Shear tape?

In the first article in the latest Rockets Magazine, the author, Bob Yanecek, used "shear tape" instead of shear pins because he was using the motor as rear coupler. I've never seen shear tape mentioned here or heard of it. Is it a new technique? Could someone explain what it is, how it's used, any pros cons vs pins, other than as above?
Could be very useful if it works as well as it seemed to for him.
thanks
Dan

It's an old technique which I still use upon occasion .

Flying this Jart, the motor was too long, to get the NC in all the way and use the normal shear pins.

So 3 small strips of masking tape was used instead . Just make sure you use enough to pass the "shake the nc and it doesn't come off" test, when picking up rocket by same.

I use it on paper rockets where shear pins would tear the airframe.

I know many old timers still using it. Kinda like friction fitting motors.......almost a lost skill these days

I use it on every flight. Never used shear pins. I use aluminum tape.

Binder Design said:

I use it on every flight. Never used shear pins. I use aluminum tape.

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Aluminum tape is really sticky and rather tough. How much do you use? Is there a trick to getting it to tear right?
Dan

pythonrock said:

Aluminum tape is really sticky and rather tough. How much do you use? Is there a trick to getting it to tear right?
Dan

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I use three to four strips on paper and composite, and two full wraps on aluminum airframes. No tricks, it always tears right at the joint.

Been doing this for years, its called FRICTION FIT!!! Tape goes on the inside. Of course there are situations when this is not feasible, but you get the idea.

Shear tape is not the same as friction fit.

jimzcatz said:

Been doing this for years, its called FRICTION FIT!!! Tape goes on the inside. Of course there are situations when this is not feasible, but you get the idea.

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Tape used instead of shear pins is better in nearly all ways except for appearance and neatness, most significantly because it has zero resistance to movement once it snaps.

Binder Design said:

I use three to four strips on paper and composite, and two full wraps on aluminum airframes. No tricks, it always tears right at the joint.

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There is a fairly specific force (depending on body tube material etc ) needed to shear nylon screws or styrene sticks for calculating BP charge. Is there a calculation you use for tape or best guesstimate from experience and ground test ?

pythonrock said:

There is a fairly specific force (depending on body tube material etc ) needed to shear nylon screws or styrene sticks for calculating BP charge. Is there a calculation you use for tape or best guesstimate from experience and ground test ?

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there's a lot of variables - kind of tape used, how well it adheres to the airframe. I have an estimate of what kind of force my 7/8" wide blue tape strips withstood from an early deployment where the nose didn't shear the tape (it's less than the documented force to break a nylon/styrene pin, but not surprisingly less). You'll need to ground test to find out what works best in your situation.

pythonrock said:

There is a fairly specific force (depending on body tube material etc ) needed to shear nylon screws or styrene sticks for calculating BP charge. Is there a calculation you use for tape or best guesstimate from experience and ground test ?

Click to expand...

I use the blow it out or blow it up method. I never use "just enough". I'm more concerned about keeping the parts together to avoid drag sep or main at apogee than I am about shearing the tape when the time comes.

Why use "shear" tape versus taping the base of the nose cone?

Both hold the nose cone on and can be adjusted.

I think one method uses pure friction the other is dependent on the shear strength of the glue.

Is the shear tape method more easily adjustable?

Or am I missing something.

I very interested because I am building a dual deploy G Force and the nose cone appears too soft to shear nylon screws. I am worried the nose cone will rip instead shearing the screws so I am seeking a different method to retain the nose cone.


So many rockets, so little time, and money.

Lowpuller said:

Why use "shear" tape versus taping the base of the nose cone?

Both hold the nose cone on and can be adjusted.

I think one method uses pure friction the other is dependent on the shear strength of the glue.

Is the shear tape method more easily adjustable?

Or am I missing something.

I very interested because I am building a dual deploy G Force and the nose cone appears too soft to shear nylon screws. I am worried the nose cone will rip instead shearing the screws so I am seeking a different method to retain the nose cone.


So many rockets, so little time, and money.

Click to expand...

When you tape the shoulder, it adds friction for the whole movement, but when you tape across the joint on the outside, once it snaps the cone moves freely and it needs less black powder. It is the same benefit as what you get from shear pins.

Ok, I'll buy that answer. Thank you.


Sent from my iPad using Rocketry Forum

I use "shear tape" rather than pins on occasion. There are a few situations where it's more appropriate (some mentioned on this thread already): paper airframes that can't hold up to pins, emergency field "macguyvering", etc.

However, shear pins are usually WAY more consistent. Once you ground test, your are pretty much sure that the same amount of force applied by a given amount of bp will work every time. Not so with tape - heat can affect the tape's glue/stick, different bits of tape can have different tear resistance, an extra bit of wrap can "just enough" or "way too much", etc. Remember that you not only want to count on "x" amount of force shearing the pins (or ripping the tape) at mains ejection, but you ALSO want those pins or tape to absolutely hold against the force of the apogee event. Otherwise your mains deploy up high, which negates the whole reason for going with dual deploy. Sure you can put LOTS of tape on to make sure it holds at apogee, and then use LOTS of bp in your mains deployment ("blow it out or blow it up"), but this is inelegant at best, and very troublesome at worst.

