Simple finless rocket in flight

[David Hall]

I'm a teacher and coach 5 junior high TARC teams. To illustrate to my students how rocket stability really does come down to CG over CP (and symmetry), I built a heavy-nosed finless rocket. The "nose cone" is a soft plastic 3" "ball pit" ball filled with 400 grams of sand. To move the CG further forward, I mounted the motor about 10.5" up the 3" rocket body tube. The tail 10.5" tube section is removable for motor access (and tail-tube replacement). The CG of the entire 34" rocket is about 9" from the tip of the nose cone. The rocket has 3 pairs of 1/2" launch lugs (for symmetry) that are equally in front of and behind the center of the rocket so as to not provide any incidental stability. I built and flew this thing in November of 2012 and used an old/odd Aerotech F-something single use motor. I'm not even sure if we knew exactly what it was; I think the label may have been scraped off. Anyway, it flew stable and straight, but was under-powered and had way too long of a delay.

I have no idea how much thrust may me lost by the nozzle blasting into a 3" tube; it has to be a bit at least. Anyone have an idea on this?

 

[TopRamen]

That's not something we see everyday.op::confused2:

 

[jadebox]

Placing the motor so far up in the body tube produces a dramatic loss of thrust known as the "Krushnic Effect." Basically, the body tube creates an inefficient nozzle for the rocket motor. For a detailed explanation, see:

https://www.nar.org/pdf/TCR1.pdf

-- Roger

 

[wfcook]

This is a really cool project. I'd love to see more detail.


Sent from my iPhone using Rocketry Forum

 

[rstaff3]

So, where do you think the CP is, and why?

 

[TopRamen]

:lol:
It's definitly "Odd".

 

[David Hall]

Placing the motor so far up in the body tube produces a dramatic loss of thrust known as the "Krushnic Effect." Basically, the body tube creates an inefficient nozzle for the rocket motor. For a detailed explanation, see:

https://www.nar.org/pdf/TCR1.pdf

-- Roger

That is just awesome! Thanks; I think it explained why it appeared under-powered.

I skimmed the PDF, but couldn't determine how much extended tube I could use before the effect kicks in...

 

[rstaff3]

That is just awesome! Thanks; I think it explained why it appeared under-powered.

I skimmed the PDF, but couldn't determine how much extended tube I could use before the effect kicks in...

The rule of thumb is you get degradation if you set the motor more than one caliber up into the tube. This may have come from the Handbook of Model Rocketry. I'd have to check.

 

[David Hall]

So, where do you think the CP is, and why?

The CP of a cylinder is right in the center. This thing is close to a cylinder.

 

[David Hall]

The rule of thumb is you get degradation if you set the motor more than one caliber up into the tube. This may have come from the Handbook of Model Rocketry. I'd have to check.

Is a caliber the radius of the tube or ???

 

[rstaff3]

Is a caliber the radius of the tube or ???

Caliber is the diameter of the body tube.

 

[rstaff3]

It is a cylinder with a hemispherical nose cone. The latter makes a big difference. In the standard models of static stability, the overall CP will be near the base of the rocket's nose cone. I'm amazed that it flew.

 

[rstaff3]

I checked 'the Handbook' and that's not where my rule of thumb came from. Stine only mention the Krushnic effect as it relates to a 2-stage rocket than blows the engine vs. the entire booster. I see other references to the same rule of thumb, but have no good source.

 

[David Hall]

Caliber is the diameter of the body tube.

Awesome. So, if I'm using a 3" body tube, I can extend the tube 2.75" behind the end of the nozzle without hurting motor performance, right?

BTW, it's pretty exciting to have just discovered this place and folks with so much knowledge!

 

[rstaff3]

Tim Van Milligan says to inset the motor by no more than 1/2 caliber. In "Model Rocket Design and Construction". No quantitative data is provided.

 

[jadebox]

I did a science fair project in the ninth grade (I think) on the Krushnic Effect. I built a test stand that measured the total impulse of a rocket motor (using a 555 timer and a digital calculator!). I hypothesized that the effect was due to the body tube acting as an extension of the rocket motor's nozzle and the recorded numbers matched the predicted ones pretty well. I was using 18mm motors in a BT-20 diameter tube. Unfortunately, I don't recall the exact effects of recessing the motor at different depths in the tube, but I think the thrust dropped dramatically somewhere between about 1 or 2 calibers in. I'm not sure the caliber rule of thumb is all that good, though, because I don't think the Krushnic effect happens much at all when the body tube is somewhat larger in diameter than the motor.

