rcktnut
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I've never seen a rocket with actively controlled fins fly as straight as a rocket without them yet, and some guys have spent a lot of money and time trying to do it.
Actively controlled rocket not flying straight
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I am not familiar with the Loki Dart so I Googled it. Looks to me like the spin is used to null out any asymmetrical thrust, not for gyroscopic effect.
Ez2cDave
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LOKI DART . . .
I think it was primarily for the actual dart's stability, but that's more speculation than speaking as an authority on such things. 1000 RPM isn't trivial gyroscopic influence.
TP
The dart’s fins would be sufficient to keep the pointy end forward, which is all they needed. There was no instrumentation, just chaff that was ejected for radar tracking to determine winds at altitude.
Neutronium95
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Many, if not most sounding rockets are spin stabilized.
One important factor to consider is balancing the rocket so that the CG is exactly on the centerline of the rocket. Also the yo-yo despin mechanisms that are commonly used to null out the roll rate wouldn't pass muster on an amateur flight since it drops weights without any recovery system.
This is a video from a series on rocket stability. My takeaway is that for optimum stability you want a quite high roll rate. If the roll rate is close to the rate at which the rocket wants to oscillate back and forth in response to a perturbation, then you'll hit a positive feedback loop that will lead to coning or worse. And a spin rate of zero is ok from a stability point of view, but might be difficult to achieve.
I think that there might be a place for spin stabilization on high performance flights in the future, but it will require a lot of development both to develop a rocket design that can be easily balanced, and that can either despin itself before recovery system deployment, or has a recovery system that will work when the rocket is spinning at a ridiculous rate. It's definitely not the right solution for this TARC team.
I think you're close to the answer there, or even have provided it. To my understanding it's primarily employed to null out lateral force disturbances whether they be internal (asymmetrical thrust) or external (asymmetrical aero loading). There are plenty of references that mention gyroscopic mechanisms for "spin stabilisation", but primarily it's a nulling tool to my understanding.
TP
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Yup. If you look at the ARC winners, almost none of them use any kind of active flight enhancement. I think one team used drag brakes a few years ago, but other than that Dave is correct... it's all about consistency, which is difficult to attain with a lot of gizmos.
This becomes very important after the rocket leaves any sensible air. The rocket needs to be as stiff as possible as any flexing will bring it to an end-on-end tumble sooner. That's the major reason they use radax joints and balancing along the longitudinal axis. And it includes the entire length of the rocket balanced around the axis, not just the CG on axis. The less flexing, the longer it keeps pointy end up and spinning around the minimum inertia axis.
Nv7 -
To elaborate a bit more on what some of the other folks have been saying...what you are trying to do is "cool", but it shouldn't be done as a ARC project if you are serious about the finals. I've participated on the TARC finals range crew at Great Meadow at least 15 times, and I can tell you from personal observation that the competition there is extremely tough. You would be flying against other teams that have refined much simpler rocket designs through multiple iterations to converge on the required performance specs, and then verified their rocket's performance via multiple flights.
Competing as a school team at ARC is a project because the effort has a definite beginning and a definite end and it produces a scoring result that may or may not qualify for the finals. From a project management perspective the concept you are working on adds complexity to the rocket which increases your project's risk to cost, schedule, scope, and performance quality (https://en.wikipedia.org/wiki/Project_management_triangle). This is because the general systems engineering process and testing required is much more complex for the type of effort you are considering ( https://en.wikipedia.org/wiki/V-model ). It also impacts the team dynamics because the more technical the project gets the more of a challenge it is to lead and manage the group.
The other thing to consider is what is the added value of the proposed system to meeting the basic performance requirements? What the customer (AIA & NAR) wants....and is willing to pay for at the finals.... is payload, altitude, and duration to meet desired specifications subject to certain design constraints. The question is does your design add value to the customer when the same performance can be accomplished with simpler designs and lower bill of material costs? The answer is no, it adds no extra value for the customer. All it does is add is risk and cost.
In summary, the best thing for ARC is KISS ( https://en.wikipedia.org/wiki/KISS_principle ). There are plenty of variables even in simple designs to keep your team fully engaged with ARC during the upcoming year. What you are looking for is a design and test process that converges on meeting performance specs and where you can build multiple rockets under a statistical process control paradigm (https://en.wikipedia.org/wiki/Statistical_process_control ) such that the individual performance of each one is very predictable before they are launched for the first time. This is easier said then done and it will keep your team fully engaged.
