SEMROC Hollow Tubing Couplers

[CenturiGuy]

Here's another burning question in my ongoing Tau Zero saga. :rolleyes:

Sure, the outside diameter of a Semroc HTC-7 is 0.715". But how much is the INSIDE diameter? :confused:

http://www.semroc.com/Store/Scripts/prodView.asp?idproduct=206


On a related note, what's the *official* (average?) Outside Diameter (O.D.) of an 18mm Estes motor? :confused:

Anybody got some digital calipers? :o


Inquiring minds want to know. CenturiGuy and BARCLONE are feverishly trying to figure out how to retain Estes motors inside Centuri/Semroc ST-7 tubes :eek: *without* using engine hooks, which can make Really Big Gouges in minimum-diameter tubes. :mad:


Thanks in advance,

[Carl@Semroc]

Sure, the outside diameter of a Semroc HTC-7 is 0.715". But how much is the INSIDE diameter? :confused:

http://www.semroc.com/Store/Scripts/prodView.asp?idproduct=206
From the P.O. - .673 ID X .713 OD X .020 Wall 1.000" Long Fishpaper

On a related note, what's the *official* (average?) Outside Diameter (O.D.) of an 18mm Estes motor? :confused:

From another PO - 2.75" LG X .500" ID X .688" OD PT HIGH BURST VIRGIN KRAFT

The .688" is the beginning tube diameter as it goes into "the machine." The sleeve is nominally 11/16" (.6875") but gets larger with wear over time. Also, depending on the moisture and compression strength, the motor can "grow" after it is ejected from the sleeve. It is not uncommon for motors to be larger than .710" when they have "cured." That is one of the reasons we always used the ST-7 size (.715") instead of the BT-20 size (.710"), because it would work with more motors. The BT-30 size was better, because it would work with ANYTHING! The nominal size as reported over the years was .690", but it is rare to find one that small.

[CenturiGuy]

Thanks for your "zippy" response, Carl!

Inquiring minds want to know. CenturiGuy and BARCLONE are feverishly trying to figure out how to retain Estes motors inside Centuri/Semroc ST-7 tubes :eek: *without* using engine hooks, which can make Really Big Gouges in minimum-diameter tubes. :mad:(heavy sigh) Okay, so that's Not Gonna Work.

Next?


Cheers,

[CenturiGuy]

(heavy sigh) Okay, so that's Not Gonna Work.After hashing it out offline with Carl and Craig BARCLONE McGraw, it looks like we may have to settle for using a Semroc HTC-7B connector:

http://www.semroc.com/Store/scripts/prodView.asp?idproduct=207

...to firmly anchor the engine hook to the Tau Zero's ST-7, even though that means slightly reworking the inner fins' root edge to allow for the 1" long exterior coupler.

The Centuri "Bandito" used a similar technique, although the fins were folded paper:

http://www.ninfinger.org/~sven/rockets/nostalgia/72cen020.html

http://www.dars.org/jimz/ka-7.htm


"--Velcome to Rocket Shyenshe." :eek:


SCIENTIST 1: "At least we're not starting from scratch."

SCIENTIST 2: "And a good thing, too, since we're fresh out of it."


Cheers,

[barone]

Jay,

Wanna avoid the gouge from an engine hook being pulled by the motor trying to kick? Try a line of glue down half the length of the engine hook, begining at the end inserted into the body tube slit, and then setting the hook into it. Once the glue dries, smear another layer of glue over it and glue a layer of paper over it so it overlaps the body tube.

Another option. Use an engine hook for an E engine. Buy coupler stock from BMS. Cut to length and install as a long engine block.

Don
NAR 53455

[CenturiGuy]

Wanna avoid the gouge from an engine hook being pulled by the motor trying to kick? Try a line of glue down half the length of the engine hook, begining at the end inserted into the body tube slit, and then setting the hook into it. Once the glue dries, smear another layer of glue over it and glue a layer of paper over it so it overlaps the body tube.Don,

Once again, you are a rocketeer and a gentleman. I especially appreciate the "line of glue" technique, which I think will most likely make it into the Tau Zero instructions. :cool: :D ;)

*Thank you!*


Cheers,

[CPMcGraw]

Don,

Once again, you are a rocketeer and a gentleman. I especially appreciate the "line of glue" technique, which I think will most likely make it into the Tau Zero instructions. :cool: :D ;)

*Thank you!*


Cheers,

Jay,

You know this is a step found in some of the earliest Estes models which used hooks, even on the internal motor tubes, and which I'll bet many of us never bothered to do? :o

[CenturiGuy]

You know this is a step found in some of the earliest Estes models which used hooks, even on the internal motor tubes, and which I'll bet many of us never bothered to do? :oI'm blushing right there with ya, buddy. :eek: :o

