APE - Minimum diameter Loki 54mm M project

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ChrisAttebery

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Hi guys,

I've started working on a 54mm minimum diameter rocket for the Loki M1378. The rocket will use a machined fin can attached directly to the motor and a subminimum diameter parachute bay/nose cone. The nose cone will slip over a machined coupler attached to the forward bulkhead on the motor.

The fin can design is complete. Since I don't have a lathe anymore I have a shop lined up to turn the fin can stock. Those parts should be ready in a couple weeks. Once they are done I'll start machining the fin can(s) on my CNC mill.

54mm Fin Can 3 Fin v17.png
 
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markkoelsch

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So the whole rocket minus fin can will be 54mm. What is the projected weight? Altitude?
 

djs

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Is this a new L3 project? I salute you sir! :)
 

mpitfield

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Is this a new L3 project? I salute you sir! :)
I would be interested as well, as I recall someone saying that a rocket where there is essentially no airframe, does not qualify as an L3 rocket. But I could easily be getting my conversations mixed up. Regardless this sounds like it will be a very cool flight, a bit like Coleman AKA Rockethunter style.
 

ChrisAttebery

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That's the idea. The projected weight is 11 lbs. The projected altitude is ~35,000'. OpenRocket isn't as happy with the design as RASaero II is. OR claims that the fins need to be 2.75" tall to be effective. RAS simulates fine at 2.25" fin height.

So the whole rocket minus fin can will be 54mm. What is the projected weight? Altitude?
 

ChrisAttebery

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I think the rules state that you can't buy a metal fin can and slap it on a motor. You can design and machine your own though.

I would be interested as well, as I recall someone saying that a rocket where there is essentially no airframe, does not qualify as an L3 rocket. But I could easily be getting my conversations mixed up. Regardless this sounds like it will be a very cool flight, a bit like Coleman AKA Rockethunter style.
 

crossfire

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What is your plans for fin attachment to motor? Sounds like a very fun project. Keep us posted.
 

ChrisAttebery

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Each fin will be held in place with 5x 4-40 countersunk screws. The fin can will abut the motor's thrust ring. I'll probably just use high temp silicone sealant to keep it from sliding forward on the motor.
 

RocketHunter

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Hi guys,

I've started working on a 54mm minimum diameter rocket for the Loki M1378. The rocket will use a machined fin can attached directly to the motor and a subminimum diameter parachute bay/nose cone. The nose cone will slip over a machined coupler attached to the forward bulkhead on the motor.

The fin can design is complete. Since I don't have a lathe anymore I have a shop lined up to turn the fin can stock. Those parts should be ready in a couple weeks. Once they are done I'll start machining the fin can(s) on my CNC mill.

View attachment 304878
Pretty fincan! Would be interested to see how you are going about attaching your upper airframe - I am also currently working on a fully sub-minimum diameter design, though with bonded on composite fins:

Stratos_Pic.jpg

Haven't decided between the M1378 and L2050 - I'm quite partial to the high thrust motors and absurd accelerations, but the M1378 gives a 15-20% increase in altitude which is equally tempting...
 

djs

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Haven't decided between the M1378 and L2050 - I'm quite partial to the high thrust motors and absurd accelerations, but the M1378 gives a 15-20% increase in altitude which is equally tempting...
The answer to this question is to make it a two stager and fly both!
 

ChrisAttebery

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Thank you. I should have an airframe adapter design to show soon.

That looks like fun. Your fins don't look very tall. Have you simmed it?

I'm considering ordering the L and M and then flying them both the same weekend.


Pretty fincan! Would be interested to see how you are going about attaching your upper airframe - I am also currently working on a fully sub-minimum diameter design, though with bonded on composite fins:

pic snipped

Haven't decided between the M1378 and L2050 - I'm quite partial to the high thrust motors and absurd accelerations, but the M1378 gives a 15-20% increase in altitude which is equally tempting...
 

RocketHunter

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Thank you. I should have an airframe adapter design to show soon.

That looks like fun. Your fins don't look very tall. Have you simmed it?

I'm considering ordering the L and M and then flying them both the same weekend.

Now that would be exciting! Would be really informative to get a comparison in altitude between the two motors on the same rocket in the same weekend.

