Class 4: Fundamentals of Rocket Propulsion

1. Class 4: Fundamentals of Rocket Propulsion Marat Kulakhmetov

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

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

4. 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)

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

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

7. 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

8. 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

9. 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

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

11. 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 • = mdotVe • Total Thrust = Pressure Thrust + Jet Thrust • Thrust produced depends on fuel, rocket nozzle, rocket altitude, etc.

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

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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

19. Over and Under Expanded Gasses Under expanded 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

20. Ambient Conditions: Pa = 101,000 Pa Example Exit Conditions: Pe = 150,000 Pa Ve = 100 m/s Density = 1.2 kg/m3 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

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

22. 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

23. Types of Rocket Engines • Pressurized Air • Solid Propellant • Liquid Propellant • Nuclear • Electric

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

25. 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

26. 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

27. 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

28. 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

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