3.5" Gemini Titan

[stevem]

I've wanted an Estes Gemini Titan since I got back into the hobby.
However I cannot pay $150 on ebay for a bag of balsa and paper.
So I'm doing the next best thing, cloning.

I could kick myself for always forgetting about pictures until after I'm well into the build.

Built from a 3.5" mailing tube (boy aren't these fun to finish!)
the nose cone is made from 1" dowel, plywood centering rings and cardstock.

I am now trying to figure out the motor mount system. Does anyone know the angle of the MMTs in the Estes kit?

I am going to power this with a pair of F21-4Ws, my first AP cluster.

Here's a pic what I have so far.

 

[Stymye]

I'm not home to measure mine but I believe the object is to have the apex of the angles meet at the CG point.

ye old rocket shoppe has the templates in the estes plan section

 

[stevem]

thanks Andy! guess I was being too simplistic in my approach - never occured to me that the angles need to converge on the CG. I was just going to use the same angle estes had.

on the same subject, what do you think if a guy just put the mmts in straight with no angle?? wouldn't that work as well, just sacrifice some scale look?

 

[stevem]

I realize that old saying 'there's nothing new under the sun" applies to rocketry. However I live in a vacuum here in NW Iowa so the only reference I have is TRF.

I was trying to figure out how to make the Gemini Titan nose cone. Of course turning balsa was my first thought but I did not have any balsa blocks big enough for this project. I tried to come up with a different method using materials I had on hand. (trying to be cheap, wife thinks I spend too much on rockets )

Since I failed to take any pictures until after the NC was assembled here's a drawing of what I came up with. I sort-of thought it was unique but I'll bet someone on TRF has done it this way before.

made of hardwood dowel, lite ply, cardstock and West Systems epoxy

PS - drawing not to scale

 

[loojack]

Looks great! Gotta love these kind of projects! What about fins? Keep us posted......loojack

 

[JStarStar]

Originally posted by stevem
thanks Andy! guess I was being too simplistic in my approach - never occured to me that the angles need to converge on the CG. I was just going to use the same angle estes had.

on the same subject, what do you think if a guy just put the mmts in straight with no angle?? wouldn't that work as well, just sacrifice some scale look?

I'm already envisioning a BT-70 Gemini-Titan of my own... what I'm thinking about the engine bells is to simulate them with cardstock skirts applied to the exterior of the MMT tubes.

The cardstock skirts can be applied at a slight angle to simulate the slight canting of the actual engine bells in the actual GT vehicle. Then, the support struts can be made out of styrene or dowels.

Of course, it will be slightly off-scale, but you really can't get around the physical dimensions of rocket motors - the thing has to fly somehow.

I think your capsule idea looks good - since the GT capsule used actual conical geometry (opposed to curved elliptical shapes) it looks to me as though it ought to work.

 

[stevem]

thanks loojack,
fins - I hate fins on scale rockets like this but since its a necessary evil I am going to make an assembly similar to the original estes kit out of clear lexan and use some sort of fastening, maybe those removable plastic rivets (like Tango Papa uses on his 3.9 Mars Lander) to fasten it to the airframe for flight.

Was the original estes kit a friction fit for the fin assembly or was there something else holding them on?

 

[JStarStar]

I think a friction fit - I have seen several comments to the effect that the plastic fin unit tended to scuff up the paint job when you slid it on over the body tube.

You could probably check out JimZ's plans and see for sure, but I think that's how they worked.

 

[stevem]

Jstarstar - thanks for the tip, I like that idea. Put the MMTs in straight and cant the nozzles!

I love this place!

 

[stevem]

Jstarstar,

Scuffing up the paint does not appeal to me at all but the only other idea I can come up with is to permanently mount clear fins on it. not very appealing either but it would save the paint.

with 2 F's in this thing I am afraid a friction fit fin assembly may be left on the pad unless it is very tight, and then the paints gonna come off for sure.

life's all about compromises and so is rocketry.

 

[JStarStar]

I think the clear fin assembly configuration I'm thinking of is a clear fin unit that snaps/bolts to the bottom of the rocket, or maybe some kind of lock-slot setup if I can figure out how to do it. Probably nylon screws on the base of the rocket would do it.

Personally, I hate hate HATE "friction fit" - it's been my experience that it doesn't work right about 48% of the time. When I have used friction fit engine mounting, it's almost a given that either a) the engine is gonna eject at ejection time and the rocket will end up in a death dive, or,
b) the friction fit works TOO well and I'll never be able to get the engine out of the rocket without either ripping out the motor mount or crinkling the body tube in half. Either way, your rocket is shot and you can never fly it again, so phooey on friction fit is my official attitude.

With friction fitting external fin sets, I'd say it's almost guaranteed (with two F's ) that either your fins are going to peel off during powered boost, or else the fin tube would indeed scuff the heck out of the paint job. Neither one a very good choice.

 

[jflis]

You could try something like this. (see attached image)

I've not ever done this, but it is something that I have developed in my head while thinking of solutions to this very problem on the Gemini design...