Yeah they're 4 fins at 1.5" tall each. Stability is accomplished with over a pound of noseweight. Minimalist composite designs are well under optimal weight, so to maximize altitude I add all the noseweight I can, then re-omptimize fin size to maintain around ~1.25 cal stability at all times. This allows for very very short fins, which are both lighter (good for stability), shorter for less drag, and much thinner (0.09") which also decreases drag.

Optimal fin root is found by plotting stability vs time for the flight. Typically the stability is lowest off of the rail and then at max speed. For a given span, short root fins seem to do best at low speeds but have large CP shifts at high mach. Longer root fins do badly at low speeds off of the rail but have less CP shift at high speed. The optimal length was when the initial stability off the rail is the same as the minimum stability at peak speed. From there, with changes in span you can choose a stability safety margin (1 cal min, 1.5 cal, etc.).

I've flown two similiar MD's (29mm & 38mm) with 4 small fins with spans of ~75% the body OD, both designed similarly, and which flew dead straight. As far as I have found, OR is the most accurate for simulating stability. The new rasaero seems to be close, but OR is also easier to enter in components to get an idea of CG and weight, so I've stuck to using OR for stability and design, and then rasaero for accurate altitude predictions.

Sorry for the long, slightly OT post lol.
 
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Aksrockets

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Rockethunter, you're rolling the dice by having spans under 1 cal. From personal experience, thats really when OR has problems with CP shift. I've seen flights that were supposedly .5-.75 cal stable at max V, go unstable. Only real common factor was the fin span. I've heard of other's similar experiences too.
If you're confident in your sims, go for it. Just reporting my experience.

Chris, I'm curious to see how you're going to do your fin can on your CNC. Do you have a 4th axis?
I just barely did a fincan similar on a manual mill, and I would have killed to have a little automation in there.

Also, do you want me to turn the fin can blanks? All the machining is on me. I still kind of owe you for blowing up that tower a few Aeronauts ago.

Alex
 

ksaves2

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Chris, If considering to use this for a flaming great L3 attempt, might I suggest you get the TAPS or "whomever" to
sign off on your fincan first. Someone might pimp you for having a shop do "some" of the work. If you can
get them onboard, you'd be good to go. On the other hand, just use another rocket and then fly this one
when the cert is "in the can". Also, I have heard some TAPs insist on seeing a rocket descending under the main
chute at sometime during the descent. They won't allow your electronic download to substantiate a safe touchdown and it must "land" within the waiver radius. Make sure your witnesses are "cool" with your recovery under
their right of "discretion" with a sight unseen flight. Since I see you are out west and have wide open spaces available, I would assume you'll be able to fly at a site with an abundant waiver radius. Looks like a fun project. Kurt Savegnago
 

ChrisAttebery

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Good info. I'll dig into it and reply tomorrow.

Now that would be exciting! Would be really informative to get a comparison in altitude between the two motors on the same rocket in the same weekend.

Yeah they're 4 fins at 1.5" tall each. Stability is accomplished with over a pound of noseweight. Minimalist composite designs are well under optimal weight, so to maximize altitude I add all the noseweight I can, then re-omptimize fin size to maintain around ~1.25 cal stability at all times. This allows for very very short fins, which are both lighter (good for stability), shorter for less drag, and much thinner (0.09") which also decreases drag.

Optimal fin root is found by plotting stability vs time for the flight. Typically the stability is lowest off of the rail and then at max speed. For a given span, short root fins seem to do best at low speeds but have large CP shifts at high mach. Longer root fins do badly at low speeds off of the rail but have less CP shift at high speed. The optimal length was when the initial stability off the rail is the same as the minimum stability at peak speed. From there, with changes in span you can choose a stability safety margin (1 cal min, 1.5 cal, etc.).

I've flown two similiar MD's (29mm & 38mm) with 4 small fins with spans of ~75% the body OD, both designed similarly, and which flew dead straight. As far as I have found, OR is the most accurate for simulating stability. The new rasaero seems to be close, but OR is also easier to enter in components to get an idea of CG and weight, so I've stuck to using OR for stability and design, and then rasaero for accurate altitude predictions.

Sorry for the long, slightly OT post lol.
 

