🚀Aerospace Propulsion Technologies Unit 5 – Rocket Propulsion: Basics & Propellants

Key Concepts & Terminology

• Propulsion generates force to move an object by expelling matter (propellant) in the opposite direction
• Specific impulse ($I_{sp}$) measures the efficiency of a rocket engine expressed as the thrust per unit of propellant consumed per second
• Thrust-to-weight ratio compares the thrust produced by a rocket engine to its weight crucial for determining a rocket's payload capacity and acceleration
• Oxidizer supplies oxygen for combustion in rocket engines enables propellant burning in oxygen-deficient environments (upper atmosphere, space)
• Ignition system initiates the combustion process in a rocket engine
  • Pyrotechnic igniters (solid propellants)
  • Hypergolic propellants (self-igniting liquid propellants)
  • Spark plugs or glow plugs (liquid propellants)
• Nozzle accelerates the exhaust gases to high velocities converting thermal energy into kinetic energy and generating thrust
• Propellant grain refers to the solid propellant charge's geometric shape affects burn rate and thrust profile

Rocket Propulsion Fundamentals

• Rocket propulsion relies on Newton's Third Law of Motion for every action, there is an equal and opposite reaction
• Thrust is generated by expelling high-velocity exhaust gases in the opposite direction of the desired motion
• Rocket engines carry both the fuel and oxidizer onboard enabling operation in atmospheres lacking oxygen (space)
• Propulsion efficiency depends on the exhaust velocity higher velocities lead to greater efficiency and specific impulse
• Staging involves using multiple rocket stages to improve overall vehicle performance and payload capacity
  • Each stage contains its own engines and propellant
  • Discarded after propellant depletion to reduce vehicle mass
• Rocket engines can be classified by propellant type (solid, liquid, hybrid) or energy source (chemical, nuclear, electric)
• Thrust vectoring allows for directional control of the rocket by adjusting the nozzle's orientation or using secondary injection of fluids

Types of Rocket Propellants

• Solid propellants consist of a solid fuel and oxidizer mixed together into a dense, stable grain
  • Commonly used in boosters and military missiles
  • Simpler design and easier storage compared to liquid propellants
• Liquid propellants store the fuel and oxidizer separately in liquid form
  • Pumped into the combustion chamber where they react
  • Allows for throttling and restart capabilities
  • Used in most modern launch vehicles and spacecraft
• Hybrid propellants combine a solid fuel with a liquid or gaseous oxidizer
  • Offer some benefits of both solid and liquid systems (safety, throttling)
  • Currently under development for future applications
• Cryogenic propellants are liquefied gases stored at extremely low temperatures (liquid hydrogen, liquid oxygen)
  • High performance but challenging to store and handle
• Hypergolic propellants ignite spontaneously upon contact between the fuel and oxidizer (hydrazine, nitrogen tetroxide)
  • Simplify ignition but are highly toxic and corrosive

Solid Propellants: Composition & Properties

• Solid propellants are composed of a fuel, oxidizer, and various additives mixed into a homogeneous grain
• Common fuel components include aluminum, beryllium, or polymers (HTPB, PBAN)
• Ammonium perchlorate (AP) is the most widely used oxidizer in solid propellants
• Additives enhance specific properties or performance characteristics
  • Burn rate modifiers (iron oxide, copper chromite)
  • Plasticizers improve mechanical properties
  • Stabilizers prevent degradation during storage
• Propellant grains are classified by their geometric shape (cylindrical, end-burning, star-shaped)
  • Grain geometry affects the burn rate and thrust profile
• Solid propellants offer high density and volumetric efficiency compared to liquid propellants
• Disadvantages include the inability to throttle or restart once ignited and lower specific impulse than liquid engines

