A few Questions

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Iceman1979

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Hi,

I'm building a couple of rockets, one for myself and one for my Wife. I'm in the design phase of it right now with some of the parts collected so for. I have some question about this that you guys would be able to answer for me.

Background: The rockets are 3" diameter by 49" tall (including nose cone, 36" body tube). I've purchased the tubes, nose cones, motor mount tube, and centering ring. The centering rings are aircraft plywood and are 1.880" thick. The phenolic MMT is 18" long. I got the MMT and centering rings, and plastic nose cone from publicmissiles.com. The BT was purchased from Staples and everything seems to be fitting together just fine.

Questions:
1. Do I need the full 18" of MMT or can I cut that back to say about 12"? It came as a MMT pre cut to that length so I'm not sure if that is supposed to be that length or if it is just that long and you can cut it down to what size you need.

2. I'm using rocsim 8 to design the rocket and it is being modeled after the Aerotech Initiator. The fins are the same size and shape but the rest of the rocket has been adjusted to the part sizes I have. Currently rocsim says my rocket is over stable because I increased the length of the rocket and the size of the fins. I can reduce the size of the fins and get it to not say overstable. The body tube and fins are going to be fiberglassed and I don't see a way to account for that other than overriding the mass of the fins and BT. I wonder if changing the mass of the Fins and BT will change my overstableness of the rocket and if so then how to I calculate the new size of the fins to be stable with out knowing what the mass is going to be?

Thanks
John
 
Ok, let's clear up something first.

You said, "The fins are the same size and shape..., " but then the next sentence you say, "Currently rocsim says my rocket is over stable because I increased the length of the rocket and the size of the fins." Do you mean the fins were increased proportionally to the body tube size?

In any case, being over-stable is not usually a bad thing. The rocket will tend to fly into the wind a little more.
 
With only a 36" body tube minus the amount the nose sets inside the body tube I'm pretty sure this rocket's not going to be flown on an I motor, that's assuming your motor tube is for 29 mm motors.

It would be ok to shorten the motor tube to 12" as that is longer than a 29-240 Aerotech case. I have hung a few cases past the motor tube end before. I used a Nomex shock cord mount protector to make sure to prevent burn through of the anchor strap.
 
Royatl,

Ok, Sorry about that confusion there. I guess that statement was an unfinished thought, not sure where I was going with that.

The fins at this point are the same size and shape of the Initiator. The BT's diameter and length have been increased to match my actual body tube.

Dave Carver,

No, i was planning on a G class motor for this rocket and the MMT is for a 29mm RMS. It looked a little long to me and if its not there for ejection gasses then it's just dead weight IMO.

Ok, I forgot question number 3. I ordered 3 centering rings but after looking around I see most MMT's with just two. Is 3 centering rings to much, should I just go with 2 rings, one at each end?

Thanks
John
 
I'd leave the motor tube long, and use the third centering ring at the top of it (reduces the volume of space the ejection charge has to pressurize).
 
The CRs are probably not 1.8in thick. If they are, replace them.

Overstable is not bad unless perhaps it is outrageous. Also, hopefully you looked at the stability calculation *while* the largest desired motor was in the rocket. The rocket will appear very overstable if you haven't inserted a motor.

The motor tube length doesn't really matter. If you keep it at least .5in longer than the tip of the casing (or even more) it will keep any laundry from uncapping the ejection charge accidentally when you stuff it in there while prepping. That leads to poor performance :)

With a 36in body tube and 18in motor tube, are you worried about fitting in the chute and shock cord? If so, you could shorten the motor tube a bit.

I would not use the 3rd CR. Not needed for strength. Depending on how thick the CRs really are, you could double up the aft CR if your motor retention system requires it; otherwise skip CR3.

