Finless ?

[rstaff3]

I'm not surprised about the Titan or a rocket with a conical nozzle, but a straight finless rocket does surprise me. Barrowman says the tube doesn't contribute to stability, and that's what Rsim is showing. In a quick look, it doesn't take much fin to move the CP a lot in a long skinny rocket. I guess if it worked it worked. I doubt in many RSOs would let it fly tho

 

[Stymye]

Rstaff,
For whatever reason, rocsim will only display the cp of the nosecone untill a fin(s) is installed. now If you just draw a bodytube the cp is obviously in the center, just a quirk of rocsim

so you have to add atleast a very tiny fin to the body tube to get rocsim to display a finless designs' cp more correctly.

 

[rstaff3]

It seems with the CP of the tube being in the middle and the NC CP moving the overall CP forward some that you'd need the CG ahead of the tube center by some distance. I guess this makes some sense.

 

[slim_t]

Yeah, I doubt an RSO would allow it. I know I wouldn't at a public launch. I've only flown these when I'm alone.

Also, I chose small rockets and small motors, for a reason. I was just testing my ideas, and was hoping if something went wrong, the motor would not burn long enough to do any damage.

I would be a lot more cautious in a mid or high powered rocket, because if something were to go wrong it would be at a much larger scale. So please know where your your Cg and Cp are, and consider it experimental and take the appropriate safety measures.

Hey, maybe we should let the guys at Apogee read this and see if they can make some changes to Rsim for the next update.

Tim

 

[slim_t]

For my long finless one, I actually found the midpoint of the entire length including the nose cone, and then put my Cg 2 inches above that. The would actually make it more than 2 inches above the actual Cp.

 

[marvSRG]

Cool stuff. Makes me wanna try it!

 

[Zippy]

Here's what I'm getting with a better Rsim. It's a dramatic improvement. Still one engine bell but I think two smaller ones will give near the same result. It also can go up to 1000' on only G64's.

If the image doesn't load try refreshing or something, Verizon's web page problems persist.

 

[rstaff3]

Originally posted by Zippy
Here's what I'm getting with a better Rsim. It's a dramatic improvement. Still one engine bell but I think two smaller ones will give near the same result. It also can go up to 1000' on only G64's.

If the image doesn't load try refreshing or something, Verizon's web page problems persist.

Sweet, can't wait to see it!

 

[rstaff3]

Originally posted by slim_t
For my long finless one, I actually found the midpoint of the entire length including the nose cone, and then put my Cg 2 inches above that. The would actually make it more than 2 inches above the actual Cp.

That should do it I guess. Taking the midpoint including the cone probably is close to the actual CP. Probably would have poor dynamic stability. What were the wind conditions when you flew her?

 

[rstaff3]

Originally posted by Zippy
Here's what I'm getting with a better Rsim. It's a dramatic improvement. Still one engine bell but I think two smaller ones will give near the same result. It also can go up to 1000' on only G64's.
If the image doesn't load try refreshing or something, Verizon's web page problems persist.

One thing to watch out for is that Rsim may actually be optimistic about the cone that is most occluded behing the BT. If the airflow goes over too fast the cone may have little effect. I built a rocket out of a large pretzel bin. A 3" tube went thru this and there were real large fins below. Flew fine on low impulse motors but on an H242 it went unstable as it got moving fast. I think the airflow at high speeds actually goes past the fins, reducing their effectiveness.

 

[Zippy]

I can't easily go back into Rsim and get the actuall speed estimates becouse it's only the demo version but If I remember correctly it was only about 160 mph so it's no speed demon with G64's. I can live with the weight being about 5 lbs. including 2.5 lbs. of lead. I might be able to shave another half pound off that becouse the transition I simmed with is a heavy solid urethane one and mine will be hollow built up with card stock and epoxy.

 

[rstaff3]

I have enjoyed this thread a lot, despite my brain fart about the CP issue. I may actually have to build something finless for the nexr HPR launch!

 

[Zippy]

I may actually have to build something finless for the nexr HPR launch!

Go for it! The reason I want to is becouse I love scale rockets and space vehicles get severly limited in modelling opportunities if I don't at least try going finless.

 

[slim_t]

Originally posted by rstaff3
What were the wind conditions when you flew her?

