Chris Collins

Tools · physics

Model rocketry

Simulate a model rocket's flight from real motor thrust curves — altitude, speed and acceleration, apogee, the right ejection delay, launch-rod speed and descent under the parachute — check its stability with the Barrowman centre of pressure and a side-view drawing, size the parachute, and look up motor classes and the NAR/Tripoli safety codes.

Added

Works with

How to use it

Pick a motor, describe your rocket, and see how high it goes, whether the ejection charge fires at the right moment, whether it is stable, and how fast it comes down. Everything is in metric or imperial (the switch at the top; remembered).

How to use it

Start from a preset or type in your own rocket. The Rocket, motor and recovery panel is shared by the tabs; every number updates as you type.

  • Flight simulates the flight straight up and shows the apogee, top speed (and Mach number), peak acceleration in g, the speed leaving the launch rod, the thrust-to-weight ratio, when the ejection charge fires relative to apogee, the ideal delay (apogee time minus burnout time) and the listed delay nearest to it, the descent rate, the descent time and how far the wind drifts it. Below are altitude, velocity and acceleration plots with burnout, apogee and ejection marked, and a CSV of the flight.

  • Stability takes the nose cone shape and length, the fins (number, root and tip chords, semi-span, sweep and position) and the balance point of the rocket without its motor. It finds the centre of pressure (CP) with the Barrowman equations, adds the loaded motor to get the centre of gravity (CG), and gives the stability margin in calibres (body diameters) at lift-off and after burnout, with a side view showing both. Measure the CG by balancing the rocket on a finger with the parachute packed; the motor is placed flush with the aft end.

  • Parachute finds the canopy diameter for a target descent rate and tabulates common sizes.

  • Motors shows the thrust curve, total impulse and class, average and peak thrust, burn time, propellant mass and specific impulse, and lets you paste or open your own RASP .eng files.

  • Safety explains motor classes and summarises the NAR Model Rocket Safety Code, with its stand-back distances and launch-site sizes, and links the official NAR, Tripoli, NFPA and FAA texts.

⛶ Full screen (or F) gives the open tab’s figure the whole screen: on Flight the altitude, velocity and acceleration plots stack one above the other at full width, on Stability the side view of the rocket, on Motors the thrust curve. It uses the browser’s full screen where it is allowed, otherwise a full-window layer (iPhone), and is redrawn as the window changes; the cards and inputs follow below. Escape leaves.

How it works

Motors. The bundled curves are Estes A8, B6, C6, D12 and E12 (digitised from the certification curves published on ThrustCurve.org; their integrated impulses match the certified totals — 8.8 N·s for the C6, 16.8 N·s for the D12), plus approximate curves for an AeroTech G80T and a Cesaroni F36 Blue Streak. For anything serious, download the certified .eng file from ThrustCurve.org and paste it in. Class letters follow NFPA 1122: A is 1.26–2.50 N·s and each letter doubles. Burn time is measured from 5% of peak thrust to 5% of peak thrust, as in NAR certification, and the average thrust is total impulse ÷ that burn time.

Flight. Newton’s second law, integrated with fourth-order Runge–Kutta: m(t)·a = T(t) − ½ρv²·Cd·A − m(t)·g. Thrust is interpolated from the curve; the motor loses propellant in proportion to the impulse delivered so far; A is the body’s cross-section; air density ρ comes from the International Standard Atmosphere for your site’s elevation and temperature. The rocket stays on the pad until thrust exceeds its weight. At burnout + delay the recovery device opens and its drag ½ρv²·Cd·A is added (parachute Cd as entered; streamer Cd ≈ 0.6 on its area, a rough figure). The flight is one-dimensional (straight up and down).