That being said, I am not discouraging experimenting with the canard system...its cool. On the contrary, go for it!....., but just not in an environment where you are trying to meet absolute performance specifications and where there is some serious scholarship bucks on the table for the winning teams.
Regards, Bill
To elaborate a bit more on what some of the other folks have been saying...what you are trying to do is "cool", but it shouldn't be done as a ARC project if you are serious about the finals. I've participated on the TARC finals range crew at Great Meadow at least 15 times, and I can tell you from personal observation that the competition there is extremely tough. You would be flying against other teams that have refined much simpler rocket designs through multiple iterations to converge on the required performance specs, and then verified their rocket's performance via multiple flights.
Competing as a school team at ARC is a project because the effort has a definite beginning and a definite end and it produces a scoring result that may or may not qualify for the finals. From a project management perspective the concept you are working on adds complexity to the rocket which increases your project's risk to cost, schedule, scope, and performance quality (https://en.wikipedia.org/wiki/Project_management_triangle). This is because the general systems engineering process and testing required is much more complex for the type of effort you are considering ( https://en.wikipedia.org/wiki/V-model ). It also impacts the team dynamics because the more technical the project gets the more of a challenge it is to lead and manage the group.
The other thing to consider is what is the added value of the proposed system to meeting the basic performance requirements? What the customer (AIA & NAR) wants....and is willing to pay for at the finals.... is payload, altitude, and duration to meet desired specifications subject to certain design constraints. The question is does your design add value to the customer when the same performance can be accomplished with simpler designs and lower bill of material costs? The answer is no, it adds no extra value for the customer. All it does is add is risk and cost.
In summary, the best thing for ARC is KISS ( https://en.wikipedia.org/wiki/KISS_principle ). There are plenty of variables even in simple designs to keep your team fully engaged with ARC during the upcoming year. What you are looking for is a design and test process that converges on meeting performance specs and where you can build multiple rockets under a statistical process control paradigm (https://en.wikipedia.org/wiki/Statistical_process_control ) such that the individual performance of each one is very predictable before they are launched for the first time. This is easier said then done and it will keep your team fully engaged.
That being said, I am not discouraging experimenting with the canard system...its cool. On the contrary, go for it!....., but just not in an environment where you are trying to meet absolute performance specifications and where there is some serious scholarship bucks on the table for the winning teams.
Regards, Bill
Nv7
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Huh, thats pretty cool! We've been seeing flight paths (sort of) like this with our rocket, although it goes sideways in addition to doing this.
Nv7
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Yeah for sure, our team has already made and flown a "traditional" TARC rocket multiple times and we believe it would do well in TARC, its reliable (although not in really windy conditions) and well tested. We are definitely going to qualify with it. We wanted to do a project that we would learn a lot with and hopefully give us an edge, in addition to being at least somewhat unique and original. Thats why I made this thread, since I think we might be missing something with our design or execution thats causing it to fly poorly, our mentors don't have much experience with this kind of thing.
Wasn't the interaction between canards and fins the reason that spin can was developed?
https://www.rocketryforum.com/threads/spin-stabilization-vs-spin-can.148945/The effects of the canards on the rear stabilisation fins are discussed.
https://www.rocketryforum.com/threads/spin-stabilization-vs-spin-can.148945/The effects of the canards on the rear stabilisation fins are discussed.
That looks like a terrific candidate for a 1:1 scale HPR project.
The rolling motion was unintentional and unwanted. But I guess it served to null out the asymmetric drag of the fins and keep the rocket from arcing over.
I tried servos and control tabs connected to an electronic RC gyro in attempts to STOP the rolling. It didn’t work. I realized that the forces causing the roll were too great.
Post in thread 'Bellyflop Recovery'
https://www.rocketryforum.com/threads/bellyflop-recovery.175503/post-2432306
Now I'm a bit confused as to your goals. Are you entering the original design and also doing this extra project (a good idea, if you can make this work). Are you planning to submit the two entries (a highly questionable idea)? Are you planning to modify a solid design by adding epic complexity in order to address one particular problem (a bad idea)?
For flights that go squirelly a short way into flight but not right away, increase the static margin by adding fin area and/or nose weight. To address weathercocking, the simplest thing is to reduce the static margin, and now you're into a classic engineering trade-off. The other very simple way to reduce weather cocking is to increase the rail exit speed, and there are two typical ways to do that. The exit speed after time t on the rail under acceleration a is a∙t, right? So increase a and/or t. Use a longer rail so the rocket has more time to accelerate, and/or use a punchier motor. While those things are not parts of the stability margin trade-off directly, they can alter the optimum solution to that trade.
