I could use just a little guidance

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Post 4 of 4

The next piece of the booster is an air frame section that slides over the top of the motor. The motor gets retained at the top of this section, and the the other functions are to keep the spinning fin can in place and to hold the rail guides. With everything assembled, the fins section spins (note blurred fins!). The upper air frame section for this first stage just slides over the top of the air frame section.

What I need to do to complete this section is to do the fillets and tip to tip carbon on the fins, add a key of some sort to the fin can so that it can't spin while the rocket is on the rail, and I suppose I should paint everything black to match the rest of the rocket.

This isn't a very pretty booster, but I think it is going to work just fine.

Jim

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Looks great Jim! Can't wait to see it fly.

P.S.
If you have issues with the tiles cracking due to edge pressure from the bag. Another option is to place blocks around the perimeter of the layup inside the bag, it will help keep stress off the edges and corners. With a big stack I usually use teflon blocks with a rounded edge at the bag interface. I have also used 1/2" thick aluminum plate that has been flycut and surfaced, works well.
 
Looks great Jim! Can't wait to see it fly.

P.S.
If you have issues with the tiles cracking due to edge pressure from the bag. Another option is to place blocks around the perimeter of the layup inside the bag, it will help keep stress off the edges and corners. With a big stack I usually use teflon blocks with a rounded edge at the bag interface. I have also used 1/2" thick aluminum plate that has been flycut and surfaced, works well.

The tile that I cracked had a 4" unsupported area on each end. I'm not sure why I thought this would work. With the 1" space for the current fins, there was not a problem.

I do have plans to laminate some fins for another flyer sometime over the next few weeks. This will require a spacer, as there is a significant amount of unsupported area. Most likely, this will happen before Hearne, but if not, the aluminum plates sound great if they are at least 1' by 2'.

In the meantime, progress continues on the spinning fin can. Fillets and tip to tip have been applied. I just need to "finish" the carbon and this part of the project will be completed - months earlier than I expected.

Jim

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Do you know how fast the fin can will be spinning?


Also, spinnerons (I've typically seen them called rollerons, but both refer to air driven gyroscopes mounted to control flaps.) do counteract spin, but they (slightly) amplify any changes in the angle of attack. I'll draw a diagram if my words don't make enough sense, but since the gyroscope maintains orientation in space, if the rocket tilts, they try and align to the prior position, which causes the air flowing by be deflected opposite of the movement of the aft end of the rocket (matching the movement of the rollerons) which pushes the rocket in the direction its movement.

That didn't make sense. Okay. Imagine a rocket moving up vertically. Then if tilts right and the aft moves left )relative to the CG). So the rollerons try to maintain orientation and stay vertical, which, since the rocket is now moving slightly right, that they intrude more into path of air. So then if you imagine the direction the rocket is traveling as up, the rollerons are tilted to the right and deflect air in that direction. This pushes the fins left and since the rocket revolves around its CG, tilts the rocket further toward the right.

However, they counteract tourque since when the rocket rotates, the fin is moving different amounts depending on the distance from rotational axis, and that coupled with the 45° hinge in a way I can't put into words right now makes the rolleron swing in the direction of the rotation, counteracting it. (The 45° angle is not necessary but they did tests and found it most efficient.)


On another side note, I considered a spinning fin can design, before seeing this part of the thread, but the logistics scared me away. I don't know how much the forces are and that determines minimum lubricant viscosity. Too much viscosity loses energy to friction and imparts some spin to the rest of the rocket, but too little causes brinelling and friction and heating. Precision of the bearings matters as well, but I don't know how precise they have to be. Bearings are measured by the ABEC scale, which goes in whole number levels 1-9, with a higher number meaning more precision. And cost. (It could also mean that the manufacturer slapped a false label on, so be wary) I do know that the rollerons on the sidewinders were ABEC 7 though.
 
Do you know how fast the fin can will be spinning?


