Electric Power Systems

Question 1

In comparison to chemical propulsion, what are the advantages of using electric propulsion systems?

Answer 1

• 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).

Question 2

What is electron bombardment and explain how is it implemented in a gridded ion thruster?

Answer 2

It is a process where neutral atoms (i.e. injected propellants) are converted to positive ions by bombarding them with electrons.

Electrons are knocked off the atoms when fast moving electrons collide with them, forming positive ions.

The ions are then accelerated through the use of a electromagnetic field to create thrust

Question 3

In terms of solar array performance, how does the silicon based solar array cell differ from
the TJ-GaAS cell?

Answer 3

Silicon solar cells convert the visible solar spectrum into electrical power. [1]

Triple Junction gallium arsenide (TJ-GaAs) solar cells are sensitive to the UV light and IR
bands of the solar spectrum. [1]

TJ cells have the highest recorded efficiencies (as high as 43% in lab tests) but it can be
more expensive compared to silicon solar cells. [2]

Question 4

What are the advantages and disadvantages of having BCR and BDR?

Answer 4

[Advantages]

-Having BCDU can regulate the Bus system (i.e. BR system), which leads to simplified thermal design of the spacecraft (e.g. electric heater circuits deliver constant heat at a fixed bus voltage). [1]

-Good electromagnetic compatibility (EMC) performance due to load regulation and bus impedance performance. [1]

[Disadvantages]

-Higher EPS weight. [1]

-Higher cost because of the BCR and BDR devices. [1]

-More solar array power required (approximately 3-13% loss in BDR and approximately 3- 10% loss in BCR). [1]

Question 5

Briefly describe Lorentz force in terms of combination of electric force and magnetic force

Answer 5

It is the combination of electric and magnetic force on a point charge due to electromagnetic fields, F = q(E + v x B):

As in the case of electromagnetic propulsion (left hand rule), the electric current and magnetic field exist (which are the causes), and they lead to the force that creates the motion (which is the effect).

Thumb: to point the direction of thrust or motion
Index finger: pointing the direction of magnetic field
Middle finger: pointing the direction of current

Question 6

State four different types of electrostatic thrusters

Answer 6

• Gridded Ion Thruster
• Stationary Plasma Thruster
• Thruster with Anode Layer
• Cylindrical Hall Thruster

Question 7

What is an electrostatic force?

Answer 7

The electrostatic force is an attractive and repulsive force between particles are caused due to their electric charges. The electric force between stationary charged bodies is conventionally known as the electrostatic force.

Question 8

Describe the advantages and disadvantages of stationary plasma thruster

Answer 8

[Advantages]
Smooth and continuous variation in plasma potential between anode and cathode
Effective exhaust velocity: 10-25 km/s

[Disadvantages]
Erosion due to plasma

Question 9

Describe the advantages and disadvantages of a thruster with anode layer

Answer 9

[Disadvantages]
Smaller channel outer diameter

[Advantages]
Lesser erosion due to plasma

Effective exhaust velocity: 10-40 km/s

Question 10

How does electromagnetic propulsion (EMP) differ from the electrothermal propulsion system?

Answer 10

[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. [1]

Question 11

Briefly describe, what is an ion and ionisation?

Answer 11

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.

Question 12

Briefly describe, what is the working principle of resistojet thruster?

Answer 12

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.

Question 13

Briefly describe the working principle of fuel cell and state one suitable space mission.

Answer 13

Two Electrodes provided with oxygen and hydrogen are placed in an electrolyte (e.g. Phosphoric acid)

Electrons flowing through the external circuit provide the desired electrical energy

spacecraft and space station power systems.

Question 14

List and briefly describe the three life-determining parameters of a satellite battery.

Answer 14

[Calendar life]
The time in which the battery or a certain part of its capacity becomes useless independent of whether the battery has been actively operated or not.

[Cycle life]
Defined as the number of charge-discharge cycles a battery can perform before parts of its nominal energy content fall.

[Depth of discharge (DoD)]
How much it can be discharged before charging (typically 85% for NiCd and NiH2, 60-70% for Li-ion cells)

Question 15

What are the propulsion requirements for the three key functions:
a) orbit achievement;
b) orbit adjustments;
and c) disposal?

Answer 15

Orbit achievement: high thrust, high efficiency, low precision. [1]

Orbit adjustment: high precision, efficiency not an issue. [1]

Disposal (de-orbit): high efficiency, high dormant reliability, low precision. [1]

Question 16

State the four different types of electrothermal thrusters.

Answer 16

• Resistojet. [1]
• Arcjet (DC, AC, Pulsed). [1]
• Electrothermal hydrazine. [1]
• Microwave electrothermal. [1]
• Pulsed electrothermal. [1]

Question 17

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’

Answer 17

mn = ms * (Pn/Ps)^0.7

https://app.box.com/s/9ubbett2rczkbc9plzh91dqnkbntmgv0/file/872749983903

Q4 part c

Question 18

what are the advantages and disadvantages of BR with DET?

Answer 18

[Advantages]
DET electronics are simple (as compared to regulated bus with MPPT) and save weight and therefore saves launch cost. [2]

[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) [2]

Question 19

Draw the essential functional blocks of an EPS for a satellite.

Answer 19

https://app.box.com/s/9ubbett2rczkbc9plzh91dqnkbntmgv0/file/872744553799

3A

Question 20

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.

Answer 20

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.

Question 21

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

Answer 21

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)

Question 22

What are the three major categories of electric propulsion? Give one example for each of the three categories.

