🚀Aerospace Propulsion Technologies Unit 8 – Electric Propulsion Systems

Fundamentals of Electric Propulsion

• Electric propulsion systems accelerate propellant using electrical energy, unlike chemical propulsion which relies on chemical reactions
• Propellant is ionized and accelerated by electromagnetic fields or heated to high temperatures
• Requires a power source (solar panels or nuclear reactors) to generate the necessary electrical energy
• Produces low thrust levels compared to chemical propulsion but can operate for extended periods
• Enables high specific impulse (Isp), which is a measure of propulsion system efficiency
  • Isp represents the amount of thrust generated per unit mass of propellant consumed
  • Higher Isp translates to less propellant required for a given mission
• Offers significant mass savings by reducing propellant requirements, allowing for more payload capacity or extended mission durations
• Suitable for missions requiring precise spacecraft control, station-keeping, or deep space exploration

Types of Electric Propulsion Systems

• Electrostatic propulsion systems
  • Accelerate charged particles using electric fields
  • Include ion thrusters and Hall effect thrusters
• Electromagnetic propulsion systems
  • Utilize the interaction between electric currents and magnetic fields to accelerate plasma
  • Examples: magnetoplasmadynamic (MPD) thrusters and pulsed plasma thrusters (PPT)
• Electrothermal propulsion systems heat propellant to high temperatures and expand it through a nozzle
  • Resistojets use electrical resistance to heat propellant
  • Arcjets employ an electric arc to heat propellant
• Field Emission Electric Propulsion (FEEP) uses strong electric fields to extract and accelerate ions from liquid metal propellants
• Helicon plasma thrusters utilize radio frequency (RF) waves to generate and heat plasma

Key Components and Their Functions

• Power processing unit (PPU) converts and regulates electrical power from the spacecraft's power source to the propulsion system
• Propellant storage and feed system stores and delivers propellant to the thruster
  • Propellants can be gases (xenon, argon), liquids (mercury, indium), or solids (Teflon, polytetrafluoroethylene)
• Ionization chamber or discharge chamber where propellant is ionized or heated
• Acceleration stage applies electric or magnetic fields to accelerate the ionized propellant
• Neutralizer emits electrons to neutralize the positively charged ion beam and prevent spacecraft charging
• Magnetic nozzle (in electromagnetic systems) directs and confines the plasma flow
• Cathodes and anodes establish the necessary electric fields and currents within the thruster

Performance Metrics and Efficiency

• Thrust ($F$) is the force generated by the propulsion system, typically measured in millinewtons (mN) for electric propulsion
• Specific impulse ($I_{sp}$) represents the efficiency of the propulsion system
  • Defined as $I_{sp} = \frac{v_e}{g_0}$, where $v_e$ is the exhaust velocity and $g_0$ is the standard acceleration due to gravity
  • Measured in seconds, with electric propulsion systems achieving $I_{sp}$ values of 1000-10,000 seconds
• Power efficiency ($\eta_p$) is the ratio of jet power to input electrical power
  • Jet power is the kinetic power of the exhaust, given by $P_{jet} = \frac{1}{2}\dot{m}v_e^2$, where $\dot{m}$ is the mass flow rate
• Thrust efficiency ($\eta_T$) is the ratio of jet power to total input power (electrical + propellant)
• Total efficiency ($\eta_{total}$) accounts for power efficiency, thrust efficiency, and other losses in the system
• Lifetime and reliability are crucial metrics for electric propulsion systems, as they often operate for extended periods

Applications in Spacecraft

• Station-keeping and orbit maintenance for satellites in geostationary orbit (GEO)
  • Counteracts perturbations and maintains the spacecraft's position
• Orbit raising and transfers, such as moving a satellite from low Earth orbit (LEO) to GEO
• Attitude control and precise pointing of spacecraft
• Deep space missions and interplanetary travel
  • NASA's Deep Space 1 and Dawn missions utilized ion propulsion
  • ESA's SMART-1 mission to the Moon employed Hall effect thrusters
• Drag compensation in low Earth orbit (LEO) to extend satellite lifetimes
• Constellation maintenance for satellite formations and swarms

Advantages and Limitations

Advantages:

• High specific impulse ($I_{sp}$) leads to significant propellant mass savings
• Enables longer mission durations and extended satellite lifetimes
• Precise thrust control and low thrust noise for accurate spacecraft positioning
• Reduced launch costs due to lower propellant mass requirements
• Potential for using alternative propellants, such as atmospheric gases or in-situ resources

Limitations:

• Low thrust levels compared to chemical propulsion systems
  • Requires longer operating times to achieve desired velocity changes
• Power-limited performance, as thrust is dependent on available electrical power
• Complex power processing and control systems
• Potential for spacecraft charging and interactions with the space environment
• Limited throttling capabilities and slower response times compared to chemical thrusters

Current Research and Future Developments

• Development of high-power electric propulsion systems (100 kW - MW range) for ambitious missions
• Investigating alternative propellants and propellant-less concepts
  • Air-breathing electric propulsion for LEO satellites
  • Miniaturized electrospray thrusters using ionic liquids
• Improving thruster efficiency, lifetime, and reliability through advanced materials and designs
• Developing compact and lightweight power processing units (PPUs) and power systems
• Studying the interactions between electric propulsion plumes and spacecraft surfaces
• Investigating the use of electric propulsion for asteroid mining and resource utilization
• Developing electric propulsion systems for small satellites (CubeSats) and nanosatellites

Comparison with Chemical Propulsion

• Electric propulsion offers higher specific impulse ($I_{sp}$) but lower thrust compared to chemical propulsion
  • Chemical: $I_{sp}$ of 200-500 seconds, thrust levels of newtons to kilonewtons
  • Electric: $I_{sp}$ of 1000-10,000 seconds, thrust levels of millinewtons to a few newtons
• Electric propulsion requires a separate power source, while chemical propulsion generates power through exothermic reactions
• Electric propulsion systems have a higher propellant efficiency, resulting in lower propellant mass requirements
• Chemical propulsion offers higher thrust-to-weight ratios and faster response times
• Electric propulsion is more suitable for long-duration missions and precise maneuvering
• Chemical propulsion is preferred for time-critical maneuvers and high-thrust applications (launch vehicles, planetary landings)
• Hybrid systems combining electric and chemical propulsion can offer the benefits of both technologies