So- Why Did My Fake Engine Bell Rip Off?

[jmmome]

When I was a high school student, I wrote research papers on "The Krushnik Effect" and "Determining the Efficiency of Ducted Systems in Model Rocketry". I don't think that either of these phenomena had anything to do with my plastic flower pot-turned fake engine bell being ripped from Marvin Martian Jr., but I thought I'd put it to guys & gals much smarter than I as to what happened.

My obvious first thought is that there was more airflow stress on the engine bell than the security that the three screws holding it to the bottom of the rocket provided. I noticed that the ends of the screws only stuck out maybe 1/4" past the bottom of the attachment points to the rocket. The answer is probably as simple as that.

However- it seemed that the St. Louis Rocketry Kaboom Krewe, who successfully flew the ten foot tall/ 400+ pound big brother of my rocket, noticed that the airflow didn't follow the shape down the bottom half of the football-shaped rocket. So I wondered if my fake engine bell was in a "dead air" zone. They were pretty certain that much of their fin surfaces weren't in contact with airflow.

I also noticed from one attached photo that the tip of the engine nozzle extends past the fake engine bell. I didn't know if some funky low air pressure area could have been set up inside the engine bell which caused the bell to be ripped off. The other attached photo shows the bell being ripped off.

Any thoughts? For me, this was one event that is kind of fun to ponder. Not really imprtant, but fun nonetheless.

 

[RocketRev]

Hello Mike,

Bell Attachment points/screws/rips/tears??????

This post raises all sorts of questions in my brain and frankly, my brain is demanding more information. I try to tell it to go slow, but well...... no response on that one. But I'm going to try and ask my question is separate replies to make it easier for me to keep up with your answers.

Your first piece of info that makes me ask questions is that there was "more airflow stress than engine bell security" and that the screws only went "1/4 inch past the bottom of the attachment points."

Here's where I think that a picture would be worth a 1000 words. Could you post a pic of the no longer connected attachment points/screws? Did the plastic from the Flowerpot Bell get ripped thru by the screws? Or what?

Brad

 

[RocketRev]

AIRFLOW question.

You mentioned that the St Louis Rocketry Kaboom Krewe "noticed that the airflow didn't follow the shape down the bottom half of the football shaped rocket."

That's a fascinating statement for which I would love to have any evidence whatsoever. What evidence do you/they have to come to that conclusion? I only ask because it is clear in your video that there was a great deal of airflow past the bell on your little brother rocket to the Kaboom Krewe's larger verison.

Along the same lines of questioning, you asked if your engine bell was in a dead air zone? Just looking at the video it clearly was not in a dead air zone. Frankly, I'm not sure there's such a thing as a dead air zone around a rocket that's under the conditions caused by a burning motor which yours clearly was.

And thirdly, you said that they came to the conclusion that "much of their fins surface was not in contact with the airflow." Again, what evidence do you or they have in order to come to that rather extraordinary conclusion?

Brad

 

[RocketRev]

Funky Low Pressure Area inside the Bell

Lastly, you were wondering about some kind of "funky low pressure area inside the bell." I too am wondering about the huge pressure differentials going on between the outside and the inside of the bell, and particularly under thrust. BTW - what motor was it flying on?

I would love to see a real wind tunnel study done on this rocket. Clearly, there's going to be a great deal of turbulence behind this rocket particularly under thrust. In the video, one can see the fins fluttering a bit, as well as the edges of the flower-pot bell fluttering. It is more likely that the uneven airflow past the bell combined with the pressure of mach-speed fluid dynamics in the gaseous outflow of the motor's nozzle causing huge pressure differentiations between the outside and the inside of the flowerpot bell that caused it to rip off. Are there any cracks in the "bell?" at either the attachment points or along the trailing edge?

Base drag in the "football" shaped rocket lowers total drag, but the convergence of all that airflow being pressurized as it passes around the airframe and that pressure being released as the airflow comes back together at the engine bell from the speed of the rocket alone could have been enough to over stress your bell attachment strategy. Adding in the pressure from the motor itself? You've suddenly got a recipe for greatly overstressing the flutter resistance of a plastic flower pot engine bell.

