Lakeroadster's "Hammerhead Shark"

[Daddyisabar]

No matter what tools you use, eventually you will push the envelope too far. I was lucky that day for me was when the Pad Fuhrer was not at the launch. I know now if mindsim is generating too much fear by the size of that lead ball in my tummy before launch. Search you feelings. Keep your mind on where you are at, what you are doing.

 

[jqavins]

Just a reminder, since this thread is rather old (and was dormant until Lake's incidental comment about upgrading to the latest OR release) that he is not relying on mindsim or OR. He has good, solid swing test results. If I recall correctly, and if my faith in the man is justified, he did so from two points of string attachment, so that both pitch and yaw stability have been verified.

In a contest between empirical data and doubtable simulations (whether in silico or in cogito) data wins.

 

[lakeroadster]

Just a reminder, since this thread is rather old (and was dormant until Lake's incidental comment about upgrading to the latest OR release) that he is not relying on mindsim or OR. He has good, solid swing test results. If I recall correctly, and if my faith in the man is justified, he did so from two points of string attachment, so that both pitch and yaw stability have been verified.

In a contest between empirical data and doubable simulations (whether in silico or in cogito) data wins.

I believe you mean "doubtable" and not "doubable"

As to the swing test... I posted the video.. I'm not sure what more a fella could do?

So watch the video, then critic it, tell me what I'm doing wrong and show me where you got the specification for "two points of string attachment, so that both pitch and yaw stability have been verified."

 

[jqavins]

1) Quite right on the typo.

2) Not a specification, just (I think) a sensible thing. With 3/4FNC designs, the rocket is the same when viewed from any angle around the axis, as far as the stability. (It looks different depending on the "phase" of the fin rotation, but that makes no difference to the CP.) So pitch and yaw are the same thing to a rocket that has that sort of symmetry.

But when there are features that give the rocket two distinct axes of symmetry, pitch and yaw are no longer the same. Attaching the string to one "side", so that the rocket's pitch axis and the ground frame pitch axis are the same may give somewhat different results from attaching the string doraslly or ventrilly, so that the rocket's yaw axis corresponds to the ground frame pitch axis. It seems like testing both ways would be worthwhile. I thought you had done that already.

The two results are probably not very different, as neither rotation is truly constrained at the end of the string. Yet there will be a small difference in how the string's pull on the rocket affects the dynamics. Small, I trust. I'm probably overthinking, since overthinking with me has passed beyond a habit and become a hobby. With your greater experience at performing swing tests, of you feel sure it's rock solid from one axis, and the difference is small, then I defer to you on the necessity of doing it both ways. If the design were borderline it would be a different story.

My concern here is in cogito, and as I wrote above, data wins.

 

[lakeroadster]

1) Quite right on the typo.

2) Not a specification, just (I think) a sensible thing. With 3/4FNC designs, the rocket is the same when viewed from any angle around the axis, as far as the stability. (It looks different depending on the "phase" of the fin rotation, but that makes no difference to the CP.) So pitch and yaw are the same thing to a rocket that has that sort of symmetry.

But when there are features that give the rocket two distinct axes of symmetry, pitch and yaw are no longer the same. Attaching the string to one "side", so that the rocket's pitch axis and the ground frame pitch axis are the same may give somewhat different results from attaching the string doraslly or ventrilly, so that the rocket's yaw axis corresponds to the ground frame pitch axis. It seems like testing both ways would be worthwhile. I thought you had done that already.

The two results are probably not very different, as neither rotation is truly constrained at the end of the string. Yet there will be a small difference in how the string's pull on the rocket affects the dynamics. Small, I trust. I'm probably overthinking, since overthinking with me has passed beyond a habit and become a hobby. With your greater experience at performing swing tests, of you feel sure it's rock solid from one axis, and the difference is small, then I defer to you on the necessity of doing it both ways. If the design were borderline it would be a different story.

My concern here is in cogito, and as I wrote above, data wins.

Easy enough to do... Here are the new video's. Tests stable no matter what the orientation is.

 

[Daddyisabar]

Come over to Hartsel on the 24th of April, the first Tripoli Colorado launch. With the Top Men launching their homebrews having thrust letters that slide way down the back side of the alphabet, I doubt they will be scared of a little hammer head shark on a kiddie motor. I have flown many mindsimed abominations there. Anyway, I need an excuse to go to see something interesting. To get off my kiester and build a canted tractor motor cluster, telescoping serpent rocket.

 

[BABAR]

Looks pretty darn stable to me. I am an Air Force guy, but I think the navy term for a launch is called “fish in the water!” Looking forward to (meataphorically) seeing this creation’s fins get wet.

 

[lakeroadster]

Looks pretty darn stable to me. I am an Air Force guy, but I think the navy term for a launch is called “fish in the water!” Looking forward to (meataphorically) seeing this creation’s fins get wet.

With any luck, the launch won't result in a "Sushi Plate"... sans the plate.

 

[lakeroadster]

@neil_w comments on this thread have lead me to spending some time looking at Barrowman Equations.

