Pre Flight Data Not Good

[Jonsayler]

I was planning having a pre flight test with my level 3 rocket on a DMS L875 DM. However, open rocket shows a thrust to weight ratio of 4.12:1 and 26ft per second off the pad. Open rocket also gives me a green light through the simulation using this motor but how? Aren't I low on both these data points?

 

[Banzai88]

Yes, you are. Your mistake is in thinking that OR cares about 5:1 T:W or minimum speed off the rod......clearly it doesn't.

 

[jderimig]

Put a 15mph cross wind and then simulate. If you rocket has high polar moment of inertia that twr and speed may be just fine. 5:1 and 40ft/per second are just guidelines and doesnt guarantee success if met and failure if not. Alot more goes into that.

 

[waltr]

Yes, you are. Your mistake is in thinking that OR cares about 5:1 T:W or minimum speed off the rod......clearly it doesn't.

Correct, OR will simply report speed off rail. It is up to you to check and decide if the flight is safe.

In OR you can plot Alt,verse Direction (East) to get an idea if flight will go straight up. Also add wind, as jderimig suggests to see how much the flight deviates.
Careful with the graph as the alt & ground distance is NOT the same scale. Hover mouse cursor over a point and write down Alt & distance. Then do the math to calc angle.

Also, check the Rail length used in OR. And, does your club have a longer rail that allows high speed off the rail.

Other option is select a higher thrust motor.

 

[Handeman]

Also, check the Rail length used in OR. And, does your club have a longer rail that allows high speed off the rail.

@waltr is absolutely correct. Check the rail length and verify what the club has where you intend to launch. Our club has 1515 pads with 10ft. rails. We also have a trailer launcher with uni-strut rail we can attach a 16 ft. 1515 rail to.

Whatever the club has available, do sims with those rail lengths. Usually the rail length is from the bottom of the rocket, so you should use that length minus the distance from the bottom of the rocket to the top rail button, for the rail length in the sim. That will give you a more accurate estimate of the speed when the top button is leaving the rail.