Finless ?

[Zippy]

I want to build a model based on a 3.5" tube, about 40-50" (haven't done the scale math yet) tall. It would have NO FINS becouse it's going to be a scale rocket and I can't bring myself to attach clear fins no matter how hard I try. Motors would be likely to be two 29mm H's. There will be engine bells that should act like minimalist fins but I'm not counting on that. For those of you that have flown finless (succesfully that is) before...Will I need to measure nose weight in ounces or pounds ? How many calibers of stabillity or static margin do you think I'll need ? Should I count down from ten then yell "Hit The Dirt !" before I press the button ?

 

[Elapid]

so i can be far away.

why not use clear lexan for fins?
if you look at the position of the CP on a finless rocket, i would think it would be mid-tube...add motors at the bottom end, and your CG is going to be waaaaaaay too far back...

good luck!

 

[Elapid]

Originally posted by Zippy
Will I need to measure nose weight in ounces or pounds ? How many calibers of stabillity or static margin do you think I'll need ? Should I count down from ten then yell "Hit The Dirt !" before I press the button ?

i hope you can answer all your questions before you decide to push the button.

 

[Stymye]

I'll take a guess,
it can be made to fly no problem, with considerable nose weight
like a bottle rocket
the trick is making it fly in a predictable direction.
as there will be no (or very little) restoring force.
so even the slightest thrust or wind deviation will have a great effect on it's trajectory.
if one motor fails to ignite it would probably cartwheel like a matchbox in the wind

 

[rstaff3]

There was a finless rocket at LDRS a couple of years ago IIRC. The strap ons, like you said, acted like 'minimalist' fins. With motors in the back, some 'fins' will be required. I'd run a sim on it using Rsim 7. If you don't have it post more specs and teflon will undoubtedly help you. I have used his techniques but would have to download the right apogee newsletters to remind myself how to do it.

 

[Zippy]

why not use clear lexan for fins?

I really hate the idea of clear fins on a scale rocket, on a R2D2 with a motor shuved up it's bottom it's okay though.

I'd run a sim on it using Rsim 7

I'll eventually do that with the demo version but I don't have enough specs yet other than it being a 3.531" diameter cardboard shipping tube with 1/8" walls. The rocket by the way is a Titan II either with the Gemini capsule or if I screw the turning up a nuke. I'm slateing this as the next HPR project after the one I'm building right now so I have plenty of time to figure it all out.
.

if one motor fails to ignite it would probably cartwheel like a matchbox in the wind

It will be going up off a rod with nearly 7' of usable length so if it goes unstable it should be high enough at the time to make for a spectaculer but safe cartwheel. Hopefully that is. Maybe I'll add some wire to my controller just to be sure it's at a safer distance.

 

[Chuck Rudy]

Originally posted by rstaff3
There was a finless rocket at LDRS a couple of years ago IIRC. The strap ons, like you said, acted like 'minimalist' fins.

That 'finless' rocket was at LDRS this year, it went home in a bag as a bunch of little parts, and it wasn't a motor malfunction, the rocket ripped itself apart all on it's own.......but it was spectaculor. I've got video of it around somewhere but it'll be winter before I get to it. 8-(

Chuck

 

[rstaff3]

The one I saw was in a rocket mag so it wasn't this years LDRS for sure. There was a pic of the boost, so at least the first part of the boost was stable. I don't remember the final outcome. It could have been a SpRocketry vs HPR mag.

Good luck Zippy, sounds like it might be a questionable flight. Depends on the motor pods but having a cluster also may be asking for trouble. BUT I've made a few questionable flights myself as I like to try odd stuff out. HPR launches are great for this as they can banish me to the far away pads

 

[Zippy]

HPR launches are great for this as they can banish me to the far away pads

The pad will be placed exactly... as far out as I'm willing to carry it into the field. I'm thinking about 50' worth of wire with me at the end and then another 50' or so to the normal flight line. It's an informal launch with probably no more than a handfull of other rocketeers there.

 

[rstaff3]

Perfect, a small launch without lots of kids everywhere is a good choice. Pics, man, pics!

 

[artu]

I've seen a few deltas fly sucessfully without fins.

Two of them were flown at naram-45.

 

[marvSRG]

I saw a finless rocket with boosters used as "minimal fins" in a SpRocketry article a few years back...it was a Delta II at NSL in Geneseo. I think it went on an I300T. AFAIK, it worked fine.

I tried a finless rocket with spin motors and much noseweight...unfortunately the spin rockets ripped themselves from the body (they were A10's) after giving it quite a spin, and one of the central 2 C6's didn't light. The result was an incredible spin but no lift (one C didn't have the power to move it). I just have to replace the spinners, though. I'll cant them upwards next time.

 

[Zippy]

Just playing with the RocSim demo I slapped togethar a quickie version of the rocket and got marginal stabillity with 6 lbs. of nose weight. 700 ft. on two H128's. I only modelled one engine bell and badly at that so I think I can get more stabillity with a little less nose weight. It looks doable so I'll probably start on it in about two months. That will give me plenty of time to get all that lead off my neighbors tires.

 

[DynaSoar]

Originally posted by Zippy
Just playing with the RocSim demo I slapped togethar a quickie version of the rocket and got marginal stabillity with 6 lbs. of nose weight. 700 ft. on two H128's. I only modelled one engine bell and badly at that so I think I can get more stabillity with a little less nose weight. It looks doable so I'll probably start on it in about two months. That will give me plenty of time to get all that lead off my neighbors tires.

