L2 low altitude rocket

[techrat]

What is the definition of a "ring"? A circular band. What is the definition of a "tube". A long hollow cylinder. Length to diameter is the defining difference.

Oh, I see. So my fins could be INFINTY diameter, but as long as the length of the fin is Infinity +1, it's now a tube, and not a ring.

 

[Off Grid Gecko]

I think this may end up somewhere in gray territory.. I'm not sure where we look up the specific rulings and precedents for this kind of thing. A couple years ago when I finally joined TRA it seemed there was a lot of discussion about whether aluminum tips on nosecones would be allowed for certain things, whether they were covered under insurance, etc. Now it seems that they're on every other HPR built, and I'm seeing a lot more aluminum in the fin can as well. If I was going to fly anything tube or ring or anything other than 3FNC or 4FNC I would prolly talk to the guy filling out the cert paperwork about it before I ever started planning to use it on a cert flight.

I only brought it up because the OP suggested this was for a cert flight, so people start talking rings and tubes and I start thinking maybe not the best idea.

On another point that was brought up, the hobby is a hobby and different for everyone. If someone wants to build a rocket just for their lvl 2 and fly it once or twice and hang it on the wall I suppose that's their business. My L2 has flown once so far and I have another motor coming for it, but it probably won't be flown every single month. I like fast skinny rockets that push the envelope a little bit, put on a good show, and will be testing trackers with my current build which is all lvl2 hardware, but I would not want to fly my current rocket for a lvl 2 cert. It's pushing altitudes that are too high for that and with our flying field it's prolly going to sit around waiting for a break in the wind before it ever goes up. But when it does it should be a good show.

A blanket statement like "don't build a rocket just for a cert flight" ... I get what you're saying. Maybe I just don't like blanket statements.

 

[jmasterj]

Tube fins will survive transonic, ring fins won't. At what length does a ring become a tube? Not sure, but I suspect it's when the length of the tube fin exceeds the diameter of the tube.

The recommendations here and elsewhere for L2 flights don't come close to transonic, so that can't be the reason.

 

[boatgeek]

Just wondering, would a ring fin be appropriate for a cert flight, or does that fall under "non-standard" designs?

It falls under a non-standard design.

@Steve Shannon Could you weigh in on whether ring fins would be allowed for a TRA cert flight? I would be surprised if it was considered "non-standard" enough to be disallowed, but I've been surprised before.

 

[DaHabes]

Oh, I see. So my fins could be INFINTY diameter, but as long as the length of the fin is Infinity +1, it's now a tube, and not a ring.

Fins cannot be of infinite diameter, that is nonsensical, and infinity+1 = infinity, so no, they cannot be infinity+1. Math 101...

 

[Steve Shannon]

@Steve Shannon Could you weigh in on whether ring fins would be allowed for a TRA cert flight? I would be surprised if it was considered "non-standard" enough to be disallowed, but I've been surprised before.

I don’t know of any reason why not. Same for tube fins.

 

[Off Grid Gecko]

Fins cannot be of infinite diameter, that is nonsensical, and infinity+1 = infinity, so no, they cannot be infinity+1. Math 101...

What if they're straight??

 

[techrat]

Fins cannot be of infinite diameter, that is nonsensical, and infinity+1 = infinity, so no, they cannot be infinity+1. Math 101...

Perhaps I was talking about the Car line, Nissan's luxury brand... I could rephrase it as "anynumber", but that wouldn't be nearly as funny as Infinity.

 

[techrat]

What if they're straight??

That just means they don't date other guys.

 

[tsmith1315]

I don’t know of any reason why not. Same for tube fins.

As long as it's simple enough that CP can be predicted accurately? This is a flight of proof, that prediction needs to be dependable.

What if they're straight?

That just means they don't date other guys.

Pretty sure they won't mate with other tubes, so that wouldn't work out.

 

[Steve Shannon]

As long as it's simple enough that CP can be predicted accurately? This is a flight of proof, that prediction needs to be dependable.

For a cert flight the flyer is required to be able to indicate where the Center of Pressure is, regardless of fin type. If in doubt I would ask how CP was determined but I’d probably be less apt to ask that for a ring fin. The ring adds to stability by shifting CP back even further than traditional fins.
The same common sense rules of design are required for ring fins as traditional fins, i.e. don’t put them in front.

 

[ThirstyBarbarian]

What is the definition of a "ring"? A circular band. What is the definition of a "tube". A long hollow cylinder. Length to diameter is the defining difference.

The difference between a ring fin and a tube fin is that a ring fin is a single ring that is a larger diameter than the body and goes around the body tube with the body centered in the ring, while tube fins are sections of tube that are attached to the outside of the body tube the way regular flat fins are, glued to the surface, not supported concentrically.

