75mm MD build for Level 3 Certification

[SoCalChris]

I'm in the beginning stages of planning my Level 3 certification rocket, and have decided to do a scratch built 75mm MD bird. Yes, this is ambitious, but I'm a firm believer that if something is worth doing, it's worth overdoing in a spectacularly ridiculous fashion.

This isn't my first MD high power rocket, I have built one that barely kissed Mach 2, so this isn't completely new territory to me. I just haven't built one of this scale. Yet.

So the initial plans are to build the rocket, fly it on a K780 motor at my home field to test the air frame and electronics and get a real world test of the stability and make sure my simulation numbers are close. Then assuming all goes well fly it on an M1350 at either XPRS 2018 or at Tripoli Phoenix depending on the timing. Assuming that goes well, I plan on launching it on an M2245 to break 40k'.

Basic stats on the rocket plans....

• Built using Madcow thin wall fiberglass tubing
• Filament wound 5.5:1 Von Karman
• Fins custom cut by PML of 0.125" G10, leading and trailing edges beveled
• Fins will be covered tip to tip in carbon fiber
• Overall length is 9'
• Empty weight will be around 8 pounds
• Tower launched
• Electronics will likely be a Missileworks RRC3 and an Altus Metrum EasyMini
• Nosecone will have a Big Red Bee GPS transmitting APRS data
• Sustainer will likely have a Big Red Bee beeline transmitter as backup, and in case there's an issue with the shock cord separating so I can find both sections of air frame.
• Plan on using structural Aeropoxy to attach the fins
• Plan on using West Systems epoxy thickened with carbon and silica for fillets
• Plan on using West Systems epoxy for TT carbon fiber on fins

Simmed speed and altitude on selected motors

• K780 - Launched at 2,000' altitude (Lucerne Dry Lake) - 10,884' AGL @ 801MPH (Mach 1.07)
• M1350 - Launched at 4,000' altitude (Black Rock) - 22,934' AGL @ 1,304MPH (Mach 1.75)
• M2245 - Launched at 4,000' altitude (Black Rock) - 42,669' AGL @ 2,083MPH (Mach 2.78)

The initial design was done in Open Rocket, then tested in RASAero II. RASAero shows the stability caliber on all of the motors to start at around 2, and increase to around 4 at Mach transition.

If anyone wouldn't mind taking a look at my files and seeing if they see any glaring issues that I'm missing, I'd appreciate it. Constructive criticism and feedback always welcome. Thanks!


View attachment L3.ork

 

[dixontj93060]

Hey why not--best wishes!

 

[rfjustin]

Use 4 fins, not 3.

 

[SoCalChris]

Use 4 fins, not 3.

What benefit would 4 fins over 3 give me in this case? I'm seeing an extra fin as more drag and more things to possibly go wrong. I seem to have the CP & CG sorted out fairly well in the current design with 3, so I don't think I need the fourth fin to move the CP back. What am I missing? Thanks

 

[Nytrunner]

I'm taking a guess that he's suggesting a way to mitigate coning on a long Min-Diam design (especially for moonburners).

 

[SoCalChris]

Doesn't the coning issue with moon burners come into play due to the grain geometry being off-center? Which causes the thrust to be at a slight angle, which causes the bottom of the rocket to kick out, which causes the fins to try to stabilize the rocket, which leads to a vicious circle of the thrust pushing the end of the rocket out and then the rocket trying to correct itself? Am I understanding that correctly? If that's the case, couldn't I avoid that by not using a motor with that grain geometry? Or am I misunderstanding it?

 

[RainierWolfcastle]

With 3 fins in a worse case scenario and corrective airflow acting at 90 degrees to one fin could that impart not only the desired change in angle (ie rocket flight changing angle, fins working to correct it) but also cause rotation since the fin area from side to side is not perfectly symmetrical? Which means any offset in thrust angle, which could be moon burned or simply nozzle imperfections/erosion would also be rotated and lead to a coning effect?

