Is the Tao spin stabilized, 2 stage rocket stable?

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Mr Rocket

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The Backstory:
Last summer after a 30 year hiatus I got back into the rocketry hobby. My son and I launched some of my 30+ year old rockets, and I was hooked again. I started doing a lot of research online, and came across a design on the JimZ website for a former Estes Design of the Month rocket called the "TAO". It is a 2 stage rocket that has some really cool features. First of all, the sustainer has no fins. It is spin stabilized by rotation introduced by canted dogleg fins in the booster section. Second, the booster has its own parachute recovery via a innovative hatch design. Third, the sustainer has rear ejection via a piston mechanism. And finally, it has a paint scheme that paints half the rocket red, and the other half green; creating the optical illusion as it spins that the rocket is actually yellow. What is not to like about this? I decided this was going to be the first rocket I would build in my return to the hobby.

I gathered all of the components and built the Tao over last winter. I was really happy with the way it turned out. Here is a picture of the completed rocket:

Tao2.JPG

The Problem
While waiting for the weather to get nice so I could launch the Tao, I started reading posts on TRF and other websites all stating that the TAO is an inherently unstable design. One post even went so far as to state that they had contacted the original designer, and even the original designer had never been able to get the rocket to fly, but had submitted the design to the Estes contest anyway. I was crushed. Here I had built a rocket that I was not comfortable was safe to launch.

How did we get here?
Recently I found a post here on TRF titled "Fins? I don't need no stink'n fins!" by Krusty. This is a really cool thread, where he is trying to build a true finless rocket by canting the motors in a 3 motor cluster to rotationally stabilize his design. Krusty is a great guy (even though I have never actually met him) and he let me hijack his thread for far too long. Cookie the Dogs Owner posted that he should look at the Tao. This led to me posting that I had built one, but had not been able to model the fins properly in Open Rocket because of the unusual dogleg shape. Another poster explained to me how to do it, so I decided to start this thread to discuss my findings (... and to give Krusty his build thread back) Thanks Krusty!

So here we go!
 
And after all of that the answer to the question "Is the Tao as presented on the JimZ website and in the Estes Design of the Month Competition stable" (at least according to Open Rocket and my best abilities to model it correctly) is... NO!

But if we accept that answer, this would be the end of the thread.

Instead I would like to post my OR model, get input from TRF members and discuss some of my ideas. So let's get started.

View attachment Tao3.ork
 
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After the initial disappointment settled in and I got over it, I wanted to see how much I would have to increase the angle of the cant on the fin to make the rocket stable. Imagine my surprise when I changed the angle from 10 degrees to 11 degrees and it instantly became stable, predicting an approximate altitude of over 1,400 feet. ONE LOUSY DEGREE! It appears this thing is right on the cusp of being stable. However, to modify this already built rocket would basically require redoing the entire fin assembly.

Idea Number 2: I changed the motor configuration from D12-0/B6-6 to D12-0/A8-5 to try to reduce the weight in the back of the rocket and move the CG forward. Again this resulted in Open Rocket predicting a stable flight, but with a 200' reduction in altitude.

Idea Number 3: I added 0.20 oz. weight in the nosecone and again was able to get a stable simulation using the D12-0/B6-6 with only a couple feet loss in projected altitude. The problem here is that because of the rear ejection, the nosecone is securely epoxied in place, and I am not sure how to get any weight into the nose without completely butchering the rocket.

At this point, I'll sit back and look forward to the wisdom of the rest of the TRF Members.

Thank you!
 
I love the TAO :) the design and how the booster parachute is hidden away behind a little hatch is just magical!

If you need to change the angle of attack by 1 degree, can you bulk out the face of the fin with a balsa wedge??

Have you seen the review on EMMR ? https://archive.rocketreviews.com/reviews/all/plan_est_doq_tao.shtml

Krusty

PS. you're welcome to hijack my threads whenever you want! That's part of the fun of working stuff out :)
 
If you need to change the angle of attack by 1 degree, can you bulk out the face of the fin with a balsa wedge??

Have you seen the review on EMMR ? https://archive.rocketreviews.com/reviews/all/plan_est_doq_tao.shtml

Krusty

PS. you're welcome to hijack my threads whenever you want! That's part of the fun of working stuff out :)

The wedge is a great idea. However, I tapered the trailing edge, so it would be a little more difficult. But still very doable.

I was considering just using the A8-5 instead of the B6-6. Sacrificing 200' to get this thing to fly as is would be worth it to me.

And yes, I did see the review on EMMR. That was what caused me to start to question this.
 
