What's the best 75mm MD simple but effective way?

[Viperfixr]

Been doing HPR since 2008, but never been to an LDRS. That changes this year! I am giddy at the thought of seeing the rocket pasture with a (insert paperwork required if going above 23k, at least one month prior) 50k waiver. I have a 54mm MD rocket that will rock a CTI L935 or AT L1000W, but I think a 75mm MD rocket is called for. I have exceeded 14k a few times, but it is time to do more.

My goals are simple: break 20k with the 54mm MD rocket (easy, I think) and to break 30k AGL with a 75mm one.

Seems most of the options these days are Wildman, which is great--I am a fan. I am debating between one of those really slick looking Binder AL fin cans [on a CF airframe?] or something like a Wildman Blackhawk 75 or Mach 3. Or, some other approach entirely? I have an unbuilt Wildman 3' FG kit that could easily be done without the motor tube. I like the WM Falcon 3, but that's more than I want to spend right now. After scouring the TRF archives, I still have a few questions.

Couple questions for those who've been down this path already:
- Fly away guides? I have a launch tower that is 4' tall. Perhaps Argonia has one for use. Either way, would you use fly away guides for such a flight?
- Much benefit going with a CF airframe vs. FG for the goal of breaking 30k (i.e. not setting a record)?
- Any thoughts on fin cans vs. traditional surface mounted fins? If later, tip-to-tip needed? Special adhesive?

Regardless, I am planning on redundant electronics via Stratologger CFs and Doghouse charge canisters w/terminals built in. I have surgical tubing for ejection charges as well. And, I have trackers-a-plenty--both BigRedBee TXs and a Featherweight GPS. Motor choice will be somewhat limited by availability, but either the CTI M840, CTI M2020 or the AT M685 would be good.

Time to pick a path!

 

[dhbarr]

If you already have a WM 3in kit then a Binder over that is certainly straightforward. Or adapting your existing fin profile to something faster.

Can't see a reason to spring for carbon unless you're really going to push an envelope, but just CF fins would probably be a middle road between what's on hand and the aluminum solution.

 

[manixFan]

I have built two 54mm MD rockets that have gone well over 20,000', and have built similar 38mm and 98mm rockets. The 38mm flew to over 14,000' and I plan to fly the 98mm at BALLS this year to over 30,000'. All have only surface mount fins with no tip-to-tip or anything other than well-sized structural fillets. All have carbon fiber fins though. I also have a 3" MD with G10 fins that uses typical tip-to-tip construction that has flown to about 20,000' on a research M motor. The key to keeping your fins on is avoiding any amount of fin flutter. One of the big advantages of the typical 3 layer offset tip-to-tip method of glassing fins is reducing the likelihood of fin flutter. G10 fins are not very stiff so they do benefit from the added stiffness of tip-to-tip. However, if you use the right shape and material you can get away with just surface mounting. But you have to prep the surface very carefully and use an epoxy that is up to the task. The CF fins that came with the 98mm CF Mongoose I bought are .308" thick - a quick sim shows a flutter speed of over Mach 5 using a very conservative value for shear modulus. G10 fins are not nearly as stiff so you have to be a lot more careful making sure they will be ok.

Using a tower is by far and away your best bet. I've had mixed success with the fly-away guides but haven't used the new version yet. They definitely require some practice and getting the fit just right. I would check with others from your area to see if you can find a tower.

Finally, the M2020 has quite a bit more impulse than the other two (about a 1,000NS) so that would likely be your best bet for altitude. However the other 2 will be a lot easier on your rocket.

Good luck,


Tony

 

[Viperfixr]

dhbarrm, yes, seemed the path of least resistance until I measured the airframe...only 36". Enough for most 54mm motors, but not the longest 75mm ones. Darn.

Tony, great comments, seems like CF fins (or AL) is a solid choice.

 

[DaveW6DPS]

I just got a Mad Cow Go Devil 75mm. It looks like it will be a lot of fun. I got the 54mm motor mount, but keep thinking about the minimum diameter version with an M650.

I may have found a reason to do level 3...

 

[BBrown]

Couple questions for those who've been down this path already:
- Fly away guides? I have a launch tower that is 4' tall. Perhaps Argonia has one for use. Either way, would you use fly away guides for such a flight?
- Much benefit going with a CF airframe vs. FG for the goal of breaking 30k (i.e. not setting a record)?
- Any thoughts on fin cans vs. traditional surface mounted fins? If later, tip-to-tip needed? Special adhesive?