Stability. Barrowman’s method (subsonic, small angles of attack) gives each part a normal-force coefficient CNα acting at a point: the nose cone 2, at 0.666 of its length for a cone, 0.466 for an ogive, 0.5 for a parabola; the body tube nothing; the fins CNα = [1 + R/(S+R)] · 4N(S/d)² / (1 + √(1 + (2LF/(CR+CT))²)), where LF is the length of the fin’s mid-chord line, acting at XF + XR(CR+2CT)/(3(CR+CT)) + (1/6)[(CR+CT) − CRCT/(CR+CT)]. The CP is the CNα-weighted average. A margin of 1–2 calibres is the usual target; less than 1 is marginal; a negative margin (CP ahead of CG) means the rocket is unstable.

Parachute. At the steady descent rate the canopy’s drag balances the weight: D = √(8mg / (πρCdv²)). 3.5–4.5 m/s (11–15 ft/s) is the usual target.

The tests check the simulation against hand calculations — burnout and coast with constant thrust and no drag (vb = at, h = v²/2g), and a coast with quadratic drag against h = (m/2k)·ln(1 + kv0²/mg) — the Barrowman terms against a fully worked example, the parachute formula, the ISA table, and the motors' impulses against their certified totals.

Limits

  • Straight-up flight only: no launch angle, no weathercocking into the wind, no multi-stage or clustered motors. Wind is used only for the drift during descent (descent time × wind speed).

  • One constant drag coefficient. Real Cd varies with speed and is much higher near Mach 1; the tool warns above Mach 0.8. Launch lugs, rail buttons and paint finish all change Cd — tools such as OpenRocket and RockSim estimate it part by part.

  • Barrowman’s equations assume small angles of attack and subsonic flight, and ignore the body tube’s lift; real rockets at larger angles are a little more stable than they predict.

  • The AeroTech and Cesaroni curves are approximate; paste the certified files for real flights.

  • Not a substitute for the safety codes, a range safety officer, or your club’s rules.

Related: physics calculators (projectiles with drag, kinematics), orbits and Hohmann transfers, fluids (Reynolds number, drag), unit conversion, log an altimeter or accelerometer.

Linking to an example

Share link copies a link with your rocket and units (and any motors you pasted). Links can be written by hand; lengths are in millimetres and masses in grams, whatever the display units. Add the tab after #: flight, stability, chute, motors or safety.

Parameter Meaning Example

motor

Motor, as listed (Estes A8, Estes B6, Estes C6, Estes D12, Estes E12, AeroTech G80T, Cesaroni F36BS)

motor=Estes%20D12

delay

Ejection delay, s (999 = none)

delay=5

mass, cg

Mass (g) and balance point (mm from the tip) without the motor

mass=30&cg=185

d, L

Body diameter and length, mm

d=24.8&L=330

cd

Drag coefficient

cd=0.6

nose, Ln

Nose shape (ogive, cone, parabolic, elliptical, haack) and length, mm

nose=cone&Ln=80

n, cr, ct, s, sw, xf

Fins: number, root chord, tip chord, semi-span, sweep, root leading edge from the tip (0 = at the aft end), mm

n=4&cr=60&ct=30&s=45

rec, chD, chCd

Recovery (chute, streamer, none), parachute diameter (mm) and Cd

rec=chute&chD=450

stL, stW

Streamer length and width, mm

stL=600&stW=50

rod, elev, temp, wind

Rod length (mm), site elevation (m), temperature (°C), wind (m/s)

rod=1000&wind=5

units

metric or imperial

units=imperial

example

A preset below; parameters given with it override it

example=e12
example= Opens

sport-c6

A small 24.8 mm sport rocket on a C6-5 with a 30 cm parachute (open)

mini-b6

An 18 mm minimum-diameter rocket on a B6-4 with a streamer (open)

big-c6

A fat 41.6 mm four-finner on a C6-3 (open)

e12

A 41.6 mm rocket on an Estes E12-4 (open)

g80

A 54 mm mid-power rocket on an AeroTech G80T (open)

unstable

The sport rocket with fins that are too small (open)

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