Clustering with a reducer in the airframe

[Fore Check]

Ok, so I'm cooking up something here, and I've run into a design quandry of sorts.

My motors will be a 4 x 24mm cluster in a BT80. This section of BT80 will be a net 8.5" long.

The airframe above this motor can will be BT70, about 30" long. The will be coupled using centering rings, not a transition. The BT70 can stick up to 4" inside the BT80, which would put the bottom end of the BT70 about 3/4" in front of the tops of the cluster mount (it will be 3.75" long for E engines.) This means that the airframe reduction will occur very close to the tops of the motors, and there will be no stuffer tubes running up through the BT70 (you can't fit 4 x 24mm in a BT70 anyway.)

I could possibly reduce the amount of BT70 that is inside the BT80 to 3" giving a bit more "relief" above the cluster mount before it is reduced a bit (again via centering rings), but would prefer not to just from an overall rigidity standpoint.


Does anyone see any problem with this? Am I asking for trouble, or should it work no problem?



Here's another idea I'm kicking around that ties in with this.

In order to keep the laundry forward in the airframe close to the NC, I was thinking about taking a 1/4" thick plywood centering ring to fit inside the BT70 that has a center hole *at least* large enough for a BT60 to pass through it. I'd glue this in the BT70 about 12 - 13" in from the forward end of the airframe. I'd lay a couple of small dowels (1/8" or so) in an "X" pattern across the opening in the aforementioned centering ring. I figure this would provide a "bottom" of sorts for wadding and laundry to stop at and stay above during a launch, but also let the ejection gasses pass through. Note that regular Estes type wadding would be used, in large loosely crumpled balls, as opposed to dog barf.

I'm opposed to using a piston arrangement because there's already a reduction in airframe above the engines, and I feel that a piston would further complicate things.

Again, any input on these ideas would be appreciated.

 

[powderburner]

Sounds like it should all work. You will have to give the insides of your 'ejection chamber' a good coating of epoxy, maybe line the inside of the BT80 with some scrap BT for a little double-wall reinforcement?

Your laundry retainer sounds like a good idea. 1/4-inch thick plywood sounds like a bit of overkill, I would bet that an Estes-type white cardboard CR would be adequate, or a Totally Tubular black fiberboard CR (I think Fliskits sells them too).

Here's another approach to getting the gas forward. In your BT50 MMT, use a 20/50 CR for a thrust block, then put a length of BT20 inside the CR projecting forward. This will not constrict the ejection gas since it comes out the front of the motor case already at that diameter. Run the BT20 forward through the BT70 (it will fit) to the laundry retainer disc. Use ejection extensions on maybe two of your MMTs and plug the other two? (or vent overboard) and you will still have redundancy on the ejection.

 

[Fore Check]

Hmmmm....

The BT20 extensions sounds like a good idea. In fact, I could run all four up through the BT70 as stuffers and the forward centering ring on the stuffers would then act as the laundry retention.

This is, of course, assuming that there would be no problem with the use of the BT20 stuffer extensions as opposed to BT50's like I'm accustomed to.

I just fiddled a bit with this idea on RocSim. For a 4 x 24mm cluster mount in a BT80 with the tubes touching, I figure a cluster radius of .690". If I make a radial 4 x 18mm mount in the BT70 at that radius, the four bt20's seem to be well centered in the 4 BT50s below. Does this sound right?

 

[powderburner]

Your info sounds right, but I am not a Rocsim expert---

I figured like this: four BT50-sized MMTs stacked together, centered in a BT80, have the individual MMTs located 0.6901 inches from the center of the BT80 (just like you said). If the BT20 stuffers are centered on the MMT, then the outside of the BT20 will be 1.05815 inches (and we all work to the nearest hundred-thousandth of an inch, right?) from the center of the BT70 or BT80. Double that radius to get a diameter of 2.1163, and this fits inside the BT70 inside diameter of 2.175, with oodles of extra room.

 

[powderburner]

To contain the ejection blast in a stuffer tube that has ANY lifetime to it, you will need to do something to protect the BT20 stuffers.

At a minimum, you should swab CA or thin epoxy through the insides and let thoroughly set. If you do this before assembly you have a much easier time inspecting. When I do this sort of thing, I use a string pulled through the tube, tied to a couple cotton balls at the far end. Soak the cotton in CA, pour some more inside the bottom end of the BT, and pull the string to wipe the CA through the insides.

