NAR HPR Rocket Design & Aerodynamics 2 — Questions and Answers
Question 1: What is the primary purpose of a boat-tail transition on a high-power rocket?
- To increase drag and slow descent
- To reduce base drag by smoothing airflow at the aft end (Correct answer)
- To provide structural support for the motor mount
- To increase stability by shifting the center of pressure aft
Correct answer: To reduce base drag by smoothing airflow at the aft end
A boat-tail transition tapers the aft body diameter inward, which reduces turbulent wake and lowers base drag.
Question 2: A rocket's center of pressure (CP) is determined primarily by:
- The weight distribution of internal components
- The geometry of the rocket's external surfaces exposed to airflow (Correct answer)
- The thrust curve of the motor
- The location of the recovery harness attachment point
Correct answer: The geometry of the rocket's external surfaces exposed to airflow
CP is a function of the rocket's external aerodynamic geometry — fin area, nose cone shape, and body diameter — not internal mass.
Question 3: Which nose cone profile generally produces the lowest drag at supersonic speeds?
- Elliptical
- Ogive
- Von Kármán (Haack series) (Correct answer)
- Conical
Correct answer: Von Kármán (Haack series)
The Von Kármán (LD-Haack) nose cone is mathematically optimized to minimize wave drag at supersonic velocities.
Question 4: If you increase fin span while keeping fin area constant, what is the expected effect on stability margin?
- Stability margin decreases because CP moves forward (Correct answer)
- Stability margin increases because CP moves aft
- Stability margin is unchanged because fin area is the same
- Stability margin decreases because CG moves forward
Correct answer: Stability margin decreases because CP moves forward
Longer, narrower fins move the CP forward relative to short wide fins of equal area, reducing stability margin.
Question 5: What phenomenon causes fin flutter, and at what flight regime is it most dangerous?
- Thermal expansion at high altitudes causing fin warping
- Aerodynamic resonance where airflow oscillations match fin natural frequency, worst near transonic speeds (Correct answer)
- Excessive motor thrust vibration transmitted through the airframe
- Recovery system deployment forces acting on fin trailing edges
Correct answer: Aerodynamic resonance where airflow oscillations match fin natural frequency, worst near transonic speeds
Fin flutter occurs when aerodynamic forcing frequency matches the fin's structural resonant frequency, typically most severe in the transonic regime.
Question 6: In Barrowman equations, which component contributes the MOST to the normal force coefficient (CNα) for a typical 4-fin rocket?
- The nose cone
- The body tube transitions
- The fins (Correct answer)
- The launch lug or rail buttons
Correct answer: The fins
Fins generate the largest CNα contribution because their large surface area produces substantial normal force in an angle-of-attack scenario.
Question 7: A rocket has a stability margin of 0.5 calibers. How should a flier interpret this?
- Acceptable — 0.5 calibers exceeds the minimum 0.25-caliber requirement
- Dangerously over-stable — the rocket will weathercock severely
- Marginally stable — generally considered borderline and risky for high-power flights (Correct answer)
- Unstable — will immediately veer off course after launch
Correct answer: Marginally stable — generally considered borderline and risky for high-power flights
A 0.5-caliber margin is considered marginal; NAR guidance recommends a minimum of 1 caliber for reliable stability in high-power flights.
NAR HPR Rocket Design & Aerodynamics 2
What is the primary purpose of a boat-tail transition on a high-power rocket?