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  #11  
Old 06-17-2010, 09:37 PM
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CPMcGraw CPMcGraw is offline
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Exclamation

Knowing that the original idea was to keep the model as close to the original kit as possible, I went back and edited out the extended length (the 5" forward section), and adjusted the size & shape of the sustainer fins, until the sustainer's stability margin was 1.22. This is an acceptable low-ball value, giving me a stable bird without going into the "overstable" extreme.

The results are shown below. Where this begins to cause concern is in the Dv numbers. If I limited the flight envelope to just the D12-0/C6-5 combination, it works out fine at 10 FPS. However, the Dv numbers are significantly higher for the C11-0/B6-4 and D12-0/B6-4 combinations, with 21 FPS and 32 FPS respectively.

The altitude numbers improve, with 713', 1080', and 1321' respectively, but with the B6-4 combos still moving upward when the deployment charge fires. The numbers suggest a good chance of explosive chute openings and probable zippers.

More development coming!

Length: 30.495"
Diameter: 1.637" (BT-60)
Fin Span: 7.14"
Weight: 2.87 oz

The model will still require a 48" x 3/16" rod.
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Name:  Sentinel-B Mk IIIA 3D.jpg
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Name:  Sentinel-B Mk IIIA 2D.jpg
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Size:  32.6 KB  
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File Type: rkt Sentinel-B Mk IIIA.rkt (68.5 KB, 58 views)
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  #12  
Old 06-18-2010, 12:09 AM
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CPMcGraw CPMcGraw is offline
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Exclamation

After seeing the simulation results of the "short" version, I went back to the "long" version and revised the sustainer fins again, reducing the overall height at the crown by about 1/2". I'm calling this the Mk IV version.

The design is starting to grow on me, and I need to start looking through my parts boxes to see what I have and what I need to order. I think I will be building this one, just to see it fly.

New revisions include:
  • Sustainer tube length returned to 23", using 5" and 18" lengths.
  • 5" tube is now at the bottom, to move the coupler away from the parachute.
  • BT-20J replaced with BT-20L, to act as a stuffer tube.
  • A third AR-2060 at the top of the BT-20L.
  • Sustainer fins reduced in crown height by 0.5".

Length: 35.495"
Diameter: 1.637 (BT-60)
Fin Span: 7.14"
Weight: 3.30 oz

Sustainer section with a C6-5 has a margin of 1.14.


C11-0/B4-4.......675'......Dv 16 FPS
C11-0/C6-5.......982'......Dv 7 FPS
D12-0/C6-5......1344'......Dv 4 FPS


Still uses a 48" x 3/16" rod.

It's getting close to build time!
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Name:  Sentinel-B Mk IV 3D.jpg
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Name:  Sentinel-B Mk IV 2D.jpg
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Size:  51.6 KB  
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File Type: rkt Sentinel-B Mk IV.rkt (73.6 KB, 65 views)
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  #13  
Old 12-27-2010, 08:16 AM
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Quote:
Originally Posted by CPMcGraw
Patience, Grasshopper...


Still waiting.
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  #14  
Old 12-28-2010, 12:40 AM
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Default One step in a thousand closer...

Here are the fin patterns for this model. Two sheet patterns, but it will take two of sheet one (sustainer fins) to make a complete set. The thickness has been raised to 1/8" from the listed 3/32".
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Name:  Sentinel-B Booster Fins.JPG
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Name:  Sentinel-B Sustainer Fins.JPG
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File Type: zip Sentinel-B.zip (10.8 KB, 62 views)
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  #15  
Old 12-28-2010, 11:12 AM
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CPMcGraw CPMcGraw is offline
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Default Another step on the journey

I have started the construction of a prototype. Something I noticed in the test-print of the fins is a slight lengthening of 1/4" in the sustainer fins, and 1/8" in the root-to-tip length of the booster fins. This is likely due to several conversions taking place from SVG to CDR to JPG formats. I'm not going to quibble over it at this point. The hand-cut first sets look good enough to work with just as they are...

A strange thing happened as I was sending the SVG file into Draw. The size of the fin was huge, like 10X what it should have been. I scaled the objects down to 10% and checked the measurements. They looked good on the screen, but the final print-out was still fractionally over-sized. This quirk has happened with several files, not all mine, so I'm guessing a scale factor is being written incorrectly in RockSim. May be something to tap Apogee about...
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  #16  
Old 12-28-2010, 12:43 PM
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Quote:
Originally Posted by blindeye2
Craig;
I was not familiar with the origional Sentinel and thought that you were converting the sustainer in your sim to a two stager. It was an interesting problem to work on.
Maybe soeone will be inspired by the discussion to build something.
Bob

I just happen to have a Sentinel kit in my "soon pile".
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  #17  
Old 01-10-2011, 04:17 PM
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The parts for this one are on order from SEMROC now. More posts soon...
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  #18  
Old 01-29-2011, 10:57 PM
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Default Construction Started!

OK, now that the parts are in, I can start the Sentinel-B. I'm working from the Mk IV revision.

This first shot is of the booster being fitted with fins. The guides are cut by hand using a matteboard product found at Office Max/Office Depot/Staples...

The engine mount will go in next. I started with the fins on this stage because it was easier to get my stiff fingers in and out of the tube without the obstruction...
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  #19  
Old 01-30-2011, 10:54 AM
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The engine mount went together quickly. Notice the glue is still wet...
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  #20  
Old 01-30-2011, 07:07 PM
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To understand the method behind the madness in this picture, refer to "The Book", 7th ed, page 167, pars 4 & 7, and page 168, par 3. I am using two lengths of 3/16" launch lug to form pressure ports that allow the build-up of gasses to escape into the area between the centering rings. Four holes at 90-degree separation will be punched into the tube wall between the fins for the gas to escape to atmosphere.

The lugs are glued to the ID of the coupler 180 degrees apart, and their tops are 1/4" below the top edge of the coupler (the mount is upside-down in the picture).

Included also is a decal I'm going to position around that opening in the tube.
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Last edited by CPMcGraw : 01-30-2011 at 07:41 PM.
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