Before you get too far, consider the motor retention system and the launch guide system. For the motor, what kind of clips/bolts/etc will you use, and how? I would not recommend using an Estes clip. In a pinch, friction fitting or Estes clips with tape wrapped tightly around them at the nozzle end would suffice, but IMO neither is a good choice. Trying to remove a friction fit motor is especially frustrating.

For guidance, launch lugs or rail buttons? Rail buttons are so much fun, if you or your club have a rail. If you use buttons, consider how to install them before you get too far into the build.

Fiberglass is not necessary or desirable. Pointless and heavy.

For good flights on a G motor, aim for about 32oz or less, in my opinion. (No expert flames, please, I know this is a totally subjective and unjustified opinion, but is provides a nice snappy launch (safer) and a higher flight is fun). Fiberglass will make that weight goal pretty difficult to achieve.

In Rocksim simulations, look at "Flight Details" (with a right click or on the Simulation menu) and scroll down to find the velocity at the end of the rod. Aim for around 45fps or more with your (weakest) intended motor. (Another case where experts can be comfortable with much smaller numbers based on experience).

Finally, make sure your wife wouldn't be happier with a new dress, an MP3 player, or a romantic weekend away! If she's dying for a rocket, congrats and send me her phone # and a 5x7 color photo.

I apologize if you know all this. I didn't mean to condescend. I just sensed from your emails that perhaps you were fairly new to rockets/motors of this particular size. If I'm wrong, then obviously you can filter my advice accordingly.

Have fun, and be sure to post photos!

Geof
 
I'd leave the motor tube long, and use the third centering ring at the top of it (reduces the volume of space the ejection charge has to pressurize).

I will second that... I am building one of similar proportions... The main tube is 3" diameter and 31" long, the engine mount tube is 18".
 
Re: leaving the motor tube long and forward CR....also helps prevent the laundry from shifting rearward. Keeping the chute and tackle near the top half of the rocket helps keep the CG in check. May not be all that important with your particular rocket, but it can't hurt.
 
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I agree with Geof on most things. It would be really boring around here if we agreed on everything.

I would keep the 18" motor tube and three CRs. One CR at the base for motor retention, one at the top of the fins, for strength, and one at the top of the MMT as an attachment point for recovery harness. I am assuming Througt The Wall fin mounting to the MMT. You might want to cut the MMT down to about 15". The 29/460 Loki case is only 15 1/8" long, but only if you feel you need the extra space for recovery.

The fiberglass is not needed if you are going to fly G motors. But, in a year or two from now, after you and/or your wife has your L1 and/or L2 and you happen to be at a large field on that perfect day with no wind, you can stuff that I motor in an see if it will really stay together. If you find you do fly an occational H or even an I in it, you might want to put a couple of layers of 4 or 6 oz glass on the tube. I only say this because you said you were getting the tubes from Staples and the shipping tubes are not always the same quality as tubes made for and sold by rocket vendors. Besides, a couple of thin coats of glass on the tubes might prevent a zipper or some other problem and maybe extend the life of the rocket a few extra years.

As for 32 oz with a G, in my opinion, that is a good max for G motors if you intend to fly H also. If you plan to stick to G as a max, I would try to stay down around 20 - 24 oz. so the larger F motors will also work for safe low altitude flights and the Gs will get some altitude.

Anyway, have fun, build light and use a large chute. Transport and landing is a LOT tougher on fins then the flights.
 
The CRs are probably not 1.8in thick. If they are, replace them.
oops, 0.1880 is the correct measurement

With a 36in body tube and 18in motor tube, are you worried about fitting in the chute and shock cord?
That was not a concern of mine, I was just asking if It was needed or not. Seems that it's not that big a deal one way or the other. I'll probably just leave it long then.

Before you get too far, consider the motor retention system and the launch guide system. For the motor, what kind of clips/bolts/etc will you use, and how? I would not recommend using an Estes clip. In a pinch, friction fitting or Estes clips with tape wrapped tightly around them at the nozzle end would suffice, but IMO neither is a good choice. Trying to remove a friction fit motor is especially frustrating.
I was going to use a metal strip bent at the end to secure the motor but I really like the aluminum motor retension systems found on PM and Apogee's website.

