# Tethys tips?

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#### BlueNinja

##### Well-Known Member
I just ordered my Tethys, and I've heard a lotta good things about it. It'll be my first HPR kit, so a lot of techniques are new to me. I still need to get some epoxy, though...

Any gotchas or tips for a newbie to HP rockets?

Thanks,

Blue N150

#### Justin Horne

##### Well-Known Member
We built one a while back, really nice!

Hints... um.. I don't think of any specific to the Tethys. As with the piston, if you got phenolic, wait till the tube is as cold as it will be, then sand the piston for the tube in that condition, and it will never be too tight. Also, after ever other flight of so, clean the inside of the tube of charge powder to keep the piston sliding well...

That's all I can thing of!

#### BlueNinja

##### Well-Known Member
I got the stock QT, ordered it from Al's so i didn't really have all the options... Oh well, I would have ordered it stock from PMl anyways. How quick do you think it could be built?

B N150

#### lalligood

##### Well-Known Member
Take a look at my EMRR review of the PML Phobos. You can apply the same techniques I used on my Phobos with your soon-to-be-delivered Tethys. Only thing I might change is to use polyurethane glue for the inner fin fillets instead of foaming the entire fin can. Same result but with less materials (& weight)...

#### BlueNinja

##### Well-Known Member
Originally posted by lalligood
Take a look at my EMRR review of the PML Phobos. You can apply the same techniques I used on my Phobos with your soon-to-be-delivered Tethys. Only thing I might change is to use polyurethane glue for the inner fin fillets instead of foaming the entire fin can.

Will Gorilla glue work? I was halfway considering foaming it, though the foam is kinda expensive.

Same result but with less materials (& weight)...

Awww, I wanna build, not take pics....

But fine, I'll post pics.

B N150

#### Missileman

##### Well-Known Member
I do have a tip for you now that I think of it.
PML has you remove the aft centering ring after you get the fins tacked into place so you can apply internal fillets.
I use what I call syringes (don't know the real name) they look like eye droppers and are used for applying CA. I cut the tip off to apply epoxy. You can get them for about 50 cents each at most hobby shops.
After appplying a set of fillets squeeze out as much epoxy from the applicator, wipe with a paper towel and hang upside down with a piece of tape. The epoxy will pool into the end of the bulb and you can reuse the dropper several times.
Works great for getting epoxy into tight places.
I have build a number of PML kits with the QT tubing and had no problem at all using epoxy, just make sure to sand the areas that the epoxy will go real good.

#### BlueNinja

##### Well-Known Member
alright

I read a PML review (CT A4?)recommending to not use the tape, push out with a popsicle stick through a fin slot? I gotta ask, how can you be sure that last fin is infront of the CR without using the CR as a guide?

#### Justin Horne

##### Well-Known Member
I suppose you could do that, but the tape works fine for me. I generally do that, then get some really hugh viscosity epoxy, put the rocket almost nose done, and just pour it in. it makes a nice fillet all the was down.

#### Missileman

##### Well-Known Member
Same here, I had no problem with the tape either. Just make sure the aft centering ring is a good fit and not too tight.
Oh and depending on what you are planning for motor retention(ie. blind nuts) you can use the retention device to pull the aft ring out.

#### BlueNinja

##### Well-Known Member
Ok... so you use teh tape to pull it off?

Maybe, since I am using a PML retainer on this, a solution would be to pre drill the pilot holes for the woodinserts, then knot up a string and put it in one... Just a thought.

#### Missileman

##### Well-Known Member
HEHE I edited my post just as you were posting. You can use the bolts threaded into the inserts.

#### joepolicy

##### Well-Known Member
I just finished my Tethys, it is a great, "easy" to build rocket. I did use a popsicle stick to push out the aft CR after the MMT epoxy had set up. It was a little tight, I had tried the tape trick PML recommends, but the tape kept breaking. I read elsewhere someone used dental floss, I have not tried that but it seems like a good idea.

Since I am using mine for a L2 attempt, I beefed mine up a bit. I roughed the inside of the tube from the front CR to the rear with a drill, extension and sanding flapper wheel. I added 12 internal fillets using Epoxy I had thickened with West 406 colloidal silica. I then glassed the fin can before adding 4lbs of weight and then PML foam to finish it off.

