I have calculated the apogee of a 2.5kN avg. thrust sounding rocket once ignoring drag and then assuming maximum constant drag after burnout. To my surprise, the results were dramatically different (~47km vs ~4km). Where should I expect the actual apogee to be within this range and why?

I have calculated the apogee of a 2.5kN avg. thrust sounding rocket once ignoring drag and then assuming maximum constant (magnitude and direction) drag after burnout. Here are the figures I used in my calculations:

Rocket:

• Carbon body and payload: 6 000 g
• Propellant: 8 496 g
• Motor (N2501) at burnout: 4 604 g
• Average thrust: 2501.8 N
• Burn time 6.09 s
• Total drag coefficient: 0.54

Other constants:

• Nominal launcher elevation angle: 75°
• Frontal area 𝐴 ≈ 0.0103 m2
• Constant air density of 𝜌 ≈ 0.631 kg m−3

To my surprise, the results were dramatically different (~47km vs ~4km). Where should I expect the actual apogee to be within this range and why?