Athena 3

Discussions for scratch builders. For the enthusiast that builds from the ground up. Tell us what you've built and what projects you working on in the space race.

Moderator: Moderators

User avatar
SpaceManMat
Astronaut
Astronaut
Posts: 2065
Joined: Mon Dec 15, 2008 9:56 pm
Location: Brisbane

Re: Athena 3

Postby SpaceManMat » Fri Dec 04, 2015 5:44 pm

Think your spot on there Viking. The process is called an aerodynamic whistle. So question is what needs to change to alter the frequency of the vortex? I'm thinking speed, but perhaps fin thickness too.
QRS: 124
AMRS: 32 L2 RSO
Highest Altitude: 13,647 feet
Fastest Flight: Mach 1.55
Largest Motor: CTI 1115J530 IM
Current Project: X Wing

User avatar
Happy Heyoka
Rocket Flyer
Rocket Flyer
Posts: 147
Joined: Wed Jul 22, 2015 7:01 pm

Re: Athena 3

Postby Happy Heyoka » Wed Mar 16, 2016 2:04 pm

Viking wrote:My theory(that means I'm totally guessing!) is that the trailing edge of the fore-fin sets up a VK Vortex trail(flutter), the gap length allows the vortex room to swing as in the animation on Wikipedia before the aft-fin separates the vortices so that each alternating one passes on opposite sides of the aft-fin creating pressure differentials on the faces of the fin which sets up a resonance in the fin which acts as a sounding board.


As a theory, I like it. Getting to the bottom of "the whistle" (for my own education, mostly) is a pet project that has been back-burnered for a while now.
(spend many hours reading about "bull roarers" and various flavours of "reed aerophones")

I like the wind tunnel idea but it would, as I understand it, have to be a pretty honkin wind tunnel to 'scale' the model accurately (maybe water?)... maybe someone with friends in high places? Oh, and there is of course the idea that maybe the change in velocity has something to do with setting up the whistle? Can you even simulate rocket stuff like that in a wind tunnel? (ie: not "steady state"??)

So, IMHO, empirical stuff to try would be various 'vortex reattachment devices' to try and make the flow laminar(ish) across the split and see if it kills the whistle (which I reckon it should) - thinking strakes along the fins or winglet/wincap gadgets on the outer edge.
Maybe a fin can with adjustable gap/replaceable fins... oh, and a whole box of the same motors for testing...

(oh, and insert obligatory note to self "yes, well why don't you just get off your arse and try it yourself" here :roll: )
Ha ha ha ha ha! You can't fool me! There ain't no Sanity Clause!
Chico Marx - A Night at the Opera (1935)

Lister
Engineer
Engineer
Posts: 967
Joined: Fri Jun 05, 2015 5:36 pm
Location: Bendigo, Victoria

Re: Athena 3

Postby Lister » Wed Mar 16, 2016 3:55 pm

Happy Heyoka wrote:I like the wind tunnel idea but it would, as I understand it, have to be a pretty honkin wind tunnel to 'scale' the model accurately (maybe water?)... maybe someone with friends in high places?

A mate in the u.s has a wind tunnel but its not big enough to fit the athena in.. how big do you think it would need to be to get useful data?

High Impulse
Rocket Crew
Rocket Crew
Posts: 281
Joined: Wed Dec 17, 2014 6:06 pm

Re: Athena 3

Postby High Impulse » Wed Mar 16, 2016 5:13 pm

You could just scale the athena down and make a model out of clay or by 3D scanning and prinitng and adjust the reynolds number accordinlgy.
L1: Callisto - H410VM

User avatar
Happy Heyoka
Rocket Flyer
Rocket Flyer
Posts: 147
Joined: Wed Jul 22, 2015 7:01 pm

Re: Athena 3

Postby Happy Heyoka » Wed Mar 16, 2016 7:35 pm

Lister wrote:A mate in the u.s has a wind tunnel but its not big enough to fit the athena in.. how big do you think it would need to be to get useful data?


at full scale it would have to be pretty fast I think... (as in, fast flow, not big tunnel)... for example, you can't just hang it out a car window - even 90 m/s (pretty modest for a rocket) is 324 km/h (I might admit to maybe half that in a car in my youth). I did think about a rig to spin a set of fins which is also a scary concept (welding some kind of safety containment etc... it'd be a project)

High Impulse wrote:You could just scale the athena down and make a model out of clay or by 3D scanning and prinitng and adjust the reynolds number accordinlgy.