For guidance, launch lugs or rail buttons? Rail buttons are so much fun, if you or your club have a rail. If you use buttons, consider how to install them before you get too far into the build.
I'm have launch lugs scoped for this rocket at this time.

Fiberglass is not necessary or desirable. Pointless and heavy
The BT is not a phenolic tube. It is just a cardboard tube .060 wall thickness and I figured it needed to be strengthened up a bit. The fins are balsa wood and I know they are to weak as is. I could go with some basswood for the fins I guess.

In Rocksim simulations, look at "Flight Details" (with a right click or on the Simulation menu) and scroll down to find the velocity at the end of the rod. Aim for around 45fps or more with your (weakest) intended motor. (Another case where experts can be comfortable with much smaller numbers based on experience).
It says 44.4fps in rocksim on a G64 which is the smallest motor I was going to use.

Finally, make sure your wife wouldn't be happier with a new dress, an MP3 player, or a romantic weekend away! If she's dying for a rocket, congrats and send me her phone # and a 5x7 color photo.
My wife is a tomboy and its like pulling nails out with your teeth to get her in a dress. Honest to god she would gladly choose a rocket over a dress (i'm not even kidding). The mp3 player she has already ;) She would go for the romantic weekend for sure but both my rockets are going to cost less to build then a weekend would where she would want to go. I'm also going to be buying a wood lathe soon and she cant wait to get her hands on it. She loves hands on projects, outdoors, etc... This started out as me building a rocket and only I only got the stuff to make two when she asked where her rocket was. I'll include a picture of her here just for you. The phone number... well thats just not going to happen, sorry.

I apologize if you know all this. I didn't mean to condescend. I just sensed from your emails that perhaps you were fairly new to rockets/motors of this particular size. If I'm wrong, then obviously you can filter my advice accordingly.
Don't apologize you are correct, this is a whole new arena for me. I used to shoot rockets with my dad years ago and the bigest thing we shot was probably a D size motor if that. I need all the help I can get here and I appreciate you going the extra mile here and offering suggestions beyond my questions.

Here is a link to two pictures of my wife. Yes this is my wife and yes that is a Glock on her side and yes the glock is her's.
My Wife
 
Ok, I have another question then... If I use the extra CR then how do you go about glueing the middle one in to the BT?

Here is what I came up with so far.
Glue the forward most CR to the MMT and then glue the MMT (with one CR into the BT, placing a CR at the end for support while the gue dries. Next I was going to take a dowel rod mark it with the length needed for the next CR and and then put some epoxy on the inside of the BT using the dowel rod. Next i was going to push in the CR to the expoxy and then add some more epoxy to the other side of the CR. and then the end CR should be easy to glue in.

What do you guys think?
 
I wouldn't necessarily glass the tube, even if it is just a mailing tube from Staples. If is is anything like the tubes sold at the post offices, and with a wall thickness of 0.060" (almost 1/16") it should be quite strong enough for a G or an H. Now, smooth... well that would be an entirely different issue. You could do just fine with a lighter, somewhat thinner wall tube, too, and save some more weight, but since you already have the parts, then go with what you have.

Resist the urge to overbuild this; you will just add dead weight and limit your motor choices (I have some experience with that). You won't need to build it like a tank in order to fly it on G through I motors.

What length of rod are you simming it in RockSim? Using a longer rod can give you a more stable flight. Also, be sure to set the launch conditions to reflect what you typically encounter at your launch site.

MarkII
 
Finally, make sure your wife wouldn't be happier with a new dress, an MP3 player, or a romantic weekend away! .

