## TECHNICAL QUESTIONS:

1. Which of Newton's Laws best describes the behavior of a rocket motor?
1. Newton's First Law: Every body continues in its state of rest or of uniform motion in a straight line unless it is compelled to change that state by forces impressed upon it.
2. Newton's Second Law: The rate of change of momentum is proportional to the force impressed and is in the same direction as that force.
3. Newton's Third Law: To every action there is always an equal and opposite reaction.
2. How does Newton's Third Law "To every action there is always an equal and opposite reaction" relate to rocketry?
1. That the blast deflector must be strong enough to push the rocket off the launch pad at ignition.
2. That a rocket flies because the rocket motor "pushes" the rocket in a direction opposite of the exhaust jet.
3. That the thrust of a rocket motor is proportional to the air density at the launch site.
3. What are the three forces acting upon a rocket during the course of its flight?
1. Thrust, rocket diameter and finish.
2. Nose cone shape, thrust and drag.
3. Gravity, thrust and aerodynamic drag.
4. What are the major factors that determine the maximum altitude of a high power rocket in vertical flight?
1. Lift-off weight, propellant weight and motor thrust.
2. Fin size, propellant weight and motor thrust.
3. Motor thrust, weight and aerodynamic drag.
5. For an inherently stable rocket, what is the relationship of center of gravity (CG) to the center of pressure (CP)?
1. The CG must be behind the CP relative to the desired direction of flight.
2. The CG must be forward of the CP relative to the desired direction of flight.
3. The CG must be in front of the fins of a rocket.
6. A 4" diameter rocket with its motor is determined to have the center of gravity (CG) four inches behind the center of pressure (CP). Is this a stable rocket?
1. There is insufficient information to answer this question.
2. No, the CP must be behind the CG for the rocket to be stable.
3. Yes, the CP is one body diameter in front of the CG.
7. The center of pressure (CP) of a rocket is generally defined as:
1. The balance point of the rocket without the motor.
2. The total area of the fins, airframe and nose cone divided by two.
3. The point at which aerodynamic lift on a rocket is centered.
8. What is the "rule-of-thumb" for a stable rocket?
1. That the center of gravity is one body diameter in front of the center of pressure.
2. That the center of gravity is at the same point as the center of pressure.
3. There is no rule-of-thumb because there are too many variables.
9. When determining the center of gravity (CG) of a rocket with a heavier motor at the launch site, one can:
1. Install the motor, recovery system and payload and determine the balance point of the rocket as it is ready for flight.
2. Balance the rocket with an empty motor because that is the condition of the rocket after motor burnout.
3. It is not necessary to test for the center of gravity when using a more powerful motor because it has more thrust.
10. What happens to the center of gravity (CG) of a rocket during a solid rocket motor's thrusting phase?
1. The Center of gravity stays the same.
2. The Center of gravity shifts forward.
3. The center of gravity shifts aft.
11. How can a statically unstable rocket be made stable?
1. Using a heavier motor.
2. Adding weight to the nose.
3. Making the rocket shorter.
12. What are three methods used to shift the center of gravity (CG) of a rocket forward?
1. Add weight to the nose, make the rocket longer, install larger fins.
2. Add weight to the nose, make the rocket longer, use a smaller (or lighter) motor.
3. Add weight to the nose, make the rocket shorter, use a smaller motor.
13. What are three methods used to shift the center of pressure (CP) aft?
1. Make the rocket shorter, use larger fins, increase the number of fins.
2. Make the rocket shorter n use smaller fins, add weight to the nose.
3. Make the rocket shorter n change the number of fins, use a longer launch rod,
14. What is the definition of coefficient of drag (Cd)?
1. A dimensionless number that represents the effect of gravity and Mach number of the rocket.
2. A dimensionless number representing the rocket configuration, Mach number and angle of attack.
3. A dimensionless number that represents the friction of the launcher and launch velocity.
15. What happens to the coefficient of drag (Cd) as the rocket approaches the speed of sound?
1. The Cd decreases.
2. The Cd stays the same.
3. The Cd increases.
16. For a subsonic rocket, what major factors affect the coefficient of drag (Cd)?
1. Motor thrust, body diameter, nosecone shape and fin shape.
2. Speed, airframe dimensions, nosecone shape and fin shape.
3. Gravity, airframe dimensions, nosecone shape and fin shape.
17. What effect does a boat tail have on a subsonic rocket's coefficient of drag (Cd)?
1. No effect, a boat tail is only a cosmetic design feature.
2. It increases the Cd by changing the airflow over the fins.
3. It decreases the Cd by reducing the base drag.
18. The flight of a high power rocket can be separated into three portions; they are:
1. Ignition, burnout and peak altitude.
2. Powered flight, un-powered ascent and peak altitude.
3. Powered flight, un-powered ascent and descent.
19. What is the thrust curve of a regressive motor burn?
1. A regressive burn has a high initial thrust relative to the ending thrust of the motor.
2. A regressive burn has a lower initial thrust relative to the ending thrust.
3. The thrust curve is flat.
20. What is the thrust curve of a progressive motor?
1. A progressive burn has a high initial thrust relative to the ending thrust of the motor.
2. A progressive burn has a lower initial thrust relative to the ending thrust.
3. The thrust curve is flat.
21. Why does a Bates grain have an essentially neutral thrust curve?
1. Because core burning motors always have a regressive burn.
2. Because the burn area of the motor remains relatively constant.
3. Because the core in the motor grain has a uniform burn area over time.
22. What is the function of a motor liner and the O-ring seals in a solid rocket motor?
1. To hold all of the parts in place prior to ignition of the rocket motor.
2. To make the motor easier to clean if it is a reloadable motor.
3. To keep the hot gasses of the motor from burning or melting the motor Case.
23. What is the most common oxidizer in commercially available high power composite solid rocket motors?
1. Ammonium Perchlorate.
2. Ammonium Nitrate.
3. Ammonium Chlorate.
24. What is NH4ClO4?
1. Ammonium Perchlorate.
2. Ammonium Nitrate.
3. Ammonium Chlorate.
25. A small hole is typically recommended near the top, but below the nosecone or payload section, of a high power rocket's booster section. Why?
