Polecat Goblin 5.5 questions

I just got one of these kits a few days ago and I'm wondering what changes if any others who have built and flown Goblins have made. It's a nice simple kit, fairly complete and comes with instructions that are well written and clear.

There have been discussions already about the fins, directions mention nothing about internal or external fillets. I have not decided yet how I will go, it will be difficult for me to not glass the fins and make fillets like I always have.

My biggest questions concern the nosecone and the use of nose weight. The instructions plainly state the rocket needs no nose weight. I downloaded a RockSim file for the Goblin but I'm not sure of its origin. It shows a nose weight subassembly at around 12 ounces and another 2 ounces of epoxy. that is in addition to the included U bolt assembly which will probably go 3 ounces or so. With a small Aerotech 54mm motor (J-460), stability is marginal and if I remove the weight CG and CP coincide. I also ran it in Open Rocket and get very similar results. I see a lot of posts questioning accuracy of Sim programs with stubby rockets.

I would really like to hear of other's experiences before I add unneeded weight or worse still seal up the cone with no weight an have an unstable flight.

thanks, George

Stubby rockets like a lot kick off the pad so the Aerotech J460 looks like a good choice. The Cesaroni J430 provides similar thrust and performance.

As to fillets, I used them inside and out. The inside ones were done with a removable rear centering ring which was later epoxied in place. I also added an Aeropack retainer.

You didn't mention dual deploy so gonna assume you're flying simple motor deploy. Mine has a nose cone av-bay set up for the Cable Cutter. The finished all up weight of the N/C is 25.5 ounces which includes 9v battery, paint, clear coat, hardware, etc. The 54mm version weighs about 90-93 ounces set up for single deploy at apogee depending on the size of the chute and blanket. When using an Aeropack 54 to 38 motor adapter the weight is about 95 ounces again depending on chute and blanket.

Have not yet flown this bad boy.

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o1d_dude said:

Stubby rockets like a lot kick off the pad so the Aerotech J460 looks like a good choice. The Cesaroni J430 provides similar thrust and performance.

As to fillets, I used them inside and out. The inside ones were done with a removable rear centering ring which was later epoxied in place. I also added an Aeropack retainer.

You didn't mention dual deploy so gonna assume you're flying simple motor deploy. Mine has a nose cone av-bay set up for the Cable Cutter. The finished all up weight of the N/C is 25.5 ounces which includes 9v battery, paint, clear coat, hardware, etc. The 54mm version weighs about 90-93 ounces set up for single deploy at apogee depending on the size of the chute and blanket. When using an Aeropack 54 to 38 motor adapter the weight is about 95 ounces again depending on chute and blanket.

Have not yet flown this bad boy.

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Hey Kit...really nice finish on your Goblin! Do you mind sharing how you filled the weave on the tube? I have a couple of their Black Brants and I'm searching for a finish like yours.

Cheers,
Michael

iqsy59 said:

Hey Kit...really nice finish on your Goblin! Do you mind sharing how you filled the weave on the tube? I have a couple of their Black Brants and I'm searching for a finish like yours.

Cheers,
Michael

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I heartily recommend the Rustoleum automotive Filler Primer. Several iterations of the prime/sand/repeat will get a pretty smooth surface. The Rusto fluorescent pink is a flat finish that can be overcoated with Rusto Crystal Clear enamel which is highly self-leveling. The end result is what you see. I've been wondering lately about fine sanding the clear and re-coating but haven't invested any time in experimenting with the process.

A club member has the Polecat Black Brant 5.5 and he's meticulous. His bird looks like it's been through an automotive paint shop and then waxed. You have a bit of work ahead of you.

Kit,
My Goblin will be built motor deploy only. I intend this one to be a simple to prep fast to fly rocket, besides I'm tired of buying altimeters and I don't like switching them between rockets. It looks like ready to go my nosecone will come out around 14 ounces. I'll be using an Aeropack retainer as well. According to Rocksim I need to add at least a pound in the tip of the nose to get 1 caliber stability with I or J motors. I'm not really sure why whoever wrote the instructions felt it could be flown with K motors and no nose weight. By the way, my kit is one of the older ones before they were made in Oregon. George

Best I can come up with: The longer the motor, the more forward the CG moves?

