ZX-21...time to cluster...

[Stones]

Thought I'd try my hand at a cluster build. Rather than just throw together a 3FNC, I decided to "scale" (somewhat) an oddball design from an old Circus World Museum cannon/rocket. The ZX-21.
Build pics will follow.

 

[Elapid]

you won't like what you find...

i did a quickie sim and the CP is located about halfway up the body tube...

the fins look neat, but at a cost.

 

[Stones]

Originally posted by Elapid
you won't like what you find...

i did a quickie sim and the CP is located about halfway up the body tube...

the fins look neat, but at a cost.

Being as the pic IS from Rocksim and you can't see the internals to know where it's stabilizing, I find it odd that you feel it's not going to be to my liking. I've run this through RS many different ways to get it the most stable, which is why it's not quite a "scale" build. The BT being a tad longer than the actual pic you see in the inset. Anyways, she'll fly just fine...
as long as all 4 motors light.

 

[Stones]

Just a few build pics...
Using a baffle for this one. Placed midway up the BT length.
Exploded view of baffle...here

 

[Stones]

The cluster...
Top view showing the engine blocks (cut from used motor casing).
Upper section of tubes are coated lightly with epoxy.

 

[Stones]

Assembled cluster...
No hooks, a T-nut for motor retention. Uses a 6-32 screw with an aluminum washer.

 

[Elapid]

you have a little nose weight?

i think it's really cool that you are going to build a semi-scale model of a cannon and expect it to fly!
you should eject a clown figure at ejection!

good luck!

 

[GL-P]

Cool!!!

 

[Stones]

Originally posted by Elapid
you have a little nose weight?

i think it's really cool that you are going to build a semi-scale model of a cannon and expect it to fly!
you should eject a clown figure at ejection!

good luck!

Just about 1oz. of NC weight added.
As I said before...it WILL fly... ye of little faith.

Here she's getting those "cannon" fins (basswood) added.

 

[radiO]

just two little questions please:

what does CP stand for?
motor retention?

...thanks!

 

[Stones]

Originally posted by radiO
just two little questions please:

what does CP stand for?
motor retention?

...thanks!

CP - Center of Pressure

Motor Retention- a way to keep the motor from leaving the airframe when the ejection charge goes off.

Check the glossary button on the top right of this site for all related rocketry terms.

 

[radiO]

gracias!

 

[teflonrocketry1]

Stones,

Cool rocket! I wonder what's inside? Could you post the RockSim file so all the rocket scientists could take a look at your design?

Bruce S. Levison, NAR #69055

 

[Elapid]

Originally posted by Stones
...it WILL fly... ye of little faith.

I have no doubt that it will fly perfectly.

when i see fins (like those) anywhere near mid-tube, i tend to think about how much better the design would fly with smaller fins, moved back farther like a *normal* rocket has. the excess finnage, forward center of pressure, and the cluster MMT are going to necessitate more nose weight and thus a further loss in performance.

it LOOKS really cool!
i'm just a bit of a performance freak...

good luck!
and did i mention that you should post pics of the flight???

 

[Stones]

Pics of the flight for sure. Always do that as one never knows if it's the last flight.

Bruce...
The RS file.
And yes...it's all right on the edge but, I feel she'll fly fine on a low wind day. All the motors gotta light...that's a given.

 

[Elapid]

now i have my doubts...sorry to be the Devil's Advocate here...
barrowman of 0.06 and rocksim stability of 0.9 is scary-tight...
that's a pinwheel waiting to happen, imo.

maybe add another ounce of nose weight and use a longer launch rod?

one thing bothers me about the simulation. if you open the details, it reports the velocity at the end of the launch rod (36") is 36 mph, but it reports for the same flight that the rocket reaches the user-defined safe flight speed (30 mph) in slightly over 37"

not the first time i've seen incompatible numbers come out of rsim.

i'd try to make it stable enough to fly on two motors.
just in case.

 

[Stones]

Originally posted by Elapid
now i have my doubts...sorry to be the Devil's Advocate here...
barrowman of 0.06 and rocksim stability of 0.9 is scary-tight...

I don't know where you are getting this info from. She sims out with (4) C-6's at 1.02 RS and .10 Barrowman. I'm going with the RS numbers as I plan on flying it on a low wind day. We get those here now and then.
Also, she'll be going off of 63" of rail. No idea where you're getting 36" from. My Bullpup 12C simmed at about the same, maybe a little less, and flew just fine.
I have a pretty good record for flying safely, as those who fly with me will attest to. I'm not about to put anyone in harms way without thinking things through. Actually, my biggest concern now is that the ejection from the 4 motors will be enough to get the laundry out with the baffle and BT volume. We shall see.

