Another Crack at "Finless" Bullet Bill

[jqavins]

Yeah, I get that wind turning no matter the direction is what matters, and I totally agree. Part of the problem, one might say, I think, was on my end: whenever you wrote "windcocking" I understood you to mean specifically into the wind. On the other hand, one could say, I think, the problem was on your end if you were misusing the word. It comes from this guy, whose one job is to always point into the wind:


ANYWAY, I was thinking about it last night as I was trying to get to sleep, and I think I've got it. We have both been right all along.

Body lift won't make it weathercock, since this rocket's stability doesn't come from body, fin or any other source of lift. So forget the CP. Mostly forget the NASA figure, since it depends on lift acting through the CP. And you were right, it will weathercock all the same.

In the mind-wandery state on the way to the edge sleep, but not the wildly disorganized state the leads to stupidity, I realized that stability and weathercocking are the same thing no matter what mechanism brings stability. Stability means turning to align the rocket with the airspeed vector, which as exactly the same thing as turning into the wind. So, stability equals weathercocking no matter where stability comes from. So you were right that it weathercocks, and I was right the mechanism is more complicated than the NASA document and figure make out (since the mechanism of stability is more complicated). I was uncertain because of the more complicated mechanism intil I realized it doesn't matter.

 

[lakeroadster]

Body lift won't make it weathercock, since this rocket's stability doesn't come from body, fin or any other source of lift. So forget the CP. Mostly forget the NASA figure, since it depends on lift acting through the CP. And you were right, it will weathercock all the same.

The rocket still has a CP, and some of the forces on the rocket act through the CP. You can't simply just "forget the CP".

Dean Black discusses the CP of finless rockets in the article he wrote in the Apogee Peak of Flight Newsletter 379: Finless Rockets Using Engine-Driven Gas-Dynamic Stabilization

 

[jqavins]

I'll read the artical before further responding.

 

[Sooner Boomer]

The rocket still has a CP, and some of the forces on the rocket act through the CP. You can't simply just "forget the CP".

Dean Black discusses the CP of finless rockets in the article he wrote in the Apogee Peak of Flight Newsletter 379: Finless Rockets Using Engine-Driven Gas-Dynamic Stabilization

I have two issues with the article.

1) it appears as though he made a rocket using two aluminum drink cans. This is not allowed at either a NAR or Tripoli launch. Out in your own back yard, you can do whatever you can get away with.
2) the gap between body section shown is less than the minimum I would use. The author describes it as the intake area, equal to the square of the body diameter. By having this gap too small, one risks developing an area of low pressure in the area below the nozzle, decreasing thrust. Having suffecient area allows the flow from the nozzle to fully entrain external air, "augmenting"* the flow through the lower tube.

* viz, Lil' Augie

 

[BigMacDaddy]

I just did the calculation for this Bullet Bill model -- my gap is around 16mm and I am providing 66% more area of gap in external shell than the area of the tube. Is this too much? Too little?

Is speed of air a factor in GDS or is it just volume of air? My recollection is that the air flow being pulled in via GDS was analogous to a fin. Does faster air equate to a larger fin? Would my current design create the equivalent of a cone fin?

I could close up a portion of the opening with a 3D printed cone. I assume this would increase the speed of air coming through the remaining / small space. Would that increase the GDS force exerted on the rear of the rocket?

 

[lakeroadster]

I have two issues with the article.

1) it appears as though he made a rocket using two aluminum drink cans. This is not allowed at either a NAR or Tripoli launch. Out in your own back yard, you can do whatever you can get away with.
2) the gap between body section shown is less than the minimum I would use. The author describes it as the intake area, equal to the square of the body diameter. By having this gap too small, one risks developing an area of low pressure in the area below the nozzle, decreasing thrust. Having suffecient area allows the flow from the nozzle to fully entrain external air, "augmenting"* the flow through the lower tube.

* viz, Lil' Augie

Write an article, it would be great to compare your data with Dean Black's. We'd all benefit from some additional data. Specifics on GDS seems to be quite rare.

In regard to using metal, it's been discussed here on TRF previously. Some folks state that it is not an issue for Tripoli launches if the design dictates / requires it. And in this case, where you have a motor firing inside a body tube, the design requires it.

 

[Sooner Boomer]

Write an article, it would be great to compare your data with Dean Black's. We'd all benefit from some additional data. Specifics on GDS seems to be quite rare.

In regard to using metal, it's been discussed here on TRF previously. Some folks state that it is not an issue for Tripoli launches if the design dictates / requires it. And in this case, where you have a motor firing inside a body tube, the design requires it.

Another finless rocket

I didn't want to hijack the other thread. This is the build sequence for a finless rocket I designed and built last August. Hopefully I'll get to fly it before this August. One main difference is that it's designed to be tube launched, rather than off a rod/rail. I've got a nice 4' heavy...

www.rocketryforum.com

I had not seen the article before yesterday.

