Plus 2 - Tres-like 2-stage cluster

[illini]

Posted a thread yesterday on my latest scratch build - a 2-stager with 4 motors in the first stage and 1 in the sustainer. Styled like the Tres, but two more motors than the Tres, hence "Plus 2" is the name. Have since worked out most issues with the design, so retracted the previous thread and restarting with this one.

The booster stage has motors in each of 3 side pods and a core. The core gap-stages to the sustainer. Note that in this 3D image the nose cones aren't shown since Rocksim doesn't have any mechanism to view nose cones on the side pods. However, these will use FlisKits NCB-20R cones...roughly BT-20 equivalents of the Tres (and Nomad) BT-55 cone.

 

[illini]

Here is the 2D line drawing.

The booster stage is heavy and probably won't tumble (probably stable), so I require a mechanism to deploy a parachute. At first, I was thinking that the way to do it would be to simply stuff a chute in one of the side pods (BT-20) and use a short-delayed motor for that pod, but the amount of space for a chute is pretty limited. Now I'm thinking it would be better to duct each of the pods into the main BT-60 tube ahead of the forward CR20-60 centering ring. The MMT assembly is not glued to either the BT-60 (aft) or BT-55 (forward). Rather it attaches to the aft CR55-60 centering ring via a length of Kevlar. The parachute will live in the space between the two CR20-60 rings (aft), protected from any gases. Pressure from the side pods ducted into the BT-60 will kick the MMT out, releasing the parachute. One question remaining is whether the side pod engines will require delay with ejection charge, or whether the pressure from zero delay motors will be enough to kick the core MMT aft and deploy the chute. I'm thinking the latter will be okay.

 

[Fore Check]

In my best imitation of the wanna-be on the Dodge Ram Hemi commercial:

"*SWEET!*"

Ah, yes. The 20R. Same nose cone as the Astron Rogue & WAC Corporal (Estes.)

I've bought *at least* 3 from BMS and have had to continually replace them because the tip keeps breaking off. I swear, the next ones I get are gonna be made of basswood. (Hey Sandman, got yer ears on good buddy?)

 

[illini]

In the process of typing the last segment of my saga, it hit me that there's possibly another mechanism to explore here. That is, glue the aft CR20-55 ring to the BT-55, but not the BT-20, forming a stop for the BT-20 as it kicks aft. This will kick the core MMT just far enough to deploy the chute, but not all the way out. Only catch is that I need somewhere for those gases to go after kicking the MMT back. Still thinking on that one.

Anyway...sustainer construction is complete and awaiting primer and paint. Will start booster construction when I return from a family reunion (aka "group therapy") in the Chicago area next week. This one has just enough interesting gadgets that it might warrant documenting with pictures as construction proceeds. So tune in next week. Same bat time. Same bat channel...err...forum!

 

[illini]

Originally posted by illini868891
Only catch is that I need somewhere for those gases to go after kicking the MMT back. Still thinking on that one.

Really must stop talking to myself...

I think I've got it. The BT-20 already has vent holes in the fore end for the gap staging. As that tube kicks aft those holes slide into the main chamber, providing a release for the pressure. It comes for free!

 

[eugenefl]

Originally posted by illini868891
Really must stop talking to myself...

I think I've got it. The BT-20 already has vent holes in the fore end for the gap staging. As that tube kicks aft those holes slide into the main chamber, providing a release for the pressure. It comes for free!

Nope, not talking to yourself at all! Interesting design. I've always wondered how best to recover the booster of a gap-staged rocket. Since you'll have some extra room down there to stuff a chute your design seems logical. Sounds like an interesting project. Keep us posted.

 

[Bill]

Originally posted by illini868891

Pressure from the side pods ducted into the BT-60 will kick the MMT out, releasing the parachute. One question remaining is whether the side pod engines will require delay with ejection charge, or whether the pressure from zero delay motors will be enough to kick the core MMT aft and deploy the chute. I'm thinking the latter will be okay.

I have to recommend against zero delay motors in the side pods. Motors do not light simultaneously and do not burn for the same length of time. If one of the side motors were to burn through while the center one is still thrusting, the MMT may slide back early and your second stage may not light reliably.


Bill

 

[illini]

Originally posted by Bill
I have to recommend against zero delay motors in the side pods. Motors do not light simultaneously and do not burn for the same length of time. If one of the side motors were to burn through while the center one is still thrusting, the MMT may slide back early and your second stage may not light reliably.