BTW - I loved the video and the pics. They really help show both the fin and bell flutter which is evidence of a great deal of high pressure turbulence at the convergence zone around the bell.

Brad

 

[jmmome]

First of all- thanks for all your observations & thoughts! I thought a good place to start would be to view the big brother's launch at LDRS 38.

The screws were still in the plastic flowerpot (see pics). There is a 3/8" centering ring inside the lip of the pot through which the screws go before entering into the centering ring base of the rocket. A layer of fiberglass on the outside bottom of the flower pot lip which attaches to the base of the rocket was also ripped away. The screws simply pulled out of the rocket base centering ring- the simple explanation is they were waaaay too short.

I have no personal evidence about the airflow, except that a member of the St. Louis Kaboom Krewe thought that the shape, at that speed, didn't give "clean air" to the fins. I concluded much too much from that statement. They apparently "borrowed' a supercomputer for about four hours to sim their rocket with NASA-grade(?) sim software (nothing we novices can buy), and that was their determination. So that's why I moved my fins forward to emerge from the fattest point on my rocket. With more power, i think mine would have flown straighter for a longer period of time.

I used an L1520 reload with about 400# max thrust, which gave me a 6.15 to 1 max thrust to weight ratio. The Kaboom Krewe's rocket had about 2,000 # max thrust compared to a 500# rocket, which gave them a 4:1 ratio. Both of our rockets were underpowered, which accounts for each of them making a "right turn". Mine went straighter for more of my flight than theirs, but both suffered that same "right turn" fate eventually.

I agree it is safe to believe the fake bell was ripped off from air flowing over it's surface. The video evidence seems pretty clear to me as well. But it was fun to ponder these other outlier possibilities. Thanks again!!!!

 

[SeanW78]

First of all- thanks for all your observations & thoughts! I thought a good place to start would be to view the big brother's launch at LDRS 38.

The screws were still in the plastic flowerpot (see pics). There is a 3/8" centering ring inside the lip of the pot through which the screws go before entering into the centering ring base of the rocket. A layer of fiberglass on the outside bottom of the flower pot lip which attaches to the base of the rocket was also ripped away. The screws simply pulled out of the rocket base centering ring- the simple explanation is they were waaaay too short.

I have no personal evidence about the airflow, except that a member of the St. Louis Kaboom Krewe thought that the shape, at that speed, didn't give "clean air" to the fins. I concluded much too much from that statement. They apparently "borrowed' a supercomputer for about four hours to sim their rocket with NASA-grade(?) sim software (nothing we novices can buy), and that was their determination. So that's why I moved my fins forward to emerge from the fattest point on my rocket. With more power, i think mine would have flown straighter for a longer period of time.

I used an L1520 reload with about 400# max thrust, which gave me a 6.15 to 1 max thrust to weight ratio. The Kaboom Krewe's rocket had about 2,000 # max thrust compared to a 500# rocket, which gave them a 4:1 ratio. Both of our rockets were underpowered, which accounts for each of them making a "right turn". Mine went straighter for more of my flight than theirs, but both suffered that same "right turn" fate eventually.

I agree it is safe to believe the fake bell was ripped off from air flowing over it's surface. The video evidence seems pretty clear to me as well. But it was fun to ponder these other outlier possibilities. Thanks again!!!!

As I was glad to help you load and carry back this rocket after it landed (I'm the guy in the grey sweatshirt in your video), I had a good view of the flight. As it arced over, into the wind, it appears to soar, supported by its wings. This occurred just after the "bell" was separated. I agree that it seems reasonable that the force was enough to rip it off and, once that occurred, the flight was more stable. The slo-mo video I got cut off just after the "bell" was separated.

 

[Steve Shannon]

I think that if you fly it again with a camera pointed at the bell, you’ll find there’s a lot of turbulence in that area.

 

[SkyFire]

It could have been Culinary Curd Extension Elasticity or Disproportionate Laceration Distress or Microscopic Fibrous Flirtation Spheres or Ocular Citric Ejaculation Trajectory.

