Balanced Moments and Roll and Coning

[kjhambrick]

Bill --

Your recent post and about radial balance and your paper at Reaction Research Society: Static Balancing of a Payload were very interesting.

I am a recent REBAR -- I used to fly rockets in the 1990s with an axial accelerometer and a pressure sensor.

In the old days. I analysed what data I could but I was only seeing events and effects in one dimension.

I left the hobby in 2000 and just recently returned and the newest flight computers and simulators are a whole new world for me.

I've got a lot to learn, relearn and unlearn.

Something I would like to learn more about is the importance of balancing the masses in the rocket, both radially as well as longitudinally,

I was once long ago a 'cable monkey' for a very talented engineer who ran a vibration analyser on rotating equipment and I understand how important radial balance can be in rotating systems.

For example a turbine engine needs to be balanced about the roll axis, otherwise it will fail catastrophically in short order.

The tires on my car need to be rotationally balanced or they will fail prematurely and so on.

You mentioned that a bullet deliberately out of balance will cone.

But each if these systems are already rolling.

My skills are not at the same 'stage' as @JimJarvis50 and his flights with active guidance ( see I could use just a little guidance for an amazing thread ) where Jim has successfully controlled tilt and roll via active guidance.

I do try to eliminate roll due to unbalanced aerodynamic forces in my rockets as much as possible by installing the fins straight and by making the airfoils as consistent as possible and other than rail giudes, I also try to avoid any 'warts' on my airframes which might introduce an imbalanced aero forces.

But in addition to aero forces, something I always wondered but that I was never able to put my finger on are the effects of radial mass imbalances along the axis of the rocket that might affect tilt and roll.

This is why your recent post was interesting to me and it reminded me of some very old questions I've wondered about.

I understand why a statically stable rocket flying off-vertical will do a gravity turn ( tilt ) -- gravity acts down thru the CG toward the center of the earth and the aero forces act in more-or-less the opposite direction, normal to the axis of the rocket thru the CP causing the ever-increasing turn as the rocket is flying.

But I don't have a feel for the other non-aero roll forces ...

If I launch a rocket that is aerodynamically radially symmetrical, which has no inherent tendency to roll due to aero forces, but the rocket's mass is radially imbalanced, will the rocket naturally tend to roll due to the radial mass imbalance ?

If not, then does radial mass balance matter ?

If so, then once roll is introduced, I can see where coning would follow.

Other than parachutes, shock cords and dog barf, my payload is an AV-Bay.

These days the electronics are very small and lightweight and LiPo Batteries weigh 'nothing' compared to the batteries from the old days.

None-the-less, the mass of the AV-Bay payload can be significant, especially in a small rocket.

I do try to locate the (x,y) plane of the accelerometer and gyro along a longitudinal center plane of the rocket ( usually purpendicular to the launch rail ).

But that is more for the quality of my data than for any other reason.

I've wondered this: Should I also try to locate my AV-Bay near the longitudinal center of gravity of the rocket ?

That point along the rockets length is a 'moving target' so maybe it should be located at the CG after burnout where the rocket spends most of it's flight time ?

But then if I fly motors with different burnout masses, that pursuit becomes impossible so I've settled on locating the AV-Bay where dual deployment works best.

But does it matter ?

I am running out of time this morning before work ... more to come ( I hope ) !

Thanks Bill !

-- kjh

EDIT: I 'lost' this post but then found it and I accidentially re-posted it ... sorry

 

[sr205347d]

If I launch a rocket that is aerodynamically radially symmetrical, which has no inherent tendency to roll due to aero forces, but the rocket's mass is radially imbalanced, will the rocket naturally tend to roll due to the radial mass imbalance ?

Not Bill, but I have some thoughts on this.

I have built and flown a number of rockets that were intentionally asymmetric in order to provide a bellyflop recovery. Although the fins were not axisymmetric, there was no direct rolling moment induced. The asymmetric weight and drag of the fins was significant, however. As you can see in the video in that post, the boost had obvious coning, or corkscrewing.

