PML AGM 256 Pitbull. Please take pity on the BAR and help

[Zoomie]

Posting with my hat in hand. Would one or more of you please be so kind as to offer some assistance and opinions ?

I have read so much that my head officially hurts. I think I'm more confused than ever. Let me explain what I think I'm trying to do, then I'll add questions at the bottom.

I have become smitten with the PML AGM 256 Pitbull ( https://publicmissiles.com/kits/scalekits ) for my first mid power build. It is a confusing choice from a number of perspectives. 38mm MMT. PML claims that it can be flown on 29mm F and G. Sooo, being the greedy, budget minded individual that I am, I was hoping to run a 29/38 conversion with reloads. I would fly a bit with the remote thought that I could potentially do an L1 cert and stand a good chance of passing.

Based on opinion here, I understand that many recommend against this idea, stating that it would be better to build a dedicated MPR, and then something else for L1.

Now here is where I get confused. When I looked at 38mm reloads (Aerotech) on PML's motor guide sheets
( https://publicmissiles.com/images/pmlalti.pdf ), the H's were pushing this thing high enough to bounce of the F1 layer of the atmosphere. I saw nothing but 5000 feet and up. I would want low and relatively slow for L1 cert, based solely on the fact that what goes up higher, lawn-darts better!

What am I missing?

So my questions are........

Am I trying to put a square rocket in a round hole ie ask it do something it was not designed to do?

Since I can't live without this kit, should I just run it with a 29/40/120 and in the event I want to make the leap to HP via L1 cert, take my chances like many others, building a second rocket dedicated to High Power only and first flight at L1 be damned?

And finally, if anyone has any recommendations on build necessities, Id be grateful. I'd rather spend money on motors than purchasing a bunch of stuff that I may or may not use.

Thank you in advance.

 

[dhbarr]

Well, if you want to keep it low, you can always add balanced removable drag+weight aft, weight forward.

 

[smugglervt]

If it's a kit you really like then I'd go with it and use the 29/38 adapter. There are some nice AT H's out there now that are Hazmat free and would work fine for an L1. If you haven't already done so, get OpenRocket (it's free) and sim your rocket with the different motors and see which falls within your comfort level.

 

[Zoomie]

Jim thanks!

I have Open Rocket. I just haven't figured out where the files are.

 

[smugglervt]

Jim thanks!

I have Open Rocket. I just haven't figured out where the files are.

K'Tesh must have one for the PML AGM 256 Bullpup. I pulled down the Rocksim file from PML's page but OpenRocket didn't like the fins.

What say you Jim?

K

 

[Exactimator]

There are as many ways to do rocketry as there are rocketeers. There’s no problem with doing an L1 cert on an MPR that’s capable of handling it. In fact if you do a search you’ll find several Estes Leviathans, MDRM’s, and other mid power rockets used for HPR.

I don’t have a PML Pitbull, but it looks like it’s an MPR/HPR rocket. PML’s site says “This kit is also designed to fly on high impulse F and G motors for Pre-Cert flights and most H and I motors for Cert 1 flights."

The RockSim file that you would use in OpenRocket is here: https://www.publicmissiles.com/images/AGM256-38mm.zip

You can also get a quick altitude and speed table based on diameter and weight at https://www.thrustcurve.org/guidepage.jsp with the Motor Guide. If you figure your rocket will weigh about 2.5 lbs, built and ready to fly without the motor, altitude is about 1,500 feet on an H165. A little over 1,600 feet on an H128. Those are very manageable altitudes where you can easily see your rocket at apogee. From the tables, F motors are too low thrust to get the rocket off the rail fast enough to be stable (again assuming the rocket weighs 2.5 lbs ready to fly, without the motor).

The advantage is if you want to feed it G motors, the flights cost less.

Once you get used to what altitudes you can still see your rocket (I’ve been able to track rockets at 9,500 feet on a clear day, lose them at 3,000 feet on other days) you can put a 38mm I motor in there and really have fun.

Five minute epoxy and some paint should be enough to get you going. You might need some BBs or lead shot for nose weight to get it balanced. Everything else will come with the kit if you add the adapter and retainer.

You'll probably find once you launch this on a G, you'll want more motor, more power. So you’ll work your way up to I’s. Then you’ll want a bigger rocket to handle bigger motors. Then bigger rockets. Then bigger motors. Within a few years you’ll be hiding your credit card bill from your wife and researching how much you can sell a kidney for. It’s all part of the experience.

 

[Steve Shannon]

Build it with the 38. As long as you fly it on a motor with similar average thrust you should be able to fly it on lower impulse motors to lower altitudes. I'm not a fan of adding weight to reduce altitude because then you change all of the loads associated with deployment also.