 

[Stymye]

seems like that would be plenty to push the laundry and cone

Heres rocksim with the motors loaded

 

[teflonrocketry1]

Originally posted by Stones
Pics of the flight for sure. Always do that as one never knows if it's the last flight.

Bruce...
The RS file.
And yes...it's all right on the edge but, I feel she'll fly fine on a low wind day. All the motors gotta light...that's a given.

You aren't kidding about being right at the edge, I get exactly 1 caliber of stability with the parachute and shock cord pushed further down in the upper airframe assembly! The longer rail should definately help out and you are correct about choosing a low wind day. This design will be prone to weathercock, a slight gust of wind should give a horizontal flight.

Bruce S. Levison, NAR #69055

 

[Elapid]

i got the 36" from your simfile.
if you double click on your C6-5 flight it gives the detailed report for the flight.

i got the .06 barrowman by loading up 4x C6-5 and hanging the motors .25" out the back to allow for motor removal.

dude...
look at my first post.
nowhere does it say it won't fly.
noplace does it say your model will be unstable.

all i said was that the CP was in a lousy position.

typically, you can look at a design and visualize the CP at the front of the fins. CG has to go in front of that, and being that the fins on this go to the midpoint of the rocket and don't reach the aft end, i decided to mention that it was going to be a tricky design to fly.

i'm certain your safety record is spotless!
i have no doubt that you are a skilled modeler.

hey...if Ky Michaelson can make a Porta-Potty fly, i have no doubt that your cannon will soar!

 

[Stewart32]

The glass is half full!

Do post flight pics.

 

[Stones]

Elapid...
I'm not taking offense to your opinions by any means. I was only commenting that I am aware of the "edge" this bird is on.
Strange about rocksim, I dl'd the file from TRF, just to make sure we're looking at the same file. When I load it up in my version, 6.05, it has the launch guide at 63" as well as the other info I posted. Might just be an applications setting difference.
Anyways, I'll post the flight pic when she goes and all post flight info as well. It's all good.

 

[teflonrocketry1]

The launch guide length is part of the "application settings" and not the rocket design file. So if Elapid had the lauch guide length set to 36" for a previous simulation that he ran, then that's what RockSim used when he opened Stones' file.

You have to be careful to specify things like engine overhang, lauch guide length and angle when trying to duplicate RockSim results. I used 0" overhang for Stones' file to make the motors butt up against the trust rings.

Bruce S. Levison, NAR #69055

 

[Stones]

Tnx for pointing that out Bruce. I thought it might be that (application settings). I hadn't paid enough attention to the thrust rings which should be 2.375 from the rear of the motor tubes, which give the motors .375 of overhang. A pic loaded.

 

[teflonrocketry1]

Stones,

I have been playing around with your design in RockSim using the RockSim equations. According to the program you should get a stable flight in a 15 to 20 mph wind, without the 1.2 ounce nose weight, but the stability margin is only 0.2 calibers!

Let us know how the first flight goes, and please post some pictures.

Bruce S. Levison, NAR #69055

 

[Stones]

Bruce,

I plan on sticking with 1 caliber (RS) just to be on the "safer" side. Tnx for taking a look at it and running it through the sim. Much appreciated.
Question...
I notice when I save a rkt file with a simulation launch on specified motor(s), and then reopen the file, the velocity and altitude at deploy are at 0.00. Yet, the info in the detailed report is there. Any clue what causes this? I have version (6.05).

 

[Stones]

Here she is, painted/decals. Just a tad over 37" long and weighs in at exactly 1 pound, fully loaded/including 1.3 oz. of noseweight.
Should lob to about 550' on a little to no wind day.

 

Very, very nice! Can't wait to see launch pics

 

[Stones]

Well...today was the day for the launch. Winds were about 5-7 mph and blue skies.
Loaded 'er up with (4) C6-5's and a 30" chute. No dogbarf or wadding as it has a AT type baffle.
Here she is wired on the rail.

 

[Stones]

I had a friend of mine hit the button so I could get a few pics. Here she is on takeoff. We were both amazed on how straight this thing flew. Slow and steady launch to around 500' or better. Chute deployed perfectly, right at apogee.