38 vs 54? 2 miles + mach breaker

Handeman said:

I keep hearing the same thing, but I'm curious, what is going on? I've been flying DD with friction fit for 8 years and haven't had any issues. Is there something I'm doing that is making that work while other have issues? I don't think I'm doing anything different than everyone else, but I don't seem to have any issues with friction fit. I never used shear pins until I got my 6" diameter FG L3 rocket. I wouldn't fly that rocket without shear pins, but I've never used them on the 4" and smaller rockets. Why does friction fit work for me and not everyone else on the 4" and smaller rockets?

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I feel it would be better with shear pins as the NC of the rocket carries the electronics and is fibre glass depends what the weight of the NC is

Yet again personal preference. 😄

Depends on the weight of the components and drag. Added weight usually culprit. Friction fit works just shear pins have a much higher success rate.

Weez said:

Depends on the weight of the components and drag. Added weight usually culprit. Friction fit works just shear pins have a much higher success rate.

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Well said 🙃🙂

You can do Mach 1.5 and 2 miles on an I216 with a scratch built 38mm minimum diameter. Including an altimeter and a GPS tracker.

Weez said:

Depends on the weight of the components and drag. Added weight usually culprit. Friction fit works just shear pins have a much higher success rate.

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It very much does depend on the weight of the components. If your electronics are in the nose, probably use shear pins.

If the nose is empty or your recovery gear sits in there, I like friction fit.

But I would only use friction fit on MD rockets, never on anything I can put a standard aeropack on.

Zebedee said:

You can do Mach 1.5 and 2 miles on an I216 with a scratch built 38mm minimum diameter. Including an altimeter and a GPS tracker.

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Yes that is very much doable. But if you are willing to spend the money, and have the waiver/recovery space, then build it for the bigger motors. My MD was built to fit the 5grain case and smaller, because it was built for being stable and achieving max altitude on an I216.

les said:

OK - decided to stay cheap

I purchased the Madcow 1.6" Tomach during their BF sale. They are kindly sending me the parts to build it as a minimum diameter - tangless fins and un-slotted body tube.

I am going to build it to accept up to a CTI 6xl case even though at the moment my biggest case is a 6 grain. (Sorry Teddy - I was going to build to support the Loki 38mm-1200 case but it is too long for the tube that comes with the Tomach)

Below is a snapshot of the Madcow generated Rocksim file that I modified. I didn't post the actual file since Madcow has it flagged as copyrighted....
I added some mass for electronics in the nose and of course changed the motor mount. I also moved the fins forward a bit to allow for the laminer flow.

The sim shows I have a stability of 3.1. I understand from other threads that the CP moves forward as you break mach so you need extra margin. Can I get by with as low as a 2.0 stability? I may cut the length down a bit to reduce weight and get a higher speed.

With a 6g J354 Rocksim shows to getting over 12000' and 1500 fps, achieving my goal. Although I do recognize that the sims tend to be optimistic.

So a few more questions - sorry....

At what point do I need to worry about heating from air friction? If I ever go to the 6xl and a j530 imax it shows a speed of 1900fps. Should I be using JB weld since it is rated for 500 degrees?

Since I don't have any of the basic tools for adding fiberglass (resin, peel ply, etc) I would like to avoid adding that. Looking at the Wildman site, even the smallest containers of resin and hardener exceeds the price I paid for the kit! From looking at other threads I don't believe I should use the standard 30 min hobby epoxy for laminating.

Since Aerofin does not appear to be available anymore, is there any other means to determine what my max speed can be with the standard 1/16" fins that the kit comes with before I have an issue with fin flutter? Will I break it if I try the imax???

To minimize drag, I plan to use a fly away guide, minimum diameter retainer, and the cable cutter. My thoughts for using the cable cutter is I can avoid shear pins projecting into the air stream as I only need the rocket to "break" in one place at apogee. Plus I own a set, might as well use it

View attachment 276478

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Your design and motor motor choice is essentially the same as the WM Blackhawk 38 which I flew last year.

• 12k feet no problem
  
• With a 6XL loaded, this rocket weighs a ton! The thin-wall stuff in the Tomach may be a lot lighter, which is good.
  
• Be mindful of the stability margin as the rocket leaves the rail, especially with crosswind. This is a rookie mistake of which I was guilty - putting a big ass motor in a long skinny rocket.
  
• Plot the graphs in OR. Mine had a few moments of negative stability coming of the rail, which I believe led to a less than ideal trajectory in the real flight. I since added some noseweight.
  
