Certification rocket - Second time's the charm!

[PenitantTango]

Hello everyone,

I'm new to the forum, so excuse any formalities in this post (and I'm open to suggestions too). I'm a AE student at UT Arlington, and I'm putting together my second attempt at a L1 certification rocket, hopefully with better results this year.

The body tube is just a 33" cardboard tube I had lying around, and I plan to laser cut some 1/8th in plywood for the fins and centering rings, then 3D print the nosecone using a MATLAB program one of my buddies made up last year. My last attempt at a L1 cert launched on a Aerotech H123 motor, so I added one of those to this years design.

Any thoughts? Suggestions? I'll be posting more pictures this week as I make the different parts and assemble everything.

 

[markkoelsch]

Hello. To cut to the chase- make your fins larger. You are right at a static margin of 1- you likely want that to be at least 1.5 and closer to 2 is better.

Good luck.

 

[djs]

I agree with Mark. Also, I can't tell from your sim file, but are the fins through the wall, or surface mounted?

 

[PenitantTango]

Sorry about that; the fins are through the wall with fin tabs between the motor mount centering rings.

 

[MikeyDSlagle]

By larger they mean span-wise, that is increasing the distance from the body tube to the tip. 1/4" or 1/2" may make all the difference in the world without changing the looks much. Shortening the root chord may increase stability a bit too. But it is stable and it will probably fly just fine as is. Stability will increase as the engine burns.

I did my level 1 on that motor. Good motor.

 

[Bat-mite]

My advice on getting L1 (because I have seen this many times): people focus on the rocket and the motor. They make sure they don't cato the motor and they make sure the rocket is stable and can handle the thrust and landing, and they have their delay timed perfectly and they've done a muillion sims.

Then they get to the field, launch it to 3000', eject the main at apogee, and watch it float away, never to be found. No L1 achieved if no rocket found.

Make sure you can recover the rocket! Get a chute release, or use DD, or build it extra-strong and use a streamer. But if you pop out a 36" chute at 3000', even if there is just a little bit of ground wind, you might be waving it bye-bye.

Good luck!

 

[PenitantTango]

Ok here's the updated design; I added half an inch to the height of the fins which made the stability much better. I also changed the shape of the nose to a Haack series rather than a parabolic since I'm only going subsonic (Mach 0.64)

 

[Bat-mite]

In your sim, you have the chutes and harness up near the nose cone. Will you be using a chute shelf to keep them up there? If not, they will fall down the tube and rest on top of the motor, moving your CG aft a little.

 

[K'Tesh]

1) Welcome to TRF!
2) I'm glad to see you're posting images (remember to post pics of the launch)
3) You can post the actual .ork file to the forums (under the "Go Advanced" button) and attach the file for us to look at.

Now, it's after midnight here in China... So, I'll be heading off to bed.

All The Best!
Jim

 

[Rex R]

thoughts; the nose cone shoulder is to short, should be a minimum f 2.5" to keep the cone from wobbling. you will need about 12" of airframe to house the chute & shock cord etc. what I would do is, make the motor tube longer by about 10" and put a third centering ring about an inch aft of the forward end, this will keep the laundry from sliding aft and reduce the amount of black powder needed to eject the chute. also moves the shock cord anchor point forward.
Rex

 

[PenitantTango]

I will be running a para cord shock cord through my tube to the inside tip of the nosecone, where I will epoxy it into place. I tie a loop in the cord just below the shoulder of the cone where I can quick-link my parachute cord to the shock cord.

Here is my most recent OpenRocket file as well.

View attachment Sullivan cert rocket 2017.ork

 

[boatgeek]

A couple of comments:

About 31 minutes* after you get your cert flight paperwork signed, you'll be looking at the rocket and wondering how big a motor you can put in it. I would make the fins a little bigger so you can go a little higher up the power chain, at least up through a full H. You might want to go to an I or baby J, but that may be a little too far.

I would also design a lug into your nose cone to tie/quick link the shock cord to. I've had really mixed results with gluing to 3-D printed parts. If you are going to glue it in, I'd make it so you have a mechanical bond (pins, nails, restrictions in tube diameter, etc.) to hold it in place even if the epoxy doesn't stick to the PLA.

