L2 rocket scratch build

[Ben Martin]

Update:

Made some progress such as reassembling the rocket with CA reinforced holes, cutting out a fin alignment guide, and adding the carbon fiber rods into the last two fins. Tomorrow I will be epoxying the fins onto the motor tube and adding interior fillets. I'll add the rear centering ring and transition on too if I have time.

 

[jjwb22101]

I respect your opinion, but I have had a lot of experience with rockets throughout high school and now in college. My L1 rocket was completed quite a few months back and I have learned a tremendous amount from it, so much that I am now working on this new rocket with the intent of L2. My view on getting my L2 is that it opens up some freedoms for experimentation that I can't have with an L1, such as 54mm motors. This rocket is not using a ballast to control the rocket's altitude, I will have at max 200 grams of noseweight in it for stability, with 100grams being much more likely. For recovery from 5000ft, I will be using a jolly logic chute release and motor deployment. I would either be using a J250 or a J270 with a 38mm to 54mm adaptor, with the J250 DMS motor being much more likely. The only issue I have right now is that the expected apogee is 16.1seconds and the motor delay is 14 seconds, however, the simulations show that the optimum is 13.3seconds so that falls within the motor.
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The use of a chute release definitely alleviates a lot of my concern about recoverability - those things are awesome. They aren't perfect though - you're still relying on the motor ejection to separate the rocket into two parts, and (I'll defer to Mark here - he's by far the expert on this stuff) those can have a good deal of variation. I second his recommendation of either the MissileWorks RRC2+ or PerfectFlite Stratologger CF - they're both excellent (I used a stratologger on my own L2, and have used an RRC2+ as a backup for competition rockets for a couple years now). As for your concern about the JLCR slipping off during flight - it can happen. I've seen it a lot more commonly than I've seen conventional dual-deploy setups have the main eject at apogee. Unsure if that's just issues with people not knowing how to pack a parachute when using the chute release, or if it's just a more common failure mode, but that is something to consider. Luckily it's not a safety issue - you'll just have to go for a hike.

Only other thing I notice is that you might be hitting the ground a little fast? 21 ft/sec is a pretty good clip, and will only increase as your rocket inevitably gets heavier (happens to the best of us - accurately estimating glue and paint weight is really difficult). I'd suggest moving to a slightly larger parachute, as that will give you a bit more leeway in terms of added weight without hitting the ground hard enough to break your fins off (general rule of thumb is to stay under 20 ft/sec, slower for hard surfaces). Launch rail clearance velocity looks good (want to keep it above ~50 ft/sec - if it dips too low, the J270 or J425 will both alleviate the issue somewhat), and from what I could see your recovery system seems well attached.

 

[OverTheTop]

Nice to see the CF rods in the fins. I hope they work since I suggested it .

Unless the rocket is being driven into a rigid and immoveable wall, the compressive force on the body tube will not be as large as the peak thrust developed by the motor.

At the point where the thrust ring pushes it will be the entire peak thrust of the motor. The forces do decrease as you move forward in the airframe due to the inertia of the airframe behind the point essentially "absorbing" some of the thrust by it being accelerated. There will always be some point where the motor applies the entire peak thrust in the rocket.

low profile plywood strakes would get you style points while adding rigidity to the airframe.

I love the style that those sort of details add .

 

[Ben Martin]

Is this all I have to purchase for the RRC2+ besides the ematches (not sure where to those)? Also, how does it compare to the stratologgercf? Are there any 3D printed sleds for the RRC2+ for 3in diameter? Pardon all the questions, I am trying to get into dual deployment at a reasonable price.

 

[Nytrunner]

Mechanical switches are optional, some fliers twist wire ends together and tape over them.

9volt holders also optional, I like to velcro them down and zip tie over them to save space. But if you don't have a stock of zip ties and velcro patches, buying a holder may be the most economical for your immediate needs.

Unless I'm mistaken, the RRC2 doesn't have a computer interface. It is totally Dip-switch driven. That being said, the Stratologger CF can fly right out of the box and I thinkit has rudimentary manual adjustment, but requires a data interface cable to adjust digitally and download data.

 

[smugglervt]

Missileworks has a 75mm modular sled for the RRC2+ that includes a battery box. I used their RRC2+ and 98mm sled and rotary switch on my first dual deploy and was very happy with its ease of setup.