For retention, I used Aeropack retainers I bought from Magnum Rockets and I installed those on both the 29 and 38mm Kwik Switch (KS) mounts with JB Weld. I then glued on the KS centering rings. I was going to use the PML KS retainers, but they did not arrive in time. I am glad I have the Aeropack retainers because that means I do not have to carry (and probably lose) the hex key or the screws that come with the PML retainer.

The only other things I did outside of "stock" was to glass the inside of the piston (I had read on TRF that they have a tendency to crack) and added 4 pounds of weight to the nose cone which I set in place with epoxy and then I added foam to really hold everything in place. I also used rail buttons instead of the brass launch lugs and I added a larger chute because of the extra weight. Honestly, I think mine is overbuilt, it seems like a tough rocket when built stock, but since it was for L2, I went a little crazy.

Have fun with your build, you will enjoy it immensely!

#### llickteig1

##### KLOUDBusters Chief Logistician
I think mine is overbuilt, it seems like a tough rocket when built stock, but since it was for L2, I went a little crazy.
Now there's the understatement of the week. The Tethys is a tough rocket when built stock. There's no particular need to add a bunch of extra weight with fiberglass and nose weight. If you'll notice from the Kit Specs chart on the PML web-site, there are a couple of the smaller kits that use .093 G-10 instead of .0625. Tethys and Sudden Rush. These are two tough stud rockets.

It can't be stressed enough: rough up the QT and G-10 where any epoxy joints will be. When you think they are rough enough, do it some more. On your internal fillets, you can do either heavy fillets on each of the 9 places where fins and MMT and fins and BT meet, both sides -- OR -- you can do lighter fillets and expanding foam. The foam is sticky stuff and adds structural strength. Use the PML Urethane 2-part foam.

If you use the piston, in addition to sanding it well, make absolutely certain that you do not get any drips or dribbles of epoxy on the inside of the BT above the motor mount. If you do, it will jamb up the piston which is very bad.

I might recommend 2 1/4" T-Nuts in the aft centering ring with clips for the 54mm motor retention. Then on the 29mm and 38mm KS tubes use slim line retainers. This will be an extra investment right now, but you will never have to buy another set for any PML rockets you own and it is by far the simplest method of retention.

If you don't get in a big hurry to get Tethys built, I can show you lots of suggestions at AIRFest.

--Lance.

#### BlueNinja

##### Well-Known Member
I went with teh Public Missiles PMR retainer, it was cheap and looked easy. Simply add woddinserts to any rocket to equip with the (omni?) ring.

#### swimmer

##### Well-Known Member
When you build your Tethys concentrate on the quality of application not the quantity of epoxy you use. All that extra epoxy does is add weight. (My .02 worth)

I had a tendency to create fat fillets. You know, if a little its good a lot must be better? I decided fat fillets are just fat and don't add a lot of structural strength. Now I use a craft stick or tongue depressor to form external fillets. The concave appearance and the adequate epoxy left behind makes a beautiful, yet structurally sound, fillet.

Don't start your build until after you see Lance's Tethys. You may want to make some modifications, like making your Tethys zipperless, before you start construction. If you have any idea you want to eventually use electronics the zipperless mod is easier now than after you complete the project.

And.......I can't find my Tethys instructions.

#### BlueNinja

##### Well-Known Member
Originally posted by Swimmer
When you build your Tethys concentrate on the quality of application not the quantity of epoxy you use. All that extra epoxy does is add weight. (My .02 worth)

I had a tendency to create fat fillets. You know, if a little its good a lot must be better? I decided fat fillets are just fat and don't add a lot of structural strength. Now I use a craft stick or tongue depressor to form external fillets. The concave appearance and the adequate epoxy left behind makes a beautiful, yet structurally sound, fillet.

Don't start your build until after you see Lance's Tethys. You may want to make some modifications, like making your Tethys zipperless, before you start construction. If you have any idea you want to eventually use electronics the zipperless mod is easier now than after you complete the project.

I was planning on having it built for airfest, but that would be jumping the gun now that I think about it.

And.......I can't find my Tethys instructions.

Thanks anyways.

