Fins and Stability graph feedback

[Peter Bech Svalgaard]

Hey
We are a group of students from the Danish Technical University. We are participating in SAC 2019 in the 30K hybrid/bi-liquid category. I attached our rocket and the stabilty graph from OR.
I've read you should aim to leave the launch rail with a stability margin of 1.5 to 2. And remain under 6 for the duration of the flight. This configuration should satisfy that.
I am not worried about the structural integrity of the fins. We have done the math and implemented an appropriate safety factor. We are using 3 mm aluminium. Plan is to round the edges of the fins to reduce drag.
What is your thoughts on this geometry? Is it acceptable that the stability margin peaks at 5.75 or is it too high?
I really dont want to screw this up.

Kind Regards Peter Bech Svalgaard

 

[Nytrunner]

Higher margin means greater susceptibility to crosswind turning it away from vertical. What horizontal motion are you getting when you simulate launch with higher windspeeds? (15-20mph or 24-30 km/hr)

Also, why does your simulation show a large rectangular profile in front of the nosecone?

 

[G_T]

It is likely that fin geometry is not nearly as stable as you think, and will be subject to coning. The area of the fins right next to a body tube are in a turbulent region and as such, are not nearly as effective as the portions which extend out into cleaner air. With the radical taper, you are losing a lot of effective area. I do not think OR takes that into account.

It does look like you have a body tube length issue in your rocket model. I'd fix that, and re-run.

Rounded ends is not a minimum or even necessarily a low drag optimization. When subsonic, a leading edge closer to elliptical would be a substantial improvement. If supersonic, then a shallow taper is better with a sharp leading edge. Then, for the trailing edge, believe it or not, just being squared off is better than rounded. A shallow sharp taper is much better. But your altitude difference for changing trailing edge shape is on the order of a thousand feet. Leading edge has greater effect if you are stuck in the transonic range or are going supersonic.

I'd suggest moving over to RasAero for your sims.

Gerald

 

[blackjack2564]

Those fins look very small, usually leading to unstable flight.
What speed did you plan your design to attain?

over 150 pounds and 7.1 in diameter [for local guys not fussing with metric] my common sense tells me fins are just too small for stable flight.

 

[Bill Hanson]

Another thing to consider is your velocity off the rail. If that’s too slow, then you can have problems with winds — and having been to the last 2 SACs, you can count on winds. There is specific guidance in the SAC rules on velocity off the rail.
Also, the elevation at Spaceport America is 1401 m / 4595 ft. That will make a difference.

Bill