Regardless, I am planning on redundant electronics via Stratologger CFs and Doghouse charge canisters w/terminals built in. I have surgical tubing for ejection charges as well. And, I have trackers-a-plenty--both BigRedBee TXs and a Featherweight GPS. Motor choice will be somewhat limited by availability, but either the CTI M840, CTI M2020 or the AT M685 would be good.

Time to pick a path!

Mark,
I’m glad to hear you are planning on joining us this coming Labor Day. The club has a 10’ tall adjustable tower that might be just the ticket for flying this project. You are more than welcome to use it. There is one caveat, we are expecting 375 to 400 high power flyers at LDRS 38 so there may be a bit of a line to access a piece of specialty equipment such as the launch tower. Find me on the field and we will figure it out.
Bob Brown
Launch Director, LDRS 38

 

[Wayco]

Hey Mark, yeah I have been down this path already.

I started with a Wildman Blackhawk 54, back before he started using thinwall fiberglass. Made several successful flights with it, but the two that made your 20k mark were on an L640 dual thrust to 22,535 ft. and the L935 Imax to 23,230 ft.

When Wildman came out with a thinwall Blackhawk I was all over it. Built it with the same epoxy and modifications as my original and it was very close to 1 lb. lighter! Imagine my surprise when I flew it on an L640 and despite a perfect trajectory, it came up several hundred feet short of its predecessor. 22,300 ft. Light is great for fast, but mass and momentum will carry the day. Speed is fun, but it also increases drag. A long burn motor will get more altitude than a fast burn if all other things are equal.
Not to be deterred, I bought into the carbon fiber with a Mongoose 54, but due to a technique I used for fiberglass, my first flight was less than perfect:

Apparently, using a spring loaded punch to mark drill holes delaminated the cf, resulting in the damage above. I negotiated a new cf tube 60" long from Curtis, and purchased a Binder design fincan. The original payload tube, avbay and nosecone have made one flight since then, just to convince myself that the Loki L2050 might be in this rockets future.

Last but not least is the Mongoose 75, which might be the ticket for your 30k flight. Mine is CF, but Sharon has the fiberglass version in her build pile.... Mine has flown five times, all successful, but the M840 got me my highest flight to date, 28,905 ft.

None of these rockets had tip to tip layups or exotic epoxies, just good surface prep and rocketpoxy. If you do a search, you might find a couple of build threads on one or two of these, but I remember that negative comments kept me from finishing the thread on the Mongoose 75, which is my most successful MD rocket.

I have flown several MD rockets at Airfest, and brought my own tower once or twice. Bob is right, it's gonna be a circus at LDRS this year, but I'm planning on bringing a couple of rockets that will require the High Altitude application, and I might be bringing my 8 ft. launch tower.

You might not know this, but one of my favorite memories is the personal tour you gave Sharon and I of Nellis AFB. Sitting on the taxiway behind that F35 was quite a treat.

So I'm really excited to hear that you are coming to the Rocket Pasture this year. If there is anything we can do to make your trip more enjoyable, don't hesitate to ask.

 

[manixFan]

To reiterate Wayco's comments, my highest altitude with a 54mm MD to date has been with the CTI L935 - 23,500' at BALLS. As a general rule of thumb, fast burn motors like heavy rockets, longer burn motors like lighter rockets. I've played around with weights in OpenRocket and it's really surprising how around the optimum mass you can change the weight by what seems like a lot and have very little effect on altitude, at least in my experience. For example, here are some sims on my 'NextGoose' (replacement 54mm Mongoose CF lookalike):

Nose wt.    L265        L935
0 oz.      22,403*    22,025
4 oz       22,264     22,136
8 oz.      22,134     22,211
12 oz.     21,970     22,255
16 oz.     21,775     22,269*
20 oz.     21,599     22,260
24 oz.     21,368     22,230

* = highest altitude

So I can add a pound of nose weight and actually improve my altitude flying on an L935, but any weight decreases my altitude on an L265. And a weight change of 12 to 20 oz yields nearly the identical altitude. Of course these are just sims and it's possible I'm doing something wrong. But it mimics my real-world experience pretty closely.

Just food for thought,


Tony

 

[plugger]

I've just returned from THUNDA 2019 which was over the weekend in Queensland. I flew a 54mm MD on a L935 successfully to just shy of 27k and roughly Mach 2.67. I did that with a FARG. It did fracture though but should be able to be repaired and reused.

My next flight was a 75mm MD on a M2245. That was not successful. I've got the AV Bay and NC back, the rest kept going despite shearing a fin off. I just watched my video from the pad, the FARG was still attached when it left the frame. I've got a lot of failure analysis ahead but I suspect the FARG was the root cause of the non nominal flight. Maybe it would have done better if I'd not flown on such a long rail (~10 ft) but you make do with what you have available when you have to fly to the other side of the country to launch.

