[plugger]

> we found the maximum altitude estimates were about 15% too high.

this often happens when the wrong rocket finish is selected. Everyone chooses a polished finish when in actuality they've just sprayed the thing with paint.

Skin drag is real.

[sandworm101]

They also seem to be treating rockets as rigid bodies. Flex of parts is real at these accelerations but the calculations get into supercomputer territory very quickly. All the math is questionable if, at any given moment, your rocket isnt actually straight.

Look into the physics of archery, which has similar accellerations albeit of a narrower tube. The choice between a heavier/stronger arrow v a lighter/flexible shaft is the entire game.

[hermitcrab]

If your rocket is flexing significantly, then it probably won't be a rocket for very long!

[sandworm101]

Shuttle was held on the launch pad after ignition for a couple seconds because of flex. The stack initially flexed out of alignment when the main engines were lit, the stack being asymetrical. Only once the nose drifted back to vertical would the srbs ignite and the stack begin to move.

[hermitcrab]

OpenRocket is intended for amateur rocketry, not NASA space shuttles.

Amateur rocketry people try very hard to make their rockets symmetrical.

Hobby rockets have to be stiff enough not to banana. I don't think it is figured into model rocketry beyond that.

[plugger]

Not really applicable here imho. Most high power high performance rockets aren't cardboard. The majority are either fibreglass or carbon fibre.

Flex doesn't really become a concern with carbon fibre until you approach Mach 3. I say this because I've built two rockets of an identical design philosophy; one for 54mm motors and one for 75mm motors. The 54mm motor rocket did M2.64 and I could fly it again today if I wanted. The 75mm rekitted itself at M3. It failed where I'd not reinforced the hand rolled airframe that I was flying. 4 wraps of 2x2 200gsm twill CF. Same number I used on the 54mm fine.

For both of these rockets the vast majority of the airframe is directly over a aluminum motor casing that is a rigid body. These things are grossly over-engineered when compared to the pressures/forces they see.

[hermitcrab]

Possibly. I don't remember the details. Also, it isn't a full aerodynamic simulation. Plus the hobby motors are not manufactured to NASA standards. So you can't expect it to be too accurate.