Open Rocket... Do I trust these numbers?

I've got a design that calls for the smallest fins (for the size the rocket) I've ever put on a rocket... I've sim'd it with E and D engines, and my stability is between 1.59 and 2.38 Cal.



I'm including the OR file below. I'm looking for opinions, would this be safe, or am I just asking for trouble? Please keep in mind, this is intended to be the upper stage of a 2 stage rocket, and not really used alone.

Thanks!
Jim

View attachment Estes Stiletto BT-60 Upscale (2 stage).ork

Anyone have experience with this kind of small finned large(ish) rocket?

Try running it in OR with 20 MPH winds.

Certainly rockets with small fins can be unstable regardless of what CG/CP margin they have. See https://www.apogeerockets.com/Wind_Caused_Instability

I'll do that when I get home tonight. Thanks for the suggestion.

[Edit] it was still showing stable [/EDIT]

this one worries me. you've got most of your fin area inside turbulent airflow(last thing I recall reading suggested that 0.5 body diameters was a min for rectangular fins)...the numbers may be right, for a very low angle of attack. if something were to hang up during staging to knock it off course...it could get ugly. I think I would go for an increased span and maybe shorten the root chord a bit.
Rex

The CP is farther forward than usual, as should be expected with such small fins. The only reason it's stable is the CG is quite far forward as well. OR/Barrowman doesn't include the body tube in stability, mostly correct but as the CP's right in the middle of the tube there's no gotcha there. So should be OK except will need to check actual CG.

Might be worth checking to see if anything is overridden in any of the part setups. This looks a bit odd.

I have built and flown a rocket such as this.
The smaller the fins, the high the take off speed has to be.
Mine went unstable right off of the launch rod.

JD

I have found OR for the most part to be conservative with respect to stability on certain designs. I simmed a Proton-M and a Soyuz and OR said to add a considerable amount of nose weight to get just on the right side of positive stability. Yet both rockets flew remarkably straight on their initial flights even with significant (+10 knot) winds. The Proton in fact flew really nicely even on an E9-4.

On the other hand, the N-1 wasn't even close to stable even though OR said it was, so it may have trouble with 'coned' designs that have no fins.

FC

One Idea I've come up with is the possibility of making the fins a smidge larger by using clear plastic (think thick vacuform container plastic) over fins shown in the sim.

OR

Using the same kind of plastic sandwiched between two pieces of 1/16" Balsa. Use holes in the plastic to allow glue from one side to bond it to the other. Painting that would be a PITA though.

On my last rocket the fins didn't stick out any further but were longer along the length of the rocket for looks, a shape less likely to stall and more effective in a stall. I did note some weathercocking in one flight and some anti-weathercocking in another with less stability margin, 1.9 vs. 1.5 cal, but in both cases it recovered rapidly.

K'Tesh said:

I've got a design that calls for the smallest fins (for the size the rocket) I've ever put on a rocket... I've sim'd it with E and D engines, and my stability is between 1.59 and 2.38 Cal.



I'm including the OR file below. I'm looking for opinions, would this be safe, or am I just asking for trouble? Please keep in mind, this is intended to be the upper stage of a 2 stage rocket, and not really used alone.

Thanks!
Jim

View attachment 162671

Click to expand...

The joker isn't so much the CP as the CG. The CP doesn't vary unless you change the physical shape of the vehicle. The CG changes every time you prep, and even during launch. CG computation also relies on data concerning materials, which may not be entirely accurate. It's a good idea to mark the computed CP on the rocket, check the CG before launch, and *then* compute your stability margin.

Larry Curcio said:

The joker isn't so much the CP as the CG. The CP doesn't vary unless you change the physical shape of the vehicle. The CG changes every time you prep, and even during launch. CG computation also relies on data concerning materials, which may not be entirely accurate. It's a good idea to mark the computed CP on the rocket, check the CG before launch, and *then* compute your stability margin.

Click to expand...

In my sims it showed the CG didn't move very far back with the larger motor. I'm also thinking of making the the rocket zipperless (perhaps not reflected in the .ork posted). With that, the laundry won't have a chance to move back, and CG won't change much at all from one launch to another. The separation point will be just behind the forward-most "fins".

I can only see the CG moving forward in my mental images of a launch as the fuel burns, so stability should increase if I'm not mistaken. Also, since this is a sustainer, it will already be moving at a pretty good clip, and not a from a dead stop.