10.1 Mission analysis and propulsion system selection

Choosing the right propulsion system is crucial for mission success. It's all about balancing performance, cost, and reliability to meet specific mission requirements. From Earth-to-orbit launches to deep space exploration, different propulsion technologies shine in various scenarios.

Chemical rockets pack a punch for quick boosts, while electric thrusters excel in long-haul missions. Advanced tech like nuclear thermal rockets and solar sails offer unique capabilities. Optimizing propulsion systems involves tweaking parameters like thrust, specific impulse, and mass to maximize performance within mission constraints.

Propulsion system selection

Mission requirements and propulsion system performance

• Mission requirements dictate the specific performance targets and constraints that a propulsion system must meet to be considered viable for a given mission
• Key mission parameters that influence propulsion system selection include required delta-v, payload mass, mission duration, available power, and mission environment
  • The required delta-v determines the total impulse and propellant mass needed from the propulsion system
  • Payload mass impacts the thrust and total impulse requirements placed on the propulsion system to achieve mission objectives
  • Mission duration influences propulsion system selection by constraining propellant storage, power system lifetime, and thruster operational lifetime
  • Available power, either from solar arrays or other power sources, limits the viable propulsion system options and their achievable performance (solar electric propulsion, radioisotope thermoelectric generators)
  • The mission environment, such as the presence of an atmosphere or the level of solar radiation, affects the suitability and performance of different propulsion technologies (electric propulsion in interplanetary space, aerodynamic control in atmosphere)

Trade-off analysis and propulsion system selection criteria

• Propulsion system selection involves a trade-off analysis between performance, cost, reliability, and technological readiness to determine the most suitable option for a specific mission
• The selected propulsion system must provide the required delta-v and thrust-to-weight ratio while meeting constraints on power, mass, volume, and mission duration
  • For missions with high delta-v requirements and short durations, such as Earth-to-orbit launches, chemical propulsion systems are typically the most appropriate choice (solid rocket boosters, liquid rocket engines)
  • Missions with low acceleration requirements and long durations, such as interplanetary probes and deep space missions, often benefit from the high specific impulse of electric propulsion systems (ion thrusters, Hall effect thrusters)
• Hybrid propulsion architectures, combining chemical and electric propulsion, can be employed to optimize performance for missions with varying thrust and delta-v requirements
• Propulsion system redundancy and reliability must be considered to ensure mission success, especially for critical maneuvers and long-duration missions

Propulsion system evaluation

Chemical propulsion systems

• Chemical propulsion systems, including liquid and solid rockets, offer high thrust capabilities but have limited specific impulse compared to electric propulsion
• Liquid propellant rocket engines use liquid fuel and oxidizer, allowing for throttling and restart capabilities, while solid rocket motors provide high thrust-to-weight ratios but lack throttling and restart abilities
  • Liquid propellants can be storable (hydrazine, nitrogen tetroxide) or cryogenic (liquid hydrogen, liquid oxygen), each with different performance characteristics and storage requirements
  • Solid propellants are typically composed of a solid fuel (aluminum) and oxidizer (ammonium perchlorate) mixed with a binder, offering simplicity and reliability but limited controllability

Electric propulsion systems

• Electric propulsion systems, such as ion thrusters, Hall effect thrusters, and pulsed plasma thrusters, provide high specific impulse but low thrust, making them suitable for missions with low acceleration requirements and available power
  • Ion thrusters accelerate ionized propellant using electrostatic forces, offering the highest specific impulse among electric propulsion options (NASA's NSTAR ion thruster, ESA's FEEP thruster)
  • Hall effect thrusters utilize a magnetic field to confine electrons and ionize propellant, providing a balance between thrust and specific impulse (Busek BHT-200, Snecma PPS-1350)
  • Pulsed plasma thrusters use pulsed discharges to ablate and accelerate a solid propellant, offering simplicity and compactness (Aerojet Rocketdyne MPPT, Mars Space PPTCUP)
• Electric propulsion systems require a power source, such as solar arrays or radioisotope thermoelectric generators, to operate, which can limit their applicability for certain missions

