Lakeroadster's "Hammerhead Shark"

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@lakeroadster One of the things you haven't shown us on the .... Hammerhead is your recovery systems. What are you using for shock cord? How are you attaching it? What size chutes do you plan on using? Inquiring minds want to know. (And if you remember where this phrase came from you too are an old fart.)

I am lakeroadster..... and I am an old fart :computer:

The recovery on the Hammerhead is a bit unconventional:

The nose cone recovery is all on it's own. It weighs 10.6 ounces. It has a 12" piece of 75# Kevlar, attached to a #7 Eagle Claw Barrel Swivel via a key ring slipped through a custom made eyebolt that threads into the 3/4" bolt that threads into the nose cone. With an 18" Estes plastic parachute it's descent rate should be about 20 fps. There is no shock chord.​
The fuselage weighs 6.2 ounces and is pretty much standard recovery fair... a kevlar chord (attached to the motor mount) with a 24" elastic chord attached, via a #7 Eagle Claw Barrel Swivel, to an 18" chute that should yield about 16 fps descent rate.​
The fins are 1/8" balsa, through wall, and are papered. The body tube is a BT-60 with a full length C-60 internal sleeve. The intent was to end up with a durable rocket in regard to damage upon landing.​
001.JPG
 
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I am lakeroadster..... and I am an old fart :computer:

The recovery on the Hammerhead is a bit unconventional:

The nose cone recovery is all on it's own. It weighs 10.6 ounces. It has a 12" piece of 75# Kevlar, attached to a #7 Eagle Claw Barrel Swivel via a key ring slipped through a custom made eyebolt that threads into the 3/4" bolt that threads into the nose cone. With an 18" Estes plastic parachute it's descent rate should be about 20 fps. There is no shock chord.​
The fuselage weighs 6.2 ounces and is pretty much standard recovery fair... a kevlar chord (attached to the motor mount) with a 24" elastic chord attached, via a #7 Eagle Claw Barrel Swivel, to an 18" chute that should yield about 16 fps descent rate.​
The fins are 1/8" balsa, through wall, and are papered. The body tube is a BT-60 with a full length C-60 internal sleeve. The intent was to end up with a durable rocket in regard to damage upon landing.​
@lakeroadster Thanks for the info. Yes, the Hammerhead head is pretty heavy and I think it is a wise decision to let it come down by itself. God, that is a big arse nut!
 
I am an old fart too. I am thinking you may want to add some elastic between that massive nose and the chute. Since you are flying without electronics, chances are you may get deployment somewhat before or after apogee, meaning that the nose is going to have some momentum. Depending on how much, you may shred your chute. Meanwhile, since all that is attached to the chute end of the elastic is the chute (as opposed to normal:attached to chute and rocket) there is no risk of “bounce back”. (Aka “Estes dent or smile”) from the recoil.

I learned this when I tried using black powder motors for a two stage, with a chute in a pod released when the segments separated. What I DIDN’T account for was that the booster was going full tilt velocity at time of staging.

Booster came down under a chute attached by only one riser! I just told everyone it was a round streamer.
 
I am an old fart too. I am thinking you may want to add some elastic between that massive nose and the chute. Since you are flying without electronics, chances are you may get deployment somewhat before or after apogee, meaning that the nose is going to have some momentum. Depending on how much, you may shred your chute. Meanwhile, since all that is attached to the chute end of the elastic is the chute (as opposed to normal:attached to chute and rocket) there is no risk of “bounce back”. (Aka “Estes dent or smile”) from the recoil.

I learned this when I tried using black powder motors for a two stage, with a chute in a pod released when the segments separated. What I DIDN’T account for was that the booster was going full tilt velocity at time of staging.

Booster came down under a chute attached by only one riser! I just told everyone it was a round streamer.

I'll take your advice... thanks! :computer:
 
Just curious, have you tried attaching the string on the top of the model and swinging that way, that would put more of a pitch angle on the direction of travel and test the stability in that direction instead of the lateral stability like you are testing swinging with it attached on the side.
 
Just curious, have you tried attaching the string on the top of the model and swinging that way, that would put more of a pitch angle on the direction of travel and test the stability in that direction instead of the lateral stability like you are testing swinging with it attached on the side.

Here ya go.. Swing test with the string top mounted.... but wrong rocket... I thought you had replied to the X-Wing thread. Bottom line.. no change in regard to stability.

Swinging this reminds me of my old U-Control airplane days.. :headspinning:

 
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:
View attachment 418454

This was always my concern with the hammerhead.

Neil I was updating this rocket to OR-22 and just for grins followed your #2 advice. I inputted the actual contoured hammer head profile. The model also reflects the 7.4 ounce nose weight, same configuration that passed the swing test with flying colors.