I had to scroll back to see which wise man mentioned this, just in case. Once my dad bought my mom the fishing rod he always wanted for her birthday. You can imagine how long that marriage lasted. :rolleyes:

Good luck with the build, Iceman. We'll look forward to seeing how the rockets progress!
 
thanks, I guess I could start an official build thread :) I have some photos of the parts I have so far and I'll be ordering some more this weekend so yeah Its time I guess.
 
Ok, I have another question then... If I use the extra CR then how do you go about glueing the middle one in to the BT?

Here is what I came up with so far.
Glue the forward most CR to the MMT and then glue the MMT (with one CR into the BT, placing a CR at the end for support while the gue dries. Next I was going to take a dowel rod mark it with the length needed for the next CR and and then put some epoxy on the inside of the BT using the dowel rod. Next i was going to push in the CR to the expoxy and then add some more epoxy to the other side of the CR. and then the end CR should be easy to glue in.

What do you guys think?
That sounds like a good plan to me. Are you doing TTW fins? Handeman suggested putting the middle ring at the top of the fins, and I agree, especially if you have fin tabs. You could glue the upper ring to the motor mount, and put two or three wraps of easy-release masking tape just above where the lower ring will go, slide the lower ring on (but don't glue it), and wrap the tube just below it with two or three more layers of easy-release tape. This will hold the ring in place temporarily as you slide the whole mount in.

When that glue has set, you can get to work on attaching the middle ring to the MMT and BT. One thing that you can do to help with this is to take some extra body tube or motor mount material, ideally with an ID that will allow it to just fit over the motor tube. Or you can use something that is the same size and just slit it down the side. Cut short lengths of this material, slip it over the motor tube, and glue it in place so that the lower edge is where the upper face of your middle ring will be. (Do all this before you glue the mount in.) If you so desire, cut another piece, slit it and slip it over the first piece and glue it on. When you do this, it will help to give the middle ring a secure hold on the tube, and, even better, it will allow you to slide the middle ring right up and have it be right where you want it.

Having the middle ring placed just above the top edge of your fin tabs (if you have them) will give you another surface to bond the tabs to.

As for the launch rod length, 36" is the default that RockSim starts with. Feel free to change that to a more realistic 48", which will give you a higher speed when the rocket reaches the end of the rod (and RockSim will display straighter flights in the 2D animation). Remember that your effective launch rod length is the distance from the lug (or upper rail button) on your rocket to the top of the rod or rail.

MarkII
 
Oh, yeah - one other thing. Built your rockets one at a time (not simultaneously). Do yours first, and completely build it before starting on the one for your wife. Then, when you built her's, you can avoid or correct all the mistakes that you made with yours. You see where I'm going with this...? ;)

MarkII
 
haha, yeah I see where you are going, thanks.

and to answer your question, yes these will be TTW fins with tabs. I even thought about making some rails out of balsa wood to glue to the MMT but I don't know how I'm going to get them in there unless I glue them to the middle CR and then glue them to the MMT once the middle CR is in place.

That might work actually. I could wrap the MMT in wax paper and glue the rails (fin alignment rails) to the CR. This will probably be hard to align once the CR is in the BT though.

Thanks
John
 
All very sound advice in previous posts..I will add my 2 plugs in as far as the CR's and motor mount and a little on motor retention...