I tried various ways to eliminate the corkscrewing, to include gyro-stabilization, but nothing worked. This post describes that effort, and includes my hypothesis that if any asymmetry in weight or drag causes the rocket to fly at a sustained angle of attack, then vortex formation along one side of the body tube will cause the coning.

This vortex formation can have other interesting effects.

 

[Spacedog49Krell]

Until we have a significant amount of on-board flight data in the 1-5 ms sps rate, the best we can do is speculate on the cause for corkscrew/coning. My opinion is that it has multiple causes. Most are aerodynamic roll coupling induced issues. A few, that I have seen, appear to be wind shear induced from a sudden change in the angle of attack at altitude.

Unfortunately, none of my high-speed data flights, over the past 4 years, have exhibited coning during the flight. A few of these flights were with asymmetric propellant grains, but with low radial mass offset.

Another possible coning related issue I've noticed from my data. High base drag rockets exhibit a high degree of dampening to roll coupled issues.

Krell

 

[wclaybaugh2]

In general, coning behavior in a rocket can be caused by either aerodynamic forces (Center of Pressure is not coincident with the rotational axis of the vehicle) or due to mass imbalance (Center of Gravity is not on the centerline).

In hobby rockets, mass imbalance issues tend to run to two classes: propellant Cg offsets and structural Cg offsets. Taking those in order:

If an otherwise fully rotationally balanced rocket is fitted with a motor that is not axially symmetric (a moon burner or similar grain design, for example) then we would expect that vehicle to cone when in free flight before ignition (assuming it is rotating) because the Cg is offset from the center-line of the vehicle due to the mass imbalance caused by the grain design. When that rocket is ignited, the coning will tend to lessen and then disappear at burnout since consumption of the propellant will bring the vehicle into rotational balance. This sort of coning can be seem in the sky as an upper stage that leaves a corkscrew exhaust trail with the corkscrew being very wide at ignition and becomeing tighter with each rotation as propellant consumption brings the vehicle into axial balance.

if a rocket is fitted with a fully axisymmetric propellant grain (a BATES grain, say) but does not have the Cg coincident with the longitudinal axis due to some structural mass offset, then it will cone due to that offset mass. Assuming the fins are close to identical in weight and are accurately placed normal to the vehicle surface then the most likely place for that structural imbalance to occur is in the payload. Assuring that the payload's pitch and yaw Cg's are in the same location is described here: https://www.rrs.org/2023/03/17/static-balancing-of-a-payload/. In flight, a rocket with this sort of mass imbalance will tend to cone with the same offset throughout the flight, that is, it will leave a corkscrew smoke trail that has about the same diameter throughout the burn.

Just to be complete, we can also consider the case where there is a mass offset but the vehicle is not rotating. In this case the rocket will not cone but will tend to gravity turn with the turn being toward whichever side of the vehicle is heavier.

Aerodynamic caused coning can be much more complex as sr205347d's post above indicates.

The most common cause of aerodynamic induced coning is probably misaligned fins; this can cause coning both because the misalignment creates a mass offset and also because one side of the rocket will "feel" more drag then the opposite side. This is further abetted by the misaligned fin or fins reacting to cross-winds differently than the other fins.

Another common issue is misalignment of the rocket body with respect to the payload (or with respect to itself if the body is built up of multiple sections of tubing). A variant of this issue is a misaligned nose cone: if the nose tip is not on the rotational axis, the rocket will cone both due to the mass offset and due to more drag occurring on one side of the nose than on the opposite side.

Finally, flexure can lead to coning: if the rocket flexes in flight (this can be a particular issue with very high L/D rockets) the differing aerodynamic forces on the vehicle will lead to coning if the vehicle is spinning.

These comments are far from exhaustive and there is a lot of detail missing in this top level analysis, but perhaps they can serve as a basis for more detailed discussion.

Bill

 

[wclaybaugh2]

Until we have a significant amount of on-board flight data in the 1-5 ms sps rate, the best we can do is speculate on the cause for corkscrew/coning. My opinion is that it has multiple causes. Most are aerodynamic roll coupling induced issues. A few, that I have seen, appear to be wind shear induced from a sudden change in the angle of attack at altitude.