This could also explain why some rockets rotate under thrust, but not after burnout? Possibly it's not spinning, but a very tight coning or spiral flight?

 

[Nytrunner]

There's been some correlation between 3 fin coning even in cases without asymmetric motors.
Of course correlation isn't always causation.

I'd be interested in rtfjustin's full reasoning behind his suggestion as well (so I can stop assuming lol)

 

[ECayemberg]

Not to be a doubting Thomas...but I don't believe that the current design with the materials listed will survive an M2245. I do believe she'll fold over while still under thrust. I've been wrong before, but I also try to help prevent high speed shreds when possible.

 

[rfjustin]

This is a really long minimum diameter rocket. Having a fourth fin (with an appropriate span length), will help reduce coning and make a shred less likely. This is for your L3, make damn sure the fins are sufficient for the job.

My .02, good luck with the project.

 

[blackjack2564]

History has proven thin wall tubing is NOT up to par, when flying a M-2245.
Would you be willing to clamp one end of your rocket,[up by forward end of payload to a workbench. Then leave the rest overhang the table top, add 750lbs on top of the fins & see what happens? Now consider dropping 750lbs weight from only 1 ft above.
That is the kind of force your rocket must handle during flight. That is how much wallop that motor has.
[see max thrust]



4 fins do not add much more,[or if designed correctly non/little at all] drag.
What ever the area of 3 fins is in sq. inches...can be done with four smaller fins of the same sq. inch surface area, though they must still be at least one caliper in span, preferably a bit more. Using a "swept" back design with mach angle leading edge, is also more mach friendly.




A longer rocket is more susceptible to column bending when fins try to correct wind shear etc.
Losing the payload, shortening the rocket & DD out of a single break design will greatly increase you chance of success. This can be done with head end deployment [putting main in NC] & eliminates payload. Also less weight & higher altitudes.

This one has on one side of 7in sled.....2 GPS missleworks & Telemega.
other side has an RRC3 and Stratologger underneath the stacked battery sled which holds 3 lips batteries.





And all of it fits in here:



Stick that on top of fincan and you have a single break DD rocket. Just enough space on top of motor for a drogue and 20 ft of 1/4in. kevlar.
This is the total package, just under 6 ft.



Finally you don't really need T-T these fins are 3/16 carbon plate, held on with just fillets of Hysol and has test flown on a 6xl green to M-2.68 I have no qualms about sticking the M-2245 sitting in my stash in it.That's what I designed it for.



I'm not telling you to do this, just showing what can be done with some effort. I bet the cost of the carbon plate, will be close if not less than all the needed supplies G-10, cloth, etc to do T-T

What ever you decide, good luck & have fun!

 

[SoCalChris]

Thanks everyone for the feedback. Based on what I've heard here and read about coning, I've made a few changes to the design to help prevent it. Mainly I've shortened the rocket by 2.5' by eliminating the top tube, and added a fourth fin. The weight saved by eliminating the top portion offsets the drag from the extra fin, and I get a slightly higher altitude than I was getting previously.

I've also switched from using the two thin wall fiberglass tubes to a single carbon fiber tube. It works out cheaper, lighter, and presumably quite a bit stronger that way.

Since the tube is CF, I plan on putting the GPS transmitter in the nosecone. For deployment, I am thinking a single event fired at apogee, which will deploy a 16" drogue. The drogue will pull out, and also act as a pilot chute for the main, which will be held together with either redundant Jolly Logic chute releases attached in series or a tender descender to be deployed at a lower altitude.

View attachment L3 - V2.ork

Any additional comments on the updated design are also appreciated.

View attachment 333225

How has having the antennas extend into the body tube worked out for you? I've considered mounting them that way, but am always afraid that they'll be ripped off by the shock cord during the deployment charge.