Use the 3 CR's..Can't hurt..Mark your motor tube for the fin positions..Measure up 1/2" from the aft end of the motor tube and glue a CR..Take your fins(you will need a 2nd pair of hands for this-sure wife will help) and set them on top of the centering ring..Slide a 2nd CR down and snug to the top of the fin tabs..TACK a couple of spots with CA on this CR..Mark each fin 1, 2 etc and the line they correspond with as well..Now, remove the fins..Cut the slots in the body tube..Test fit your fins in the slots and mark them to the fin numbers that fit the best-you want them snug..Remove the fins..Drill a half circle of about 1/8" on each side of the fin slots at about the midpoint..Glue that 2nd CR to the motor tube, but fillet ONLY on the forward side..Slide the motor tube(once the glue/epoxy has set) into the body tube as a test fit, lining up the fin lines previously marked with their corresponding slot, slide in the fins..Things should be pretty secure..Remove..Now, glue your 3rd CR to the front of the motor tube(now would be a good time to attach an eye bolt to this CR for your recovery system)..Place a ring of glue/epoxy forward of the fin slots and slide your motor tube in..Insuring the fin lines again line up to their appropriate slots..Let glue/epoxy set up..Now 1 fin at a time with the rocket laying horizontal, 'butter' epoxy to the root edge of the fin, then slide it into the slot..Now, here is where those half circles you drilled earlier come into play..Using a syringe, inject some epoxy thru one of the half circles, then in a rocking motion move the front up and down to get the epoxy to 'flow' the length of the fin tab..Set horizontal, with the fin at about the 2 o'clock position to allow the epoxy to 'even up' on the fin/motor mount joint and once epoxy sets repeat with the other fins..And then do the same on the other side of each fin(injecting)..Once done with that you can then add a fillet on the aft centering ring..The injecting puts nice secure fillets internally at the fin/motor tube joint..

A little long winded but it works like a champ!

Motor retention..Well, I will let the pictures speak instead of me adding another chapter to my book!LOL..Parts list:

2 threaded inserts
2 bolts
Some nuts as spacers
2 washers

Looking forward to seeing pictures as you progress! Oh, and welcome to TRF!
 

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Couple more comments.

I don't understand the 3CR problem. If you're going to use all three, why not glue all 3 CR to the motor tube, then insert the whole thing into the body, then do the fins? This assumes that you pre-slot the tube, that the fin tabs have notches cut out, and that you haven't forgotten to attach the shock cord to the forward ring. Maybe I'm missing something here. (Note: Pantherjon posted while I was writing. I now understand that the concern is about interior fillets, I think? If so, I don't think they are critical. I would just make sure the tabs make good contact with the motor tube and the CRs, then coat everything in epoxy and stick it in. You can drop some extra epoxy on the CRs and MT using a q-tip or anything else to make sure it is all good and wet. Many G kits have surface-mounted fins; we're not going for our L3 here.)

Pantherjon also shows one of the best, cheapest ways to get positive motor retention. I've done it his way many times, except I replace the washers with little clips you can get from the hardware store. With about the same frequency, I reverse the setup. Then the head of the bolt is on the forward side of the CR. The head is epoxied to the CR. There is no threaded insert. The shaft passes downward through the CR and sticks out the back. On goes a clip, and then a nut to tighten it down. Both ways work great and save $20 compared to a specialized product.

Regarding fiberglass, I still say "no" but it could depend what you mean by "just cardboard". If it is like a tube from toilet paper or paper towels, throw it away. If it is like a mailing tube and somewhat rigid, you're probably okay. A body tube is very very rarely the point of failure of a rocket, in my experience.

Regarding the total weight, I'm not so sure about Handeman's 20-24oz limit unless it is for beer. He's an expert, but that seems pretty light to me. I'm working on a 3fnc design right now at 16.3oz empty and am getting 1700ft+ on most big Fs and up to 3000 on a G. For me that's verging on too high, depending on whether you want to find your case and your rocket. Of course, I actually think your concern should be on the other end: to avoid making it too heavy (e.g., > 30-32oz).

Regarding your wife and whether she wants a rocket......if that's her gun, then you better make doubly sure!

Geof
 
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Where: Pictures > words

https://www.rocketryforum.com/showthread.php?t=93

Shameless self-promotion alert!

This was my first attempt at making my own fin can and self-slotted body tube so I was really slow and cautious about doing all that. It shows about how I went with 3 CR's like you are thinking: one each on the fin tabs and one towards the front end of the motor tube.