Unfortunately, none of my high-speed data flights, over the past 4 years, have exhibited coning during the flight. A few of these flights were with asymmetric propellant grains, but with low radial mass offset.

Another possible coning related issue I've noticed from my data. High base drag rockets exhibit a high degree of dampening to roll coupled issues.

Krell

You are far from the only credible observer to report that rockets with offset propellant grains did not cone....

Although Occam's Razor compels me to suspect that coning did occur but wasn't observed, I'm also inclined to consider alternatives: is there some physics that would prevent or offset the expected coning of a rocket?

You observe that the examples where you did not see coning involved small propellant offsets which seems to me to imply that the severity of any mass offset induced coning would be smaller and would tend to be mitigated more quickly as the propellant was consumed. That seems--to me--to be one possibility as to how a vehicle that should cone did not observably so do.

Are there other physical explanations as to why a vehicle that should cone does not do? Are there vehicle design features that offset or prevent coning?

Bill

 

[wclaybaugh2]

Bill --

Your recent post and about radial balance and your paper at Reaction Research Society: Static Balancing of a Payload were very interesting.

If I launch a rocket that is aerodynamically radially symmetrical, which has no inherent tendency to roll due to aero forces, but the rocket's mass is radially imbalanced, will the rocket naturally tend to roll due to the radial mass imbalance ?

No. It will, however, tend to gravity turn in the direction of the side that has the mass offset.

If not, then does radial mass balance matter ?

That Gravity Turn could be a reason it matters....

If so, then once roll is introduced, I can see where coning would follow.

Other than parachutes, shock cords and dog barf, my payload is an AV-Bay.

These days the electronics are very small and lightweight and LiPo Batteries weigh 'nothing' compared to the batteries from the old days.

None-the-less, the mass of the AV-Bay payload can be significant, especially in a small rocket.

I do try to locate the (x,y) plane of the accelerometer and gyro along a longitudinal center plane of the rocket ( usually purpendicular to the launch rail ).

But that is more for the quality of my data than for any other reason.

I've wondered this: Should I also try to locate my AV-Bay near the longitudinal center of gravity of the rocket ?

Longitudinal location doesn't much matter WRT coning, but axial location is critical: the Cg of the Av Bay needs to be on the logintudinal axis of the vehicle. This can mean the acceleration sensors are not on the centerline unless the Av Bay is balance weighted to that it's Cg is on the same center line as the sensor chip(s).

That point along the rockets length is a 'moving target' so maybe it should be located at the CG after burnout where the rocket spends most of it's flight time ?

Again, not really an issue for coning and possibly impractical for most rockets.

But then if I fly motors with different burnout masses, that pursuit becomes impossible so I've settled on locating the AV-Bay where dual deployment works best.

But does it matter ?

No for avoiding coning.

I am running out of time this morning before work ... more to come ( I hope ) !

Thanks Bill !

-- kjh

EDIT: I 'lost' this post but then found it and I accidentially re-posted it ... sorry

 

[kjhambrick]

Thanks for the replies Krell and Bill.

I am stuck at work this eve so I'll reply early tomorrow morning.

Good stuff.

Thanks again

-- kjh

 

[wclaybaugh2]

Until we have a significant amount of on-board flight data in the 1-5 ms sps rate, the best we can do is speculate on the cause for corkscrew/coning. My opinion is that it has multiple causes. Most are aerodynamic roll coupling induced issues. A few, that I have seen, appear to be wind shear induced from a sudden change in the angle of attack at altitude.

Unfortunately, none of my high-speed data flights, over the past 4 years, have exhibited coning during the flight. A few of these flights were with asymmetric propellant grains, but with low radial mass offset.

Another possible coning related issue I've noticed from my data. High base drag rockets exhibit a high degree of dampening to roll coupled issues.

Krell

I’d like to pick up on that last observation which seems intuitively correct.

What do we think the physics are in this case? Does exhaust on produce less dampening than exhaust off?