 

[Ben Martin]

Missileworks has a 75mm modular sled for the RRC2+ that includes a battery box. I used their RRC2+ and 98mm sled and rotary switch on my first dual deploy and was very happy with its ease of setup.

Is this what you are referring to? https://macperformancerocketry.com/...orks-altimeters/products/copy-of-power-switch

Would I need anything else besides the RRC2+ and switch? Also, where can I but the ejection charges from?

 

[smugglervt]

Yup, that's it and you can also buy it direct from Missileworks:

https://www.missileworks.com/store/#!/RRC2-75mm-Modular-Sled-System/p/33363834/category=9045099

You will also need wiring. I used JST connectors.
My ejection charge cannister mounting kit came from Binder Design (Doghouse Rocketry ones) where you can also get the wiring kits:

https://binderdesign.com/store/page29.html

 

[Ben Martin]

Yup, that's it and you can also buy it direct from Missileworks:

https://www.missileworks.com/store/#!/RRC2-75mm-Modular-Sled-System/p/33363834/category=9045099

You will also need wiring. I used JST connectors.
My ejection charge cannister mounting kit came from Binder Design (Doghouse Rocketry ones) where you can also get the wiring kits:

https://binderdesign.com/store/page29.html

Missileworks offers a package with 2 bulkhead terminals, switch, and 9V snap. Should I go for that? Assuming that I would still need to get wires from somewhere.

 

[smugglervt]

That's one way to go if you plan on using the terminal blocks or you can just get the sled and pick up a package of 9V snap connectors on Amazon and get your charge cannisters and rotary switch already pre-wired at Binder Design.

 

[rcktnut]

If you wait a couple of weeks, should be able to save some money. Most rocketry vendors have BF or BS or CM sales.

 

[boatgeek]

Missileworks offers a package with 2 bulkhead terminals, switch, and 9V snap. Should I go for that? Assuming that I would still need to get wires from somewhere.

If you need wires, the twisted pairs from Cat5 cables work pretty well from altimeter to charges and are widely available. From battery to altimeter, I'd slightly prefer something with a larger gauge, but Cat5 will work in a pinch. Charge wells can be as simple as a PVC pipe cap screwed to the bulkhead.

 

[smugglervt]

What I like about the Doghouse charge kit is that it is a charge well and wiring terminal all in one

 

[Ben Martin]

Update: Epoxied all of the fins to the motor tube, creating small interior fillets which I may add onto tomorrow.

Tomorrow I will be doing the fillets, putting in the rear centering ring, and attaching the retainer/transition.

It's all coming together nicely.

 

[Ben Martin]

Do you guys think that interior fillets are necessary on this build or would exterior be enough? The fins epoxied onto the motor tube and the notches in the centering rings.

 

[OverTheTop]

I normally would not bother with internal fillets with traditional materials as the bond strength is usually high and predictable.

Given that these fins are 3D printed and I have seen quite variable bonding on printed fins I would add the extra fillets.

 

[jlabrasca]

... where can I but the ejection charges from?

I think 18 y.o. is legal to purchase 4F black powder (or similar) for the ejection charge, but maybe yours could be the generation that never has to?

Do you guys think that interior fillets are necessary on this build or would exterior be enough? The fins epoxied onto the motor tube and the notches in the centering rings.

Be wary of asking adhesive questions on this forum. Some folks have opinions. <smile>

By "Notches" I am guessing you mean dados in the centering rings?


After talking about this with more experienced fliers (and fliers with engineering expertise) this is how I tend to think about fillets.

Grossly oversimplified, and certainly a little simple-minded.

For a cert flight, where you are worried about ground-hit speed, the major concern would seem to that you will pop a fin on impact (or while transporting the rocket to the field). With dados in the centering ring to keep the fin from rotating parallel to the long axis of the rocket, and without a good idea of the shear strength or peel strength of the bond between the 3D printed fins and the motor tube, I would probably do internal fillets to the body tube to harden the fin against radial forces.

If the fins only make contact with the rear centering ring (if you don't have the fin tabs pinned between forward and aft rings), then the internal body tube fillets might harden the fins against being wrenched out (rotating perpendicular to long axis of the rocket).

 

[Bat-mite]

Obtaining black powder sounds like it is getting more difficult all the time. Call local gun stores. Fortunately I have a seller within 15 miles of my house.

 

[Ben Martin]

I normally would not bother with internal fillets with traditional materials as the bond strength is usually high and predictable.