I'm trying to get my head wrapped around why this rocket swing tests rock solid stable, yet Open Rocket shows it to be unstable.

If I remove the hammerhead fin, the rocket has a stability caliber of 1.97

With the hammerhead fin, the rocket has a stability caliber of -0.559

So follow me on this.. With the hammerhead in place, the large vertical stabilizer fin can be removed with almost no change in stability? (from -0.559 to -0.592)
But without the hammerhead fin, if the large vertical stabilizer fin is removed, it radically changes stability? (From 1.97 down to .878)

I've looked at the "Tools" / "Component Analysis" tab to try to wrap my head around what the equations are doing.

It seems that the way Open Rocket does the analysis the hammerhead fin so radically drives instability, that it overshadows the stability provided by the large vertical stabilizer fin.

 

[burkefj]

Open rocket will only show you the worst case CP, that could be the top view or the side view in an asymmetric rocket. with the hammerhead fin in place the CP is dominated by the top view, so removing the stabilizer doesn't have any impact because the CP is dominated by the top view not the side view, once you remove the hammerhead now the CP is dominated by the side view which only has the one vertical fin not the two fins like on the top view.

When I'm designing my rocket gliders I find it helpful to do two simulations,
One with just the vertical fin and a copy of that at 90° to check the center of pressure, then another version of the top view fins or wings and copies of those at 90° as well so you have two symmetric simulations one with the wings and the other simulation with the vertical stabilizers if that makes sense so you can look at the CP change separately if you tweak wing fins or stabilizer fins.

@neil_w comments on this thread have lead me to spending some time looking at Barrowman Equations.

I'm trying to get my head wrapped around why this rocket swing tests rock solid stable, yet Open Rocket shows it to be unstable.

If I remove the hammerhead fin, the rocket has a stability caliber of 1.97 View attachment 510834

With the hammerhead fin, the rocket has a stability caliber of -0.559 View attachment 510835

So follow me on this.. With the hammerhead in place, the large vertical stabilizer fin can be removed with almost no change in stability? (from -0.559 to -0.592) View attachment 510839
But without the hammerhead fin, if the large vertical stabilizer fin is removed, it radically changes stability? (From 1.97 down to .878) View attachment 510841

I've looked at the "Tools" / "Component Analysis" tab to try to wrap my head around what the equations are doing.

It seems that the way Open Rocket does the analysis the hammerhead fin so radically drives instability, that it overshadows the stability provided by the large vertical stabilizer fin.

 

[lakeroadster]

Open rocket will only show you the worst case CP, that could be the top view or the side view in an asymmetric rocket. with the hammerhead fin in place the CP is dominated by the top view, so removing the stabilizer doesn't have any impact because the CP is dominated by the top view not the side view, once you remove the hammerhead now the CP is dominated by the side view which only has the one vertical fin not the two fins like on the top view.

When I'm designing my rocket gliders I find it helpful to do two simulations,
One with just the vertical fin and a copy of that at 90° to check the center of pressure, then another version of the top view fins or wings and copies of those at 90° as well so you have two symmetric simulations one with the wings and the other simulation with the vertical stabilizers if that makes sense so you can look at the CP change separately if you tweak wing fins or stabilizer fins.

Very interesting. Thanks for taking the time to explain this.

 

[burkefj]

No problem that's a good way to find out which view is dominant for the center of pressure calculation for airplane like configurations.

Very interesting. Thanks for taking the time to explain this.

 

[neil_w]

Unfortunately, what you don't get from OR right now is a breakdown of stability on different axes, so yeah you have to be very conscious of that stuff with airplane-like designs.

 

[Daddyisabar]

Asymmetry SUCKS! That is why you safely fly 3-4FNC!

My inner RSO is quelled.

 

[Daddyisabar]

Unfortunately, what you don't get from OR right now is a breakdown of stability on different axes, so yeah you have to be very conscious of that stuff with airplane-like designs.

The mindsim for this one required going into Aboriginal Dream Time. Had to Tune In, Turn On and Drop out. How did those old analog dudes do it with their wind tunnels, slide rules and liitle models attached to the end of a booster rocket?

This one ended up with a fair bit of nose weight in the lower pod requiring poor boy dual deploy.



The whole world truely does revolve around the CG-CP Axis: And like W said, it's an Axis of Evil!

 

[lakeroadster]

Anybody else here a data miner? One of the thing I love about Open Rocket is the ability to export the simulation data into Excel.

Here's a screen shot (nerd alert) of the data, once manipulated and organized. I've also attached the std plot.

 

[Daddyisabar]

When Open Rocket, Rocsim, paper cut outs and swing testing isn't doing it for you, what do you do?... JUST GET OUT THE DIDGERIDOO!

A nice big bonfire and some soothing Aboriginal tunes will do the trick! If that is not possible then it is some chem dog or refreshing ale and old Henry John Deutschendorf records played on the stereo in your parent's basement. Rocky Mountain High and I'm Leaving on a Jet Plane are a good start.