Bill

 

[cerving]

If you are running redundant altimeters and you have both on one side of the sled and both batteries on the other side, that can be a problem if the batteries are significantly heavier than the altimeters (which they often are nowadays). Ditto for the hardware... U-bolts, charge wells, etc. Putting them on opposite sides will help balance your rocket axially.

 

[rocket_troy]

I’d like to pick up on that last observation which seems intuitively correct.

What do we think the physics are in this case? Does exhaust on produce less dampening than exhaust off?

Bill

this perhaps?

https://www.rocketryforum.com/threads/coning.74977/post-817895
Mind you, I didn't know fin restoring forces had a frequency.

TP

 

[SolarYellow]

Mind you, I didn't know fin restoring forces had a frequency.

Makes sense. Simple harmonic motion of a viscously damped (not dampened) system.

When there's a perturbation that causes the rocket to pitch/yaw, at 0 AOA, there is no restoring force from the fins. As the AOA increases, the restoring torque provided by the fins increases, slowing the pitch/yaw until it reverses and driving it back toward zero. The rocket then has rotational momentum in the opposite pitch/yaw direction, and the fins have zero restoring torque as the rocket crosses through 0 AOA, so it has to go back to the opposite angle and be driven back toward 0 AOA again. Just like a pendulum. When the frequency of this oscillation happens to match up to the roll rate, the result is coning.

It's a matching problem, which can be addressed by various means of "unmatching" the roll rate and oscillation frequency. You can reduce the tendency to induce roll. There's data that suggests four fins have less tendency to induce roll than three fins due to the better-balanced forces regardless of pitch/yaw perturbation angle. You can intentionally induce a greater roll rate by angling the fins slightly. Many of the early, unguided sounding rockets did this. Someone earlier this year posted about doing that and getting up to around 2200 rpm, at which point their airframe reached its rotating shaft critical speed and shredded quite spectacularly. You can overdo things, apparently. Bigger or smaller fins or reprofiling (airfoiling) them would affect the restoring torque vs. angle relationship. Changing the distribution of mass longitudinally in the rocket would change the rotational inertia in pitch/yaw and therefore the frequency of oscillation.

 

[rocket_troy]

Makes sense. Simple harmonic motion of a viscously damped (not dampened) system.

When there's a perturbation that causes the rocket to pitch/yaw, at 0 AOA, there is no restoring force from the fins. As the AOA increases, the restoring torque provided by the fins increases, slowing the pitch/yaw until it reverses and driving it back toward zero. The rocket then has rotational momentum in the opposite pitch/yaw direction, and the fins have zero restoring torque as the rocket crosses through 0 AOA, so it has to go back to the opposite angle and be driven back toward 0 AOA again. Just like a pendulum. When the frequency of this oscillation happens to match up to the roll rate, the result is coning.

It's a matching problem, which can be addressed by various means of "unmatching" the roll rate and oscillation frequency. You can reduce the tendency to induce roll. There's data that suggests four fins have less tendency to induce roll than three fins due to the always-balanced forces regardless of pitch/yaw perturbation angle. You can intentionally induce a greater roll rate by angling the fins slightly. Many of the early, unguided sounding rockets did this. Someone earlier this year posted about doing that and getting up to around 2200 rpm, at which point their airframe reached its rotating shaft critical frequency and shredded quite spectacularly. You can overdo things, apparently. Bigger or smaller fins would affect the restoring torque vs. angle relationship. Changing the distribution of mass longitudinally in the rocket would change the rotational inertia in pitch/yaw and therefore the frequency of oscillation.

Good explanation. Thanks.

TP

 

[kjhambrick]

Makes sense. Simple harmonic motion of a viscously damped (not dampened) system.

When there's a perturbation that causes the rocket to pitch/yaw, at 0 AOA, there is no restoring force from the fins. As the AOA increases, the restoring torque provided by the fins increases, slowing the pitch/yaw until it reverses and driving it back toward zero. The rocket then has rotational momentum in the opposite pitch/yaw direction, and the fins have zero restoring torque as the rocket crosses through 0 AOA, so it has to go back to the opposite angle and be driven back toward 0 AOA again. Just like a pendulum. When the frequency of this oscillation happens to match up to the roll rate, the result is coning.