* Or less

 

[boatgeek]

Forgot to add that a 3" rocket to 3,000 feet will be nice and visible for the entire flight. Above 4,000 will get a little more iffy but certainly doable. However, advice to use a Chute Release or DD is good, especially if you have a small field or rivers, trees, etc. nearby. If you do use a Chute Release, try flying it a few times on other rockets so you get the hang of it.

 

[K'Tesh]

I will be running a para cord shock cord through my tube to the inside tip of the nosecone, where I will epoxy it into place. I tie a loop in the cord just below the shoulder of the cone where I can quick-link my parachute cord to the shock cord.

Here is my most recent OpenRocket file as well.

Thanks for the file... I haven't looked at it yet (it's not even 5:30am here in China yet, I'm still waking up)...

I'm a little... um... concerned... yeah... concerned... about para cord being used as a shock cord "through my tube". Para cord isn't well known for heat resistance. I'd only use it as my sole shock cord if there was a baffle mounted behind it. Now I need to look at the file and see if you mention baffle.... Nope... No baffle.

I would STRONGLY recommend using Kevlar for the shock cord that remains inside the body tube, and save the para cord for the part that would be outside of the rocket.

Also... Epoxy (and other hard drying glues) will make the cord brittle, and brittle cords = bad results. You don't want to use glue on knots if it can soak into the fibers. White PVA (Elmers) is OK for the thread used by Estes, Semroc, and other companies parachutes, because it's small and will dry quickly and won't soak in.

You could use something like the "U Loop" to do the attachment for the nosecone.

My L1 also used a 3D printed nosecone, and you can see how I attached the cord in my build thread...

Basically, I used a length of body tube (cut down to create a coupler), and glued that inside the exposed nosecone, I attached my nose weight to the bulkhead with the idea that it would prevent it from pulling apart upon deployment. It worked well for that, but my attachment wasn't the best, and a hard landing allowed the weight (a 16oz can of Diet Coke) to break the nosecone upon landing (I got the part back, and was able to use some British duct tape (Thanks Ron!) to repair and get cert'd.

As an OR Chuck Norris... I noticed that your parachute, CRs, body tube, fins all seem to be default thicknesses. For OR to simulate your rocket properly, you should make your dimensions accurate. Oh, and 40" for a shock cord seems a tad short.

Best of Luck!

 

[Rex R]

right, 40" shock cord and a 12" chute...at 40" your nose cone isn't going to stay attached to the rocket and your chute might not deploy. you should be thinking more along the lines of 15'(180"). while a 12" chute will help get the rocket down in a hurry, landing at 30 ft/sec is inviting damage. while I have used 1/16" ply for centering rings, I was building a mid-power bird(that flies nicely on an F52), consider 0.125" thick rings. one thing I have noticed is that rockets seldom build out underweight, don't lock yourself into one particular motor before the rocket is built and painted.
Rex

 

[Rex R]

oh right. how are you going to retain the motor?
Rex

 

[PenitantTango]

Thanks for the file... I haven't looked at it yet (it's not even 5:30am here in China yet, I'm still waking up)...

I'm a little... um... concerned... yeah... concerned... about para cord being used as a shock cord "through my tube". Para cord isn't well known for heat resistance. I'd only use it as my sole shock cord if there was a baffle mounted behind it. Now I need to look at the file and see if you mention baffle.... Nope... No baffle.

I would STRONGLY recommend using Kevlar for the shock cord that remains inside the body tube, and save the para cord for the part that would be outside of the rocket.

Also... Epoxy (and other hard drying glues) will make the cord brittle, and brittle cords = bad results. You don't want to use glue on knots if it can soak into the fibers. White PVA (Elmers) is OK for the thread used by Estes, Semroc, and other companies parachutes, because it's small and will dry quickly and won't soak in.

You could use something like the "U Loop" to do the attachment for the nosecone.

My L1 also used a 3D printed nosecone, and you can see how I attached the cord in my build thread...

Basically, I used a length of body tube (cut down to create a coupler), and glued that inside the exposed nosecone, I attached my nose weight to the bulkhead with the idea that it would prevent it from pulling apart upon deployment. It worked well for that, but my attachment wasn't the best, and a hard landing allowed the weight (a 12oz can of Diet Coke) to break the nosecone upon landing (I got the part back, and was able to use some British duct tape (Thanks Ron!) to repair and get cert'd.

As an OR Chuck Norris... I noticed that your parachute, CRs, body tube, fins all seem to be default thicknesses. For OR to simulate your rocket properly, you should make your dimensions accurate. Oh, and 40" for a shock cord seems a tad short.

