DIY Rail Buttons for MPR & LPR

[Peregrine Falcon]

I was having an issue getting something to work (question in another thread) and decided to post an experimental solution over here since I have one.

Below are pictures of what I came up with for rail buttons for large LPR and some MPR rockets. I'm interested in opinions to see how well people think they'll work. I haven't flown this one yet to test it but hope to soon.

Construction (per button) is one #8 nylon screw, 2 #8 nylon nuts and one 1/4" washer. The screw head wasn't enough to hold onto the rail by itself as it could work it's way out so I added the washer. The washer is actually glued onto the screw head (CA).

As you can see in the end-view picture there is a nut on each side of the BT to hold everything in place. On the foward nut I was able to clip off a little excess screw with precision pliers.

I didn't get the aft button as narrow as I wanted due to the tight quarters (as you can see). It still worked, the forward button is tight enough that the little bit more play in the aft button doesn't make much difference.

After getting the screws and nuts positioned right I fed a little thin CA into the joint between the screw and top nut hoping it would feed all the way through and cement the whole button together.

Advantages: lighter than a rail-button with a steel screw (maybe?) and a little more practical for lighter/smaller rockets. No ordering parts, you can get these from a local hardware store and not pay or wait for shipping.

Disadvantages: they're still a little bulky in size for real small rockets but rods still work well for those anyway; likely not workable for HPR rockets due to weakness and potential they could break off.

I priced the same setup on McMaster Carr and if you buy the pieces in 100 packs you can build a set of buttons this way for under $1.

[Peregrine Falcon]

aer-o-dy-nam-ics: atmospheric interactions with moving objects.

Not sure I get the full point of your comment. I didn't think it would be any less aerodynamic than regular rail buttons. I don't fly competition so a loss in altitude due to increased drag isn't a big thing for me.

What would be HUGE for me is if the aerodynamic properties (rather lack their of) of that design destabilized the flight. If that is what you're implying please clarify so I can try something else.

HUGE problem #2 would be if it wasn't strong enough and broke off before it reached the top of the rail; likely causing a destabilized flight or who knows what else.

[SMR]

HUGE problem #2 would be if it wasn't strong enough and broke off before it reached the top of the rail; likely causing a destabilized flight or who knows what else.

That would be my only concern... nylon bolts are what a lot of us use for shear pins, they definitely are not as strong as a metal screw would be, so the possibility of breaking one off is real. One scenario would be crosswind + tall rocket = lot of torque on rocket which creates shear force on top button. Nothing to do with size of motor installed.

You can put that in the same drawer as open eye-bolts. Saves a little money, probably be okay most of the time, but the one time it fails is catastrophic. So, I would recommend not using a nylon bolt. The nylon washers and a roller of some sort would be fine with a real screw up the middle, though. Or, invest in a few actual rail buttons and have one less thing to worry about at T minus zero.

[dixontj93060]

You can actually get all the exact parts used for the nylon rail buttons sold on railbuttons.com from McMaster-Carr, and yes, very cheap per rail button. Only problem is that they don't have the full complement of Delrin spacers/washers (at least last time I checked). But given how cheap they are you can afford to switch out the nylon every five or six launches (or so)--I've already bought my "lifetime" supply

[MarkII]

Back in the late 1960s, Estes Industries sold a product called "C-rail," which was meant to be used with the LPR kits that they produced at the time. It apparently was quite a bit smaller than the 80/20 rail that we are all familiar with now. There were no rail guides that were sold for it; instead, the builder created a T-shaped brace out of hardwood that was surface mounted. The other recommended method was less scale-like but far more versatile, though. It involved gluing the launch lug onto a hardwood stand-off. This made it possible to launch the rocket off of either C-rail or a standard launch rod. I never had any, but I would sure like to find some type of comparable stock today. No luck so far, though. I don't know how popular C-rail ever was, but Estes only sold it for a few years.

[stickershock23]

I order 2 nylon washers 1 nylon spacer and a package of 8/32 screws from Mcmaster.

You get 100 buttons for about $20 including shipping. they fit perfect and have never had one fail.

[georgegassaway]

I use Plastruct "H" Beam as rail lugs. The 3/8" wide "H" Beam works well with 80/20 type rails that have 1/4" slots. It is a loose fit, but that is what I like about it - very little sliding friction or binding compared to "buttons" I have tried out.

- George Gassaway

[geof]

You can actually get all the exact parts used for the nylon rail buttons sold on railbuttons.com from McMaster-Carr, and yes, very cheap per rail button. Only problem is that they don't have the full complement of Delrin spacers/washers (at least last time I checked). But given how cheap they are you can afford to switch out the nylon every five or six launches (or so)--I've already bought my "lifetime" supply

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I order 2 nylon washers 1 nylon spacer and a package of 8/32 screws from Mcmaster.

I don't know much about sizings of washers, spacers, screws/bolts. Is either of these descriptions enough for me to find the exact items on McMaster-Carr to fit 1010 rails?