(keeping in mind I have no formal training in fluid dynamics...)

it's my understanding that this is particularly problematic - fine details don't necessarily scale even if you adjust flow rate? I get this idea from the number of papers I've read that are trying to understand why UAV designs are problematic when scaled to "micro" vehicles. Feel free to point me at appropriate reading material if I'm off on the wrong track here...

the other thing that just popped into my head (re-reading Viking's comment) is what about dynamic effects of the fin _actually_ fluttering - it might seem stiff but >300km/h it could actually be operating as a reed).

Anyway, it occurs to me the more I write about this the dumber I feel :?

Maybe we should just start a fund to buy Lister 25 or 50 identical motors, a tiny smoke generator and a high speed camera and tell him to get back to us with an answer... 8)
Ha ha ha ha ha! You can't fool me! There ain't no Sanity Clause!
Chico Marx - A Night at the Opera (1935)

Lister
Engineer
Engineer
Posts: 967
Joined: Fri Jun 05, 2015 5:36 pm
Location: Bendigo, Victoria

Re: Athena 3

Postby Lister » Wed Mar 16, 2016 8:08 pm

From the way I understand the whistle it is mainly due to resonance. If I can just use the fin section (even just a fore and aft fin mounted in a section of airframe) to do the wind tunnel test on then i can make sure it will fit and send it over to the states and get Dave to test it.. His wind tunnel is a commercial one so it should be able to reach the required air speeds and I can get some actual data on the subject.. from the launches I have done with the athena series I havent seen any speed in which they dont whistle except for ultra low speeds. I might try hanging the fin section out the car window at 100kmph and see what it does.

is anyone else going to build an Athena/Thor? I have a spare airframe and fins here for a thin wall fibreglass Athena.2" with a 38mm MMT if anyone wants one.

User avatar
OverTheTop
It's only money...
It's only money...
Posts: 2623
Joined: Sat Jul 21, 2007 2:20 pm
Location: Melbourne, Australia

Re: Athena 3

Postby OverTheTop » Wed Mar 16, 2016 8:16 pm

Reynold's number is one of the "similarity coeffecients" that can be used to compare the aerodynamics of different sized objects. It is basically a relation between the viscous effects and the inertial (Newtonian) effects. It can be used to determine if two different sized objects have similar aerodynamics. In the case of getting a faster flow, you can use a slower flow but at a higher absolute pressure to get a similar Reynolds number.

I was looking for a nice simple explanation but have not been able to find a nice canned definition. Here is the wiki entry:
https://en.wikipedia.org/wiki/Reynolds_number

I would recommend a great book on aerodynamics if you are keen. Check out this thread here for the recommendation:
viewtopic.php?f=52&t=5432
TRA #13430
L3
"Everybody's simulation model is guilty until proven innocent" (Thomas H. Lawrence 1994)

RedPhenix
Parachute Packer
Parachute Packer
Posts: 44
Joined: Mon Mar 23, 2015 11:00 am

Re: Athena 3

Postby RedPhenix » Wed Mar 16, 2016 8:42 pm

Hi guys

Aluminite dye - the reddest of reds. About $6 on eBay.

Cheers

N
TRA L3 #14659, AMRS L3 # 183

User avatar
Digit
Rocket Crew
Rocket Crew
Posts: 411
Joined: Sun Feb 21, 2016 1:23 pm
Location: Ipswich, QLD

Re: Athena 3

Postby Digit » Wed Mar 16, 2016 8:50 pm

I think i got to hear the above mentioned whistle first hand at our club launch just been. First two launches of the day were HPR and straight after motor burnout you could hear a cool sound similar to a turbo charger winding down. Is that what you are talking about?
QRS:169
AMRS:108 (L2)

QRS Committee Member

Lister
Engineer
Engineer
Posts: 967
Joined: Fri Jun 05, 2015 5:36 pm
Location: Bendigo, Victoria

Re: Athena 3

Postby Lister » Wed Mar 16, 2016 9:28 pm

Thanks mate. I'll check it out.

Sort of, the athena's I have sound like the hollywood bomb falling sound, alot of rockets whistle for different reasons.. it was at my first launch I heard a rocket whistle and wanted to make one that would reliably do it.. the athena seemed like the best off the shelf candidate, I have been hooked on whistling rockets ever since, all 3 sizes I have all make a slightly different whistle.. seems to be the thicker fins give a louder whistle.. but all the fins are ply wood laminated with FG which apparently helps to produce a good whistle. I will be trying all fibreglass fins in the wind tunnel tests along side the wood and FG.

