Activity

Well there’s no point in discussing this anymore with someone that is 100% convinced. But for the rest of you, remember those assumptions that are used to calculate the Center of Pressure? I looked them over last night and I…   Read more»

Take a look at Freeze Frame. It had a 10 pound camera 3 inches lateral distance off of the centerline and it was stable. Another example is cluster where one motor doesn’t light. The may have curved trajectories but if…   Read more»

I forgot to bring that up Ed. Depending on the motor and more likely with the thick webbed motors, you will have a higher % of Erosion. Every motor has some percentage of erosion, especially with the long small diameter…   Read more»

I wonder if anyone has messed around with a helical offset core? That would give it the same burn geometry (at least in 2D), but eliminate the weight imbalance. As long as it were a perfect helix, the core could…   Read more»

I realize there is a mixing chamber at the bottom of a motor that uses medusa nozzles, but if you line the slot up precisely with a nozzle port, it would seem to me that there would be a bit…   Read more»

Elvis Jr.,You highlight an excellent example of experience versus theory and pure science. This comes up many times in this hobby whether it’s fin design, EX motors, electronics or whatever.I’ve long heard JW swear off medusa nozzles. In fact he’s…   Read more»

Concerning the medusa nozzle…Relaying statements from my dad, the medusa nozzle should not effect the flow of gases going through the nozzle. I have yet to find out any specific reason why the medusa nozzle is used, being more efficient…   Read more»

Another cause of cartwheels that almost nobody ever talks about is a lateral mass offset. If the center of mass is a little bit away from the centerline, then the motor thrust will make your rocket pitch over. I had…   Read more»

I wonder if anyone has messed around with a helical offset core? That would give it the same burn geometry (at least in 2D), but eliminate the weight imbalance. As long as it were a perfect helix, the core could…   Read more»

Not to beat a dead horse here, but I think the c-slot combined with a medusa nozzle is a recipe for an erratic flight with a minimum diameter rocket. I can’t tell you how many rockets I’ve seen this on…   Read more»

Yes, you can really notice the mass offset in the Ellis G37 motors when you roll them, since they have an offset core.

Excellent point Adrian. One thought that brings to mind is it seems to me that C-slot and moonburner motors more often than not “wobble” or “wiggle” or whatever in flight, particularly in minimum diameter rockets. Sometimes even when the rocket…   Read more»

Thanks Doug. The rocket im building now it stable in Rocksim. Yet my SSSS was stable in rocksim but never flew straight, everytime it did cartwheeles. Im assuming its because i just didnt have enough stability from such small fins….   Read more»

Don’t forget two things – the principle of moment arm in leverage and laminar flow. Air flow close to the airframe is different than airflow further out. Also, stubby fins in terms of semi-span relative to airframe diameter have less…   Read more»

Most, but not all, of the equations are based on a physics MIT graduate student named James Barowman who, with others, came up with them in the late 1960’s. Estes an Centuri produced booklets with graphs for you to design…   Read more»

Thanks Doug. The rocket im building now it stable in Rocksim. Yet my SSSS was stable in rocksim but never flew straight, everytime it did cartwheeles. Im assuming its because i just didnt have enough stability from such small fins….   Read more»

I’ve never seen the formula, but if the fin semi-span is less than the airframe diameter, the root has to go up a LOT to counterbalance reductions in semi-span. It is NOT linear.W

Check out these sites:http://my.execpc.com/~culp/rockets/Barrowman.htmlhttp://www.webcalc.net/calc/0225.phphttp://www.apogeerockets.com/Education/rocket_stability.aspDoug

So what about a small semi-span with a long root edge? do they counter balance?

It’s not just fin area that matters, but semi-span. With a greater semi-span, you can do with a lot less fin area at the tradeoff of greater drag due to increased frontal area.Warren