Consider what you can do to make the engine bells act differentially on the air flow according to the angle of attack. What sort of surface might add to their stability figures?

Also, you might be able to get some spin stability by canting the engines. That doesn't necessarily mean canting the bells, so your cosmetics would remain intact.

Both Gemini and MIRV noses will be very draggy and make stability problems worse. Consider trying the satellite shroud. See www.ast.lmco.com/SSC/ news/2003/oct1803.shtml for an example. It looks very Delta-ish.

The Titan II gave off a really nifty orange smoke. If you get this thing off the ground, consider placing a pan of orange marking powder under your blast deflector. I got to put stuff in them but never got to see one fly except on film.

 

[jetra2]

Zippy,

If you don't want to add 'fins' in the sense of normal fins, then how about a clear bowl at the end that provides enough pressure to move the CP back to a reasonable position so you don't have to add 6 POUNDS of noseweight.

Like THIS or THIS

Of course, the bowl would be clear.

Jason

 

[slim_t]

Basically just plan on a bit more weight than the motors.
I have flown a simple 36" long BT20 with nose cone finless on 18mm motors. For a straight tube the Cp is near the midpoint of the rocket, so the nose will just need to weigh more than the motor. Of course how much more depends on the distance you want between your Cp and Cg. And as mentioned before, a small amount of wind could have a larger effect than with a finned rocket.

I have a small (about 12" tall) Titan III that I've built to fly finless on a 13mm motor. I put the Cg near the top of the side boosters. I haven't flown it yet, but will soon. I'll let you know how it goes.

Of course these may not be of much signifigance compared to the size your planning, but the principle's the same. But a failure would be a bit scarier in your case. I would plan on a good deal of nose weight.

Tim

 

[rstaff3]

By any way I know to calculate CP the BT does not affect the CP, other than positioning other elements such as NCs, fins, etc. For a NC on the top of a tube the CP is somewhere in the NC itself. I'd like to see an explanation of how a rocket with no fins and an engine in the rear can be stable.

 

[Zippy]

In my quickie RocSim the CP is right inside the nose cone. If I remember right it moved back into the body a bit when I added the single bell. It was just a stubby conical nose cone. The engine bells will be as close to scale in appearence as I can get but pretty draggy plus all the struts and what not add more drag. In RocSim I just used a small transition for the bell. If I really, really have to I might make some sort of detachable fins or even a bowl araignment but thats an absolute last resort. Maybe some really small fins just to get the weight down.

 

[Silverleaf]

I truly think that a long body with no fins will work, if you offset the motors with sufficient nose weight, or barring that, a payload section with sufficient weight.

Truth of the matter is, Rocksim the heck out of it, and then build a scale version of it using a set of A's as your power source so you can see just how it will fly. Cheaper and easier that way before you invest all the time in building the big one and have something major go wrong.

Cheers,

 

[Ozymandias]

Speaking of finless rockets I used to have a rocket called the Drag Demon. It used a conical shroud made from a plastic funnel for stability. It didn't fly very high but it did fly stable. I won the NAR Junior design contest for that design too. I would use some clear polycarbonate fins on your rocket. It's better to be safe than sorry. It will still look like it has no fins to the spectators. You don't want to give the RSO a heart attack do you?

 

[Stymye]

I threw together a "very" rough outline just to see where rocsim puts the cp..the shape of the engine shroud had a dramatic effect

heres a screen shot

 

[slim_t]

Just got through launching a couple of rockets.
That's always a pick me up.

Back to the discussion. About Rocksim saying the Cp is in the NC.
This can't be correct. I tried this too a while back and just figured Rsim won't calculate it correctly unless you put fins on it. The BT absolutley affects the Cp. If not then why would longer rockets be more stable, other than shifting the Cg? Longer rockets also shifts the Cp because of the added area of the BT.

Try this: In Rsim design the BT with a NC. You'll have the Cp showing in the NC. Then add some really small fins, like toothpicks. I'm guessing the Cp will shift back alot. Now we all know that toothpick size fins will not have that much affect on the Cp, so what happened? My guess again is that Rsim just isn't programmed to deal with finless designs.

Cp is basically just the center of the surface area, or skin, of the rocket. It shows the centroid of the force that wind will apply to the rocket. Having a Cg above the Cp provides more surface area behind the Cg and therefore more force from the wind and a stabilizing effect. The only reason it's complicated is the symmetry of the rocket.

Tim

 

[slim_t]

Sorry for the long post.
Now for the supporting evidence to my thesis above.

Here is what I just got through flying.

This is a 34" Bt 20 with a NC. The NC is full of lead weights and clay. The Cg is about 2 inches above the midpoint of the rocket.

Tim

 

[slim_t]

We have ignition....

 

[slim_t]

....and liftoff....

 

[slim_t]

a bit farther up and still stable

There was barely any wind at all. This was on a B4-4 to about 150' and landed about 50' from the pad under a 10" chute.

 

[slim_t]

Next up was the Titan.
I built this from a cardstock model and simply installed a 13mm MMT in the main body.
The nose is loaded with lead weights and clay. The Cg is just at the base of the side booster's NC.
Here's a static photo.

Tim

 

[slim_t]

and here she is at ignition...

 

[slim_t]

...liftoff was quick, but you can see the smoke trail is straight.

This flew on an A10 to only about 100', so you can tell it's a bit heavy. It came down safely on a 10" chute.

 

[slim_t]

I took this after the flight to give an idea of it's size and to show where the nose seperates.

Tim