Airframe separation deployment

[dnl2]

I'm not sure what you would call this type of parachute deployment and airframe separation. I want to use the coupler as a payload bay to start becoming familiar with altimeters, but still use the motor for ejection. I have a phenolic Public Missile airframe and coupler. The problem I have is that the coupler is extremely tight in the airframe. Is this common for couplers? In order to separate the rocket in the pictured manner I would think the coupler needs to be snug but not tight. What is typically done here? Sand down the exterior of the coupler, or sand the interior of the lower airframe? I am assuming that couplers are used a payload bays. Is it that payload bays have a smaller outer dimension than couplers? Am I missing something? Thanks.

[Wayco]

"Snug but not tight" is just about right, at least that's what I did with the coupler in my LOC Nuke Pro Max. I wanted to use it later (after my L-1 certification) for an AV-bay, so I sanded down the paper coupler and added two bulkheads held together with all-thread. I have no experience with phenolic, but I'm assuming it would work much the same. Your payload bay is on the other side of the coupler in your drawing. For dual deploy, it holds another parachute and the associated rigging.

[MarkII]

Yeah, it should have the same kind of fit as the nose cone does. Enough friction to keep the two sections together up you pick up the rocket by the upper section and shake it a little. Not so much friction that it you have to struggle to get them apart by hand.

[bill2654]

This is a typical set up for dual deploy. Actually you can do without the 1st joint above the fin can, I built it that way so I could go with motor deploy or electronic deployment. All my joints are either friction fit or held together with plastic rivets.

[cbrarick]

Friction fit is NOT a good idea for dual deploy....Use rivets if you want it to stay together. You don't want the apogee (or motor) charge causing the ebay to blow off the payload bay (or the shock of the separation causing it to pull out). Don't forget to use shear pins for the nose cone to prevent that from coming off early either.

[MarkII]

Friction fit is NOT a good idea for dual deploy....Use rivets if you want it to stay together. You don't want the apogee (or motor) charge causing the ebay to blow off the payload bay (or the shock of the separation causing it to pull out). Don't forget to use shear pins for the nose cone to prevent that from coming off early either.

I'm not seeing the logic here. Why would the two sections that separate at apogee need to be riveted together? Why would the separation of two riveted-together sections be less violent or stressful than the separation of two friction-held sections? It seems to me that it would be the other way around, since the riveted-together joint would require quite a bit more force to separate than a friction-held joint. The purpose of using rivets is to create a stronger, less-prone-to-separation joint, isn't it? The electronics bay needs to be firmly affixed to one of the sections, but if it is so strongly fastened to both sections, how would they be able to separate?

[bill2654]

I should have made myself more clear. Plastic rivets for the joints I DONT want to separate and friction fit for those I DO.

[cbrarick]

That makes more sense Bill!

the deal is that the aft connection between the ebay and the fin can should be friction fit (larger rockets use shear pins, too) which is what separates at apogee (either electronically or with the motor delay). The connection between the ebay and the payload tube is riveted together so it won't come apart. The connection between the nose cone and the payload bay (your mains come out here on the altimeter) is friction fitted (rivets again are good).

Get "Modern High Powered Rocketry" by Mark C. and read it - it should be required reading for level 1......

[bjphoenix]

I'm not seeing the logic here. Why would the two sections that separate at apogee need to be riveted together? Why would the separation of two riveted-together sections be less violent or stressful than the separation of two friction-held sections? It seems to me that it would be the other way around, since the riveted-together joint would require quite a bit more force to separate than a friction-held joint. The purpose of using rivets is to create a stronger, less-prone-to-separation joint, isn't it? The electronics bay needs to be firmly affixed to one of the sections, but if it is so strongly fastened to both sections, how would they be able to separate?