My Goblin was one of the Oregon kits. Believe mine was one of the first ones shipped.

Speaking of Oregon...you might want to call Polecat and ask them about your concerns. They usually answer the phone.

It seems like you would be right about the CG moving but the longer the motor, the farther aft it moves (not drastically, a few tenths of an inch). RockSim calculates CP at 27.61 from the tip, that's with an extra 16 ounces in the nose. I think you're right about contacting Polecat.

gltruett said:

It seems like you would be right about the CG moving but the longer the motor, the farther aft it moves (not drastically, a few tenths of an inch). RockSim calculates CP at 27.61 from the tip, that's with an extra 16 ounces in the nose. I think you're right about contacting Polecat.

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That depends on where the CG of the motor is located. If it is fwd of the CG of the whole rocket, then the whole rocket CG moves fwd with a longer motor. But, of course, you knew that. I'm just explaining it for myself.:smile:

Igotnothing said:

That depends on where the CG of the motor is located. If it is fwd of the CG of the whole rocket, then the whole rocket CG moves fwd with a longer motor. But, of course, you knew that. I'm just explaining it for myself.:smile:

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The Goblin 5.5 has a 9" long motor tube of which 1/2" protrudes from the rear centering ring. The forward end of the motor tube protrudes 2-1/2" beyond the leading edge of the four fins. Typically the CP is just in front of the fins in most rockets. The longer the motor, the more forward the CG shifts. An Aerotech J460 is what...9" long? So at least 2" of the motor extends beyond the CP, right? A 5G Cesaroni 38mm J430 is almost 15" long of which 8" of motor are forward of the Cp...just saying.

Stubby rockets seem to do well at 0.5 caliber of stability if you kick them off the pad with authority. As the motor burns stability increases, right? In a 5.5" diameter rocket 0.5 caliber is 2-3/4" beyond the CP.

Have you considered using over-sized quick-links to attach the shock cord to the nosecone? I've been known to use the 3/8" ones for balance as they're fairly heavy. Have also used 1/4lb washers (check your local industrial supply house) attached to the backside of the nose cone bulk plate secured by the forged eye bolt in my Crayon rocket.

I still think a phone call would be a good idea.

o1d_dude said:

Stubby rockets like a lot kick off the pad so the Aerotech J460 looks like a good choice. The Cesaroni J430 provides similar thrust and performance.

As to fillets, I used them inside and out. The inside ones were done with a removable rear centering ring which was later epoxied in place. I also added an Aeropack retainer.

You didn't mention dual deploy so gonna assume you're flying simple motor deploy. Mine has a nose cone av-bay set up for the Cable Cutter. The finished all up weight of the N/C is 25.5 ounces which includes 9v battery, paint, clear coat, hardware, etc. The 54mm version weighs about 90-93 ounces set up for single deploy at apogee depending on the size of the chute and blanket. When using an Aeropack 54 to 38 motor adapter the weight is about 95 ounces again depending on chute and blanket.

Have not yet flown this bad boy.

View attachment 262129

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I would have known that was yours just from the paint alone...

Well, Old Dude and I have similar paint schemes on this. Interesting...

My Open Rocket file on this tells me that I have 24 oz. of nose weight, (I just weighed the nose cone, it's a total of 44 oz., show that sounds about right).

I have very minimal fillets on the fins. They are epoxied to the rings, and the motor tube, with a minimal external fillet. The theory is, the entire fin will be knocked loose on a hard landing, instead of snapping off. Makes for an easier repair, yes I know this from experience... YMMV.

