Friction fit - Drag Separation

[Sailfish1957]

I completed a LOC IRIS 3 and have an AV bay in the payload section. This is not a dual deployment set up. My question is: How tight of a friction fit should there be between the booster and payload section. The Coast Guard red racing strip decal seen in the upper part of the rocket in the attached photo, is about 5 inches above the coupling. Right now, the fit is tight and certainly passes the “Shake Test”. I will be using 38 mm motors in the H or I range. I figure that the charge will be plenty powerful - but could some advice.

 

[jmasterj]

You'll be fine. The charges in standard HP motors are more than enough for that length of 3" tube.

It looks great!

 

[Sailfish1957]

You'll be fine. The charges in standard HP motors are more than enough for that length of 3" tube.

It looks great!

Thanks!

 

[Handeman]

Since it's motor ejection, you don't need much friction fit at all. The only thing is to make sure the booster, payload and nose cone are all attached to each other with a shock cord. Both joints may open when the ejection charge goes off but it won't matter as long as everything is attached.

 

[Sailfish1957]

Since it's motor ejection, you don't need much friction fit at all. The only thing is to make sure the booster, payload and nose cone are all attached to each other with a shock cord. Both joints may open when the ejection charge goes off but it won't matter as long as everything is attached.

Thanks! That is done. The NC is fastened with screws tapped in. All is connected with shock cord.

 

[bjphoenix]

For a straight rocket it seems that most of the drag is on the payload section in effect holding it in place against the booster section, so friction isn't needed that much. If you had large fins, additional side tubes or some other shape on the booster that would give it more drag than the nose cone, maybe drag separation would be an issue. I might be wrong about this because I'm not considering base drag, I don't know how much the base drag is compared to drag on the nose cone.

 

[neil_w]

For a straight rocket it seems that most of the drag is on the payload section in effect holding it in place against the booster section, so friction isn't needed that much. If you had large fins, additional side tubes or some other shape on the booster that would give it more drag than the nose cone, maybe drag separation would be an issue. I might be wrong about this because I'm not considering base drag, I don't know how much the base drag is compared to drag on the nose cone.

I don't think you can generalize like this. Many/most of the rockets that use shear pins to avoid drag separation are 3FNC or 4FNC designs. The mass of the nose section plays a big role (noses with a lot of weight can be quite dense, and have a lot of momentum).

 

[Handeman]

I don't think you can generalize like this. Many/most of the rockets that use shear pins to avoid drag separation are 3FNC or 4FNC designs. The mass of the nose section plays a big role (noses with a lot of weight can be quite dense, and have a lot of momentum).

I think most rockets that use shear pins to avoid drag separation do it because that is what is suggested by so many on this forum and because it is suggested so much, it's now become "common knowledge" that you need to use shear pins, not because they are actually needed to prevent drag separation. I've never seen anyone post an actual analysis of drag on various parts of 3FNC or 4FNC rockets that indicated that shear pins would be required to prevent drag separation. I think this is similar to the idea that you have to have swivels on parachutes.

Just my thoughts...

 

[Steve Shannon]

I don’t use shear pins for most small rockets, but I make sure that the rocket requires some force to pull apart. I have sacrificed a rocket to drag separation on a high thrust motor. It’s not enjoyable. Judging how much friction is sufficient is sometimes difficult. Using a shear pin is a bit more predictable. with experience both ways work. I’ve heard about shear tape as well but I haven’t done it.

 

[Voyager1]

You might want to consider pressure vents in you airframe. One just below the NC shoulder (you might want to pop the NC at some later stage); and one just below the avbay/payload coupling in the booster, particularly if you’re planning on high altitude. In your case, this one is the more important one. Does the avbay have pressure sense vents, assuming that you’re installing an altimeter - at least for logging?

 

[waltr]

I don’t use shear pins for most small rockets, but I make sure that the rocket requires some force to pull apart. I have sacrificed a rocket to drag separation on a high thrust motor. It’s not enjoyable. Judging how much friction is sufficient is sometimes difficult. Using a shear pin is a bit more predictable. with experience both ways work. I’ve heard about shear tape as well but I haven’t done it.

I think High Thrust motor is a key to getting drag separation and needing shear pins.
Plus many other points in the design as has been mentioned.

Also look at the negative acceleration value in a sim at burn-out. The higher the velocity at burn-out the high the negative acceleration is due to Drag increases by V squared. Now drag of various parts verse their mass (*V = momentum) has a greater affect.

 

[rellis2]

I had a Semroc Aero Dart that had many good flights. On it's last flight, the motor started chuffing at ignition and it vibrated it's way off the rod puking off the friction-fit nose cone and parachute about ten feet away. Then, with it's body tube empty it straightened out and flew a beautiful parabola, augering in a quarter mile away. Drag separation can happen anywhere.

 

[Buckeye]

I think most rockets that use shear pins to avoid drag separation do it because that is what is suggested by so many on this forum and because it is suggested so much, it's now become "common knowledge" that you need to use shear pins, not because they are actually needed to prevent drag separation. I've never seen anyone post an actual analysis of drag on various parts of 3FNC or 4FNC rockets that indicated that shear pins would be required to prevent drag separation. I think this is similar to the idea that you have to have swivels on parachutes.

Just my thoughts...

Agree. "Drag Separation" sounds cool and sexy and is another myth propagated by over-builders.

