NAR HPR Aerodynamics & Stability 2 — Questions and Answers
Question 1: What does a negative static margin indicate about a rocket's stability?
- The rocket is overstable and will weathercock aggressively
- The rocket is unstable and will likely tumble or veer off course (Correct answer)
- The rocket has neutral stability and flies unpredictably
- The rocket's CP is exactly at the CG
Correct answer: The rocket is unstable and will likely tumble or veer off course
A negative static margin means the Center of Pressure is ahead of the Center of Gravity, causing the rocket to be aerodynamically unstable.
Question 2: How does adding nose weight affect the stability of a marginally stable rocket?
- It moves the CP forward, decreasing stability
- It moves the CG forward relative to the CP, increasing stability (Correct answer)
- It increases fin area, improving stability
- It has no effect on stability margin
Correct answer: It moves the CG forward relative to the CP, increasing stability
Adding nose weight shifts the CG forward, increasing the distance between CG and CP and thus improving the stability margin.
Question 3: What is the primary aerodynamic purpose of boat-tailing a rocket's airframe?
- To increase base drag by expanding the wake
- To reduce base drag by smoothing airflow at the aft end (Correct answer)
- To move the CP rearward for greater stability
- To increase the rocket's frontal area
Correct answer: To reduce base drag by smoothing airflow at the aft end
Boat-tailing tapers the aft section of the airframe, reducing the size of the turbulent wake and thereby decreasing base drag.
Question 4: Which fin configuration is most effective at minimizing roll coupling during flight?
- Three fins with slight cant angle
- Four fins arranged symmetrically with no cant (Correct answer)
- Two fins mounted 180° apart
- Six fins with alternating cant angles
Correct answer: Four fins arranged symmetrically with no cant
Four fins arranged symmetrically with no cant produce balanced lift forces and minimize induced roll torque.
Question 5: What aerodynamic phenomenon causes a rocket to 'weathercock' into the wind?
- The CP being behind the CG causes the nose to turn away from the wind
- The CP being ahead of the CG causes the nose to turn into the wind (Correct answer)
- Increased drag from the fins in crosswinds
- The CG shifting rearward during motor burn
Correct answer: The CP being ahead of the CG causes the nose to turn into the wind
When CP is ahead of CG (unstable condition), aerodynamic forces rotate the nose into the wind, a behavior called weathercocking.
Question 6: How does a rocket's fineness ratio (length-to-diameter) affect skin friction drag?
- A higher fineness ratio always reduces total drag regardless of conditions
- A higher fineness ratio increases wetted surface area and thus skin friction drag (Correct answer)
- A lower fineness ratio increases skin friction by creating more turbulence
- Fineness ratio has no effect on skin friction drag
Correct answer: A higher fineness ratio increases wetted surface area and thus skin friction drag
A longer, slimmer rocket has more wetted surface area exposed to airflow, increasing the total skin friction drag force.
Question 7: What is the effect of increasing fin span on a rocket's static stability margin?
- It moves the CG rearward, reducing stability
- It moves the CP rearward, increasing the stability margin (Correct answer)
- It moves the CP forward, decreasing the stability margin
- It has no effect on the stability margin
Correct answer: It moves the CP rearward, increasing the stability margin
Larger fins generate more restoring force and shift the CP rearward, increasing the distance between CG and CP (stability margin).
NAR HPR Aerodynamics & Stability 2
What does a negative static margin indicate about a rocket's stability?