NAR HPR Rocket Design & Aerodynamics 3 — Questions and Answers
Question 1: What is the 'transonic dip' effect on a rocket's stability margin?
- Stability increases dramatically as the rocket passes through Mach 1
- CP shifts forward near Mach 1 due to changing aerodynamic coefficients, temporarily reducing stability (Correct answer)
- CG shifts aft due to propellant burn-off at transonic speeds
- Fin flutter causes a visible dip in altitude data near Mach 1
Correct answer: CP shifts forward near Mach 1 due to changing aerodynamic coefficients, temporarily reducing stability
Near Mach 1, the normal force coefficients of fins change, causing CP to migrate forward and temporarily reducing the stability margin.
Question 2: What is the purpose of a drogue parachute in a dual-deployment recovery system?
- To provide full deceleration from apogee to landing
- To stabilize the rocket in a nose-down attitude and slow descent from apogee to low altitude before main deployment (Correct answer)
- To prevent the rocket from drifting downrange by increasing drag at apogee
- To protect the main parachute from ejection charge hot gases
Correct answer: To stabilize the rocket in a nose-down attitude and slow descent from apogee to low altitude before main deployment
The drogue deploys at apogee to slow the rocket and keep it stable during high-altitude descent before the main parachute opens at lower altitude.
Question 3: Which factor most directly affects the 'weathercocking' tendency of a rocket in a crosswind?
- Motor total impulse
- Stability margin — higher margin increases weathercocking tendency (Correct answer)
- Fin material (aluminum vs. plywood)
- Launch rod length
Correct answer: Stability margin — higher margin increases weathercocking tendency
A more stable (higher margin) rocket has a stronger tendency to turn into the wind (weathercock) because its CP is farther aft of CG.
Question 4: When computing stability with Rocksim or OpenRocket, why does the software show two CG positions?
- One for ascent and one for descent trajectories
- One for the loaded (full propellant) configuration and one for the burnout (empty) configuration (Correct answer)
- One for sea-level and one for altitude air density
- One including the launch rail and one without it
Correct answer: One for the loaded (full propellant) configuration and one for the burnout (empty) configuration
As propellant burns, CG shifts; software shows both full-load CG (launch) and burnout CG to ensure stability throughout the flight.
Question 5: What is 'roll coupling' and why is it a concern for high-power rockets?
- The tendency for a rolling rocket to develop pitch and yaw oscillations due to gyroscopic effects (Correct answer)
- Mechanical binding of the fin can when fin tabs are too tight
- Coupling between two separate motor stages causing ignition timing errors
- The connection method between upper and lower airframe sections
Correct answer: The tendency for a rolling rocket to develop pitch and yaw oscillations due to gyroscopic effects
Roll coupling occurs when a spinning rocket's gyroscopic forces couple with pitch/yaw modes, potentially causing violent coning or divergence.
Question 6: For a given fin geometry, switching from 4 fins to 3 fins of equal individual area will generally:
- Increase total fin CNα contribution because fewer fins have less interference drag
- Decrease total fin CNα contribution because total fin planform area is reduced (Correct answer)
- Have no effect on CNα because the number of fins doesn't matter, only total area
- Increase stability by reducing body-fin interference at the root
Correct answer: Decrease total fin CNα contribution because total fin planform area is reduced
Total fin normal force scales with total planform area; three fins have 75% of the area of four identical fins, reducing CNα and shifting CP forward.
Question 7: A 'minimum diameter' rocket design places the motor directly inside the body tube with no motor mount tube. What is the primary aerodynamic benefit?
- Eliminates the need for fins entirely
- Reduces frontal cross-sectional area to an absolute minimum, dramatically lowering pressure drag (Correct answer)
- Moves the CP rearward for maximum stability margin
- Allows the rocket to reach subsonic speeds faster for easier recovery
Correct answer: Reduces frontal cross-sectional area to an absolute minimum, dramatically lowering pressure drag
Minimizing body diameter reduces frontal area and thus pressure (form) drag, enabling much higher velocities for a given motor impulse.
NAR HPR Rocket Design & Aerodynamics 3
What is the 'transonic dip' effect on a rocket's stability margin?