Model Rocket Altitude Calculator (Simple)
Estimate a model rocket’s peak altitude from motor thrust, burn time and rocket mass using a simple drag-free two-phase model — an optimistic upper bound.
How it works
Boost: a = (F − mg) ÷ m · v = a·t · y₁ = ½·a·t² · Coast: y₂ = v² ÷ (2g) · Apogee = y₁ + y₂
This is the textbook drag-free estimate: during the burn the motor pushes with more force than gravity, so the rocket accelerates upward; after burnout it coasts, trading its speed for height until gravity brings it to a stop at apogee. Because it ignores air resistance entirely, the number it gives is an optimistic upper bound — real rockets lose a great deal of altitude to drag, which grows with the square of speed, so a light, fast rocket can come in at half this figure or less. It also assumes constant average thrust and constant mass; use it to compare motors and sanity-check a build, not to predict the exact peak.
Worked example
A 50 g rocket (0.05 kg) on a 5 N motor burning 1 s weighs 0.05 × 9.81 ≈ 0.49 N. Net accel = (5 − 0.49) ÷ 0.05 ≈ 90 m/s², giving a burnout speed of about 90 m/s at 45 m up. Coasting adds 90² ÷ (2 × 9.81) ≈ 415 m, for a drag-free apogee near 460 m — real altitude would be well below this.
Frequently asked questions
Why is my real altitude lower than this?
This model ignores air drag, which is the single biggest force slowing a model rocket after launch. Drag rises with the square of speed, so a fast, light rocket loses the most, and real apogees are commonly half of the drag-free estimate or less.
What if the thrust is less than the weight?
The rocket cannot lift off — it needs more thrust than its weight to accelerate upward. The calculator flags this; the fix is a more powerful motor or a lighter rocket. A healthy safety margin (thrust several times the weight) also keeps the rocket stable off the launch rod.
Where do I find thrust and burn time?
Motor manufacturers publish them. The letter code (A, B, C…) gives the total impulse and the number after it is the average thrust in newtons; the burn time comes from the motor’s thrust curve on its data sheet.
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This calculator is for educational purposes. Double-check important results before acting on them.