[emoji1010] Steve Shannon [emoji1010]

 

[75Grandville]

Zoomie,

When I looked at the entry in the pdf table you linked to, it does show some extreme altitudes for an H. I think those might be too high.

Plugging in the 2.6" diameter, 38mm motor mount and 29 oz dry weight for the AGM 256 Pitbull on thrustcurve.org, it looks like an H will take you to between 1800 and 2500 feet. I think you should be safe with an L1 on this rocket. Build it with the 38mm mmt and adapt away! 4000 feet should require a J. If you build it very well, consider a Loki K1127 for your L2:y:

I pulled up an Estes Argent in OpenRocket (Dry weight 17.7 oz.) for comparison. An H128W sims to 2285 ft, and and I200W sims to 3254 ft. Further confirming my hypothesis that the altitudes in the table you referenced are off!


@Exactimator - no fairs posting my answer while I am still typing it myselfs!!!

 

[Exactimator]

Zoomie,

When I looked at the entry in the pdf table you linked to, it does show some extreme altitudes for an H. I think those might be too high.

Plugging in the 2.6" diameter, 38mm motor mount and 29 oz dry weight for the AGM 256 Pitbull on thrustcurve.org, it looks like an H will take you to between 1800 and 2500 feet. I think you should be safe with an L1 on this rocket. Build it with the 38mm mmt and adapt away! 4000 feet should require a J. If you build it very well, consider a Loki K1127 for your L2:y:

I pulled up an Estes Argent in OpenRocket (Dry weight 17.7 oz.) for comparison. An H128W sims to 2285 ft, and and I200W sims to 3254 ft. Further confirming my hypothesis that the altitudes in the table you referenced are off!

I noticed that too. I'm not sure how PML calculates their tables, but it looks incorrect.

And 75Grandville, the important thing to remember here isn't that we're helping a BAR. It's that I WON!

BOOM!

 

[Buzzard]

Zoomie,
I built a Pit Bull a few years back, but haven't finished it as I got way too much epoxy on the strakes that needs to be sanded down and other projects took priority.
My biggest error was early on by not planning for motor retention. I had followed the instructions and had the motor tube flush with the end of the boattail. in hind
sight I would have moved the motor tube back to install a threaded retainer like an Aero Pack 38mm. I ended up drilling a hole and putting in some threated rod and
will use a washer and nut for motor retention.
I concur with the thought of building for 38mm motors and using an adapter for 29mm motors. In other threads there have been discussion of not using the piston
system and using a parachute protector instead. That will be the route I go when I get around to finishing this build.

Chas
Charles

 

[Zoomie]

I cant thank you guys enough ! amazing information and exactly what I was looking for. You have no idea how many hours I read because no one wants to be the guy that gets called out for being lazy!

I shall honor all of you by becoming a supporting member.

And you can all hold your head high knowing that you are NOT contributing to the delinquency of a BAR. I would have done it without anyone's assistance.....it just would have ost me a lot more money to figure it out without your assistance. So technically, each and every one of you are providing a valuable public service by helping a new fools get the biggest bang for his rent money.......I mean disposable income.

Yeah. My wife should buy that !

 

[75Grandville]

For tailcones/boattails, I prefer the Giant Leap slimline retainers: https://giantleaprocketry.com/products/components_retainers_adapters.aspx

I've got one in my Jayhawk, and the end of the retainer is flush with the end of the tailcone - nothing protrudes outside of the rocket.

 

[Zoomie]

Ill check it out, Bill. Thanks.

Well, PML isn't exactly slow to ship. I had a tracking number within 2 hours of ordering !

 

[Zoomie]

I ran the file in OR that Exactimator was kind enough to suggest. There are no rear fins ! I'll send an email to PML and see if they have any ideas.

 

[K'Tesh]

K'Tesh must have one for the PML AGM 256 Bullpup. I pulled down the Rocksim file from PML's page but OpenRocket didn't like the fins.

What say you Jim?

K

Nope... No Pit Bull/Bullpup file.

To do it justice I'd need a lot of info on it... Scans of the fins (along with ruler), optional scans of decals (again with ruler), data on the nosecone, body tube(s?)(Length, OD, ID)(including motor tube), instructions, CR thicknesses (fins too), and materials. Weights would be something that you'd need to edit for accurate sims, as I don't have access to a scale anymore.

 

[Zoomie]

K, please don't go through the Herculean effort. I have enough info to operate it safely.

 

[K'Tesh]

K, please don't go through the Herculean effort. I have enough info to operate it safely.

Looks like fun...

Found the instructions and the .rkt file... It's just a matter of checking it against the instructions.

The fins didn't come through as OR doesn't like attaching fins to transitions.