 

[blackjack2564]

First off all I did was move the sweep angle slider under fin design, change them to what I suggested. Notice I gained over 3000ft in Alt. by THAT alone! No other change. [click on it 3 x's for full size]



Then I shortened airframe by 10 inches. [my overall length is 67.75...yours 78.25 Total difference is 10.5 in.] By doing that you gain another 2500ft. Went from original 43,000 something to OVER 49,000![over a mile gain] with 2 simple mods, if you are trying for Max altitude, I would suggest trying this really simple way to get there. You can also make your fins a bit smaller:smile:



You still have 7.5 in. of space to get you gear in. If you need more, after reading what I am going to show next, REVERSE what I did to get more room in NC & recess the BP up into coupler, you will then have 4.5 in recess + 7.5 =12in of space to work with....more than the 10in I cut off! [Re-read this part after the whole explanation] I still have a 7in. interior av-bay.

Look at my drawing. Note the front BP on av-bay is recessed back from front edge of coupler 4.5in & the antennas are going forward into NC. BP rests on ledge created by gluing a 1in section of a sliced coupler inside av-bay.

Coupler is 11.5 long Vent band 2in,to keep more of av-bay out of fincan.



How I pack everything with no danger of 'Ripping' antennas off. [Antenna'a and BP mounts, extensions all purchased from E-Bay. There is detailed construction of all here:

https://www.rocketryforum.com/showt...2-stage-sounding-rocket&p=1605121#post1605121

You will find I also had a tiny bay mounted in tip of NC to hold another GPS! I replaced the metal tip with phenolic tip. Hole drilled in it so 6in antenna could extend straight all way through tip within 1/4in of said tip, no RF interference. That cone held 3 GPS and 3 altimeters, it was a electronics test bed. After flying it 3 times.we found there was no advantage of tip mounted unit.
The new version just has 2 altimeters & 2 GPS all in av-bay to keep it simple.

Heres how I did it:
60 inch Spherachute EASILY packs in a burrito/kevlar 12in shield.



15ft of 1/4 kevlar is ribbon folded, laid on top of burrito,2/3 of cord is in front & 1/3 on top.
Note that ONLY the 12in tail. which is wrapped in tape [to protect from charge blast] is leading out back of package & hooked up to bay.
Then inserted into coupler between wire whip antennas, there is tons of room to do this.
You can see how I tapered the front of coupler, to fit further into cone, just spun it on lath/sanded taper.



Here the cone is added...screwdriver shows how far the chute/cord/nomex goes into cone.
There is 9 in. left in front for the GPS bay shown by black ring in this unpainted pic.



That's the basics...details are in that link above, along with pics of what can happen to a payload [not needed] when traveling at M-3.

Now I think you can see how recessing the BP into coupler on drogue side. [opposite of mine] will give you needed room to DD out of fincan. You will have a ton of left-over space in NC..LOL

My carbon tube in Quasi Isotropic-laid...0-45-90-45 ...17 layers of carbon uni-prepreg, heat cured for 8hrs at various temp, rolled with 750 degree high temp epoxy. If interested in that process & where to get it...PM me.

Good luck have fun...hope all this gets you higher/closer to your goals.

Edit: if you read several of the posts down from where link starts, you will see I switched coupler/vent band from tapered carbon to fiberglass. This solved the GPS antenna from being blocked by carbon
I forgot to mention you come in at 384 grams [correction ounces!] vs 359 [mine] that's a whopping 1.56 lbs lighter!
I used 25 ft of 1/4in kevlar and a 15in drogue in my fincan. I can still shorten it by several inches to optimize even more.
Friction fitted the motor. Re-think those chute releases...hate to see ya chase a rocket due to main at 40,000 + ft!
Might land in Gerlach..LOL

 

[SoCalChris]

Quick update on this.... After talking around on my club's mailing list I found another member who is doing something similar for his L3. He pointed me to the Wildman Falcon kit, which was almost identical to what I'd come up with, save some minor differences on fin dimensions.