But I guess this is all part of rediscovering rocketry, right? :rolleyes: ;) :D


Even if that means reinventing the flying wheel. ("Hey, Zeke! Get a load of *this!*") :eek:


Cheers,

[barone]

Jay,

You know this is a step found in some of the earliest Estes models which used hooks, even on the internal motor tubes, and which I'll bet many of us never bothered to do? :o


What? Is someone not "read a step, do a step"? :rolleyes:

Don
NAR 53455

[Ltvscout]

From another PO - 2.75" LG X .500" ID X .688" OD PT HIGH BURST VIRGIN KRAFT

The .688" is the beginning tube diameter as it goes into "the machine." The sleeve is nominally 11/16" (.6875") but gets larger with wear over time. Also, depending on the moisture and compression strength, the motor can "grow" after it is ejected from the sleeve. It is not uncommon for motors to be larger than .710" when they have "cured." That is one of the reasons we always used the ST-7 size (.715") instead of the BT-20 size (.710"), because it would work with more motors. The BT-30 size was better, because it would work with ANYTHING! The nominal size as reported over the years was .690", but it is rare to find one that small.
I've come across many Estes motors that had a bulge in them at the ejection end from the machine ramming the powder in. Those are the worst for trying to fit into BT-20.

[ghrocketman]

Why not use "friction-fit" and DITCH the motor hook altogether ?
That completely eliminates the possibility of a hook tearing up the tube....I NEVER use motor hooks on minimum diameter rockets EVER....due to ugliness alone. If a kit calls for one, I leave it off.
One way that could work is to use an "E" series length hook and glue a "D" size adapter or stack of 3 motor blocks aft of the forward hook edge that are usually used forward of the hook as a motor block.
This would prevent the hook from tearing the tube in the case of a "12ga Shotgun" ejection charge.

[CPMcGraw]

What? Is someone not "read a step, do a step"? :rolleyes:

Don
NAR 53455

You mean, we're supposed to do all the steps? ... in sequence? :eek:

[CPMcGraw]

Why not use "friction-fit" and DITCH the motor hook altogether ?

That was sort of the problem we were running into, GHR. Jay and I are using friction-fit motors in these two Mk I prototypes, and we are both just not getting enough friction to hold the motors in-place. We've had identical blow-outs at deployment, and we're just trying to prevent this from happening in any future kit release.

It wasn't disasterous for either of us, but if a motor blows out without tossing the laundry, a ballistic Tau Zero can still prang rather hard from 300'...

[falingtrea]

One thing that I have used is Tyvek. You may have seen it used as wristbands for places. You can get this at just about anyplace that sells mailing supplies. A lot of mailing evelopes are made out of Tyvek. You can probably get it free from the post office. I have used it as a sheath for motor hooks. CA holds well to it. And paint will stick once you rough up the surface a little. I have even used Tyvek for fin joint strengthening.

[snaquin]

That was sort of the problem we were running into, GHR. Jay and I are using friction-fit motors in these two Mk I prototypes, and we are both just not getting enough friction to hold the motors in-place. We've had identical blow-outs at deployment, and we're just trying to prevent this from happening in any future kit release.

It wasn't disasterous for either of us, but if a motor blows out without tossing the laundry, a ballistic Tau Zero can still prang rather hard from 300'...

I friction-fit motors in most of my rockets and here's something you may not have considered. Leave a short piece of motor mount tube extending from the rear of the rocket to tape the motor to. If no motor tube is used you can move the fins forward about 1/4" and leave about 1/4" of the motor to overhang the mount. Put a wrap or two of 1/4" masking tape around the motor and that will act as a thrust ring to keep it from moving forward. Tape over the extended motor tube or body tube with 1/2" masking tape directly to the motor to hold it in.

I'm almost finished with the rocket below and I'll be posting a thread on it soon. It is near minimum diameter Semroc LT-125 and you can see 1/2" of motor tube Semroc LT-115 extended from the rear to tape the 29mm motor to.

It may not be feasible to modify a future prototype of the Tau Zero in this manner but this simple method does work very well.

[CenturiGuy]

I friction-fit motors in most of my rockets and here's something you may not have considered. Leave a short piece of motor mount tube extending from the rear of the rocket to tape the motor to.
[SNIP]
It may not be feasible to modify a future prototype of the Tau Zero in this manner but this simple method does work very well.Steve,

Thanks for your excellent suggestion! Unfortunately, the guy who designed the Tau Zero ("Just WAIT 'til I find him!") made the mistake of mounting the fins right at the end of the body tube. If I manage to track him down, I think he and I will have a little talk about that very thing... :eek:

[CenturiGuy]

Carl,

One final question on this topic.


If HTC-7 measures thusly:
From the P.O. - .673 ID X .713 OD X .020 Wall 1.000" Long Fishpaper
....Would HTC-7B have these dimensions?

