38 vs 54? 2 miles + mach breaker

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les

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I'll start with the question, then a little history

I want to build a rocket that will go 2 miles (or at least 10K feet)
I also want it to break mach 1

There are several options for this with a minimum diameter rocket - both as kits and as scratch built.
I've simulated a few with Rocksim that will meet my criteria.

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

My thoughts are to mount the Egg Finder in the nose followed by the Raven. I am still trying to decide whether to do a "standard" DD or use the cable cutter.


Some history.
I do have my L2, although I tend to prefer LPR/MPR. I can do a lot more flights for less money using these over an HPR flight.
My record so far is 8330' and about 0.8mach on an L1000.
I also prefer SciFi, Fantasy, Oddroc over 3FNC.
And I like to see the entire flight. Hopefully the Egg Finder will help me with the recovery.

To achieve my goal I will need a 3FNC that will probably go out of sight -

My main reason for doing this is once or twice a year I go to events where I am asked to display some of my rockets.
So I am mostly doing this rocket for "bragging rights". Look - you can build rockets that go this high and this fast!

Assuming I get it back successfully, I doubt I will fly it again. And I don't think I will continue to push the envelope and go for 3 miles and/or break mach 2.

I also have not done any fiberglassing, so I want to be able to build stock and not have to fiberglass anything to hold it together (including tip to tip)
Which to me seems another good reason to stay with 38mm. The 54 may provide too much stress without reinforcement.

I recognize that with a 38mm minimum diameter things will be tight, but I should be able to fit everything.

I also hope to use the fly away guides to minimize drag during the flight.

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

Thanks in advance for feedback
 
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When in doubt, pick the bigger mount and build/buy an adapter if that's what you want to start with.
 
When in doubt, pick the bigger mount and build/buy an adapter if that's what you want to start with.

While I normally completely agree with this sentiment, his stated goal is 2 miles up. It will be tougher for a 38mm big J to push the bigger bird. It's by no means impossible, but by staying with a 38mm he has tons of choices.

I'm in the same boat - I haven't broken mach yet, nor 10,000 feet. I'm scratch building this to do it:

View attachment ThisCouldBeQuiteGood_v3.ork
 
While I normally completely agree with this sentiment, his stated goal is 2 miles up. It will be tougher for a 38mm big J to push the bigger bird. It's by no means impossible, but by staying with a 38mm he has tons of choices.

I'm in the same boat - I haven't broken mach yet, nor 10,000 feet. I'm scratch building this to do it:

View attachment 275645

Brainfart...was thinking non-MD. My bad.
 
Initial thoughts....

4 grain 54 in an MD
38mm 1200 in a 38 mount in a 54mm body
the CTI case you have in an MD


The easiest build that gets cleanly over that, will be the 54 MD.
Cheapest will be a 38MD
 
I also have not done any fiberglassing, so I want to be able to build stock and not have to fiberglass anything to hold it together (including tip to tip)
Which to me seems another good reason to stay with 38mm. The 54 may provide too much stress without reinforcement.

I'm about to stick a 54/2800 into a 54mm MD Madcow Tomach to take a crack at M2. I didn't do any tip to tip, and everyone I've talked to, and everything I've seen tells me this will be fine. Fins are 1/8, which is pretty beefy. You could go thicker if you wanted to be "super sure" and still clear M1 and 10K on a 4 grain 54
 
I would say go for 54mm, and a large one. If you are aiming at Mach and 2miles, then a 54mm L265 Longburn in most 3" airframes (non fiberglass) would do it, with ease, and you would show off a 10 second burn!!
You would probably hit 15K and Mach 1.2

That would be a memory for life.
 
If you're only going to fly it once don't buy a bunch of new gear. You have a case that should push a MD bird to your goal. Start by downloading a Rocksim file for a couple 38mm MD birds and see what combinations will work.

If you decide to go 54mm you'll probably only need a 3 grain motor. I just used a 4 grain K740 to push a 7lb 3" rocket over M1 and 10k'.
 
Les,,
I know cost per flight is very important to you. ..
I would like to see you fly this build and enjoy it...
If you build it 38 you can fly it every now and again
for less than if you build it 54...
This sounds like if you build it 54 it's going to spend it's days on a shelf..

Oh,, I have a lot of Aerotech and Loki hardware you're welcome to borrow,,
so build what is coolest to you instead of around a particular case....

