Engine-Driven Gas-Dynamic or Induction Stabilization Streamlined Finless Rockets

[BigMacDaddy]

A while back I built a BT60 streamlined rocket that flew successfully (originally posted in the "More Half-Baked Designs" thread but reposted below). I originally called this finless but there seem to be some debate about that (it does not have any fins that protrude beyond the body tube diameter but does have a pseudo-ring-tail that is the same diameter as the body tube but larger than the boat tail and there are supports that connect this to the boat tail. Anyway, I think this rocket uses Engine-Driven Gas-Dynamic or Induction stabilization and I would like to do more experiments in this area.

https://paperzz.com/doc/9449939/induction-stabilization-of-model-rocketshttps://www.apogeerockets.com/education/downloads/Newsletter379.pdf
I am considering making a BT50 version w/ 18mm engine and/or a BT20 version with 13mm mini-engine. I liked the boat tail leading to ring-tail design that I did on the BT60 version I posted previously. I really do not understand the difference between induction stabilization and GDS. The top paper shed some new light for me.


I wonder if this can ever be as efficient as a simple set of small fins on a minimum diameter body tube. It seems that because the ring tail / body tube needs to be larger than the engine to allow air mixing / suction that it is not possible to do this with a minimum diameter body tube. Does that seem true to everyone? Is there something else I should try to make this more effective / efficient or is this relegated to an oddroc demonstration model?

Here is the original rocket and launch:

 

[GlenP]

I think the concept is sound, using the engine exhaust combined with captured airflow, but the effect only works while the motor is producing thrust. You might have enough momentum going to keep the rocket going, but during coast, you will probably lose some of that stability. Instead of a cylindrical ring fin/duct, you could shape that into a slightly expanding conical/nozzle so that it gives some stabilization during coast, might not take much of a change in dia to get that effect, and it might actually help with the mixing and expansion of the thrust to give a little extra push during a portion of the thrust also.

 

[aerostadt]

We discussed several times on TRF. This is one of them:
https://www.rocketryforum.com/threads/new-gds-document-from-dean.136251/

 

[BigMacDaddy]

We discussed several times on TRF. This is one of them:
https://www.rocketryforum.com/threads/new-gds-document-from-dean.136251/

Thanks -- I came across that document online but not that thread... I need to take more time to process the difference between induction stabilizing and GDS.

 

[BigMacDaddy]

We discussed several times on TRF. This is one of them:
https://www.rocketryforum.com/threads/new-gds-document-from-dean.136251/

One thing that confuses me about that write-up is that the induction stabilization is stated to be different than GDS because:



However, the Ikaros provided as an example of induction stabilization seems to show a rocket with intakes quite close to the rear of the rocket and not particularly close to the CG (assuming that the back end of the rocket is empty but the front end is more or less solid and full of propellant and flare fuel as shown in the diagram).



Anyway, I still do not understand the difference between GDS and induction stabilization. Does it look like my rocket is using GDS or induction? I did include nose weight but not sure if I needed to. How could I modify my design to better use induction and reduce the nose weight needed? Do I just need to make the rear tube that the engine fires through longer (2-3x the body tube diameter according to the article)?

 

[aerostadt]

Arguably, I am thinking that a change in gas momentum, which causes a force on the rocket air frame, can come from two sources: 1) the first is due to the internal acceleration of the incoming outside air by the rocket motor itself (thrust augmentation), 2) the deflection of the incoming outside air by the air frame itself (fin-less). As I look at some of the designs with the motor placed at the center and at the tail, I think one could argue that thrust augmentation could be present in either design. In reality I don't think thrust augmentation is appreciable by rocket exhaust either way. (This last statement can be a vast discussion in itself. I have seen arguments for this effect in both the model rocket world and the professional world including my former employer. Theoretical derivations show tremendous increases in thrust augmentation, but my colleagues and I concluded this was assuming complete mixing of cold incoming air with hot motor exhaust in a very short distance. It also is difficult to envision by control volume analysis how a straight tube could create forward thrust.)

