Shearing Rail Buttons...on purpose

[AlphaHybrids]

Does anyone have any experience shearing rail buttons off on purpose? I was thinking using nylon machine screws to hold them to my rocket and then two hose clamps at the top of the rail to remove the rail buttons.

I would score the nylon machine screws at the rail buttons/airframe joint so they would snap off easier.

I've seen fly away guides, but have also seen them doink fins when coming off and I don't want that to happen.

Rocket is 104" tall, 3" diameter, N1100.

Edward

 

[djs]

My concern about this is if you had some slightly off axis thrust or misaligned buttons that the torque coming from the rail would snap the buttons prematurely. Maybe a tower launcher would be better?

 

[OverTheTop]

The idea has merit. If the energies work out (calculations) and no off-nominal launches set up something unsafe then it is possibly a good idea .

Make sure you consider all failure modes before committing to this.

 

[FredA]

Been tempted to try this myself.
My idea was to just put a razor knife at the top of the slot as an attempt to cut the screws off as they leave the rail.

 

[rharshberger]

Been tempted to try this myself.
My idea was to just put a razor knife at the top of the slot as an attempt to cut the screws off as they leave the rail.

I had a similar thought myself.

 

[manixFan]

As djs says, it sure seems like just using a tower would be a lot easier. I can see a lot of ways an idea like this could go wrong and only one way for it to go right. Flying an N motor on intentionally weakened rail guides just seems like a worrisome idea. Plus you'd have a rail that you'd have to make sure no one else tried to use otherwise the results could be disastrous.


Tony

 

[FredA]

"Flying an N motor on intentionally weakened rail guides just seems like a worrisome idea"

So is flying an N motor with fly-away guides....I know personally that can be a problem.
Just trying to suggest alternatives to towers.

And yes, I fly off our team pads 99% of the time, so trying something like this could be done easily.

 

[AlphaHybrids]

Yes, a tower would be a better idea. Though building a 20' tower for this rocket is a bit more than I want to undertake. If I go with the hoseclamp method I'd sharpen the one edge of the clamp. I don't see how someone would load a rocket onto the rail if the top has a hose clamp on it, but I'm sure someone could try.

I did a test of the idea last night with machined delrin set screws that the rail button then threads onto. I was able to hang 25 pounds off the button before it snapped. I can't forsee an off axis thrust coupled with a binding of a rail button that would cause it to snap when traveling up the rail. I'll do some drop tests with two buttons on a PVC pipe and weight attached to see how cleanly they separate. Then I'll do some moment bending as well.

Edward

 

[SCP]

What about when the first one shears off? Hope there is velocity to keep things stable and headed "up" by that time..........it seems like quite a bit bigger "shock" to the event than the fly away method. I am all for learning about a new method though!

 

[AlphaHybrids]

My standard is 3 rail buttons. The first will be as far up as possible. The bottom two are 1.5" apart starting about 1" from the bottom. At the top of the rail the rocket should have a velocity of just over 125 feet/second.

Well, I've seen fly away rail guides hit a rocket's fin and take it off, rendering the rocket fatally unstable and flying uncontrolled over the range.

Edward

 

[OverTheTop]

You probably have a higher chance of breaking (or damaging) the buttons when racking it up. More care needed when fitting it to the rail and hauling vertical.

 

[manixFan]

Yes, a tower would be a better idea. Though building a 20' tower for this rocket is a bit more than I want to undertake. ...<snip>

I don't understand why it would have to be 20' tall? How tall of a rail are you planning to use? In my experience with towers they can be shorter than a corresponding rail since they provide guidence for more of their length than a rail does. I flew a 60" 54mm rocket off a 6' tower at BALLS this year twice, on a L265 and on a K750. Both flights were nominal.

If you decide to go this route I think it would be very prudent to try it with a much smaller rocker/motor first to verify that it works. One issue I can see is that the first rail button that get cut is now captive in the rail groove and the second rail button crashes into it and is unable to be sheared off cleanly by the sharpened edge. I'm not sure how you solve that issue unless I'm missing something.


