NAR HPR Rocket Design & Construction 3 — Questions and Answers
Question 1: What is the typical recommended wall thickness for a phenolic or fiberglass airframe tube used in a Level 2 high-power rocket?
- 0.010–0.020 inches
- 0.060–0.125 inches (Correct answer)
- 0.250–0.375 inches
- 0.500 inches or more
Correct answer: 0.060–0.125 inches
Level 2 rockets typically use tubes with wall thickness of 0.060–0.125 inches to balance weight, strength, and cost for typical L2 motor impulse ranges.
Question 2: What is the significance of the 'stability margin' being expressed in calibers rather than absolute length?
- Calibers are easier to calculate than centimeters
- Calibers normalize stability relative to body diameter, allowing comparison across rocket sizes (Correct answer)
- NAR requires caliber measurements for certification
- Calibers account for fin flutter margins
Correct answer: Calibers normalize stability relative to body diameter, allowing comparison across rocket sizes
Expressing stability margin in calibers (body diameters) normalizes the measurement, making it meaningful across rockets of different sizes.
Question 3: Which construction method produces the strongest fin-to-body attachment for high-power rocketry?
- Gluing fins to the exterior airframe surface only
- Through-the-wall (TTW) fins bonded to the motor mount tube with internal fillets (Correct answer)
- Wrapping fins with fiberglass tape only
- Using machine screws through the airframe
Correct answer: Through-the-wall (TTW) fins bonded to the motor mount tube with internal fillets
Through-the-wall fins extend through the airframe to bond directly to the motor tube, distributing loads over a larger area and greatly increasing joint strength.
Question 4: What is 'ballistic coefficient' and why does it matter in high-power rocketry?
- The ratio of motor thrust to rocket weight at liftoff
- A measure of a rocket's ability to overcome aerodynamic drag, affecting terminal velocity and altitude (Correct answer)
- The coefficient of drag at supersonic speed
- The ratio of fin area to body cross-sectional area
Correct answer: A measure of a rocket's ability to overcome aerodynamic drag, affecting terminal velocity and altitude
Ballistic coefficient (mass divided by drag area) determines how well the rocket maintains velocity against drag, affecting maximum altitude and terminal descent speed.
Question 5: When using RockSim or OpenRocket, what does a Barrowman stability calculation assume about the rocket's flight regime?
- Supersonic flight only
- Subsonic, non-spinning, small angle of attack flight (Correct answer)
- Hypersonic reentry conditions
- Flight with active guidance
Correct answer: Subsonic, non-spinning, small angle of attack flight
The Barrowman equations are valid for subsonic, non-spinning rockets at small angles of attack and become inaccurate at transonic or supersonic speeds.
Question 6: What is the purpose of a 'vent hole' in a rocket's nose cone or payload section?
- To reduce nose cone weight
- To equalize internal pressure with ambient pressure during ascent and descent (Correct answer)
- To allow electronics wiring to pass through
- To release combustion gases
Correct answer: To equalize internal pressure with ambient pressure during ascent and descent
Vent holes prevent pressure differentials from building up inside sealed sections, which could cause the airframe to bulge, fail, or prematurely eject components.
Question 7: Which epoxy characteristic is most important when bonding carbon fiber components in high-power rocket construction?
- Fast 5-minute cure time for convenience
- Low modulus (flexible) formulation to allow movement
- Compatibility with carbon fiber surface energy and high shear strength (Correct answer)
- High filler content for gap filling
Correct answer: Compatibility with carbon fiber surface energy and high shear strength
Carbon fiber has low surface energy and requires epoxy specifically compatible with composites, with high shear strength to exploit carbon fiber's structural properties.
NAR HPR Rocket Design & Construction 3
What is the typical recommended wall thickness for a phenolic or fiberglass airframe tube used in a Level 2 high-power rocket?