Also, spinnerons (I've typically seen them called rollerons, but both refer to air driven gyroscopes mounted to control flaps.) do counteract spin, but they (slightly) amplify any changes in the angle of attack. I'll draw a diagram if my words don't make enough sense, but since the gyroscope maintains orientation in space, if the rocket tilts, they try and align to the prior position, which causes the air flowing by be deflected opposite of the movement of the aft end of the rocket (matching the movement of the rollerons) which pushes the rocket in the direction its movement.

That didn't make sense. Okay. Imagine a rocket moving up vertically. Then if tilts right and the aft moves left )relative to the CG). So the rollerons try to maintain orientation and stay vertical, which, since the rocket is now moving slightly right, that they intrude more into path of air. So then if you imagine the direction the rocket is traveling as up, the rollerons are tilted to the right and deflect air in that direction. This pushes the fins left and since the rocket revolves around its CG, tilts the rocket further toward the right.

However, they counteract tourque since when the rocket rotates, the fin is moving different amounts depending on the distance from rotational axis, and that coupled with the 45° hinge in a way I can't put into words right now makes the rolleron swing in the direction of the rotation, counteracting it. (The 45° angle is not necessary but they did tests and found it most efficient.)


On another side note, I considered a spinning fin can design, before seeing this part of the thread, but the logistics scared me away. I don't know how much the forces are and that determines minimum lubricant viscosity. Too much viscosity loses energy to friction and imparts some spin to the rest of the rocket, but too little causes brinelling and friction and heating. Precision of the bearings matters as well, but I don't know how precise they have to be. Bearings are measured by the ABEC scale, which goes in whole number levels 1-9, with a higher number meaning more precision. And cost. (It could also mean that the manufacturer slapped a false label on, so be wary) I do know that the rollerons on the sidewinders were ABEC 7 though.

I don't really expect the "spincan" to be spinning all that fast, if at all. From my videos, it appears that a little roll occurs and the canards try to correct this, and that starts the control reversal problem. If that correction is instead met with a turn of the spincan, then it might stop there.

My spincan design is dirt simple. There are about 160 ball bearings (5/64") sitting between the faces of two fiberglass tubes. For making one, it helps to be able to roll tubes to specific diameters. It is also important that the faces of the tubes in contact with the bearings be very square. I published the laser method that I use a few years ago, and that gets the surfaces square enough.

I've tried putting as much weight as I can on the bearings, and so far, I cannot detect any change in how the bearings work. That is, rotation seems to remain almost frictionless. I expect about 100 pounds of force at the most during the flight, and I should be able to figure out a way to test with that load. I expect it to work though.

I saw another guided flight in the last week where I suspect that control reversal occurred (the jury is still out on that one pending more review of the data). If this problem proves to be the norm, then a spincan might be something of a requirement for forward canards for higher velocity flights. I would like to be able to show a simple approach that works.

Jim
 
I decided to make a short video on the design of my spincan on the premise that it may not be possible to do it after the flight this fall. I think the spincan concept maybe anywhere from helpful to essential for those working on active stabilization, and the purpose of the video is to show that it isn't all that difficult.

https://youtu.be/CtMtY_vSzXQ

The spincan in the video will be the first stage of my three-stager for Balls. I think it will work.

Jim
 
Thinking out loud.....Why don't you need bearings at the top of the fun can also?
Won't the can bind on the top spacer?
 
Thinking out loud.....Why don't you need bearings at the top of the fun can also?
Won't the can bind on the top spacer?

If I was a rocket scientist, I could probably calculate this, but my feeling is that the fin section will stay pressed down on the bearings through the flight. Another way of saying this is that the fin section won't "drag separate" during the short coast period (the flight with these fins involved is 3 seconds of boost and 3 seconds of coast). I base this on the relatively low mass but high drag of the fins. If that did happen, it's possible that there would be some additional friction, but I think the fin can would still spin, as the force trying to turn the fins is quite high.

This video best shows the problem that I'm trying to correct. Note the counter spin during the boost that goes away as soon as the booster is separated.

https://youtu.be/Abod_JBxnM8

Jim
 
Spincan, I like that.


