M982 Excalibur Artillery Shell (BT-80 Version)

[BigMacDaddy]

Mark's @mccordmw thread made me aware of the M982 Excalibur Artillery Shell https://www.rocketryforum.com/threads/scratch-build-excalibur-artillery-round.170369/ and inspired me to do a scratch-build myself with a BT-80 tube. I commented a couple of times there but did not want to threadjack so starting my own thread (also mine is low-powered, his will use some huge motor that is out of my league / price range).

I decided to do the 1b version since I like the look of the fins / tail.

I wanted to keep weight down so I could launch on 24mm D or E engines and that means trying to keep nose weight down. So I decided to try a design that recessed the motor and will try to create some GDS to help with stability. Also made the front canard fins pivot and made the rear fins and fin can larger to make things more stable (that was the biggest compromise). I posted a picture of the final rocket as-built in a group shot yesterday but it is included below again along w/ some 3D renderings.


Openrocket seems to indicate this model will be stable with a D12-5 engine with 1oz of weight in nose. This does not take into account GDS and does not take into account that the canards can move (i.e., counts them as static fins).


If this model flies stable I will try a version with rear fins closer to scale. Below is a mock-up of the current model vs. the closer-to-scale model (on right). Looks like I also need to extend the fake canard fin storage slots in the nose cone.



With the smaller fins I need about 4oz in the nose to make it similarly stable.

 

[mccordmw]

Is your schematic showing a bit of a tapered tail? Won't that cut base drag? Careful of the stability hit there when shrinking the fins.
I don't know how much GDS will contribute to stability relative to the fins.

 

[BigMacDaddy]

Is your schematic showing a bit of a tapered tail? Won't that cut base drag? Careful of the stability hit there when shrinking the fins.
I don't know how much GDS will contribute to stability relative to the fins.

Yes, A little bit of a tapered tail. The 1b version seemed to have a little bit of a tapered tail from the pictures I could see but it was hard to tell how much (so many fins makes it difficult to measure).

Yeah, I am a bit worried if I drop the size of the fins but will try version with larger fins first. I wanted to launch today but glue had not dried on motor block that I forgot to glue in when I made the model.

 

[BABAR]

don't forget the launch lug!

Looks cool!

 

[jqavins]

Would you kindly show and explain some detail on how you made the spring-out fins work?

Is your schematic showing a bit of a tapered tail? Won't that cut base drag? Careful of the stability hit there when shrinking the fins.

A boat tail will reduce the stability, but it has nothing to do with base drag. It has to do with the aerodynamic lift as described in Barrowman (and implemented in RS and OR). Those don't include base drag in stability calculations, and do show the effect of boat tails.

Boat tails also reduce drag, enough to have a substantial effect on performance, but not enough to have a significant effect on stability in most cases. Really, base drag isn't important to stability except in really short, fat rockets. IMO, it's discussed on the forum for more than it's worth lately.

 

[BigMacDaddy]

Would you kindly show and explain some detail on how you made the spring-out fins work?

Sorry should have been more clear -- I just made the canards so that they can pivot. I used a similar approach to what I did on the Fritz-X. Couple of small 3D printed pieces that fit on either side of a fin tab that sticks through a 6mm hole. I assemble and wick a bit of thin CA glue into the outside edge. This time I had an issue with it running into the hole and had to sit rotating things for 30 minutes so that glue would not set up in the parts I wanted to move.


I considered trying to make the rear fins spring loaded and fold in but realized that it was going to take a fair amount of effort, the fins would likely be in the way of the engine blast, and I was unlikely to try a tube launch anytime soon.

 

[jqavins]

Ah. They looked like the spring out, and I want to take on some tube launches. I guess I'll have to work it out. (Probably something with a large enough body tube diameter to keep the mechanism clear of the exhaust, and operated by orthodontic rubber bands.)

 

[bjphoenix]

Here's a similar thing that I spotted at an airshow 10 years ago. It might be a little harder to model.

 

[Raco123]

Very Nice

 

[teepot]

Here's a similar thing that I spotted at an airshow 10 years ago. It might be a little harder to model.View attachment 502117

Here is my version of a RPG rocket. I made this very soon after I started scratch building. I have a 4' PVC tube I was going to use but the rear ended up to wide. So when it flies the fins are already down. My next attempt at a tube launch was a 60mm mortar round. The fins were so small and the nose weight to little. It spun as soon as it left the tube. I have added more nose weight and added a ring fin. It will be tube launched. Just haven't gotten around to it yet. I think the artillery round with the drop down fins would be the easier of the two to make. I have seen the pop out ones. It involved springs to open the fins after it left the tube.

 

[BigMacDaddy]

I finally had a chance to launch this model (and a number of others) and it was nice and stable on a D12-5.



In my next iteration I would probably either make the front fins static (I do not think they rotated at all -- design did not work as well as my Fritz-X design) or maybe try to actually make the fins fold into nose cone (and might as well make the ones in rear fold as well).