Two-Stage Sentinel [Estes #1987]

[CPMcGraw]

This is a thread for the development of a two-stage version of the OOP Estes kit. A question was raised about the possibility of modifying the design, and I've got a few results of some RockSim testing.

In the original post, I suggested the forward fins were too small, and the model would be unstable without some additional fin area, such as pop-out tabs. RockSim confirms the upper section, using stock fins, would be completely unstable. The static margin with just a B6-6 is a minus 0.14.

What I will try to do is adjust the length and fin area slightly until I reach a happy compromise without degrading the original appearance.

Stay tooned...

[CPMcGraw]

Here's the first attempt at turning this design into a true two-stage rocket...

Length: 35.495"
Diameter: 1.637" (BT-60)
Fin Span: 7.14"
Weight: 3.16 oz


C11-0/B6-4......651'......Dv 13 FPS
C11-0/C6-5......932'......Dv 16 FPS
D12-0/C6-5.....1270'......Dv 9 FPS


All flights require a 48" x 3/16" rod.

Enjoy!

[tbzep]

It shouldn't take too much tweaking to make it stable. Kody (Intruder) built a gap staged Nike-X and we know how short the sustainer section would be along with those tiny triangle shaped fins. It was stable without weight when it flew naked, but unstable after it was painted using the same motor. All it took was a little bit of nose weight and it flew great.

[tbzep]

You can change the fin profile slightly, widening them a little at the aft end, along with changing the length like you mentioned to help the CP a little. Move the motor forward, slightly recessing it in the BT-60 and gap stage it. It will only change the CG a tiny amount, but every little bit helps when it comes time to add nose weight. It has a plastic nosecone, so getting the minimum amount of weight as far forward as possible won't be any trouble. IIRC, it has 1/8" fin stock. You could remove a little weight from the aft end of the sustainer by using something similar to contest grade 1/16" or 3/32" balsa. Or just dump a bunch of shot in the nose and use a D12-0 booster. ;)

[blindeye2]

The second stage alone has a margin of 1.31 in your design. What woul be the min accectable margin for the two stage version? Your 2 stage version has a margin of 2.76
but the fins are very unsymmetric compared with the the first stage. I have modified your booster design to a more symmetrical look, but the margin is only 1.18. The margin can be increased slightly by lengthening the booster from 3 to 3.5 inches.

Bob

[rokitflite]

All I wanna know is if we are actually going to see a real model built and flown by the end of this thread or is it going to be all rocsim theory? :(

[CPMcGraw]

All I wanna know is if we are actually going to see a real model built and flown by the end of this thread or is it going to be all rocsim theory? :(

Patience, Grasshopper...

[CPMcGraw]

...the fins are very unsymmetric compared with the the first stage. I have modified your booster design to a more symmetrical look...

Bob,

It looks good, but the idea of the original thread was to make the Estes kit into a two-stage design. I interpreted that to mean "keep the appearance the same". My version only lengthened the sustainer and enlarged the fins (with some minor shape changes) to achieve good stability, but retained the original booster fins for continuity.

Your version qualifies as a "Sentinel-C", having its own personality. The flight numbers look to be nearly the same as my "-B" version.

[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

[CPMcGraw]

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

Bob,

In my first attempt, I just cut a 3" section off the sustainer and converted it into a booster, and left the sustainer at 15". The problem was the stability margin of the sustainer. I didn't want to add any ballast to the rocket (it's still just "dead weight", after all), opting instead to increase the fin area and the tube length.

The original was just a single-stage rocket "dressed up" like a two-stage AIM. It used a standard 18mm engine mount, which I slid up into the sustainer. The booster needed the added power of the 24mm engines, so that's what I used to get the stack off the pad.

As I mentioned to rokitflite, be patient. I'm getting interested in it as a build project. The altitude numbers are not beyond the boundaries of my local field.

[CPMcGraw]

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.

[CPMcGraw]

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!

[rokitflite]

Patience, Grasshopper...

Still waiting.

[CPMcGraw]

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".

[CPMcGraw]

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...

[jeffyjeep]

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".

[CPMcGraw]

The parts for this one are on order from SEMROC now. More posts soon...

[CPMcGraw]

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...

[CPMcGraw]

The engine mount went together quickly. Notice the glue is still wet... :)

[CPMcGraw]

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.

[CPMcGraw]

Next is the sustainer engine mount. I'm using a BT-20L tube to form a stuffer tube. The subassembly is upside-down in this photo. At the top is a third centering ring. It is not glued in place yet, as the tube needs to be glued into the 5" BT-60 first (the coupler is involved).

[El Cheapo]

Are those ply it fiber centering rings?

[CPMcGraw]

Are those ply it fiber centering rings?

These rings are SEMROC's AR-2060 matteboard rings, laser-cut. The rings in the booster are their AR-5060's. All of the main parts used in this build are from SEMROC. The balsa used for the fins is from Lone Star Balsa in Texas (I bulk-order balsa as I also build RC airplanes).

[CPMcGraw]

The lower sustainer subassembly is coming together, with the long mount now glued into the 5" BT-60. The AR-2060 at the 'top' has a "V" notch in it for passage of the Kevlar thread, but at this point is still not glued in place.

[CPMcGraw]

At this point in the build, I need to mention that I made a couple of changes in the sustainer fins, relative to what is called out in the Mk IV plan file. First, I reduced the thickness to 3/32 from 1/8". Second, I increased the root-to-tip-peak measurement by 1/16", for a maximum width of 1.188". When loaded up with one of the engines in my simulations, these two simple changes improved the stability margin from 1.14 to 1.31 without being highly noticeable. I just wasn't comfortable with such a low static margin for a model this size.

This is a large model for 18mm and 24mm engines. It's really impressive when I put the stack together and get to see the full length on my workbench. Photos are forthcoming! :D

[CPMcGraw]

OK, I'll tease you a bit...

Here's the full stack fitted together, but not glued, and without the sustainer fins. Notice the comparison between Sentinel-B and Night SHADO...

Those are 17" monitors in the background.

[CPMcGraw]

Progress so far:


The Kevlar thread was double-looped around the top of the stuffer tube, and the upper centering ring glued in place.
The upper and lower sustainer tubes have been glued together (upper centering also glued inside upper tube).
The screw eye was inserted into the nose cone and glued in place.
The sustainer fins are now being attached.


This has been a much quicker and easier build than the Night SHADO. In my mind, I'm envisioning a 2" version... :eek:

[CPMcGraw]

The sustainer fins are now attached, and all that remains is:


Cut vent holes in the booster tube.
Glue the booster mount in place.
Attach the launch lugs to the sustainer.
Prime & paint.
Decals.
Parachute and harness.
Engines.


The last one is proving to be the most difficult... :(

[CPMcGraw]

Some decal images for the model.

[CPMcGraw]

The booster vents have been cut, and the booster mount has been glued in place. Last step in the construction is the placement of the lugs.