• Surface mount fins are OK with a good epoxy. No additional FG cloth needed. The BH38 came with "RocketPoxy", or whatever it is called. Good stuff, and the temperature rating seems fine.
  
• However, your fins are only 1/16" because they were designed for additional TTW strength. Now after removing the TTW, the fins seem a little wimpy. The BH38 fins are a beefy 1/8", I think. They are rock solid.
  
• You can countersink the screw heads to keep them flush to the airframe. I use set screws to hold the av-bays in place and maintain flushness. Got this idea from Crazy Jim.
  
• Friction fit with masking tape in small fiberglass tubes is a pain for me. The tape just rolls away. Shear pins are just easier, cleaner, and more consistent. I use one in the nose.

Les,, You're a good man,, REALLY well thought out BEFORE you move forward...
I normally don't like to build in restrictions,, but for your purposes here I don't think the Loki 38 / 1200 case is necessary...
And with an adversity to fiberglassing the fins,, it would probably be worse then that....

I always put all of the tracking equipment in the nosecone ( redundant )..
So my nosecones are usually heavy...
It's the weight of the nosecone and the length of the booster's harness that determines whether or not shear pins become necessary.....
I'd venture a guess that anybody that has consistent success with friction fitting a nosecone on high fast flights likes to be certain to have enough length in their booster's harness....

Teddy

Buckeye said:

[*]Surface mount fins are OK with a good epoxy. No additional FG cloth needed. The BH38 came with "RocketPoxy", or whatever it is called. Good stuff, and the temperature rating seems fine.

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I was looking at the instructions for the BH38. It states it comes with Pro-line epoxy which is rated at 500 degrees. The Rocketpoxy is rated for 225 degrees. That is why I was considering using JB Weld as it is also rated for 500 degrees.

I have seen other build threads where people used the JB weld - just wasn't certain if they were doing so for the temperature rating or not.
Or can I use my standard hobby 30 minute epoxy?


One other comment/question. Even if I breakdown and buy a small west systems set of resin and hardener, I looked up its specs. Not sure if it is the same thing, but it defines its "heat deflection temperature" of only 123 degrees. And an Ultimate Tg of 139 degrees. So if you do tip to tip isn't this epoxy the weakest link if you get aerodynamic heating?

Weez said:

Personal preference for sure, I use shear pins for both the nose and the middle. Seen too many drag separations and the resultant carnage.

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I use shear pins on everything I want to stay together.

markkoelsch said:

I use shear pins on everything I want to stay together.

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+1....

JB Weld is very heat resistant.....
And brittle....
I would not use it for this purpose...

Teddy

And now I am very confused. I recall seeing some threads that claimed OR is a bit better at transonic than RS, so I finally downloaded OR (I've owned RS for years)

I also understand OR will import a RS file, which it appears to have done

I tried a simulation and am seeing differences

weight - RS - 57 oz OR - 71.5 oz
Max altitude - RS 12647 OR - 1234 feet - an order of magnitude difference!
Max speed - RS 1520 fps OR 548fps

I even added weight to the RS sim to get the mas to 72 oz. It still shows a max alt of 12811 and max speed as 1267fps

So why the huge difference?


WAIT - Started comparing files. RS shows CG at 19.5", OR shows it way back at 33" and rocket is unstable. I did notice for the components there is a statement that the mass is over ridden by stage. Not sure what that means...

Found where/how to eliminate override mass. Much better now.

Please ignore this post

You need to setup the recovery parameters in OR and you'll get the right numbers, see this all the time. OR is better at transonic in my experience. Yesterday I flew my Tomach 54mm and OR simmed at ~ 19k ft while RS simmed ~ 16k ft. Actuall was 19147 ft by gps altitude and 18886 by baro. Glad I didn't fly the L935 which simmed at 19.8k in RS, the waiver was for 20k ft!

FWIW I used system 3 t88 structural epoxy on the fin roots and filets were proline 4500. I fiberglassed tip to tip with west systems after "painting" the outer 1/2 inch of the fins with proline 4500. Heat issues will mostly be the fin edges and any protrusions from the body tube and the tip.

Mach one won't be that big a deal but as you go up in speed it will manifest.

You could get away with using jb weld, just thin with acetone or alcohol wherever you need to for workability. In your case you could use any good strong epoxy for the fin roots and jb weld for the fillets.

Someone mentioned thicker fin material and I second that.

Not necessarily the only way nor the best way but it works for me.