It's a matching problem, which can be addressed by various means of "unmatching" the roll rate and oscillation frequency. You can reduce the tendency to induce roll. There's data that suggests four fins have less tendency to induce roll than three fins due to the better-balanced forces regardless of pitch/yaw perturbation angle. You can intentionally induce a greater roll rate by angling the fins slightly. Many of the early, unguided sounding rockets did this. Someone earlier this year posted about doing that and getting up to around 2200 rpm, at which point their airframe reached its rotating shaft critical speed and shredded quite spectacularly. You can overdo things, apparently. Bigger or smaller fins or reprofiling (airfoiling) them would affect the restoring torque vs. angle relationship. Changing the distribution of mass longitudinally in the rocket would change the rotational inertia in pitch/yaw and therefore the frequency of oscillation.

Great summary @SolarYellow !

And thanks for the Coning link @rocket_troy !

And @cerving is onto something -- try to balance your AV-Bay as much as possible when you lay it out -- which was the point of Bill's paper.

And @Spacedog49Krell is right. Without high-rez N-DoF data we're just guessing at what's what ...

Thank you all !

I am getting old so I am expected to repeat myself: I don't like my rockets to roll at all ...

This is because, from watching a lot of flights, a rocket that does not roll does not corkscrew either.

So maybe another way to reduce or even eliminate roll might be to install @JimJarvis50's Spin Can, an important component of Jim's ground breaking VOS System ( Jim's Spin Can design video is here and ( his Spin Can in action starting at 2:34 )

I've been pondering ...

Could a Spin Can be installed for passive roll control on a small diameter MPR / HPR ?

Yes ... probably ... maybe ... I don't know ...

How small could it be made ?

How much would it weigh ?

And would it work as expected without active tilt -and- roll contol via Jim's VOS canards or @VTS SkunkWorks VTS Ailerons or @georgegassaway's Sunseeker Canards?

I don't know ..

Would a spin can actually reduce induced drag due by reducing the normal aerodynamic force ( aka lift ) that the rocket experiences ?

I don't know ... maybe a passive spin can would actually steal kenetic energy from the flying rocket ...

It is a nebulous idea at this point and it might require some delicate machine tool work that I am not equipped to perforn.

Or maybe the Spin Can could be 3D printed ?

Or maybe it could be constructed with forged composite parts ?

Any which way it is actually constructed, it would have to be small, light weight and precisely rotationally balanced.

But it 'feels' like it should work in passive mode ...

The good news is I believe with a modern 6 DoF recording flight computer, we've now got the means to measure and record such effects.

Anyhow, a quick hit-n-run post while I ponder the concepts in Bill's and Krell's Posts above ...

Thanks all'Y'all !

-- kjh( Einstein's Flatlander awakening in a 6-DoF world )

 

[kjhambrick]

Not Bill, but I have some thoughts on this.

I have built and flown a number of rockets that were intentionally asymmetric in order to provide a bellyflop recovery. Although the fins were not axisymmetric, there was no direct rolling moment induced. The asymmetric weight and drag of the fins was significant, however. As you can see in the video in that post, the boost had obvious coning, or corkscrewing.

I tried various ways to eliminate the corkscrewing, to include gyro-stabilization, but nothing worked. This post describes that effort, and includes my hypothesis that if any asymmetry in weight or drag causes the rocket to fly at a sustained angle of attack, then vortex formation along one side of the body tube will cause the coning.

This vortex formation can have other interesting effects.

Holy cow, @sr205347d

Your bellyflop recovery was beautiful !

Did it always work as intended ?

Do you mean that your rocket did not roll when it coned ?

I can't tell from the video ...

I wonder if your canards could be deployed near apogee to commence the belly flop action ?

EDIT: does your belly flopping rocket glide horizontally as it comes down ?

You've definitely given me much more to ponder ...

Thanks !

-- kjh( this place ( TRF ) is a treasure trove )

 

[sr205347d]

Did it always work as intended ?

No, many flights were 'learning opportunities.' Look through the rest of that thread. My floppers are very reliable now.

