V2 for my L3

Nick@JET said:

Chris I feel for you - I'm n the same boat - my onboard died sitting on the pad too long - so I have a crappy cell video of take off - and my memory and that's it for my big flight arggg. 😖

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I know the frustration too. My daughter swiped the SD card out of my möbius before my L2 flight.

My younger child had a first attempt at Junior L1 denied because the parachute was a parawad. Despite not having damage and being marginal on unsafe descent speed, the witness decided not to pass the flight because the same failure would be unsafe on another rocket. A beautiful flight the next day passed.

Depending on what the failure was, I can see applying the same reasoning here, depending on the L3CC. I could get behind passing a failed altimeter a lot easier than getting behind a design/build/wiring error.

All that said, congrats on a safe flight!

So any idea yet why the main deployed at apogee?


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Best guess is it was basically slung off by the spin. The rocket was too high to clearly see what was happening, so everything is just a guess. Really, I have no good data to go on. Just observation from ground. This thing is getting 2-4 more shear pins, a shorter burning motor, a rewrite to allow for main deploy at apogee in flight profile, and a second try this winter sometime.

If you think the rotation did in the pins, could you just key the separation point to take the torque but still allow the separation with fewer pins?

Cl(VII) said:

Best guess is it was basically slung off by the spin. The rocket was too high to clearly see what was happening, so everything is just a guess. Really, I have no good data to go on. Just observation from ground. This thing is getting 2-4 more shear pins, a shorter burning motor, a rewrite to allow for main deploy at apogee in flight profile, and a second try this winter sometime.

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You figure out why both altimeters blew at the same time? No delay between the two or just a random event they blew at the same exact time? Kurt

You had a crap-load of nose weight.
What was your shear-pin plan?
What burn-out de-acceleration and apogee G-loading did you plan to accommodate without deployment?

FredA said:

You had a crap-load of nose weight.
What was your shear-pin plan?
What burn-out de-acceleration and apogee G-loading did you plan to accommodate without deployment?

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I had four 4-40 pins, so roughly 150 lbs of force to break them. With a roughly 9 lb NC that should have handled about 17G. Certainly enough for normal deceleration. The damage to the main recovery train (swivels stretched) and not the drogue train at least implies that the event that stretched the main train was not from loading due to drogue deploy. Because you couldn't have transferred more energy into the NC than the drogue separation had alone. I could go on forever guessing, but in the end my only hard data (alt traces) says that deployment charges were not at fault.

I am left to remediate the issues I can guess about, and try one more time. It probably is only one more time too. I'm not into flying the same or similar flight profiles with the same rocket over and over, and the aft end of this thing got charred real good, including the fins, so attempts with big motors will not be infinite...gotta make the next one really count.

You clearly deployed the main at high speed.
That's one of three things:
- Drag separation on the way up.
- Horizontal deploy at speed.
- Late deploy on the way down.

Pick each scenario and see if it matches the altimeter logs.
My money is on a big gravity turn with the apogee charge shaking lose the whole enchilada with significant sideways velocity.

17G's is not very much actually.....I target at least 30G's at apogee.

Is it at all possible you reversed your charges ? I have had that happen when I was rushing .

3stoogesrocketry said:

Is it at all possible you reversed your charges ? I have had that happen when I was rushing .

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Nope. Checked it four times ahead of flight, and verified after. I made this my only flight that day, so there was no rushing. Everyone was also very nice to give me space to work without distraction, so no problem there either.

FredA said:

You clearly deployed the main at high speed.
That's one of three things:
- Drag separation on the way up.
- Horizontal deploy at speed.
- Late deploy on the way down.

Pick each scenario and see if it matches the altimeter logs.
My money is on a big gravity turn with the apogee charge shaking lose the whole enchilada with significant sideways velocity.

17G's is not very much actually.....I target at least 30G's at apogee.

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Fred, Do you mean the rocket hit apogee, nose flopped down, the apogee charges blew simultaneously and the excess momentum force when the harness hit the end of travel caused the main to break the shearpins causing the main to come out? Certainly plausible.

I had a 4 inch diameter cardboard rocket motor ejection fail due to grease plugging the hole. Rocket came in ballistic and since it was single deploy I had no shearpins. Four inches in diameter the recovery system fit very loosely. So this long necked thing is heading for Terra Firma and it's obvious to me that gravity,
late in the terminal phase of the death dive, allowed the recovery laundry to fall inside the tube and push the nosecone off. That resulted in a zipper and the
"Covert Recovery" chute (remember them?) exploded. The rocket was knocked sideways 50 to 100 feet up and landed flat in dirt. The fincan survived and the rocket was repaired to a DD configuration. That's an example of a gravity deployment of a chute at the last minute!

Kurt

Care to share your altimeter data?