I did a simple control volume derivation that shows deflection of incoming air by the air-frame could produce a side force(fin-less) for a small perturbation in the rocket's flight path.

 

[GlenP]

Arguably, I am thinking that a change in gas momentum, which causes a force on the rocket air frame, can come from two sources: 1) the first is due to the internal acceleration of the incoming outside air by the rocket motor itself (thrust augmentation), 2) the deflection of the incoming outside air by the air frame itself (fin-less). As I look at some of the designs with the motor placed at the center and at the tail, I think one could argue that thrust augmentation could be present in either design. In reality I don't think thrust augmentation is appreciable by rocket exhaust either way. (This last statement can be a vast discussion in itself. I have seen arguments for this effect in both the model rocket world and the professional world including my former employer. Theoretical derivations show tremendous increases in thrust augmentation, but my colleagues and I concluded this was assuming complete mixing of cold incoming air with hot motor exhaust in a very short distance. It also is difficult to envision by control volume analysis how a straight tube could create forward thrust.)

I did a simple control volume derivation that shows deflection of incoming air by the air-frame could produce a side force(fin-less) for a small perturbation in the rocket's flight path.

That is a great description, and somewhat similar to the explanation of how a wing produces a lift force equivalent to the change in the momentum of the surrounding air.

 

[DaHabes]

I have flown Polaris A3 models scaled for 13mm and 18mm engines (not minimum diameter, airframe is one BT size bigger) using GDS. The flights are stable until motor burnout and then they start tumbling as expected. My goal is to scale up to where I can use long burn engines (24mm Apogee E6, 29mm Apogee F10, 29mm AT G12ST & H13ST, 38mm AT H45W). If those work, then scale up further to a 1/10 scale on a 5.5" airframe that could handle anything from 54mm to 98mm long burn motors. It's a journey...

 

[aerostadt]

I have flown Polaris A3 models scaled for 13mm and 18mm engines (not minimum diameter, airframe is one BT size bigger) using GDS. The flights are stable until motor burnout and then they start tumbling as expected. My goal is to scale up to where I can use long burn engines (24mm Apogee E6, 29mm Apogee F10, 29mm AT G12ST & H13ST, 38mm AT H45W). If those work, then scale up further to a 1/10 scale on a 5.5" airframe that could handle anything from 54mm to 98mm long burn motors. It's a journey...

Thanks for the information. Where was the motor positioned in the model, near the c.g. or at the aft-end? Any pictures?

 

[GlenP]

To keep it going straight after boost, and to keep the finless appearance, it would need like a bit of base drag at the bottom of the lower cylinder. Like a disc fin just slightly larger in dia or maybe a short transition, or conical “fin” that would not detract from the rocket appearance too much. Even though you are adding some drag, it might go higher overall if you keep it straight compared to a tumbling coast, or it will keep it low and slow.

 

[BigMacDaddy]

I did a simple control volume derivation that shows deflection of incoming air by the air-frame could produce a side force(fin-less) for a small perturbation in the rocket's flight path.

Thanks for doing this -- I need to use this as a lesson to help my kids appreciate the practical applications of math -- my HS math is just too atrophied for this project!

 

[BigMacDaddy]

I have flown Polaris A3 models scaled for 13mm and 18mm engines (not minimum diameter, airframe is one BT size bigger) using GDS. The flights are stable until motor burnout and then they start tumbling as expected. My goal is to scale up to where I can use long burn engines (24mm Apogee E6, 29mm Apogee F10, 29mm AT G12ST & H13ST, 38mm AT H45W). If those work, then scale up further to a 1/10 scale on a 5.5" airframe that could handle anything from 54mm to 98mm long burn motors. It's a journey...

I had put a fair amount of nose weight in my model since I was not confident in the GDS and wanted it to be stable regardless. It basically just started drifting backwards till the parachute deployed (I guess it was so symmetrical that there was nothing to make it tumble and CG was likely a bit in-front of CP anyway).

I'd like to try the same thing with less or no nose weight.