The wedge is a great idea. However, I tapered the trailing edge, so it would be a little more difficult. But still very doable.

I was considering just using the A8-5 instead of the B6-6. Sacrificing 200' to get this thing to fly as is would be worth it to me.

And yes, I did see the review on EMMR. That was what caused me to start to question this.

200' less for a stable flight is a fair trade-off :) Esp. when you're talking about the difference between 1,400' and 1,200'

I wonder about the build quality of the EMMR post - since his fins tore off on the first flight. That would have a huge impact on the over-all stability.

The other thing - packing the parachute could cause problems if the weight isn't evenly distributed around the circumference of the rocket. I'm looking at using a streamer on my finless rocket - just so I can pack it more evenly.

Krusty
 
I wonder about the build quality of the EMMR post - since his fins tore off on the first flight. That would have a huge impact on the over-all stability.

There was a warning in the instructions not to change any of the details of the design as it may influence the stability. I noted that he used 2 vent holes vs. 3 for the piston ejection, and it did not look like from the wreckage and the description that he constructed the upper portion of the piston correctly. It seemed like he did not use the piece of coupler to link the 2 centering rings together allowing them to bend past the lower piece of coupler used as a stop. (If anyone decides to build the Tau this was one area where I remember them not being clear, and having to reread them multiple times to make sure I built it correctly) All of that made me wonder how strictly the remainder of the instructions were followed.

The other thing - packing the parachute could cause problems if the weight isn't evenly distributed around the circumference of the rocket. I'm looking at using a streamer on my finless rocket - just so I can pack it more evenly.

Krusty

I was thinking about your streamer last night. Because the rocket is spinning the Tau instructs you to pack the parachute by wrapping it around the piston opposite the direction of rotation. That way the rotation of the rocket helps unfurl the chute. It also uses swivels to keep the chutes from tangling. Have you considered this? I was wondering if you we going to use a paper or mylar streamer? It seemed like the mylar might be more durable and less likely to tangle.
 
The wedge is a great idea. However, I tapered the trailing edge, so it would be a little more difficult. But still very doable.
In that case, I would attempt the following:
1) Sand off the paint on the face to be built out (obviously.)
2) Build up the desired new shape with foam spackle. The is very light and can be spread on like frosting then easily (perhaps too easily) shaped with sandpaper. Such a build-up will conform to the tapered trailing edge and the rounded leading edge alike then be reshaped with new rounded and trailing edges.
3) Because the spacke is very soft and weak, you'll want to have some sort of strengthening and hardening coat on the surface. I use this foam for non-structural fin fillets, and there I just skim the surface with Aerogloss. For the larger surface you might want something stronger. I don't know what, but can offer two suggestions to experiment with:
a) Thin CA with no kicker. Brush it on then spray with the separate kicker to set it. Since the CA will have soaked a little into the surface, it would probably work pretty well if the heat from curing doesn't wreck the whole thing. The problem with this (for me, at least) is that I don't know where to get it.
b) There is a product called Git Rot, which is a two part epoxy that's nearly water thin. It's meant for strengthening punky wood that's well dried out, and is sold at marine supply stores (e.g. West Marine.) I keep thinking it would have plentiful applications in rocketry, but I haven't tried it yet.
c) Hey, maybe Aerogloss will be good enough.​
Experiment with some scrap before attacking the Tao's fins.
4) Sand and paint the hardened surface as usual. (Is it just me, or do others sometimes think that "rocket building" really ought to be called "rocket sanding?")​
 
First of all, even if it doesn't fly right, that Tao sure is pretty!

I've been intrigued by the design for a long time, but also a little intimidated. I like the rear-eject parachute deployment system for the upper stage; I like the booster recovery system behind the little hatch; I just fear that's too many outside-the-box design elements on one rocket at one time. I've considered building an otherwise conventional rocket with the Tao's upper-stage recovery system to see if thatt works, then building an otherwise conventional two-stager with the Tao's booster design to see if that works, and only then taking a stab at an all-up Tao. You are obviously more ambitious than me, for which i salute you.
 
I was thinking about your streamer last night. Because the rocket is spinning the Tau instructs you to pack the parachute by wrapping it around the piston opposite the direction of rotation. That way the rotation of the rocket helps unfurl the chute. It also uses swivels to keep the chutes from tangling. Have you considered this? I was wondering if you we going to use a paper or mylar streamer? It seemed like the mylar might be more durable and less likely to tangle.

I have some crepe paper streamer that is fire resistant - so I don't need to use any additional wadding :) I am going to use a short length of kevlar to attach the streamer to the rocket body via a swivel and I will attach the nose cone to the other end.