Do you mean that your rocket did not roll when it coned ?

It doesn't roll before the coning starts. Then it rolls with the coning. Play that video clip full screen and you can see.

I wonder if your canards could be deployed near apogee to commence the belly flop action ?

Yeah, but why? Any deployment mechanism will increase the moment of inertia, affecting the dynamic stability of the descent. Some of my rockets would oscillate in pitch with increasing amplitude until the AOA dropped low enough to regain stability in forward flight, resulting in a lawn-dart landing.

EDIT: does your belly flopping rocket glide horizontally as it comes down ?

If the CG is too far aft, it will glide backwards (downwind of course ). I don't like that because of damage from hard landings on the tail.

Glad you like it!

 

[tsmith1315]

not dampened

Thanks for that, I was starting to develop a tic…

 

[kjhambrick]

<<snip>>
Yeah, but why ( EDIT: why deploy canards at apogee) ? Any deployment mechanism will increase the moment of inertia, affecting the dynamic stability of the descent. Some of my rockets would oscillate in pitch with increasing amplitude until the AOA dropped low enough to regain stability in forward flight, resulting in a lawn-dart landing.

I was wondering about maybe deploying canards instead of a drogue and then a main at X-feet AGL ... maybe ???

If the CG is too far aft, it will glide backwards (downwind of course ). I don't like that because of damage from hard landings on the tail.

One would definitely need to tune the trim for a good glide.

Glad you like it!

I sure do !

Your thread is on my TRF 'do list' now

Thanks again !

-- kjh

 

[wclaybaugh2]

I've been pondering ...

Could a Spin Can be installed for passive roll control on a small diameter MPR / HPR ?

Yes ... probably ... maybe ... I don't know ...

Probably.

Professional sounding rockets typically have a spin “keep out zone” between three and six rps (180 - 360 rpm). This is the “zone” when pitch roll coupling can occur in those vehicles. Professionals tend to cant the fins to induce around 8-9 rps, thus avoiding pitch yaw coupling and assuring any small thrust, mass, or aero imbalances are averaged out by the spin. Also note that such rockets are dynamic spin balanced as standard practice.

A handful of hobbyists / amateurs have used Air Force Datcom techniques to estimate the aero coefficients of a high performance hobby rocket and combined those with a careful 6 DOF mass model to estimate the spin rate at which pitch roll coupling will occur. I have seen two such analyses and both fell within the professional three to six rps “keep out” range.

*If* keeping away from 3- 6 rps is a valid rule of thumb for hobby scale rockets, then a roll control system that keeps roll at 1-2 rps might avoid coning as well as avoiding any gravity or powered turning that might occur with no spin and either mass or thrust offsets.

Also note that if spin below three rps avoids pitch roll coupling then that would explain why some hobby rockets do not appear to cone even though they have flown with mass offsets.

Bill

 

[kjhambrick]

Until we have a significant amount of on-board flight data in the 1-5 ms sps rate, the best we can do is speculate on the cause for corkscrew/coning. My opinion is that it has multiple causes. Most are aerodynamic roll coupling induced issues. A few, that I have seen, appear to be wind shear induced from a sudden change in the angle of attack at altitude.

Unfortunately, none of my high-speed data flights, over the past 4 years, have exhibited coning during the flight. A few of these flights were with asymmetric propellant grains, but with low radial mass offset.

Another possible coning related issue I've noticed from my data. High base drag rockets exhibit a high degree of dampening to roll coupled issues.

Krell

Krell --

I agree -- high-rate accelerometer and gyro data is essential if we want to see what is going on up there.

But it's not magic -- all we can measure from within an AV-Bay are the effects of real-world causes of any flight anomolies that we might experience.

But in the end, all we can see are the effects -- the real causes of those effects are liable to be hidden from us.

High speed video of the flights ( from the ground and from the rocket ) are probably just as important to have

-- kjh

 

[SolarYellow]

Probably.

