Estes Goblin Question

[Lawndart]

Just finished building one and, boy it looks like the motor sticks way out the back. Is it possible that the dimensions are wrong in the instructions? Don't remember having to putting in so much nose weight either. Motor tube was longer than necessary, as well. Unless I messed it up (which I readily admit possible).

 

[samb]

I couldn't find the instructions for the latest version on Estes web site. How far is "way out the back" ? The classic instructions on JimZ has the motor tube flush with the aft end of the airframe and the motor protruding aft about 1/4 inch (the width of the thrust ring in the forward end of the motor tube. Is the nose cone plastic ? New materials/weights might have necessitated changes to get it balanced for stable flight.



https://www.spacemodeling.org/jimz/k-55.htm

 

[Lawndart]

New kit with plastic nose cone. Motor sticks out ⅝". Uses two Estes slabs of clay for nose weight.

Normally I leave the engine hook and block out so I have the option of longer motors. But this will fly high enough on an Estes Mighty D.

 

[samb]

New kit with plastic nose cone. Motor sticks out ⅝". Uses two Estes slabs of clay for nose weight.

Normally I leave the engine hook and block out so I have the option of longer motors. But this will fly high enough on an Estes Mighty D.

... And streamer recovery the way the rocket gods intended ! You might send a pm to The Chief Designer, Jumpjet and ask him about it. He pops in here fairly regularly.

 

[Kirk G]

I built one not all that long ago (maybe two years? Has it really been that long?)
and I don't recall any excessive amount of rear extension, nor any excessive nose cone weight.
Everything went together well and I thought the instructions were spot on.
I have flown it several times without problem, until I used a D12-7 instead of a D12-5 and it striped the streamer off...but even then, it went OK as I found all the parts on the ground in cut hay field.

 

[samb]

Kirk G - Can you quantify [not] "any excessive amount of rear extension" ? That data point might help Lawndart determine if his 5/8 inch is correct or not. The Estes international hieroglyphic instructions can be hard to interpret sometimes.

 

[JumpJet]

I don't have access to my cad files at home on the Goblin. Do us a favor and scan the page that should show the clearly marked side view of the motor mount. It should show the dimension were the slot needs to be cut for the engine hook to go in. I believe the motor should stick out 1/2". Back in the day they used Balsa Nose Cones and LEAD disk Weights to balance the model. You should however be able to use just one of the pieces of clay to get a stable model.

Well based on the old set of instructions from back in the day they don't show lead weights. I guess I imagined seeing it then.


John Boren

 

[Initiator001]

I built my Estes Goblin with the motor mount flush with the back of the body tube and put the clay in the nose cone.

Flies without any issues.

 

[K'Tesh]

When I get home I will see if I can sim up the modern version.

 

[Micromeister]

I've built and rebuilt my 4th original Solid Balsa Nose Goblin Twice. The aft end of the motor mount is flush with the end of the Airframe tube. Motor hook extends 1/2" below the body.

I've not seen the new plastic nose version but if the blown Polystyrene nose cone is as light as I figure then 2 pats of clay may be required. I've always flown my Goblin's with 2" x 120" orange crape streamers to get them back for D12-5 out of sight flights.

 

[K'Tesh]

Ok... Got home, and found that I had somehow failed to upload the images from the Goblin (7237) version.

Here's what it looks like:



And the K-55 version orientated the same way for comparison:



Markings for both are based on the facecards, catalog photos, or instructions that were used to sim them.

 

[DeltaVee]

So to resurrect this thread for a minute. I just cloned my goblin (got a semroc nose cone, had the original fin pattern and the original decals because I used to fly em unfinished!). My original goblin was indeed a freebie kit. It says in the plans that the balance point should be 3.625" (10.375 from the nose) from the rear of the rocket. Fine. Alot of estes plans (eg. the A-20 demon) state a figure for the balance point with a motor installed... but not the goblin plans. I guess it's implied, right? But it doesn't *say* in full flight configuration on the instructions, does it now?

My original had a balsa nose cone and I sure as heck never ever added nose weight (I did finish the nose cone but that was it!). I flew it several times until my brother jammed an aerotech e-15 into it and DIDNT put in the streamer! Never saw it again because I couldn't see it come down. Sigh. Now I have a clone all painted up but openrocket says the Cg (w/o motor) is 9 inches from the nose. With a motor it's 10.8" from the nose (D12-7). This seems to be about right with the actual model. With no added weight (the Cg is ahead of the Cp in this config by .781 diameters of the tube) the string test simply fails totally. As I recall, it failed miserably when I was 15 years old too!! The reality is that I'm seeing a Cg about 11 inches from the nose (giving me 3 from the back end.

I'm wondering what the aero/astronautical engineers opinions on this matter are. I sure cannot start the swing test at the speed Openrocket is suggesting off the rod, so without nose weight it starts looking dicey. But I *know* my original flew fine without it... but the swing test on this indicates it really enjoys flying backwards! I know that nose weight is not a terrible option evidently 9-10 grams of weight (.31-.32 oz) is enough for my new goblin to easily pass the swing test.

