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  #1  
Old 05-14-2016, 05:38 PM
JediBoss JediBoss is offline
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Default Gemini Titan Gemini 8 John Pursley Special

Good Evening All,

Just wanted to show you all a beautiful model scratch built by John Pursley of Accur8 Space Models, and a master builder. It flies on 2 A10-3T motors, and flies finless. This is his prototype. If you want more information, contact him at johnpursley@accur8.com.
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Old 05-14-2016, 07:37 PM
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ghrocketman ghrocketman is offline
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How does it fly finless ?
Are the engine mounts severely canted to impart spin-stabilization ?
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Old 05-14-2016, 07:45 PM
JediBoss JediBoss is offline
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He told me that as long as the CG is ahead of the CP like for conventional finned rockets that it will fly straight and true.
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Old 05-14-2016, 10:28 PM
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mojo1986 mojo1986 is offline
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Is it just me, or do the engines look smaller than true scale?
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Old 05-15-2016, 07:48 AM
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Quote:
Originally Posted by ghrocketman
How does it fly finless ?
Are the engine mounts severely canted to impart spin-stabilization ?


John has done a lot of work on flying finless rockets. I've seen his actively stabilized gimbaled motor finless models using a model airplane anti-crash stabilization system and it's VERY impressive. He also mentioned that he's done some stuff with passive finless stabilization as well, including test models of his actively stabilized rockets, in a smaller scale. The key is keeping the weight well forward, since the CP is near the center of the rocket on a finless design. This means very "nose heavy" compared to a regular model rocket, as you have to counterbalance the weight of the motors in the rear, as well as move the CG as far forward as practically possible.

Also, it works better with rockets that have a long, slimmer upper stage. It can be made to work with a long straight-cylinder rocket, like the Gemini-Titan or Peacekeeper. However, "hammerhead" designs like the Ares I wouldn't work well, because the upper stage is a larger diameter (thus more surface area) which would move the CP *VERY* far forward, and make finless stabilization pretty impractical, if not impossible.

Later! OL J R
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Old 05-15-2016, 10:23 AM
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Royatl Royatl is offline
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as long as there's no pitch/yaw disturbance, a rocket should go in one direction and one orientation with no need for corrective action (i.e. fins). Once that disturbance is introduced, however, all bets are off. More nose weight helps keep things straight but, if a rotational disturbance begins, the nose weight amplifies the problem. It is all about inertia.

For short model rocket flights in calm winds, you can get away with it, if you don't mind some wobbly-bobbly as it slows down.
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Old 05-15-2016, 03:19 PM
johnpursley johnpursley is offline
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There's a WHOLE LOT of misconceptions being presented about in these messages about finless rockets...mostly, I suspect, from folks who've never flown finless rockets. Let me clarify, if I may.

Finless rockets obey and react EXACTLY the same way as finned rockets (though in many cases not as quickly) so long as the CG is ahead of the CP. The most obvious GENERAL difference is that the CG on finless rockets must be closer to the middle of the rocket because the CP is closer to the middle.

Finless rockets are typically less affected by wind than finned models. In windy conditions finless models tend to translate more in their flight path rather than rapidly change vector as many (over) stable finned rockets may do...finned rockets generally change vector rather than translate. This is based on my experience deliberately flying quite a few finless models in windy conditions...not just speculation.

Spinning or artificial "active" systems are no more required for finless models than for finned models. Again, this I know from experience. In fact, unless you have a very "fast and smart" active system, the model still has to be stable by CG ahead of CP

It is TRUE, however, that finless rockets don't aerodynamically dampen out out oscillations as readily as finned rockets because the CP of a finless rocket doesn move much, if at all, as angle of attack changes whereas (at sub transonic speeds anyway) the corrective forces of a finned rocket become much greater more quickly as the angle of attack increases...which is also an explanation of why some (but not most) finless rockets seem to "bobble" as they slow at apogee...at slow speeds and high angles of attack the forces to keep the model on a steady course aren't (generally) as great as for finned rockets.

---John Pursley

Last edited by johnpursley : 05-15-2016 at 03:37 PM.
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Old 05-15-2016, 03:25 PM
johnpursley johnpursley is offline
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To the post about the Gemini Titan engines looking "small"...

The engines on this model are actually larger than "scale" even when using 13motors. On this model the max diameter is just a tad larger than scale but the combustion chambers are significantly larger.

--- John Pursley
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  #9  
Old 05-15-2016, 03:42 PM
johnpursley johnpursley is offline
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If anyone is thinking about Gemini Titan...in just about any scale, I can do vinyl (not the heavy stuff you see with the self-stick decals in many kits) skins up to about BT-101. I can also do skins for a very realistic looking Gemini capsule.

The model that Jediboss has presented is "skinned" from the tip of the nose to the aft edge of the body tube.

I may be listing Gemini Titan skins on eBay soon or you can contact me direct at johnpursley@accur8.com for more info...or just to chat!

---John Pursley
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Old 05-15-2016, 10:21 PM
johnpursley johnpursley is offline
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Ol JR pretty much has his head wrapped around things with respect to finless. Just since posting here I've gotten a couple of direct emails about using "guidance" to make "hammerhead" models stable. One was specifically talking about the SpaceX Falcon. As JR stated, that big payload shroud at the front is pretty much the finless equivalent of putting fins at the front. The Falcon 9 might not be so severe as it is a very, very long and slim vehicle...but I would only attempt flying something like the Falcon finless if it were a fairly large model...say, 5-6 feet long.

Why? Because it's easier to make the critter stable (without an active system) because the proportion of the engine weight is a much lower fraction of the total model weight than with a smaller model. The CG will "naturally be fairly far forward and just a little nose weight (to bring the CG ahead of the CP) will go a long way toward offsetting the mass of the motor in the rear. PLUS, a long big model has a much lower natural frequency of oscillation. Still, I would "boilerplate" a smaller model of any finless vehicle away from crowds before "going for it" with a crowd pleaser.

BTW...I boilerplated a BT-70 Gemini Titan years ago with A10 motors (the one presented in this thread is a BT-80 based model). AND...though he didn't spcifically say, Jediboss is the owner of the model and the photos presented here are those that I sent him as the model was nearing completion, they are not of the completed model that he has in his hands now! I added the 7 systems tunnels and (what I call) "clutter" around the scale engines to make it look a bit more realistic since those photos.

---John Pursley
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