Made from cardboard, plastic, and poplar - no metal. (sorry about lousey paint job, it's all I had at the time)



[edit] oops, there the article is, in post #24, from May 3, '21. five years after I built the rocket.
[edit 2] I said I wasn't going to build any more rockets this year. I may just have to build another one of these. I just happen to have the parts, including screw-on retainers.

 

[BigMacDaddy]

Finally put in a bit of time to assemble the latest version and launched today. Went with more nose weight and a slightly more recessed E12-6 engine (should have been an E12-4 since delay was too long).

It flew quite well -- except for ejection charge not going off till it was 20 feet from the ground.

I am thinking about making another revision with a smaller ring tail part and a more recessed engine -- trying to better utilize GDS. This has been a fun model to play around with.





You cannot see much from the photo but there is some engineering that goes into the design of the internal frame here.

 

[cwbullet]

Impressive. Did it require nose weight?

 

[jmasterj]

Went with more nose weight and a slightly more recessed E12-6 engine (should have been an E12-4 since delay was too long).

Impressive. Did it require nose weight?

 

[BigMacDaddy]

Impressive. Did it require nose weight?

Thank you... As to nose weight, oh yes... a lot... so not sure it is really GDS as much as it is pushing the CG forward into the nosecone...

200g nosecone...



I just attached parachute to nosecone on this model. The body just bounces for recovery.

 

[Daddyisabar]

All this mind simming is blowing my mind!

 

[BigMacDaddy]

All this mind simming is blowing my mind!

This is basically just an internal traction motor...

 

[Daddyisabar]

This is basically just an internal traction motor...

As long as the Anti Traction League dosen't take me out I'm OK. So scary.

 

[lakeroadster]

It flew quite well -- except for ejection charge not going off till it was 20 feet from the ground.

What did the simulation show in regard to apogee and altitude at ejection. Maybe it's just the perspective of the video, but it still looks like it's flying more horizontal than vertical.

 

[BigMacDaddy]

What did the simulation show in regard to apogee and altitude at ejection. Maybe it's just the perspective of the video, but it still looks like it's flying more horizontal than vertical.

I stopped simulating this since I have no confidence in the simulation for this design.

Mostly that is the perspective of the video. It flew on a bit of an angle but mostly vertical. I think the angle was mostly due to the rod leaning over due to the heavy rocket -- i.e., the sideways direction it flew is directly opposite the launch rod / launch lug placement.

 

[lakeroadster]

I stopped simulating this since I have no confidence in the simulation for this design.

Mostly that is the perspective of the video. It flew on a bit of an angle but mostly vertical. I think the angle was mostly due to the rod leaning over due to the heavy rocket -- i.e., the sideways direction it flew is directly opposite the launch rod / launch lug placement.

Flying into the wind... with the wind... none of the above?

It does look great, without any fins, clear or otherwise.

I'd be tempted to do a 3 motor cluster to get the motors out nearer to the body tube, to help with the GDS.

 

[BigMacDaddy]

Flying into the wind... with the wind... none of the above?

It does look great, without any fins, clear or otherwise.

I'd be tempted to do a 3 motor cluster to get the motors out nearer to the body tube, to help with the GDS.

I had positioned it about perpendicular to wind but it was not very windy today so not sure that wind had much of an impact.

I do wish I understood better GDS. If I made the slot smaller (or partly filled it in around the circumference) would be make the air flow faster and thus create greater stability?

I was thinking of moving the slot back 20mm or so -- the current design assumed that the rear section would work a bit like a ring tail once air was pulled in through the slot. However, if it is mostly the air flow of the air being sucked in that creates stability then moving the slot back would equate to moving fins back.

 

[lakeroadster]

I had positioned it about perpendicular to wind but it was not very windy today so not sure that wind had much of an impact.

I do wish I understood better GDS. If I made the slot smaller (or partly filled it in around the circumference) would be make the air flow faster and thus create greater stability?

I was thinking of moving the slot back 20mm or so -- the current design assumed that the rear section would work a bit like a ring tail once air was pulled in through the slot. However, if it is mostly the air flow of the air being sucked in that creates stability then moving the slot back would equate to moving fins back.

To optimize GDS the thrust of the motor needs to pull in the air outside the skirt of the rocket through the annular gap, and force it out the bottom of the rocket. Unless the motors are closer to the body tube... that ain't gonna happen.

 

[KenECoyote]

I'm not completely sure what the objective is with this but what do you think would happen if you placed some forward-facing air-scoops, just below the nosecone, that would funnel airflow into the body tube and over the internal fins and out the back?

Or would that defeat what is being attempted here?

I tried that a bunch of years ago, but imho it's different from gds.

There seems to be a lot of confusion for many over how gds works and at that time I came up with this drawing of what I believe is happening and how it stabilizes a rocket:

EDIT ADD: So the motor thrust vectors the air over the rear basically treating it almost like a ring fin, however I believe it's also creating a circular type of low pressure around it to help center the bottom of the rocket around the thrust and bottom ring (in this case).