.761 ID X .781 OD X .020 Wall 1.000" Long Fishpaper


Please confirm or deny. Thanks, and cheers,

[Carl@Semroc]

Carl,

One final question on this topic.


If HTC-7 measures thusly:

....Would HTC-7B have these dimensions?

.761 ID X .781 OD X .020 Wall 1.000" Long Fishpaper


Please confirm or deny. Thanks, and cheers,

The ordered dimensions are:

HTC-7B - .761 ID X .801 OD X .020 Wall 1.000" Long Fishpaper

but we have one "bad" lot that is slightly bigger that we did not throw away that has a little more play for an engine hook. We use it for the pop-pod on the Swift-BG (unreleased).

[Tweener]

An observation on friction fit mounts:

I wrap my engine just front of halfway with plain old 1/2" wide masking tape (more than I need) and test-fit, tear, test-fit, tear until I can just twist it all the way to the engine block. At this point, the only way I am going to get it back out is with a ram-rod (usually a 3/16" launch rod) from the front, and even that might break something! After launch when the casing is hot, it takes much less effort to remove it - though a ram is still often needed. I think that is one of the problems with the friction mount. It may seem tight enough before launch, but it will loosen from the heat expansion prior to ejection.

[CPMcGraw]

...the guy who designed the Tau Zero ("Just WAIT 'til I find him!") made the mistake of mounting the fins right at the end of the body tube. If I manage to track him down, I think he and I will have a little talk about that very thing... :eek:

Jay,

I just ran a quick sim of the TZ with the fins and ring components shifted forward 0.25". The performance doesn't seem to be hurt, and in fact is improved with every motor when compared to the version with the hook and sleeve.

This might have to become Prototype IV...

[Green Dragon]

I'm almost finished with the rocket below and I'll be posting a thread on it soon. It is near minimum diameter Semroc LT-125 and you can see 1/2" of motor tube Semroc LT-115 extended from the rear to tape the 29mm motor to.

That looks suspiciously like a Crown LASOR rocket to me :-) .

noting the motor mount tube sticking out - my bro always left the 29mm mount hang out the back of the LASOR 134's, but filled it as a 'mini-boatail', not as a 'tape over top ' retainer (still could tape over the boatail, but ruins the aerodynamic plus, I bet, lol )

~ AL

[CenturiGuy]

I just ran a quick sim of the TZ with the fins and ring components shifted forward 0.25". The performance doesn't seem to be hurt, and in fact is improved with every motor when compared to the version with the hook and sleeve.I wonder if that extra 1/4" helps smooth out the airflow, a little bit like a boattail. Hmm...


This might have to become Prototype IV...Hmm. Although shoving the fins forward *and* using a hook and sleeve...

Craig, buddy, you make my head hurt. ;) :p

Although *your* Prototype V could be *my* Prototype II... :eek: :rolleyes: :D :cool:


I'll have to mess with the RockSim file some more.


Cheers,

[CenturiGuy]

The ordered dimensions are:

HTC-7B - .761 ID X .801 OD X .020 Wall 1.000" Long Fishpaper

but we have one "bad" lot that is slightly bigger that we did not throw away that has a little more play for an engine hook. We use it for the pop-pod on the Swift-BG (unreleased).Hmm... What are the (ballpark) measurements for those "slightly bigger" couplers? Those would be a lot easier to slip over an ST-7 with an engine hook in it.


Curious,

[Carl@Semroc]

Hmm... What are the (ballpark) measurements for those "slightly bigger" couplers? Those would be a lot easier to slip over an ST-7 with an engine hook in it.


Curious,They appear to be about .003" oversize. Measuring tubes is an art that I have never mastered. I can get about any result I want (or don't want.)

I need to send you some to play with. What is the final length of the ST-7?

[snaquin]

That looks suspiciously like a Crown LASOR rocket to me :-) .

noting the motor mount tube sticking out - my bro always left the 29mm mount hang out the back of the LASOR 134's, but filled it as a 'mini-boatail', not as a 'tape over top ' retainer (still could tape over the boatail, but ruins the aerodynamic plus, I bet, lol )

~ AL

:)

AL,

Lasor 134 thread coming to a forum near you - soon .....

That mini-boatail is a pretty cool idea too!

[CenturiGuy]

They appear to be about .003" oversize. Measuring tubes is an art that I have never mastered. I can get about any result I want (or don't want.)

I need to send you some to play with.I'll contact you off-forum about that.


What is the final length of the ST-7?I think Craig is lobbying for 10". I wanted to keep it at your standard 9", but Craig has this cool idea about shoving all of the fins forward 1/4", which also messes with the stability a little bit, and *then* you factor in the added rearward weight of the the coupler and engine hook... ("worn out" emoticon)

Like I told Craig, I need to mess with the RockSim file some more. Once I decide, I'll let you know.


Thanks again, as always,