Tony

 

[AlphaHybrids]

My rail will be 16' tall. I have guidance for all but ~3" of the rocket. I tested out a similar setup last night with 12' of rail. I used a 2.5" PVC pipe as the 'airframe'. The actual OD is 2.875. The pipe was 3' long. I placed one button at the top and two buttons at the bottom, 1.5" apart. I added mass at the bottom to bring it up to 25 pounds. The rocket will weigh ~30lbs. The drop was ~10' feet before the buttons hit a non-sharpened hose clamp. Math says that the velocity was ~20 feet/second. The delrin set screws snapped cleanly in each of my five tests before I ran out of delrin to turn more set screws. These had a very thin groove at the interface. Tonight I'm going to try with COTS nylon set screws, and also hanging a weight perpendicular to the buttons.

Edward

 

[heada]

When they shear off, is there anything left protruding from the airframe or does it shear clean to the airframe? This is a very interesting idea and a novel solution. I applaud your out of the box thinking.

 

[AlphaHybrids]

When they shear off, is there anything left protruding from the airframe or does it shear clean to the airframe? This is a very interesting idea and a novel solution. I applaud your out of the box thinking.

In the tests last night I have the groove even with the airframe/button interface. They snapped relatively cleanly. The part where it wasn't grooved was torn a bit. I could do tests tonight with the shear/groove line set slightly below the airframe to have everything inside the airframe after snapping. There would still be turbulence, but nothing protruding from the airframe. I also might give a try at some polycarbonate screws as they are more brittle and will snap cleaner instead of tearing like the delrin ones.

I'd even guess you could probably use brass screws for this. Make them a two piece affair and connect them together with a solder joint. (I'd probably actually face two blank pieces of stock, solder them and then thread them instead of trying to line up threads when soldering.) Then that solder joint will become your clean snap line.

Edward

 

[manixFan]

Would be very helpful if you could post a photo of your setup at the top of the rail. I'm having a hard time visualizing where your cutting blade is relative to the rail buttons and what happens to the buttons once they are sheared off.

I understand your desire to shed the rail buttons. I fly a lot of MD rockets and don't want to mount buttons on them so I use a tower. Cutting/shearing the screws to drop the guides is an interesting idea, but one that seems to add a potentially dangerous point of failure. When your N motor lights, you'll be hitting those guides with 250lbs (or more) of thrust. How do you verify they won't shear at that point? If there is a crosswind that causes lateral force against the rail at what point does the thrust/drag force overcome the shear strength of your weakened screws?

Or imagine a hard chuff that bounces the rocket on the rail - we've all seen those. The rockets scoots up the rail and then drops back down. Could that cause the screws to shear? What then if the motor ignites?

I'm not saying your idea can't work, just that there are a variety of failures modes that need to be considered. Think of it from an RSO standpoint - if something goes wrong they have to defend their decision to let you fly that rocket.


Tony

 

[AlphaHybrids]

Imagine a hose clamp around a 1010 rail tightened. It shears off the buttons, the buttons slide under the hose clamp.

As you hopefully see, I'm going about trying to test and verify this idea before just throwing it on a rail. This is a lot more than I see when people propose new ideas and their first test is when they fly their rocket.

I've tested the shearing in a drop test that simulates flight. I'm hanging weight off the side of three buttons tonight to quantify how much force before something snaps. I've quantified how much it takes a single button to shear in single shear. I agree it ultimately is up to the RSO on the flight.

Hard chuff - I agree that is an issue. That issue can be mitigated by proper motor testing and igniter design. This is a hybrid motor. I have 10 successful static tests of this motor. All of them come up to full thrust in less than a fifth of a second. The rocket is supported by a L-Bracket that the weight of the rocket rests on that instead of buttons.

Quick calculation on crosswind. If I take my maximum crosswind to be 10 mph (which is very high, I'll likely want less than 5mph for this flight) the force generated on the rocket is ~12 pounds. I assumed a cross sectional area of 3.17 square feet (which is about 20% more than there is). If this load is spread equally on all guides that is 4 pounds per guide. This is less than what I observed the individual shear of each button.

I'm doing this research to see if it can be an effective alternative to a tower. I agree towers are a good solution, but rails are much more ubiquitous at launches. Fly away guides are good, but I'm much more concerned with one of those taking off a fin because I've seen it happen, and in videos I've seen them come very close. I feel much less confident in those because the way it releases is less controlled. How far it flies away depends on the speed of the rocket, the moment it releases, the grip it has on the rocket, etc. And what if the grip of the fly away guides slip and are impaled on the front edge of the fins? You could what if every single scenario to death. Engineering is about quantifying the risks and seeing if they can be minimized.

