Cause of RUD at Max-Q

[MattM]

I'm a member of a rocketry team at Purdue University, and our team's rocket broke apart right before burnout, probably supersonic. I thought we broke a fin, but the fincan was recovered with all 4 fins still solidly attached. Any ideas on the cause of failure? Video footage, wreckage, and .ork are attached.

I should say that the rocket was supposed to be spin stabilized (bad idea from the start, I know), and the failure probably had something to do with that, as OpenRocket predicted a 2200 RPM spin at Max-Q. I'd still like to understand the exact cause of failure though, as our team is looking towards a space shot in the (far) future, and spin stabilization is very attractive for that type of launch.

Rocket design:
54mm min diameter, flying on a K540M with the Aerotech adapter in a 54-2800 case.
Full fiberglass
Fincan was recovered fully intact, other than a zippered body tube

Avionics:
AltusMetrum Telemega and EasyMini, on fully redundant circuits
Featherweight GPS
Head-end traditional Dual Deploy

 

[cwbullet]

Did you have vent holes in your airframes?

I'm a member of a rocketry team at Purdue University, and our team's rocket broke apart right before burnout, probably supersonic. I thought we broke a fin, but the fincan was recovered with all 4 fins still solidly attached. Any ideas on the cause of failure? Video footage, wreckage, and .ork are attached.

I should say that the rocket was supposed to be spin stabilized (bad idea from the start, I know), and the failure probably had something to do with that, as OpenRocket predicted a 2200 RPM spin at Max-Q. I'd still like to understand the exact cause of failure though, as our team is looking towards a space shot in the (far) future, and spin stabilization is very attractive for that type of launch.

Rocket design:
54mm min diameter, flying on a K540M with the Aerotech adapter in a 54-2800 case.
Full fiberglass
Fincan was recovered fully intact, other than a zippered body tube

Avionics:
AltusMetrum Telemega and EasyMini, on fully redundant circuits
Featherweight GPS
Head-end traditional Dual Deploy

 

[mikec]

Hard to tell, rocket is clearly spinning fast before burnout and then makes a hard turn around max-Q. I doubt OR does a good job of modeling the AoA behavior of fins with so much rotation/cant. I suspect you just went unstable at max-Q.

 

[Rocket86]

That was spinning really fast. If there was a coning component the rapid bending moments could have caused the coupler to fail. I've seen a tube fracture like that right at the coupler from a slight coning near max-Q.

 

[Grog6]

Someting clearly shifted the cog at maxq, that may have been the cause of the split body tube; It ripped from nose cone pressure. Something shifted under pressure, it threw the nc out to the side, and the rotation tore it apart.
Possibly internal pressure bumped the nc off just after bo , but I'd still think there would be enough to hold it on.

 

[user 30299]

I'm a member of a rocketry team at Purdue University, and our team's rocket broke apart right before burnout, probably supersonic. I thought we broke a fin, but the fincan was recovered with all 4 fins still solidly attached. Any ideas on the cause of failure? Video footage, wreckage, and .ork are attached.

I should say that the rocket was supposed to be spin stabilized (bad idea from the start, I know), and the failure probably had something to do with that, as OpenRocket predicted a 2200 RPM spin at Max-Q. I'd still like to understand the exact cause of failure though, as our team is looking towards a space shot in the (far) future, and spin stabilization is very attractive for that type of launch.

Rocket design:
54mm min diameter, flying on a K540M with the Aerotech adapter in a 54-2800 case.
Full fiberglass
Fincan was recovered fully intact, other than a zippered body tube

Avionics:
AltusMetrum Telemega and EasyMini, on fully redundant circuits
Featherweight GPS
Head-end traditional Dual Deploy

Do you have close-up pictures of the rocket fully assembled and as individual sections before assembly?

 

[rharshberger]

What was the margin of stability? If it was really close to 1 caliber combined with CG shift from propellant burn and in the transonic/mach range that could be the result.

 

[tfish]

I'm leaning to an electronics event..spoofed by spinning up the rocket? Photos look like 2 rapid pyro events and not a vehicle failure.
My 2 cents..




Tony

 

[Tractionengines]

+1 What Tony said above at failure point. "How does rapid spinning effect the baro sensor"? Anyone have any idea...

BUT then my other question/comment: it looks like you are using a "sparky" motor. Is that correct? I wonder if the spinning caused metal to build up on the ID of the motor burn, and then that cleared the nozzle as chucks, a few times in the flight. The 3rd of which caused enough off axis thrust to bend / fail your coupler.

1st. Expelling of "material" see the "puff" a few feet behind rocket.


2nd expelling of "material"



3rd expelling of "material". It sticks and causes off axis thrust.