Since this is a rotating component with "vanes" intended to stabilize the vehicle's flight, Can this now be referred to as a Turbo-Stabilized rocket?
 
Amazing work Jim, in the video, what tube came out of the sustainer a few seconds after booster separation?

The stabilization looks perfect, instant.
 
Amazing work Jim, in the video, what tube came out of the sustainer a few seconds after booster separation?

The stabilization looks perfect, instant.

Yes, that's the stabilization spool. In an actual flight, the second stage motor would light at the moment the spool departs. In the test flight, there's no second stage motor (just 20 pounds or so of congealed BB's simulating the motor weight).

Jim
 
In the last stabilization flight...

https://youtu.be/Abod_JBxnM8

..., the sustainer stabilized with respect to roll once the booster fell away. At that point, both sets of canards were at their maximum deflection (7.5 degrees) for yaw/pitch. Even so, however, the rocket didn't turn very fast. Based on the video, I calculated something like a 15 degree turn over a 10-second period. I don't really want the rocket to turn very fast, but that's not fast enough.

I wish I had the capability to calculate all of the factors that go into quantifying this problem. But I don't. So, I have to rely on feel (and on the assistance of the folks here at TRF). The configuration of my three stage rocket, at the time guidance is required, is shown in the pic. The distance between the canards and the CG is 57". On the test rocket, it was 34". On the other hand, the three stager will weight roughly twice as much. Do these more or less offset for a given turning force? The test rocket had a static margin of 1.6 calibers whereas the three stager margin is 3.3 calibers. I guess that will work against me?

Currently, the maximum deflection of the canards is 7.5 degrees. A rocket angle of 7.5 degrees will produce a 7.5 degree deflection in the canards. At higher rocket angles, however, the canard angle is capped at 7.5 degrees to avoid stalling the canards. I suspect this maximum angle could be a little higher.

Another thing that I can do is to increase the size of the canards. The version used in the test flight was pretty small due to concerns about overwhelming the servos. I haven't seen any indication of instability in the canards in any test flights to date, including flights where the canards were larger, so I have built a slightly larger set (they have about 50% more total area). The first pic of the canards shows the old and the new shapes. I think they will be OK, but there is obviously a point at which they will become too big.

Roughly, the canards will be controlling roll at speeds up to 1,100 ft/s and then controlling yaw/pitch as the speed drops from 900 to 700 ft/s.

Thoughts appreciated.

Jim

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Jim, first question- you have not flown your ball bearing fin can yet, right? That is something I would love to see before the full flight, but not sure if that is feasible?

My thoughts are along the line of increasing the maximum deflection from 7.5 degrees to 10 degrees. Even then, considering the size of the fins relative to the rocket it may not have enough command authority to over come the roll. The larger canards might help there too.

Any way to tell how close to their torque limits the Servos are now? As you said, they have shown no fluctuations etc.
 
Jim, first question- you have not flown your ball bearing fin can yet, right? That is something I would love to see before the full flight, but not sure if that is feasible?

My thoughts are along the line of increasing the maximum deflection from 7.5 degrees to 10 degrees. Even then, considering the size of the fins relative to the rocket it may not have enough command authority to over come the roll. The larger canards might help there too.

Any way to tell how close to their torque limits the Servos are now? As you said, they have shown no fluctuations etc.

I just don't see a test of the fin can before Balls. It has to fly on a 98mm motor and I don't have one that would provide a flight in the correct velocity range. If Balls is cancelled, I'd figure out how to do a flight before the next Balls. On the other hand, flying to just under Mach, I think the worst case is that it doesn't spin. If that happens, it's no worse than fixed fins.

I was considering 10 degrees. Actually, the canards are plenty large for roll control. It's yaw/pitch where they seem limited.

I'm sure it is possible to calculate the CP of the fins and the torque along a specified axis. I have some chance of solving that problem, and the result could be compared to the servo specs. I have done test flights with larger canards, and their present shape is the result of several "trimming" sessions. They would not have flown quite as fast though.