[BODY] ~~~~ Streamer ~~~~ [NC>

Krusty
 
@jqavins - Thanks for the help on the modeling of the fins in the OR simulation.

As far as the fins, if I decide to modify them, I was thinking I would sand the paint off, laminate a balsa wedge to the face of the cant to increase the angle and then fill the gap with wood filler. Then seal, seal, sand, repeat, repeat, repeat:wink:
 
First of all, even if it doesn't fly right, that Tao sure is pretty!

I've been intrigued by the design for a long time, but also a little intimidated. I like the rear-eject parachute deployment system for the upper stage; I like the booster recovery system behind the little hatch; I just fear that's too many outside-the-box design elements on one rocket at one time. I've considered building an otherwise conventional rocket with the Tao's upper-stage recovery system to see if thatt works, then building an otherwise conventional two-stager with the Tao's booster design to see if that works, and only then taking a stab at an all-up Tao. You are obviously more ambitious than me, for which i salute you.

Thanks for the compliment. But I did not think I was being that ambitious when I started because I thought I was building a proven design, and just needed to follow the directions. I am married, so I am pretty good at that :wink: It was only once I realized the design needed to be revised that I realized how much work I had ahead of me, but I think I have learned alot.

The basic build was not that bad. I am building a QCC Explorer right now which has been much more time consuming. If you want to do it, I would advise just to go for it. Just make sure you come up with a way of slightly increasing the rotation, or moving the CG forward before you start.
 
I did a little futzing with the design. Here are a couple of variations for whatever they're worth:

One with an 0.2 oz nose weight and streamers instead of parachutes: View attachment 134671

I agree with the 0.2 oz weight in the nose and see no reason why you could not use streamers. It's not like you need to worry about breaking a fin on the sustainer :wink:

Another with four fins instead of three: View attachment 134672

The only problem I see here is if you keep the rear compartment for the chute. On my Tao, I used a 6" chute instead of the 12" in the plans and fitting the chute in there is tight. Going to 4 fins would decrease the size of the compartment from a third of the circumference to a quarter of the circumference.

I see you switched the motors to 18mm. Does anyone know why the original designer chose to use D's for the booster, and B's for the sustainer?
 
Does anyone know what kind of factor of safety Open Rocket uses in their simulations? I mean if it says I need to add 0.2 oz nose weight to be stable, should I add 0.4 oz, or if it says I need an 11 degree angle on the fin, should I make it 13 degrees?
 
I've been thinking about stability - specifically stability around bullets.

With a rocket - the way we increase stability is to increase fin area and increase weight in the nose.

But a bullet is backwards -

bullet_5.jpg

The weight is at the back and uses both spin and drag for stability.

Could the problem be around the way we're thinking? By adding more weight to the nose - could we in fact be making the sustainer *less* stable???

Krusty
 
Going to 4 fins would decrease the size of the compartment from a third of the circumference to a quarter of the circumference.

I hadn't even thought of that issue. Maybe on the 4-fin we'd just go to tumble recovery--it should tumble nicely.

I see you switched the motors to 18mm. Does anyone know why the original designer chose to use D's for the booster, and B's for the sustainer?

I simmed it using an 18mm in the booster, but the motor tube is still a 24.
 
Could the problem be around the way we're thinking? By adding more weight to the nose - could we in fact be making the sustainer *less* stable???

Krusty

I don't think so. It's still about the relationship of CP to CG. Because the bullet is very short and wide (compared to our rockets) and because the mass is fairly consistent, even though the CG is behind the CP, it is only by about 1 caliper. This makes it much easier for the rotation and drag to correct for the instability caused by having CG behind CP. In the Tao, once the booster drops off, the CP is in front of CG by 4.12 calipers. By adding the 0.2 oz nose weight it reduces the deficit between CG and CP to only 3.47 calipers. So we are definitely not making the sustainer less stable.

I think that might be why the original designer of the Tao used the nested staging. It moves the mass 0.75" forward of traditional staging and the larger diameter of the 24mm motor takes weight further from the center axis, increasing the moment of inertia caused by the rotation.

:surprised:
 
What I would recommend is simply adding the required nose-weight to make the rocket stable. Regarding accessing the inside of the front of the rocket what I
would do is to drop a single bead of Epoxy or PVA in and drop the required weight into that.
 
drop a single bead of Epoxy or PVA in and drop the required weight into that.

Thanks. I think at this point I have decided to do both adding the weight to the nose cone and using the A8-5 engine in lieu of the B6-6. At least for the first launch:horse:

I was thinking of putting a BT-5 tube down through the BT-20 tube of the piston to pour the epoxy through because I don't want to risk getting any Epoxy on the piston.

