New Design - Yellow Jacket

[Feyd]

This is a four-engine cluster loosely based on a few other designs. Didn't come out exactly like I wanted it to in RockSim, but it's close enough for simulations.

The centering rings are supposed to be similar to a 4-engine cluster set sold by Semroc. The lower middle body tube (used as a spacer but really only necessary for the fins) is supposed to have a nosecone on it, but RockSim wouldn't let me put it there because of the way I built it. Same for the upper body tube spacer.

Also, the four body tubes are supposed to go into the upper body tube about 2 inches but I couldn't figure out how to do that.

Flies well to an altitude of about 1500' on four C7 engines.

[Solomoriah]

I have a three-engine rocket design I call Baffler. Getting the ends of the tubes inside the body in OpenRocket required breaking them up into short "inside tubes" and separate exposed tube sections; likely a similar method would work in Rocksim.

To be clear, Baffler has two sections of exposed tubing with a middle outer tube section. Doing it with a straight cluster is pretty easy, but going out and in and out again was challenging.

[CPMcGraw]

This is a four-engine cluster loosely based on a few other designs. Didn't come out exactly like I wanted it to in RockSim, but it's close enough for simulations.

The centering rings are supposed to be similar to a 4-engine cluster set sold by Semroc. The lower middle body tube (used as a spacer but really only necessary for the fins) is supposed to have a nosecone on it, but RockSim wouldn't let me put it there because of the way I built it. Same for the upper body tube spacer.

Also, the four body tubes are supposed to go into the upper body tube about 2 inches but I couldn't figure out how to do that.

Flies well to an altitude of about 1500' on four C7 engines.

Five BT-20's will fit inside a BT-70 by "fudging" the placement. You have to use "inside tubes" with the "cluster" button, and adjust the position of each tube out from where RS places them. Using the "pod" feature was a good try, and should have worked. I spotted where you missed unchecking the boxes for "make this pod ejectable","calculate the angle", and "calculate the distance from center".

I changed out your G10 glass fins for more-common 1/8" balsa, to reduce the mass behind the CG. I also reduced them a bit in overall size (scaled down) to get the deployment closer to apogee.

I got the nose cone onto the central fin-mount tube.

Study the attached revision and images. I think this is what you were looking for...

Length: 28.50"
Diameter: 2.247" (BT-70)
Fin Span: 8.37"
Weight: 3.75 oz


(4) B6-6........781-793'......Dv 9-20 FPS.......36" x 3/16" launch rod
(4) C6-7......1553-1589'......Dv 13-32 FPS......36" x 3/16" launch rod


Nice attention-getter!

[Doug Sams]

Five BT-20's will fit inside a BT-70 by "fudging" the placement. Why stop there? So will seven :D Doug .

http://www.doug79.com/stuff/BT70-w-7xBT20.gif

.

[John Brohm]

Why stop there? So will seven :D Doug .

http://www.doug79.com/stuff/BT70-w-7xBT20.gif

.

Well, only if one uses a little butter. Not to split hairs (Lord knows I can't afford to do so anymore!), but 3 x 0.736" = 2.208"; the ID for BT-70 is 2.175" which means that 7 engine cluster is interfering by 0.033", or 0.0165" on the radius. That sounds a little tight to me, and would likely mean a little distortion somewhere on the back end there.

Ouch.

[Doug Sams]

Well, only if one uses a little butter. Not to split hairs (Lord knows I can't afford to do so anymore!), but 3 x 0.736" = 2.208"; the ID for BT-70 is 2.175" which means that 7 engine cluster is interfering by 0.033", or 0.0165" on the radius. That sounds a little tight to me, and would likely mean a little distortion somewhere on the back end there.

Ouch.Agreed, John :D But between sanding the tubes and a little distortion, it'll fit :)

The thing to keep in mind is, even with only five BT-20's, there's still interference, as indicated by your 3x0.736 observation. So I figured, if he's gonna have some, he might as well go big :)

More seriously, this is an old combo that's apparently been done many times over the years. Normally, looking at all the interference in the drawing, I wouldn't recommend it, but having seen it reported way back in the rmr days, I concluded the interference was manageable. That said, of all the BT-70 setups I've played with, that one has escaped my builds. Go figure :confused:
http://www.doug79.com/BT70mms/6mmas-p.jpg

Diving into the engineering a bit more, there are a couple ways to reduce the interference. One idea is to remove a strip, about 0.3" wide, from each BT-20 where each contacts the BT-70. Gluing it in place, the user would need to take care to keep all the slotted tubes open fully, but it's do-able. I might stick spent motor cases wrapped in foil into the tubes while the glue sets.

Another idea would be to get a custom cluster ring made with the center hole cut for BT-20 and the 6 outboard holes slightly smaller, right at the size of the motors, and forgo the outboard tubes altogether. Bill at BMS has cut stuff like this for me a couple times :)

Doug

.

[Feyd]

Thanks, Craig, that was almost exactly what I was trying to do. Still have lots to learn about RS. I almost always forget to uncheck the "ejectable" button on pods.

I'll have to put this one in my queue. I've got lots of BT20 tubes to play with.

Maybe I'll put an engine in the central bottom tube too ;)
If I do happen to start building it, I'll post pictures.

Thanks again!

[CPMcGraw]

Thanks, Craig, that was almost exactly what I was trying to do. Still have lots to learn about RS. I almost always forget to uncheck the "ejectable" button on pods.

I'll have to put this one in my queue. I've got lots of BT20 tubes to play with.

Maybe I'll put an engine in the central bottom tube too ;)
If I do happen to start building it, I'll post pictures.

Thanks again!

For this one, I'd leave the central tube free (like it is). It has some interesting lines, and the performance numbers could be improved in other ways. Build it like it shows, and report back on how well it flies.