Given that these fins are 3D printed and I have seen quite variable bonding on printed fins I would add the extra fillets.

Seems like my 30min epoxy sticks VERY well to PETG when it's roughed up. I'll probably add small internal fillets just to be safe.

 

[Ben Martin]

Update:

ALL fins have interior fillets, both on the motor tube and body tube. Tomorrow I will be cleaning it up for the rear centering ring then attaching the transition/retainer. I'll have to get more epoxy for the exterior fillets unless I just do relatively small ones.

 

[Ben Martin]

Update:

ALL fins have interior fillets, both on the motor tube and body tube. Tomorrow I will be cleaning it up for the rear centering ring then attaching the transition/retainer. I'll have to get more epoxy for the exterior fillets unless I just do relatively small ones.

Photo of completed fins. Tape was used to indicate the completion of interior fillets.

 

[Ben Martin]

Tempted to just sand down everything and launch it without paint for the first few launches to keep the weight down.

 

[boatgeek]

That is a legit option. You might want to prime and quickly sand the exterior just so you get rid of most/all of the little hairs on the outside of the tubing. Getting the 3-D printed stuff smooth will take a bunch more work and may not be worth it until you see how it flies.

If you don't paint, change the surface textures in OpenRocket appropriately. They have a pretty big impact on your altitude and delays.

 

[Ben Martin]

That is a legit option. You might want to prime and quickly sand the exterior just so you get rid of most/all of the little hairs on the outside of the tubing. Getting the 3-D printed stuff smooth will take a bunch more work and may not be worth it until you see how it flies.

If you don't paint, change the surface textures in OpenRocket appropriately. They have a pretty big impact on your altitude and delays.

The 3D printed stuff is pretty dang smooth, but a coat of paint would certainly help smooth the entire thing. The thing I am more concerned about is the weight as it's already coming in at 20fps without paint. Want to keep the 3D printed stuff exposed as a "showpiece" so maybe a clear enamel is a good option.

 

[Nytrunner]

Good luck on the recovery (that's sincere, not sarcastic).
I remember an SLI cert day and the three 2.6" rockets with 54mm L2 motors were never seen again that we heard.

 

[jlabrasca]

Tempted to just sand down everything and launch it without paint for the first few launches to keep the weight down.

Fly it unpainted. Make it earn its colors <grin>.

Do you have everything else ready to go for the attempt? Cert motor in-hand? Exam corrected and signed off?

Don't forget to turn on the JLCR. And check that your don't trick it into an early deployment when you close up the rocket. I read about that issue on the forum and (I think) I was able to replicated it with my L2 rocket on 2 tries out of 7 in ground testing. I drilled a small pressure-relief port before the attempt, just to be sure.

And yes, good luck.

 

[OverTheTop]

Don't forget to turn on the JLCR. And check that your don't trick it into an early deployment when you close up the rocket. I read about that issue on the forum and (I think) I was able to replicated it with my L2 rocket on 2 tries out of 7 in ground testing. I drilled a small pressure-relief port before the attempt, just to be sure.

Have you got one breather hole in the chute compartment area? That should get you around the JLCR accidentally triggering and also prevent dh/dt (rate of change of height) blowing the NC off. Only a small hole needed.

 

[Ben Martin]

Have you got one breather hole in the chute compartment area? That should get you around the JLCR accidentally triggering and also prevent dh/dt (rate of change of height) blowing the NC off. Only a small hole needed.

That's part of what I'm doing today, thanks for reminding me.

 

[Ben Martin]

Fly it unpainted. Make it earn its colors <grin>.

Do you have everything else ready to go for the attempt? Cert motor in-hand? Exam corrected and signed off?

Don't forget to turn on the JLCR. And check that your don't trick it into an early deployment when you close up the rocket. I read about that issue on the forum and (I think) I was able to replicated it with my L2 rocket on 2 tries out of 7 in ground testing. I drilled a small pressure-relief port before the attempt, just to be sure.

And yes, good luck.

I don't have the motor or exam as there should be a vendor and don't I get the test onsite? I'll be flying it on L1 54mm motors to test it before flying on a small J motor for L2.

 

[Rex R]

you might want to drop a line to the host club just to be sure that they bring the exam(s) to the launch. you will need proof of your L1 status prior to taking the test(temp NAR member card will work). good luck and have fun .
Rex