Two-Stage Sentinel [Estes #1987]
 

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...

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!

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.

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. ;)

Sentunel Margine of Stability
 

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

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...

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.

Should have looked befor I leaped
 

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.