Drag Seperation?

[ThreeJsDad]

I was watching a video of a club launch and a rocket comes apart on the climb. It looked to still be under impulse. I believe it was an MPR. Does this happen very often?

I was under the impression it happened primarily during the coast and larger motors because of the speed needed for this to occur.

I am asking because someone can be heard saying "drag separation" on the video.

Straighten me out...

 

[tim cubbedge]

It usually happens at motor burnout. Cause is typically from payload too loose in booster. With HP rockets, shear pins are usually used. Mid power, can usually be eliminated by friction fitting.

 

[Speaknoevil]

Drag separation typically occurs at burnout, when you get the change in acceleration and the fore part of the rocket has some weight to it, i.e., momentum to continue at the same pace before the burnout occurs. Moral: when in doubt, use shear pins.

 

[dhbarr]

Motor burnout is not always the same thing as when the motor produces significantly less thrust.

A highly regressive motor could easily drag separate a fluffy cardbird before burnout -- all that is required is for the total drag on the aft section to exceed thrust + retention.

 

[Speaknoevil]

Motor burnout is not always the same thing as when the motor produces significantly less thrust.

A highly regressive motor could easily drag separate a fluffy cardbird before burnout -- all that is required is for the total drag on the aft section to exceed thrust + retention.

And why I qualified the statement with "typically" as in the predominant conditions in which the behavior is observed. Other cases do exist.

 

[K'Tesh]

My theory as to why the Big Daddy seems to lawn dart so frequently is based on drag separation. The lightweight, high drag of the fin can section causes the nosecone to separate enough to allow the gasses of the ejection charge to vent, rather than popping the nosecone off. The rocket turns turtle, and the rocket then darts.

 

[manixFan]

I was watching a video of a club launch and a rocket comes apart on the climb. It looked to still be under impulse. I believe it was an MPR. Does this happen very often?

I was under the impression it happened primarily during the coast and larger motors because of the speed needed for this to occur.

I am asking because someone can be heard saying "drag separation" on the video.

Straighten me out...

Hmm, a link to the video would be very helpful. Unless of course it's not online. Without more visual information, it's very hard to tell. I do a lot of rocketry failure analysis via video and have seen all kinds of things cause early separation. But without the video it's just guesses.


Tony

 

[ThreeJsDad]

Hmm, a link to the video would be very helpful. Unless of course it's not online. Without more visual information, it's very hard to tell. I do a lot of rocketry failure analysis via video and have seen all kinds of things cause early separation. But without the video it's just guesses.


Tony

Hey Tony, I will see about posting a link with a rough estimate of where to look time wise. I was just surprised to see a DS issue with an MPR that wasn't a hard core feather weight (you know, something insanely fast).

 

[PSLimo]

Hey Guy's,

Thought you would like to see a video of drag separation while still under impulse. It's a 5.5" diameter Army Hawk on a K-550W. The motor has a 3.5 second burn and the drag separation is after about 2 seconds.



I used shear pins for the main tube too after that.

Phil

 

[ThreeJsDad]

Hey Guy's,

Thought you would like to see a video of drag separation while still under impulse. It's a 5.5" diameter Army Hawk on a K-550W. The motor has a 3.5 second burn and the drag separation is after about 2 seconds.



I used shear pins for the main tube too after that.

Phil

I would be interested to read what people think as to why that happened.

 

[llickteig1]

Hey Guy's,

Thought you would like to see a video of drag separation while still under impulse. It's a 5.5" diameter Army Hawk on a K-550W. The motor has a 3.5 second burn and the drag separation is after about 2 seconds.

I used shear pins for the main tube too after that.

Phil

This probably is NOT drag separation, per se. This separation is due to pressure differential between the outside atmosphere and the air trapped inside the airframe.

Notice the coupler is pushed apart under thrust until it is far enough for air to escape. Then when the rocket decellerates, it comes apart.

Shear pins are good. You also need a 1/8” or so hole in each airframe section to allow the pressure to equalize.

 

[Speaknoevil]

I would be interested to read what people think as to why that happened.

