Stability of The Pumpkin King

When I built Rocketry Warehouse's Flying Pumpkin Kit a year ago I hadn't gotten the memo about the "Flat Plate Base Vortex Effect" on short fat rockets and ended up with a nose weighing 4 lbs. With the added weight it was plenty stable and flew well, drag separation notwithstanding, but it certainly wasn't a cheap date and required J motors.

Jump ahead to Oktoberfest at Jean Dry Lake this year when the delay went long and shredded a 52" skyangle chute.

I decided to make the beast dual deploy with a new nose cone and a section of fiberglassed cardboard tube. Since it is not really a short, fat rocket anymore with a length to width ratio of 8:1, should the FPBVE play any part towards calculating the CP?

Below are pictures of the finished rocket and the Open Rocket screen captures of it with no motors, with a CTI K600 and with and without the FPBVE.

Thank you for your time,

Miles

Why would the FPBVE go away when the rocket gets longer? It will have less effect on CP in a longer rocket, but the effect would still be present until you do something to change it, like adding a boattail.

Handeman said:

Why would the FPBVE go away when the rocket gets longer? It will have less effect on CP in a longer rocket, but the effect would still be present until you do something to change it, like adding a boattail.

Click to expand...

Good point, but I won't be able to count on the FPBVE until the rocket is going fast enough. I was planning on launching it on a 12' rail and/or using a high thrust motor.

As much as I didn't want to add more weight, there is a 1 pound removable weight curing even now. That would bring it to 1 cal. of stability with any of the simulations I have done and allow me to use a lower thrust motor. The the pad weight with a CTI K360 is 15.9 lbs. and only sims to 2729'. If it survives that, I have some 5 grain 54mm motors on order from Wildman's Black Saturday sale that should take it to about 4K.

Miles

I asked this exact question a while ago but never got the response I was looking for. I was designing a modified Madcow Squat and I wanted to stretch it a bit and wondered how far I could go and still retain the base drag of a squatty design. I ended up adding 7" to it and it looks like it's not going to matter any more because when the chute is in the nose cone and the smallest motor I have is in the tail, I still get a GC ahead of the CP although not a full caliber. I think the simple answer is that nobody really knows. I still don't because the rocket isn't finished and flying yet but I have a good feeling about it, the thing just looks right. I simply wanted to avoid adding lead to the nose. I have seen and read of a few Squats meeting their end due to drag separation and want to miss out on that experience.

grouch said:

I asked this exact question a while ago but never got the response I was looking for. I was designing a modified Madcow Squat and I wanted to stretch it a bit and wondered how far I could go and still retain the base drag of a squatty design. I ended up adding 7" to it and it looks like it's not going to matter any more because when the chute is in the nose cone and the smallest motor I have is in the tail, I still get a GC ahead of the CP although not a full caliber. I think the simple answer is that nobody really knows. I still don't because the rocket isn't finished and flying yet but I have a good feeling about it, the thing just looks right. I simply wanted to avoid adding lead to the nose. I have seen and read of a few Squats meeting their end due to drag separation and want to miss out on that experience.

Click to expand...

On its second or third flight, this rocket as a squat Flying Pumpkin, drag separated and came down in 4 pieces. The 52" skyangle chute and snapped kevlar harness came down a 1/4 mile away, the body tumbled and knocked a fin loose and the 4 lb. nose cone came in ballistic and left a sizable dent in a hard packed dirt road. Some sanding and epoxy and it was ready to fly again (I love fiberglass rockets), but this time with three 2-56 shear pins. All of my dual deploy rockets and a couple motor deploy have shear pins. The temperature fluctuations here in the desert make it difficult, for me in any case, to get the fit just right with tape. The only downside is ground testing the BP charges. I know a lot of people don't have that option. I didn't figure I had a choice with the Flying Pumpkin, since it weighed more than 10 lbs w/o a motor.

It doesn't sound like you modified Madcow Squat will be unstable as long as it gets up to speed before leaving the rod/rail.

Miles

I still don't know how to really understand the base drag either. I'm debating on whether to add a bit of nose weight to my Minie Magg which is almost as short as the pumpkin. It was stable but veered with the wind, I may try another flight in calm weather to see before adding weight.

The Pumpking King flew on a CTI K360 to 2923' on 12/28/14.
The Open Rocket simulation estimated the apogee at 11 seconds but looks like the drogue charge and the motor charge (13 second delay) went off at about the same time and sheared the pins holding the nose cone, so the main came out at apogee.

Because this really wasn't a short stubby rocket anymore, I didn't trust the "Flat Plate Base Vortex Effect" and added about a pound to the nose cone. All of the motors I intend to fly now have a stability of at least one caliber.



Here are some pics and videos from the flight:







[video=youtube;ea6ksSx_LSY]https://www.youtube.com/watch?v=ea6ksSx_LSY[/video]

[video=youtube;K7_Zl8dykz4]https://www.youtube.com/watch?v=K7_Zl8dykz4[/video]