New to rocketry, first time designing a rocket with rail buttons. Apogee's tutorial said to put the top button above the CM. My CM falls right on a transition piece, and I would need a standoff to make it above the CM. Physics wise, I don't see the reason why it needs to be above the CM, in fact keeping them lower keeps both on the rail longer while it picks up speed off the rail. I want to put them near the bottom and about 13 in from the bottom on a 27 in rocket, CM is about 14 in from the bottom.
Does the button need to be above the CM? Why?
There's CG(Center of Gravity), and CP(Center of Pressure. What's CM? Bottom will called "Aft", top or nose is referred to as "forward". Use a pencil to mark these, the first habit to get into. Is when you open the box, before anything. Sand down all the tubes, inside and out.
Apogee has videos on this too(CG/CP), you need to understand what both are, 100%. To be safe and successful.
Either way, you'll want your rail buttons or launch lugs spaced out equally from the CG(Center of Gravity), when the rocket is done. I will build the rockets, then before finally assembly install or glue the backing plates in permanently. Being mindful of the rail button placement(to avoid assembly issues). I usually wait until right before final aft CR(Centering Ring)assembly to glue the aft one in permanently. You can drill the holes as soon as you find your CG before assembly, or right before primer. But I prefer to do them first. Center of Pressure comes into play during flight, I can't explain all of this out. Check youtube.
You have 1010(usually size #8 machine screws)rail buttons, which a little lesser known fact is they can support up to 75 lbs. Some also work on 1515(usually #10 screws) rails. I myself, sometimes will use both 1/4" lugs and 1010 rail buttons. Or, 1010 and or 1515 buttons(So you don't have to wait to use a rail launch days, or need a longer rail, my L2 is 91" long). It really depends on the build, motor being used, fly weight, flying conditions that day, thrust to weight ratio, etc...You can also use some of the 1515 hardware with some of the 1010 buttons, depending on whose you are using. So you could have one set of well nuts, or two 1515 well nuts, 2-#10 machine screws and use 2 of the 1010 "buttons", and switch them out if you need too. I also use a small amount of LocTite on the threads.
I usually just add 1010 to one side, and now 1515 to the other. 1010 rail extrusion is 1.0" x 1.0". Most High or even Low power clubs(I'd call the club or check the site for equally info)have one 1010 rail)will have several 8 ft -1010 rails, and or 10 or 12 ft 1515(it depends on the club)rails, then a Unistrut Tower maybe, or at least one 1010 rail you'll have to find out. Most clubs have one, or guys build there own to take.
The 1515, is 1.5" x 1.5". That rail is obviously thicker, and will be longer for bigger, heavier rockets. Keep in mind the minimum pad distance(L1/L2/L3) to the flight line. Or If you have a heavier rocket with a lower thrust to weight ratio a longer rail is what you want, or the RSO may tell you no-go. You'll be very disappointed when you realize you spent months on a build or design that isn't stable or too heavy for the motor you have.
I'm pretty old school as well(not braging different techniques were used), I can usually look at it or hold it, and tell you if it will fly or not. You see a cool design, an RSO sees a weapon capable of Mach One with it stopping when it wants to, or when it has too its. Either way, if its unsafe or unstable, you wont even make it to the pad. So do your homework, and you'll be fine.
There's many online calculators from Thrustcurve.org to a black powder calculator for DD(dual deployment). The whole point of a rail or rod and its length. Is to keep the rocket going straight when it exits the rail at a fast enough speed that it will stabilize and continue to fly straight up. That being at a certain velocity leaving the rail to prevent "weather cocking". That's when your rocket isn't going fast enough on its on yet to properly use the fin area for stability. Download OpenRocket, and watch a YouTube tutorial on it. Thats another great spot for resources. Stay away from certain Facebook pages and advice, trust me on that.
Many companies sell rail buttons, I use rail-buttons.com. I still have some good(full kits they sold, R-B.com is great too!) 1010 Doghouse Rocketry rail buttons I got 10 plus years ago too, but they are running out. Some rail buttons will be a machine screw and nylon parts, other companies may include well nuts too. They are sold in 1 pc or 3 pc from R-B.com. You'll also want backing plates, I usually use a piece a half inch to 3 quarters of an inc squared. Then a 5/16"(depending on the button)hole for the well nuts in the plate. The machine screw holes in the airframe I use a bit just bigger than the machine screw threads, it usually a 1/8" drill but. There's many threads about rail buttons, as even above some are listed. One thing guys do, is during assembly of the motor tube and centering rings. They permanently put the rail buttons in first, not realizing they should have done the buttons after that.
