adjusted and optimised to EOL
operational conditions (wasting part of the higher BOL power performance), as can be seen
from the Figure 4-2 ‘area of shunt regulation’.
What is electron bombardment and explain how is it implemented in a gridded ion thruster?
It is a process where neutral atoms (i.e. injected propellants) are converted to positive ions
by bombarding them with electrons. [1]
Electrons are knocked off the atoms when fast moving electrons collide with them, forming
positive ions. [1]
How does electromagnetic propulsion (EMP) differ from the electrothermal propulsion
system?
[Electromagnetic propulsion]
It employs the principle of accelerating an object by the utilisation of a flowing electrical
current and magnetic field.
[Electrothermal propulsion]
It is a form of propulsion where electrical power (usually generated by a solar array) is used
to heat a fluid or gas.
Briefly describe, what is an ion and ionisation?
Ion is a charged atom or molecule.
It is charged because the number of electrons do not equal the number of protons in the
atom or molecule.
Ionisation is the process of electrically charging an atom or molecule by adding or removing
electrons.
Briefly describe, what is the working principle of resistojet thruster?
Resisto-jet works by heating the propellant through an external power source to high speed.
Heating is usually achieved by passing electricity through a resistor consisting of a hot
incandescent filament.
Thermal energy released is coverted into kinetic energy by a nozzle with high expansion
ratio.
In general, the Arcjet (N2H4) thruster is used for a combination propulsion system on
satellites. It has the disadvantage of having chemical erosion problems, which can be
intensified at higher specific impulses. Describe what is meant by the term ‘chemical
erosion’.
It is sometimes related to the problem of flow accelerated corrosion (FAC).
By definition, the rate of FAC depends on the flow velocity.
An increase in specific impulse results in an increase in exit velocity. Thus leading to
intensified FAC (chemical erosion).
Arcject thruster exhibits substantial erosion of the cathode due to extended operation at
very high cathode temperature levels and exposure during this time to highly reactive
species formed from trace impurities (hydrazine typically used as propellants).
