Development and Principles of Rocketry Methods of Operation Lecture Four

Forces on a Rocket When a rocket lifts off there are 3 main forces acting on it. Thrust and Drag are trying to ‘crush’ the rocket, rather like pushing on opposite ends of a drinks can. and when these forces are at a maximum, it is called “Max Q”. DRAG DRAG Thrust Weight Drag WEIGHT THRUST THRUST

Forces on a Rocket When a rocket lifts off there are 3 main forces acting on it. DRAG Thrust Weight Drag Depends on the amount of propellant and oxidiser being burned. Rocket weight plus the payload and fuels equals total weight. Depends on the aerodynamic shape of the rocket and its velocity. Remember – when Thrust and Drag are at a maximum, it is called “Max Q”. WEIGHT THRUST

Forces on a Rocket At Lift Off the rocket is stationary DRAG At Lift Off the rocket is stationary Drag is zero because the rocket isn’t moving. To lift the rocket, thrust must be more than weight. The thrust to weight ratio of a launch vehicle is Thrust divided by Weight (normally in the range of 1.2 to 1.6) WEIGHT THRUST

Forces on a Rocket Once the rocket is moving DRAG Once the rocket is moving Thrust must be greater than drag plus weight. Drag increases as the rocket gains speed. But weight decreases as propellant is burned and the acceleration increases. WEIGHT THRUST

Rocket Structure One common structure attaches all components to a tubular frame The frame comprises of - Struts running the length of the body Ribs running around the body The Fairing protects the Payload And the engines, having moving parts, are attached to the frame using support structures. Support Structures Ribs Fairing Struts Engine Payload

Rocket Structure Huge fuel tanks are required One common structure attaches all components to a tubular frame Huge fuel tanks are required to store the Liquid Hydrogen, because Liquid Hydrogen has a very low density. Support Structures Ribs Fairing Struts Engine Payload Fuel Tank Fuel Tank

Rocket Structure Operation (usually Helium) “Blowdown System” Fore Tank Aft The fuel and oxidizer are stored in tanks inside the rocket body. Tanks are pressurized with an inert gas (usually Helium) to force the fuel and oxidizer towards the engine. This arrangement is called a “Blowdown System” Inert Gas Propellant Oxidiser Engine

Like aircraft, rockets can roll, pitch and yaw. Guidance Like aircraft, rockets can roll, pitch and yaw. The roll axis is about the direction of travel, but the pitch & yaw axes are aligned with the fins or any gyroscopes fitted to the rocket. Fins are control surfaces, aligned with the pitch and yaw axes, and they are deflected to produce aerodynamic forces. The AIM-9 Sidewinder used two sets of fins to steer the missile. FIN

Like aircraft, rockets can roll, pitch and yaw. Guidance Like aircraft, rockets can roll, pitch and yaw. Robert Goddard inserted vanes into the exhaust plume, allowing the rocket to be steered in both pitch and yaw. Engineers then mounted the whole engine on gimbals, and tilted the complete motor using hydraulics. VANE GIMBAL

Like aircraft, rockets can roll, pitch and yaw. Guidance Like aircraft, rockets can roll, pitch and yaw. Another technique used on larger rockets and for manoeuvring in space, is to have small thrusters fitted onto the rocket. These are mounted on the craft to give a sideways thrust. THRUSTER

Action and Reaction the action produced by a rocket engine In terms of Newton's Laws, the action produced by a rocket engine is caused by its fast exhaust flow. Remember the balloon ! Engine efficiency can therefore be improved by using different propellants. Reaction The balloon moves in the opposite direction Action Air leaves the balloon at high velocity Compressed Gas

Engine Principles Rocket Engine Performance Thrust x duration of burn We can compare propellant performance by determining how much thrust can be obtained in 1 second, by burning 1 kg of propellant. We call this “Specific Impulse” which is a measure of the efficiency of the propellant. Specific Impulse is defined as :- Thrust x duration of burn Weight of propellant burned

Check of Understanding In terms of Newton's Laws, the action produced by a rocket engine is caused by its? Reaction against the atmosphere Ability to operate without atmospheric oxygen Reaction against the pull of gravity Fast exhaust flow

Check of Understanding Which part of a launch vehicle protects the payload? The fairing The skin The motor case The support structure

Check of Understanding When a rocket is in flight, as fuel is used the total weight falls and acceleration is? Reduced Increased Doubled Not affected

Check of Understanding Which of the following gases is used to pressurise blow-down systems? Oxygen Helium Hydrogen Nitrogen

Check of Understanding In a rocket engine what does the term Specific Impulse quantify? Acceleration Efficiency Energy Power

Check of Understanding A blow-down system works by: Using gaseous propellant to force the oxidiser out of the tanks Using gaseous oxidiser to force the propellant out of the tanks Using pressurised inert gas to force the oxidser out of the tanks Using pressurised inert gas to force the propellant out of the tanks

Check of Understanding Huge fuel tanks are required for Liquid Hydrogen because? It has a very low density It has a very high density It boils at low temperatures It freezes at low temperatures

Check of Understanding The thrust to weight ratio of a launch vehicle is normally in the range: 1.6 to 2.0 1.2 to 1.6 1.0 to 1.2 0.6 to 1.0

Check of Understanding The point in a flight at which the forces trying to crush a rocket are at their greatest is called: Lift Off Max Q Ignition Burn Out

Development and Principles of Rocketry End of Presentation