More new (interesting) kits coming from Estes

BABAR said:

Gap ignition wouldn’t be hard, but given length of booster/position of fins, how do you keep booster from coming in ballistic? Rig a streamer to come out of sustainer?

Click to expand...

I would do some variation of this:
https://www.rocketryforum.com/showthread.php?p=1737212

There are probably other options, but that's the most straightforward I've seen.

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BABAR said:

Gap ignition wouldn’t be hard, but given length of booster/position of fins, how do you keep booster from coming in ballistic? Rig a streamer to come out of sustainer?

Click to expand...

One way would be to "adjust" the CG/CP position of the booster so that alone, it is unstable, but the stack is stable. You would need to add weight to the base of the booster, and weight to the nose of the top stage.

Considering that the booster will already have a motor casing in the bottom and little in the top, that doesn't seem to be too hard to sort out...

EXPjawa said:

Considering that the booster will already have a motor casing in the bottom and little in the top, that doesn't seem to be too hard to sort out...

Click to expand...

Don't forget that it also has fins. The CG/CP relationship is more important than just the CG. A sim would show if the booster alone is stable.

Here's an example. It's a Terrior Orion I've got buried at the back of the build pile. It's a lot like what we're talking about - long booster with ducted (gap) staging. Look at the difference between the two sims.

View attachment Terrier-booster-24mm-burned engine weight.ork booster w/added weight - unstable

View attachment Terrier-booster-24mm-burned engine.ork booster w/just burned engine - stable

OK, I modeled it in Rocksim after opening the kit and measuring the parts. The Red Nova, as a standard build:





I was kind of surprised to see that they only used a 3" long motor tube, rather than the 3.5" version that would accommodate E motors. But even on a D12, it shows around 870'. An AT E15 puts up around 2000'...

The would-be booster section, retaining the 3" long motor mount:


Though this diagram doesn't specifically show mass objects for some reason, this is modeled with .75 oz of burned D12 motor casing in the mount. With a .56 stability margin, it ought to tumble, right?

The two-stage sim model:


Doesn't look too bad. It helps that the sustainer motor is near the CG. One could build this version just by adding a coupler and an 18mm mount for BT60, everything else is the same and everything in the kit is used (including the 1/2 oz of clay in the cone).

sooner.boomer said:

One way would be to "adjust" the CG/CP position of the booster so that alone, it is unstable, but the stack is stable. You would need to add weight to the base of the booster, and weight to the nose of the top stage.

Click to expand...

Can definitely make the booster unstable, but with those small fins, the tumble drag is likely to be borderline to begin with, and adding mass only makes the recovery impact velocity worse. Even with an optimal tumble and NO added weight, with those small fins probability of a cracked fin is high.

I'm with Lithosphere on this one, need some way to add a streamer or other recovery device.

If you put the sustainer's engine mount rear centering ring a bit more forward, you have a small "space within a space" around the motor mount but inside the body tube of the sustainer. Might stow a small nomex streamer in this space (or other non-flammable material that can survive a brief "broiling" by the sustainer motor when it lights and the booster separates). Need a flame-proof cord attachment to connect with the booster.

Just my thoughts.

EXPjawa said:

OK, I modeled it in Rocksim after opening the kit and measuring the parts. The Red Nova, as a standard build:

I was kind of surprised to see that they only used a 3" long motor tube, rather than the 3.5" version that would accommodate E motors. But even on a D12, it shows around 870'. An AT E15 puts up around 2000'...

The would-be booster section, retaining the 3" long motor mount:


Though this diagram doesn't specifically show mass objects for some reason, this is modeled with .75 oz of burned D12 motor casing in the mount. With a .56 stability margin, it ought to tumble, right?

Click to expand...

For some reason, I was thinking that the "booster" section of the original model was a lot longer. You might be right - the unweighted booster might be unstable enough to just tumble. The open-ended body tube pointing in the direction of flight will create a bunch of drag. It should stop flying quickly. And, I guess, there's a danger in making the booster *too* unstable - it could actually become stable in a fins-forward configuration.

I may consider building a drop-test mock up for the sake of research. Sticking such a booster together shouldn't be difficult from what I have on hand, and I can either use it or convert it to a rocket later on if it survives. I would also consider re-making the booster fins from basswood, to make them a little more robust on landing. That would also add a little mass to the tail, but not a lot. Updated sim puts that at around .45:1...