'Wrongway' build: rear eject 2x cluster rocket

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matthewdlaudato

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Somewhat of an odd-roc, but on the surface, it looks normal. Twist is that the chute will eject from the rear. This will be a quickie build thread - rocket is mostly assembled at this point, will post the current state picture when I get home tonight.

Here is the design from OR:

wrongwaydesign.jpg

Ignore the CG location - it will be considerable closer to the aft; I estimate around 2-caliber stability.

The engine pods have a 3/16 x 3/4" slit near their forward end, mating with a similar slit in the main body tube. The pod noses are fixed and do not blow, as is the forward nose. The ejection gases should vent into the slits and pressurize the aft section of the main body (there is a bulkhead with a shock cord attachment just forward of the slits that should keep the gasses aft). If all goes well, the 'rear ramjet' will be forced out along with the cord and chute.
 
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Here are the parts, and the already partially constructed rocket. Been using 5-minute epoxy and occasional wood glue for construction. I'm liking the Loctite 'heavy duty' 5 minute epoxy for these quick builds - it comes in two containers, simple 1:1 mix, and more than enough working time for attaching a fin or a tube-to-tube as I've done here.

wrongwayparts.jpg
 
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Mostly complete. Decided to make the forward bulkhead assembly modular - 2x2-56 nylon screws going into brass inserts in the bulkhead plywood. Still need to get shock cord installed and decide how to fix the nose in place (plastic rivets, probably).

ImageUploadedByRocketry Forum1411517910.694436.jpg
 
Very COOOOOL design.
I really like the aft nc look.

Thanks - come to Amesbury to see it fly! Pretty sure it will fly the 'right way' on the up part, and rear eject should be a crowd pleaser, assuming all goes well.

Edit: should add, coming from you that is high praise Scott - you have some very cool designs!
 
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Thanks - come to Amesbury to see it fly! Pretty sure it will fly the 'right way' on the up part, and rear eject should be a crowd pleaser, assuming all goes well.[/QUOTE.... Sandy (wife) and I are going. Thats the plan anywhoo. Cross me fingers all goes well between now and then.
 
Matt, I don't know what kind of camera you have, but if it can white balance, you can eliminate that yellow overtone from your pics before you shoot. Just an FYI.
 
Matt, I don't know what kind of camera you have, but if it can white balance, you can eliminate that yellow overtone from your pics before you shoot. Just an FYI.
Snapped those with the iPhone 4S. I really should get in the habit of using my good camera (D3100 Nikon). Not sure if the fact that there is so much actual yellowish beige colors in the subject are affecting this.
 
Somewhat of an odd-roc, but on the surface, it looks normal. Twist is that the chute will eject from the rear. This will be a quickie build thread - rocket is mostly assembled at this point, will post the current state picture when I get home tonight.

Here is the design from OR:

View attachment 185133

Ignore the CG location - it will be considerable closer to the aft; I estimate around 2-caliber stability.

The engine pods have a 3/16 x 3/4" slit near their forward end, mating with a similar slit in the main body tube. The pod noses are fixed and do not blow, as is the forward nose. The ejection gases should vent into the slits and pressurize the aft section of the main body (there is a bulkhead with a shock cord attachment just forward of the slits that should keep the gasses aft). If all goes well, the 'rear ramjet' will be forced out along with the cord and chute.

I did a similar bird 2 yrs ago. It had 3 pods and 3 fins. I did the slits and re-eject like you as well. On mine, I added a baffle. I put a CR (BT60 - BT20) just below the slits. Then added a chunk of BT20 that run upward. The ejection gasses come out the slits, hit the BT20, then go upward and then down through the center via the BT20.

Word of caution, with the rocket landing on the nose, the tube is more susceptible to getting crinkled on landing. I used a smaller chute one day do to the wind conditions and closeness of corn. The rocket barely missed the corn (good thing) but the body tube was crinkled (bad thing). My advice is size the chute for 15-17 FPS.
 
Word of caution, with the rocket landing on the nose, the tube is more susceptible to getting crinkled on landing. I used a smaller chute one day do to the wind conditions and closeness of corn. The rocket barely missed the corn (good thing) but the body tube was crinkled (bad thing). My advice is size the chute for 15-17 FPS.

That's good feedback CZ Brat - thanks. Current plan is to use one of my 14" CATO chutes. We fly on grass fields so all landings tend to be soft (as long as the chute actually deploys, that is!)
 