-- Roger

 

[rstaff3]

I'm not sure the caliber rule of thumb is all that good, though, because I don't think the Krushnic effect happens much at all when the body tube is somewhat larger in diameter than the motor.

-- Roger

I can see that the larger the BT diameter the less effect there may be. While I can't back it up, I think the one caliber rule is still reasonable. It would be easy to test if you had a test stand.

 

[jadebox]

I can see that the larger the BT diameter the less effect there may be. While I can't back it up, I think the one caliber rule is still reasonable. It would be easy to test if you had a test stand.

I guess the rule works in the sense that .... if you recess the motor by no more than .5 to 1.0 caliber into the body tube, you won't be affected by the Effect. But, it doesn't tell you how far that you can really get away with recessing the motor.

-- Roger

 

[rstaff3]

Agreed. I see this discussed so often that it surprises me there isn't hard data somewhere. I've seen a couple of references (including one from the link you provided) to reports done in the '60s, but nothing newer.

David, to muddy the water, there are a lot of references to the fact that if you add slots in the tube to let air pass by the motor that you can eliminate (mitigate?) the effect.

I still am not sure why your rocket would be stable. I did a quick Rocksim (just from eyeballing the photo and using the info in the first post) and the lugs do help a little. But not near enough to provide a stable model. That being said, I've seen a rocket barely lift off the rod and hover until the motor burned out. So much for hard and fast rules like minimum launch speed. I'm not sure that event, or your finless flight, would be reliably reproducible.

Great discussion though.

 

[David Hall]

Agreed. I see this discussed so often that it surprises me there isn't hard data somewhere. I've seen a couple of references (including one from the link you provided) to reports done in the '60s, but nothing newer.

David, to muddy the water, there are a lot of references to the fact that if you add slots in the tube to let air pass by the motor that you can eliminate (mitigate?) the effect.

I still am not sure why your rocket would be stable. I did a quick Rocksim (just from eyeballing the photo and using the info in the first post) and the lugs do help a little. But not near enough to provide a stable model. That being said, I've seen a rocket barely lift off the rod and hover until the motor burned out. So much for hard and fast rules like minimum launch speed. I'm not sure that event, or your finless flight, would be reliably reproducible.

Great discussion though.

The ultimate test is to fly a rocket on a string. Even though I am the technology teacher at my school, I put more faith in the string test than rocket sim programs and require my students to fly their loaded rockets on a string before first flight. There's nothing like a real world test.

I did this with the finless rocket. I tied a string around the CG so that the rocket would hang level and swung it around in a circle. It tracks nose into the wind just like a rocket should. Flying it on a string into the wind on a windy day is even better because the rocket's stability does not have to fight the torque of inducing a 360 spin for every lap the rocket takes. This is especially true with heavy loaded TARC egg lofters that have almost no dead air onboard.

I'm going to rebuild the finless rocket with shorter a tail and will certainly test it on a string first.

 

[bill_s]

Barrowman's equations for stability ignore the body tube entirely, which while better than the cardboard cutout method which weighs the tube equally with everything else, is obviously not completely true. Once there is angle of attack, the tube comes into play; I expect Open Rocket shows this. I would guess that almost any rocket consisting of a weighted nose cone and a lightweight tube (no motor) would be stable as the aerodynamic effect of the tube will be higher than its weight.

This finless rocket is helped by the nose cone being as minimal as possible, and I expect the rear launch lugs do help it out substantially.

Flying it on a string into the wind on a windy day is even better because the rocket's stability does not have to fight the torque of inducing a 360 spin for every lap the rocket takes.

I agree.

 

[rstaff3]

The ultimate test is to fly a rocket on a string. Even though I am the technology teacher at my school, I put more faith in the string test than rocket sim programs and require my students to fly their loaded rockets on a string before first flight. There's nothing like a real world test.