Edward

 

[AlphaHybrids]

Last night I did some more tests of just straight vertical drops with less and less weight until they didn't snap. After my second test I realized that the piece that shears needs to be much simpler. Grooving a plastic screw is not as repeatable, or accessible to fabricate as I'd like. With that in mind I'm switching to some OTS nylon set screws from McMaster. To insure that they snap cleanly drilling a hole through the middle of the set screw is much, much more repeatable than grooving the outside. Using 10-32 threads I can control the amount of material left between the minor diameter and drilled out cylinder. This also builds on the previous work the HPR has documented in shear pins. I'm also re-designing the rail buttons so that they have a bevel on the side that they slide under the hose clamp. Once I get the new setup I'm also going to experiment with hose clamp distance to the end of the rail so momentum of the button will keep it going and it will exit the T-slot.

Edward

 

[heada]

Are you still considering a soldered threaded brass rod or was that ruled out for the same repeatability issues? How are you using the set screws to hold the buttons on versus what I've seen as standard button head or flat head screws? (button head on 3 piece, flat head on single piece)

 

[AlphaHybrids]

The brass rod has the same issues as grooving. You have to get the plane just right, it soldered just right, etc. Lots of futzing. The buttons I machined and tapped for 10-32 threads. They screw on to the set screw in the PVC pipe airframe. They are typical 1515 buttons. I'm going to re-make them to be double beveled.

Edward

 

[heada]

So you're treating the set screws as studs that the rail buttons screw onto rather than using a screw through the button into the airframe?

 

[AlphaHybrids]

Yes. That is correct.

Edward

 

[heada]

Any more testing with this? I'm very interested in how well the weakened set screw sheared. Also if the shearing force would have any impact on the stability/trajectory off the rail.

 

[AlphaHybrids]

I got my pack of 100 10-32 nylon set screws and have used approximately 50 of them testing. I've been using a #44 bit to drill through the set screw. Just a bit shy of the hex to install which is a 3/32" hex wrench. I've been using my standard three buttons and a drop of 6 feet until the first button contacts the hose clamp. I'm using 3 feet of 2.5" SCH 80 PVC as my 'airframe'. Just the airframe is ~4.2 pounds. I added an additional 5 pounds of lead balls inside.

I have been able to shear all three buttons consistently and more than once chip the end of the PVC on my garage floor. I haven;t been paying attention to the impact on the concrete deviation, but I could put a piece of cardboard below the rail and note impact location. The set screws shear a little more 'rough' than a groove, but are much easier to manufacture.

Edward

 

[Random Flying Object]

Where do you plan to launch?

 

[AlphaHybrids]

Northern Colorado Rocketry, when I'm confident that it will work. If I can't get repeatable results, I'll pursue other options. I'm currently going through my airframe bin to see what I have that would be smaller and I could fly on a K-ish motor as a proof of concept.

Edward

 

[cerving]

How about having retractable rail buttons that pull out to mount on the rail and retract flush after leaving the rail? Obviously it won't work with an MD rocket, but it would fun to play with.

 

[heada]

How about having retractable rail buttons that pull out to mount on the rail and retract flush after leaving the rail? Obviously it won't work with an MD rocket, but it would fun to play with.

I'm envisioning spring loaded rail buttons but wouldn't they need too much room near the aft centering ring? At a minimum you'd need 1/2" or so just for the rail button itself so at most your airframe is an inch wider than your motor. More than likely you'd be an inch or more for the button so you're multiple inches between airframe and motor. It is an interesting idea.

Steel threaded insert as the core for a delrin/plastic button and held on with an electromagnet inside the airframe? Flip a switch and the electromagnet turns off and the buttons fall away. Again, couldn't be done with MD.

 

[Walldiver7]

Edward I like your idea (and testing of this concept). I have flown many rockets from towers and have never appreciated what the process has done to my paint jobs. FARGs (Fly Away Rail Guides) have broken.. been lost for a time, and have a tendency to slide back onto the leading edge of the fin ...which can cause the FARG to apply more pressure/friction to the button-rail interface. Rail guides that shear off at the top of the rail appear to be THE solution. Please keep us posted. Pictures would be greatly appreciated.....

 

[Ez2cDave]

So, basically, after all this discussion, it is safe, unless it's not - LOL !

The effect of launch lugs / rail buttons diminishes, as rocket size increases.

Personally, I would never, potentially, compromise safety at the expense of a relatively small amount of performance, especially on an HPR rocket.

Dave F.