Winged and forward winged designs unstable for rockets?

The Rocketry Forum

Help Support The Rocketry Forum:

KenECoyote

Well-Known Member
TRF Supporter
Joined
Mar 19, 2015
Messages
2,986
Reaction score
234
Managed to go launch the Tomahawk this morning! Sorry, gray skies so vid was poor and YouTube is poorer. Rocket rose up beautifully straight and from what I could tell did a nice small cruise (as planned).

IMG_20210331_071035769.jpg



For this build, I filled about half of the small nose cone with clay and did a swing test jic since I wanted to be safe and also fly off A10 later. My system is more of a smoothing and slight correction rather than "save from upside down flight" (although that may be possible).

I also tried to build this "scalish" first using the Quest kit and then checking actual missile pics, however the body unintentionally came out too slender.
 
Last edited:

Jeff Lassahn

Well-Known Member
TRF Supporter
Joined
Jul 5, 2020
Messages
321
Reaction score
350
Location
Portland OR
I wonder how much effect is caused by the mass pushing on the forward fins, and how much is just because they're allowed to move. If you mount a forward fin on a pivot so it can rotate into the airstream does that nearly eliminate any lateral forces it would otherwise cause, basically making it not contribute to the CP?
 

KenECoyote

Well-Known Member
TRF Supporter
Joined
Mar 19, 2015
Messages
2,986
Reaction score
234
I wonder how much effect is caused by the mass pushing on the forward fins, and how much is just because they're allowed to move. If you mount a forward fin on a pivot so it can rotate into the airstream does that nearly eliminate any lateral forces it would otherwise cause, basically making it not contribute to the CP?
Good question and one I've thought about already and I'll test in the future...currently having too much fun with these. In my opinion and having observed the flights, it does make a difference even if it is slight, that's all it takes. The nosecone one when added to another rocket seems to make it fly much nicer and just pivoting forward fins shouldn't do that I'd think.
If it was as simple as just letting the wings/fins pivot, you'd see it much more common but I don't recall ever seeing it in LP/MP. Will test and compare further when I get a chance!
 

KenECoyote

Well-Known Member
TRF Supporter
Joined
Mar 19, 2015
Messages
2,986
Reaction score
234
Additional testing to come includes:
-Similar rocket without forward fins launched side-by-side or following with one with forward fin system to compare (need to build and do at club launch)
-Same rocket with freely pivoting fins to compare vs pendulum weighted
-Upscaling* so I can better observe flight and coast (the Tomahawk really took off too quickly and I need another person to film farther away). One reason I'm powering through small scale tests first is to make sure the system/design is safe first.
-Additional designs and improvements. Right now I have 5 versions in mind.
-Additional types of winged rockets to build and test

Gimmie a few years! lol

*Note: One reason I started on this was that I've always wanted to upscale the Quest Tomahawk...possibly to HP. However my Quest build had a tendency to zoom off into an arcing horizontal flight (similar to some of my other winged rockets when built so it flies without rotation/twist) and I didn't want that...I wanted straight up (as much as possible), arc over & coast and ejection. This 1/2A Tomahawk did just that for me, so I want to start building a BT80 version soon.
 
Last edited:

kbRocket

Well-Known Member
TRF Supporter
Joined
Nov 14, 2017
Messages
141
Reaction score
159
Location
Portland, OR
I think you have invented something cool and useful, but it is not a stabilization system. It is a way to allow the forward fins to not contribute as much to the Cp because they pivot to allow air to flow around them.

Lets think about forces. You can break down the forces on a rocket into gravity, thrust, drag, and normal force. There are lots of guides and technical reports with diagrams and discussions. The treatment and pictures in Sampo Niskanen's documentation for open rocket: http://openrocket.sourceforge.net/techdoc.pdf are clear and easy to follow. This is on p. 24 of the pdf or p. 15 of text.

1617556878857.png


Gravity is a sneaky one. If you hold the rocket in your hand, like in post #60, gravity does make the fins turn. However if you take away your hand, and motor thrust, and air, and let the rocket fall freely the fins will not turn because every component of the rocket will fall with the same acceleration. So this isn't going to help with stabilization.