 

[Micromeister]

Forecheck:
Most if not all my 4 D12 cluster models use a single BT-55 stuffer centered in a 3/32" ply upper bt-80/55 centering ring epoxied to a standard BT-80 coupling epoxied to the forward end of the Clustered motor tubes ring. stopping the BT-55 about 14" from the forward end of the airframe. Heres a pic of the BT-80/60 stuffer/motor mount used in my 3X OT.
Hope this helps

 

[Micromeister]

Forecheck:
heres' a neat motor retainer system I've found that eliminates the possible spring clip motor ejection problem i've seen a few times.
basically it consists of a 10/24 or 1/4-20 aluminum threaded insert installed flange down (sometimes needing some grinding with a 1/2" dremal drum sander to fit correctly). The insert is set in epoxy between the motor tubes. Retainging screw is a stainless steel truss head machine screw or a round or pan head machine screw with 1" standard stainless steel flat washer soldered and expoy covered. makes for very quick motor changes
Hope this helps.

 

[Silverleaf]

What a neat idea !!

I really like this method of retention, and it looks good to. Many thanks for posting that image. 8)

Cheers,

 

[DynaSoar]

Originally posted by Fore Check
Ok, so I'm cooking up something here, and I've run into a design quandry of sorts.

My motors will be a 4 x 24mm cluster in a BT80. This section of BT80 will be a net 8.5" long.
.....
Again, any input on these ideas would be appreciated.

I don't see why the geometry should matter, at least with respect to recovery. The charges will exert pressure which will be spread evenly through the interior.

Perhaps more room between the motors and the 70 would be good -- less chance for motor boogers to fly up into the chute.

I used a baffle type thing to keep my chutes forward too. In 24mm birds I use a 20/50 centering ring with an inch of bamboo rod glued on top of it inserted far enough for everything to fit. The wadding keeps the shock and shrouds from falling down farther. Haven't tried it on larger birds yet.

You sure the motor tubes won't fit in the 70? If they're just carrying ejection charge there's no reason for them to be perfectly round all the way up.

If pressure might be a problem, don't fly it with four ejection charged motors. Use two booster engines and two regular. Then only two of the tubes need to go anywhere near the 70.

 

[astronboy]

If you are in need of an engine mount, or a stuffer tube that has some protection built-in, consider the Totally Tubular T-50mf. It is a thick walled motor tube that has a 24mm interior diameter (sized like a bt-50) and a .99 outer diameter.

Here is the kicker: it is lined with foil!! This should protect against charring.

TT also carries a centering ring to fit this thick walled, foil lined 24mm tube in a BT-70

Here is the link to TT's listing:

https://www.wooshrocketry.org/misc/tt.htm


I hope that this helps!!

Fred

 

[Micromeister]

Astronboy is right, If you look closely at the motor tubes in the retainer pic I posted you can see what these foil lined motor tubes look like, It's great stuff for 24mm motors!

 

[Fore Check]

Originally posted by Micromister
Forecheck:
Most if not all my 4 D12 cluster models use a single BT-55 stuffer centered in a 3/32" ply upper bt-80/55 centering ring epoxied to a standard BT-80 coupling epoxied to the forward end of the Clustered motor tubes ring. stopping the BT-55 about 14" from the forward end of the airframe. Heres a pic of the BT-80/60 stuffer/motor mount used in my 3X OT.
Hope this helps

It looks like what I have in mind of going from a 4x24mm BT80 cluster to a BT70 airframe is quite close to what you have in this pic, only you're going to a BT60. It sounds like this will work just fine - I'll just paint the inside of the BT70 with epoxy for the first few inches above the engines, or simply install a 4" BT70 tube coupler at that end of the tube and give that lowest centering ring and the lip of the tube/coupler an epoxy coating. The use of a piece of BT80 coupler in there between the engines and the airframe/stuffer is a good idea.

Thanks, all! This will make things easier.

I've not had much trouble with the motors kicking in the clusters - but that's a cool motor retention idea. I haven't flown my Atlantis yet, and it's a friction fit 3x24mm cluster. You can see from this pic that it will be a relatively easy retrofit on an otherwise completed model. I'm sure I'll use your idea here.

https://www.rocketryforum.com/attachment.php?postid=88853

 

[GlassfibreMan]

Re foil lined tubes.

I have used that very same tube and I can tell you that the foil is very quickly vaporised by the heat of the ejection charge. When I say quickly I mean after one or two flights.

Ray's Rocketry

 

[GlassfibreMan]

BTW I know it's nothing strictly to do with the subject but here's how I got 4 'D' clusters into a BT70 - by half blending on the airframe and tapering in. It flew well - as long as the motors lit.

 

[astronboy]

Repeat after me.... Deuce, Tres, Quatro!!