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You should be able to look at your accelerometer data and see if you had any events in excess of 17G's.
I've seen tons of flights with apogee events that cause measured accelerations in the 10's of G's, some pegging the 50G limit.
So I really question the 17G limit you choose.
But without seeing the data, that's a guess.

Your evidence of stressed recovery clips says those clips were involved in a high speed deployment.
Maybe you had a big turn and hit apogee while still moving fast as may happen on a V2 with lots of nose.
Or you tossed the chute early or late.....but clearly it wasn't a "stop at apogee and turn around" event nor anything that would happen just because the nose was pointing down (that's only 1 more G.)

Show us the data.................

FredA said:

You should be able to look at your accelerometer data and see if you had any events in excess of 17G's.
I've seen tons of flights with apogee events that cause measured accelerations in the 10's of G's, some pegging the 50G limit.
So I really question the 17G limit you choose.
But without seeing the data, that's a guess.

Your evidence of stressed recovery clips says those clips were involved in a high speed deployment.
Maybe you had a big turn an hit apogee while still moving fast as may happen on a V2 with lots of nose.
Or you tossed the chute early or late.....but clearly it wasn't a "stop at apogee and turn around" event nor anything that would happen just because the nose was pointing down (that's only 1 more G.)

Show us the data.................

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2 RRC3 altimeters, so no accelerometer data. Sure, I'll share the traces. I nailed the main decent rate too as the target was 15 ft/s, just not from 9100 ft. These do look a little jumpy at the top. Typically the traces are super smooth.

Altimeter 1:




Altimeter 2:

I am not familiar with RRC3 data. Why two graphs for each altimeter? They appear to have the same data graphed differently??


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Lowpuller said:

I am not familiar with RRC3 data. Why two graphs for each altimeter? They appear to have the same data graphed differently??


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The second for each is just zoomed in on apogee

Sorry ---- not much help without accelerometer data.

But it does look like you hit baro apogee - that says nothing about velocity.

But the stress recovery and the major slope change says you deployed going sideways at high speed.

Again, my money is on a big gravity turn and hard enough drogue deploy that it shook the main loose.

On the second graph for Alt 1, it looks like you used the AUX output or at least had an AUX event at apogee. Am I reading that wrong? If you did program the AUX output, what was it used for?

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timbucktoo said:

On the second graph for Alt 1, it looks like you used the AUX output or at least had an AUX event at apogee. Am I reading that wrong? If you did program the AUX output, what was it used for?


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I did. It was apogee +1 sec. Backup alt was apogee +3 sec. I had the channels, so why not use them. Main has three channels too.

So is it possible that you had the AUX channels reversed?


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timbucktoo said:

So is it possible that you had the AUX channels reversed?


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No, I know they were correct. I wanted them to be hooked up wrong...my kingdom for an easy, clear answer.

OK, so here is my best guess, and I have no way to prove it...maybe my fantastic conjecture is a better way to frame it.

I know the rocket had a big time spin...can't quantify because no video, but enough that I was cringing (probably 2-3 rps). Also, spin doesn't quite describe it. It wasn't a perfect down the Z-axis spin...more of a tight spinning wobble of the type that give the corkscrew smoke trail.

As the motor burns the CG comes forward, not a lot, but about an extra 4" which puts it near the middle of the body tube. As the rocket slowed toward apogee the fins do less and less, and the desire to go straight loses out over the desire to spin about the CG. This causes the angle off the z-axis that the rocket wants to spin around to become larger and larger which builds the centrifugal forces exerted on/by the really heavy NC (most of the weight is in the very tip also, making it particularly bad). This increases the force on the pins which break, and the NC and remaining bt/boat tail assemblies fly away from each other with a great deal of energy resulting in the deformed swivels down the nose-cone to body tube kevlar cord. This event would also have sheared the aft pins.

Since this would occur very close to apogee where the aerodynamic forces are essentially gone, maybe just as the drogue charge fires too it would be indistinguishable on the altimeter traces from deployment.

The boat tail is heavy too, but it's CG is much closer to the overall CG of the rocket...probably within a foot or so at burnout, so the seperation force is far lower than on the NC where the CG is within 10 inches or so of the tip (probably twice as far from the overall CG as the boat tail CG is.

This (and multiple charges simultaneously which I have ruled out) are the only ways I can think to get enough energy to deform the swivels ONLY in the main cord train, and not on the parachute arm of the Y-harness. I was looking at them again last night, and I got REALLY lucky that they stayed together. I was using ROSCO 12/0 which are 1200 lb TEST. I am switching to swivels with 600 lb WORKING LOAD LIMIT (which should mean at least 3000 minimum breaking strength, probably more like 6000 because it is rigging equipment). Once again, the devil is in the details, and in this case the terminology. I'm ignoring shock load here, but that is probably too in the weeds. Basically, more margin on swivels is the goal.