The difference has to do with whether the section of ring/tube is concentric with the body or attached to the body non-concentrically. You could make a tube fin rocket with tubes short enough to be considered rings, or a ring fin rocket with a ring long enough to be considered a tube.

 

[ThirstyBarbarian]

The simplest way to make a rocket with lots of drag is to pick a large diameter, not odd surface treatments or other unusual designs.

Rings and tube fins are probably relatively easy to make, but not so easy to sim. Surface treatments might work, but would also be harder to sim.

Diameter is pretty straightforward. If you double the diameter of a rocket, then the area of a cross section is quadrupled. It goes up by the square of the radius. That adds up fast. The area of the body tube goes up by a multiple of the radius — double the diameter = double the surface area of the tube.

If you want a simple to make, simple to sim rocket, make a standard design large-dimmer rocket.

 

[JackC]

For a cert flight the flyer is required to be able to indicate where the Center of Pressure is, regardless of fin type. If in doubt I would ask how CP was determined but I’d probably be less apt to ask that for a ring fin. The ring adds to stability by shifting CP back even further than traditional fins.
The same common sense rules of design are required for ring fins as traditional fins, i.e. don’t put them in front.

Years ago, I certified L3 with a ring fin rocket Search Google- EMRR Galadriel -

Several years later, my TAP told me my rocket would not pass muster because it would be considered an odd-roc. If you use the trick in Rocksim or OpenRocket to simulate the ring fin by using six fins that are square and the height of your ring, you should get an accurate CP for it. If up to me, I would be fine with a ring fin under TRA or NAR cert rules.

 

[techrat]

If you use the trick in Rocksim or OpenRocket to simulate the ring fin by using six fins that are square and the height of your ring, you should get an accurate CP for it. If up to me, I would be fine with a ring fin under TRA or NAR cert rules.

I've been out of this hobby for so long that perhaps I'm not up on all the ins and outs, but is that what's holding things back?
Are the only rockets allowed to fly for a cert "simm-able"? Would I have to "sim" a rocket to get a cert?

 

[Zeta]

everything and the kitchen sink .... what was the OP again ...LOL

 

[Steve Shannon]

Years ago, I certified L3 with a ring fin rocket Search Google- EMRR Galadriel -

Several years later, my TAP told me my rocket would not pass muster because it would be considered an odd-roc. If you use the trick in Rocksim or OpenRocket to simulate the ring fin by using six fins that are square and the height of your ring, you should get an accurate CP for it. If up to me, I would be fine with a ring fin under TRA or NAR cert rules.

The odd roc rule was intended to prohibit the use of things like spools and pyramids. Ring fins are not prohibited at any level. I even sent a note as a sanity check to the TAP chair.
A ring fin rocket requires just as much attention to structural detail as a 3 FNC. After all, a ring fin rocket is basically a 3 FNC with a ring attached to the ends of the fins.
As a TAP and L3CC I would have no problem signing off on one.

 

[techrat]

As a TAP and L3CC I would have no problem signing off on one.

OK, I think this is settled then -- so for a L2 attempt, a LOC IV modded with a Ring Fin would be acceptable.
The ring fin would add more than enough drag to keep the flight low and slow, but enough to make an L2 cert flight acceptable.

 

[JackC]

The odd roc rule was intended to prohibit the use of things like spools and pyramids. Ring fins are not prohibited at any level. I even sent a note as a sanity check to the TAP chair.
A ring fin rocket requires just as much attention to structural detail as a 3 FNC. After all, a ring fin rocket is basically a 3 FNC with a ring attached to the ends of the fins.
As a TAP and L3CC I would have no problem signing off on one.

I agree. As a L3CC, I would have no problem signing off on one.

 

[JackC]

I've been out of this hobby for so long that perhaps I'm not up on all the ins and outs, but is that what's holding things back?
Are the only rockets allowed to fly for a cert "simm-able"? Would I have to "sim" a rocket to get a cert?

For L3 certification, yes you would need to sim the rocket and show the results to the TAP or L3CC. Those should be in your certification document.

For L1or L2, you don’t need to do it but it’s a good thing to do. Since most cert rockets are first flights, the simulation is the only way you can project what your flight characteristics will be. Also, a certification officer may ask for it, so you should be prepared.

 

[JackC]

A great thing about a ring fin for a cert is the rocket goes straight up and comes straight down. I flew a M1400 in my ring fin and it flew to 11,111 feet (I love that number) and landed about 75 yards from the launch pad. At times it landed maybe 10 yards from the flight line. For a cert, it makes recovery a breeze!