#### BlueNinja

##### Well-Known Member
For redundancy, I was wishing to be able to use a timer in this rocket for charge deployment. I noticed the PML payload bays, QT, I would probably only need 9", however it adds 7 oz to the rocket, meaning no G flights pretty much period unless the payload section could be taken off and on. I'm checking out LOC
right now.

B N150

EDIT: The LOC e-bays are expensive, however I could get a pretty long payload bay for $7, so that isn't too bad... Has anyone used the LOC bay before? what hardware does it come with, or is it just a tube, coupler and bulkhead? #### llickteig1 ##### KLOUDBusters Chief Logistician A stock Tethys on a G motor ... hmmm. wRASP shows launch rod velocity with a G64 and 6' rod to be 36 ft/sec. Pretty slow in my book, so be wary of G flights In my mind needing a redundant charge for deployment won't really be necessary. Make sure you keep the grease out of the ejection charge well on your motor and the motor in the rocket -- if the motor lights the ejection charge will work. Perhaps plus or minus a couple of seconds from apogee, but it will work. Getting a timer to pop at apogee is a crapshoot anyway. A more common failure is blow-by, and things not coming out are not a problem in that case. I can also show you a good electronics bay / payload bay set-up in my modular Tethys. Maybe I'll take some photos today and post them later. --Lance. #### BlueNinja ##### Well-Known Member Yes, it is kinda slow... I'll do a mass override with my weight on rsim after it's built and all that, then determine whether or not a G flight would be safe. I have an idea for a "charged piston" that I'll tell ya about at airfest. B N150 #### gregzo ##### Well-Known Member I have the second EMMR review for the Tethys. https://www.rocketreviews.com/reviews/kits/pml_tethys.html My build was quite heavy - w/o a paylaod it would never fly on a G which was my origianl plan. I'm very happy with it however. Since my L1 cert flight - I've added an 18" payload with a altimeter to use my modiied Tethys for my L2 cert. Just waiting for weather to cooperate...... Great Rocket!!! Good Luck. #### NewEntity1 ##### Well-Known Member Thread hijack alert! I wanted to ask Lolligood about his Phobos... I bought a 38" mmt Phobos w/ standard Quantum tubes at the last ROC launch; I won't be ready to start building it until I finish a couple of other rockets first, though. I have a few questions about it, however... 1) How long of a motor tube do I need for a 720 ns 38mm case? This would be the largest I would fly the Phobos on ever (say, for instance, an AT I132, which is a 100% I). 2) What grit should I use to rough-up the QT tube and fins? I figure something fairly course, for a deep "bite". 3) Since I have the 38mm mount, the PML retainor won't work. You mentioned using an Aeropack retainer...would a Slim-Line retainor work as well? They are not quite as expensive as the Aeropack ones (though still costly). 4) Would a 29mm slimline retainer work on the PML 38-29mm adaptor? Thanks in advance to your answers to these questions. And BN150, good luck with your Tethy's #### Justin Horne ##### Well-Known Member I'm one of the people that doesn't think you need a full length motor tube ( IE, one the covers the entire casing.) I think go for 10-12 inches, and you'll pretty much be fine.... And im sure you could get away with less.... Oh, and no, the slimline won't work on the adapter, as the adapter has no tube 'sticking out' to glue the slimline onto.This is where i'd say put an Aeropack on, then use the aeropack adapter, and you are guaranteed (well, darn close) to get everything back... #### BlueNinja ##### Well-Known Member Maybe, you should use a 17" tube (extended KS length). #### jetra2 ##### Well-Known Member I agree with you on the motor tube thing Justin. It's just a matter of imagining how the motor transfers its thrust through the airframe. When you think about it, the thrust ring pushes on the end of the motor tube, which pushes on the aft CR (hope ya got good fillets!) and then the CR distributes the load throughout the airframe. The reasoning I see behind a longer than normal BT is using a hybrid motor, which means you need support for entire case+NOS tank, but usually, unless you have an extra long motor case, like the 38/1080 (longer than the others!), you want support for it, because I guess you don't want it wiggling around in the tube. I am a fan of middle-length motor tubes. Not too long, but not short, either. Jason #### Ozymandias ##### Well-Known Member The Tethys is a great first sport kit. The only real advise I can give you is read, understand, and follow the directions. The only real problem I have with PML kits is the method for removing the aft centering ring to apply inner fillets to the fins and motor mount tube. Before you begin building the kit give the aft ring a little sanding to make it easier to remove. You'll thank me later. Have fun and keep the pointy end up! #### NewEntity1 ##### Well-Known Member Oh, and no, the slimline won't work on the adapter, as the adapter has no tube 'sticking out' to glue the slimline onto. Actually, the 38-29mm adaptor I have has a thrust ring built into the end of it, which sticks a half inch out the end of the 38mm mount tube. The motor hardware inside the 29mm adapter sticks out a further half inch, making the whole thing look like an inverted step pyramid . I'll show a picture of it once I free up the current load of pics on my camera. It's not like the LOC adapter, which is just a 29 mount tube with 2 centering rings on it. I would hate to waste this adapter I already paid$12 for (Whats Up Hobby's only sold them pre-assembled). Unfortunately, I'll still have to buy the Aeropack adaptor and retainer eventually, because while I could put a slim-line on the 29mm adaptor, I would NOT be able to put a slim-line on the 38mm tube and ever be able to use the 29mm adaptor afterwards, again due to this adaptor having a built in thrust ring on the end. So it looks like I'm stuck with paying $25 for an Aeropack, plus another$20 for the Aeropack adaptor, regardless . Sheez, thats about as much as I paid for the entire rocket!!!

