Model Rocket- What does the 'v' in drag=kv^2 really mean?

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SUMMARY

The discussion centers on determining the correct value for 'v' in the drag equation, drag = kv^2, when predicting the peak altitude of a model rocket. The consensus is that 'v' should represent the velocity of the rocket relative to the surrounding air, incorporating all forces acting on the rocket, including thrust, gravity, and drag. The user, Andrea, initially confused about whether to consider the motor's force or the actual velocity, clarified that the velocity must account for air resistance. Ultimately, it was advised that incorporating drag into calculations may be complex due to the presence of two unknowns: the force due to drag and the actual velocity.

PREREQUISITES
  • Understanding of the drag equation and its components.
  • Basic principles of physics related to forces and motion.
  • Familiarity with model rocket dynamics and thrust calculations.
  • Knowledge of air resistance and its impact on projectile motion.
NEXT STEPS
  • Research the effects of air resistance on model rocket flight using computational fluid dynamics (CFD) simulations.
  • Learn about the impulse-momentum theorem to better understand thrust calculations.
  • Explore advanced drag coefficient calculations for various rocket shapes and sizes.
  • Investigate methods for experimentally measuring drag forces on model rockets during flight tests.
USEFUL FOR

This discussion is beneficial for model rocket enthusiasts, aerospace engineering students, and anyone involved in the design and optimization of rocket performance, particularly in understanding the dynamics of drag and thrust.

Andy24
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Hello! I am predicting the peak altitude of a model rocket based on some ground tests. I know that Total Force on the rocket=Thrust-mg-kv^2 but am stuck as to which value for 'v' to use? Is it the velocity of the rocket with the effects of gravity and air resistance taken into account or the velocity purely from the motor (without any air resistance or gravity taken into account)?

Your help is appreciated, thanks.
 
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The velocity at any given time is the rate of change of position over time, how fast the object is actually going. There is no such thing as "the velocity purely from the motor": the motor does not give a velocity, it gives a force. The velocity will be the result of all the forces acting on the object (and its previous velocity if it is already moving).
 
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DrClaude said:
That's reminds me that I forgot to add that it is the velocity with respect to the surrounding air.

Thanks for your response.. I understand now. I actually calculated the velocity based on some ground testing (where I found out the impulse of the motor) and was hoping to incorporate drag.. This may give you context: https://www.physicsforums.com/threads/coefficient-of-drag-on-a-model-rocket.871810/
Thanks for your help though, I think I might just leave incorporating drag into my calculations as I have 2 unknowns- the force due to drag as well as the actual velocity. Thanks again,
Andrea
 
Andy24 said:
Hello! I am predicting the peak altitude of a model rocket based on some ground tests. I know that Total Force on the rocket=Thrust-mg-kv^2 but am stuck as to which value for 'v' to use? Is it the velocity of the rocket with the effects of gravity and air resistance taken into account or the velocity purely from the motor (without any air resistance or gravity taken into account)?
In this context, the terms "drag" and "air resistance" are synonymous.
 

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