You've "nailed" it! 
Lakeroadster's "Hammerhead Shark"
- Thread starter lakeroadster
- Start date
Help Support The Rocketry Forum:
- Joined
- Mar 27, 2013
- Messages
- 22,537
- Reaction score
- 14,953
NICE!!!
Did you know you could kinda sim the shape of the nose's hammer in OR? (at least cosmetically) Fin width is the "span" of the fin, shape is half of the cross section. The fin is attached to a PBT of 0 length and diameter, offset by the length of the fin root.
Did you know you could kinda sim the shape of the nose's hammer in OR? (at least cosmetically) Fin width is the "span" of the fin, shape is half of the cross section. The fin is attached to a PBT of 0 length and diameter, offset by the length of the fin root.
Attachments
Last edited:
lakeroadster
When in doubt... build hell-for-stout!
Thanks... much appreciated.
My main reason for using OR is for simulation of the rocket flight.
Any idea if the modification you outlined creates a more or less accurate flight simulation?
What K'Tesh is proposing there is for visuals only. It will not simulate correctly.
lakeroadster
When in doubt... build hell-for-stout!
Nytrunner
Pop lugs, not drugs
Is that airfoil shape a result of the new developer version?
- Joined
- Mar 27, 2013
- Messages
- 22,537
- Reaction score
- 14,953
No, like Neil said, it's cosmetic only.
No, it's a result of me trying to figure out how to make a wing shape and playing with body tube diameters. You can do it with 15.03.
https://www.rocketryforum.com/threads/offset-wings-fins-in-openrocket.138593/#post-1662595
And now... Back to your regularly scheduled program.
Awww. Now I really wanna build that.
lakeroadster
When in doubt... build hell-for-stout!
- Joined
- Mar 27, 2013
- Messages
- 22,537
- Reaction score
- 14,953
If you look at the .ork file I created, you'll notice that it's not an airfoil, but a teardrop shape.An airfoil... on "The Hammerhead" would likely result in the rocket looping into a spectacular lawn dart.
Now.. where to place the ring...
- Joined
- Aug 27, 2011
- Messages
- 11,618
- Reaction score
- 6,257
I theeeeeeenk a teardrop technically IS an airfoil, just a neutral one. Although for that matter, straight edge rectangles are ALSO airfoils, just neutral and draggy and inefficient ones.
A flat sheet may not be considered an airfoil, an airfoil being the shape applied to modify a flat sheet for reduced drag. But obviously it generates lift when it has a non-zero AoA (or else lots of people's rockets wouldn't be stable). A teardrop may or may not be an optimal neutral airfoil for drag since "teardrop" itself is ill defined; it's a lot better than a flat sheet in any case.
lakeroadster
When in doubt... build hell-for-stout!
Per NASA,
airfoil: A streamlined surface designed in such a way that air flowing around it produces useful motion.
I'm thinking...
a straight edge rectangle wouldn't qualify as "streamlined"
if it's neutral it's not "producing" useful motion
Untrue; lift is a function of both shape and angle of attack. A symmetrical (or neutral) airfoil is streamlined and produces lift when at a non-zero angle of attack which is useful. Also, one could easily argue that the smooth motion of air around a symmetrical airfoil is useful in that it greatly reduces turbulence and thus drag as compared to a flat and squared off slab.
Nytrunner
Pop lugs, not drugs
We make symmetrical airfoils on fins all the time, (at least hope they're symmetrical and produce no lift/spin)
Thus my question to Tesh about how he produced the front. If 0-thick tuebs are involved its a no from me. I make sims for data, not drawings
It's not the phantom tubes that are the problem here. It's that the whole hammer is an ultra-wide fin (or pair of fins). OR simply is not designed to sim fins like that.
lakeroadster
When in doubt... build hell-for-stout!
My point exactly. K-Tesh's Open Rocket visuals are awesome. But for my purposes I don't care that the hammerhead doesn't "look" right in the Open Rocket simulation, I'm wanting simulation data.
It's simulated as a trapezoidal fin set, with fin tabs that go to the vessel C/L, made from spruce.
Would you agree that, if for no other benefit, having the hammerhead in the simulation adds weight in the correct location? Sure, a mass component could just be used but one would speculate that OR would also add drag based on the fin set configuration... but how would one know for sure?
I may use a regular nose cone to prove the flight worthiness of the rocket. Then after a successful launch, install the Hammerhead to see how it affects the flight.
The .ork file is below.
Attachments
Last edited:
One could make two versions, one with the hammerhead as you have it, flat and thick, and the other no hammerhead and a mass object instead. Then you know how much drag OR believes it's adding. Not how accurate that is, of course, but you'd know it's adding something.
