Goblin (4") assembly and motor recommendation

[vincarroll]

Good morning,

I’m in the early stages of building a Goblin with my kids (our first high power rocket). I’ve attached a few photos and wanted to double check that we’re doing things correctly before applying epoxy. There’s not much of the aft end of the motor tube protruding, and the hardware at that end is the limiting factor. Does this look correct?

We’re planning to go for our level 1 certification with this rocket. What would be a good motor (single use) for this endeavor? I’m not sure if the dimensions of the launch site (white's Fam in Durham, CT) but it’s not prohibitively small.

Thanks in advance for any advice!

 

[Rob Campbell]

Is this the LOC 4 inch Goblin? You need to decide which type of motor you want to certify on, reloadable or single use. Reloadable are more effort as the motor has to be assembled, but are less expensive as the case and closures are reusable. With single use, all you need to do is set the delay time.

I recommend a good simulation program like Open Rocket (free download) or Rocksim (available for purchase from Apogee) and try different motors. For a Level 1, I recommend a motor that won't take the rocket higher than 2,000 feet. This way, you will see the entire flight and have a low risk of the rocket drifting away.

I obtained my Level 1 cert on an Apogee Zephyr flying on an Aerotech H-123 (reloadable) with an 8 second delay. In September, we had several Jr. Level 1 certs with the kids flying Zephyrs on Aerotech H100 (single use) motors.

Good luck with the build and cert flight.

 

[MarsLander]

It looks correct. Nice rocket.

I'm sure it is just because you are dry fitting, but make sure that the tabs on the fins that go through the centering rings (CR) are flush with the CR on the back and front (tabs should go all the way through the CR). Top one in pic below is not through, bottom one is.


Can't tell if you already did it or not, but you might put a bit of epoxy on the well nuts for the motor retention so they don't come loose. Pic looks like you did?

 

[vincarroll]

Is this the LOC 4 inch Goblin? You need to decide which type of motor you want to certify on, reloadable or single use. Reloadable are more effort as the motor has to be assembled, but are less expensive as the case and closures are reusable. With single use, all you need to do is set the delay time.

I recommend a good simulation program like Open Rocket (free download) or Rocksim (available for purchase from Apogee) and try different motors. For a Level 1, I recommend a motor that won't take the rocket higher than 2,000 feet. This way, you will see the entire flight and have a low risk of the rocket drifting away.

I obtained my Level 1 cert on an Apogee Zephyr flying on an Aerotech H-123 (reloadable) with an 8 second delay. In September, we had several Jr. Level 1 certs with the kids flying Zephyrs on Aerotech H100 (single use) motors.

Good luck with the build and cert flight.

It's a 4" Goblin, sorry I didn't specify. And in terms of reloadable vs. single use, I was thinking of using the latter. Thanks Rob!

 

[neil_w]

I recommend a good simulation program like Open Rocket (free download) or Rocksim (available for purchase from Apogee) and try different motors. For a Level 1, I recommend a motor that won't take the rocket higher than 2,000 feet. This way, you will see the entire flight and have a low risk of the rocket drifting away.

I of course endorse this recommendation, but also just for quick motor selections thrustcurve.org will do the job nicely. You can then go into OR or Rocksim and fine-tune the delay and such.

 

[Antares JS]

It's a 4" Goblin, sorry I didn't specify. And in terms of reloadable vs. single use, I was thinking of using the latter. Thanks Rob!

Do you have an adapter to adapt your motor mount down to 38mm? That looks like a 54 and I don't think there are any H or I 54mm single use motors.

I also endorse the recommendation to download OpenRocket and give it a try. Using a simulation software is a valuable skill that you can use throughout your time in rocketry. You'll find yourself loading your simulation models before every launch to make sure Rocket X will fly safely on Motor Y.

 

[vincarroll]

It looks correct. Nice rocket.

I'm sure it is just because you are dry fitting, but make sure that the tabs on the fins that go through the centering rings (CR) are flush with the CR on the back and front (tabs should go all the way through the CR). Top one in pic below is not through, bottom one is.
View attachment 624749

Can't tell if you already did it or not, but you might put a bit of epoxy on the well nuts for the motor retention so they don't come loose. Pic looks like you did?