Thanks for the info . As for the main 38mm mount tube, I'll take your advice and go with about a 12" tube length max, I think.

Edit: Come to think of it, a slim-line would not fit on the thrust ring of the adaptor I have anyway, because its wider than 38mm! doh!

#### Justin Horne

##### Well-Known Member
I know, i've got the 38/29 adapter. What i meant was you couldn't put a slimline on it. Did i interpret your question wrong??? Sorry if i did...

#### lalligood

##### Well-Known Member
Originally posted by NewEntity1
1) How long of a motor tube do I need for a 720 ns 38mm case? This would be the largest I would fly the Phobos on ever (say, for instance, an AT I132, which is a 100% I).
The stock length MMT tube should be fine. Hope you got a big field for that rocket-motor combo

Originally posted by NewEntity1
2) What grit should I use to rough-up the QT tube and fins? I figure something fairly course, for a deep "bite".
100-150 grit should be adequate. The roughed up areas will pretty much becovered with epoxy and/or sanded smooth once you're done with the external fin fillets.

Originally posted by NewEntity1
3) Since I have the 38mm mount, the PML retainor won't work. You mentioned using an Aeropack retainer...would a Slim-Line retainor work as well? They are not quite as expensive as the Aeropack ones (though still costly).
No problems with the Slimline. I chose the Aero Pack because it was a threaded adapter instead of requiring snap ring pliers. Both work great!

HTH,

#### NewEntity1

##### Well-Known Member
Thanks for the info I'll probably use 100 grit then...I tend to save the 150 and 220 stuff for smoothing after fillits have already been made.

No problems with the Slimline. I chose the Aero Pack because it was a threaded adapter instead of requiring snap ring pliers. Both work great!
Justin Horne pointed out that the slim-line would not fit on the 38-29mm adaptor I have, though :/. It originally sounded like he was saying it would fit because the adaptor doesn't stick out the end, but it turns out he just meant that the part that does stick out is too wide for the Slim-line.

It looks like at this point that I could use either the Slim-line or the Aeropack on the main 38mm mount tube, but that if I use either, I will not be able to use the 38-29mm adaptor I bought with the rocket.

This leaves me with either a) my own custom retainor that would wrap over both the adaptor and the motor itself (I know a way to do this...but its not pretty )

A third option would be to file the thrust ring off this adaptor I have, and glue the Aero-pack or Slim-line retainer where the thrust-ring used to be, using the Aeropack/Slim-line itself as the new thrust ring.

#### lalligood

##### Well-Known Member
I bought & have used the Aero Pack 38-29 adapter. If you think the retainers are bulletproof, wait till you see their adapters! It wasn't cheap (almost as much as the retainer), but it's so easy to use. It also maintains that clean look of the retainer making almost impossible to tell what kind of motor is loaded up in the rocket.

Just a happy Aero Pack customer's view...

BTW, I know that newentity kidded about hijacking this thread as he had questions about his Phobos but everything in this thread is completely applicable to a QT-based Tethys too

HTH,