I guess what I'm really asking here is: did any of "youse guys" fly the goblin back in the day w/o any nose weight added?

 

[hcmbanjo]

From the post in August 2017 -
You might mean the longer engine hook (with finger tab) extends farther out the back.
The original hook didn't have the extension and allowed the model to stand on the fin tips.

No nose weight in the first Estes Goblin:
https://www.spacemodeling.org/jimz/k-55.htm
John Boren has explained in past posts that Estes now requires the C/G to be 1 1/2 calibers ahead of the C/P.

 

[DeltaVee]

From the post in August 2017 -
You might mean the longer engine hook (with finger tab) extends farther out the back.
The original hook didn't have the extension and allowed the model to stand on the fin tips.

No nose weight in the first Estes Goblin:
https://www.spacemodeling.org/jimz/k-55.htm
John Boren has explained in past posts that Estes now requires the C/G to be 1 1/2 calibers ahead of the C/P.

That's what I thought... that said, with a D12-7 openrocket suggests that the Cg leads by .764 calibers! Like I said, without any noseweight, string test flies backwards. .32 oz of nose weight fixes it well (openrocket then says 1.27 Caliber)

 

[hball55]

I know this discussion is old/stale, but since I just built the Goblin, I decided to add to it. I built this rocket in Rocksim and left out the motor block and engine hook, and added a 24mm PSII motor retainer. My simulation, which I am attaching, shows that no nose weight is needed. Even with an Aerotech F35 motor, my CG is 3-6/8" from the rear of the rocket (Estes recommends the CG never be less than 3-5/8" from the rear of the rocket). The lowest caliber I get to is 1.11, with the Aerotech F35 motor.

My only worry is will any of these motors rip the fins off my rocket, lol. I did use some Rocketpoxy for the fillets, which aren't very big.

Oops! Edited Rocksim file, as I have my motor mount extend past the end of the body tube by .25", changing my stability numbers. The F35 engine now brings my stability to .98 marginal. This is still within my limits, as the rocket is quickly made stable by motor burn. New Rocksim file replaces the old file.

 

[Buzzard]

My original Goblin from its initial release met its end from a D13 (original form of the D12). I used the nose cone on a clone Goblin that I have been flying for years. Bought the SEMROC version and have an Estes version that I bought at HL. I have replaced the plastic cone with a balsa one. Over the decades, I have accumulated at least a half dozen of BNC-55AO cones from SEMROC and Balsa Machining Services. To go with the BNC-60AO cones...
I may have a problem...

Chas

 

[DeltaVee]

FWIW whatever openrocket SAYS, is kind of irrelevant since my Goblin clone flights perfectly, every time, even without the minor nose weight I attached. Go figure. It's tempting to toss a F39 or E18 but I think I'd rather not lose the hardware. har har har

 

[hball55]

I built my Estes Goblin with the motor mount flush with the back of the body tube and put the clay in the nose cone.

Flies without any issues.

View attachment 325011

You used one of my favorite shades of yellow, Caterpillar yellow. I used it on my Aerotech Arreaux and it goes very well with black.

 

[dr wogz]

You used one of my favorite shades of yellow, Caterpillar yellow. I used it on my Aerotech Arreaux and it goes very well with black.

Growing up, dad had a Datsun B210 station wagon..

He also had a 'heavy mechanic' freind.

One summer day, they painted the car.. "Caterpillar yellow" Always thought it was an actual colour.. (And I guess it really is..) I later learned he was a main mechanic at the local Caterpillar factory.. (Guess where the paint came from..)

 

[Back_at_it]

I know this discussion is old/stale, but since I just built the Goblin, I decided to add to it. I built this rocket in Rocksim and left out the motor block and engine hook, and added a 24mm PSII motor retainer. My simulation, which I am attaching, shows that no nose weight is needed. Even with an Aerotech F35 motor, my CG is 3-6/8" from the rear of the rocket (Estes recommends the CG never be less than 3-5/8" from the rear of the rocket). The lowest caliber I get to is 1.11, with the Aerotech F35 motor.

My only worry is will any of these motors rip the fins off my rocket, lol. I did use some Rocketpoxy for the fillets, which aren't very big.

Oops! Edited Rocksim file, as I have my motor mount extend past the end of the body tube by .25", changing my stability numbers. The F35 engine now brings my stability to .98 marginal. This is still within my limits, as the rocket is quickly made stable by motor burn. New Rocksim file replaces the old file.

I built mine with the motor mount flush to the body tube and left out the motor retainer but did use the motor block. I prefer to friction fit on this model. As for stability, I once lost the stock nose cone and had to replace it. I forgot to add nose weight to the new cone and the first flight on a D12-7 was a bit unstable. Still went vertical but it was all over the sky. I backed down and flew it on C6-5's the rest of the weekend and added 1/2oz of nose weight when I got home. Now it flies nice and stable.