NAR HPR Propulsion Systems & Motor Classification 3 — Questions and Answers
Question 1: What is 'specific impulse' (Isp) as it applies to rocket motors?
- A measure of propellant efficiency expressed in seconds (Correct answer)
- The total impulse divided by motor weight
- Average thrust multiplied by burn time
- The ratio of oxidizer to fuel in the propellant
Correct answer: A measure of propellant efficiency expressed in seconds
Specific impulse (Isp) measures propellant efficiency as thrust produced per unit weight flow of propellant per second, expressed in seconds.
Question 2: Which motor thrust curve type delivers the majority of its thrust near the beginning of the burn?
- Regressive (Correct answer)
- Progressive
- Neutral
- Plateau
Correct answer: Regressive
A regressive thrust curve delivers peak thrust early in the burn, which then decreases as the propellant grain burns away.
Question 3: For NAR Level 2 certification, what is the maximum motor class a certified rocketeer may purchase and fly?
- L (Correct answer)
- K
- M
- J
Correct answer: L
NAR Level 2 certification authorizes the purchase and use of motors up through and including L-class (total impulse up to 10,240 N-s).
Question 4: What is the function of the forward closure in a reloadable motor casing?
- It seals the nozzle end of the motor and retains the propellant grain (Correct answer)
- It houses the igniter and initiates the burn
- It contains the ejection charge for recovery deployment
- It provides structural attachment between motor and airframe
Correct answer: It seals the nozzle end of the motor and retains the propellant grain
The forward closure seals the forward (top) end of the motor casing, retaining chamber pressure and the propellant assembly.
Question 5: Which of the following best describes a 'progressive' thrust curve?
- Thrust increases throughout the burn as the grain surface area grows (Correct answer)
- Thrust remains constant from ignition to burnout
- Thrust peaks early then gradually decreases
- Thrust oscillates between high and low values during burn
Correct answer: Thrust increases throughout the burn as the grain surface area grows
A progressive thrust curve shows increasing thrust over the burn duration due to a grain geometry that exposes more burning surface area as it burns.
Question 6: What is the primary hazard associated with motor 'CATO' (catastrophic failure) during a high-power launch?
- Explosive rupture of the motor casing producing high-velocity shrapnel (Correct answer)
- Incomplete propellant burn causing the motor to misfire
- Over-pressurization of the recovery bay ejecting the parachute too early
- Nozzle erosion causing the rocket to veer off-course
Correct answer: Explosive rupture of the motor casing producing high-velocity shrapnel
A CATO results in catastrophic over-pressurization and structural failure of the motor casing, releasing fragments at high velocity.
Question 7: What does the 'O' motor class total impulse range span?
- 20,481–40,960 N-s (Correct answer)
- 10,241–20,480 N-s
- 40,961–81,920 N-s
- 5,121–10,240 N-s
Correct answer: 20,481–40,960 N-s
O-class motors cover 20,481–40,960 N-s total impulse, sitting between N-class and P-class in the doubling sequence.
NAR HPR Propulsion Systems & Motor Classification 3
What is 'specific impulse' (Isp) as it applies to rocket motors?