Friction fit for staging. Best Practices?

[Faroutspacenut]

About to start building my first 2-stager. LOC Sandhawk 2.6" (38mm) on top of a 3" Terrier Booster. These are both cardboard kits. There won't be room on the second stage for a motor retainer if I want my IC to fit, so I'm planning on friction fit. I use mostly CTI motors.

Since the motor tube is cardboard, I'm wanting to be mindful of excess wear on it, or having a tape-wrapped motor casing stretch out the tube.

I was thinking of possibly painting the bottom inch or so of the motor tube, that way if I wrap any tape around it to give the motor a little bit of positive retention, it won't peel the naked tube. Or possibly a wrap of aluminum tape?

The kit instructions says to have the motor tube flush with the aft end of the body tube, but I'm thinking of having it stick out an inch or so - allowing me to give it a wrap or two of tape.

I suspect I'll be electronic dual-deploy on this, but it would be nice to have motor eject as a backup if that's possible.

 

[caveduck]

Have you considered an MD style retainer (threaded rod extending forward from the motor closure to a fitting in the upper stage tube)? I believe CTI has officially blessed tapping an eyebolt or rod into the forward closure section of their reloads (CJ recently posted a link). Tape retention on a 38mm makes me a wee bit nervous. I'm making such a rig for the upper stage of a Madcow Nike-Apache. That way you can securely position the aft end of the motor wherever you want.

 

[Faroutspacenut]

Have you considered an MD style retainer (threaded rod extending forward from the motor closure to a fitting in the upper stage tube)? I believe CTI has officially blessed tapping an eyebolt or rod into the forward closure section of their reloads (CJ recently posted a link). Tape retention on a 38mm makes me a wee bit nervous. I'm making such a rig for the upper stage of a Madcow Nike-Apache. That way you can securely position the aft end of the motor wherever you want.

It's definitely something to consider - was hoping to avoid that kind of retention for this build - my goal is to keep it as simple (and stock) as possible. I hear you on your concern about friction fit with a 38 though. For a moment I was thinking I might go with a 29mm instead - which would give me plenty of room for a retainer and conduit for air start wires - but that would mean all new centering rings AND fins since the fin tabs won't be long enough...

I have that same MC kit. Didn't want it to be my first 2-stager so going with a lighter, "easier" setup for my first go at it.

 

[BABAR]

I like the extra security of having an external band of tape around the engine/body tube junction. I haven’t yet experienced a failure when I do this, although I also have pretty good luck with just putting a few folded tape pieces around the engine and using friction fit only.

Here’s how I do it when I plan ahead.

Put the fins on so there is still a 1/2 inch of just 360 degree rocket tube at the tail end.

Build and paint the rocket as desired.

Put a 1/2 inch piece of clear cellophane tape around the base of the rocket. Press it really tight. Colored Mylar tape also will work really well, and adds a splash of, well, color.

Now when I put the motor in, I can overlap a single wrap of cellophane or masking tape over the exposed engine casing and tail or the rocket, in addition to my standard internal friction fit. Generally I can still slide this into my booster section, even with minimum diameter, such as BT-20 to BT-20 or BT50 to BT-50.

After the flight, I can take off this outer layer of tape and the internal base layer stays in place, protecting the paint underneath.

Nothing is perfect, but I have yet to experience a failure with this technique. Largest I have used this for in two stage is a D, I have used it on single stage up to a G. As always YMMV.

 

[JimJarvis50]

I don't know what sustainer motors you might want to use, but if a case with a reload, reload and spacer, or reload and two spacers would fit your flight range, you could glue in the longer case. You would need access to the top of the motor tube to push stuff out, and an appropriately sized dowell to push out the spacers. I have done this.

Jim

 

[blackjack2564]

Simplest solution I use is:
When building cut motor mount as short as reasonable.
Measure length say 6in.
What ever motor you wish to fly , wrap several layers of tape at 6in from rear of motor case this will be a "reverse" thrust ring.
Build motor/reload, don't put rear closure on.
Drop loaded case into motor tube from FRONT.
Screw on rear closure...now motor is captured between rear closure and tape ring on case.Can't move forward or to rear when ejection fires.
You may need a dowel/pvc pipe...to put pressure on front of motor after flying, to hold in place while unscrewing rear closure to remove.

Sustainer from one of my 2-stagers below, motor mount tube just long enough to catch fin tabs through wall.

 

[dshmel]

Another great idea from CJ !!

 

[Faroutspacenut]

Simplest solution I use is:
When building cut motor mount as short as reasonable.
Measure length say 6in.
What ever motor you wish to fly , wrap several layers of tape at 6in from rear of motor case this will be a "reverse" thrust ring.
Build motor/reload, don't put rear closure on.
Drop loaded case into motor tube from FRONT.
Screw on rear closure...now motor is captured between rear closure and tape ring on case.Can't move forward or to rear when ejection fires.
You may need a dowel/pvc pipe...to put pressure on front of motor after flying, to hold in place while unscrewing rear closure to remove.

Sustainer from one of my 2-stagers below, motor mount tube just long enough to catch fin tabs through wall.

View attachment 376096

That's simply brilliant. Problem solved! Thank you CJ as always for your sage wisdom!

 

[Faroutspacenut]

I don't know what sustainer motors you might want to use, but if a case with a reload, reload and spacer, or reload and two spacers would fit your flight range, you could glue in the longer case. You would need access to the top of the motor tube to push stuff out, and an appropriately sized dowell to push out the spacers. I have done this.

Jim

Thanks Jim - I like this idea too. If only I had a CTI case to spare!

 

[Faroutspacenut]

Simplest solution I use is:
When building cut motor mount as short as reasonable.
Measure length say 6in.
What ever motor you wish to fly , wrap several layers of tape at 6in from rear of motor case this will be a "reverse" thrust ring.
Build motor/reload, don't put rear closure on.
Drop loaded case into motor tube from FRONT.
Screw on rear closure...now motor is captured between rear closure and tape ring on case.Can't move forward or to rear when ejection fires.
You may need a dowel/pvc pipe...to put pressure on front of motor after flying, to hold in place while unscrewing rear closure to remove.

Sustainer from one of my 2-stagers below, motor mount tube just long enough to catch fin tabs through wall.

View attachment 376096

Ok I've been giving this some thought - assuming the motor case isn't long enough to reach the motor tube from the top, what's the trick to getting it in there? Would I need to consider cutting the body tube just above the fin tabs and glue in a coupler that I can attach to the new upper section with pop rivets? (This way I could also use AT DMS motors loaded from the aft end and secured with a tape ring at the top) My only concern here is I'm also planning on using a conduit for my air start wires...

LOC assures me friction fit should be fine, but I like an extra level of safety having some positive retention.