RocketSim CP question

[SPONGE]

When I enter my rocket profile into RocketSim it shows that, according to the two mathmatical models, my design is overstable. But the cardboard cutout model shows it to be unstable. Which of these models am I to belive? I really don't want to try to add weight to the nose to make the cutout model stable, as it would decrease the performance and put the total weight over a pound. Is it normal to see such a large variance between CP models?

 

[KermieD]

This is actually probably a question for rstaff3, but I'll take a shot since I'm here. My own personal experience is that the RockSim formula is the most accurate, followed by Barrowman, then the cutout. There are some instances in which this is not the case, but I'm not sure what they are. You might want to look at the Apogee website, as Tim has a ton of informational material on there for RockSim.

As to whether it's common or not to have widely differing CP's...well, it's not exactly common, but it's not that uncommon either. It seems like the more you deviate from the 3FNC "standard" looking rocket, the more of a difference you're going to find.

If you're really worried about it, do a swing test (unless your rocket is 30 lbs or so ).

 

[KermieD]

Originally posted by SPONGE
I really don't want to try to add weight to the nose to make the cutout model stable, as it would decrease the performance and put the total weight over a pound.

Don't forget to add the motor, too, both for CG purposes and for FAA notification purposes.

 

[rstaff3]

I'll comment on what KermieD said...

I personally have come to trust the Rsim numbers, which I believe to be better than the Barrowman, especially as you deviate from a standard 3/4FNC rocket. I never use the cardboard cutout, it is basically useless once you have the other two. One word of caution: I have had the Rsim numbers go nuts on weird designs, especially those modeled with lots of transitions. If this WAY off from the Barrowman you might question it. This often corrects itself if you edit a part (any part) and return to the sim.

Swing tests aren't perfect either, but if you have concerns you should perform one. Also like he said, there is tons of free info on Tim's site. Make sure you sign up for his newsletter!

 

[SPONGE]

I figured the mathmatical models would be more accurate that a cardboard cutout. I feel pretty good about it since I would rather be a little overstable. I think with the motors and recovery gear I will be pushing a pound. It'll be finished tomorrow night.

 

[Brian Barney]

Rocksim and barrowman calculations are very good when there is little or no wind, but if you have a high wind, you will need to use the cardboard cutout for the CP. We had alot of stable rockets go unstable at the last launch because there was a contstant 20mph wind with gusts up to 40mph. We had a wind guage and launched in the short lulls. Because the instability was becoming so common, we moved the estes rods out to the HPR pads 500' out

Rocksim is pretty good normally, but be careful with conduits and such, I had a scratchbuilt rocket go unstable at motor burnout because rocksim didn't understand the external conduits (although I simulated them by having thick fins the same shape and location as the conduits)

-Brian Barney

 

[rstaff3]

That is the difference between static and dynamic stability. I have had pretty good luck with the Rsim estimates, even on odd designs, up to 15mph or so. You can set the wind speed in the program but I usually do not.

In 40 mph gusts, I'm not sure the cardboard cutout is a guarantee either, and think the extra distance is a good idea. It's not uncommon for me to fly a lpr or mpr rocket off of a high power pad. I just lug my BSR-based pad next to whatever rack I want!

 

[KermieD]

I actually got one almost sorta/kinda right! BTW...what is a BSR-based pad?

 

[KermieD]

Originally posted by rstaff3
I'll comment on what KermieD said...

I have had the Rsim numbers go nuts on weird designs, especially those modeled with lots of transitions.

kinda like coffee-cup rockets?

 

[rstaff3]

Originally posted by KermieD
I actually got one almost sorta/kinda right! BTW...what is a BSR-based pad?

Sorry I was too cryptic....this is one of my pet-peeves in others, and here I go doing it

BSR=Black Sky Rail

 

[rstaff3]

Originally posted by KermieD


kinda like coffee-cup rockets?

As a matter of fact, that sim does it too