 

[MaxQ]

I have become smitten with the PML AGM 256 Pitbull <snip> PML claims that it can be flown on 29mm F and G. Sooo, being the greedy, budget minded individual that I am, I was hoping to run a 29/38 conversion with reloads. I would fly a bit with the remote thought that I could potentially do an L1 cert and stand a good chance of passing. Based on opinion here, I understand that many recommend against this idea, stating that it would be better to build a dedicated MPR, and then something else for L1. Now here is where I get confused. When I looked at 38mm reloads (Aerotech) on PML's motor guide sheets <snip> I saw nothing but 5000 feet and up. I would want low and relatively slow for L1 cert, based solely on the fact that what goes up higher, lawn-darts better! What am I missing? So my questions are........
Am I trying to put a square rocket in a round hole ie ask it do something it was not designed to do?
Since I can't live without this kit, should I just run it with a 29/40/120 and in the event I want to make the leap to HP via L1 cert, take my chances like many others, building a second rocket dedicated to High Power only and first flight at L1 be damned?
Thank you in advance.

I must be looking at a different PML altitude/motor chart....
I followed your link to the PML chart (pg. 5 - 3rd line) and the 38mm "H" motors appear to predict altitudes of 2,323 feet (H73J) to 3,720 feet (H112J).

Yes, I see the "I" motors in the chart (pg.6 - line 3) predict 4,888 feet (I-154J) to 5,319 feet (I-211W).
But if your question is about Level 1 cert with an "H" (I think that is what you said) and not an "I" - the altitude looks more like 2,300 - 3,720 feet.

All that side, and those are all estimates at best - I think the advice given is right on.

Build it stock with 38mm motor mount - the adaptor for 29mm gives you freedom to multiply your motor selection and go lower as the field and weather conditions warrant.
You'll find plenty of motor choices to keep you busy for awhile.
There may be recommended mods for this kit out there, but I think the most important thing for a Level 1 is to make sure that the recovery harness is top notch...no elastic, get a nomex sheath for the harness if the kit doesn't have that PML piston...harness connections tight.

I did my Level 1 back in the day with a 38mm 29 oz. rocket (Loc Precision Loc IV) that was an F, G, H motor recommended rocket.
Kept it simple to get the Level 1 quickly - no electronics/no dual deployment, just motor ejection at apogee (there is where the considerations come into play).

Flew it on a G, and then an H to cert.
The H was a single use Vulcan H100 Smoky Sam (well - that dates me) flew almost out of sight .......it was a LONG walk to recover, fortunately there were no tree lines at Culpeper at the time to worry about.
I think you were on the same track, given your concern about altitude, and having no dual deployment (if I read you correctly).
Hey...if you like this hobby, you'll eventually be building a dual deployment bird sooner or later, and keeping this sexy Pit Bull 38mm safe for no/low wind days.

https://shop.locprecision.com/product.sc?productId=121

 

[Zoomie]

Max, awesome post and thanks for taking the time!

The he allure of an F,G,H rocket for me is being able to test fly a bit.

Id prefer to keep it low for a Cert flight. Less potential chance for issues and besides, I don't want the stress of doing a half mile walk wondering if a fin snapped on landing!

she'll be here tomorrow. While I'm anxious to get started, I intend to be very methodical as I want my intro in to MPR to be successful.

And we wont talk about me buying a Striker to stash in the closet to egg me on to Cert!

 

[MaxQ]

Max, awesome post and thanks for taking the time!

The he allure of an F,G,H rocket for me is being able to test fly a bit.

Id prefer to keep it low for a Cert flight. Less potential chance for issues and besides, I don't want the stress of doing a half mile walk wondering if a fin snapped on landing!

she'll be here tomorrow. While I'm anxious to get started, I intend to be very methodical as I want my intro in to MPR to be successful.

And we wont talk about me buying a Striker to stash in the closet to egg me on to Cert!

One of the first bullets you'll have to bite is that investment in motor reload hardware.
Not sure what you have acquired so far....
I was kind of kit crazy when I started, but the vendor (Ken Allen - Performance Hobbies) suggested I buy a Dr. Rocket hardware set.
Don't recall what it cost me at the time, (and I think they went out of production - to be replaced by Rouse Tech as an alternative) but Ken was right, that 29-38mm hardware set was one of the best investments I made.
Took me through many years of flying, right up to Level 2.

A look at the motor charts here shows lots of options, from F to J in 29mm to 38mm.
29/40-120 and 29/60-120 hardware will give you some nice F thru G options to tailor your flight profile.
All a matter of choice.

https://www.aerotech-rocketry.com/u...4604-8ab9-5d77c22a4976_AT Catalog Updated.pdf
https://www.rouse-tech.com/monstermotors.htm

And then there is Cesaroni..............
https://www.pro38.com/products/pro29/pro29.php
https://www.pro38.com/products/pro38/pro38.php

 

[AlfaBrewer]

I ran the file in OR that Exactimator was kind enough to suggest. There are no rear fins ! I'll send an email to PML and see if they have any ideas.