ChrisAttebery

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I've designed a fixture to hold the fin can that will allow me to rotate it 30 degrees at a time. First I'll machine the areas between the fins with a ball end mill. Then I'll rotate the fins perpendicular to the spindle, drill and countersink the screw holes. Then once all of the holes are drilled I'll cut the slots for the fins.

You don't owe me anything, but if you're up for some turning I would appreciate any help you could give me. I'll send you a PM.


Chris, I'm curious to see how you're going to do your fin can on your CNC. Do you have a 4th axis?
I just barely did a fincan similar on a manual mill, and I would have killed to have a little automation in there.

Also, do you want me to turn the fin can blanks? All the machining is on me. I still kind of owe you for blowing up that tower a few Aeronauts ago.

Alex
 

ChrisAttebery

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I'm hoping that the booster for my Punisher will show up by next spring. If so I'll try for L3 again with the Punisher.

This project is just something that I've wanted to do. I'd rather not tie it to a cert attempt however if I do need to use it as a cert attempt I'll run it by my TAPs.



Chris, If considering to use this for a flaming great L3 attempt, might I suggest you get the TAPS or "whomever" to
sign off on your fincan first. Someone might pimp you for having a shop do "some" of the work. If you can
get them onboard, you'd be good to go. On the other hand, just use another rocket and then fly this one
when the cert is "in the can". Also, I have heard some TAPs insist on seeing a rocket descending under the main
chute at sometime during the descent. They won't allow your electronic download to substantiate a safe touchdown and it must "land" within the waiver radius. Make sure your witnesses are "cool" with your recovery under
their right of "discretion" with a sight unseen flight. Since I see you are out west and have wide open spaces available, I would assume you'll be able to fly at a site with an abundant waiver radius. Looks like a fun project. Kurt Savegnago
 

RocketHunter

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Rockethunter, you're rolling the dice by having spans under 1 cal. From personal experience, thats really when OR has problems with CP shift. I've seen flights that were supposedly .5-.75 cal stable at max V, go unstable. Only real common factor was the fin span. I've heard of other's similar experiences too.
If you're confident in your sims, go for it. Just reporting my experience.

Chris, I'm curious to see how you're going to do your fin can on your CNC. Do you have a 4th axis?
I just barely did a fincan similar on a manual mill, and I would have killed to have a little automation in there.

Also, do you want me to turn the fin can blanks? All the machining is on me. I still kind of owe you for blowing up that tower a few Aeronauts ago.

Alex
Not to get too off topic, but I feel the OR vs. RASaero stability argument is relevant to the construction of this rocket/rockets like it, so i'll continue the discussion of that. Taking mine as an example, here are the OR and RASaero CG and CP graphs:

Stratos_OR_Stability.jpgIMG_8707.jpg

For RASaero II, the stability is constant in the subsonic region around 2.5 cal, and then slowly decreases by ~ 1 caliber at mach 3, though because of the motors CG shift at peak speed the overall stability is ~5 calibers!

For OR, stability is initially low (just over a caliber) as it takes off at low speed, increases dramatically by quite a few calibers up through the transonic region then dramatically decreases at high mach as the CP moves forward, back down to around 1 caliber.

Its pretty amazing to me how wildly different these predictions are. I personally prefer OR as it seems to take into account things like the lack of effectiveness of the fins at low speed (you can shorten the length of the launch rail, and see the initial stability drop, sometimes to near zero for an instant which is usually a good predictor of weathercocking), rather than simply using a constant subsonic CP like RAS seems to do.

RASaero II still seems like it over estimates stability - there's so little CP shift at high mach, and 5 calibers of stability at mach 3 with a rocket with 4 fins 70% the diameter of the rocket seems a a bit to good to be true. IDK YMMV.

My biggest takeaway is that it seems to be a very hard thing to predict, and that simulations are best supplemented with empirical data and testing. What would be great would be someplace where people could post their design files with stability predictions, and then post the results of the flight (unstable, weather cocking, arrow straight, coning, etc.). Alex, if people posted actual data for the flights you describe where they have run into stability problems, future flyers might be able to use that info to makeup for the faults in the software that may have lead to those failures in the first place.
 

Andrej

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Hi guys,

I've started working on a 54mm minimum diameter rocket for the Loki M1378. The rocket will use a machined fin can attached directly to the motor and a subminimum diameter parachute bay/nose cone. The nose cone will slip over a machined coupler attached to the forward bulkhead on the motor.