User avatar
Digit
Rocket Crew
Rocket Crew
Posts: 411
Joined: Sun Feb 21, 2016 1:23 pm
Location: Ipswich, QLD

Re: Athena 3

Postby Digit » Wed Mar 16, 2016 9:59 pm

Yea that's the sound I'm trying to describe you gave a better reference. You could always pillage the whistles from a Vortex football (they sell them at target etc) and mount them in your airframe lol.

QRS:169
AMRS:108 (L2)

QRS Committee Member

RedPhenix
Parachute Packer
Parachute Packer
Posts: 44
Joined: Mon Mar 23, 2015 11:00 am

Re: Athena 3

Postby RedPhenix » Thu Mar 17, 2016 8:54 pm

Hey, nice job on this rocket Luke. This will sound woozy, but a h or an I will allow us to the whistle more.

Noel
TRA L3 #14659, AMRS L3 # 183

RedPhenix
Parachute Packer
Parachute Packer
Posts: 44
Joined: Mon Mar 23, 2015 11:00 am

Re: Athena 3

Postby RedPhenix » Thu Mar 17, 2016 8:55 pm

Hey, nice job on this rocket Luke. This will sound wussy, but a h or an I will allow us to the whistle more.

Noel
TRA L3 #14659, AMRS L3 # 183

Lister
Engineer
Engineer
Posts: 967
Joined: Fri Jun 05, 2015 5:36 pm
Location: Bendigo, Victoria

Re: Athena 3

Postby Lister » Thu Mar 17, 2016 9:51 pm

Couldnt agree more mate :) This has a 54mm MMT soni can fly it on some bigger stuff... same with the athena 4 which I will use for my L2 and should give us a nice whistle.. might try and rack all the athena's at once and play us a tune :)

I did look into the vortex whistle but im not sure how good it would sound at high speed.. would be worth putting one on a rocket for shits n giggles but the athena whistle is a nice even pitch and has a sound that would make my pommy grandparents duck for cover :P

Lister
Engineer
Engineer
Posts: 967
Joined: Fri Jun 05, 2015 5:36 pm
Location: Bendigo, Victoria

Re: Athena 3

Postby Lister » Tue Apr 05, 2016 12:19 pm

I have changed the motor im using for my L2, instead of an L265 im using a K650PI motor so I can hear the athena's whistle better. The scratch built 3" athena I have has a few things im not 100% happy with, the airframe is a bit smaller than a standard 3" so I cant interchange things between my rockets, the fins are only made from 4mm plywood laminated with 290gsm, which isnt quite as strong as I would like for the more powerful motor. And the nose cone on it is a plastic off the shelf nose.. I want my L2 to be all scratch built.

so I decided to just remake the athena 3 the way I want it.

I had a red fin panel that needed a few more layers to get it strong enough to use for fins.. Noel was good enough to give me a bunch of CF off cuts he had from some of his projects so I found some bits big enough to cover the fins. I covered each sode of the fins with a layer of 200gsm CF and the strength it gave them is phenomenal. I also post cured them over night and have been left with very stiff yet lightweight fins.. they dont look the best as it was done with off cuts but they will be painted anyway.
2016-04-04 13.24.55.jpg
2016-04-04 13.24.55.jpg (110.52 KiB) Viewed 1197 times

2016-04-05 11.54.22.jpg
2016-04-05 11.54.22.jpg (169.32 KiB) Viewed 1197 times


I made a 3" nose cone a while back that I will finish off and use for the athena.. it was made on a wooden mandrel that I turned on the lathe. Then it was coated in epoxy and sanded smooth, coated in furniture wax which I use as mould release and the FG was laid up over that.. then I put it on the lathe and sanded the FG so the surface was even and then applied a gel coat to give it a nice surface to paint and made a wooden tip as I couldnt find aluminium cheap enough to be able to use for the tip.

I borrowed a full length blue tube coupler to roll the airframes on, have one tube rolled, still need another tube and found a length of 54mm airframe that I had laying around and will use that for the MMT on it.
2016-04-05 11.55.27.jpg
2016-04-05 11.55.27.jpg (109.7 KiB) Viewed 1197 times


I have a 3" x 10" coupler I made for the other athena, once I add another layer to make it fit inside the 3" tube I will get started on making the AV bay for it.

Once I have done my L2 with it I plan on using a 3"-2" transition I made as an inter stage coupler and having an athena 2" as a sustainer and seeing how high I can push it.


Return to “Scratch Built”

Who is online

Users browsing this forum: No registered users and 3 guests