Last question. What is the core geometry? If not a centered circular core burner. Then the off center propellant mass would be changing dynamically during the burn, at some RPM versus, weight, versus stability shift; they coupled together and made a large oscillation.

 

[dhbarr]

Was it a Medusa? Did you just have the one spacer in the case? Did you have anything attached to the front end of the case?

 

[5x7]

Looks like an off balance washing machine.

 

[David Schwantz]

Looks like you blew the chute under thrust, it then zippered the tube and broke the coupler all at once.

 

[Rschub]

Looks like an off balance washing machine.

Shaft whirl is a well known dynamic behavior, although apparently not in Rocketry. You should ask for advice from your mechanical vibrations professor. That bent coupler is a large red flag in my opinion.

 

[rharshberger]

I'm leaning to an electronics event..spoofed by spinning up the rocket? Photos look like 2 rapid pyro events and not a vehicle failure.
My 2 cents..

View attachment 568195
View attachment 568196

Tony

Tony definitely has some good insights, I might agree with him unless better info comes to light.

 

[rfjustin]

Tony definitely has some good insights, I might agree with him unless better info comes to light.

This. I agree with @tfish analysis here.

 

[kramer714]

Shaft whirl is a well known dynamic behavior, although apparently not in Rocketry. You should ask for advice from your mechanical vibrations professor. That bent coupler is a large red flag in my opinion.

Did you ever try and test the different components for stability at 2200 RPM? Did you look at the DYNAMIC rotational balance?

 

[cwbullet]

I am still interested to see if you drilled vent holes, if it did expel the chute on the way up, the lack fo a vent hole in the body tube could cause pressure to push the chute out.

 

[texasorbit]

Hello,

Recently me and some friends launched this rocket.



It was scratch built, dual deploy, and made entirely from fiberglass.

The fins were slightly canted (tilted laterally) to induce a spin on the rocket. I'm unsure how precisely they were canted, ie some may have been tilted slightly more than others, although I don't think you could see much of a difference by just looking at it. I'm wondering if this maybe caused the rocket to be unstable and turn then break up as shown below.

The rocket flew well for two seconds, then spontaneously took a sharp turn and broke apart. Below is a video of the failure as well as two pictures of the damage.

View attachment FailureShort.mp4



I think that due to the sharp turn, the rocket took on a very high angle of attack and promptly broke up under the stress caused by that angle.
So I don't think the thing which caused the sharp turn also directly caused the break up, but rather something caused the turn which in turn caused the break up, but I'm not completely sure.

Again, the two possible reasons for the failure I've been able to think of are:
- Instability due to uneven fin cants
- Faulty commercially bought motor (AeroTech K270M motor in a 54mm-2800 motor case)

What do you all think the cause of the failure may have been? Thank you

 

[sr205347d]

I should say that the rocket was supposed to be spin stabilized (bad idea from the start, I know), and the failure probably had something to do with that, as OpenRocket predicted a 2200 RPM spin at Max-Q. I'd still like to understand the exact cause of failure though, as our team is looking towards a space shot in the (far) future, and spin stabilization is very attractive for that type of launch.

The spin is likely the exact cause of the failure. Long ago, I was a grad student at Purdue and had a course in spacecraft dynamics. I remember that in general, a long skinny object will not remain stable spinning along its long axis. This because, with any slight perturbation, the mass towards the ends of the object will eventually be pulled outwards from the axis of rotation by "centrifugal force." Even in space. This would cause the rapid departure that put your rocket at high AOA. Even a spin stabilized bullet will eventually tumble.

You might want to think about having an internal spinning mass (like a top or gyroscope toy) to stabilize your space shot.

 

[AllDigital]

What did the motor case and nozzle look like after you recovered it? Was there any burn-through of the case or around the top of the nozzle? One frame of the video looks like you have two exhaust streams, like a burn-through, but it could also be the result of a pyro event.

 

[wclaybaugh2]

I'm a member of a rocketry team at Purdue University, and our team's rocket broke apart right before burnout, probably supersonic. I thought we broke a fin, but the fincan was recovered with all 4 fins still solidly attached. Any ideas on the cause of failure? Video footage, wreckage, and .ork are attached.

I should say that the rocket was supposed to be spin stabilized (bad idea from the start, I know), and the failure probably had something to do with that, as OpenRocket predicted a 2200 RPM spin at Max-Q. I'd still like to understand the exact cause of failure though, as our team is looking towards a space shot in the (far) future, and spin stabilization is very attractive for that type of launch.