Next we have thrust and drag. When the motor is powered up, the rocket is pushed with something like 5 to 20 gs of acceleration. Then the motor burns out and atmospheric drag decelerates the rocket body. Think of what happens with the gravity assist pendulum hooked to your fins under these conditions: during acceleration and deceleration the forces on the pendulum and hence fins is opposite. They can't both be stabilizing.

Finally we have the atmospheric normal forces, aka lift. Your fixed fins at the aft end of the rocket are contributing to Cp. The pivoting fins (with their axel above the fin Cp) allow the fin to twist to be parallel to the airflow. These fins will make a little drag, but their pivoting prevents the kind of lift that went into the Barrowman equations/cardboard cutout model/every simulation program. Cn -> 0 as alpha -> 0. The Cp that is produced by these methods assumes rigid, not pivoting fins. Take your cardboard cutout model and twist the two big middle fins so you are looking at their edge. That is what they look like to the wind. There will be a small drag force, but it is applied to the rocket body through the fin axel, which is almost right at the Cg, so it will apply a little drag and negligible torque to the rocket body.

The cool thing is that by allowing the large midsection fins to pivot, they don't contribute to Cp and don't destabilize the rocket if they are near or behind the Cg. You can use this to make some cool rockets that are not otherwise possible.

You are probably better off omitting the pendulum weight entirely and just using a straight axel. The pendulum isn't helping. It is being overwhelmed by the atmospheric forces twisting the fins. Put its mass into the nose cone instead.
 

KenECoyote

Well-Known Member
TRF Supporter
Joined
Mar 19, 2015
Messages
2,986
Reaction score
234
I think you have invented something cool and useful, but it is not a stabilization system. It is a way to allow the forward fins to not contribute as much to the Cp because they pivot to allow air to flow around them.

Lets think about forces. You can break down the forces on a rocket into gravity, thrust, drag, and normal force. There are lots of guides and technical reports with diagrams and discussions. The treatment and pictures in Sampo Niskanen's documentation for open rocket: http://openrocket.sourceforge.net/techdoc.pdf are clear and easy to follow. This is on p. 24 of the pdf or p. 15 of text.

View attachment 458406

Gravity is a sneaky one. If you hold the rocket in your hand, like in post #60, gravity does make the fins turn. However if you take away your hand, and motor thrust, and air, and let the rocket fall freely the fins will not turn because every component of the rocket will fall with the same acceleration. So this isn't going to help with stabilization.

Next we have thrust and drag. When the motor is powered up, the rocket is pushed with something like 5 to 20 gs of acceleration. Then the motor burns out and atmospheric drag decelerates the rocket body. Think of what happens with the gravity assist pendulum hooked to your fins under these conditions: during acceleration and deceleration the forces on the pendulum and hence fins is opposite. They can't both be stabilizing.

Finally we have the atmospheric normal forces, aka lift. Your fixed fins at the aft end of the rocket are contributing to Cp. The pivoting fins (with their axel above the fin Cp) allow the fin to twist to be parallel to the airflow. These fins will make a little drag, but their pivoting prevents the kind of lift that went into the Barrowman equations/cardboard cutout model/every simulation program. Cn -> 0 as alpha -> 0. The Cp that is produced by these methods assumes rigid, not pivoting fins. Take your cardboard cutout model and twist the two big middle fins so you are looking at their edge. That is what they look like to the wind. There will be a small drag force, but it is applied to the rocket body through the fin axel, which is almost right at the Cg, so it will apply a little drag and negligible torque to the rocket body.

The cool thing is that by allowing the large midsection fins to pivot, they don't contribute to Cp and don't destabilize the rocket if they are near or behind the Cg. You can use this to make some cool rockets that are not otherwise possible.

