Electric Propulsion Flashcards
What is the principle of an Electric Propulsion System?
Charged Matter can be accelerated by electric or magnetic fields.
Mention the three main types of EP based on their Isp and what are these different types distinguished by?
- Electrothermal
- Electromagnetic
- Electrostatic
These families are distinguished by:
1) How electric energy is coupled.
2) How propellant is subsequently accelerated.
Mention the different types of EPs for each EP family based on:
1) Produced Power
2) Efficiency
3) Specific Impulse (Isp)
4) Thrust generated
5) Propellant Used
Why not increase the Isp as much as possible in EPs?
- Required beam power is proportional to Isp2 which means:
- Large amount of energy necessary
- Subsequently Mass increase
- Isp ∝ to meps BUT ∝-1 to Δmprop
What is the main difference between electrothermal and chemical thrusters?
The temperature of the combustion chamber (Tc) is provided by electric energy. Energy is directly coupled to the propellant.
Properties for electrothermal propellants
- High Cp (specific heat at constant pressure)
- Storability
- Non Corrosive
- Low Molecular mass
- Low Cost
- Low tendency to frozen flow losses
Draw a simple schematics of how the two type of electrothermal thrusters work:
Mention the advantage and disadvantages of Resistojet Thrusters
- Advantages:
- Simple design
- Low electronics requirements
- Multitude of propellants
- Disadvantages:
- Thermal loads on the structure limit max Isp to 1000s
- Heat losses ↓ efficiency and operative lifetime
When are Resistojets useful in terms of mission requirements?
Useful when a high amount of energy is available and low thrust and low Isp are desired (attitude control in S/C)
What problem of the resistojets is solved by the arcjet and how?
Instead of heating the whole structure (HIGH thermal loads) the arcjet only heats the core & surrounding gases protect the structure.
What’s an Arc or discharge?
It is defined as the current developing between two electrodes under an inert gas that produces a PROLONGED electric DISCHARGE
What is the Paschen Curve?
Describes the BREAKTHROUGH VOLTAGE (voltage necessary to start a discharge) between two electrodes in gas depending on Pressure & Gap Length.
Different types of arc/discharges.
What type is better for EP?
What is the benefit of Electrostatic thrusters vs Electrothermal ones?
How does electrostatic thrusters generate thrust?
Avoids complications of excessive heat or thermal loads by directly converting the electric energy → kinetic energy
They generate thrust by converting propellant into plasma & accelerating the charged particles in an electric field
What’s plasma and what are their units of measurement?
Quasineutral gas of neutral and charged particles which exhibits collective behaviour.
It is measured in eV (electronvolts)
What are the types of electrostatic thrusters? (4)
- Hall
- Field Emission Electric Propulsion (FEEP)
- Ion Thruster: Grid and Kaufman Type
- Colloid
Describe the principle and how a Gridded Ion Thruster works
PRINCIPLE:
- A large difference between Vplasma and VAccel for high efficiency and high currents.
- Uses grids with various voltages to accelerate ions
- Vde (Voltage deacceleration) decreases the total voltage difference to reduce necessary power
- Has a high accel. efficiency & ↑ Thrust for acceptable power consumption.
- ION OPTICS IMPORTANT
Principle and Characteristics (4) of FEEP
-
PRINCIPLE
1. Based on the possibility of directly extracting ions from liquid metals (LMIS) by strong Electric Fields (109 V/m) - CHARACTERISTICS
- Very high and scaleable Isp
- No moving parts
- High Density Propellant
- Service-free
Based on its needle, what are the three main types of FEEP?
- Solid Needle
- Capillary
- Porous
What are the applications for FEEP?
- Focused Ion Beam
- Potential Control
- Station Keeping
- Ultra-high precision Temperature applications (scientific purpose)
- Clusters of SATs, formation flight.
Difference between Needle and Capillary Emitter
- Needle Emitter
- ↑ Electric Field Density
- Lower Voltage
- ↑ Propellant use efficiency
- Complex Manufacture
- Capillary Emitter
- ↓ Electric Field Density
- High Voltage
- ↓ Propellant use efficiency
- Simple Manufacture
What is the Taylor Cone?
Conical equipotential surface as a result of a balance between ELECTRICAL SURFACE TENSION stresses and LIQUID PRESSURE
How does a Taylor Cone forms?
- Cone Formation:
A large voltage is applied to the needle as the solution is pushed through with a syringe pump.
As the charged solution exits the needle, a “Taylor cone” is formed where the electrostatic repulsion is high enough that a very fine stream of solution erupts from the needle.
LOW CURRENTS ARE IN GENERAL BETTER FOR A LIFETIME.
AVOID PULSATING MODE
What are the OP characteristics of FEEPs?
- Impedance is a good way to characterize an emitter
- Impedance of a LMIS is a combination of ohmsche and fluid resistance
- Low Impedance → High Currents
- High Impedance → Increases the mass efficiency at given tank volume
- High impedance beneficial to cluster concept
Describe the Functionality of a Hall thruster
- Electrons are generated by the neutralizer outside of the discharge chamber and sucked into thruster
- Crossed axial electric and radial magnetic fields forces the electrons on an ExB drift and Azimuthal direction (Hall Current) such that the neutral atoms are ionized and accelerated.
Advantages and Disadvantages of Pulsed Plasma Thruster (PPT)?
- ADVANTAGE
- Low complexity → Low Cost
- Lower Power Requirement
- Reliable
- Suitable for miniature
- Important in future
- DISADVANTAGE
- Very complex physics
- Low performance (2 →9% of efficiency)
