Class 4: Fundamentals of Rocket Propulsion Marat Kulakhmetov

Did Buzz Aldrin punch a reporter? http://www.youtube.com/watch?v=KaUqaVj51w4&feature=related

Gases Gas is made up of molecules that fly in random directions Molecules collide with other molecules and with the walls

Temperature Temperature describes how fast the molecules move At higher temperature, molecules move faster At room temperature air molecules move at 500m/s ( 1118.46 mi/h)

Pressure How much momentum gases transfer to the surface How often and how fast the molecules hit the surface

Density How much stuff is in a volume Solids are high density Gases are low density

Ideal Gas Law Temperature, Pressure, Density describe what molecules do so they have to be related. P = Pressure ρ = Density R = Gas constant T = Temperature

Thrust Most vehicles need to overcome weight and drag by generating thrust There are many ways of generating thrust: Internal Combustion Engines Usually found in cars They are small but require an oxygen supply Jet Engines Usually found on fast airplanes They can generate a lot of thrust but they still need an oxygen supply Rocket Engines These generate enough thrust to escape Earth’s gravity and often carry their own oxidizer so they work in space

4 Stroke Internal Combustion Engine Use gas expansion to rotate the cam shaft The cam shaft turns the wheel or propeller and makes cars or airplanes go forward There is no ground or air in space, rockets cant use this

Jet Engine Compressor Bypass Fan Combustor Turbine Compresses air Accelerates Air Combustor Burns air Turbine Powers compressor and Bypass Fan Needs Air Max Speed: 3000 mi/h

Thrust Typically, rocket engines produce two types of thrust: Pressure Thrust Generate higher pressure behind the rocket = (Pe-Pa)*A Jet Thrust Pushes the rocket forward by throwing gasses out of the back = mdot Ve Total Thrust = Pressure Thrust + Jet Thrust Thrust produced depends on fuel, rocket nozzle, rocket altitude, etc.

Pressure Forces Air wants to go from high pressure to low pressure Pressure Force ( P1 – P2) * A Remember that Pressure = Force / Area

Momentum Forces Action-Reaction If you throw something out one way it will push you the other way If the rocket nozzle throws gases down, the gasses push the rocket up

Rocket Nozzles Nozzles push on high gasses and accelerate them out the back In return, the gasses push on the nozzle and accelerates it forward

Control Volume It is usually easy to study gas flows using control volumes Forces on the rocket could be calculated by only looking at control surfaces Fpressure =(Pe - Pa ) Ae Fgas = ρ Ue2 Ae

Isentropic Nozzles Rockets usually use converging-diverging nozzles. These could also be called isentropic nozzles The thrust through the C-D nozzle depends on chamber pressure, ambient pressure, and nozzle shape

Converging Section Upstream of the nozzle, in the combustion chamber, the gas velocity is small All fluids (water, air, etc.) accelerate through a converging section The fastest they could get in the converging section is Mach 1

Diverging Section If the gases reached Mach 1 in converging section then they will continue accelerating in the diverging section If the gasses did not reach Mach 1 in the converging section then they will decelerate in the diverging section This is why our water bottle rockets only had converging section

Over and Under Expanded Gasses Perfectly Expanded Over Exanded Separated Performance of the nozzle depends on outside pressure Outside Pressure changes with altitude Over and Under expanded gasses are not as efficient

Lets Calculate Rocket Thrust and acceleration Ambient Conditions: Pa = 101,000 Pa Exit Conditions: Pe = 150,000 Pa Ve = 100 m/s Density = 1.2 kg/m3 Example Area = 0.05 m^2 Mass = 0.5 kg Lets Calculate Rocket Thrust and acceleration A = F/m = 3050 / 0.5 = 6100 m/s^2

Water Bottle Rocket Debriefing Why did rockets filled with water go higher than those filled with just air? Changes Exit Pressure Constant Ambient Pressure Constant Exit Velocity Assumed Constant Air Density = 1.2 kg/m^3 Water Density = 1000 kg/m^3

ISP ISP is used to classify how well a rocket performs Low ISP = need a lot of fuel to achieve thrust High ISP =do not need as much fuel to achieve same thrust

Types of Rocket Engines Pressurized Air Solid Propellant Liquid Propellant Nuclear Electric

Compressed Air Compressed air leaves out of the back of the rocket The air pushes the rocket forward

Solid Propellant Propellant is initially in the solid state and it becomes a hot gas during combustion Pros: Simple Cheap Easy to store Can be launched quickly Cons: ISP only 150-350 Cannot turn off after ignition Cannot throttle during flight

Fuel and Oxidizer are both stored separately in liquid form Liquid Propellant Fuel and Oxidizer are both stored separately in liquid form Pros: Better performance (ISP 300-460) Cons: More complex Requires pumps or pressurized gas tanks Heavier

Nuclear Nuclear Reactor heats working gas that is accelerated through a nozzle Pros: Isp 800-1000 Cons: Requires shielding, can be heavy It’s a NUKE

Electric Two types: Arcjet: Electricity is used to superheat the gases Ion Thrusters: ionized (charged) atoms are accelerated through an electro-magnetic field Pros: ISP 400-10,000 Cons: Thrust usually <1N VASIMR

Video http://www.youtube.com/watch?v=YOSBzFSZUx4&feature=related