1. This hole allows excessive ejection charge pressures to vent to reduce shock cord stress.
2. The hole is used to give air pressure readings for on-board altimeters.
3. The hole vents internal air pressure as the rocket gains altitude to prevent premature separation.
26. What happens when changing to smaller or fewer injector orifices in an ideal hybrid rocket motor (assume the oxidizer weight stays the same)?
1. The total impulse decreases and the average thrust increases.
2. The total impulse stays the same and the average thrust increases.
3. The total impulse stays the same and the average thrust decreases.
27. What happens when changing to more or larger injector orifices in an ideal hybrid rocket motor (assume the oxidizer weight stays the same)?
1. The total impulse decreases and the average thrust increases.
2. The total impulse stays the same and the average thrust increases.
3. The total impulse stays the same and the average thrust decreases.
28. What is the oxidizer most commonly used in a commercial hybrid rocket motor?
1. N2O
2. N2O4
3. NO2
29. What is the nominal tank pressure of a nitrous oxide hybrid motor at 75°F?
1. 100 psi
2. 750 psi
3. 1500 psi
30. Above what temperature does pressurized nitrous oxide change to a gas?
1. 97°F
2. 75°F
3. 37°F
31. A rocket with a motor cluster consisting of a central composite motor and four black powder motors using thermalite igniters or electric matches:
1. will result in all motors starting about the same time.
2. will result in the composite motor starting first followed by the black powder motors.
3. will result in the black powder motors starting first followed by the central composite motor.
32. What typically happens to a marginally stable rocket with a hybrid motor during the thrusting phase?
1. Nothing.
2. The rocket may become more stable.
3. The rocket may become less stable.
33. In general terms, the specific impulse of a rocket motor is:
1. The total thrust force of a motor throughout its action time.
2. The total impulse divided by unit weight of propellant.
3. Dependent on the diameter and length of the propellant grain.
34. In general terms, the total impulse of a rocket motor can be described as:
1. The product of the average motor thrust and its burn time.
2. The product of the propellant weight and its burn time.
3. The product of the propellant weight and the motor thrust.
35. The average thrust of a rocket motor is 100 Newtons and the burn time is 4 seconds, what is the total impulse?
1. 25 Newton-seconds
2. 400 Newton-seconds
3. 400 newtons
36. Which motor has a higher total impulse?
1. J200
2. J400
3. K200
37. Which motor has a higher average thrust?
1. J200
2. J400
3. K200
38. What is the difference between a J640 and a J320 high power rocket motor (assume full 1280 Newton-second J motors)?
1. The J320 burns out twice as fast as the J640.
2. There is no difference between the motors, the numbers are manufacturer reference only.
3. The J640 burns out twice as fast as the J320.
39. Which of the following has a total impulse in the J motor range?
1. It = 600 Newton-seconds
2. It = 1000 Newton-seconds
3. It = 1290 Newton-seconds
40. What is a Newton?
1. The amount of force required to accelerate one pound one foot per second per second.
2. The amount of force required to accelerate one kg, one foot per second per second.
3. The amount of force required to accelerate one kg, one meter per second per second.
41. What does the motor designation I220-8 mean?
1. The motor is in the I impulse range with an average thrust of 220 Newtons and an 8 second delay from motor ignition.
2. b. The motor is in the I impulse range, having a total impulse of 620 Newton-seconds with an average thrust of 220 Newtons and an 8 second delay from motor burn-out.
3. The motor is in the I impulse range with an average thrust of 220 Newtons and an 8 second ejection delay from motor burn-out.
42. What is the difference in kinetic energy between two identical rockets, one descending at 30 feet per second, the other descending at 60 feet per second?
1. Cannot be determined without the weight.
2. Two times as much energy.
3. Four times as much energy.
43. The equation for determining the energy of a moving body (such as a rocket) is:
1. E = 1/2 mv2
2. E = ma2
3. E = mv3
44. What is the purpose of a launch rod, rail or tower?
1. To keep the rocket pointing in the right direction prior to flight.
2. To control the rocket's flight long enough to allow aerodynamic stability.
3. Both a and b.
45. What is the purpose of a launch lug?
1. To add drag to the rocket at launch.
2. To guide the rocket along the launch rod or rail.
3. Both a and b.
46. A rocket with a motor cluster consisting of a central composite 54mm J415 motor and four 29mm G80 composite motors using thermalite igniters or electric matches:
1. will result in all motors starting about the same time.
2. will result in the J415 motor starting first followed by the G80's.
3. will result in the G80's starting first followed by the J415.
47. What can happen if all the motors of a cluster do not ignite at launch?
1. Nothing, the rocket is inherently stable.
2. The rocket may not fly straight.
3. The rocket will shred.
48. What is a shred?
1. A failure of the rocket air frame during boost resulting in destruction of the rocket.
2. A failure of the recovery system during boost.
3. A failure of the motor causing early ejection.
49. What is a cato?
1. A failure of the rocket resulting in failure of the air frame during boost.
2. A failure of the recovery system during boost.
3. A failure of the motor causing flight termination.
50. What is the primary requirement for a rocket motor ignitor?
1. It must transfer sufficient heat to the propellant to assure ignition.
2. It must produce hot, high velocity gasses to assure ignition.
3. It must have a high resistance to be reliable.

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.