Jim
 
Hi Jim. I wouldn't be concerned about the fins stalling. All that means is that the drag goes through the roof. Remember you still get lift, and the fins are not that big. If you need the lift for control moments then it is available as long as your servo still has control and is not pegged at the limit.

The other limit you have is the strength of the fin construction. As long as it can take the maximum bending moment from the forces at max AoA without snapping you should be good to go.

Look up the lift for a flat plate. You get lift to quite a high AoA, but the drag goes up significantly at the stall angle and higher.

FYI, optimum maximum deflection if you are worried about supersonic drag is where one face of the front diamond is parallel to the air stream.

You are making great progress! Mine is stalled at systems integration currently :(.
 
Hi Jim. I wouldn't be concerned about the fins stalling. All that means is that the drag goes through the roof. Remember you still get lift, and the fins are not that big. If you need the lift for control moments then it is available as long as your servo still has control and is not pegged at the limit.

The other limit you have is the strength of the fin construction. As long as it can take the maximum bending moment from the forces at max AoA without snapping you should be good to go.

Look up the lift for a flat plate. You get lift to quite a high AoA, but the drag goes up significantly at the stall angle and higher.

FYI, optimum maximum deflection if you are worried about supersonic drag is where one face of the front diamond is parallel to the air stream.

You are making great progress! Mine is stalled at systems integration currently :(.

Good input. Thanks!

Jim
 
Any update on launch at black rock?

Sent from my SM-G950U using Rocketry Forum mobile app

Sorry to be slow responding. We just got back home tonight.

The flight wasn't successful. It turns out one of the stabilization system servos went "bad" right at motor burnout. I checked the offending servo tonight and it indeed has more "play" than it should (and more than the other three servos). I don't know whether this happened before, during or after the flight - all are possible - but you can clearly see where the servo goes bad in one of the flight videos. Prior to that, the system was doing a good job of roll control, and the spin can was working as intended.

Obviously, I'm very dissapointed in the outcome, but the flight will generate a lot of helpful information. Everything recovered fine. I'll post more information and videos when I have a chance.

Jim
 
Shoot but great that the hardware recovered OK. Fix the dern thing for the future. Kurt

So, I had a servo fail in the Balls flight, and it's time to fix the dern thing. You can see it fail in the video. Crap.

https://youtu.be/G-Iwi-UF2ck

Now that I have a replacement servo in hand, it's obvious that I need better servos. The replacements have about 5 times more torque and much less "slop" in the movement. Like em!

There are two issues that need to be managed. First, the new servos are larger and getting them installed in the available space is a challenge. Second, it will be necessary to precisely locate the new servos so that they fit into the axle bearings in the air frame. Both problems are at least partly solved. It turns out that there is exactly enough room to mount the servos. Another millimeter one way or another and I would have to be sanding down the case somewhere.

I have so far been able to reproduce the center mounting plate (with thinner plywood than the original since the servos are wider). This attaches to the air frame and must be positioned exactly so that each servo sits "low" by a few thousands of an inch in the air frame bearings. With this, I can use a few layers of tape under each servo to get the exact elevation. Getting this piece made successfully means that the module can be built - I just have to fabricate it.

Jim

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Maybe you can find a way to put those stabilizers in double shear so a little play doesn't cause a complete failure. Not a lot of room to work with, though...
 
Wow. That was a great video, even though it was explaining a flight failure. Thanks for providing the analysis.


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Good explanation, Jim. See you next year!


Steve Shannon

It's a date! With the pre-training we've been through, we should be able to launch in about an hour, plus or minus. That'll be good, as I'm hoping to have as second project and it won't go as fast.

Jim
 

Well, plus or minus....

We had to do it twice. The first day, we were up against the clock the whole time. We had fun, but it was pretty stressful. The second day, everyone knew what to do, what tools were needed, where stuff went, and many things were done independent of me. It was kind of amazing how smoothly it went. Maybe it took more than an hour, but by Balls 2022, we should have it nailed.

Jim
 
I must say your patience with this project is admirable. I've seen folks get snippy because they didn't have time to launch their 7th rocket for the day....
 
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