Apparently the rivets and ejection charge are designed to work together. IOW the rivets hold the sections together until the ejection charge fires, then the ejection charge shears the rivets and separates the sections. They are also called "shear pins" for this reason. On a single deployment rocket you don't want the sections to separate early. When the engine burns out, if your fins and lower section have a lot of drag then that can pull the lower airframe out of the upper airframe and cause premature deployment. On a dual deployment rocket you don't want drag separation either, and you also don't want the shock or forces from the first deployment to cause the other joint to separate prematurely. You may be able to achieve this with friction but the problem with friction is that it is too variable. You would have to sand the components to get them to fit just right then when you get to the launch field the difference in humidity or temperature or whatever could cause the fit to be different. With carefully selected shear pins you can get it to work the same way every time.

[new2hpr]

I think there's just some terminology confusion going on here. In general, most refer to "rivets" as the plastic rivets used to make a removable connection that is NOT to come apart, as opposed to "shear pins" which are usually nylon screws or styrene rod that are used to make a connection that will separate with more force than a deployment shaking, but less than the BP charge.

Carry on the discussion and clarify which you're talking about. All can be of more help if the confusion is removed.

-Ken

[cbrarick]

Rivets are used to hold rocket parts together until we remove them. They are designed to stay together during flight, including withstanding ejection charges. After all, wouldn't want to blow off both ends of the payload bay, right?
Shear Pins are generally small nylon screws or polystyrine rods designed to break under the force of the ejection charge. They just prevent drag separation, or while under drogue, prevent the main chute from falling out of the rocket.....

[El Cheapo]

Not sure how well it'll work with phenolic but for paper, I soak the coupler and inside of tube with thin CA then sand smooth. It'll keep them from binding. Have some pics in a Mean Machine thread here. Everything else is friction fit.

I also ran screws through the coupler/nose cone in my L1 in case I need to add weight. The coupler to bt is friction fit. Has worked like a charm.

[bjphoenix]

Rivets are used to hold rocket parts together until we remove them. They are designed to stay together during flight, including withstanding ejection charges. After all, wouldn't want to blow off both ends of the payload bay, right?

In my rockets I've used steel screws for this, but none of mine have been over 4" diameter.

[cjl]

In my rockets I've used steel screws for this, but none of mine have been over 4" diameter.

Steel screws work great on larger rockets too. The exact method doesn't matter, so long as it achieves the goal.

[El Cheapo]

Steel screws work great on larger rockets too. The exact method doesn't matter, so long as it achieves the goal.

I'm kind of a stickler for weight and try to use aluminum screws, nylon washers and aluminum nuts instead of steal. I've found my local Ace has a decent selection of all.

[cjl]

I figure the difference between the weight of #6 aluminum and #6 stainless screws matters a lot less to me than the headache when I inevitably strip the aluminum ones.

(The weight's pretty minuscule anyways)

[El Cheapo]

I hear ya cluckiin' big chicken. I only do snug at best if going into a nut on the other side. I haven't seen the need to to crank down on the coupler to nose cone for my application. A big L3 rocket would be a bit different for sure. Would love to find some aluminum allen heads but have not been so lucky as of yet.

[dnl2]

Don't the rivets and sheer pins do damage to cardboard body tubes?

El Cheapo Not sure how well it'll work with phenolic but for paper, I soak the coupler and inside of tube with thin CA then sand smooth. It'll keep them from binding. Have some pics in a Mean Machine thread here. Everything else is friction fit.

Does the CA strengthen the tubes enough?

[hardinlw]

On lighter rockets, I've used strips of the Tamiya 1/8" yellow masking tape run lengthwise in place of shear pins. It's the same effect as a shear pin but with a lower breaking point. Strips of Monokote trim can also be used. Essentially, you want something that actually breaks at a very repeatable force instead of a friction fit that can vary all over the place depending on temperature, humidity, etc.

[SRBell]

Don't the rivets and sheer pins do damage to cardboard body tubes?





Does the CA strengthen the tubes enough?

