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boatgeek

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Usually, detective stories ask whodunit. In this case, I know who but not how. Check out the video below, starting at -2:40:10 on the right side of the progress bar (sorry, I don't know exactly what time into the video).

https://www.facebook.com/NASAStudentLaunch/videos/1398675543519390/

This is the high school flight that was mentioned in the other SLI thread. The rocket goes up, arcs over, and the drogue doesn't deploy at altitude. That's when things get weird. Every other time I've seen this, the rocket came in ballistic until carnage ensues, either at main deployment or impact with the ground. In other words, it's stable on the way up, so it's stable on the way down with the nose pointing into the wind (ie down). This rocket just falls in a flat spin at a reasonable speed until the main deploys.

All I know about the rocket is that it's a 6" diameter and the payload was an autonomous glider. The payload was not finished in time to fly, so it was intended to fall under parachute separately from the main rocket. Could shifting weight in the payload section change stability that much? Are there other explanations?

In other SLI news, the next flight in the video is from the high school team that is objectively the best in the competition, since their my students. :wink: It also demonstrates the importance of backup charges. The main charge took the nose cone off but didn't get the chute out. This was despite ground testing and a successful prior flight.

In other, other news, many thanks to anyone here from UA-Huntsville for lending our students an altimeter after ours got dorked during setup. You are awesome for saving our ability to fly.

Finally, please keep this one positive and reasonably on topic so it doesn't get locked like the last one.
 

djs

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This is the high school flight that was mentioned in the other SLI thread. The rocket goes up, arcs over, and the drogue doesn't deploy at altitude. That's when things get weird. Every other time I've seen this, the rocket came in ballistic until carnage ensues, either at main deployment or impact with the ground. In other words, it's stable on the way up, so it's stable on the way down with the nose pointing into the wind (ie down). This rocket just falls in a flat spin at a reasonable speed until the main deploys.
I'm assuming you're referring to the flight that ends at 2:40:20 or so- the one just before the time you've had. It looked kinda cool- they were really lucky!

I had a rocket do this where instead of arcing in, it went into a flat spin and somehow didn't turn into a lawn dart. I think it's just a lucky draw when it comes to aerodynamics- nothing with payloads shifting.

Here's the onboard video of mine, although the picture after it arcs over is more nauseating than edifying :)

[video=youtube;ZciVg09BExU]https://www.youtube.com/watch?v=ZciVg09BExU[/video]
 

Nytrunner

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That was an unquestionably bizarre descent.
I want to say I witnessed a puff from a Drogue charge going off, but the separation failed. Haven't had a chance to review the video to confirm

My only hypothesis is that once the motor burned out, the CG somehow ended up in a position where it was relatively stable ......in a horizontal setting (?)
Possibly due to payload mass positioning? Fin positions during fall accounting for the spinning descent?


and now I can't help but wonder if I could design a weight scheme that would repeat it :)
L2 cert with helicopter recovery anyone?
 

Flyfalcons

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If the fins are stalled then they may not be able to provide enough corrective force to bring the rocket back to stable flight. Much like how some airplane designs can be put into an unrecoverable spin.
 

boatgeek

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If the fins are stalled then they may not be able to provide enough corrective force to bring the rocket back to stable flight. Much like how some airplane designs can be put into an unrecoverable spin.
That was what I thought at first, but it appeared to have forward airspeed all through the flight. The announcer did say it was coming back in the tailslide, so maybe that was an illusion?
 

Flyfalcons

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It was definitely starting to arc over, but it looks like it might have lost enough airspeed for the fins to remain effective before the nose arced over enough. It looks like when that happened, the rocket started its tail slide.
 

rharshberger

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That was what I thought at first, but it appeared to have forward airspeed all through the flight. The announcer did say it was coming back in the tailslide, so maybe that was an illusion?
How about this theory, that particular rocket was about 5" and a fairly long cardboard/blue tube airframe and fairly lightweight iirc (they test flew it under my obsevation as RSO at one of TCRs launches). The airframe is long and wide and once horizontal provides some lift (?) then if the fins provide only slightly more lift, then the rocket turns into a glider so long as the angle of attack is slightly upward on the fin end.