If you need to, or want to, you can dive further into flight dynamics. When a deviation from nominal flight occurs, and the fins act to restore it, how fast that happens depends on a bunch more than the static margin. And the rocket is likely to overshoot perfectly vertical then oscillate a little and damp out. You can go pretty deep into this stuff, and there is room there for optimization. As far as I know, that's most often done maximum altitude in light or no wind, but I'm confident that there's room there for decreasing weathercocking as well (at the cost of the best possible altitude in low wind).
All that flight dynamic stuff is not worth diving into unless the simpler methods - high rail exit speed and appropriate static margin - aren't getting you where you need to go. The advantage of going that route is that you still end up with a rigid rocket, i.e. one with no moving parts.
@Nv7 --
I've watched the videos of your flights with the canards and the flight pattern ( a procession like a top winding down -- aka a corkscrew ) is about what one should expect for a constant but imbalanced roll moment introduced forward of the CG.
Do you have videos of your traditional TARC rocket in flight without the canards ?
A video showing the weathercocking would be great to see ...
one Q: have you tried setting your launch angle downwind to reduce the initial AoA when it is windy ?
-- kjh
Nv7
Well-Known Member
Our plan is to qualify using our traditional rocket and to use an actively controlled (separate) rocket at nats.
I'll try adding some weight to the nose and see what happens!
Nv7
Well-Known Member
I would say it turns about 25deg into the wind and then continues straight. Enough to make our altitude go from 820ft to 790ft.
We don't live in a windy area so we usually wait for the wind to settle down before we launch, so when this happens we suspect it is because there are different winds up high than on the ground. Due to this we can't really know how to angle the launch rod. This is one of our old traditional designs that didn't do as well as our current one, but it still did fly straight in the past.
Neutronium95
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What motor are you flying on? I'd be inclined to use the highest thrust motor possible to reduce weathercocking.
Or if you can be a little more adventurous:
https://www.rocketmotorparts.com/product/super-loki-motor-assembly--single
So ...
If I glue fins on the Dart, would that count as building the rocket myself for a Level 3 Cert ?
-- kjh
Ez2cDave
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Nv7
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Because our simple rocket doesn't handle wind very well (which there is a lot of in D.C) and we want to try to develop an actively controlled rocket
Ez2cDave
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Personally, I would just redesign the simple rocket ( adjust length or change fin size ), rather than flying something so complex, with so many possible "failure modes". Remember, you also have the "tolerance range" of performance for the rockets motors, which is completely beyond anyone's control.
Explorer I was spin stabilized. Not long after it reached orbit the radio signal strength varied. It was discovered that it was no longer spinning about the roll axis but had shifted to spinning about the yaw axis. Anything spinning about an axis which doesnt have the highest moment of inertia will nutate into a spin about the axis with the highest moment of inertia due to pertabations. Sorry if this is redundant to the discussion above.
Evan Gates wrote a paper evaluating the effect of spin stabilazation on altitude. He did this in Jr. High, I think. I can post a copy if there is interest. I tried to develop a finless spin stabilized rocket that was like a bullet, but never got it to work.
I had one that was a 2 stage with stage 1 spinning stage 2. Stage 2 was finless.
The second one was pre-spun with a router and was finless. Neither worked - hey it was fun trying!
Evan Gates wrote a paper evaluating the effect of spin stabilazation on altitude. He did this in Jr. High, I think. I can post a copy if there is interest. I tried to develop a finless spin stabilized rocket that was like a bullet, but never got it to work.
I had one that was a 2 stage with stage 1 spinning stage 2. Stage 2 was finless.
The second one was pre-spun with a router and was finless. Neither worked - hey it was fun trying!
To summarize: Spinning will never prevent weathercocking off the rail. A longer rail, higher thrust motor, and/or reduced stability margin can minimize it.
However, you can use an off the shelf RC airplane gyro and forward canards to correct the rocket’s attitude to vertical. (Still drifting with the wind.) It has been done, at least once.
Well, I suppose you could 3D print a helical tower, or figure out some other means to pre-spin the rocket, in which case, the wind off the pad would cause coning (corkscrewing) rather than weathercocking. But you would still lose altitude.
Ez2cDave
Well-Known Member
(1) Increase Fineness Ratio.
(2) Reduce Fin Size.
(3) Increase Velocity Off the Rail.
(4) Tilt pad WITH the wind.
Dave F.
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