Some FBD analysis reveals a simple relationship between mass and drag of the two halves:

https://www.rocketryforum.com/threads/shear-pins-on-a-8-dia-bird.153740/post-1901747
Yes, the typical 3FNC/4FNC will not meet this criterion for separation. Vent holes are probably not needed either, as most joined rocket tubes have plenty of natural leakage, though vent holes do make packing the laundry easier.

 

[Antares JS]

Shear pins are not generally needed to prevent drag separation. In my mind, shear pins have up to three purposes:

-Holding break points together between smooth fiberglass parts or other materials that don't have much friction between them.

-Holding your second break point together in a dual-deploy rocket to ensure it doesn't open when the first event happens and everything is being jerked around. (Even then, friction fit is usually good enough for smaller rockets.)

-Greater consistency in the force required to separate two parts, since friction fit can be a bit subjective.

My few fiberglass rockets, my level 3 prototype, and my level 3 rocket are my only rockets that use shear pins.

 

[rellis2]

Maybe 'Momentum Separation' is a better term.

 

[Banzai88]

Shear pins are not generally needed to prevent drag separation. In my mind, shear pins have up to three purposes:

-Holding break points together between smooth fiberglass parts or other materials that don't have much friction between them.

-Holding your second break point together in a dual-deploy rocket to ensure it doesn't open when the first event happens and everything is being jerked around. (Even then, friction fit is usually good enough for smaller rockets.)

-Greater consistency in the force required to separate two parts, since friction fit can be a bit subjective.

My few fiberglass rockets, my level 3 prototype, and my level 3 rocket are my only rockets that use shear pins.

This!

 

[Sailfish1957]

You might want to consider pressure vents in you airframe. One just below the NC shoulder (you might want to pop the NC at some later stage); and one just below the avbay/payload coupling in the booster, particularly if you’re planning on high altitude. In your case, this one is the more important one. Does the avbay have pressure sense vents, assuming that you’re installing an altimeter - at least for logging?

Thanks and Yes, I have put in the vents in the exact locations you describe as I knew that I would need to relieve pressure. I do not have pressure sensing vents. I did not put an altimeter on the sled, just a FW GPS Tracker, battery, and mag switch. I plan to buid another sled that will have different components as I progress this one.

 

[PSLimo]

Hey Guy's,

I disagree that drag separation isn't a thing.

That said, I flew a 5.5" Army Hawk on a AT K-550W without shear pins. The onboard video shows the drag separation that almost ended in disaster. This rocket had a 1/4" vent hole so it wasn't over pressurization.

Take an onboard look at what happens when the motor loses thrust and the 5 lb nosecone wants to continue and the very draggy fins put the brakes on. You can see at the end of the video those huge fins are floating above the drogue.

 

[Antares JS]

I disagree that drag separation isn't a thing.

It's absolutely a thing, but it's less common than some people seem to think. Unless he has a crazy heavy nose, I don't think the guy in the OP needs to worry about it. Your rocket with a 5-lb nose and those giant fins, sure I can see drag separation being a problem.

 

[ChicagoDave]

Agree. "Drag Separation" sounds cool and sexy and is another myth propagated by over-builders.

Some FBD analysis reveals a simple relationship between mass and drag of the two halves:

https://www.rocketryforum.com/threads/shear-pins-on-a-8-dia-bird.153740/post-1901747
Yes, the typical 3FNC/4FNC will not meet this criterion for separation. Vent holes are probably not needed either, as most joined rocket tubes have plenty of natural leakage, though vent holes do make packing the laundry easier.

The faster you change altitude and the higher your go the more likely you are to experience pressure separation. Also, the tighter the fit the more likely you are. In my case pressure separation sheared three #2 shear pins and spit out the main under thrust. There is a video of this rocket shredding due to pressure separation somewhere on YouTube. Interestingly, you may note there is no payload bay in the photo below. That is because it was completely shredded by the Kevlar harness.

In my experience, having proper sized vent holes is critical to the safe operation of a fiberglass rocket.





I have also lost a Goblin due to drag separation when I did not properly affix the weighted nosecone. The draggy fins and nose weight combination proved too much for the friction fit of my nosecone.



So in my personal experience, both drag separation and pressure separation are possible. You may be lucky with drag separation and not hit it with a typical rocket design. But please be mindful of the possibility with larger / heavier nosecones and draggy fins.

Also please consider properly venting any enclosed airspace in your rocket to avoid the possibility of pressure separation. Both of these problems can be easily avoided by carefully planning for the safe operation of your rocket utilizing well-documented construction techniques such as venting and using shear pins. The cost of using these techniques is negligible compared to the risk of your rocket plummeting from altitude in an uncontrolled way.

 

[PSLimo]

It's absolutely a thing, but it's less common than some people seem to think. Unless he has a crazy heavy nose, I don't think the guy in the OP needs to worry about it. Your rocket with a 5-lb nose and those giant fins, sure I can see drag separation being a problem.

I agree and don't think the OP will have any problems with that rocket design.

Dave also makes good points as I had no problems on its first flight on a tamer AT-415W.

Seeing it on video is a reality check that there's lots going on that we don't see during a flight.

 

[Sailfish1957]

Hey Guy's,

I disagree that drag separation isn't a thing.

That said, I flew a 5.5" Army Hawk on a AT K-550W without shear pins. The onboard video shows the drag separation that almost ended in disaster. This rocket had a 1/4" vent hole so it wasn't over pressurization.

Take an onboard look at what happens when the motor loses thrust and the 5 lb nosecone wants to continue and the very draggy fins put the brakes on. You can see at the end of the video those huge fins are floating above the drogue.

View attachment 545688

Wow!