Looking into the Falcon some more, I found that it was designed by one of the members of my club, and one of the guys who I was going to ask to be one of the TAPS for my project. So, I talked to him this weekend about it. I'm making a few minor changes to the kit (Single break dual deploy, and swapping the 3/16" G10 fins for 1/8" CF), and that's what my project will be.

I got the kit ordered, and will hopefully begin building fairly soon. Thanks everyone for the design tips, and helping point me in the right direction. Once I start, I'll do a build thread for this beast.

 

[ChrisAttebery]

The Falcon is a solid kit. I think upgrading the fins is a good plan. Make sure you use the recommended epoxy to attach them. Keep in mind that an M2245 flight will damage the fins possibly to the point of making it a one time use rocket.

My advice is to keep your build simple and keep your goal in mind. Get your L3 cert and then go hog wild. I tried to include too many cool but unnecessary features (and had a bit of bad luck) in my Punisher 4 build and wound up taking several attempts to get my L3. My Blackhawk 75 (Falcon 75 clone) also had a fin crack loose because I wanted to use the epoxy I had on hand, and biased my build more towards performance than reliability. A better epoxy, bigger fillets and/or additional re-enforcement would have reduced the chances of the fillet cracking loose.


My L3 sagas are posted here:
https://www.rocketryforum.com/showthread.php?131084-Chris-Punisher-4-L3-build

https://www.rocketryforum.com/showthread.php?137544-Wildman-Blackhawk-75

 

[SoCalChris]

Hi Chris, David and I talked a bit on both issues of epoxy and protecting the fins. For epoxy, I'll be using Aeropoxy ES6209 to attach the fins, and will create the fillets with Hysol high temp epoxy. As far as the fins, he is having Tim at Wildman see about custom making them using carbon fiber with high temp resin, or I will have them made using high temp carbon fiber (Possibly using this stuff from Dragon Plate). Additionally, we discussed protecting the leading edge with either a coating of high temp epoxy, or someone else suggested coating the leading edge with stainless steel tape. I haven't personally heard of the stainless tape being used, and I'd be concerned about air getting under it at the speed it will be travelling, but I will look into it some more.

What size fillets did you use on your Blackhawk? What epoxy did you use that failed? And what reinforcements would you recommend? We talked about tip to tip carbon fiber, but he highly discouraged that with what I'm planning.

 

[ChrisAttebery]

I used 1/2" radius fillets using RocketPoxy. Before I fly it again I'll add a 2" wide strip of CF tape over the fillets. In hindsight I should have ordered Hysol. My recommendation is to use a 5/8" radius fillet with the Hysol.

What is the thinking for using 6209 to attach the fins and then using Hysol? Why not use Hysol to attach them too? I agree that tip to tip isn't necessary with the CF plate. The failure point is going to be the at the root.

 

[patelldp]

The use of Hysol appears to have been reinvigorated by my Dynacom Scorpion build thread a few years ago. I posted that they called for it back then, included some figures on the glue that is wasted in the mixing nozzles, and showed how to mix it without the nozzle. Between the FWFG nose cones and the recommended use of Hysol, it's funny how what was old is new again.

Hysol's (E-120HP) biggest benefit is that it has known properties which used to be a bigger benefit back in the Dynacom era. It has a 5900psi tensile strength, Aeropoxy 6209 has an 8,000psi tensile strength, and RocketPoxy has a 7,600psi tensile strength. Neither of the other two report a shear strength, but Hysol's is 4,300psi.

Looking at those figures and taking them as fact, Hysol really isn't the best option out there anymore. Its primary benefits now are likely consistency/repeatability due to the large manufacturer and dispensing gun method, convenience, and nostalgia. It isn't even cheaper than the others. So, I'm thinking nostalgia.

Did I mention that one of the Hysol fillets on my Scorpion cracked on landing on the first flight?