Also, I have been reading several threads about adding weight to the nose cone. Several mention making sure you wash the inside of the NC to remove any mold release agent and to rough up the surface with sand paper. Since it is too late to do either of these without removing the NC, I also read about a technique to drill a hole through the nose cone and inserting a balsa skewer all the way through. This would provide a mechanical anchor that would be locked in place when the epoxy is poured around it. Has anyone tried this?

Also, I read you need to put the nosecone in an icebath while the epoxy cures to keep the heat from deforming the nosecone. Thoughts?

Then we call for a heads up launch and let it fly :duck:
 
Also, I read you need to put the nosecone in an icebath while the epoxy cures to keep the heat from deforming the nosecone. Thoughts?

I've epoxied weight into plastic nose cones many times and never had a problem with deformation - it just doesn't get hot enough, or at least not in the quantities that you're going to put into nose cones.

Krusty.
 
Also, I have been reading several threads about adding weight to the nose cone. Several mention making sure you wash the inside of the NC to remove any mold release agent and to rough up the surface with sand paper.

Also, I read you need to put the nosecone in an icebath while the epoxy cures to keep the heat from deforming the nosecone. Thoughts?

I've never had any problems with mould release agent messing up my gluing, although it's probably best to use Epoxy as I've found that PVA doesn't adhere to plastic very well. Regarding putting it in an ice bath to stop the melting, that's probably a good idea.
V
 
I think all of this is simply intriguing! It makes me almost want to get one of these! I had never thought to even attempt or look for a finless sustainer on a rocket! sounds like this rocket has numerous ingenious design characteristics!!!
 
I think all of this is simply intriguing! It makes me almost want to get one of these! I had never thought to even attempt or look for a finless sustainer on a rocket! sounds like this rocket has numerous ingenious design characteristics!!!

Nate,
I could not agree more. It is a really cool rocket. If you decide to build one I have 3 pieces of advice
  1. Have some plan to adjust it for stability
  2. Find a source for the PNC60AH nose cone. Estes no longer sells it as a part. It only comes in kits. I got lucky and happened to stumble upon a kind soul who had access to a couple and was willing to part with one. Otherwise, you will need to kit bash a Der Red Max or Mean Machine or similar.
  3. There is an error in the parts list that says you only need one BT60 x 3/4" when in reality you need 2. I have speculated this may have been the cause of the ejection failure on George Pike's Tao as it is not visible in the wreckage.
:2:
If you give it a try, please post back here and let us know the outcome.

Hoping all your rockets fly high and straight!
 
I worked up an "entry level" variation that uses a conventional streamer in place of the piston arrangement in the upper stage, and does away with the parachute in the booster stage and relies on tumble recovery. I call it "Ba Gua," which is the name of "a symbol used to represent the [T]ao and its pursuit," according to them what knows these things. Seemed fitting, as the point of this design is to be a first step in "pursuing" (building) an all-up Tao.

The annoying part is that I can't get Open Rocket to complete a simulation--no matter what engine combination I use, even if I zero out wind and turbulence and set the time step to 0.0025s, I get a "simulation exceeds limits" error. :facepalm: Any ideas on how to correct it?

View attachment Ba Gua (Pursuit of Tao).ork
 
I had an idea over night that it might have something to do with the piston being removed. This additional weight in the sustainer might help the rotation.

The attached OR file adds a streamer to the booster to get rid if the recovery glitch, and a full length coupler in sustainer to increase the moment induced in the sustainer.

View attachment Ba Gua (Pursuit of Tao)_Rev.ork

Did not have time to fully check it out, but it appears to have helped.

Let me know how it goes.:pop:
 
I had an idea over night that it might have something to do with the piston being removed. This additional weight in the sustainer might help the rotation. The attached OR file adds a streamer to the booster to get rid if the recovery glitch, and a full length coupler in sustainer to increase the moment induced in the sustainer....

Did not have time to fully check it out, but it appears to have helped.

Let me know how it goes.:pop:

Now we're getting somewhere! Using your revision, I was able to get some engine combinations to successfully simulate.

View attachment Ba Gua (Pursuit of Tao)_Rev with sims.ork

I then went back to my original and added a phantom streamer to the booster, and increased the nose weight to one ounce. This one sims out rather nicely, with everything from a "low and slow" option with a C11/A8 that attains 672 feet and 145 mph to a 1,731 foot apogee and 240 mph top speed on a D12/C7 combination.

View attachment Ba Gua (Pursuit of Tao) Rev2.ork
 
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