 

[KenECoyote]

Finally put in a bit of time to assemble the latest version and launched today. Went with more nose weight and a slightly more recessed E12-6 engine (should have been an E12-4 since delay was too long).

It flew quite well -- except for ejection charge not going off till it was 20 feet from the ground.

I am thinking about making another revision with a smaller ring tail part and a more recessed engine -- trying to better utilize GDS. This has been a fun model to play around with.

View attachment 564957



You cannot see much from the photo but there is some engineering that goes into the design of the internal frame here.

View attachment 564958

Great work and good flight!

I think there's a really good balance of gds and nose weight here.

 

[BigMacDaddy]

I tried that a bunch of years ago, but imho it's different from gds.

There seems to be a lot of confusion for many over how gds works and at that time I came up with this drawing of what I believe is happening and how it stabilizes a rocket:
View attachment 564995
EDIT ADD: So the motor thrust vectors the air over the rear basically treating it almost like a ring fin, however I believe it's also creating a circular type of low pressure around it to help center the bottom of the rocket around the thrust and bottom ring (in this case).

Thanks Ken -- good picture.. that is what I think is happening also (and apparently the amount of air moving through that rear tube is much more than just what the engine pushes).

I wonder -- smaller or larger rear ring tail part (which trades off recess depth on the front) and also wonder constraining the opening or leaving it more or less fully open.

 

[BDB]

Very interesting project. The easiest way to test if the stabilization is due to GDS would be to tape over the gap and launch it.

 

[Cape Byron]

Very interesting project. The easiest way to test if the stabilization is due to GDS would be to tape over the gap and launch it.

That's probably an idea that was way to clever for me to think of. Well done that man.

 

[jqavins]

If anyone is taking bets, my money says that with the gap covered over it will be unstable. Bigger gap, smaller gap, move it forward, move it aft, I have no idea. Take the gap away completely so there is nothing but nose weight going for you and I bet it fails dramatically.

Why? The details of how GDS works are a subject about which I know nothing. That GDS works is certain. Without GDS you have a short rocket stabilized only by nose weight, which is also well know to not work.

 

[lakeroadster]

The other clue, well in my mind at least, that this rocket is not developing Engine Driven Gas Dynamic Stability is that the rocket should go unstable as soon as the thrust stops... and it doesn't.

 

[jqavins]

For what it's worth:

https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.apogeerockets.com/education/downloads/Newsletter379.pdf&ved=2ahUKEwjFp8Tw2qv9AhVPEFkFHbe5Cv4QFnoECBgQAQ&usg=AOvVaw3jXl9YmbvTi0_iioflcd90

The other clue, well in my mind at least, that this rocket is not developing Engine Driven Gas Dynamic Stability is that the rocket should go unstable as soon as the thrust stops... and it doesn't.

Well, you have a point.

 

[lakeroadster]

Well, you have a point.

and it's on top of my head.. lol..

Yeah, this rocket isn't following the data outlined in the Apogee newsletter 379. It's principals of stabilization fall more into the category of a pyramid rocket, than GDS, IMO.

From page 6 of the above referenced newsletter.

The resistance to changes in pitch and yaw angles during the thrust-powered portion of the flight gives induction stabilized rockets different flight characteristics than conventional model rockets. ​

​

Induction rockets launched at angles other than vertical still tend to maintain near-constant pitch and yaw angles. This feature makes it possible to launch inductors at somewhat larger angles off the vertical without the rocket following a gravity curve into the ground. ​

​

Delay times should be kept small for such lower elevation launches, however, since the unusual “nose up” attitude will not be maintained after engine thrust ceases unless the rocket has a strong spin. ​

​

Noseup flight attitudes tend to cause “lofting”: extended horizontal range due to the generation of lift and associated altitude gain due to that lift. ​

​

Once again, post-thrust lofting is only pronounced in inductors that remain stable after thrust termination by virtue of a strong spin.​

 

[KenECoyote]

Thanks Ken -- good picture.. that is what I think is happening also (and apparently the amount of air moving through that rear tube is much more than just what the engine pushes).

I wonder -- smaller or larger rear ring tail part (which trades off recess depth on the front) and also wonder constraining the opening or leaving it more or less fully open.

View attachment 564997

That's likely most easily answered through testing. Theory wise you'll get all sorts of opinions and imo there's so many factors here (each which is variable and has variable effect) that it's really hard to tell for sure.

BTW There's also a base drag effect I'm sure.

 

[BigMacDaddy]

Very interesting project. The easiest way to test if the stabilization is due to GDS would be to tape over the gap and launch it.

Yes, fully or partly covering the middle gap has potential for testing.

However, it does not eliminate the potential that GDS is having an impact -- GDS can work when air is pulled in through the base of the rocket as well as if there are mid-body intakes.