Geof

[Peregrine Falcon]

One scenario would be crosswind + tall rocket = lot of torque on rocket which creates shear force on top button. Nothing to do with size of motor installed.

That was definitely something I wouldn't have expected without being told. I would have figured the force of the motor would be the major factor. The tallest rocket in my build queue currently would be over 40" tall w/a BT-80 tube. I was planning on using regular rail buttons for that though, I do have 2 sets on hand.

I was looking for something to work better with smaller rockets, the one in my pictures above is under 36" tall and the lower tube with the "buttons" attached is a BT-55. The solution I think I've settled on for smaller rockets (BT-50/55? and down) is a 6' by 3/16" rod I can get from a local hardware store for $5-6; I have a PVC launch pad already and a friend gave me a drill chuck he didn't need last night.

[Peregrine Falcon]

I don't know much about sizings of washers, spacers, screws/bolts. Is either of these descriptions enough for me to find the exact items on McMaster-Carr to fit 1010 rails?
Geof

Yes and no. To share my experience and ask a few questions too... 8-32 is a dimension easily found on McMaster Carr. The site has explanations of the dimensions, the first number refers to the size, the second number refers to the thread count; the first number is the big one to watch. You would want to find #8 washers, nuts etc with anything else you get to go with a #8 screw.

The spacers and washers could be several different dimensions. I used a 1/4" washer on top but you could get away with smaller washers IF you used a spacer. The size of the screw itself would allow enough play that the head or washer (if too small) could work it's way out of the rail. The spacer increases the size around the screw giving the top less play to get out of the rail allowing for a smaller washer or screw head on top. The 1/4" washer is enough to keep it from coming out REGARDLESS of the shaft below; personally I think it's almost too big around though. Spacer length should be measured, would need to be small. I don't have that measurement on hand to provide but putting a ruler up to a rail cross section would get you that answer easily. McMaster offers multiple dimensions as search paramaters.

My question is why rail button sets nor the homeade combinations above use nuts to secure the button to the rocket? I can imagine that to make it work without a nut you put some kind of wood backing inside the rocket for the screw to attach to. Wouldn't a nylon nut (w/metal screw) be easier and more secure? CA could still be used to bond them securely and keep the button from working it's way out. I suppose on HPR models if you're using plywood as backing it would work either way but I can't see getting away with that using balsa.

MarkII: That Estes C rail sounds interesting, would definitely work. I remember seeing another member on this form mention that he made his own rail once with 2 pieces of aluminum angle. You could use that method to create a rail with a custom sized channel allowing for much smaller "rail buttons" usable on smaller rockets. I already made custom balsa extrusions for surface mount and potential use in a 1010 rail but I was too affraid they'd break. They were still a little too big and bulky looking for smaller rockets so I think I'm going to head toward the 6' rod I mentioned above.

[Rex R]

I note that; Johnson metals lists 'open seam tubing' that could be used as c-rail. hth
rex

[geof]

My question is why rail button sets nor the homeade combinations above use nuts to secure the button to the rocket? I can imagine that to make it work without a nut you put some kind of wood backing inside the rocket for the screw to attach to. Wouldn't a nylon nut (w/metal screw) be easier and more secure? CA could still be used to bond them securely and keep the button from working it's way out. I suppose on HPR models if you're using plywood as backing it would work either way but I can't see getting away with that using balsa.

Assuming that your first sentence is missing a "neither", I can suggest a few reasons from personal experience. First, anything that passes all the way through the airframe wall might cause the chute to snag. A few times, I have made a smooth glob of epoxy or epoxy putty to smooth out the protrusion. Second, for mid-power rockets, the buttons don't have to be bombproof. I have sometimes trimmed the bolt so it is flush with the airframe interior, then epoxied the bolt in the hole and the bottom of the button to the airframe surface. Third, ideally I would prefer to sink my button bolts through the wall where they can go into the (plywood centering rings). I'm sure other people have good reasons as well, and I would also be interested about the fraction of people who do/don't use something like a nut, split by whether they are mpr or hpr.

Geof

[Peregrine Falcon]

Oops on the grammar, you caught what I meant though and thanks for the answer. I guess I hadn't thought about the chute snagging. I cut the leftover bolt as flush as I could get it and I'm not that worried about things snagging on the nut but may be I should be.

Future models (including my current build thread) will use a parachute bucket eliminating this worry.

[MarkII]

For LPR and MPR, George's suggestion of Plastruct H beam (available from a hobby shop) sounds interesting. Just surface mount it to the airframe.

McMaster-Carr has technical drawings right on their web site of just about every hardware item they sell. These provide exact dimensions.

I have sometimes daydreamed about making my own miniature unistrut out of a combination of various pieces of brass or aluminum stock to use as a low-power (even MicroMaxx) rail. (For example, square brass tubing with two lengths of brass angle soldered onto opposite sides of the tube so that their right angle sections almost, but not quite, meet.) Open-seam tubing? That might be the ticket. For C-rail, a third type of guide that Estes showed was a pair of pan-head nylon screws. These might have been durable enough for LPRs.