Rocket design:
54mm min diameter, flying on a K540M with the Aerotech adapter in a 54-2800 case.
Full fiberglass
Fincan was recovered fully intact, other than a zippered body tube

Avionics:
AltusMetrum Telemega and EasyMini, on fully redundant circuits
Featherweight GPS
Head-end traditional Dual Deploy

Matt:

The vehicle was obviously coning as seen in the images; that is caused by a non-axisymetric Cg.

Typically, in sounding rockets, one wants the spin to be around 7-10 hertz (less than 600 rpm); this is because on the one hand you need to spin to average out the consequences of non-axisymetric thrust and Cg but you also want to avoid resonance which typically occurs in the 3-6 hertz range...best if you do the appropriate analysis in advance to determine you vehicle's actual coupling spin rate and then make it spin faster than that...but not too much faster.

In my experience, your failure is most likely a result of a vehicle structural failure caused by the high aerodynamic forces induced by coning.

You can avoid this issue by making certain that the rocket and the payload are fully balanced with Cg's that are both at the same location in two orthogonal planes on the longitudinal axis and, separately, balanced in rotation, also at the same Cg location. If the three axis Cg's are not at the same location, one will have this sort of failure unless the vehicle is just plain hell for stout compared to the forces it sees in flight.

Bill

 

[thzero]

https://www.rocketryforum.com/threads/cause-of-rud-at-max-q.178481/#post-2404771

 

[rharshberger]

We now have two threads on this very same rocket, your team needs to pick one representative for questions about it.

FWIW the motor appears to function normally, unless you have pictures of a casing that was cato'd, or burn through, the puffs you saw prior to breakup might have been pieces of casting tube being ejected, they do that and make a popping/coughing sound when it happens (and its pretty common if the grains are not glued into the liner).

 

[Steve Shannon]

This is what happened to me when I attempted to spin stabilize a polycarbonate rocket. It was flying straight, then it started coning, and then it just folded over.

 

[Kallahan11]

It looks like in the video an ejection charge goes off before the rocket takes a hard turn. Did you have a the motor ejection charge as well as your electronic ejection charges. I had a K2050 set off the motor eject right at burnout with an un-drilled delay, I believe hot gasses got past the delay due to mis-asembly on my part. In my case it was obviously the ejection charge as the airframe was coated in soot but both electronic ejection charges clearly happened after separation.

 

[FredA]

Show us the flight data.....

 

[FredA]

Show us the flight data.

 

[cautery]

TWENTY-TWO HUNDRED RPM.

Spinning up.... rocket = bullet... mass not accounted for laterally from bore centerline. At some given RPM and velocity the lateral mass/spin moment(s) caused the progressive material failure at the avbay/switch band interface. The airframe came apart and the recovery system zippered BOTH ends at approx. same time, and re-kitted properly. Recovery mostly stayed with avbay remains. Booster remnants came down a la Newton's apple.

But freaked out electronics due to the the rapid spinning, or insufficient or OVER-provision of venting at the high spin rates are also good explanations. One or more of those failure domains was GOING to present first...

It was just a glance and I can't see anymore.... but it's easy to see, physics killed that bird. Or, rather an inattention to physics killed it.
Who told you to spin that rocket like that?

 

[Steve Shannon]

Hello,

Recently me and some friends launched this rocket.

View attachment 568225

It was scratch built, dual deploy, and made entirely from fiberglass.

The fins were slightly canted (tilted laterally) to induce a spin on the rocket. I'm unsure how precisely they were canted, ie some may have been tilted slightly more than others, although I don't think you could see much of a difference by just looking at it. I'm wondering if this maybe caused the rocket to be unstable and turn then break up as shown below.

The rocket flew well for two seconds, then spontaneously took a sharp turn and broke apart. Below is a video of the failure as well as two pictures of the damage.

View attachment 567962
View attachment 567963
View attachment 567964

I think that due to the sharp turn, the rocket took on a very high angle of attack and promptly broke up under the stress caused by that angle.
So I don't think the thing which caused the sharp turn also directly caused the break up, but rather something caused the turn which in turn caused the break up, but I'm not completely sure.

Again, the two possible reasons for the failure I've been able to think of are:
- Instability due to uneven fin cants
- Faulty commercially bought motor (AeroTech K270M motor in a 54mm-2800 motor case)

What do you all think the cause of the failure may have been? Thank you

Spinning a rocket causes a bending moment which is repeated at the rate of spin. That bending moment may eventually cause the coupler to fail, just the way it looks in your one photo. Once that happens the rocket comes apart, the chute comes out at high speed, and bad things follow. Because rockets fly fast, nearly everyone who tries to spin stabilize greatly overestimates the fin cant needed.

 

[mrwalsh85]

Is this for a SLI competition?