Super Star Trooper: Estes Star Trooper Two-Stage Conversion

[smstachwick]

Having proven the concept of my Super Gnome, and also having lost the sustainer at my Jesmond Dene Park launch, I decided to construct a successor. One cheaper to purchase (and replace if necessary), with a little bit of a more-involved build and hopefully even more ridiculous performance.

Behold: The Super Star Trooper!



The stock Star Trooper is 3 inches shorter than a stock Gnome, but the booster on this one is a little longer. I’d guess it’s still about 2 and a half inches shorter than the Super Gnome.

Now, this kit isn’t exactly hard to find, but if I were to guess, I’d say Estes is planning on pulling the plug on it soon. It’s only $6.99 on their website and my local independent hobby store (Discount Hobby Warehouse) couldn’t seem to place an order of their own, so I got both kits that I used for this build from Penrose.

In terms of construction, it’s a little bit of a step up from the Super Gnome. The motors are friction fitted, with a coupler between the lower and upper body tubes doubling as a motor block. The really nice thing about the lower body tube is that it’s precisely 45mm long, the length of an Estes mini motor. Therefore, the tube for the booster didn’t have to be cut down to size. The booster is literally just the second kit’s lower body tube and fins glued on. The sustainer’s motor serves as the stage coupler and extends into the booster’s body tube. It just might be the most brutally simple staging method possible.

The shock cord in the Gnome was run through a slit in the body tube and glued onto the exterior, with the forward launch lug ring covering it. In the Star Trooper, the mount is identical to those in Estes’ larger kits, just scaled down, and it’s a bit tricky to fold it down and get it installed. The shock cord rubber resists getting a nice epoxy bond, so I just declared “F it!” and threw it together with CA glue.

The glue-on balsa fins got yet more use out of my Estes fin alignment big. They ended up straight but they missed the centerline a bit, so I’m considering pitching the plastic guide and just going with printout-cutout guides from PayloadBay.com going forward. I don’t think their leading/trailing edges are precisely aligned either, but it should be close enough to not adversely affect flight characteristics.

The nice thing about the individual fins, however, is that I could align the booster launch lug where I wanted it, interdigitating the fins on the rod and hopefully moving my CP back.

Stability feels a little tricky with those fins being pushed a bit forward on the tube. CG with motors (but no wadding) appears to be somewhere in the middle of the sustainer’s fins. Hopefully it’s far enough forward that a decent pack job and keeping the sustainer limited to a 1/2A3-4T will prevent it from skywriting. If not, I’ll have to crack open the nose and put some clay in it.

First flights are scheduled for the morning with DART. The sustainer will fly alone on an A10-3T, followed by the A10-0T / 1/2A3-4T combo if recovery is successful. If it survives launch day, I’ll take it home and give it a nice paint job.

I’ve practiced this kind of staging enough that I’m confident the sustainer will ignite, but if not, it should be light enough to just tumble like the Super Gnome did. Crossing my fingers for nothing truly frightening!

 

[CalebJ]

Nice build! I've always appreciated the simplicity and great flying of the Star Trooper.

 

[jmasterj]

Good luck with the A10s!

 

[smstachwick]

Launch summary:

My Super Star Trooper was a success! It catapulted itself towards the dog park when flown as a single stager, but the lug stayed on. I may have to make some adjustments to it before trying that again, but that at least validated the glue joints on the fins and shock cord mount.

After briefly discussing the two-stage configuration’s estimated stability and demonstrating that the upper lug was still good, the RSO cleared it to it fly at its full rated total impulse. The upper stage lit and separated without a hitch, and both components were recovered on the field.

Flight video:

View attachment 493758

 

[GlenP]

Seems like the booster came down nearly ballistic, but just a slight shift in where the motor fits in the tube might be enough to get it to tumble a bit. You can do some test throws or swing test on a string of the booster alone to find that sweet spot, with a spent engine casing. If you find that you have to shift the booster so far forward and you push the sustainer out of the booster tube too far for a secure connection, you can always add an external coupler.

 

[smstachwick]

I think perspective in the video is tricky. From what I remember seeing out of my peripheral vision, the terminal velocity was more akin to a nose-down featherweight recovery like an Estes Mosquito.

Still, just for safety’s sake, I’ll play around with it. I don’t want to move the casings too far back with the stability the way it is, and I don’t think this arrangement lends itself well to gap staging, but I’ll see what I can get away with.

I think I’ll experiment with the booster by itself first, then try to cut out and move the tube coupler/motor block to match the ideal position.

What do you think? Do I send the booster CG forward or aft?

 

[GlenP]

With the booster and spent engine after a launch, give it a toss over the grass. If it flies straight but engine first, then you need to move the engine further up the tube. You want cg and cp nearly on top of each other. With the engine sticking out the back seems like it will want to fly engine first, cg is too far back to tumble.