Today was a first for the Titan, and there wasn't much wind at all, just light breezes. The other has flown before but always in little to no wind.

I'd like to see a higher powered finless design myself. Since the Titan worked out well, I might have to build a bigger one someday.

Good luck to those going for it now.

Tim

 

[Zippy]

Today was a first for the Titan, and there wasn't much wind at all, just light breezes. The other has flown before but always in little to no wind.

The wind can be a problem here in Florida near the west coast. Sometimes it's dead calm though and I hope to get lucky with that. Usually I launch LPR in the early part of the day then progressivly work towards HPR. If I get a good wind window I may hurl the Titan II skyward first though. Prep time is usually zero since I do it all the night before and only have to place it on the rail and connect the clips. Of course my last HPR flight made me walk out to the pad like five times before I could get the continuity to stick. So much for timing.

 

[eugenefl]

Originally posted by Zippy
Of course my last HPR flight made me walk out to the pad like five times before I could get the continuity to stick. So much for timing.

OT: Did you ever get the continuity "bug" worked out?

I suppose you and Pat (pdooley) have something in common - he builds rockets that have a tendency to go finless midflight and you're just wanting to go finless from the get-go!

 

[Zippy]

Originally posted by eugenefl
OT: Did you ever get the continuity "bug" worked out?

No, I still don't know what it was. I would have continuity when I started the count down and then nothing. Time after time as you know. It wasn't the ignitor though since I never replaced it and it did eventually work. One of these days I'm gonna grind the teath off my alligator (clips) becouse I suspect they might have something to do with it.

I suppose you and Pat (pdooley) have something in common - he builds rockets that have a tendency to go finless midflight and you're just wanting to go finless from the get-go!

I did enjoy Pat's flights that day. It's just a hobby (no matter how pashionate) so we shouldn't take it too seriously. If my finless goes kaput I hope we get some entertainment value out of it at least. That will make the countless hours of finishing worthwhile since I want it to look at least as good as my SatV. If it survives it might be at least temporarily retired like the SatV.

 

[teflonrocketry1]

Actually the side pods on the Titan rocket behave as fins! You might want to look over my article at: https://www.apogeerockets.com/education/downloads/Newsletter119.pdf

The long skinny finless model is taking advantage of the body lift or boyancy effect on the body tube.

Wide and relatively short finless model rockets take advantage of base drag for stability like a spool rocket.

It seems to me that the effect of a body tube on CP would be easy to take into accout (body lift and base drag). Someone should try to incorporate these effects into a simulation program. I have tried doing this with base drag and simulating spool rockets in RockSim but I only have a limited amount of data to confirm what I think is happening.

Bruce S. Levison, NAR #69055 Section #535 Tri City Sky Busters

 

[slim_t]

Originally posted by teflonrocketry1
Actually the side pods on the Titan rocket behave as fins!

This is what I was thinking as well. But wouldn't it only help along one axis? I haven't read your article yet, it's downloading now. But if you look at it from the side, or if wind hit it perpendicualr to the side, wouldn't it act similar to a single tube?
If the wind hit perpendicular to the front, then I believe the side pods would help a good bit, due to the surface area they provide.
Just curious what your thoughts are on this.

Another idea I had, yet more complicated, was to put motors in the side pods as well, and maybe cant them to help with stability, maybe even induce a spin. This might be more feasible in a larger model. My concern with that though would be getting the side pod motors equally canted and positioned with respect to the central booster.

Just brainstorming.

Tim

 

[rstaff3]

Ah this thread makes me wonder if I had just added a tad more noseweight if some of my odd-balls might have flown better.

 

[Smokin' Rockets]

I've seen a few HPR finless rockets. All failed. A fast motor helps stapilize it. One was stable during motor burn but spun as soon as it shut off. That rocket was VERY nose heavy. The amount of nose weight you would need would be very impracticle. Sim programs will not be accurate for this type of rocket.

.....Bill

 

[teflonrocketry1]

Originally posted by slim_t
This is what I was thinking as well. But wouldn't it only help along one axis? I haven't read your article yet, it's downloading now. But if you look at it from the side, or if wind hit it perpendicualr to the side, wouldn't it act similar to a single tube?
If the wind hit perpendicular to the front, then I believe the side pods would help a good bit, due to the surface area they provide.
Just curious what your thoughts are on this.