 

[Nytrunner]

The use of Hysol appears to have been reinvigorated by my Dynacom Scorpion build thread a few years ago. I posted that they called for it back then, included some figures on the glue that is wasted in the mixing nozzles, and showed how to mix it without the nozzle. Between the FWFG nose cones and the recommended use of Hysol, it's funny how what was old is new again.

Hysol's (E-120HP) biggest benefit is that it has known properties which used to be a bigger benefit back in the Dynacom era. It has a 5900psi tensile strength, Aeropoxy 6209 has an 8,000psi tensile strength, and RocketPoxy has a 7,600psi tensile strength. Neither of the other two report a shear strength, but Hysol's is 4,300psi.

Looking at those figures and taking them as fact, Hysol really isn't the best option out there anymore. Its primary benefits now are likely consistency/repeatability due to the large manufacturer and dispensing gun method, convenience, and nostalgia. It isn't even cheaper than the others. So, I'm thinking nostalgia.

Did I mention that one of the Hysol fillets on my Scorpion cracked on landing on the first flight?

A club member did L3 at Airfest that was MD Mach 3+ single-tube/single-break.

Amusingly enough he was going to use Hysol, but his source to get some free fell through. He ended up using random 30 minute on the root to stick them in place, then JB weld fillets under the tip to tip. Thing was solid as a rock (even though the engine block paint on the nose and fins looked like someone had attacked it with a cheese grater)

 

[patelldp]

A club member did L3 at Airfest that was MD Mach 3+ single-tube/single-break.

Amusingly enough he was going to use Hysol, but his source to get some free fell through. He ended up using random 30 minute on the root to stick them in place, then JB weld fillets under the tip to tip. Thing was solid as a rock (even though the engine block paint on the nose and fins looked like someone had attacked it with a cheese grater)

Every scenario is different and can still yield success. I was surmising on how Hysol was reincarnated in the rocketry community despite its inferior properties. I intend to use Hysol on my own 76mm carbon rocket because I have some left over and already own the application gun. It's convenient.

 

[SoCalChris]

What is the thinking for using 6209 to attach the fins and then using Hysol? Why not use Hysol to attach them too?

My understanding of this, and thinking behind my decision is that the Aeropoxy provides a superior bond, at room temperature. According to my model in RASAero II, the rocket will spend 13.5 seconds between Mach 1 and Mach 2, and 5.5 seconds above Mach 2 with a maximum speed of Mach 2.84. The Aeropoxy will give the fin bond the greatest strength overall, while the high temperature Hysol epoxy will protect the Aeropoxy from deforming and losing it's bond while the rocket is experiencing higher temperatures from the speed and motor burn.

 

[ChrisAttebery]

The use of Hysol appears to have been reinvigorated by my Dynacom Scorpion build thread a few years ago. I posted that they called for it back then, included some figures on the glue that is wasted in the mixing nozzles, and showed how to mix it without the nozzle. Between the FWFG nose cones and the recommended use of Hysol, it's funny how what was old is new again.

Hysol's (E-120HP) biggest benefit is that it has known properties which used to be a bigger benefit back in the Dynacom era. It has a 5900psi tensile strength, Aeropoxy 6209 has an 8,000psi tensile strength, and RocketPoxy has a 7,600psi tensile strength. Neither of the other two report a shear strength, but Hysol's is 4,300psi.

Looking at those figures and taking them as fact, Hysol really isn't the best option out there anymore. Its primary benefits now are likely consistency/repeatability due to the large manufacturer and dispensing gun method, convenience, and nostalgia. It isn't even cheaper than the others. So, I'm thinking nostalgia.

Did I mention that one of the Hysol fillets on my Scorpion cracked on landing on the first flight?

This is great info. Thanks for posting it Dan.

 

[patelldp]

The Aeropoxy will give the fin bond the greatest strength overall, while the high temperature Hysol epoxy will protect the Aeropoxy from deforming and losing it's bond while the rocket is experiencing higher temperatures from the speed and motor burn.