As K'Tesh said, OR doesn't attach fins to tailcones. What I've done in similar cases is to attach the fins to the body tube and then adjust the location back to the transition/tailcone. It's not perfect, as you lose the fin area near the tailcone, but it gets CP in the ballpark.

View attachment AGM256-38mm.ork

 

[Zoomie]

One of the first bullets you'll have to bite is that investment in motor reload hardware.
Not sure what you have acquired so far....
I was kind of kit crazy when I started, but the vendor (Ken Allen - Performance Hobbies) suggested I buy a Dr. Rocket hardware set.
Don't recall what it cost me at the time, (and I think they went out of production - to be replaced by Rouse Tech as an alternative) but Ken was right, that 29-38mm hardware set was one of the best investments I made.
Took me through many years of flying, right up to Level 2.

A look at the motor charts here shows lots of options, from F to J in 29mm to 38mm.
29/40-120 and 29/60-120 hardware will give you some nice F thru G options to tailor your flight profile.
All a matter of choice.

https://www.aerotech-rocketry.com/u...4604-8ab9-5d77c22a4976_AT Catalog Updated.pdf
https://www.rouse-tech.com/monstermotors.htm

And then there is Cesaroni..............
https://www.pro38.com/products/pro29/pro29.php
https://www.pro38.com/products/pro38/pro38.php

I am currently in the process of learning about reloadables. Leaning toward AT and would grab cases in 29mm and 38mm to cover broadest range of loads.

That said, looking at Cearoni and Loki as well.

Will ll continue to read and study until something sticks in my brain.

 

[Zoomie]

As K'Tesh said, OR doesn't attach fins to tailcones. What I've done in similar cases is to attach the fins to the body tube and then adjust the location back to the transition/tailcone. It's not perfect, as you lose the fin area near the tailcone, but it gets CP in the ballpark.

View attachment 290759

you rock sir! This gives me something far closer.

still trying to learn the basics of OR so I can motor info over and review.

Sincerest thanks!

 

[AlfaBrewer]

You'll need to measure the rear fins and adjust the OR file. I eyeballed the measurements from a picture.

 

[Zoomie]

Roger sir.

It's on the truck for delivery so I can do it as soon as it arrives !

 

[75Grandville]

There are lots of discussions on reloads. The best advice I can offer is: Find out what your local vendor stocks!

 

[Zoomie]

OK, this might be one of those moments reflecting upon historical life choices.

Disclaimer - The OP publically admits that he should have spent a little more time learning to fondle his abacus and slide rule......and a little less time testing recreational substances. You were right, dad.

With that out of the way, I'm about to fling myself on a number 2 pencil.

I DL'd the .ork file above with the hopes of plugging in accurate fin dimensions. I started attempting to take measurements and that's where it CATO'd. Between fin shape and understanding how much fin surface is exposed once mounted in the boat tail, I am seriously considering abandoning model rocketry and embracing my secret love for ancient Hawaiian basket weaving.

How in Hades, does one understand or calculate the exposed wing surface/dimensions. What am I missing? Or do I just have to wait until I build it and then take measurements. And if this is the case, how to I measure the relief of the boat tail?

Am I making this more complicated than necessary?

Here's some pics so I can explain my confusion........

Here is the fin on 1cm graph paper.

Here are a bajillion pics of the boat tail.........

 

[K'Tesh]

As K'Tesh said, OR doesn't attach fins to tailcones. What I've done in similar cases is to attach the fins to the body tube and then adjust the location back to the transition/tailcone. It's not perfect, as you lose the fin area near the tailcone, but it gets CP in the ballpark.

View attachment 290759

When it comes to fins mounted to transitions, my trick is to create a "Phantom Body Tube" that is the diameter of the motor tube, then attach the fins (with tabs and all) to that. That way you maintain the fin area near the tail cone, and OR's guesstimates as to the weight.

 

[Zoomie]

When it comes to fins mounted to transitions, my trick is to create a "Phantom Body Tube" that is the diameter of the motor tube, then attach the fins (with tabs and all) to that. That way you maintain the fin area near the tail cone, and OR's guesstimates as to the weight.

OK, I'm with you.

Can I just dry fit fins in to Boat Tail, with engine mount installed, then take measurements from it?

 

[K'Tesh]

OK, I'm with you.

Can I just dry fit fins in to Boat Tail, with engine mount installed, then take measurements from it?

I'm sure you could, but it'd be hard to translate that into something that OR would be able to handle.

Oh, and for ease of measuring the fins from a scan, it's best if the ruler is along the root edge of the fin... BTW... are they really 1/16" thick?