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Class 2: Advanced Rocket Concepts

Class 2: Advanced Rocket Concepts. Marat Kulakhmetov. Intro Video. http://www.youtube.com/watch?v=13qeX98tAS8. Water Bottle Rocket Debriefing. Did some rockets tumble? Did some rockets wobble? Did some rockets flip over? Maybe some rockets were unstable. Fun Video.

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Class 2: Advanced Rocket Concepts

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  1. Class 2: Advanced Rocket Concepts Marat Kulakhmetov

  2. Intro Video • http://www.youtube.com/watch?v=13qeX98tAS8

  3. Water Bottle Rocket Debriefing • Did some rockets tumble? • Did some rockets wobble? • Did some rockets flip over? • Maybe some rockets were unstable

  4. Fun Video • http://www.youtube.com/watch?v=B47XEFw5l6w

  5. Stability • Stability refers to how likely an object will return to its initial position or orientation if it is disturbed • Stable – Object returns to initial position • Neutrally Stable – Object does not move • Unstable – Object continues moving away from its initial position

  6. Moments • Moment describe the object’s tendency to rotate • Moment = Force * Perpendicular Distance • In the example above, the moments generated by the two weights generate 20 N*m and -20 N*m. They are balanced • Moments are usually calculated about their center of gravity (CG) • Unbalanced moments on a rocket will cause the rocket to tumble.

  7. Center of Gravity (CG) • Location where the forces will balance • CG = Moment / Total Weight • Example: • Moment = 10 * (0) + 20 * 3 = 60 N * m • Total Weight = 10 + 20 = 30 N • CG = Moment / Total Weight = 60 / 30 = 2 m X = 0 X = 2 X=3

  8. Calculating CG of complex 2D and 3D Shapes Beer, Russell, Johnston, DeWolf Mechanics of Materials

  9. Example: Moments of a Rocket X = 0 5 7 11 13 14 20

  10. Example Continued X = 0 5 7 11 13 14 20

  11. Example Continued X = 0 5 7 11 13 14 20

  12. Example Continued • Moment = 1152.2 • Mass = 103 • CG = Moment / Mass • = 1152.2/103 = 11.19 cm X = 0 5 7 11 13 14 20

  13. How about complex Fins? B3=2 • Break it up into a triangle, rectangle and triangle • Area 1 = ½ *b1 * h = 5 • Area 2 = b2 * h =5 • Area 3 = ½ * b3 * h=5 H=5 B1=2 B2=1 • Total Area = Area 1 + Area 2 + Area 3 = 15 • Mass1 = Total Mass * Area 1 / Total Area = 1 • Mass2 = Total Mass * Area 2 / Total Area =1 • Mass3 = Total Mass * Area 3 / Total Area =1 1 1 1 1 2 3

  14. How about complex Fins? b3 • Part 1 is a triangle • Centroid 1 = b1/3 =.66 • Part 2 is a rectangle • Centroid 2 = b2/2 = 0.5 • Part 3 is a triangle • Centroid 3 = b3/2 =.66 h b1 b2 • Moment Fin = Mass1 * (b1 – Centroid 1) + Mass2 * ( b1 + Centroid 2) • + Mass3 * ( b1 + b2 + Centroid 3)= 7.5 • CG Fin = Moment Fin / Total Fin Mass =2.5 1 1 1 1 2 3

  15. Example Continued • Moment with fins = 1152.2 +(2.5+14)*3 • Mass = 103+3 • CG = Moment / Mass =11.34 cm X = 0 5 7 11 13 14 20

  16. Moments on a Rocket without Fins • If : • Rocket has no fins • Thrust is aligned • Rocket pitched a little • Moment = -1*Lift * x • This rocket will keep pitching and fly out of control X y x

  17. Fins • Little Drag Lots of Drag

  18. Moments on a Rocket with Fins • If : • Thrust is aligned • Rocket turned a little • Moment = -1* Lift *x + Fin * x1 • If Fin * x1 > Lift * x , the rocket will right itself X X1 Fin Force

  19. Fins • Fin force = • Larger Area = More force provided by fins • Larger Velocity = More Force provided by fins • Fin Moment = Fin Force * Distance • Larger Force = Larger Moment • Larger Distance = Larger Moments • For stability, we want large fins as far away from CG as possible. • If fins are too large they create more drag

  20. Aerodynamic Center • Calculating aerodynamic center will require Computational Fluid Dynamic (CFD) analysis. • We will estimate that the aerodynamic center is at Fin centroid • We calculated that this is at 16.5cm X = 0 5 7 11 13 14 20

  21. 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

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

  23. Momentum Forces • Action-Reacting • 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

  24. 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

  25. 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

  26. 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

  27. 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

  28. 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

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

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

  31. 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

  32. Solid Propellant • Propellant is initially in the solid state and itbecomes 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

  33. 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

  34. 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

  35. 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 • VASMIR

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