 

[lakeroadster]

One thing that confuses me about that write-up is that the induction stabilization is stated to be different than GDS because:

View attachment 492867

However, the Ikaros provided as an example of induction stabilization seems to show a rocket with intakes quite close to the rear of the rocket and not particularly close to the CG (assuming that the back end of the rocket is empty but the front end is more or less solid and full of propellant and flare fuel as shown in the diagram).

View attachment 492865 View attachment 492866

Anyway, I still do not understand the difference between GDS and induction stabilization. Does it look like my rocket is using GDS or induction? I did include nose weight but not sure if I needed to. How could I modify my design to better use induction and reduce the nose weight needed? Do I just need to make the rear tube that the engine fires through longer (2-3x the body tube diameter according to the article)?

Neat thread. Now that ^^^ is a finless rocket! * I wonder if anybody here has ever made on of these? (of course not out of aluminum... just a simple C6 powered BT-50 or BT-20 based rocket, maybe foil lined in the exhaust area).

Also, another question, what motor did you use for your finless rocket? Looks like a lot of flames.

*(Note to self.. put this on the build list)

 

[BigMacDaddy]

Also, another question, what motor did you use for your finless rocket? Looks like a lot of flames.
*(Note to self.. put this on the build list)

I used a C5-3 since the rocket was a bit heavy. If I was more confident in the GDS or induction stability I could get rid of some (all?) of the nose weight and go to a C6-3 or C6-5.

 

[GlenP]

... I still do not understand the difference between GDS and induction stabilization. Does it look like my rocket is using GDS or induction? I did include nose weight but not sure if I needed to. How could I modify my design to better use induction and reduce the nose weight needed? Do I just need to make the rear tube that the engine fires through longer (2-3x the body tube diameter according to the article)?

I think they are very similar, but according to their definition in the article the induction stabilization refers to the configuration where the outside air is induced through a vent into a secondary mixing tube closer to the c.g. of the rocket and the engine is mounted ahead of that induction vent. Your design looks like that, but your engine and induction vent is located farther aft on the rocket, and your induction tube is somewhat shorter and not as large dia,the article suggests a much larger dia for the mixing tube than the motor, more like 4x in Fig3, This might draw in more air and change the direction of its momentum, but the force acts closer to the c.g., since the moment arm of the stabilizing torque is not as high, so you need a more massive amount of air in that larger induction tube.


The gas-dynamic-stabilization or pseudo-fin method is when the engine is near the bottom of the rocket and recessed slightly into a larger tube, with or without vents. While the induction of air might not be as high, it has a longer moment arm about the c.g. so the stabilizing torque might be sufficient for stability. Your design also resembles that configuration somewhat too, you have much more venting and a bit longer tube aft of the engine than what they show as an example of GDS. Maybe a variation on the vented version shown in Fig 2 of the Apogee article?


That is how I understand their description of the two terms in the articles linked above, yours does have some traits of both, maybe you could make up your own term for this hybrid induction system, it is not exactly one or the other, but very cool regardless in how you have incorporated characteristics of both.

 

[BigMacDaddy]

Moved here...

I am also really curious about the limits / factors about creating a GDS stabilizing effect. I am guess it is about volume but also speed of gasses being pulled through vents as well as the positioning of those vents (the articles I read liken these airflows to those created by fins so I am assuming that further back and better distributed around rocket would = better stabilizing effect).

I also wonder what happens if you combine with smaller fins (i.e., seems that GDS would allow you to use smaller fins or less nose weight). For example, would GDS be a significant stabilizing factor in this design:

 

[Y3kankerous]

Moved too.....
I'm going to say yes!

As long as the distance from the motor to the ducts and then to the tube exit is right for the tube dia.

The reason I'm saying that is that's one of the designs described in that PoF article not because I can really claim to have a handle on it.

I am also basing my analogy for the effect of the duct lateral flows on the air flows around fins, but I can't work out if this is a good or bad analogy. What about a straight thru GDS roc (open tube with axial motor mount struts) with all the lateral stabilisation at the fore? I can't make myself think that won't be stable because it should still oppose perturbations?!?! I will def build one soon to find out...