I did this with the finless rocket. I tied a string around the CG so that the rocket would hang level and swung it around in a circle. It tracks nose into the wind just like a rocket should. Flying it on a string into the wind on a windy day is even better because the rocket's stability does not have to fight the torque of inducing a 360 spin for every lap the rocket takes. This is especially true with heavy loaded TARC egg lofters that have almost no dead air onboard.

I'm going to rebuild the finless rocket with shorter a tail and will certainly test it on a string first.

OK this then is the answer to my question in the original post. I guess you can't argue with success. I'm anxious to to see the results of your ongoing experiment.

 

[rstaff3]

Barrowman's equations for stability ignore the body tube entirely, which while better than the cardboard cutout method which weighs the tube equally with everything else, is obviously not completely true. Once there is angle of attack, the tube comes into play; I expect Open Rocket shows this. I would guess that almost any rocket consisting of a weighted nose cone and a lightweight tube (no motor) would be stable as the aerodynamic effect of the tube will be higher than its weight.

This finless rocket is helped by the nose cone being as minimal as possible, and I expect the rear launch lugs do help it out substantially.

Yeah, these are good points.

 

[rstaff3]

David, please post the weight of the your new version along with the pertinent dimensions.

 

[Daddyisabar]

With enough power and nose weight, good weather conditions and a long and stable launch platform almost anything will fly. I tell the folks that fins are over rated on model rockets. No fins or two fins way up front are cool. They just look at me funny. I tell them to put the motors up front as the Chinese did in the first place, good for CG. They look at me even funnier. I tell them I like to shape my airplane fins with fire in flight, that burning off little bits on the back end of you rocket is just fine. That one didn't go over as well.

I like the photo of the exhaust - looks like a Argus pulse jet flame on a Buzz Bimb. All that lovely rocket power being sucked away and just burnin' the inside of the tube is cool. Take and put some kinks in it near the top, add a conical nose cone (ACME SPITFIRE like), paint it flat grey and call it the Flyin' Stovepipe. Nice, powerful and kicky motors with short delays - gotta love those oddrocs!

Just be sure to tell them you are a certifide rocket scientist and you know exactly what you are doing. It is all science baby!

 

[rstaff3]

I tell them to put the motors up front as the Chinese did in the first place, good for CG. They look at me even funnier.

How about ALL fin AND motors on top?

 

[Daddyisabar]

How about ALL fin AND motors on top?

View attachment 163739

If the all High Power Christmas tree fliers would do it that way there would be no cool Christmas tree crash videos! That one looks kid tested, mother approved. Canted motors solve a lot of problems but the loss of efficiency is more than the hardened model rocket engineer can take. The motor needs to go straight down, no exceptions. All the precious thrust be utilized and none lost to silly cant angles, stinkin' nose weight or Krushnic effect. The rocket must not incur any permanent heat damage and must be computer simulated or swing tested to ensure safety. Symmetry of at least three fins at the bottom of the rocket is the way to go. Aerodynamic nose cones and slick paint jobs RULE. That is what they tell me are the Hallmarks of good model rocket design. I can't argue with all their book learnin' and certified status, so I just say if they let me fly it there is a chance of a really cool crash video.

 

[jeffgeraci]

Hasn't anyone ever seen a bottle rocket fly straight??? No fins at all, just a length of stick. You can clearly see on this rocket that there's sufficient drag created by the length of airframe, and the added weight of the nose keeps it on its trajectory. No mystery here, just good fun. Thanks for sharing your odd roc.

 

[Daddyisabar]

They keep telling me nose weight and more power will not solve all my problems. They keep telling me Money will not solve all my problems. Well, maybe not, but solving 99% of them either way ain't bad!

 

[UpsilonAerospace]

Hasn't anyone ever seen a bottle rocket fly straight??? No fins at all, just a length of stick. You can clearly see on this rocket that there's sufficient drag created by the length of airframe, and the added weight of the nose keeps it on its trajectory. [snip]

I admire the stability of these rockets, and I want to build something like this some time (oversized bottle optional)...

[video=youtube_share;O4u9s74goG0]https://youtu.be/O4u9s74goG0[/video]

...although I am a bit worried that the RSO won't allow anything non-conventional out of me after my soccer field rocket experiments.