ANSWERS

1. c. Newton's Third Law. Applying a force in one direction always results in an equal force in the opposite direction.
2. b. The rocket motor's thrust causes the rocket to accelerate in the direction opposite the motor's thrust. Thus a rocket motor pushes only on the rocket, not on the air or launch pad.
3. c. Gravity, thrust and drag are the forces acting on a rocket.
4. c. The motor thrust, weight and aerodynamic drag are the primary forces considered when determining the altitude of a rocket. Please note that the weight of the rocket must consider the lift-off weight and the weight at burn-out to be complete.
5. b. The center of pressure (CP) is where the aerodynamic lift, due to the rocket being at a non-zero angle of attack, is centered. For an aerodynamically stable rocket with the CP behind the center of gravity (CG)n the lift which is centered aft of the CG will create a corrective moment to return the rocket to zero degrees angle of attack. Conversely, if the CP is ahead of the CG the lift will attempt to turn the rocket around so that the CP will again be behind the CG. This resultant "tumbling" is characteristic of an unstable rocket.
6. b. The rocket is not stable because if the rocket rotated around its center of gravity (CG), the greater aerodynamic force forward of the CG would cause the rocket to rotate even farther, resulting in an unstable flight.
7. c. The center of pressure (CP) is the point on the rocket where the aerodynamic lift is centered, This means that aerodynamic lift, if the rocket is at a non-zero angle of attack, forward of this point is balanced by the aerodynamic lift aft of that point.
8. a. Keeping the center of gravity (CG) one body diameter in front of the center of pressure (CP) typically allows an adequate margin for rocket stability.
9. a. Measuring the center of gravity (CG) by balancing the rocket requires that the rocket be prepared as though ready for flight. It is especially important to check when using a heavier motor than previously flown.
10. b. As the propellant burns the motor gets lighter and thus moves the balance point or center of gravity (CG) forward, This is why a marginally stable rocket will "act squirrelly" at launch, then stabilize and fly straight.
11. b. Adding enough weight to the nose will shift the center of gravity (CG) forward of the center of pressure (CP).
12. b. Moving the CG forward requires judicious design changes. The following are given as "rules-of-thumb," n Adding weight to the nose moves the CG forward by counterbalancing the rocket. Think of the rocket as a lever' making the rocket longer shifts the CG forward by making the lever longer. Using a smaller (or lighter) motor reduces the weight aft thus shifting the CG forward.
13. a. Moving the CP aft requires judicious design changes. The following are given as "rules-of-thumb." increasing the total fin area will move the CP aft. This can be accomplished by increasing the area on each fin and/or increasing the number of fins. The CP can also be shifted aft by making the rocket shorter. This alone is generally not preferred because the CG is also shifted aft and CP/CG stability relationship may be compromised.
14. b. The coefficient of drag (Cd) is a number that is used in equations for calculating the aerodynamic performance of a rocket. Values that make up the Cd are the rocket configuration (nose cone shape, airframe diameter(s), transition sections, fin size and sharpen etc.), the rocket velocity as Mach number and the angle of attack.
15. c. The coefficient of drag (Cd) increases and can be greater than 1 as the rocket exceeds Mach 1.
16. b. As speed increases, the drag number changes. The length and diameter of the rocket factors into the total surface area, The nose cone shape effects the airflow over the front of the nose cone. The fin shape and fin area factor into the total surface area.
17. c. A boat tail reduces the drag for a subsonic rocket by reducing the base drag resulting from the discontinuity of the air flow as it leaves the end of the rocket.
18. c. The three phases of flight of a high power rocket: (1) Powered flight - the period of time when the rocket motor is producing thrust against gravity and drag. (2) Un-powered ascent - the period after powered flight where the rockets momentum allows the rocket to coast to peak altitude and is effected by gravity and drag, (3) Descent - the return of the rocket to earth effected by gravity and drag.
19. a. As the regressive motor burns, the thrust decreases or regresses because the burning surface area of the propellant decreases. This is typical of slotted grains.
20. b. As the progressive motor burns, the thrust increases or progresses because the burning surface area of the propellant increases. This is typical of core burning motors.
21. b. As the motor burns from the core out, the ends of the grains are also burning making the grains shorter, This results in a relatively constant surface area.
22. c. The liner serves to keep the burning propellant (typically >5000°F) from touching the motor case (aluminum melts at 1075-F) while the O-rings seal the ends to keep the hot gasses where they belong, that is going out of the nozzle.
23. a. Ammonium Perchlorate is NH₄CIO₄ and is used in practically all modern solid rocket motors.
24. a. NH₄CIO₄ is the chemical formula for Ammonium Perchlorate.