I did some sims and I think I could get my Pro38 case in a rocket to go 11000'+ and M1.7. However, that is only 4 seconds above M1, so I don't think mach friction heating will amount to much at the fillet. Might at the fin LE. Try a thin strip of aluminum tape on the LE.

Did you complete this bird and fly it?

BTW I had a Tomach 54 that went to 21500' on an L640-DT. Peaked at Mach 1.8. I was very disappointed that there was no Mach rash, just a bit of a rub from the rail at launch . The paint was automotive two-pack.
Even the vinyl decals were largely unscathed.

FYI the build thread is here:
https://forum.ausrocketry.com/viewtopic.php?f=6&t=4758

Hmm - I responded to this but it's not showing

But no, I wanted to do a custom AV bay using a 3D printer. Unfortunately, I got burned through Kickstarter with the Tiko.

A friend recently got a printer and has offered to help me so I'll need to kick this back off

Building for altitude and Building for speed are two different challenges. A slower heavier rocket can fly higher than a faster lighter rocket because it has much more inertia. Also, it is a lot easier and far less expensive to build a 38mm to achieve that goal than to build a 54mm. The extra diameter of airframe will require K-level firepower instead of J. The parts and motor will cost a lot more money.

With a 38mm minimum diameter rocket you should be able to do both, but instead of a 6-grain CTI case, you will most likely need a 6XL. You need the extra firepower more for speed than for altitude. A minimum diameter 38mm should hit 10,000' relatively easily. My 54mm Scarab can hit 10,000' on a 38mm J530. It won't get anywhere near Mach 2, though. It is more of a challenge to break mach 2.

On the downside, with a J530 in a minimum diameter rocket, you are going to need a 20,000+ foot waiver. Otherwise you will need to design a rocket that is light, but has a lot of base drag so it can slow down easier. That means larger fins that are easier to shred along with significant ballast which is going to slow your down.

I am currently working on a 3-rocket project that will get me to 100,000' on M power by using a 2-stage design. I will fly the first of the three this summer to set a complex I record using 2 6-grain 29mm motors and a 2-stage minimum diameter design. I will hit mach 1.6 and 18,000.' The image below is of the sustainer for the next rocket which will be 38mm. By itself it should hit 20,000' and break Mach 2. It will be CF with tip-to-tip fins. The booster for this is 54mm and will go after a complex K altitude record. In the 2-stage configuration it will push Mach 2.9 and fly to 40,000'. I am showing just the sustainer to demonstrate that you should be able to do this, but you better plan for 20,000' instead of 10,000 with a 38mm minimum diameter rocket.



Giant Leap has a 38mm minimum diameter Mariah kit that should hit your goals. You might want to look at that rocket as well.

Mach+ birds need to be more stable even than sub-mach ones. You should design it to have at least 2 ca. of stability instead of one because as it moves past mach, the CP moves forward. While a mach shred is a glorious flight, it does cost a lot of money because you tend to need a plastic bag to recover the tiny pieces. Download RASAero software which is specifically designed for mach speeds.

You may want to buy the most 'value' up front .. the one that hits the goal and you get everything back.

My two highest altitude flights were in Wildman Rockets .. 11k in a Dark Star 3 K260 & Punisher 3 in K1440 went 15k and definetely busted MACH.

If i were to do over would start playing with a Fiberglass 54mm MD or the PunisheR. . Test flight to half mile and see the electronics work is never wasted either . Even with DMS motors , losing a rocket can be pricey when you factor in retainer , tracker, chutes and the rocket costs.

Kenny

One more thing... You haven't discussed electronics. It doesn't go Mach 2 or break 10,000' unless you have a record of it. You need an altimeter/accelerometer that is small enough to fit into your airframe and is capable of reading speeds past Mach 1. Your Stratologger or other barometric altimeter won't do. You are going to need a Raven 3 or similar that is accurate at mach speeds. And you are going to need to secure some components because this rocket will pull well past 50 G's. $150+ for that. And you will definitely need a tracker. GPS is preferred but a Beeline tracker should work. Another $100 and a HAM licence for that.

EeebeeE said:

One more thing... You haven't discussed electronics. It doesn't go Mach 2 or break 10,000' unless you have a record of it. You need an altimeter/accelerometer that is small enough to fit into your airframe and is capable of reading speeds past Mach 1. Your Stratologger or other barometric altimeter won't do. You are going to need a Raven 3 or similar that is accurate at mach speeds. And you are going to need to secure some components because this rocket will pull well past 50 G's. $150+ for that. And you will definitely need a tracker. GPS is preferred but a Beeline tracker should work. Another $100 and a HAM licence for that.