Advanced propulsion technologies

• Advanced propulsion technologies, such as nuclear thermal rockets and solar sails, offer unique capabilities for specific mission profiles but have varying levels of technological readiness
  • Nuclear thermal rockets use a nuclear reactor to heat a propellant, providing high thrust and specific impulse, but face challenges in development and political acceptance (NERVA, Project Timberwind)
  • Solar sails use the pressure of solar radiation to generate thrust, enabling propellantless propulsion for long-duration missions, but are limited by their low thrust and dependence on solar proximity (IKAROS, LightSail-2)

Propulsion system optimization

Propulsion system design parameters

• Propulsion system optimization involves adjusting design parameters and operating conditions to maximize performance while satisfying mission constraints
• Key parameters for optimization include propellant selection, thrust level, specific impulse, power input, and propulsion system mass
  • Propellant selection impacts the achievable specific impulse, storage requirements, and compatibility with the propulsion system components
    • Bipropellant systems offer higher performance than monopropellant systems but require more complex storage and feed systems
    • Cryogenic propellants provide high specific impulse but pose challenges in long-term storage and management (liquid hydrogen, liquid oxygen)
  • Thrust level optimization balances the required acceleration with the available power and propellant mass, considering the mission timeline and trajectory constraints
  • Specific impulse optimization involves selecting the operating conditions and propulsion technology that provide the highest exhaust velocity while meeting thrust and power limitations

Propulsion system performance optimization techniques

• Power input optimization ensures that the propulsion system operates efficiently within the available power budget, considering the power source capabilities and mission duration
  • Solar electric propulsion systems must optimize their operating point based on the available solar array power, which varies with distance from the sun
  • Nuclear electric propulsion systems have a constant power output but must manage the heat generated by the reactor and power conversion system
• Propulsion system mass minimization is critical for reducing overall spacecraft mass and improving mission performance, requiring careful design and material selection
  • Lightweight materials, such as composites and advanced alloys, can be used to reduce the mass of propulsion system components (propellant tanks, thrust chambers)
  • Optimizing the propulsion system architecture, such as using a common propellant for attitude control and main propulsion, can reduce the overall system mass and complexity

Propulsion system for mission profiles

Earth-to-orbit and interplanetary missions

• Earth-to-orbit missions, such as satellite launches and crewed missions, typically rely on chemical propulsion systems due to their high thrust capabilities and the need to overcome Earth's gravity well
  • Solid rocket boosters are often used in combination with liquid rocket engines to provide additional thrust during the initial launch phase (Space Shuttle, Ariane 5)
  • Liquid rocket engines, such as the Merlin engine used by SpaceX's Falcon 9, provide the main propulsion for the launch vehicle and can be designed for reusability
• Interplanetary missions, such as Mars exploration and outer solar system probes, often employ a combination of chemical and electric propulsion systems to optimize performance and mission flexibility
  • Chemical propulsion is used for Earth escape and initial interplanetary trajectory insertion, while electric propulsion is used for efficient in-space maneuvering and orbit insertion around the target body (Mars Science Laboratory, Psyche mission)
  • Gravity assist maneuvers, using the gravitational pull of planets to alter the spacecraft's trajectory, can be combined with propulsion systems to reduce the required delta-v and propellant mass (Voyager missions, Cassini-Huygens mission)

Satellite orbit maintenance and deep space exploration

• Satellite orbit maintenance requires periodic propulsive maneuvers to counteract perturbations and maintain the desired orbital parameters
  • Electric propulsion systems, such as Hall effect thrusters and ion engines, are well-suited for satellite station-keeping due to their high specific impulse and low propellant consumption (Boeing 702SP, Eutelsat 172B)
  • Chemical propulsion systems, particularly monopropellant hydrazine thrusters, are also used for satellite orbit maintenance when higher thrust levels are required (Intelsat 10-02, Astra 2E)
• Deep space exploration missions, such as New Horizons and Voyager, rely on a combination of propulsion systems and innovative mission design to achieve their scientific objectives
  • Radioisotope thermoelectric generators provide long-lasting power for propulsion and scientific instruments in the outer solar system, where solar power is insufficient (New Horizons, Cassini-Huygens)
  • Advanced propulsion technologies, such as electric sails and laser-powered propulsion, are being developed to enable more ambitious deep space missions and reduce travel time (NASA's Interstellar Probe concept, Project Starshot)