Another idea I had, yet more complicated, was to put motors in the side pods as well, and maybe cant them to help with stability, maybe even induce a spin. This might be more feasible in a larger model. My concern with that though would be getting the side pod motors equally canted and positioned with respect to the central booster.

Just brainstorming.

Tim

You are partly right about the side pods they do stabilize the Titan design more along one axis. But for overall stability there are several other things to consider. The valleys between the tubes and the air stream rushing through them lends stability by helping cancel forces perpendicular to the thinner profile of the airframe. You can think of the side pod equivalent to a cross (actually simulated as a "Y") attached by one of its arms at the same place as the pod attachment root. Essentially a side pod is like a "Y" shaped fin with with large fillets, the attachment point of the arms of the "Y" are at the center of the side pod tube.

Also helping the stability situation is the additional base drag and body lift created by the addtional tubes. I have yet to come up with a good way to simulate the effects of body tubes on the CP of the rocket but I think these two factors (base drag and body lift) are the major players.

Body lift is the same as, or similar to, boyancy (almost the same formula). So when a long skinny finless rocket leans in one direction the boyancy in that direction increases because its volume with respect to the horizontal plane perpendicular to gravity increases. Thus, boyancy (or body lift) if large enough can be a restoring force aginst any lateral force.

The base drag works to stabilize wide diameter finless designs like spool and disk shaped rockets. Essentailly a base vortex drags the CP behind the base of the model.

Both of these factors are present in all rockets, but only one has to be exploited to make finless designs fly stable. So spool and disk rockets eploit the base drag effect for stability, and bottle rockets and other long finless designs take advantage of body lift or boyancy.

These are what I believe are the effects of a body tube on the CP of a rocket; do you think I missed anything?

Bruce S. Levison, NAR #69055, Section #535 Tri City Sky Busters

 

[Ray Dunakin]

I think Finns should be allowed at all launches, same as any other ethnic group.

 

[slim_t]

Originally posted by teflonrocketry1
do you think I missed anything?

I think you pretty much covered it.
I like how you compare it to boyancy. I never looked at it that way, but now that you mention it, it makes sense.
It helps me understand things in a more 3D sense, rather than just 2D. I tend to simplify it, almost to the point of the cardboard cutout method in a mathematical sense, but now I think I can deal with the symmetry a little better. I hope I'm making sense. I guess what I'm trying to say is, I usually just look at the 2D plane that is the least stable, and design around that, and the rest will take care of itself. But now maybe I can stretch the limits even further. Thanks for the info.

Tim

 

[mike_bar]

Originally posted by 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. (snip.)

This is logical, good point Andy.

Originally posted by jetra2
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

Jason has a good idea, the Larry Brand design is proven stable,
as is this (Centuri?) finless rocket .

Originally posted by 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.

Dick, I believe you are partially correct. The Barrowman calculate ignores the affect of the sustainer tube in it's calculation of Center of Pressure (CP). The sustainer tube does have an effect on stability, especially when angles of attack are considered. Barrowman, VCP and Rocksim all ignore the contribution of the sustainer tube to the overall CP of a rocket. I believe my understanding is correct, but we should ask a “real” rocket scientist like Bob Krech! Bob may have covered this previously. I will ask him to contribute here.
Regards,
Michael

 

[bobkrech]

Mike

No matter how the rocket is designed, you must insure that the CG is ahead of the CP, or the rocket will not be stable.

I think Bruce pretty much said what I would have said. Body lift and base drag can stabilize a rocket, but you should do your homework before you fly a rocket with a nonconventional design.

The only additional comment I would say is that you spin stabilize a cluster rocket by canting the motors. The base drag and spin stabilize bullets and artillery shells by moving the CP aft.

Also if you do not have fins to assist in the maintenance of the zero angle of attack flight vector, you must have a high thrust to weight ratio to insure sufficient velocity to insure that the drag stability is in effect when the rocket leaves the rod. If not, there is a good chance of skewering someone near the pad.

Bob Krech

 

[mike_bar]

Thanks Bob, and to others.
I enjoyed reading this thread and have learned a lot...