 

[MaxQ]

"I think that if you fly it again with a camera pointed at the bell, you’ll find there’s a lot of turbulence in that area."


At 1:10 in the original video, camera appears to show the bell wobbling in the airflow.......

 

[OverTheTop]

I am willing to guess dynamic pressure from the flight, combined with some turbulence possibly creating a resonance, exceeded the strength of the screw fixings. Didn't look to be much length at all trying to hold it on, and it was mostly the lead-in of the screws. Thoughts?

 

[mrwalsh85]

AIRFLOW question.

You mentioned that the St Louis Rocketry Kaboom Krewe "noticed that the airflow didn't follow the shape down the bottom half of the football shaped rocket."

That's a fascinating statement for which I would love to have any evidence whatsoever. What evidence do you/they have to come to that conclusion? I only ask because it is clear in your video that there was a great deal of airflow past the bell on your little brother rocket to the Kaboom Krewe's larger verison.

Along the same lines of questioning, you asked if your engine bell was in a dead air zone? Just looking at the video it clearly was not in a dead air zone. Frankly, I'm not sure there's such a thing as a dead air zone around a rocket that's under the conditions caused by a burning motor which yours clearly was.

And thirdly, you said that they came to the conclusion that "much of their fins surface was not in contact with the airflow." Again, what evidence do you or they have in order to come to that rather extraordinary conclusion?

Brad

Come down when we fly Marvin in November, Brad I think you will enjoy! And we will be putting your system to good use with a VERY safe distance when we fly Marvin!

 

[jmmome]

As I was glad to help you load and carry back this rocket after it landed (I'm the guy in the grey sweatshirt in your video), I had a good view of the flight. As it arced over, into the wind, it appears to soar, supported by its wings. This occurred just after the "bell" was separated. I agree that it seems reasonable that the force was enough to rip it off and, once that occurred, the flight was more stable. The slo-mo video I got cut off just after the "bell" was separated.

Thank you for the help!!!!!!!!!!!!!!! It was a fun flight- a bit underpowered, but exciting nonetheless. The flight was a classic example of an over-stable rocket with huge fins, and it clearly weathercocked into the wind after powered flight was done.

Here is my next weird project- an 8 feet tall, 8" dia version of the Gary Larson cartoon. Partially completed.

 

[jmmome]

I am willing to guess dynamic pressure from the flight, combined with some turbulence possibly creating a resonance, exceeded the strength of the screw fixings. Didn't look to be much length at all trying to hold it on, and it was mostly the lead-in of the screws. Thoughts?

Screws were woefully too short. Didn't give that part too much thought, clearly.

 

[jmmome]

I think that if you fly it again with a camera pointed at the bell, you’ll find there’s a lot of turbulence in that area.

I thought that the fake engine bell area would be in "dead air" but i was SOOOOO wrong!

 

[SeanW78]

Thank you for the help!!!!!!!!!!!!!!! It was a fun flight- a bit underpowered, but exciting nonetheless. The flight was a classic example of an over-stable rocket with huge fins, and it clearly weathercocked into the wind after powered flight was done.

Here is my next weird project- an 8 feet tall, 8" dia version of the Gary Larson cartoon. Partially completed. View attachment 463232View attachment 463233

If you're up for it, I'd be geeked to come check out this next build some time. I'm likely just a few miles up 23 from your area.

 

[jmmome]

If you're up for it, I'd be geeked to come check out this next build some time. I'm likely just a few miles up 23 from your area.

When I'm a bit farther along- sure!

 

[boatgeek]

I thought that the fake engine bell area would be in "dead air" but i was SOOOOO wrong!

"Dead air" behind a spherical shape is a hot mess of turbulence. That's what makes it less effective for fins, and would also mess with anything in the area.

 

[RocketRev]

Come down when we fly Marvin in November, Brad I think you will enjoy! And we will be putting your system to good use with a VERY safe distance when we fly Marvin!

What's the date in November?

 

[mrwalsh85]

What's the date in November?

Uncertain at this time. Probably early November, but don't hold me to that. I don't like flying closer to the holidays as we often have enough to do!

 

[RocketRev]

Let me know when you find out.