I have seen this type of recovery on several Mean Machines and a 3" lightweight BSD Thor - not intentional recoveries via that method.
 
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Nytrunner

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How about this theory, that particular rocket was about 5" and a fairly long cardboard/blue tube airframe and fairly lightweight iirc (they test flew it under my obsevation as RSO at one of TCRs launches). The airframe is long and wide and once horizontal provides some lift (?) then if the fins provide only slightly more lift, then the rocket turns into a glider so long as the angle of attack is slightly upward on the fin end.

I have seen this type of recovery on several Mean Machines and a 3" lightweight BSD Thor - not intentional recoveries via that method.
So large angle of attack -> Cp shifts forward
Motor burnout -> CG also shifts forward (with weighty payload in the front)

Both these happen in just the right proportions and you get a giant spinning log?
 

Flyfalcons

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You have to think of the rocket as flying backwards, with gravity working to continually pull it in one direction (down). Flying backwards, it is unstable, but not so unstable that it can overcome gravity's pull and turn itself around.

At the 0:22 mark of my video here, there is another rocket that did the same thing. In this case, the slow flat spin down actually saved the rocket since the delay was so long that it would have crashed if it arced over and came in ballistic.

[video=youtube;YDte1G86fAo]https://www.youtube.com/watch?v=YDte1G86fAo[/video]
 

DAllen

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Usually, detective stories ask whodunit. In this case, I know who but not how. Check out the video below, starting at -2:40:10 on the right side of the progress bar (sorry, I don't know exactly what time into the video).

https://www.facebook.com/NASAStudentLaunch/videos/1398675543519390/

This is the high school flight that was mentioned in the other SLI thread. The rocket goes up, arcs over, and the drogue doesn't deploy at altitude. That's when things get weird. Every other time I've seen this, the rocket came in ballistic until carnage ensues, either at main deployment or impact with the ground. In other words, it's stable on the way up, so it's stable on the way down with the nose pointing into the wind (ie down). This rocket just falls in a flat spin at a reasonable speed until the main deploys.

All I know about the rocket is that it's a 6" diameter and the payload was an autonomous glider. The payload was not finished in time to fly, so it was intended to fall under parachute separately from the main rocket. Could shifting weight in the payload section change stability that much? Are there other explanations?

In other SLI news, the next flight in the video is from the high school team that is objectively the best in the competition, since their my students. :wink: It also demonstrates the importance of backup charges. The main charge took the nose cone off but didn't get the chute out. This was despite ground testing and a successful prior flight.

In other, other news, many thanks to anyone here from UA-Huntsville for lending our students an altimeter after ours got dorked during setup. You are awesome for saving our ability to fly.
Actually, Mr. Upton mentions the phenomena which I've seen before (assuming I am looking at the correct launch):

https://www.rocketreviews.com/backslider-glider.html

As I understand it, rockets need to be at velocity to get it's stability. Rockets that are more overstable will be less likely to backslide or tailslide whereas small finned rockets with relatively long and narrow bodies will backslide or even flat spin on descent. I've even heard of people making rockets to do this intentionally.

Finally, please keep this one positive and reasonably on topic so it doesn't get locked like the last one.
It's actually easy to keep threads from being locked. Stay on topic.
 

boatgeek

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Actually, Mr. Upton mentions the phenomena which I've seen before (assuming I am looking at the correct launch):

https://www.rocketreviews.com/backslider-glider.html

As I understand it, rockets need to be at velocity to get it's stability. Rockets that are more overstable will be less likely to backslide or tailslide whereas small finned rockets with relatively long and narrow bodies will backslide or even flat spin on descent. I've even heard of people making rockets to do this intentionally.
That makes sense. From an analytical perspective, rockets that do this probably have relatively low stability and relatively high drag to weight ratios. The first condition helps keep them from aligning to forward airflow. A long rocket with a relatively high pitch/yaw moment of inertia is probably helpful here as well. The second condition slows the rocket to a stop or near stop when it is arcing over so it is less likely to maintain enough airspeed to align to airflow.

Come to think of it, both of those would explain why tube fin rockets often show the tailsliding behavior. Lots of drag aft and a fair amount of weight at the end to increase MoI.

Thanks to everyone who responded!
 
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