Both rockets were carbon airframes and fins. Fillets only, no t2t. As mentioned, the 54mm was fine throughout the flight with no t2t but one of the fins popped when it hit the ground. Given that experience I now plan on using at least one layer of t2t in future on MD birds.

My use of FARGs is over. I'll post a photo of the 75mm FARG once I dump all my media from the weekend but I can tell you that it's mangled significantly. The rods are bent and nothing is square anymore. I'd definitely listen to Tony's advice if I were you and use a tower if at all possible.

drew

 

[manixFan]

Wow, that is a great flight on the L935, I just ordered one for BALLS this fall. I doubt I'll get anywhere near that altitude though. Sorry to hear about your troubles with the FARG. Will be interesting to hear your findings after you look at the media of the flight. Also interesting to note the fin popped off on landing, definitely a concern on my 4" MD with the much heavier empty motor case compared to a 54mm rocket. I'll have to give that issue some thought.

Thanks for the report,


Tony

 

[rfjustin]

Couple questions for those who've been down this path already:
- Fly away guides? Yes indeed, use it! I've used the 75mm and 98mm from Wildman/Additive Aerospace, and they just work. Towers are a PITA.
- Much benefit going with a CF airframe vs. FG for the goal of breaking 30k (i.e. not setting a record)? IMO, Nope
- Any thoughts on fin cans vs. traditional surface mounted fins? If later, tip-to-tip needed? Special adhesive? Nothing wrong with fin cans, never used them myself. IMO, surface mount the fins, good size fillets (proline 4500?), then do CF and glass t2t with Aeropoxy and post cure the fin can. From my experience, t2t is needed for landing/recovery and necessarily the up part.

 

[plugger]

Hi OP,

I thought I'd chime in on the rest of your questions.

Much benefit going with a CF airframe vs. FG for the goal of breaking 30k (i.e. not setting a record)?

Personally, I think so. I know that from a pure physics perspective the adage of "build light for max V but build for optimum mass for max altitude" is correct. But in our specific use case the inefficient mass fraction of the over engineered cases and closures we use from a hobby perspective do a great job of limiting our performance in both altitude and velocity. This is where CF provides an advantage over FG as you can build the tail end of your rocket to be incredibly light (when compared to FG). This allows you to build an light vehicle overall with most of the non-motor weight where you want it, at the top of the rocket. This has a value add in so far that it allows you to build a light vehicle which still has a stability margin above 1.5 during the duration of the motor burn.
For instance, my L935 flight from a couple weeks ago went to 26,850 ft AGL. The airframe + fins + fillets total weight of that rocket was 325 grams. The majority of the vehicle weight (excluding the motor) was in the AV bay and NC.

Any thoughts on fin cans vs. traditional surface mounted fins? If later, tip-to-tip needed? Special adhesive?

Surface mounted fins work fine up to around Mach 2.5 but if you're going to approach Mach 3 then you'll want t2t or a fin can. That's my experience at least. That said as I wrote earlier if you only do filleted fins you might survive the boost/flight only to pop a fin during recovery.

Wow, that is a great flight on the L935, I just ordered one for BALLS this fall. I doubt I'll get anywhere near that altitude though. Sorry to hear about your troubles with the FARG. Will be interesting to hear your findings after you look at the media of the flight. Also interesting to note the fin popped off on landing, definitely a concern on my 4" MD with the much heavier empty motor case compared to a 54mm rocket. I'll have to give that issue some thought.

Thanks for the report,

Tony

Cheers Tony. If it wasn't for your airframe layup videos on Youtube I would still be in the dark ages. Teflon coated peel ply is amazing!

Here's a little build thread and flight report for my L935 flight.

https://forum.ausrocketry.com/viewtopic.php?f=10&t=6216

 

[Nytrunner]

Cheers Tony. If it wasn't for your airframe layup videos on Youtube I would still be in the dark ages. Teflon coated peel ply is amazing!

I thought Tfish was video-tony

Manix, do you do tutorials also?

 

[plugger]

I thought Tfish was video-tony

Manix, do you do tutorials also?

Shoot, you're probably right. If so sorry for the confusion.

 

[manixFan]

I thought Tfish was video-tony

Manix, do you do tutorials also?

I wish I had the knowledge of tfish Tony! I haven't done any tutorials for rocketry, but one of these days...