Neil, are you saying that if one takes a 3FNC design but makes the fins an inch thick the simulation results would be compromised? And if so, in what way?
lakeroadster
When in doubt... build hell-for-stout!
It's definitely worth putting in the simulation because it has an enormous affect on both mass, CG, and CP. As always the sim may not be perfect but it'll give an idea.
I just took a look at your file and propose two changes to the hammerhead.
1) Change the shape to airfoil. That's closer to what you have than rounded. Doesn't change the apogee estimate very much.
2) Attach the hammerhead to a zero-length, zero-diameter tube in front of the nose cone. The way you have it now, you're getting a very optimistic view of CP, because the fin tabs are not being factored in and they're quite large (they're assumed by OR to be inside the rocket, and therefore not affecting CP). Putting them in front is slightly pessimistic because some of the hammerhead is hidden in the nose. But better to be a bit conservative.
When I make this change, CP ends up right on top of CG, at least with the included F motors:
This was always my concern with the hammerhead.
You get a "thick fin warning", which I *think* refers to the miscalculation of the fin/body effects when the fin is too thick relative to the body. I don't know for sure but I wouldn't trust it to simulate correctly on a fin 2 inches wide and 1/4" tall, for a whole host of reasons.
Nytrunner
Pop lugs, not drugs
Whenever you light this thing, make sure you're behind something solid!
Ugh. This late in the game the immediate thought is to add nose weight. But with the CG already so high it would take a lot of nose weight to move it enough more. The basic problem is much more high CP than low CG, so it really needed bigger fins.
What kind of F motor is in it? If it's Estes's 29 mm BP F, then you might benefit by going composite. The Estes motors weigh about 100 grams. AeroTech 24 mm F motors weigh about from 50 to 70 grams. With the motor being further from the present CG than the nose, each gram removed from the tail will have a bigger effect than each gram added to the nose.
lakeroadster
When in doubt... build hell-for-stout!
Econojet F42-8T
lakeroadster
When in doubt... build hell-for-stout!
Thanks for your help Neal. Better to know now.
No worries. Worst case scenario the hammerhead nose cone will be for show and I'll turn another nose cone for flight. In that case it'll be a great performer on a D12
I'll make the modifications to the nose cone to maximize the volume for more weight. Once it's bored out I can weigh it and then see how much actual weight can be added.
I once had dreams of flying oddrocs on 24mm D12s but always ended up with F24 composites. Darn nose weight and drag just suck the thrust. The most important factor is to fly on a calm day. Any disturbance causing lift on that nose cone and it is Hammer Time!
lakeroadster
When in doubt... build hell-for-stout!
I decided to go ahead and proceed with adding the ballast and then do a swing test.
For ballast I used a 3/4" x 3" long Hex Head Machine Screw, 7.4 ounces, since that's what I had on hand. I tapped the nose cone so the bolt would thread into it.
I installed the Econojet F42-8T motor and the swing test shows the rocket to be stable. I did (7) swing tests, every time the rocket went nose into the wind.
The CG is about 7" back from the nose cone... that pretty much matches the Open Rocket Model once I updated the ballast to reflect what I'm using.
It appears that the CP in the revised Open Rocket model that Neil suggested, attaching the hammer to a faux tube, isn't anywhere accurate in relation to the swing tested CP of the as built rocket?
I updated the old simulation that had the hammer as just regular fins, but up front, and it says the rocket is stable and the CG is also accurate. Looks like that model is more accurate, based on the swing test.
Thoughts?
And here is a swing test without the added ballast in the nose cone... and the motor is installed.
String moved to the new CG.
For ballast I used a 3/4" x 3" long Hex Head Machine Screw, 7.4 ounces, since that's what I had on hand. I tapped the nose cone so the bolt would thread into it.
I installed the Econojet F42-8T motor and the swing test shows the rocket to be stable. I did (7) swing tests, every time the rocket went nose into the wind.
The CG is about 7" back from the nose cone... that pretty much matches the Open Rocket Model once I updated the ballast to reflect what I'm using.
It appears that the CP in the revised Open Rocket model that Neil suggested, attaching the hammer to a faux tube, isn't anywhere accurate in relation to the swing tested CP of the as built rocket?
I updated the old simulation that had the hammer as just regular fins, but up front, and it says the rocket is stable and the CG is also accurate. Looks like that model is more accurate, based on the swing test.
Thoughts?
And here is a swing test without the added ballast in the nose cone... and the motor is installed.
String moved to the new CG.
Attachments
Last edited:
Looks good, pretty clearly stable, I'd say. I'm curious, did you try without the nose weight?
lakeroadster
When in doubt... build hell-for-stout!
That's the same thing my wife asked...
No I didn't... but I will. Stay tuned for coming attractions..
Similar threads