That's good feedback CZ Brat - thanks. Current plan is to use one of my 14" CATO chutes. We fly on grass fields so all landings tend to be soft (as long as the chute actually deploys, that is!)

I crumpled my rocket on sod, on a 12".
 
I crumpled my rocket on sod, on a 12".

Not sure I could go much bigger than, say, a 16". I'll know the recovery weight tonight - if it looks like it will come down hot, perhaps I can add a stiffener coupler or two in the upper body.
 
Not sure I could go much bigger than, say, a 16". I'll know the recovery weight tonight - if it looks like it will come down hot, perhaps I can add a stiffener coupler or two in the upper body.

Also, how do you plan on keeping the chute in the rocket during boost, but ensuring it comes out during ejection? On my last rear eject, I made a simple "Friction Calibration" rig. Details of it are in this build thread.

https://www.rocketryforum.com/showthread.php?67272-Estes-V2-Steam-Punk-Forward-Mounted-Canted-Cluster-build-thread
 
Also, how do you plan on keeping the chute in the rocket during boost, but ensuring it comes out during ejection? On my last rear eject, I made a simple "Friction Calibration" rig. Details of it are in this build thread.

https://www.rocketryforum.com/showthread.php?67272-Estes-V2-Steam-Punk-Forward-Mounted-Canted-Cluster-build-thread

It is certainly a concern. My basic calculation (similar to yours) shows that a slightly tighter fit on the plug than I might normally use on a nose should suffice to allow friction to overcome force due to acceleration, and that the 2x worth of ejection charge will compensate on ejection. I like your rig a lot - thanks for sharing.

Edit: should add, I will use weighed 'bag o junk' since I have no lead plates laying around!
 
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Glued the launch lug on. Final mass of rocket with chute is 92g (3.3oz). Mass of chute and plug is 20g (.02kg).

Used this clamped to the plug to successively add mass - the small wrench, last to be gently 'dropped' on to the hook, caused separation.

ImageUploadedByRocketry Forum1411698920.948222.jpg

Friction test showed separation between 550-600g (1.2-1.3lb, around 5.9N force). OR says 180 m/s^2 max acceleration. So:
F=ma=.02 kg x 180 m/s^2=3.6N. This should be a good enough margin to keep the plug in place under thrust.

Combined volume (body plus 2 pods) is ~ 20 in^2, area of the BT-55 tube is ~1.3 in^2. Ejection charge from 2xB6 would need to pressurize only to 1 psi to pop the plug - ejection should not be a problem according to one of the common charge calculators on the 'net. (The total ejection charge would only need to be .02g - much less than in 2 18mm motor from what I've read.)

Seems like this should fly and deploy as designed.


NAR #91867, L2
Member CMASS, MMMSC, RIMRA, CATO
 
Glued the launch lug on. Final mass of rocket with chute is 92g (3.3oz). Mass of chute and plug is 20g (.02kg).

Used this clamped to the plug to successively add mass - the small wrench, last to be gently 'dropped' on to the hook, caused separation.

View attachment 185341

Friction test showed separation between 550-600g (1.2-1.3lb, around 5.9N force). OR says 180 m/s^2 max acceleration. So:
F=ma=.02 kg x 180 m/s^2=3.6N. This should be a good enough margin to keep the plug in place under thrust.

Combined volume (body plus 2 pods) is ~ 20 in^2, area of the BT-55 tube is ~1.3 in^2. Ejection charge from 2xB6 would need to pressurize only to 1 psi to pop the plug - ejection should not be a problem according to one of the common charge calculators on the 'net. (The total ejection charge would only need to be .02g - much less than in 2 18mm motor from what I've read.)

Seems like this should fly and deploy as designed.


NAR #91867, L2
Member CMASS, MMMSC, RIMRA, CATO

My V2 had same cross section and needed a slight higher pressure to eject the plug, and it worked fine on (2) C6-5s and (2) B6-0s. I have full confidence your ejection will work as intended.
 
My V2 had same cross section and needed a slight higher pressure to eject the plug, and it worked fine on (2) C6-5s and (2) B6-0s. I have full confidence your ejection will work as intended.

My confidence is growing as well - thanks a lot for the comments and suggestions, CZ Brat. It will fly tomorrow at the CMASS Launch in Amesbury, MA. Pictures and/or video if I get my act together.
 
Starting to get the rocket painted. Two coats of high build Duplicolor grey primer, followed by a coat of plain white primer.

ImageUploadedByRocketry Forum1411742319.346279.jpg

Not sure what the final paint scheme will be - was thinking I might try my hand at a camouflage look.
 
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