How to figure base drag

[David Schwantz]

Is there a formula for base drag on a rocket? Building a fin less rocket, and need to determine if I will have enough drag.

 

[dhbarr]

If you're using OpenRocket or RocSim you can simulate it.

https://www.apogeerockets.com/education/downloads/Newsletter154.pdf

https://www.apogeerockets.com/education/downloads/Newsletter158.pdf

 

[David Schwantz]

I have rock sim, OR will not work on my laptop. Adding a cone to the design as rock sim states will not work in this case.

 

[dhbarr]

Is there a specific reason a 3:1 massless basecone won't work for your situation ?

 

[David Schwantz]

Part of the rocket is of a open frame work design. since the base will be several inches below the airframe and tail cone I do not think that will simulate the effect.

 

[neil_w]

(a) it sounds like you may have a rocket that won’t reliably simulate

(b) why won’t OR work? Have you tried the packaged installer?

 

[David Schwantz]

OR will not work with vista.
The base of the rocket is below the end of the tail cone. all the base drag is located there. It is a very short, stubby rocket, 2 to 1 ratio. Should be stable, but am wanting to know if more drag will be needed.

 

[neil_w]

Just to confirm, you tried the packaged installer?

Can you post a side view of the rocket?

 

[David Schwantz]

I did try the installer, even another suggestion from some one else. Don't remember what, but did not work.
Can post pic of rocket when it is assembled again. Should have all the covering on it this weekend and can reinstall the frame.

 

[David Schwantz]

Well here she is. RTF. 54mmt, will fly with a Quark for deployment. 16'DIA.
The access hatch on the side where you would normally crawl in a grain bin from is actually a chute cannon. Want her to come down horizontal so as not to destroy the frame work.

 

[dhbarr]

Are there any clear panels there?

 

[David Schwantz]

If you are referring to panels for fins, nope.

 

[dhbarr]

If you are referring to panels for fins, nope.

That's going to do backflips, pretty sure. That lovely conical aft transition almost entirely else eliminates the base drag when the motor is making thrust.

 

[rharshberger]

That's going to do backflips, pretty sure. That lovely conical aft transition almost entirely else eliminates the base drag when the motor is making thrust.

My mindsim concurs, you basically have less base drag due to the tailcone effect and the open frame structure will not create any appreciable drag.

 

[dhbarr]

EDIT: duplicate post

 

[David Schwantz]

Well there is only one way to find out for sure Although Rocksim gives me a 1.6 margin and average altitude of 350'.

 

[neil_w]

I loaded it into OR and it had some unrecognized tube fins (?) and a ring tail. So without the framing at the bottom CP was off the charts to the left. Of course I could be wrong (have been plenty of times in my life), but if I were RSO I would not let it fly unless given some really compelling evidence. As mentioned by others above, with that tail cone you're not going to get much if any benefit from base drag. So to me it looks like a basic finless rocket that will skywrite.

Clearly, though, something in the Rocksim model is giving it some stability. Unfortunately without the program I can't see which component(s) might be helping get you positive stability margin. Could you show a side view or something? Which components are entered as tube fins? I don't see anything there that looks like a tube fin.

 

[David Schwantz]

I used tube fins for the frame work and just made them solid. The ring tail was made as 1/2" height, 2" wide, again solid, even though it is an "L" shape. According to my sim the "ring tail" was far enough below the cone to give it the stability. Tried the rocksim file I posted, won't open. Will try posting it again later. it is now 4am and I just put todays brisket on the barbie

 

[David Schwantz]

Ok, she flew on the 12th of July. K1100, bout 400' or so. perfect chute out of the cannon, but bounced on landing and need to straighten some of the metal work.

 

[Mugs914]

That thing is flippin' awesome!!!!! Well done!

 

[David Schwantz]

thanks much, Mugs. Dave.

 

[PatD]

Ok, she flew on the 12th of July. K1100, bout 400' or so. perfect chute out of the cannon, but bounced on landing and need to straighten some of the metal work.

Need to leave the dents in it, make it look like most of the grain bins out here in Montana.

 

[David Schwantz]

Thanks Pat, might just do that. But I will straighten the frame on the bottom. Not bad, one screw popped out and a couple cross ties got bent a little. She WILL fly again

 

[PatD]

Really nice. Got any pics on how you put it together? Inquiring minds want to know. If you had a thread on it, I missed it.

 

[David Schwantz]

Maybe a couple.
Hatch
Frame work legs
chute cannon
NC
Side view
NC inside showing carbon fiber lay up
NC outside after aluminum covering
Lower transition during cure
A really big CR with lightning holes

 

[PatD]

Is that an aluminum tape finish? The cone pics look like it might be. Excellent finish. Did you run the aluminum angle all the way up?
Edit: OddRocs fascinate me.

 

[David Schwantz]

It is aluminum tape. Good eye. Thanks. I would have to go and look at the brand again, but this is the thinnest I have found. It is .0015 thick. I have used the 3M 425 stuff lots. But it is .00425 thick and much heavier. But that way it will stretch more before tearing and you can burnish it down into concave surfaces better. I also use .010 thick stuff to make panels and hatches out of. The legs run up through the top bulkhead. About a couple inches down from the NC.

 

[Walter Longburn]

16'DIA

Wow, that's pretty big.

 

[Greg Furtman]

That may be the strangest rocket I've ever seen. And it actually flies! Dave, did you have any kind of a recovery system other than gravity?

 

[David Schwantz]

70" Top Flite chute out of a cannon on the side. I did it like that so the chute was at the balance point and it would come down horizontal so as not to mangle the legs and frame work.