You are probably better off omitting the pendulum weight entirely and just using a straight axel. The pendulum isn't helping. It is being overwhelmed by the atmospheric forces twisting the fins. Put its mass into the nose cone instead.
Ah, good thoughts; however I think there's something missing here...assume the rocket takes off on a 45 degree angle, I'd argue that gravity does have an effect...of course the greater the thrust, the less the affect of gravity (also the more weight in the pendulum, the more it may affect the flight), but overall I believe there is an effect and even a tiny amount over a distance does change the path imo. Also falling is a different diynamic than thrusting upwards. One of the problems I considered is that the pivoting forward fins/wings would flutter on coast and in my Tomahawk build I created a version that locks on coast (unfortunately it recently froze due to ejection debris...I'll build a better enclosed system next).

I'll do more testing, but so far everything I've seen and based on my exp launching hundreds of small rockets (including a lot of experimentals), it does make a noticeable difference in having the rocket go straighter (for the more conventional designs) as well as actually seeing it working on the crazier extreme designs.

Today's Tomahawk launch was a very straight and beautiful flight albeit short...delay was too long even though same motor was fine earlier. I had painted and added vinyl decals which added significant weight to the rocket.

For my upside down rocket, it's currently too heavy and I have to build another lighter version, but in this example you can see it actually working to try to keep the rocket upright after it goes almost horizontal...quite unusual "flight"that somewhat defies what you'd normally see in a top-heavy rocket under thrust that goes sideways I believe. This one may have also had binding on the rod since previous launch on the same motor had better speed.
 

KenECoyote

Well-Known Member
TRF Supporter
Joined
Mar 19, 2015
Messages
2,986
Reaction score
234
FYI - lost my job of 16 years this week, so I'm further pre-occupied with other things, but do plan to continue this as "therapy". I'll also try to build similar without the pendulum (but with equal weights) for a good comparison, but have a lot of things going on now...I may slow some parts down/put on backburner. Thanks for your help, input and attention everyone!

However I must say I REALLY enjoy the building of the systems, rockets and testing (plus seeing the beautiful flights...mostly lol), so I hope everyone understands why I prioritize my time in this way. Lots of testing on things like this...safety first, small scale, small motors, good flight first, other systems and improved versions, fixing problems, upscale, comparisons, etc.
 

kbRocket

Well-Known Member
TRF Supporter
Joined
Nov 14, 2017
Messages
141
Reaction score
159
Location
Portland, OR
Ken
This is a hobby. If you're having fun then you're doing the right thing. You're also advancing rocketry in an area and sharing what you learn. Thanks.
Sorry for your job loss. It's difficult, but it's also an opportunity.
kb
 

JohnCoker

Well-Known Member
TRF Supporter
Joined
Apr 13, 2013
Messages
2,047
Reaction score
699
Gotta applaud the experimentation here; great work!
 

KenECoyote

Well-Known Member
TRF Supporter
Joined
Mar 19, 2015
Messages
2,986
Reaction score
234
Ken
This is a hobby. If you're having fun then you're doing the right thing. You're also advancing rocketry in an area and sharing what you learn. Thanks.
Sorry for your job loss. It's difficult, but it's also an opportunity.
kb
Thanks KbR...you've got great points there! :)
 

KenECoyote

Well-Known Member
TRF Supporter
Joined
Mar 19, 2015
Messages
2,986
Reaction score
234
I've been pondering the last "Gnikiv" launch. In person it was extremely unusual in that it seemed to be "hovering" sideways a bit while the motor was thrusting...what initially came to mind was Mary Poppins lol. While very brief, if you take a look at the video from 06 on, you'll see that it actually pointed downwards at one point and managed to recover upwards. I noticed almost the same sideways hover a bit on the prior Gnikiv launch.

I may try some hack-job lightening and then fly off A10-0T motors to see if I can remove the negative angle of attack (right-side-up?) and replicate & extend the "hover". That'd be cool.

Note: I did see a spin on both of my last launches, so I'll try to get that out of the system best I can on follow up launches. I think the spinning actually hindered my system rather than helped.
 

DrewW

Well-Known Member
Joined
Aug 23, 2020
Messages
224
Reaction score
145
At a recent launch the top designer for the largest model rocketry company in the world held the Backburn Triplane and then I handed him the nose cone. With out saying a word he immediately broke out into a sustained belly laugh. Yes, silly airplane oddrocs take copious amounts of no good, stinking, performance robbing nose weight. Recovery is on multiple chutes on seperate bits if you have a nose cone that separates. These rockets are like a really cheap hospital; no Recovery Room!

