Help analyzing a flight

On this flight of my dx3 (j250) there's a little wobble about 50 foot off the pad. I can't put my finger on where that comes from. You can see it tilt over and then come right back, if it was weather cocking I wouldn't expect it to straighten out so quickly. Could it have had something to do with the launch rail and buttons? Maybe an illusion from the camera angle?



countdown starts at about 0:15

How fast was it going at ~1sec of flight time?

Wind could have caught it initially, but you're still accelerating and vertical velocity corrected it.

Looks like it could a Cg shift due to acceleration...reduced propellant weight combined with your recovery gear settling rearward. Then acceleration takes over and it smoothes out.

Do you remember which way the wind was blowing? Is the direction of the tilt consistent with weather cocking?

Nytrunner said:

How fast was it going at ~1sec of flight time?

Wind could have caught it initially, but you're still accelerating and vertical velocity corrected it.

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i don't have it, i did do some simulations to verify the velocity off the rail was ok. I'll see if i can find those numbers

Cameron Anderson said:

Looks like it could a Cg shift due to acceleration...reduced propellant weight combined with your recovery gear settling rearward. Then acceleration takes over and it smoothes out.

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I bet that's it, sitting on the pad cp/cg difference was about 4" (1 caliber). Total burn time on that J250 is 2.8 sec so one second into the flight about 30% of the propellent is gone and since it was the cardboard DX3 the motor made up the bulk of the mass. I bet the CG coming forward made it twitchy. That's something i haven't read much on, the dynamic nature of the center of gravity during boost. For example, where should it end up at and how fast should it get there?

mbeels said:

Do you remember which way the wind was blowing? Is the direction of the tilt consistent with weather cocking?

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If you look at the igniter smoke the wobble was consistent with the wind direction. I think the CG moving forward on a light airframe at 1 caliber of stability (if that's the right phrase) made for a twitchy rocket.

Might not have been enough velocity off the rail...?

Someone on a recent thread called this "a rocket being a rocket." There are a lot of factors that can cause departures from a perfectly vertical flight, and it's maybe impossible to tell what they might have been in any particular case.

If you have good TTW, reasonable stability margin, aren't flying on a windy day, and have a reasonably long, reasonably rigid rail, you've done about all you can do.

mikec said:

Someone on a recent thread called this "a rocket being a rocket." There are a lot of factors that can cause departures from a perfectly vertical flight, and it's maybe impossible to tell what they might have been in any particular case.

If you have good TTW, reasonable stability margin, aren't flying on a windy day, and have a reasonably long, reasonably rigid rail, you've done about all you can do.

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lol that was my line.

With out a ground camera looking up it's hard to say.

The rail could be loose and move back and forth as the rocket leaves the rail, could be coning, my DX3 tends to do a spin (cone) off the pad as it gains speed and can look like back/forth wiggle from the flight line, for my L2 on a Loki J320r and a CTI J530 IMAX it did this until about 200'.

The Loki H160, AT 242 launches on my DX3 weather cock with the slow velocity. More of a slow back and forth oscillation with weather cock.

I thought about recovery gear moving but it's all packed tight with a laundry shelf, you can document the cg with and without propellant to see the shift.

I have come to believe the coning is a flight characteristics of a longer air frame with only 3 fins. My L1/2 rockets do this, my Lowtech 1-4 scratch built rockets do this. I have a few 4 fin rockets and they are hard to upset, they all fly fairly straight even in wind.

A simulation with my DX3 and a J250w nets 63ft/s off a 86" rail, liftoff weight: 123oz, Apogee about 3,582'.

~John

First 3 are the J530 IMAX flight, and the 4th is the AT H242t, wind on both flights was from the left about 6-8mph

GalantVR41062 said:

With out a ground camera looking up it's hard to say.

The rail could be loose and move back and forth as the rocket leaves the rail, could be coning, my DX3 tends to do a spin (cone) off the pad as it gains speed and can look like back/forth wiggle from the flight line, for my L2 on a Loki J320r and a CTI J530 IMAX it did this until about 200'.

The Loki H160, AT 242 launches on my DX3 weather cock with the slow velocity. More of a slow back and forth oscillation with weather cock.

I thought about recovery gear moving but it's all packed tight with a laundry shelf, you can document the cg with and without propellant to see the shift.

I have come to believe the coning is a flight characteristics of a longer air frame with only 3 fins. My L1/2 rockets do this, my Lowtech 1-4 scratch built rockets do this. I have a few 4 fin rockets and they are hard to upset, they all fly fairly straight even in wind.

A simulation with my DX3 and a J250w nets 63ft/s off a 86" rail, liftoff weight: 123oz, Apogee about 3,582'.

~John

First 3 are the J530 IMAX flight, and the 4th is the AT H242t, wind on both flights was from the left about 6-8mph

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my stratologgerCF beeped out a 3,700' apogee so your simulation is pretty accurate (i need to get/make a datacable). It was a very straight flight so the wiggle is more of a curiosity than anything else. Main deployment was at 700' and it ended up landing maybe 500 yards from the pad.

For the "cause" of the wobble in this flight, I have heard three interesting possibilities: 1) that the CG moved forward with propellant burn and 2) that the CG moved backwards when the recovery system settled backwards in the air-frame during high G thrust, and 3) that the rocket did some "coning" before it was going fast enough to become fully stable.

I actually like all three ideas, they all show that somebody is actually putting some thought into their response. Any of the three possibilities is a viable "cause" of a momentary wobble in an otherwise stable vertical flight. And its just as possible that all three of the ideas ( the two CG moving one forward and one backwards which might very well have canceled each other out, and the one coning before full stability idea) were operating on the stability of the rocket all at the same time. Under high acceleration/G-forces almost any recovery system can experience some compression no matter how "tight" the rocketeer thinks his recovery system was packed.

But it is also just as possible that there was no actual wobble at all. It could be a simple optical illusion as the original post postulates. The rocket does "leave" the video momentarily rising faster than the camera-person is keeping up. So its hard to tell. Plus a side view in a hand held device is very difficult to calibrate for movement.

I have my doubts about the CG moving forward causing the rocket to become twitchy. If the CP:CG ratio was just 1 caliper at lift off, then it is VERY doubtful that the CG moving forward during motor burn would move the CG far enough forward to cause any instability from the rocket becoming over stable. And remember that during acceleration the CP is moving backwards in almost all rockets. Generally speaking, the faster the rocket the further back the CP moves, but it would take well over mach speed to cause the CP to move so far back to engender an over-stabilization of the rocket. And that's just not probable on a J-250 (baby J motor) within the first second of flight. It would be more possible at the end of burnout when the rocket is moving at its highest velocity. But "coning" is far more likely a momentary problem than over-stabilization from a high speed move of the CP towards the back of the rocket.

Just my 2 cents

Brad