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I have an eggfinder and a Raven 2 that will be installed...

les said:

I have an eggfinder and a Raven 2 that will be installed...

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My mistake, I noticed that at the beginning of your thread after I posted this. If the Raven manual suggests epoxying down the capacitors above 50 G's, you will want to do that.

EeebeeE said:

My mistake, I noticed that at the beginning of your thread after I posted this. If the Raven manual suggests epoxying down the capacitors above 50 G's, you will want to do that.

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I asked Adrian about this specifically, and he said that suggestion was really aimed at landings on hard pack surfaces where it could see very high instantaneous Gs (like several hundred to 1000). It certainly won't hurt to do it, but it's not necessary for normal flight loads.

If the Raven manual suggests epoxying down the capacitors above 50 G's, you will want to do that.

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I tack the supercapacitors down with Loctite 401 and primer, regardless of the expected accel during flight. It is the ejection charges that create the spikes in acceleration usually.

To the original question

Here's my 38mm MD
Takes a J570 with easy to 11K to 12K on a nice day.
RRC3 with a poly lipo running it and a tracker in the NC( crazy if you don't )

And make sure the airframe has a vent hole or two.
Fins are made of carbon fiber honeycomb sheet , 1/8 thick, with fiberglass edges, tough stuff. and light....


yea yea I know it split fin, but know this it's max speed as recorded by the RRC3 was 1942mph (mach 2.5 ) at Orangeburg and mach 2.2 at Red Glare
and yes I had to replace the decals a few times.

dlb said:

To the original question

Here's my 38mm MD
Takes a J570 with easy to 11K to 12K on a nice day.
RRC3 with a poly lipo running it and a tracker in the NC( crazy if you don't )

And make sure the airframe has a vent hole or two.
Fins are made of carbon fiber honeycomb sheet , 1/8 thick, with fiberglass edges, tough stuff. and light....


yea yea I know it split fin, but know this it's max speed as recorded by the RRC3 was 1942mph (mach 2.5 ) at Orangeburg and mach 2.2 at Red Glare
and yes I had to replace the decals a few times.

Click to expand...

As mentioned, the baro RRC3 altimeter is not good for speed measurement due to resolution and mathematical derivation issues. What is the mass of your MD38? A sim of my Wildman Blackhawk 38 on a J570 is only around Mach 1.5. I trust the sim over a baro-derived speed calculation.

Buckeye said:

As mentioned, the baro RRC3 altimeter is not good for speed measurement due to resolution and mathematical derivation issues. What is the mass of your MD38? A sim of my Wildman Blackhawk 38 on a J570 is only around Mach 1.5. I trust the sim over a baro-derived speed calculation.

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I'm inclined to agree. The split fins cause a lot of turbulence (it's why they whistle) that tend to slow a rocket down. And it just doesn't generate enough firepower to shut the speed down. You cannot trust a baro altimeter past mach. Download the data and plot the curve. If the velocity doesn't show steady increases, but instead show spikes, then it didn't go that fast.

EeebeeE said:

One more thing... You haven't discussed electronics. It doesn't go Mach 2 or break 10,000' unless you have a record of it.

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The more I look at altimeter data, the less I am inclined to declare it the official "record" of the flight. Collecting data is fun, but baro and accel altimeters have their issues. A reasonable simulation is just as good or better in determining speed and altitude.

Buckeye said:

The more I look at altimeter data, the less I am inclined to declare it the official "record" of the flight. Collecting data is fun, but baro and accel altimeters have their issues. A reasonable simulation is just as good or better in determining speed and altitude.

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As long as you can accurately input drag. I think it takes a combination of things to actually come up with a "best guess" of what happened.

les said:

I can achieve it with 38mm, but should I go to 54mm?

Now - I have limited budget and I know the 38mm will be a lot cheaper - both for the rocket and the motor.

I already have a 6 grain 38mm CTI case, Egg finder, Raven 2, and a cable cutter to work with.
To go to 54 will require me to purchase a new case (more money) or use the AT DMS

Anything I have not considered or should be aware of before I make a final decision?

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One option you left out . . . Borrow 54mm hardware on the field, until your budget permits purchasing your own !

Dave F.

THe one benefit of going 54mm is that you can use a MARSA flight computer. Plus you can fly your rocket with an Aerotech L1000. Awesome motor.