Answer 22

i) Electrostatic propulsion [1]
(e.g. Ion Bombardment, Hall effect thrusters)

ii) Electromagnetic propulsion [1]
(e.g. MPD, Variable specific-impulse plasma thrusters)

iii) Electrothermal propulsion [1]
(e.g. Resistojet, Arcjet thrusters)

Question 23

Describe what type of propulsion subsystems are used for each of the four tasks: i) Apogee Injection

Answer 23

• 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

Question 24

Describe what type of propulsion subsystems are used for each of the four tasks: ii) Orbit Control

Answer 24

• 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

Question 25

Describe what type of propulsion subsystems are used for each of the four tasks: iii) Attitude control

Answer 25

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.

Question 26

Describe what type of propulsion subsystems are used for each of the four tasks: iv) Additional tasks such as course corrections and braking manoeuvres

Answer 26

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.

Question 27

suggest four possible improvements to increase the Solar array’s operating point current (Iop). This effectively increases the array’s electrical power.

Answer 27

a) Increase the number of series-connected cells per string.

b) Increase the number of parallel solar cell strings.

c) Increase the active area of the solar cell .

d) Increase the cell efficiency by increasing the short circuit current ; short circuit current increase proportionally to the photon intensity.

Question 28

What is a solar sail and explain briefly its physical principle for generating sailing propulsion.

Answer 28

• Solar sail comprises of 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]

Question 29

What are the major advantages and disadvantages of an electric propulsion system as compared to conventional chemical propulsion system?

Answer 29

[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

Question 30

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”

Answer 30

It is sometimes related to the problem of flow accelerated corrosion (FAC).

By definition, the rate of FAC depends on the flow velocity. [1]

An increase in specific impulse results in an increase in exit velocity. Thus leading to intensified FAC (chemical erosion). [1]

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). [3]

Question 31

With the calculated value of mp from (i) and the derived mp/ms ratio, suggest a way forward to achieve a higher velocity increment ( DeltaV) for the satellite.

(Mp/Ms) = 1 - e^(-DeltaV/Ve)

Answer 31

Instead of monopropellant system, use bipropellant or electrical propulsion system to increase the velocity increment (v).

Electrical propulsion system in particular is capable of producing large DeltaV using small amount of propellant (hence mp/ms ratio can be comparatively smaller than the other systems).

Question 32

For a spacecraft, power (energy) storage is usually realised by battery. State two other energy storage systems.

Answer 32

Flywheels and regenerative fuel cells.

Question 33

Figure 3-1 shows the working principle of a solar sail. It is known that the solar radiation exerts a pressure on the sail due to reflection. Copy the figure on your answer sheet and write down what each of the arrows represent (as depicted by (?) in the figure).

https://app.box.com/s/9ubbett2rczkbc9plzh91dqnkbntmgv0/file/432683823847

Q3C

Answer 33

-Reaction Force

-Reflected momentum

-Incident momentum

-Momentum Change

Question 34

The solar sail material is typically aluminised temperature-resistant material (LaRCT-CP1). What are the requirements for solar sail material?

Answer 34

Besides being lightweight, the material must be highly reflective and able to tolerate extreme temperatures.

Question 35

What are the advantages and disadvantages of regulated bus (BR) with MPPT?

Answer 35

[Advantages]
MPP of the Solar array is automatically extracted (Solar array power can be utilized whenever needed), ideal solution for missions with strong variations in solar intensity.
[1]

[Disadvantages]
Requires more circuit design effort than DET (higher weight and cost)

Question 36

What are the three main characteristics of an electric propulsion systems?

Answer 36

Use much less propellant than chemical rockets because they have a higher exhaust speed (operate at a higher specific impulse). [1]

Due to limited electric power the thrust is much weaker compared to chemical/nuclear rockets (not a method suitable for launches from the Earth’s surface). [1]

Electric propulsion can provide a small thrust for a long time (ideal for deep space exploration). [1]

Question 37

In general, an electric propulsion system (EPS) is used for low-thrust manoeuvres. List five advantages and/or applications for using an ESP in lieu of chemical propulsion system

Answer 37

• Attractive mass saving (less propellant needed). [1]
• Applications requiring high velocity increments:
  —For North-South station keeping (50m/s per year). [1]
  —For orbit raising (1 to 5km/s). [1]
  —For orbit transfer from LEO to GEO (6km/s). [1]
• Attitude control in LEO satellites (e.g. international space station). [1]

Question 37

the MPD thruster uses hydrogen or lithium as propellant. State the respective performance characteristics (e.g ionisation energy, power requirement and exhaust velocity) and advantage/disadvantage of using these propellants.

Answer 37

[Lithium]
-Very low first ionisation energy
-Low power requirement (typically 0.5 – 5 MW)
-Moderate exhaust velocities (20-70 km/s)
-Reduces the cathode erosion problem.

[Hydrogen]
-Low first ionisation energy
-Multimegawatt power requirement (),
-High specific impulse operations
-High exhaust velocities
-Easier to supply to the thruster as a gas (ease of handling)
-Less likely to condense on spacecraft surfaces.

Question 38

What are the 4 subsystems for electric propulsion Tasks

Answer 38

a) Apogee Injection
b) Orbit Control
c) Attitude Control
d) Additional tasks such as course corrections and braking manoeuvres

Question 39

Describe the advantages and disadvantages of a Cylindrical Hall Thruster (CHT)

Answer 39

CHT has lower surface-to-volume ratio (potentially smaller wall losses in the channel)
Ion thrust is generated by the Lorentz Force
Power range: 50-300 W
Thrust 2.5 to 12 mN
Efficiency 15 to 32%