Professional sounding rockets typically have a spin “keep out zone” between three and six rps (180 - 360 rpm). This is the “zone” when pitch roll coupling can occur in those vehicles. Professionals tend to cant the fins to induce around 8-9 rps, thus avoiding pitch yaw coupling and assuring any small thrust, mass, or aero imbalances are averaged out by the spin. Also note that such rockets are dynamic spin balanced as standard practice.

Gotta wonder what the critical speed for the shaft rotation is on these airframe/payload combos, and whether it's also a consideration in that 3-6 rps "keep out zone."

 

[kjhambrick]

Probably.

Professional sounding rockets typically have a spin “keep out zone” between three and six rps (180 - 360 rpm). This is the “zone” when pitch roll coupling can occur in those vehicles. Professionals tend to cant the fins to induce around 8-9 rps, thus avoiding pitch yaw coupling and assuring any small thrust, mass, or aero imbalances are averaged out by the spin. Also note that such rockets are dynamic spin balanced as standard practice.

A handful of hobbyists / amateurs have used Air Force Datcom techniques to estimate the aero coefficients of a high performance hobby rocket and combined those with a careful 6 DOF mass model to estimate the spin rate at which pitch roll coupling will occur. I have seen two such analyses and both fell within the professional three to six rps “keep out” range.

*If* keeping away from 3- 6 rps is a valid rule of thumb for hobby scale rockets, then a roll control system that keeps roll at 1-2 rps might avoid coning as well as avoiding any gravity or powered turning that might occur with no spin and either mass or thrust offsets.

Also note that if spin below three rps avoids pitch roll coupling then that would explain why some hobby rockets do not appear to cone even though they have flown with mass offsets.

Bill

Bill --

I would love to read that analysis for the hobby rocket if you have a link.

As for Jim Jarvis' Spin Can, the idea is to let the fins spin on a 'frictionless' bearing so the rocket does not roll due to aero forces.

Of course, there are no frictionless bearings so I imagine the rocket's roll will eventually catch up with any spin in the Spin Can.

But given the relatively short time to apogee of our hobby rockets, a Spin Can might be able to postpone rocket roll long enough that it does not matter because the chute is out.

-- kjh

EDIT: p.s. when you mentioned the expensive spin balance machines, I eventually thought of my local 'Discount Tires' and their tire balancing machine

I wonder ... ??? ...

 

[SolarYellow]

Something like this will get truly frictionless support for balancing.

https://www.amazon.com/Aluminum-Magnetic-Propeller-Balancer-Electric/dp/B06WWPQ1JJ
This will have a bit of friction, but may be a little more practical for our purposes. It all depends on getting geometrically accurate fixturing to hold the components.

https://www.amazon.com/Du-Bro-499-Tru-Spin-Prop-Balancer/dp/B0006N72Y8
Also, note that each segment of the rocket should be balanced by itself. For example, if there is a payload bay in the NC with an imbalance and a matching imbalance in the fin can, but they are clocked 180 degrees apart, then a static balance of the assembled airframe would not reveal it. However, in free air, the spin axis of the rocket would be more aligned with the c.g.s of the individual components, which would be at an angle to the airframe's geometric axis.

This discussion makes me hate launch lugs and rail buttons even more.

 

[kjhambrick]

Also, note that each segment of the rocket should be balanced by itself. For example, if there is a payload bay in the NC with an imbalance and a matching imbalance in the fin can, but they are clocked 180 degrees apart, then a static balance of the assembled airframe would not reveal it. However, in free air, the spin axis of the rocket would be more aligned with the c.g.s of the individual components, which would be at an angle to the airframe's geometric axis.

Yes, static .ne. dynamic balance !

This discussion makes me hate launch lugs and rail buttons even more.

Yes, me too ... have you tried the Sacrete in a bucket Launch Tower yet ?

 

[SolarYellow]

Yes, me too ... have you tried the Sacrete in a bucket Launch Tower yet ?

Of course, I have to polish that turd until it becomes awesome. Pretty much have the plan all outlined. But no, I haven't actually done it yet. Probably won't be doing any actual building of anything until sometime in November, the way things are going. And that's actually a good thing. What's happening for the next month-plus is better than rockets.