Best of Luck!

That's definitely an interesting way of mounting the cord... I had previously just tied the lower end to an eye bolt screwed into the upper centering ring, tied a loop for attaching a quick link for the chute about 12-24" from the upper end, and taped the end of the cord to the inside of the (upside down) cone, where I could then pour in epoxy and any weights I needed to mount.

oh right. how are you going to retain the motor?
Rex

On the lower centering ring I will drill a bolt (or maybe two) through the wood, then sandwich a washer between two nuts right at the extruding end of the motor.

 

[OverTheTop]

I've had really mixed results with gluing to 3-D printed parts.

Me too. Caution!

Check out my 3D NC here. You are welcome to steal any ideas off it.
https://forum.ausrocketry.com/viewtopic.php?f=6&t=5019


If you want your rocket to be a bit more robust during heavy landings, consider moving the fins a little further forward.

 

[PenitantTango]

Update from today's work:

Got the tube painted and finished up the nosecone for printing, definitely a challenge getting the internal ribs the way I wanted them. I've attached the .stl file for anyone still interested.

View attachment Sullivan nosecone completed.STL

 

[PenitantTango]

For anyone still following this thread, I'm just about done with the build and I plan to launch this Saturday (Sept 16, 2017) at the DARS high power launch site. Really liking how the whole thing came together.

 

[samb]

Did you get a Chute Release then ? Tomorrows wind forecast and direction may keep this one grounded without it. We really have to respect the northern field boundary because we don't have carte blanche to recover rockets north of Stiff Chapel Road. Now having said that, forecasts change and winds change. I would bring it along with something else to fly.

 

[A-N-D-R-E-W]

Good luck on your flight! Thanks for posting pictures and let us know how it goes!

 

[Bat-mite]

Soooooo ... what happened?

 

[PenitantTango]

Well the launch was a mixed bag. It wasn't as big as I was expecting, but then again I went to AirFest 2016 in Kansas last year, so I guess my expectations were really quite high. The DARS L2's and 3's were really easy to get along with, and even donated some of that insulation/ dog barf everyone seems to use. Since it was pretty windy, I meant to use a chute release borrowed from one of the DARS guys, but I seem to have forgotten to attach it to my rocket, as I found it on our table later! I opted to use one of the lower power H100 DMS motors as I was worried about it floating away in the wind. Turns out a 30" chute is still pretty big, because after breaking line of site behind a treeline I never saw it again. Spent the better part of 4 hours looking for it, wading through waist high soybean (?) plants. I left my contact information with the family renting the land, and they said they'd call me if they ever found it, but my hopes are pretty small.

(I have a great video of the launch but I think I'd need to upload it to YouTube?) Anyway, I've added a few pictures of it on the rail.

 

[Bat-mite]

My advice on getting L1 (because I have seen this many times): people focus on the rocket and the motor. They make sure they don't cato the motor and they make sure the rocket is stable and can handle the thrust and landing, and they have their delay timed perfectly and they've done a million sims.

Then they get to the field, launch it to 3000', eject the main at apogee, and watch it float away, never to be found. No L1 achieved if no rocket found.

Make sure you can recover the rocket! Get a chute release, or use DD, or build it extra-strong and use a streamer. But if you pop out a 36" chute at 3000', even if there is just a little bit of ground wind, you might be waving it bye-bye.

Good luck!

Sorry to hear it, truly. But I had to quote my own post for posterity.

 

[fyrfytr310]

Sorry to hear it, truly. But I had to quote my own post for posterity.

Sound advice.

Sorry for the loss on this cert attempt.



Sent from my iPhone using Rocketry Forum

 

[K'Tesh]

Sorry to hear of the attempt's loss. Hopefully it'll be found soon, and intact. If not... The 3rd times's the charm!

Name suggestions for a 3rd attempt (should it be necessary)... Murf, Murph, or Murphy... or Murphy's Law.

 

[AlfaBrewer]

Sorry to hear you didn't recover the rocket. I guess its a good thing you changed from the H123 to the H100. It sucks to lose a rocket, but it really hurts when it takes a case with it.


Come back out with Triplicate Act 2. This time I'll check to make sure you have a Chute Release before I start the countdown.

 

[Rex R]

nother name suggestion; Finn Agle law
Rex