I don't like to hedge my bets with "more than enough" bp to do the job. A little more is fine, but overdoing it is overdoing it. Too much ejection force can cause other problems. I like the consistency of shear pins over tape for this reason.

But like I said, tape is appropriate at times, and (as noted by a few posting here) can certainly work.

Either way, remember the rule - ground test, ground test, ground test.

s6

CarVac said:

When you tape the shoulder, it adds friction for the whole movement, but when you tape across the joint on the outside, once it snaps the cone moves freely and it needs less black powder. It is the same benefit as what you get from shear pins.

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A similar effect can be achieved by applying the tape only to the most forward part of the nose cone. After a short travel, the force drops of. On a side note, this is the same reason why friction fitted motors should have it's tape as far forward as feasible. This will maximize the extraction energy for a given peak extraction force.

In my opinion, a bigger reason for using shear tape/pins vs. friction fit is reproducibility. This is probably most important when dissimilar materials are used. For example a phenolic coupler in a quantum tube airframe will exhibit more friction when temperatures are lower. This will also affect couplers wrapped in tape. There are other variables too, like the cleanliness of the joint etc. On the other hand, the shearing forces are influenced by less variables.

Reinhard

Reinhard said:

A similar effect can be achieved by applying the tape only to the most forward part of the nose cone. After a short travel, the force drops of. On a side note, this is the same reason why friction fitted motors should have it's tape as far forward as feasible. This will maximize the extraction energy for a given peak extraction force.

In my opinion, a bigger reason for using shear tape/pins vs. friction fit is reproducibility. This is probably most important when dissimilar materials are used. For example a phenolic coupler in a quantum tube airframe will exhibit more friction when temperatures are lower. This will also affect couplers wrapped in tape. There are other variables too, like the cleanliness of the joint etc. On the other hand, the shearing forces are influenced by less variables.

Reinhard

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I always put tape lengthwise, and if I am concerned about it peeling off I wrap the ends with more tape, oriented around the tube.

They never fail by adhesion, only by snapping, and the cross section where it breaks is absolutely repeatable.

stealth6 said:

I don't like to hedge my bets with "more than enough" bp to do the job. A little more is fine, but overdoing it is overdoing it. Too much ejection force can cause other problems. I like the consistency of shear pins over tape for this reason.

But like I said, tape is appropriate at times, and (as noted by a few posting here) can certainly work.

Either way, remember the rule - ground test, ground test, ground test.

s6

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How high do you fly?

Binder Design said:

How high do you fly?

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Most of the time between 2000' and 4000'. I also fly 5000'-7000' here and there.
My record altitude - which I achieved just this last weekend - is just under 12,000'. This rocket was a 3" Darkstar kit-bash on a Loki K350 - and I use shear pins on it.

s6

stealth6 said:

Most of the time between 2000' and 4000'. I also fly 5000'-7000' here and there.
My record altitude - which I achieved just this last weekend - is just under 12,000'. This rocket was a 3" Darkstar kit-bash on a Loki K350 - and I use shear pins on it.

s6

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Ok, that explains it. Ground testing is no good when flying above 25K' or thereabouts. That's why I use extra BP on anything approaching 20K'.

Binder Design said:

Ok, that explains it. Ground testing is no good when flying above 25K' or thereabouts. That's why I use extra BP on anything approaching 20K'.

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How much more, about? Do you calculate what would be needed for a 'normal' altitude and then add a certain %?

Viperfixr said:

How much more, about? Do you calculate what would be needed for a 'normal' altitude and then add a certain %?

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Mark

Understanding how the BP combustion process occurs makes high altitude deployment simple. The key is confinement until all the BP has ignited. You do not need to add any additional BP to you charge if you hermetically seal the BP container and allow the internal combustion pressure to build up to ~100 PSI or more before the container ruptures. There are other ways to confine the charge, but hermetic confinement is what NASA and DoD use for motor ignition and pyro charge initiation. https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19710020870.pdf

You want to use an e-match sealed inside the bottom of a centrifuge tube, then load in the BP charge and add wadding between the top of the BP and snap the top down to slightly compress the BP charge. This prevents the BP from moving away from the e-match. You then want to put two pieces of electrical tape in an x pattern over the closed cap and down the side of the centrifuge tube and then put a couple of wraps over the tape on cylinder. This will allow all the powder grains to ignite before the tube ruptures insuring 100% combustion of the BP.

I recommended this method to the FIT JAMSTAR Project in 2002-2003. https://projects.fit.edu/jamstar/index.html

The team did a lot of documented verification testing. https://projects.fit.edu/jamstar/html/testing.html

This sealed BP charge technique has been documented to work at over 100 Kft. https://projects.fit.edu/jamstar/html/hazard_chamber.html

Prepare your charges properly and they work 99+% of the time (e-match limited). You might as well roll the dice if you do it any other way......

Bob

Viperfixr said:

How much more, about? Do you calculate what would be needed for a 'normal' altitude and then add a certain %?

Click to expand...

50% more. Containment with surgical tubing. If I were going 40K'+, I'd use a sealed container with burst disk.

I have been watching this thread and it came time to do some ground testing on a 3" DD scratcher made from mailing tubes.

Putting tape on the outside of a rocket is just wrong!!!! It looks very much bad.

I'm surprised and impressed at how well three strips of masking tape held the joint and they broke cleanly with no damage to the tubes or the finish. It's ugly but it works.