As a rule of thumb, fat (larger diameter rockets) stay low. Estes crayon, Freefall, Big Daddy, Der Red Max are examples. If you launch one of those on their recommended smaller motors, they won't go too high. On the flip side, a small Estes gnome will go pretty high on a small A motor.
You can poke around the Estes website and look at the recommended motors and altitude ranges for different rockets. For instance, with a Der Red Max Estes website says
Recommended Engines : B6-2, (first launch), B4-2, B4-4, B6-4, C6-5 Projected Max Altitude: 600 ft. (183 m)
You can use thrustcurve.org to get a table of altitudes for different rockets and motors from the MOTOR GUIDE section.
Again, using a Der Red Max, for example:
Body diameter: 1.64 inches
Dry Weight: 2.4 oz (68g) This is the weight without the motor.
MMT diameter: 18mm (this is the diameter of the motors it can take)
MMT length: This doesn't really matter. Just put in 6"
Complexity: simple
Finish: average
Moto Mfr: Estes Industries
Motor type: All
Certified by: All
You get this table:
.............Motor ................Weight .......Launch .......Velocity ......Accel .........Altitude .......Time ....Delay
| Motor | Weight | Launch | Velocity | Accel | Altitude | Time | Delay |
---|
| Estes B6 | 2.9oz | 49ft/s | 137ft/s | 14.0G | 256ft | 4.0s | 3s |
W | Estes C5 | 3.3oz | 56ft/s | 211ft/s | 23.2G | 584ft | 5.8s | 4s |
W | Estes C6 | 3.2oz | 49ft/s | 219ft/s | 13.9G | 597ft | 6.0s | 4s |
If you really wanted to be safe, you could get a B6-4. at 256 feet max altitude, that will hardly drift. On a windless day, you could do the C6-5 and send it up to 600 feet.
Your grandson can change the finish selection and see how putting a nice smooth paint job on affects the performance.
A Big Daddy is 3" around and takes the big 24mm C and D motors. Lots of smoke and noise with not too much altitude. Those are fun for small fields.