View attachment 624750

Yeah, I did epoxy over the well nuts. I'll make sure the finb tabs are flush with the CR as you advised. Thanks for your input!

 

[vincarroll]

Do you have an adapter to adapt your motor mount down to 38mm? That looks like a 54 and I don't think there are any H or I 54mm single use motors.

I also endorse the recommendation to download OpenRocket and give it a try. Using a simulation software is a valuable skill that you can use throughout your time in rocketry. You'll find yourself loading your simulation models before every launch to make sure Rocket X will fly safely on Motor Y.

The kit comes with a 38 mm and 29 mm adapter. Thanks for your response!

 

[Antares JS]

The kit comes with a 38 mm and 29 mm adapter. Thanks for your response!

Rather nice of LOC to do that. Good luck with your eventual cert flight.

 

[thzero]

I'd go with thurstcurve and openrocket to dual in.

It's slightly smaller, and fins arent as big so less drag, but my 4" minniemagg from LOC runs nice on a h128.

Singleuse... I could see a h180.

 

[mtnmanak]

Yeah, I did epoxy over the well nuts. I'll make sure the finb tabs are flush with the CR as you advised. Thanks for your input!

From the picture, it looks like you epoxied over the screws as well. How are you going to get the retention clips off to put the motor in the tube?

 

[vincarroll]

I'd go with thurstcurve and openrocket to dual in.

It's slightly smaller, and fins arent as big so less drag, but my 4" minniemagg from LOC runs nice on a h128.

Singleuse... I could see a h180.

Thanks for your advice!!

 

[vincarroll]

From the picture, it looks like you epoxied over the screws as well. How are you going to get the retention clips off to put the motor in the tube?

Ugh - I didn’t realize that the screws would have to come off. This is my first HPR and the directions are a bit unclear. Do you know if it’s possible to remove a screw that has been epoxied? Thanks for catching this!

 

[DigBaddy]

Ugh - I didn’t realize that the screws would have to come off. This is my first HPR and the directions are a bit unclear. Do you know if it’s possible to remove a screw that has been epoxied? Thanks for catching this!

Bust out the dremel. Then get new T-nuts and screws, drill two more holes 90 degrees from the originals and you'll be back in business.

 

[vincarroll]

Bust out the dremel. Then get new T-nuts and screws, drill two more holes 90 degrees from the originals and you'll be back in business.

Will do - thank you!!

 

[neil_w]

Do you know if it’s possible to remove a screw that has been epoxied? Thanks for catching this!

Anything is possible if you’re willing to use enough force.

 

[augendoc]

Heat will soften the epoxy. Otherwise, you’ll have to grind off the screw heads, grind the protrusions flush, and re-mount the hardware 90 degrees.

As for heat, try putting a soldering iron point on the screw head notch and give it enough time to get the screw hot enough to soften the epoxy just around the screw threads, then back the screw out. Once everything has cooled, chase the threads in the well nuts with a tap to clean them out and get rid of any residual epoxy in them. I can’t speak to the strength of the epoxy remaining between the well nut and centering ring, but it will probably be good enough.

Good luck and let us all know how it goes!

 

[mtnmanak]

Hopefully some helpful tips here:

https://www.mountainmanrockets.com/index.php/hpr-primer/

 

[mtnmanak]

Ugh - I didn’t realize that the screws would have to come off. This is my first HPR and the directions are a bit unclear. Do you know if it’s possible to remove a screw that has been epoxied? Thanks for catching this!

Luckily, you didn't epoxy the centering ring in yet. As others have noted, just grind/cut off the hardware and reinstall new tee nuts (easy to get at your local Home Depot or Lowes).

Another option is to cut the screw heads off, remove the clips and make new holes for new tee nuts.

If all else fails (or you inadvertently ruin/break the centering ring), you can just make a new centering ring. If you don't feel like your manufacturing skills are up to that task yet, give LOC a call - I am sure they will sell you a new centering ring with no issues.

 

[Titan II]

+1 using heat if they do not back out without it. In the future, just put a release agent on the screw the epoxy will not stick to...chap stick, wax, oil of any variation, vasolene...you get the point.