I also have an altimeter bay between 2 of the fins. This is accessed through a hole in the top centering ring, with a 1/2 centering ring screwed to it with the altimeter hanging down from the PML mount. There is a switch on the side of the rocket, a wiring harness to the switch is accessed through the centering ring. The idea is to fly this as dual deploy, with a Defy Gravity Tether. So far, I've only got one flight on this, using a J-180 with the altimeter deploying the main only.
I'm trying to attach photos of this, hopefully, there will be something to look at here.

Phil L.

gltruett said:

It seems like you would be right about the CG moving but the longer the motor, the farther aft it moves (not drastically, a few tenths of an inch). RockSim calculates CP at 27.61 from the tip, that's with an extra 16 ounces in the nose. I think you're right about contacting Polecat.

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That depends on how long the motor is.

Phil,

o1d_dude is right, you only need about .5 calibers of stability in OR for a kit like this. Here's an article that explains how to fake it in the simulators:

https://www.apogeerockets.com/education/downloads/Newsletter86.pdf

Look at page 6.

Maxitout said:

Well, Old Dude and I have similar paint schemes on this. Interesting...

My Open Rocket file on this tells me that I have 24 oz. of nose weight, (I just weighed the nose cone, it's a total of 44 oz., show that sounds about right).

I have very minimal fillets on the fins. They are epoxied to the rings, and the motor tube, with a minimal external fillet. The theory is, the entire fin will be knocked loose on a hard landing, instead of snapping off. Makes for an easier repair, yes I know this from experience... YMMV.

I also have an altimeter bay between 2 of the fins. This is accessed through a hole in the top centering ring, with a 1/2 centering ring screwed to it with the altimeter hanging down from the PML mount. There is a switch on the side of the rocket, a wiring harness to the switch is accessed through the centering ring. The idea is to fly this as dual deploy, with a Defy Gravity Tether. So far, I've only got one flight on this, using a J-180 with the altimeter deploying the main only.
I'm trying to attach photos of this, hopefully, there will be something to look at here.

Phil L.

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Yeah, I knew something about that when I built it, just wasn't comfortable with about it. So, I guess it will just haul around some excess ballast, oh well. Still flew good!
I also have a 3" Goblin, (flies great on the H400), and had a 4" Goblin, but lost that during a drag race on it's first flight. Had a K1200 in that one....

Phil L.

Yesterday I picked up some lead shot at my weekly Friday visit to Sportsman's Warehouse thinking it would come in handy if I need some ballast for my Goblin 5.5.

Today I fitted out the Goblin with a 20' One Bad Hawk shock cord, three 1/4" quicklinks, 48" Fruity Chutes classic w large blanket, Aeropack motor adapter, and a 5g Ceasaroni case with J316 reload to verify the CG vs CP relationship. The balance point of the all up Goblin was about 2-1/2" ahead of the fin/airframe junction which is typically the CP on most rockets. This implies 0.5 caliber of stability.

For the record, my nose cone with av-bay installed weighs 25.5 oz and the all up bird (motor included, ready to launch) weighs 118.5 ounces. Without motor and adapter, the Goblin weighs 97.5 ounces. I have no 54mm hardware yet so I'm not able to check stability at this time. Descent rate under the 48" Fruity Chutes should be 19 fps.

I'd recommend doing this exercise using your intended gear to verify the CP/CG relationship of your Goblin.

This is my L2 cert ship so I really want to be sure of things. At this point, it doesn't look like any nose cone ballast is necessary with the J316 and motor adapter. Should I need ballast for bigger 54mm J motors, I can always add it later though the 3" diameter av-bay hole.

Hey guys, I got a 5.5" Goblin about a 5 months ago and have yet to fly it. I am flying it at Midwest Power next weekend for my L2 but I was wondering if any of you guys bought the Electronics bay extension? I did and I'm just wondering if you have seen or heard of anyone who has flown with the extension. I'm just a little worried about the stability. When you guys fly, 1st what motors do you use, and 2nd, do you add nose weight? The extension I added is about a foot long and looks sweet! I am planning on flying on a CTI J760. According to rocksim it's supposed to go over a mile. Thanks!