The fin can design is complete. Since I don't have a lathe anymore I have a shop lined up to turn the fin can stock. Those parts should be ready in a couple weeks. Once they are done I'll start machining the fin can(s) on my CNC mill.

View attachment 304878
Chris you might consider ditching the fin can and laser welding the fins straight to the motor casing. That way your OD won't increase resulting in lower cross section and lower drag. That's what I did on one of my rockets (photo attached).

Laser welded fins.jpg
 

Buckeye

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Not to get too off topic, but I feel the OR vs. RASaero stability argument is relevant to the construction of this rocket/rockets like it, so i'll continue the discussion of that. Taking mine as an example, here are the OR and RASaero CG and CP graphs:

View attachment 304944View attachment 304945

For RASaero II, the stability is constant in the subsonic region around 2.5 cal, and then slowly decreases by ~ 1 caliber at mach 3, though because of the motors CG shift at peak speed the overall stability is ~5 calibers!

For OR, stability is initially low (just over a caliber) as it takes off at low speed, increases dramatically by quite a few calibers up through the transonic region then dramatically decreases at high mach as the CP moves forward, back down to around 1 caliber.

Its pretty amazing to me how wildly different these predictions are.
Wow. Agreed.

RASaero II still seems like it over estimates stability - there's so little CP shift at high mach, and 5 calibers of stability at mach 3 with a rocket with 4 fins 70% the diameter of the rocket seems a a bit to good to be true. IDK YMMV.
Perhaps this is why RAII complains about anything less than 2 calibers stability anytime during the flight. It is adding a huge safety factor on top of a liberal result???

My biggest takeaway is that it seems to be a very hard thing to predict, and that simulations are best supplemented with empirical data and testing. What would be great would be someplace where people could post their design files with stability predictions, and then post the results of the flight (unstable, weather cocking, arrow straight, coning, etc.).
Agreed, this is tough. I am starting to do this for my designs, and I may post about it soon. However, my flight results are limited and perhaps merely anecdotal.
 

watheyak

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Exciting project! I'm watching with interest.

There was a lot of discussion of high mach flights and stability with the N5800 and Carvac & co. From those discussions came a trend towards four fins with a semi-span at least equal to one caliber. The simple answer was that three fins lead to coning as the CP shifted forward. Someone else can explain it better than I, for sure.

I'd highly recommend going back and reading about that flight, paying attention to the discussion after it's failed flight. Some good stuff there. The name of the project is on the tip of my tongue...
 
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eggplant

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Exciting project! I'm watching with interest.

There was a lot of discussion of high mach flights and stability with the N5800 and Carvac & co. From those discussions came a trend towards four fins with a semi-span at least equal to one caliber. The simple answer was that three fins lead to coning as the CP shifted forward. Someone else can explain it better than I, for sure.

I'd highly recommend going back and reading about that flight, paying attention to the discussion after it's failed flight. Some good stuff there. The name of the project is on the tip of my tongue...
I believe it was called "Bare Necessities".
 

ChrisAttebery

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I've been playing with sims over the last couple days. One thing that has been pretty consistent is that RAS's altitude is 25-35% higher that OpenRocket. It is also more optimistic about the CP location. I found that the taper of the leading edge of the fin can made a huge difference in the altitude in the sims. They are both claiming over 40k' now. I think I'll probably switch to 4 fins, 8" root, 2" tip, 2.25" height. It's about 8oz heavier than optimum, but there really isn't anywhere I can shave weight at this point. I could go to a CF fin can, but then I've got to worry about heat damage. I like the idea of being able to replace a fin and fly again the next day.

54mm M.jpg

54mm M OpenRocket Stability.jpg

54mm M RASaero II Stability.jpg
 
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watheyak

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TRAPHX, or Tripoli Phoenix always has a 50k waiver, and plenty of enabling altitude junkies to egg you on. Kinda far from Gilroy, but an option.
 

CoyoteNumber2

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True - can you fly Loki motors at FAR (not California 'CSFM classified') ?
Hmm, that I'm not sure about. You can fly anything you make yourself, which naturally is not CSFM-approved, so I wouldn't guess it would be a problem. I'll ask next time I go. Do you fly at FAR?
 
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