Thin CA is great for tacking parts together, Google it. Hobby Lobby sells it too, again BSI is top notch. Thin is basically water, about only good for "tacking" or inside the upper airframe to prevent zippering from a kevlar shock cord. You can make or buy "zipper stoppers" too. CA usually comes in Thin, Medium, and Thick. Have you heard the saying: "measure twice, cut once?".
Well with rocketry for me, you'll measure about 10 times, hold the knife on it, then do that about 50 more times until you decide to finally score or cut it,
. That happens to all of us. Making templates, using a paper towel, or aluminum foil rolls to practice techniques of high power building on spare or junk stuff first can really help you later. Then start using high power build techniques, and adhesives for low and mid power builds. Take your time, if you have any doubts just stop. Its also very easy to mess up, but guess what? Its even easier to fix it, when you start to look ahead and prepare. You'll have many failures but your success comes from learning from those, learn and adapt.
Now what glue do you use?!?! I use Titebond II for general assembly, Titebond Quick& Thick for fillets, slightly watered down Elmer's carpenters glue for all couplers one side at a time until they dry. Then a mix of 5 min and 30 min epoxy from Hobbylinc or BSI(Bob Smith Industries) from Hobby Link in person. Elmer's and BSI I get at Hobby Lobby, Titebond from Walmart, etc..5 min epoxy I'll use for couplers, everything else I use 30 min epoxy.With glues or epoxy you'll chose when and where you use them, even plain old JB Weld can be used with fiberglass or plastic model rocket components for outer fin fillets. That can survive Mach flights, its been done recently too go check out Rocket Vlogs on YouTube by Braden C.
It depends on what material you are using, that will decide what you need. It makes no sense to use epoxy with cardboard and plywood when the material you are using isn't even as strong as the bond you made. But, you can decide for yourself. I like using epoxy for cardboard couplers if necessary, you'll see why, and also for coating parts or the inside airframe itself from ejection blasts and carbon residue.
That should be some info to get you started, the forum is full of posts like this, even if you Google exactly what you are looking for, something will be in the search results regarding the forum Id almost bet on it. Take some time and look through the different topics, you can also be a supporter if you wish. I will asap, as I've been a member of this site since 2012, for me personally its time to give some back. Anyways, read, ask, read, go observe in person, read, and read some more. I'm not sure how far you want to go, plan ahead. Preparation is the key for this hobby. Its a very large world once you step into it, welcome to the rabbit hole my friend. You won't be disappointed.
I am really a fan of this book below too, even though its for high power it will educate you more than enough for low or "mid" power builds. Theres also books on low power Rocketry or in general. Think, ask, be safe, execute, repeat. Be mindful, for many guys saftey is number one. It should be, anyone thats doing something stupid to jeopardize the hobby, should be called out respectfully and appropriately. Especially if it's high power. You'll also have the best luck joining a club, but first either NAR or TRA, or both of you want, if you haven't already. Most clubs are not super expensive with memberships and you'll learn more in one day then you will in 10 years reading online. CG, is center of gravity, CP is center of pressure. Remember them?
One last thing, if you don't know your CG/CP for the rail button install( which as mentioned above, they should be equally spaced out from the CG, or Center of Gravity). The rule for me, is one inch up from the aft airframe forward, and one inch above the top CR. That's if I'm unsure of exactly where the CG will be. You can also use more than 2 rail buttons, if you need too. Once your centering rings are thick enough, you can even drill into them for the buttons. That's a little ways out yet, I'd highly again. Suggest this book, it will answer all of your questions. He uses OpenRocket too in for 2 builds from L1 to L3. It's a high power book, but it has everything you'll need as your going forward. Just my opinions and suggestions..
Have fun, be safe.
Michael B.
Apologies for any grammar errors, ill fix them shortly.
another year, another rail button thread