On my DD rockets I've used thin tin sheet, I've heard others use brass as well. For example on the AV bay and the sustainer I've used a router bit to take down the outside of the AV bay and the inside of the sustainer tube. So when I glue in the strips of tin it's flush inside and out. Then drill through and thread in 2 56 nylon screws. They'll shear without damaging your tubes. Of course I'm a total newbie to rockets so if there's a better way I'm sure others will chime in.

[doug_kosty]

On lighter rockets, I've used strips of the Tamiya 1/8" yellow masking tape run lengthwise in place of shear pins. It's the same effect as a shear pin but with a lower breaking point. Strips of Monokote trim can also be used. Essentially, you want something that actually breaks at a very repeatable force instead of a friction fit that can vary all over the place depending on temperature, humidity, etc.

This seems like a great idea. I am working on rebuilding an Executioner to try out DD. Think I will steal this concept!

[Handeman]

I know there is a place for shear pins and they are needed some times, but I also thing they get over used a lot too. I've been flying HPR and DD for over four years now on rocket up to 4" and I've never used shear pins and have never needed them.

I might consider them when I get up to 5.5" rocket, but probably not until i'm up to 7+ inches. I just don't think they are needed in the smaller sizes, friction fit works just fine.

[cjl]

I would argue that shear pins aren't just beneficial to give a tighter fit - they also give a much more consistent fit than friction alone. Shear pins give you a very repeatable deployment, where the same amount of force is needed every time, while friction fitting does not.

[Handeman]

I would argue that shear pins aren't just beneficial to give a tighter fit - they also give a much more consistent fit than friction alone. Shear pins give you a very repeatable deployment, where the same amount of force is needed every time, while friction fitting does not.

I can understand why that would be, but I've never seen any practical need for the extra consistent shear pins. I've never had any problems with inconsistent deployment with friction fit. Granted I've only use up to 4", but if the charges went off, they always deployed properly. I do NOT follow the blow it out or blow it up method. None of my dual deploys snap hard to the end of the harnesses and the friction fit nose cone has never been an issue with coming off early or not coming off during main deploy.

I just find that I have consistent deployments with friction fit and have no need of the added complexity of shear pins. With my luck, my first time using them, they wouldn't shear and end up jamming between nose cone and BT and prevent deployment.

[Reinhard]

I would argue that shear pins aren't just beneficial to give a tighter fit - they also give a much more consistent fit than friction alone. Shear pins give you a very repeatable deployment, where the same amount of force is needed every time, while friction fitting does not.

One advantage of shear pins is that they remove very little energy from the separation process.
W = F * d
The distance, over which the force is applied is very short, therefore the amount of mechanical work "lost" in the process of breaking them is very small.

Friction fit on the other hand causes a bigger energy loss for the same breaking force.

Reinhard

[qquake2k]

I'm not sure what you would call this type of parachute deployment and airframe separation. I want to use the coupler as a payload bay to start becoming familiar with altimeters, but still use the motor for ejection. I have a phenolic Public Missile airframe and coupler. The problem I have is that the coupler is extremely tight in the airframe. Is this common for couplers? In order to separate the rocket in the pictured manner I would think the coupler needs to be snug but not tight. What is typically done here? Sand down the exterior of the coupler, or sand the interior of the lower airframe? I am assuming that couplers are used a payload bays. Is it that payload bays have a smaller outer dimension than couplers? Am I missing something? Thanks.

I too do what ElCheapo does. I flood the outside of the coupler and inside of the body tube with thin CA, then sand them both smooth. I do this for my mid separation zipperless rockets, and it works great. I usually mount the coupler on my lathe and sand/polish them that way, but you could easily do it by hand. Just takes a little extra elbow grease. I'm not sure, though, that you need the CA with phenolic. You can probably sand/polish them as is. And don't forget, you'll need a bulkhead in the coupler, too.

[donperry]

http://books.google.com.jm/books?id=...page&q&f=false

the book that was mentioned earlier is on google books.