The Dornier 217 N utilizes powdered Tungsten in the nose. So desperate was the oddroc builder for maximum mass in a small volume. Many, many problems to overcome on each model. Asymmetric drag and weight, widely spaced pods (Asymmetric Thrust,) stability issues, exhaust flame burning fins in flght, poor boy dual deploy, ridiculous canted tractor motors, lack of computer simulation and ability to swing test. Use the dark arts and you will fly the airplane oddrocs you love!
I checked and private citizens can own depleted uranium. New opportunities in compact nose weight. Say goodbye to your inefficient tungsten!
 

Daddyisabar

Oddrocs Rule!
TRF Supporter
Joined
Jan 23, 2009
Messages
4,772
Reaction score
347
Location
Littleton Colorado
I checked and private citizens can own depleted uranium. New opportunities in compact nose weight. Say goodbye to your inefficient tungsten!
You could use the DU in a nice mix in the motor on the bottom of the rocket!
 

kbRocket

Well-Known Member
TRF Supporter
Joined
Nov 14, 2017
Messages
141
Reaction score
159
Location
Portland, OR
Say goodbye to your inefficient tungsten!
Tungsten is more dense than depleted uranium and it is easily available from Amazon as fishing weights, pinewood derby car weights, and TIG welding electrodes.
 

DrewW

Well-Known Member
Joined
Aug 23, 2020
Messages
224
Reaction score
145
Tungsten is more dense than depleted uranium and it is easily available from Amazon as fishing weights, pinewood derby car weights, and TIG welding electrodes.
@kbRocket you are correct, but then I guess you already knew that. Good job fact checking, and teaching me something.
 

KenECoyote

Well-Known Member
TRF Supporter
Joined
Mar 19, 2015
Messages
2,986
Reaction score
234
Two updates!

1) I may have a most excellent chance at a new job....I did a search on Monster.com and the first company I worked at (I've only worked at two! Lol) had a posting for what sounds like the same job I had at company #2! Crossing my fingers, but that'd be amazballs! :D

2) I went to the park to try Tommy Viking on an A10-3T. I actually had some concerns about how it would launch and almost went to a secluded small park instead. My concerns weren't necessary and it flew BEAUTIFULLY! Very fast and straight and perfect ejection and deploy around apogee....one of the nicest flights I've seen in a long time! :wos_love:
Sorry yet again...the video doesn't do it justice!
The design for me is now confirmed viable for upscale, so I'll start on that as well as a small control version with freely pivoting unweighted wings to compare.
 

KenECoyote

Well-Known Member
TRF Supporter
Joined
Mar 19, 2015
Messages
2,986
Reaction score
234
Some other ideas I want to try building...

X-15 with less nose weight than available kits


V-1


X-1


I believe the last two are generally understood to be extremely difficult to launch straight without a lot of heavy metals in the nose, controlled guidance or some complex system, so if I can build these and get a good flight using my Viking bulk BT-20s I think that would be pretty cool and also quite a challenge. Gimmie a month or so! :)
 

KenECoyote

Well-Known Member
TRF Supporter
Joined
Mar 19, 2015
Messages
2,986
Reaction score
234
Just an update that I launched the Tomahawk again on A10-3T at a club yesterday and there was about 7-10mph wind. Skies were a bit overcast and with a small fast rocket, I couldn't capture it well on video. The rocket took off the rod very quickly and then took a sudden angle into the wind...then instead of normal weather cocking (where a rocket's path curves into the wind until it is heading into it) the rocket maintained the same angle going up in a straight path, which was very unusual, but cool...to me it does show a significant difference and that the system is working imo.

