Rocket control and balancing surfaces

INTRODUCTIONy A stable rocket is one that flies in a smooth, uniform y y y y

direction. Unstable rockets are dangerous because it is not possible to predict where they will go. Concept of centre of mass. Spinning about CG :- a way to obtain stability.(Frisbee example) Types of spinning based on the axis about which it take place:-roll, pitch and yaw.

VARIOUS SPINNING MOTIONS POSSIBLE FOR A ROCKET DURING FLIGHT

ROLL

PITCH

YAW

y For rockets roll and pitch are most important-any movement in

these directions can cause the rocket to go off course. y Concept of centre of pressure:-only when air is passing around the moving rocket. y COP:- A point about which the surface area on one side is the same as the other side. y It is extremely important that the center of pressure in a rocket be located toward the tail and the center of mass be located toward the nose.

A Wind Vane

y Control systems for rockets are intended to keep a rocket stable

in flight and to steer it. Small rockets usually require only a stabilizing control system. Large rockets, such as the ones that launch satellites into orbit, require a system that not only stabilizes the rocket, but also enable it to change course while in flight. y Control surfaces are of two types:-Active and passive. y Active surfaces:- can be moved during flight to stabilize and steer the craft. y Passive surfaces:-fixed devices on rockets exterior whose presence imparts stability to the rocket.

Passive Controly Sticks:- simplest of all passive control surfaces. Used in ancient

Chinese fire-arrow. Disadvantage:1-A proper initial launch system was required. Self stabilization starts only after a minimum speed has been attained. 2-fire arrow stick added much dead weight to the rocket thus reducing its range considerably.

y Fins:-clusters of lightweight fins mounted around the lower end

near the nozzle. Large surface area of fins easily kept the COP behind the CG. Advantages:- Lightweight and can be streamlined. Disadvantage:-Much more drag which limits the range of the rocket.

ACTIVE CONTROLSy Most common active controls are:

1- vanes 2- movable fins 3-canards 4- gimbaled nozzles 5-vernier rockets 6- fuel injection 7-attitude-control rockets

Movable fins and Canardsy Tilt like rudders to deflect the air flow and cause the rocket to

change course. y Canards are mounted on the front end of the rocket while the tilting fins are at the rear. y Motion sensors on the rocket detect unplanned directional changes, and corrections can be made by slight tilting of the fins and canards. y smaller and lighter and produce less drag than the large fins.

Exhaust gas deflecting devicesy Vanes-

1-small finlike devices that are placed inside thE rocket engine. 2-Tilting the vanes deflects the exhaust, and by action-reaction the rocket responds by pointing the opposite way.

VANES

Exhaust gas deflecting devices(Contd.)y Gimbaled Nozzles:

1-able to sway while exhaust gases are passing through it. 2-tilting the engine nozzle in the proper direction, the course of the rocket can be changed.

Exhaust gas deflecting devices(Contd.)y Vernier Rockets:

1-small rockets mounted on the outside of the large engine. 2-When needed they fire, producing the desired course change.

ROCKET ENGINES AND THEIR PROPELLANTS

INTRODUCTIONy Propellant:-fuel and Oxidizer. y Rockets unlike Jets have to carry oxygen. y Types of propellants:-Basically two types

1-Solid Propellant:- fuel and the oxidizer combined together in the chemical itself. 2-Liquid Propellant:a)gases chilled into liquids b)fuel and oxidizer kept in different containers. c) Fuel and oxidizer are mixed together in the engine.

Solid Propellant enginey Basic parts are-

1-Nozzle 2-A case:- usually a relatively thin metal lined with insulation. 3-Insulation:- to keep propellant from burning through. 4-Propellant 5-Igniter

Features of solid propelled enginesy Only the surface of the propellant burns. y Generally propellant ignited at lower end and burning proceeds

gradually from one end to the other. y To get more thrust hollow core is used-increased surface of propellant available for burning. y Star shaped cores used for very high thrust.

IGNITERS( FOR SPE)y Ignition using fuse:-

-Ignite too quickly and can harm the rocketeer. y Ignition using electricity: -electric current heats up special wire inside the rocket. -wire raises the temperature of the propellant in contact to the combustion point. -more reliable and safer. y Ignition using rocket engine:-used for large rockets. -small engine inside the hollow core blasts a stream of flames and hot gas down from the top of the core and ignites the entire surface area of the propellants in a fraction of a second.

Nozzley opening at the back of the rocket that permits the hot expanding

gases to escape. y Narrow part throat and beyond that -exit cone. y Purpose:1)to maximize the thrust by increasing the accel-eration of the leaving exhaust gases. 2)cuts down the opening through which the gases A typical nozzle used in rockets can escape.

Liquid Propelled Enginesy Much more complicated. y Main parts:-

1-Separate storage tanks for fuel and oxidizer. 2-Pumps 3-Combustion chamber 4-Nozzle

Liquid Propelled Engines(Contd.)y Fuel-usually kerosene or liquid hydrogen. y Oxidizer-liquid oxygen. y Combustion Chamber- here fuel and oxidizer are combined and

burnt. y Injectors( nozzles) present on roof of the CC spray and mix the propellants. y Pumps- used for pumping the propellants against the high combustion chamber pressure. y Expanding gases in the combustion chamber escapes through the nozzle at the lower end.