 

[DaHabes]

would GDS be a significant stabilizing factor in this design:

I don't think so, the fins would provide more than enough stability, GDS wouldn't add much.

Without fins, GDS (or induction stabilization) will lose stability after burnout. I have in my mind an idea to build a Polaris A3 for a long-burn engine with fins embedded in the airframe around the MMT that pop out after motor burnout, so it stays stable until apogee. I would combine fin deployment (likely with servos) with apogee deployment using a Eggtimer Proton or Quantum and a JLCR.

 

[DaHabes]

Thanks for the information. Where was the motor positioned in the model, near the c.g. or at the aft-end? Any pictures?

I have fins on the design so Rocksim will do a reasonable sim until burnout. From there I can determine altitude at burnout, which isn't much below apogee once it starts tumbling.

 

[BABAR]

As Heinlein said, TANSTAAFL.

There is no question GDS is cool and challenging. as for efficiency, two issues, with the second one pretty devastating.

first and foremost, does GDS WHILE under thrust have less drag than a standard finned rocket? The idea that it is the ultimate in minimum diameter (well, maybe not, can you MAKE it minimum diameter or do you need space for a motor tube) DOES get rid of those pesky draggy fins, but at what cost?

second, even if the answer to the first point is YES, it is more efficient under thrust, the fact that it immediately goes unstable in coast phase seems kind of like an efficiency deal breaker, although @DaHabes concept of deploying fins at coast phase is intriguing. One lightweight method for this would be to use a zero delay motor, this could be hooked up with a burn string or other fin release/deploy system as the motor immediately blows out at end of thrust phase. Hmmmm, now how do we deploy the chute? Maybe a sustainer motor that continues to hoist the (now finned) segment of the rocket?

 

[lakeroadster]

As Heinlein said, TANSTAAFL.

There is no question GDS is cool and challenging. as for efficiency, two issues, with the second one pretty devastating.

first and foremost, does GDS WHILE under thrust have less drag than a standard finned rocket? The idea that it is the ultimate in minimum diameter (well, maybe not, can you MAKE it minimum diameter or do you need space for a motor tube) DOES get rid of those pesky draggy fins, but at what cost?

second, even if the answer to the first point is YES, it is more efficient under thrust, the fact that it immediately goes unstable in coast phase seems kind of like an efficiency deal breaker, although @DaHabes concept of deploying fins at coast phase is intriguing. One lightweight method for this would be to use a zero delay motor, this could be hooked up with a burn string or other fin release/deploy system as the motor immediately blows out at end of thrust phase. Hmmmm, now how do we deploy the chute? Maybe a sustainer motor that continues to hoist the (now finned) segment of the rocket?

Spin tabs.. I Thunk! GDS will work using spin tabs so the rocket spins like a bullet from a gun.

​

 

[BigMacDaddy]

Spin tabs.. I Thunk! GDS will work using spin tabs so the rocket spins like a bullet from a gun.

View attachment 565827

View attachment 565828View attachment 565829View attachment 565830View attachment 565831View attachment 565833View attachment 565834
​

Where is your CG on Thunk? Front or back half of the rocket?

I wonder if it would be possible to induce a spin based on the way that the rocket pulls in air. I played with that with the Bullet Bill model but finally gave up on since I did not know if it worked and it made the printing and everything more complex. The idea was to use canted vents that air would be pulled through by the GDS effect.

 

[lakeroadster]

Where is your CG on Thunk? Front or back half of the rocket?

I wonder if it would be possible to induce a spin based on the way that the rocket pulls in air. I played with that with the Bullet Bill model but finally gave up on since I did not know if it worked and it made the printing and everything more complex. The idea was to use canted vents that air would be pulled through by the GDS effect.

View attachment 565854

My mind sim says unless the fins are outside the o.d. of the body, you probably won't get much spin... but... ​

Here's the CG per Open Rocket... As measured it's about 5.62 with a D12-5.​

​