 

[Spacedog49Krell]

Krell --

I agree -- high-rate accelerometer and gyro data is essential if we want to see what is going on up there.

But it's not magic -- all we can measure from within an AV-Bay are the effects of real-world causes of any flight anomolies that we might experience.

But in the end, all we can see are the effects -- the real causes of those effects are liable to be hidden from us.

High speed video of the flights ( from the ground and from the rocket ) are probably just as important to have

-- kjh

True, data is not magic. It requires a lot of work to interpret what it means. If the AV-bay location does not give you a clear picture of what the rocket is doing, then move the sensors to where they do give a better picture. A 240 frame/second video gives you a snapshot of what the rocket flight looks like every ~4 ms. You still need to interpret the image. An Arduino based flight computer can record 10 channels of data every 4 ms. I have less interpretation with the digital data. I still need to understand what the data means, but I can apply equations to the data for understanding.

The trend in flight computers is to higher data recording speeds. SparkyVT has been flying 1000Hz systems for 5-6 years, I've been flying 500Hz systems for 4 years. Blue Raven does 500Hz and there are other systems at 400Hz. I use video as confirmation of my interpretation of the data. On-board video is an important tool. I'm looking at the drone barebones 4K video boards for my next avionics bay.

 

[wclaybaugh2]

Bill --

I would love to read that analysis for the hobby rocket if you have a link.

As for Jim Jarvis' Spin Can, the idea is to let the fins spin on a 'frictionless' bearing so the rocket does not roll due to aero forces.

Of course, there are no frictionless bearings so I imagine the rocket's roll will eventually catch up with any spin in the Spin Can.

But given the relatively short time to apogee of our hobby rockets, a Spin Can might be able to postpone rocket roll long enough that it does not matter because the chute is out.

-- kjh

EDIT: p.s. when you mentioned the expensive spin balance machines, I eventually thought of my local 'Discount Tires' and their tire balancing machine

I wonder ... ??? ...

Kjh:

I have seen two such hobby rocket level analyses.

One was for the ‘96 RRS Dart and was paper only, which I returned to the person who did that work..

The other was for the Saad Mirza / Coleman Merchant / Princeton University Space Shot attempts. That was provided to me privately and not for distribution.

The DATCOM software is available only to DOD contractors that are US persons. Thus that part of these two analyses can’t be passed on w/o DOD permission,,,which regular civilians are not likely to get.

Tire balancing machines are not, so far as I have been able to parse, suitable for long, thin, and light weight (compared to a wheel / tire) hobby rocket payloads. But I’d love to be proven wrong on that….

Bill

 

[wclaybaugh2]

Gotta wonder what the critical speed for the shaft rotation is on these airframe/payload combos, and whether it's also a consideration in that 3-6 rps "keep out zone."

Good question. I have no idea but my information comes from the NASA Wallops Sounding Rocket Payload Users Guide, which might discuss this, although I do not recall such discussion.

Bill

 

[SolarYellow]

Good question. I have no idea but my information comes from the NASA Wallops Sounding Rocket Payload Users Guide, which might discuss this, although I do not recall such discussion.

Bill

https://sites.wff.nasa.gov/code810/files/SRHB.pdf

 

[kjhambrick]

https://sites.wff.nasa.gov/code810/files/SRHB.pdf

Nice !

Thanks @SolarYellow !

-- kjh

 

[Spacedog49Krell]

I’d like to pick up on that last observation which seems intuitively correct.

What do we think the physics are in this case? Does exhaust on produce less dampening than exhaust off?

Bill

SolarYellow presented a good explanation of coning. The rockets fins are the strong corrective force. I'm looking at base drag is a possible resonant damping coefficient. The low-pressure region behind the vehicle dampens the overcorrection of the fins. Even when the Input frequency/Natural frequency =1. It's a theory that needs further work.

It started as a project to study launch exhaust flow patterns of finless cone and pyramid rockets when launched close to a flat plate flame deflector producing a low-pressure venturi effect region. It morphed into a base drag pressure measurement study of large diameter single engine rockets.

The limited data I've collected is trending to greater dampening after engine burnout.