Liquid Propellants: Types & Characteristics

• Liquid propellants are categorized as monopropellants or bipropellants
  • Monopropellants decompose and release energy without an oxidizer (hydrazine, hydrogen peroxide)
  • Bipropellants consist of separate fuel and oxidizer components (kerosene/LOX, LH2/LOX)
• Cryogenic propellants (liquid hydrogen, liquid oxygen) offer high performance but require insulated storage tanks and careful handling
• Storable propellants remain liquid at room temperature (RP-1, hypergolic propellants)
  • Simplify storage and handling but have lower performance than cryogenics
• Liquid propellant engines employ either pressure-fed or pump-fed systems to deliver propellants to the combustion chamber
  • Pressure-fed systems use pressurized tanks but are limited in chamber pressure and thrust
  • Pump-fed systems allow for higher chamber pressures and thrust levels
• Throttling and restart capabilities are major advantages of liquid propellant engines
• Challenges include complex plumbing, turbopumps, and the need for precise flow control and mixing

Propellant Performance Metrics

• Specific impulse ($I_{sp}$) is a key performance metric for rocket propellants
  • Represents the thrust produced per unit of propellant flow rate
  • Higher $I_{sp}$ indicates better propellant efficiency and vehicle performance
• Characteristic velocity ($c^*$) measures combustion performance independent of nozzle design
  • Depends on combustion temperature, gas properties, and chamber pressure
• Density specific impulse ($\rho I_{sp}$) considers both the $I_{sp}$ and density of a propellant
  • Important for volume-constrained applications (upper stages, spacecraft)
• Propellant mass fraction is the ratio of propellant mass to total vehicle mass
  • Higher mass fractions are desirable for improved vehicle performance
• Mixture ratio (oxidizer-to-fuel ratio) affects combustion temperature, $I_{sp}$, and engine design
  • Optimized for specific propellant combinations and mission requirements
• Adiabatic flame temperature is the maximum theoretical temperature achieved during combustion
  • Limited by dissociation and heat losses in practical engines

Combustion Processes in Rocket Engines

• Combustion in rocket engines involves the rapid oxidation of the fuel, releasing heat and gaseous products
• Efficient combustion requires proper mixing, atomization, and vaporization of the propellants
  • Injectors introduce and mix the propellants in the combustion chamber
  • Atomization breaks up liquid propellants into small droplets for faster vaporization
• Ignition sources initiate the combustion process (igniters, hypergolic propellants, or pyrotechnic devices)
• Combustion instabilities can occur due to pressure oscillations or acoustic resonance
  • Addressed through careful injector design and combustion chamber geometry
• Solid propellant combustion occurs at the exposed surface of the grain
  • Burn rate depends on chamber pressure and grain temperature
  • Progressive, regressive, or neutral burning profiles based on grain geometry
• Liquid propellant combustion is influenced by factors such as droplet size, mixing efficiency, and residence time
• Hybrid propellant combustion involves the vaporization of the solid fuel and mixing with the oxidizer
  • Offers throttling and shutdown capabilities but can suffer from uneven burning and low regression rates

Safety & Environmental Considerations

• Rocket propellants pose various safety risks due to their energetic and reactive nature
  • Solid propellants are sensitive to shock, friction, and electrostatic discharge
  • Liquid propellants may be toxic, corrosive, or cryogenic, requiring special handling
• Proper storage, handling, and transportation procedures are essential to mitigate risks
  • Insulated tanks for cryogenic propellants
  • Ventilated and isolated storage areas
  • Personal protective equipment for personnel
• Environmental concerns include the release of combustion products and unburned propellants
  • Ammonium perchlorate (AP) in solid propellants can lead to acid rain and groundwater contamination
  • Hydrazine is highly toxic and can cause environmental damage if released
• Green propellants are being developed to reduce the environmental impact of rocket launches
  • Hydrogen peroxide (H2O2) as a monopropellant or oxidizer
  • Liquid methane (LCH4) as a fuel with lower carbon emissions
  • Ionic liquids and gelled propellants for improved safety and reduced volatility
• Proper disposal and remediation techniques are necessary for spent propellants and contaminated materials
• Launch site selection considers environmental factors and downrange safety to minimize potential impacts