Bill

Thanks, Bill. I have the same concerns. I don't have a good answer to this, but if the core is still thrusting when one of the side pods burn through, then that thrust will oppose the pressure in the BT-60 chamber. Will it be enough to keep the MMT in place until staging occurs? Don't know, but even if it was okay, could get a bit dicey as core thrust tails off during staging.

Another concern I had was that Rocksim pegs velocity at booster burnout at around 400 feet/sec (using 4 C6's in the booster). Deploying a chute at that speed (although slowed down somewhat by the upper stage "kickback") might be a bit of a problem.

I'm leaning toward using delayed motors in the side pods and kicking the core MMT entirely out (connected with Kevlar). Another advantage of this is that replacing the core MMT is straightforward since it is removable. I can also make the core modular...a 24mm core could be fun!

 

[Fore Check]

Why not make one of the side pods like a separate rocket? You know, with a shock cord, ejectable nose cone, and recovery system?

Here's what I mean. Put zero delay C6 engines in 2 of the the three side pods, with your zero delay booster in the center.

In the third side pod (the one with the standard - type recovery system, including a streamer, a shock cord, and a bit of wadding) put a C6-3 engine.

Surely the central booster engine will light the sustainer before a 3 second delay on the side pod expires. Then, after separation, that ejection charge kicks the cone and streamer for the booster while it's on the way down. If you're worried about 3 seconds being too short, go with a 5.


This way you can keep your side pods "independant" of the each other and the central tube. It will take away a lot of variables from your gap staging arrangement, making for more reliable stage ignition.

 

[illini]

Thought of that...original design was that way. Only concern is that the side pods are only 6" long. With 2.75" going to engine and ring and ~.5" going to nose shoulder and screw eye, that leaves less than 3" for wadding, chute, and shock cord. On the plus side, definitely more straightforward. Maybe enough so as to warrant lengthening side pods to accommodate. On the other hand, I'm kind of liking the idea of having a modular core MMT.

 

[Fore Check]

If you're up for the challenge, I say "Go for it!"

I, however, am lazy - and would lengthen the side pods a bit.

Be sure to post some pics when you get crankin' on this one!

 

[teflonrocketry1]

illini868891

Can you post the RockSim file for your design I am having trouble visualizing how this would stage properly. If the motor mount is not connected to the either the BT-60 or BT-55 how would the outer BT-60 section with the fins and side pods stage?

Bruce S. Levison, NAR #69055, Section #535 Tri City Sky Busters

 

[illini]

Be happy to once I get home (out of town at the moment). Basically, the core MMT is a 14" BT-20 (vented, as is the BT-55 and BT-60 that encapsulate it at the top). 1/4" of the sustainer motor sits inside the BT-20. The 20-55 rings are glued to the BT-20, but not the -55, and the fwd ring rests against the stage coupler.

BTW, I used your side pods article to adjust the Cp. When you look at the Rocksim file I'd especially appreciate your looking over that part of it. Will post the file Sunday or Monday.

 

[illini]

Originally posted by teflonrocketry1

Can you post the RockSim file for your design I am having trouble visualizing how this would stage properly. If the motor mount is not connected to the either the BT-60 or BT-55 how would the outer BT-60 section with the fins and side pods stage?

Here is the RockSim file.

 

[illini]

Finally getting around to doing some building on this. First, here is the updated Rocksim file. This one uses booster fins the same size as the Tres (previously they were downsized) since I want the booster to work with a 24mm core motor as well.

 

[illini]

Here are all of the booster tubes cut to size. Three 6" BT-20's for side pods. 3" BT-55 for the forward transition. 14" BT-20 for the core/gap-staging motor. 13" BT-60 for the core airframe.

 

[illini]

The upper transition BT-55 has 3 vent holes 120 degrees apart between the two centering rings. The outer BT-60 vent holes will also be between the rings on the outer skin. Note the Kevlar that will attach to the ejectable core motor mount.

 

[illini]

This shows the slots cut into the side pods and BT-60 to duct the side pod ejection gases into the core for ejecting the motor mount with parachute (connected to rest of booster via shock cord). The slots are 1.25" long starting 3.25" from the rear. Building tip: The slots in the Estes tube marking guide are perfect for guiding your X-acto knife to make nice clean vents.

 

[illini]

...and the side pods glued onto the BT-60.