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  #1  
Old 05-06-2020, 08:03 PM
RobVG RobVG is offline
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Default Scale Starship

First time building a scratch built. Successfully scaled it from pictures. BT 80 tube was just right for a 35cm height.

Just looking at it, the Space X Starship looks like it should fly but I'm starting to realize it's not a fin stablized rocket. I just started using Openrocket and CG was behind CP. Tried a few things. Added 4cm of length and moved the fins back a little. Better but still unstable.

Couldn't find a function to add weight to a specific part of the rocket so I played with the nose cone mass. Found if the nose cone weight was 5oz more I get a stability of 1.5. That's not gonna work.

If I add weight to just the tip of the nose cone do you think I can get away with less added weight?

I really don't want to but would adding a fourth fin help?

Does more fin area help?

It'll be hard to make it look right if I have to make too many changes.
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Old 05-06-2020, 08:06 PM
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tbzep tbzep is offline
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You could add larger clear fins, removable or permanent. At a decent distance, you don't even notice them in flight. Earlier Saturn V kits had clear fins that slipped over the standard ones and made for a nice stable flight.
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Old 05-06-2020, 08:29 PM
RobVG RobVG is offline
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Quote:
Originally Posted by tbzep
You could add larger clear fins, removable or permanent. Earlier Saturn V kits had clear fins that slipped over the standard ones


I like that idea!
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Old 05-06-2020, 08:45 PM
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tbzep tbzep is offline
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Quote:
Originally Posted by RobVG
I like that idea!

Removable fins have been done several ways. Off the top of my head...

Centuri: Slits cut in clear material to slip over the fin. (Saturn V)
Estes: Two fin halves glued together with the root unglued to create an envelope that slips over the fin. (Saturn V)
Estes: Slots cut into model for fins to hook into. (Gemini Titan)
Estes: Full fin-can that slips over the finless BT, (Titan II)
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Old 05-06-2020, 09:53 PM
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mwtoelle mwtoelle is offline
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Quote:
Originally Posted by tbzep
Removable fins have been done several ways. Off the top of my head...

Centuri: Slits cut in clear material to slip over the fin. (Saturn V)
Estes: Two fin halves glued together with the root unglued to create an envelope that slips over the fin. (Saturn V)
Estes: Slots cut into model for fins to hook into. (Gemini Titan)

That was only used on the K-39 Semi-Scale Saturn V.

Quote:
Estes: Full fin-can that slips over the finless BT, (Titan II)

Used on the K-21 Gemini-Titan and the K-28 Thor Agena-B.

One method that you did not mention was the twist on clear plastic fin unit used on the 1980s Gemini-Titan and Titan II from Estes. The recently discontinued Estes crayon rockets used the same pieces shot in black plastic instead of clear. Transparent plastics tend to be more brittle than nontransparent plastics of the same type.
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  #6  
Old 05-06-2020, 10:03 PM
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Quote:
Originally Posted by mwtoelle
That was only used on the K-39 Semi-Scale Saturn V.


Used on the K-21 Gemini-Titan and the K-28 Thor Agena-B.

One method that you did not mention was the twist on clear plastic fin unit used on the 1980s Gemini-Titan and Titan II from Estes. The recently discontinued Estes crayon rockets used the same pieces shot in black plastic instead of clear. Transparent plastics tend to be more brittle than nontransparent plastics of the same type.

Woops. Mix up. Yes, the full can was the K-21 GT. You can get reproductions on eBay and some guys here have reproduced them. I've seen some scratch GT's with notched fin inserts and that imagery was floating in my head.

After thinking about it, the clear fin can was also on the Beta Launch Vehicle and the MX Missile.
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Last edited by tbzep : 05-06-2020 at 11:00 PM.
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  #7  
Old 05-07-2020, 09:01 PM
RobVG RobVG is offline
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Openrocket is fun!

Moved the fins back a little and stability went up a tiny bit. Adding a fourth fin increased it from -.096 to .141.

Increasing fin dimensions by 25% actually made it worse- -.286. I'm guessing it was the added weight? Adding a 4th fin brought it up to -.088.
Weird. In that case it doesn't look like it's the weight of the fins. IDK

I do not want to add a 4th fin.

The CG is really close to the CP. I'm hoping adding a "reasonable" amount of weight to the very tip of the nosecone will make it stable.
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  #8  
Old 05-09-2020, 01:49 PM
RobVG RobVG is offline
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Well...

It's working on paper.

Lengthened the body tube 3.5" . It's not to scale but still recognisable as a Starship. The cool thing is, I don't have to cut the other 14.2" tube I bought (2 pak).

Adding 3 clear plastic fins- 6 total. Thanks for the Idea!

Have to add 2 oz. to the nose. Thinking I could drop in some split shot fishing weights followed by elmers glue.

Stability = 1.2

I did find out how to add and position an "unspecified" mass to the rocket, thanks to YouTube.
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