Of note is that I had started on this as a means to figure out how to build an upscale of the Quest Tomahawk cruise missile and not have to worry about the forward fins affecting the flight/making it curve horizontal. I feel I've accomplished that on a small scale (even more so given the forward wings are 3-4x that of the Quest rocket). Of significant note is that at yesterday's club launch, the person parked next to me had an upscale of the Quest Tomahawk cruise missile!!! WHOA! What were the chances? I showed him mine and explained my plans and he also said he observed my flight to be going straight up. His rocket later launched on the HP pads using an F motor and in the same wind, it took the typical "curve into the wind" and had somewhat low apogee that was almost horizontal. He had a JL Chute release set for 300', but it didn't go off and he felt maybe because the rocket didn't go much higher than 300'! I have to say his build was GORGEOUS and I've got my work cut out for me if I'm going to build one similar scale to his, but I will and then maybe we can do a drag race and compare. COOL!

His rocket (he's not on forums):

Upscale Tomahawk Barclay.jpg
 
Last edited:

KenECoyote

Well-Known Member
TRF Supporter
Joined
Mar 19, 2015
Messages
2,986
Reaction score
234
Here's also a video demo of my latest version of my MAARS stabilization system that I plan to use on the upscale Tomahawk. This is the one with the "cruise lock" that locks the forward fins after boost while on coast and worked on my Viking Tomahawk build until the ejection charge fouled the lock. I can't wait to build it in a larger scale to better see it in action!


Also of note is that this system lends itself to be put into a nose cone and can be moved between rockets. :)
 

lakeroadster

Lonewolf.... No Club
Joined
Mar 3, 2018
Messages
2,558
Reaction score
1,377
Location
Central Colorado
I will eventually, but I have to get Rocksim or similar first and learn it.
Suggestion: download Open Rocket. It's free and you'll be knocking out sim's the same day. Then if you're modifying kits, download K'Tesh's already built simulations here: K'Tesh's OpenRocket Files... Then modify the simulation as needed.

 
Last edited:

KenECoyote

Well-Known Member
TRF Supporter
Joined
Mar 19, 2015
Messages
2,986
Reaction score
234
Suggestion: download Open Rocket. It's free and you'll be knocking out sim's the same day. Then if you're modifying kits, download K'Tesh's already built simulations here: K'Tesh's OpenRocket Files... Then modify the simulation as needed.

Thanks! I was actually thinking of doing just that to try it. However I was also weighing getting the Rocksim since I have some old custom files from when I last flew some big custom rockets. I'm also in the middle of looking for a new job, so that has to be the priority, but I'd still like to build some small test rockets because that's what I really enjoy and I can use some happy times during these crappy times. ;)

Regarding the OR/Rocksim, it would be useful for determining where the CP is so I can try to get marginal stability as a test bed for safe launches and to see if I can get smooth straight flights; however because of the pivoting frontal fins, it may not be able to help much more beyond that and may be a bit too conservative. We'll see.

BTW - if anyone has any diagrams of wild winged rockets from those programs showing the CP & CG, please post here and I'll try to build a few of them. I recently saw the V-1 Buzz bomb info and found that helpful as a basis for my next build.

Best,
KEC
 

jadebox

Roger Smith
TRF Sponsor
Joined
Jan 18, 2009
Messages
5,753
Reaction score
267
You might want to try eliminating the weights. Allowing the forward fins to pivot more freely may make the system work better since it will reduce their effect on the CP.
 

lakeroadster

Lonewolf.... No Club
Joined
Mar 3, 2018
Messages
2,558
Reaction score
1,377
Location
Central Colorado
Regarding the OR/Rocksim, it would be useful for determining where the CP is so I can try to get marginal stability as a test bed for safe launches and to see if I can get smooth straight flights; however because of the pivoting frontal fins, it may not be able to help much more beyond that and may be a bit too conservative. We'll see.
You could add mass inside the rocket at the location of the pivoting fins, but not actually model the fins if they pivot, since pivoting fins don't affect CP location.
 

BABAR

Builds Rockets for NASA
TRF Lifetime Supporter
TRF Supporter
Joined
Aug 27, 2011
Messages
8,322
Reaction score
3,100
You could add mass inside the rocket at the location of the pivoting fins, but not actually model the fins if they pivot, since pivoting fins don't affect CP location.
curious, i would agree that pivoting fins would alter CP much less than fixed fins, but I would expect just from a drag standpoint they would have some effect on CP. Am I out to lunch on this, @neil_w ?
 
Top