 

[jqavins]

Who's "first rule of aerodynamics" is that with a big enough motor, anything can fly? I guess the second rule is that with enough mind altering practices and intoxicants anything is stable.

Unfortunately, what you don't get from OR right now is a breakdown of stability on different axes, so yeah you have to be very conscious of that stuff with airplane-like designs.

That's another point in favor of RS, and therefore another feature that OR must add.

 

[neil_w]

That's another point in favor of RS, and therefore another feature that OR must should add.

Fixed.

 

[lakeroadster]

Who's "first rule of aerodynamics" is that with a big enough motor, anything can fly? I guess the second rule is that with enough mind altering practices and intoxicants anything is stable.

That's another point in favor of RS, and therefore another feature that OR must add.

Now, now now Joe. Don't be rude. Daddyisabar has proven his theory, time and time again, by not just talking the talk... but by walking the walk.

But our wizard is a guy named Vern, he wrote the book on checking stability, literally. And therein lies the reasoning behind a swing test. It's not simulated data... it's real data. And adding weird fins isn't beyond the scope or limitation of the string.

Will it go 'round it circles? Will it fly high like a bird up in the sky?

 

[OverTheTop]

Who's "first rule of aerodynamics" is that with a big enough motor, anything can fly?

The Mirage fighter jet was one of the early birds that that adage was applied to, IIRC. I'm sure there were others before that.

 

[Daddyisabar]

Who's "first rule of aerodynamics" is that with a big enough motor, anything can fly? I guess the second rule is that with enough mind altering practices and intoxicants anything is stable.

That's another point in favor of RS, and therefore another feature that OR must add.

Our old RSO (RIP) would say to the youglings and neophytes "Don't be influenced by or listen to the mindsimmers, they are a silly lot. Be like Spock; pure logic, science and mathematics rule. If you haven't achieved the needed result then you haven't done all the math required. DO THE MATH!"

All the stuff about the "Spice" is just fiction. Rocket science is the only path. Trust the swing tester, cardboard cut outs not so much, simulations only if no garbage in. Wind tunnels rule.

Funny looking Oddrocs are, by nature, towards the garbage end of the spectrum and are always viewed with a high degree of suspicion. No country for old Oddrocs. 3-4FNC today, 3-4FNC tomorrow, 3-4FNC forever! The mindsim method is unsound, to be dispatched with prejudice, just like we did with old Colonel Kurtz.

So, if you doubt your courage or your oddroc then go no further, for you face certain death! The atmosphere is a cruel mistress with big, pointy teeth! A creature so foul and a mean streak a mile wide! Look at the bones!

Those were sure fun days.

 

[Daddyisabar]

The Mirage fighter jet was one of the early birds that that adage was applied to, IIRC. I'm sure there were others before that.

The F4 Phantom was the ultimate example.

 

[lakeroadster]

The F4 Phantom was the ultimate example.

Glides like a brick.

 

[Daddyisabar]

Anybody else here a data miner? One of the thing I love about Open Rocket is the ability to export the simulation data into Excel.

Here's a screen shot (nerd alert) of the data, once manipulated and organized. I've also attached the std plot.

View attachment 510895 View attachment 510897

Your nerd card is fully functional!

 

[lakeroadster]

That's another point in favor of RS, and therefore another feature that OR must add.

It's free software.. beggar's can only ask... demands are for paying customers.

 

[Greg Furtman]

So, if you doubt your courage or your oddroc then go no further, for you face certain death! The atmosphere is a cruel mistress with big, pointy teeth! A creature so foul and a mean streak a mile wide! Look at the bones!

Those were sure fun days.

I love your satire!

 

[Daddyisabar]

It's free software.. beggar's can only ask... demands are for paying customers.

When it comes to a high end launch cost is not an option. First Class, Bourgeois rules. Rocsim is just a toy for the Top Men. I've heard the real software is in that big concrete building with no windows in Waterton Canyon, just outside Littleton, CO. Just a bit hard to get on the Lockheed Martin campus and then into the building ... and I don't think holding your oddroc outside the facility and begging for entry will do.

 

[lakeroadster]

When it comes to a high end launch cost is not an option. First Class, Bourgeois rules. Rocsim is just a toy for the Top Men. I've heard the real software is in that big concrete building with no windows in Waterton Canyon, just outside Littleton, CO. Just a bit hard to get on the Lockheed Martin campus and then into the building ... and I don't think holding your oddroc outside the facility and begging for entry will do.

Maybe strap the oddroc to the horn of a Bighorn Sheep and have him sneak it into the facility. "Nothing to see here... it's just me, "Cam", I'm here for the local college mascot tour."

 

[Daddyisabar]

Maybe strap the oddroc to the horn of a Bighorn Sheep and have him sneak it into the facility. "Nothing to see here... it's just me, "Cam", I'm here for the local college mascot tour."

That would work until you ran into a guy who graduated from CU Aerospace. He would demand Ralphie!