Teddy
 
A well-built 38mm MD should have no trouble with the speed or altitude you want I'd think provided it can take the largest 38mm motors. My Rocketry Warehouse Go Devil 38 sims to 13k+ at >1100mph on a motor like the AT J510. Personally I'm afraid to push anything I've built past mach yet, so I've been looking more at long-burn J's to keep the speed lower and just go high, the CTI J150 sims to 11.6k at 740mph so that's more what I'm considering the limit for my kit right now, though my first goal is to break 1 mile with it. I made a shot at October Skies a few weekends ago on an I285 that sim'ed to ~5500 ft @ 540mph, but the fly-away rail guide rode the rocket to apogee which was reduced to 3821 ft (500mph) with the extra drag. So I'll take another stab at my next launch.
 
If you're only going to fly it once don't buy a bunch of new gear. You have a case that should push a MD bird to your goal. Start by downloading a Rocksim file for a couple 38mm MD birds and see what combinations will work.

+1 to this. Look at Wildman Blackhawk 38 (no FG needed) or PML Cirrus Dart (FG included) as a basis for your project that you can use with your existing gear.

I'll give the same advice I gave this guy in a similar thread.

https://www.rocketryforum.com/showt...high-Rocket-suggestions&p=1513480#post1513480
 
Les,

I had the same problem, I could not hit 10k. I tried with a limited budget to hit10k but failed. so I got a CTI K 1200 and case and put it in my 4in fiberglass Little JOhn(madcow) and pushed the button!

9629ft mach 1.3!

Dang it!!! so close!

On the salt flats in Utah for Hellfire 20 I finally made it to 10k!

Vertical Assault CTI K590 to 12,555 ft (personal record)
My L3 bird 30 pound pad weight Aerotech M 650 to 10,850 ft

Ill always remember those two days of Hellfire 20!

Goodluck
 
Downloaded the Rocksim file for the Wildman Blackhawk 38. Simmed it with a CTI J354 (using your existing 6 gr 38mm case) to 13k and M1.65 (for a launch in Utah at 4,400 ASL).


Note: I read the OP as you didn't want to apply fiberglass to the rocket. If FG rockets are not OK, then please disregard.

Note 2: With a Loki K1127, it sims to 18k and M2.28:wink:
 
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Les,

If your goal is to reach Mach 1 and 10K and make it a non-3FNC, you may NEED to go 54 mm. If you add "and cheap", you may not be able to do all 4. Most sci-fi, fantasy and odd-roc designs are very draggy. Many will require extra NC weight, which will hurt altitude some, and lower your top speed a lot. You may need to remove/reduce one of the four stated requirements.
 
Since either will work, I'd wait and see what pops up on the Black Friday sales.
 
Note: I read the OP as you didn't want to apply fiberglass to the rocket. If FG rockets are not OK, then please disregard.

I have no issue building a FG rocket, I have built several. I just have not applied FG cloth, resin, etc to strengthen a rocket.

Les,

If your goal is to reach Mach 1 and 10K and make it a non-3FNC, you may NEED to go 54 mm. If you add "and cheap", you may not be able to do all 4. Most sci-fi, fantasy and odd-roc designs are very draggy. Many will require extra NC weight, which will hurt altitude some, and lower your top speed a lot. You may need to remove/reduce one of the four stated requirements.

I prefer SciFi, etc, but I recognize I will have to do a 3FNC for this attempt.

I did a rough sim of the 1.6" Tomach from Madcow on the J354. Sims to 12600' and 1500 ft/sec
 
I have no issue building a FG rocket, I have built several. I just have not applied FG cloth, resin, etc to strengthen a rocket.



I prefer SciFi, etc, but I recognize I will have to do a 3FNC for this attempt.

I did a rough sim of the 1.6" Tomach from Madcow on the J354. Sims to 12600' and 1500 ft/sec

Because sims aren't usually very accurate until you have a few flights and can dial in the parameters, I would take that one at 11,000 and 1350 ft/sec. It still meets your goal.

I'm not sure I would recommend a fiberglass rocket if you want to stay with smaller motors. They are heavy and take more motor to get the same flight. Since you don't plan on flying this much, long term durability isn't as much of an issue. I would go with LOC tubing, 1/4" (6mm) thick aircraft ply fins in a delta shape with leading and trailing edges tapered about 3/4" to 1". You might want to do a tip2tip fiberglass. You say you haven't done that before, this might be the perfect time to learn.