Would you believe the OR-22 model shows a stability of 2.87 in this configuration? And that sounds right, because the swing test was rocket solid stable.

Here are the (2) different simulation screen shots and OR files.

2022-03-19 Hammerhead OR22 Flight Sim Profiled Head.jpg 2022-03-19 Hammerhead OR22 Flight Sim Head Fin With Large Fin Tabs.jpg
 

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  • Hammer Head BT-60 With HHCS Ballast Aero Head AS BUILT.ork
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  • Hammer Head BT-60 With HHCS Ballast AS BUILT.ork
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Would you believe the OR-22 model shows a stability of 2.17 in this configuration?
Yes, because you implemented the fins (4" thick) in a way that OR doesn't really understand. Here's a more straightforward implementation, with fins attached to the nose. This might be close to what your previous model was:
1647740405534.png
I'm not entirely sure I believe these results either; CP seems a bit more forward than I'd expect. And I understand that your model passes the swing test. All I can tell you for sure is that your OR model with the 4" thick fins was producing nonsense results.

[edit] I should elaborate.

When OR looks at fins for CP calculations, it looks at the side area. The thickness is mostly used for drag calculation. So when you have a fin implemented as 4" thick, OR doesn't know that the gigantically thick fin overhanging the parent object actually presents a flat aerodynamic surface. Rather, it sees the comparatively tiny cross section (your fin "shape") for CP calculation. And hence you get those results.

It is important to remember that OR (or Rocksim) do not see the entire finished shape of the rocket as would, for example, a CAD or CFD program. Rather, each component is treated with a set of approximations based on assumptions of "reasonable" shape and configuration. Once you violate those assumptions, you will get unrealistic results.
 
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Yes, because you implemented the fins (4" thick) in a way that OR doesn't really understand. Here's a more straightforward implementation, with fins attached to the nose. This might be close to what your previous model was:
View attachment 510242
I'm not entirely sure I believe these results either; CP seems a bit more forward than I'd expect. And I understand that your model passes the swing test. All I can tell you for sure is that your OR model with the 4" thick fins was producing nonsense results.

[edit] I should elaborate.

When OR looks at fins for CP calculations, it looks at the side area. The thickness is mostly used for drag calculation. So when you have a fin implemented as 4" thick, OR doesn't know that the gigantically thick fin overhanging the parent object actually presents a flat aerodynamic surface. Rather, it sees the comparatively tiny cross section (your fin "shape") for CP calculation. And hence you get those results.

It is important to remember that OR (or Rocksim) do not see the entire finished shape of the rocket as would, for example, a CAD or CFD program. Rather, each component is treated with a set of approximations based on assumptions of "reasonable" shape and configuration. Once you violate those assumptions, you will get unrealistic results.

I was talking to the owner of the manufacturing company I worked at a couple decades ago about how great our new Pressure Vessel Design Software was. His comment was to never put too much faith in such things, they will never be more valuable than the knowledge gained through real world results.

I'm curious @neil_w , have you ever done a swing test? It's really useful on designs that fall outside the 3/4fnc rocketry realm.

The reason I ask is what I've found from building many odd rocs is that if you perform swing tests on each rocket, being of various rocket designs, you not only gain the knowledge of if the rocket is stable, but also how stable the rocket is.

It's an acquired skill. Marginally stable rockets will dance on the end of the string, pitching up and down. Rockets that are very stable are arrow straight. The Hammerhead seems to me to be very stable based on this.

Perhaps the big hammer on the nose is creating vortices that add stability beyond the capabilities of OR? OR does throw error messages sometimes stating that thick fins may not be modeled accurately... but it doesn't do that on this simulation... which I also find strange?

The actual powered flight should be educational.
 
I was talking to the owner of the manufacturing company I worked at a couple decades ago about how great our new Pressure Vessel Design Software was. His comment was to never put too much faith in such things, they will never be more valuable than the knowledge gained through real world results.
Not putting in too much faith: good advice. Even better: understanding the limitations of the software to know *when* to put faith in it, and when not to trust it.

I'm curious @neil_w , have you ever done a swing test? It's really useful on designs that fall outside the 3/4fnc rocketry realm.
No, for various reasons. None of my rockets fall so far into odd-rock territory that I feel the need, to be honest.

Perhaps the big hammer on the nose is creating vortices that add stability beyond the capabilities of OR? OR does throw error messages sometimes stating that thick fins may not be modeled accurately... but it doesn't do that on this simulation... which I also find strange?
I won't pretend to understand the peculiarities of the Hammerhead's aerodynamics. Make sure to get some video of the flight. :)
 
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