NAR HPR Propulsion Systems & Motor Handling 3 — Questions and Answers
Question 1: What is 'specific impulse' (Isp) as applied to rocket motor performance?
- The thrust produced per unit weight flow rate of propellant, measured in seconds (Correct answer)
- The total impulse divided by the motor's total mass including casing
- The burn time multiplied by average thrust in Newton-seconds
- The propellant weight expressed as a fraction of total motor weight
Correct answer: The thrust produced per unit weight flow rate of propellant, measured in seconds
Specific impulse (Isp) is a measure of propellant efficiency: thrust divided by propellant weight flow rate, with units of seconds.
Question 2: When a reloadable motor casing shows visible cracks, bulging, or heat discoloration after a flight, the correct action is to:
- Retire the casing from service immediately and do not reload it (Correct answer)
- Reload it one more time and inspect after the next flight
- Sand the discolored area and pressure-test before next use
- Contact the manufacturer for a waiver to continue using it
Correct answer: Retire the casing from service immediately and do not reload it
Any structural damage or severe heat discoloration on a reloadable casing is grounds for immediate retirement to prevent catastrophic failure.
Question 3: The 'thrust curve' of a rocket motor graphically shows which relationship?
- Thrust output versus time during the burn (Correct answer)
- Motor temperature versus altitude during ascent
- Propellant mass versus burn duration
- Nozzle pressure versus ambient air pressure
Correct answer: Thrust output versus time during the burn
A thrust curve plots thrust (in Newtons) on the Y-axis versus time (in seconds) on the X-axis, describing how thrust changes throughout the burn.
Question 4: A 'regressive' burn profile in a motor means that thrust:
- Starts high and decreases over the course of the burn (Correct answer)
- Starts low and increases over the course of the burn
- Remains constant throughout the entire burn
- Spikes sharply at ignition and then cuts off instantly
Correct answer: Starts high and decreases over the course of the burn
A regressive burn profile produces higher initial thrust that tapers downward as propellant is consumed, due to decreasing grain surface area.
Question 5: Under what circumstance is it permissible to fly a high-power motor that is not on the NAR/TRA certified motors list?
- It is never permissible; only certified motors may be flown at sanctioned launches (Correct answer)
- With written permission from the launch director on a case-by-case basis
- If the motor was previously certified but has since been discontinued
- If the flyer holds Level 3 certification and assumes personal liability
Correct answer: It is never permissible; only certified motors may be flown at sanctioned launches
NAR and TRA require all motors flown at sanctioned events to be currently certified; no exceptions exist based on certification level or other conditions.
Question 6: What does 'CATO' mean in the context of high-power rocketry motor events?
- A catastrophic motor failure during or before liftoff, causing the motor to rupture (Correct answer)
- A motor that burns correctly but the rocket fails to achieve stable flight
- A delayed ignition sequence that fires the motor after the countdown
- The certification authority's rejection of a motor test submission
Correct answer: A catastrophic motor failure during or before liftoff, causing the motor to rupture
CATO refers to a motor that fails destructively—rupturing the casing rather than producing normal thrust—and poses a serious safety hazard.
Question 7: What is the purpose of a 'delay element' in a high-power motor designation such as 'J450DM-14A'?
- It creates a timed delay between motor burnout and ejection charge firing for recovery deployment (Correct answer)
- It slows the initial thrust buildup to protect the rocket airframe at launch
- It delays ignition after the launch controller fires to allow personnel to clear the pad
- It provides a secondary burn phase for additional altitude after coast phase
Correct answer: It creates a timed delay between motor burnout and ejection charge firing for recovery deployment
The delay element burns after motor burnout to allow the rocket to coast to apogee before firing the ejection charge that deploys the recovery system.
NAR HPR Propulsion Systems & Motor Handling 3
What is 'specific impulse' (Isp) as applied to rocket motor performance?