25. c. Air pressure external to the rocket decreases as the rocket ascends. Trapped (higher) pressure within the rocket can prematurely separate the rocket. The hole vents this internal pressure to prevent separation. Note: The hole size is dependent on the size of the rocket and volume of air to be vented; larger airframes require larger holes. Use caution in locating the hole so the nose cone or payload coupler does not block the hole. Be sure to position the hole such that ejection charge pressure is not vented before recovery system deployment.
26. c. Smaller or fewer injector orifices allows a lower average oxidizer flown reducing the average thrust. Since the same amount of oxidizer is being used, the total impulse remains the same.
27. b. Larger or more injector orifices allows a higher average oxidizer flow, increasing the average thrust. Since the same amount of oxidizer is being used, the total impulse remains the same.
28. a. N₂O or nitrous oxide, also called NOX.
29. b. Nitrous Oxide liquefies at 750 psi at room temperature.
30. a. At 97°F the NOX has changed state to a supercritical gas.
31. c. black powder motors do not have a significant start up time and will ignite as soon as a flame front is encountered. Ammonium perchlorate-based composite motors require heat and pressure To start the combustion process and generally require at least a half-second before ignition occurs.
32. c. As the CG of the hybrid motor shifts aft, so does the CG of the rocket which may result in an unstable flight.
33. b. specific impulse is a term used to define the efficiency of a rocket propellant and is the total impulse derived from a given mass of propellant.
34. a. Total impulse is the amount of thrust produced by a motor over its action time. For instance, a motor may produce 10 pounds of thrust for 4 seconds resulting in a total impulse of 40 pound-seconds.
35. b. Multiply the average thrust (100 Newtons) by the burn time (4 seconds) to get the total impulse of 400 newton-seconds.
36. c. The J motor has a range of 641 to 1280 Newton-seconds and the K motor has a total impulse range of 1281 to 2560 newton-seconds.
37. b. Even though the total impulse of the K motor is greater than the J motor, the J motor's average thrust is 400 Newton's versus the K motor's 200 Newtons.
38. c. The burn time is determined by dividing the total impulse (J = 1280) by the average thrust of each motor. The burn time for the J640 is: 1280 Newton-seconds divided by 640 Newtons = 2 seconds, and for the J320 is: 1280 Newton-seconds divided by 320 Newtons = 4 seconds.
39. b. A J motor is in the range of 640.01 to 1280 Newton-seconds. Therefore, a 1000 Newton-second motor is a midrange J. The 600 Newton-second motor is an I motor and the 1290 Newton-second motor is a K motor.
40. c. The newton is an international (metric) unit of force and is the force required to accelerate one kg (2.2 lbs) one meter (39 inches) per second per second.
41. c. This is an I motor with a total impulse range of 320.01 to 640 Newton-seconds, an average thrust of 220 Newton's and an ejection delay of 8 seconds from burn-out.
42. c. Energy is determined by the equation E = 1/2 mv². From this it is important to note that for objects of the same mass. one moving twice as fast has four times the energy as the slower one.
43. a. Energy is determined by the equation E = 1/2 mv². From this it is important to note that for objects of the same mass. one moving twice as fast has four times the energy as the slower one.
44. c. The purpose of the launch rod, rail or tower is to guide the rocket at the beginning of its flight to allow it to gain sufficient velocity for a stable flight. This is achieved when the air flowing over the rocket and its fins allows the rocket to correct its flight by forcing rotation around the rocket's center of gravity,
45. b. The launch lug attaches the rocket to the launch rod or rail allowing the rocket to be guided by the rod or rail at launch.
46. c. Composite (Ammonium Perchlorate) motors require heat and pressure to ignite. The motor core diameter is smaller in the 29mm G80 motors and heat and pressure is more concentrated resulting in faster ignition of the motors.
47. b. Not having ignition of ail clustered motors results in the thrust being unsymmetrical. This unbalanced thrust may force the rocket to fly in an unanticipated arc that will not achieve a vertical flight.
48. a. A shred happens when the rocket is improperly built or has a rocket motor too powerful for that particular rocket. The typical shred sequence is that the velocity of the rocket has increased to a point where airframe, fins or other structural parts cannot take the loads. When that part fails, it typically causes the rocket to become unstable resulting in the rapid destruction of the rocket.
49. c. A cato is short for catastrophic motor failure. This occurs when the nozzle, forward bulkhead or casing fails. The immediate result is abrupt termination of thrust which results in the rocket failing.
50. a. A motor ignitor must deliver sufficient heat to the propellant to get it ignited. This may be in the form of hot gas, hot burning particles, a hot wire or a combination of all three.