BTW, since you have very specific goals for this rocket, I would highly recommend you scratch build exactly what you want/need.
 
The scaling ratio is the rocket cross-section squared [(2.3/1.6)^2 ~ 2.07] which represents the ballistic similarity factor describing both drag and mass values for the 2 similar rocket with different MDs.

A well-designed 38 mm minimum diameter rocket (1.6" actual OD) weighing 2 pounds (w/o motor) launched with a >=750 Ns J (17% J) motor will exceed both Mach 1 and 2 miles. (Most Pro38-6G reloads work. Cost ~$63 for Pro38-6G reload, Pro38 starter kit ~$110)

A well-designed 54 mm minimum diameter (2.3" actual OD) rocket weighing 4 pounds (w/o motor) launched with a >=1550 Ns K (21% K) motor will exceed both Mach 1 and 2 miles. (Most Pro54-4G reloads work. Cost ~$113 for Pro54-4G reload, Pro54 starter kit ~$220)

Results based on Thrustcurve.org motor guide using a Cd = 0.6. If the Cd is lower, the total impulse required to obtain 2 miles will be lower. The cost of the 4G 54 mm motor and reload will be about double that of a 6G 38 mm motor and reload.

Bob
 
A bird in hand is worth two in the bush so to speak, I'd go with the 38 mm unless you have future plans for the 54 mm hardware in which case it wouldn't hurt to acquire it now. See how this hobby can really suck you in!
 
Since MD rockets and going higher is what I like to do, I figured I might as well share my two cents.

I recently built a 38mm rocket that fits the CTI 38mm 5 grain case. The rocket is designed to break at least 13k (hoping for 15), and weighs in at just 9.5oz.

If I were you, I would use your 6 grain case, stick it into a FWFG airframe, use 3/32" G10 fins, stick all your electronics in your nose cone, and use the cable cutter. That whole configuration should stay under 1.5 pounds. And like Bob said, as long as your vehicle stays under 2 pounds w/o motor, reaching your goal should be very, very easy.
 
Quick sim on a Space Ark with a large J puts her at Mach 1.5ish and 7.3K ish feet. It's got the SF theme, goes fast enough, and gets you a mile and a half in w/o glass. I'm pretty sure you could get even closer --maybe even find that extra 2,500 feet-- with a thinner, less draggy model.

Maybe even downscale that to a 2.6" airframe, keep the 38mm MMT...


Later!

--Coop
 
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 :)

tomach rocksim.jpg
 
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No shear pins on the nose cone? This could end up in a nasty drag separation. If your electronics are going to be in the nose cone it will make the NC pretty heavy this will make a drag separation a more likely thing.

My thoughts are put some shear pins on the NC better safe then sorry I would rather have a little more drag than a shredded rocket.

I would love to see this build! Good luck! 😄
 
No shear pins on the nose cone? This could end up in a nasty drag separation. If your electronics are going to be in the nose cone it will make the NC pretty heavy this will make a drag separation a more likely thing.

My thoughts are put some shear pins on the NC better safe then sorry I would rather have a little more drag than a shredded rocket.

I would love to see this build! Good luck! 😄

Friction for the nose. Make it tight, but not to tight. Use an energetic enough charge to pop it off.

My 38mm MD uses a friction fit nose and a 0.8g charge to dump the laundry.
 
Friction for the nose. Make it tight, but not to tight. Use an energetic enough charge to pop it off.

My 38mm MD uses a friction fit nose and a 0.8g charge to dump the laundry.

I guess it's personal preference. Hold the rocket buy the nose and shake it to make sure! When you get it do a build thread I love MD and Mach breakers currently I'm building a 54mm for my first Mach breaker. 😜
 
Personal preference for sure, I use shear pins for both the nose and the middle. Seen too many drag separations and the resultant carnage.
 
Personal preference for sure, I use shear pins for both the nose and the middle. Seen too many drag separations and the resultant carnage.

Same!

I like the consistency of shear pins😄
 
+1 for shear pins, I tried friction fitting NCs many times in my early DD days and had many failures.
 
+1 for shear pins, I tried friction fitting NCs many times in my early DD days and had many failures.

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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