The first part is confirmed, the AeroPoxy is the stronger of the 2 epoxies. It appears that the second part is true as well, the Hysol has a Tg of 90C, RocketPoxy has a 66C, and the ES6209 is unreported.

RocketPoxy - https://www.glenmarc.com/datasheets/EPOXY/RP_G5000_DATASHEET.pdf
ES6209 - https://www.ptm-w.com/aeropoxy/AEROPOXY Product Bulletins/AEROPOXY ES6209 Bulletin.pdf
Hysol E-120HP - https://tds.us.henkel.com/NA/UT/HNAUTTDS.nsf/web/8FD3ABAA09A649FE882571870000DB2F/$File/EA%20E-120HP-EN.pdf

 

[Andrew_ASC]

Call Aeropoxy and ask for a TG just because it is unreported does not mean they do not have it. Tell em' you want it on a rocket at Mach 2+. The poor sales person will flip out and get an engineer. It isn't superior when you have zero data for that to compare with, bud. You may get a shocker when you hear how low TG some of their products are. They wouldn't sell me an epoxy for a M1.7 application. I told them on the phone lives were on the line. And I damn well meant it, because they were literally resting on a TG value keeping a fin on and a casing bonded in a 0.3s to Mach 1.5 application 200ft away from people. They gave a list of competitors. I might be an idiot for using Cotronics 4700 on an L-1 MD multistage, but it had TG value listed and exceeded these other guys specs. Heard the 4525IP is the room temp version non oven cure. Aeropoxy is good on paper at room temperature. With Aero heating.... You don't want it. Call Aeropoxy. They will get numbers you don't want to hear. Not trying piss on Aeropoxy products. Fine AIRCRAFT glue.
First thing they want is how hot is your ROCKET going to get when its supersonic. When they don't sell you the epoxy... You realize something about how important a TG number is. Don't settle for unreported.

 

[Andrew_ASC]

The first part is confirmed, the AeroPoxy is the stronger of the 2 epoxies. It appears that the second part is true as well, the Hysol has a Tg of 90C, RocketPoxy has a 66C, and the ES6209 is unreported.

If you forced me to pick something of Aeropoxy, Rocketpoxy, or Hysol. I'll rest with Hysol's printed TG value. It makes me fuming mad, knowing a fin could literally rip off well above room temperature from friction with the air, yet some people don't understand. It's a number on a sheet with a random meaningless meaning to non engineer or non engineer student until a fin rips off in flight from a thermal load. At face value take the Aeropoxy at room temperature only until you call and get a proper TG value to compare with. Mach 2+ is not room temperature. This is why hobby rockets fall apart. It wasn't that people are dumb, they never had the chance to learn the difference. When you go real economical on an epoxy you start gimping yourself on the thermal performance of the epoxy and the tensile strength above room temp.

Once TG is exceeded the epoxy changes viscosity and loses bond strength with surface. I had no idea what TG until I called an epoxy factory about it and they walked me through how important it was. Whatever brand epoxy you all pick for a high performance application will have serious real consequences from a real TG value whether you know the value or don't know it. Sorry.

/RANT

 

[Andrew_ASC]

You wouldn't put a blowtorch (A thermal load) to a rocket motor casing with live fuel grains inserted. Why do people recommend epoxies without TG values listed that could fail if the application exceeds the value?????

 

[patelldp]

Alcohol?


Sent from my iPhone using Rocketry Forum

 

[Andrew_ASC]

TG is life.... If you gotta guess a thermal loading, pick a higher TG. Epoxy is cheap. Rockets are expensive. Lives are irreplaceable.

 

[Steve Shannon]

Alcohol?


Sent from my iPhone using Rocketry Forum

Something.

Or we haven’t had enough …

 

[OverTheTop]

I forgot to mention you come in at 384 grams vs 359 [mine] that's a whopping 1.56 lbs lighter!

I think there was a units issue somewhere in this sentence. Grams, or Ounces?

Thanks for some great experience in this thread Jim (and everyone else). Instructive and helpful as always.