You make a tube of clear plastic that is open along one side (like a seam) where the plastic is bent to form a lip that can take a screw. You attach 3 fins permanently to this fin unit and the 4th fin is pinched between the lips with the screws (the image will make this paragraph make more sense, beleive me ... )

anywho, it's a thought. It has the advantage of not damaging the paint on the rocket because it doesn't slide on and off

jim

 

[Bill]

Originally posted by stevem
Built from a 3.5" mailing tube (boy aren't these fun to finish!)

Not a bad choice. That makes it approximately the same scale as the Estes Mercury-Atlas.

There was some good ideas for clear fins and fin repleacements in the very similar thread https://www.rocketryforum.com/showthread.php?threadid=12269


Bill

 

[stevem]

Bill, thanks for the thread links, I don't know how I missed JStarStar's thread? I guess I should have posted on that one instead of starting this thread. Sorry 'bout that.

Jim, now that is a novel approach and I'm going to give that some thought. good idea!

On the other thread Bill refers stymye talks about making his without fins at all.

In order to do this I imagine you need a LOT of nose weight.

Would this work? If a guy could get by without fins that would sure be nice.

I didn't put any fins on my Ariann 5 and the swing test was good (with a good amount of nose weight added) but the first and only flight failed due to a booster not lighting so I still don't know if it would fly without any fins.

 

[powderburner]

Originally posted by stevem
what do you think if a guy just put the mmts in straight with no angle?? wouldn't that work as well, just sacrifice some scale look?

Obviously the best approach would be to have the individual motor thrust lines oriented to point through the rocket's c.g. That way, no matter what happens with the motors, you are not introducing any further de-stabilizing moments (or torques, for you non-engineering types). The key here is that the c.g. you should be aiming your thrust lines through is the c.g. with the opposite motor fully loaded (un-ignited). Not that this will make a whole huge difference, but as long as we are getting down to nit-picking then let's go all the way?

Installation of your MMTs in a straight-ahead orientation (MMT axis aligned parallel to rocket's longitudinal axis) will introduce a moment equal in magnitude to the motor thrust x the offset of the thrust axis. The moment generated by one motor will try to turn the rocket from the desired flight path. This is not a problem if all motors ignite at exactly the same time and generate exactly the same thrust because all of the individual MMT moments will add together and cancel.

The first failure mode you need to worry about (probably the primary launch mode) will be non-simultaneous initiation of the motors. One motor will reach peak thrust at a different time than the other(s). In most cases the timing difference will be quite small and the opportunity for one motor to 'turn' the rocket (before any subsequent thrust peaks 'turn' it back) will be minimal. However, if you are going for a no-fins rocket this could still be important. Pointing the MMT throught the rocket c.g. will minimize or eliminate this problem.

The second failure mode you might have is complete failure to ignite one (or more) of the motors. The same comments as above also apply here. Alignment of individual MMTs parallel to the rocket longtdnl axis will cause a moment that tries to turn the rocket, alignment of the MMTs pointed through the rocket c.g. will not cause a turning moment. The main concern in the case of motor non-ignition is that the rocket is now operating with less-than-ideal thrust, and will not perform to the nominal altitudes for recovery deployment. With a two-motor design, failure to initiate one of the motors means you have lost fully half of the thrust. You need to run some sims for this case to make sure that deployment occurs before impact.

 

[stevem]

great info powderburner - using AT F21-4w's in the past has shown me that getting 2 to light off at the same instant might be tricky. After reading your info I thnk its best to cant the mmts to the CG.

can't sim it - don't have rocksim.

 

[loojack]

Steve, How about an update? How 's the project comming?.....loojack

 

[stevem]

here ya go loojack -
work has been slow on this one and I am still struggling with the whole fin thing.

here's a shot of the completed mmt assembly

 

[stevem]

and here's the rocket as it sits now with all painting done. I just need to make up some decals for the rest of the markings.
You can see how big it is sitting next to my Estes Saturn V.

 

[powderburner]

Might not be much of a difference, but I think it is an improvement.

Originally posted by jflis
You could try something like this. (see attached image)
. . . .
You attach 3 fins permanently to this fin unit and the 4th fin is pinched between the lips with the screws

The concept that Jim posted will theoretically work----if everything is absolutely perfect. That is, if the diameter of the plastic wrap tube, after cinching down tight on that fourth (loose) fin, is juuuust the right size to remain snug on the outside of the BT without crushing or slipping.

I think it would be better to do the following: Bolt the fourth fin under the fasteners **outside** of the tube joint, leaving a small gap between the ears of the plastic wrap tube. That way, you have some adjustment space (note "gap" in my sketch) to get the right fit on the BT, and the fourth fin will still be held snug.

 

[Lugnut]

Originally posted by stevem
and here's the rocket as it sits now with all painting done. I just need to make up some decals for the rest of the markings.
You can see how big it is sitting next to my Estes Saturn V.

Wow that s a beauty! I just finished watching the Gemini DVD series. That looks great. Hows it progressing?