Lesson 9: Solid Rocket Propulsion Basics

1. Lesson 9: Solid Rocket Propulsion Basics Dr. Andrew Ketsdever

2. Solid Rocket Motors • A solid rocket motor is a system that uses solid propellants to produce thrust • Advantages • High thrust • Simple • Storability • High density Isp • Disadvantages • Low Isp (compared to liquids) • Complex throttling • Difficult to stop and restart • Safety

3. Solid Rocket Motors • Solid rocket motors are used for • Launch vehicles • High thrust (high F/W ratio) • High storage density • Ballistic Missiles • Propellant storability • Excellent aging • Quick response • storability • high F/W ratio)

4. Solid Rocket Motor Components

5. Thermal Insulation • Design involves: • Analysis of combustion chamber environment • Stagnation temperature • Stagnation pressure • Propellant gases (material compatibility) • Selection of insulation material • Material thickness determination for various areas of the motor case • For the cylindrical part of the case, the walls are only exposed to hot combustion gases at the end of the burn

6. The Nozzle • The design of the nozzle follows similar steps as for other thermodynamic rockets • Throat area determined by desired stagnation pressure and thrust level • Expansion ratio determined by ambient pressure or pressure range to allow maximum efficiency • Major difference for solid propellant nozzles is the technique used for cooling • Ablation • Fiber reinforced material used in and near the nozzle throat (carbon, graphite, silica, phenolic)

7. Ablation • Meteorite • Re-entry speed of 10 - 20 km/sec • Extreme heating in the atmosphere • Ablation and internal energy modes cooled the meteorite through its fall • Ablation gas cloud • Dissociation • Internal energy deposition • Stony-Iron Classification • (95% of all meteorites)

8. Ignition System • Large solid motors typically use a three-stage ignition system • Initiator: Pyrotechnic element that converts electrical impulse into a chemical reaction (primer) • Booster charge • Main charge: A charge (usually a small solid motor) that ignites the propellant grain. Burns for tenths of a second with a mass flow about 1/10 of the initial propellant grain mass flow.

9. Propellant Grain • Two main catagories • Double Base: A homogeneous propellant grain, usually nitrocellulose dissolved in nitroglycerin. Both ingredients are explosive and act as a combined fuel, oxidizer and binder • Composite: A heterogeneous propellant grain with oxidizer crystals and powdered fuel held together in a matrix of synthetic rubber binder. • Less hazardous to manufacture and handle

10. Conventional Composite • Fuel • 5-22% Powdered Aluminum • Oxidizer • 65-70% Ammonium Perchlorate (NH4ClO4 or AP) • Binder • 8-14% Hydroxyl- Terminated Polybutadiene (HTPB)

11. Fuels • Aluminum (Al) • Molecular Weight: 26.98 kg/kmol • Density: 2700 kg/m3 • Most commonly used • Magnesium (Mg) • Molecular Weight: 24.32 kg/kmol • Density: 1750 kg/m3 • Clean burning (green) • Beryllium (Be) • Molecular Weight: 9.01 kg/kmol • Density: 2300 kg/m3 • Most energetic, but extremely toxic exhaust products

12. Oxidizers • Ammonium Perchlorate (AP) • Most commonly used • Cl combining with H can form HCl • Toxic • Depletion of ozone • Ammonium Nitrate (AN) • Next most commonly used • Less expensive than AP • Less energetic • No hazardous exhaust products

13. Binders • Hydroxyl Terminated Polybutadiene (HTPB) • Most commonly used • Consistency of tire rubber • Polybutadiene Acrylonitrile (PBAN) • Nitrocellulose (PNC) • Double base agent

14. Additives • Used to promote • Curing • Enhanced burn rate (HMX) • Bonding • Reduced radiation through the grain (darkening) • Satisfactory aging • Reduced cracking