 

[jmasterj]

I'd try just backing one of the screws out. You didn't use JB Weld on them, so the epoxy/metal bond might be weak enough to allow that, especially if you heat it up with a heat gun or something first.

 

[waltr]

LOC actually has a very good build video for the Goblin.

This thread asks about cert motors for the Goblin:
https://www.rocketryforum.com/threads/need-advice-on-h-motor-for-level-1-cert.184277/#post-2536288

 

[vincarroll]

Bust out the dremel. Then get new T-nuts and screws, drill two more holes 90 degrees from the originals and you'll be back in business.

Will do - thank you

The kit comes with a 38 mm and 29 mm adapter. Thanks for your response!

Heat will soften the epoxy. Otherwise, you’ll have to grind off the screw heads, grind the protrusions flush, and re-mount the hardware 90 degrees.

As for heat, try putting a soldering iron point on the screw head notch and give it enough time to get the screw hot enough to soften the epoxy just around the screw threads, then back the screw out. Once everything has cooled, chase the threads in the well nuts with a tap to clean them out and get rid of any residual epoxy in them. I can’t speak to the strength of the epoxy remaining between the well nut and centering ring, but it will probably be good enough.

Good luck and let us all know how it goes!

Thanks for your great advice! I got my soldering iron out this afternoon and was able to get the screws out. It worked great, and I was able to remove/melt off all the epoxy on the screws. I’ll clean everything up, then repeat this step properly. Thanks again for your help!

 

[John Kemker]

I built one of these, meaning to use it for my L1 recert. Later, I managed to get hold of a different kit to use for that task. However, I did learn something that I'd do different, if I was going to rebuild the LOC 4" Goblin: Pick a different method of motor retention. I like both the Aeropac as well as the Slimline retainers. If you're going to use one, you would need to leave some of the MMT hanging out the back to epoxy (JB Weld preferred) the retainer to. Of course, if you'd rather use the LOC adapter/retainer system, go ahead. Free advice is worth what you pay for it. [grin]

 

[vincarroll]

I built one of these, meaning to use it for my L1 recert. Later, I managed to get hold of a different kit to use for that task. However, I did learn something that I'd do different, if I was going to rebuild the LOC 4" Goblin: Pick a different method of motor retention. I like both the Aeropac as well as the Slimline retainers. If you're going to use one, you would need to leave some of the MMT hanging out the back to epoxy (JB Weld preferred) the retainer to. Of course, if you'd rather use the LOC adapter/retainer system, go ahead. Free advice is worth what you pay for it. [grin]

I was actually wondering if a retainer might be preferable to the z clips. I’ve used the former in the past, but haven’t had any experience with the z clips. Checkout the 2 you mentioned. Thanks John!

 

[dpower]

I was actually wondering if a retainer might be preferable to the z clips. I’ve used the former in the past, but haven’t had any experience with the z clips. Checkout the 2 you mentioned. Thanks John!

I've used the LOC Z-Clips, they work great. A nice low-cost alternative to threaded retainers.

 

[Tractionengines]

I was actually wondering if a retainer might be preferable to the z clips. I’ve used the former in the past, but haven’t had any experience with the z clips. Checkout the 2 you mentioned. Thanks John!

DON'T use a retainer on this rocket...without understanding the effects that change will cause.

The adapters mount to the same place as you have the z-clips.

If you put a retainer on; you WILL NOT be able to use the flanged adapters that come with the kit.

 

[ShadowSpire48]

Ditch the Z clips and get an Aeropack retainer. I know z-clips have been used since the dawn of rocketry but they're clunky and ugly. The adapters LOC uses will add more weight to the rocket because they're thick wood, Aeropack is aluminum and the adapters for it are stupid easy to use and all nest together (I've stacked from 75mm to 38mm for my L2).

 

[waltr]

The Z-clips work perfectly and are easy to use and cheap.
Sure the LOC Adapters may be slightly heavier but work well.
I've flown my Goblin with 29 & 38mm motors using the provided adapters and Z-clips.

 

[Zbench]

I’m not a fan off the z clips either for the reasons stated. Another no one mentioned is they rust and just look nasty on a nicely finished rocket.