Also, this theory supports Fred's observation/data interpretation of a sideways deployment. The spinning about the axis of travel (Z-axis of the flight path) would give the 50' up and down osculation observed in the alt data before the chute fully inflated.

All the scenarios lead to the same strategy going forward. Upgrade swivels, tighten up part fitment, remove camera and rewrite doc so main deploy at apogee is acceptable part of flight profile. Also, gonna punch it for a shorter time since my still photos show rotation was not a big thing for first 1.5 sec of flight, and because I never fly the same rocket on the same motor twice.

You can have your opinions about if I should have passed by the letter of the law, but the more I look into this the more I believe I deserved to fail the cert. I made mistakes, and only luck got me by on them.

A spin with a wobble will load every movable joint in a rockets, causing the nosecone to work its way out, especially if the shear pins don't have sufficient margin. Rocking the nosecone works against a single shear pin at a time. Based on what you've said your shear pins were marginal. I blew my first L3 flight by having too light of nose cone shear pins.
I've seen quite a few apogee events that were right at 29-30 gees.
The wording that says inadvertently deploying main at apogee is no cause for failure really needs to be clarified. I think the reason that the main deploys, i.e. was it a mistake or was it a fluke event that could happen to even the most experienced flyer, needs to be considered.
You're doing all the right things and by doing so you'll come out of this with a lot more knowledge.



Steve Shannon

Chris,

A few thoughts...mere thoughts nothing more nothing less.

1. You'll get it next time!

2. I don't use swivels on big stuff anymore. I've personally stretched one or two...worst was on an XXL chute attached to a 11.5" diameter Nike. Nominal deployment, but it killed the swivel. I've seen them fail on others' projects as well. I'd personally rather have twisted line than failed recovery, so I've eliminated them from my recovery systems...one less potential point of failure.

3. Spin: So many things can cause spin. Moonburners are notorious due to the offset core (that's a lot of mass off of centerline, which is exaggerated during the second half of the burn. I'd expect spin from a minimal diameter rocket on a moony; don't know if that was the direct cause on a bigger bodied V2. Could've been camera, could've been stronger bevels on one side of the fin(s) causing a lifting moment on the fins, who knows. Stability shift could've been a culprit as you mentioned as well. I've seen a lot of V2's fly over the years, some great, some awfully unpredictable! The nose of our 12" V2 weighs 35 or 40 lbs...it flies straight so long as it's got enough thrust! For comparison, I'd try a Bates grain motor next time...see if it makes a difference.

4. Shear pins: Agree with Fred, others that said you could probably use more holding force. The fit of the cone matters; the fit of the shear pins matter (don't want wobble slop), etc. People rip on me for using a drill and tap on cones...not gonna change me though!

Just a cool project that'll secure the cert next time for sure! Go get 'em; great job so far!

Thanks to everyone for the encouragement and experienced advice.

I know using swivels is "controversial" here.
I'll add my 2-cents:
Every rocket my team builds has two or three of those ROSCO 12/0 swivels sewn into the harness.....we buy them in bulk.....every rocket, big and small.

We have, on occasion, stressed the crap out of them -- stretching them till the eye's are nearly closed.
We've pulled on them hard enough to snap the 1-inch TN sewn to them.
We've NEVER broken a swivel.....sure they don't look nice and don't turn any more, but we've never had one fail.

They are stronger than the TN and Quick-Clips, so don't be shy about using them.

FredA said:

I know using swivels is "controversial" here.
I'll add my 2-cents:
Every rocket my team builds has two or three of those ROSCO 12/0 swivels sewn into the harness.....we buy them in bulk.....every rocket, big and small.

We have, on occasion, stressed the crap out of them -- stretching them till the eye's are nearly closed.
We've pulled on them hard enough to snap the 1-inch TN sewn to them.
We've NEVER broken a swivel.....sure they don't look nice and don't turn any more, but we've never had one fail.

They are stronger than the TN and Quick-Clips, so don't be shy about using them.

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That is good to hear. Glad I wasn't really flirting with disaster there. In any event I have ordered some beefier swivels, and since they will be in front of the CG they will only help with stability. I will be making up for 1 grain worth of weight from the motor since grain five was in front of CG previously.

ECayemberg said:

Chris,


4. Shear pins: Agree with Fred, others that said you could probably use more holding force. The fit of the cone matters; the fit of the shear pins matter (don't want wobble slop), etc. People rip on me for using a drill and tap on cones...not gonna change me though!

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Why would people grief you for tapping your shear pin holes?

Why would people grief you for tapping your shear pin holes?

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+1. I'm curious on this one too!