Army Hawk's are notorious for drag separation. Drag from large fins lead to quick change in acceleration after burn out coupled with the typically heavy nose cone (to adjust the CG forward of CP again due to large fins) which has momentum and does not stop moving up at burn out.

Lance may know more though as I was not at this launch. Also if the drag separation happened before burn out, then, yes, there must have been something else going on.

 

[llickteig1]

Large-ish airframes - you said 5.5" - are really prone to this type of separation. There is a lot of volume of air trapped in there

 

[David Schwantz]

Drag separation typically occurs at burnout, when you get the change in acceleration and the fore part of the rocket has some weight to it, i.e., momentum to continue at the same pace before the burnout occurs. Moral: when in doubt, use shear pins.

My 29mm Go devil has 12 oz of lead in the nose, as No evil states, unless I have the NC fit perfectly it WILL seperate on burnout. The lower airframe is very sleek, so this is due strickly to the NC weight.

 

[Speaknoevil]

My 29mm Go devil has 12 oz of lead in the nose, as No evil states, unless I have the NC fit perfectly it WILL seperate on burnout. The lower airframe is very sleek, so this is due strickly to the NC weight.

"Due strictly" is not quite correct. Even though you have a sleek finish, the body still has drag.

 

[David Schwantz]

I actuallyyy have 2, one built for speed , the other for altitude. the speedy one has no weight in NC, has never had a seeeperation. And NC is loose. hate them damn sticky keys.

 

[Speaknoevil]

I put together a spreadsheet sometime back based on John Derimiggio's formula as shown in this post: https://www.rocketryforum.com/threads/calculating-nose-cone-drag-separation.48617/#post-473376. Here is a link to the spreadsheet if it is of any interest: https://drive.google.com/open?id=0B4LURzRDtzD3TGZwM3NhZ1VuVHM.

 

[PSLimo]

Hey Guy's,

That's my Army Hawk and it had many flights. The separation was from the drag of the large fins and the nosecone with 5lbs of shot in it still wanting to cruise when the motor started losing thrust. The airframe was also vented so no pressure build up there.

At MWP 5 the main chute got tangled and the fin can was almost flying and actually landed well after the drogue chute as you can see in this pic with no damage:



Here's the pictures of the K-550W launch in the video






The photo credits go to Preston Nobil on that flight back in 2009.

Phil

 

[Speaknoevil]

Phil, you said 2s point in flight, maybe 2.5s looking at the curve?

 

[Speaknoevil]

BTW, I have a 6" Polecat version of this, maybe an MWP drag race is in order?

 

[PSLimo]

Speaknoevil,

That's about right, at 2+ seconds the motors thrust curve dropped and the nose wanted to keep on flying. One of newtons laws which answers the original question on the post was can drag separation happen under thrust. The airframe had 2, 1/4" holes in it as I'm well aware of pressure build up even in these low and loud flyers.

I sold that rocket and it was a Polecat version, I thought it was a 5.5" frame but it was 10+ years ago so I stand corrected.

Don't want to hijack a thread but I'm always up for a drag race, I still have a scratch built 7.5" MGM-140 that has that desirable 1:1 rocket to flame ratio on a K-550W. I'll shoot you a PM.



Phil

 

[Speaknoevil]

Figured out that Phil's Army Hawk is my Army Hawk. I purchased it from him back in 2015. I reconfigured the av-bay, changed up the camera a bit, and did a complete repaint/decal job. Oh, and I was wrong, it is a 5.5" airframe.

 

[llickteig1]

It's enlightening that a change in thrust would cause vented sections to push apart. This evidence should be part of the training video.

Highly weighted nosecones demand special attention for sure. We always find lost heavy nose cones in the list and found - often with other damaged components like broken recovery harnessed, splintered centerings, parachute shroud lives with no canopy, etc.

Shear pins are your friend.

 

[PSLimo]

It's enlightening that a change in thrust would cause vented sections to push apart. This evidence should be party of the training video.

We'll have to agree to disagree on that one. It's the reduction in thrust that will make the more dense and aerodynamic nosecone section pull apart from the draggy finned section. I didn't think it would be an issue on a slow moving "K" flight but that video was an eye opener.

BTW, The Hawk looks great with a fresh coat of paint and decals!

Phil