I have also watched many of Tony's (tfish) tutorials. I think anyone who takes the time to do a really useful tutorial like those solely for the benefit of other hobbyists is to be highly commended. Really makes a difference to lots of less experienced folks like myself.


(not tfish) Tony

 

[DEmery]

Use a tower if you can get one. I have used fly-away guides on my 3" Wildman with no motor tube, just the body as a minimum diameter. The guides from Additive Aerospace have been redesigned, and they survived the last flight on a K560W.

 

[Steve Shannon]

Use a tower if you can get one. I have used fly-away guides on my 3" Wildman with no motor tube, just the body as a minimum diameter. The guides from Additive Aerospace have been redesigned, and they survived the last flight on a K560W.
View attachment 382179

That looks familiar …

 

[OverTheTop]

- Any thoughts on fin cans vs. traditional surface mounted fins? If later, tip-to-tip needed? Special adhesive?

I would slot the airframe and sit the fins into the slots (very close fitting slots). This is a huge improvement in structural integrity over just butting the fins on the surface of the tube. If I were building this it is what I would do.

 

[manixFan]

I would slot the airframe and sit the fins into the slots (very close fitting slots). This is a huge improvement in structural integrity over just butting the fins on the surface of the tube. If I were building this it is what I would do.

Can you give any empirical examples of this? I’ve had this discussion with other folks and in the end the consensus seems to be that for minimum diameter slotting the airframe weakens it rather than strengthens it. There’s no supporting internal structure for the airframe to keep it from flexing between the slots. Based on some very simple testing that seemed to be true, but that was a long time ago. Having just built a 98mm MD, I thought about this issue quite a bit and figured if slotted was better, more folks would do it. But of course that’s not always true.

Thanks,


Tony

 

[KenRico]

In reading this thread I am reminded of an ol CJ trick he would use if he thought Max Q exceeded Delta Shred .

He would add additional coupler through out the airframe that did not already have a coupler or motor hw present ..ie : below the nose shoulder and above the traditional avbay seat.. above the motor hw below the avbay seat.

I have no hard data to support this but CJ may have some more detail of when he thought it was helpful

Kenny

 

[OverTheTop]

Can you give any empirical examples of this?

As requested: https://forum.ausrocketry.com/viewtopic.php?f=6&t=4758
Mach 1.8

Note that I put the engine thrust to the airframe at the top of the motor, so the aft section is in tension. The "weakening" of the airframe if properly bonded (and not taking the thrust) is minimal. You have effectively bonded, through very thin glue joints, two very strong materials. Composites work far better that way, in tenstion. Also, the Euler buckling load on the upper section of airframe is higher, due to the shorter length in compression.
https://en.wikipedia.org/wiki/Euler's_critical_load

Here is an example of it being done and using an improved method that involves tabs to set the depth and location of the interface:
https://forum.ausrocketry.com/viewtopic.php?f=6&t=5019&start=30
Mach 2.14

 

[Titan II]

Using slotted tube for fins....

 

[manixFan]

As requested: https://forum.ausrocketry.com/viewtopic.php?f=6&t=4758
Mach 1.8

Note that I put the engine thrust to the airframe at the top of the motor, so the aft section is in tension. The "weakening" of the airframe if properly bonded (and not taking the thrust) is minimal. You have effectively bonded, through very thin glue joints, two very strong materials. Composites work far better that way. Also, the Euler buckling load on the upper section of airframe is higher, due to the shorter length in compression.
https://en.wikipedia.org/wiki/Euler's_critical_load

Here is an example of it being done and using an improved method that involves tabs to set the depth and location of the interface:
https://forum.ausrocketry.com/viewtopic.php?f=6&t=5019&start=30
Mach 2.14

Wow, that is some great info! Really some great engineering and design throughout.

I'm near retirement and I've been telling my wife I want a real workshop once I retire. After seeing all the wonderful gadgets in your photos, I realize I've been thinking on way too small a scale. Thanks for sharing those links, although my wife my feel otherwise!


Tony

 

[OverTheTop]

I'm near retirement and I've been telling my wife I want a real workshop once I retire. After seeing all the wonderful gadgets in your photos, I realize I've been thinking on way too small a scale. Thanks for sharing those links, although my wife my feel otherwise!

Thanks. The tube slotting could be accomplished with a Dremel but it would be a long process. The mill allows the tube to be jigged straight and then the slots are straight too. Note that slots are cut undersized and the slot then opened up (last 0.1-0.2mm, 4-6thou) and fitted for each individual fin. This takes very little time with a file.

There are other ways of slotting tubes that I have seen that should give acceptable results.