 

[Off Grid Gecko]

A great thing about a ring fin for a cert is the rocket goes straight up and comes straight down. I flew a M1400 in my ring fin and it flew to 11,111 feet (I love that number) and landed about 75 yards from the launch pad. At times it landed maybe 10 yards from the flight line. For a cert, it makes recovery a breeze!

Maybe I should add a 12" ring to my DX-3 just for the funzies.

 

[ThirstyBarbarian]

I've been out of this hobby for so long that perhaps I'm not up on all the ins and outs, but is that what's holding things back?
Are the only rockets allowed to fly for a cert "simm-able"? Would I have to "sim" a rocket to get a cert?

It’s not required, but the original goal expressed by the OP was to keep the flight to 1,500 - 2,000’, and you aren’t going to know that without a sim. My feeling is that if you take a LOC IV, spray it with truck bed coating or flock it like a Christmas tree, there’s no way you are going to know what it will do, and I’m guessing it will go to more like 4,000’ on a J, not 2,000. How would you know?

I’m also not sure the LOC IV with a ring fin added would keep it under 2,000’ either. I don’t know much about that or how to sim it. It would not be my approach. And if I were asked to be a witness on the flight or sign the paperwork, I’d suggest the flyer work with someone else.

My feeling is that if you want to certify for L2, you should build an L2 rocket, not try to hack your L1 rocket for L2 with gimmicks.

 

[Zeta]

One of the senior guys in our club once told me that he puts a "skirt" on the rocket when he needs to keep the flight lower ie: low cloud deck. Hence there is no modification the physical rocket.

 

[Alan15578]

Do you know where this aspect ratio is/was documented? IE, in a book, from a website etc? Need to source it in a report for class

thanks

Ya know, it is perfectly acceptable to credit data and ideas form credible people on the internet. Steve Shannon is more authoritative than most. TRF is not a peer reviewed journal, but most of the regulars are degreed professionals who will chime in with a correction, clarification, extension, or exception, when something is posted that is not quite right. It is also better to credit "some guys I eat lunch with" than to plagiarize an idea as your own.

Many things are such common knowledge that there is no need to attribute a source. The 10/1 fineness ratio is commonly mentioned and I think it can even be found in the old technical section of the Estes catalog. Engineers do a lot of trade studies and optimizations to determine things. Some things are determined by practical matters such as the size of underpasses, tunnels, and other logistical issues. Even without the underlying engineering analysis, you can often get a close enough starting point by simply doing a survey of previous designs and looking at where various parameters tend to fall.

 

[Hobie1dog]

I really dig the shag carpet treatment suggested by @BDB

now all I can think of is Dumb and Dumber with him wearing that dog suit.

 

[Tractionengines]

So what would you call the Celtic Thunder.... ring or non standard tubes...

 

[JackC]

So what would you call the Celtic Thunder.... ring or non standard tubes...
View attachment 540821

I would say tube fin. Ring fins have supports that go through the wall from the motor mount to the inside of the ring.

 

[MetricRocketeer]

I flew a Warlock for my L2. It went to around 2,000’ on a CTI J290. The Doorknob is another great L2 rocket. It’s basically a longer version of the Warlock and will stay lower on the same motor.

Hi @ThirstyBarbarian and anyone else,

I am not saying your are wrong about the Warlock's and the Doorknob's being basically longer and shorter versions of each other.

However, could you please explain that statement in a little greater detail. Yes, LOC/Precision produces both rockets, each of which has the same diameter, and the shape of the fins is the same in both kits. On the other hand, the Doorknob has four fins while the Warlock has three. Moreover, the Doorknob has two body tubes and the Warlock has only one.

Aren't those important differences? Maybe they're not. I am asking a genuine question. I don't know, but I am curious as to your opinion.

Thank you.

Stanley

 

[jmasterj]

Hi @ThirstyBarbarian and anyone else,

I am not saying your are wrong about the Warlock's and the Doorknob's being basically longer and shorter versions of each other.

However, could you please explain that statement in a little greater detail. Yes, LOC/Precision produces both rockets, each of which has the same diameter, and the shape of the fins is the same in both kits. On the other hand, the Doorknob has four fins while the Warlock has three. Moreover, the Doorknob has two body tubes and the Warlock has only one.

Aren't those important differences? Maybe they're not. I am asking a genuine question. I don't know, but I am curious as to your opinion.

Thank you.

Stanley

I don't think those are particularly big differences, besides the length. The bigger difference, of I remember correctly, is that the Doorknob has LOC's modular motor mount system and the Warlock does not.