And I'm surprised that I'm the first to say this but if you are going with just G power on wood and paper rocket, just about any yellow glue should ge fine. I used Titebond III on mine but TB II or just plain Elmer's carpender's glue would work. Epoxy is more dead weight.

To add some strength to your balsa fins, try laminating them with paper and thinned glue. Or switch to basswood.
 
Thanks for all the wonderfull advice guys. I'm all ears, well maybe eyes but you know...


I guess I was just concerned about the logistics of assembling a 3 CR setub and having it glued in properly.

Pantherjon,
You touched on some ideas I had for making sure I had the fins pre-aligned and numbered. The rest answered alot of unknowns for me too, Thanks for painting that picture verry well.

geof,
My wife was sitting right next to me when I wrote that reply. She laughed so hard when I showed her the pictures I posted. She was scared at first saying "oh, god which picture did you put up of me" When she saw the pictures she that it was fitting. The Glock is her's, I got it for her to cary but we never got around to getting her carry permit in Memphis. We now live in Orlando, FL and she is pushing for us to go get them for here. Also, after I wrote that reply she reminded me that we needed to find a shooting range down here. The only regret I have is after i tought her how to shoot she can now out shoot me, argh.

I will probably leave the BT un glassed. I guess I was thinking it needed it for a G class motor, of course I also wanted to try out vacuum bagging :D Having worked with R/C airplanes I understand the value of keeping it light and if the fiberglass is not really needed then I guess I can skip that part.

The fins I'm pretty sure I'm going to glass. I don't want to have to be replacing fins any time soon on these rockets especially with TTW fins.

A side note here, I'm not to worried about the altitude for these rockets yet as long as it leaves the launch pad safely I'm fine. I've been playing around with the motors in rocksim and I'm seeing 1700 - 2K on the altitude depending on the motor. I'm going to assume that 1500 - 1700 is more like it and thats fine with me. I don't have an altimiter or a dual stage recovery ejection system so not going to high will be a plus for now.

These two rockets are meant to be a starting point for me to get into the MPR and eventually HPR as well as a building refresher for me too.

Thanks
John
 
Judo,

Were you at the NEFAR rocket launch? I was reading your thread and I saw a picture of the 4 alike rockets and it reminded me of some rockets I saw there. In fact i have a picture of 4 similar rockets.

I'm including a picture I took at the NEFAR February launch.

NEFAR 02-07-09 Photon Probes.jpg
 
I'll claim laziness as I haven't read all the posts above. But, if you're doing TTW the easiest approach is to just leave off the last (bottom/closest to the nozzle end of the motor and the aft end of the rocket) centering ring until the very end. Glue the top one onto the MMT and put your eyebolt or U-bolt into it, glue the middle CR onto the MMT at the point just forward of the fin tabs, then glue the whole thing in place. THEN put in your fins and you have easy access to the fin/tube joints for doing internal fillets if you choose to do so (it also helps you make sure you have good fin to MMT contact). Put the last centering ring on LAST, after all the fillets are done. If you're using blind nuts for retention, put those into the last CR before you put it into place. Consider leaving enough room to mount an AeroPack or other commercial retainer onto the MMT to keep your options open.

Easy!
 
Thanks for that. I had seen this before but didn't know what it was called, now I do :)

Thanks
John
 
The balsa wood is in my closet, forgot to take a picture of that but it's nothing special just a sheet of balsa wood.

I'm hoping to be able to order the rouse-tech RMS and the motor retension system this weekend. I decided to go with the nice aluminum motor retension system this time around. I just like the way it looks and how easy it is to attach.

I'll be going by the hobby shop hopfully after work to pick up some fiberglass and building supplies. I'm going to test something I found on the internet. Using fiberglass and CA glue on the fins. If that doesn't work to my satisfaction then I'm going to facuum bag it with epoxy

I'll post some pics of the goodies later :)
 
Any suggestions for fiberglass oz weight for covering the fins? 2oz, 4oz or something else?
 
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