Confusing L2 Fail

[DTH Rocket]

Hello everybody,

I haven't been on TRF for a while, but recently I tried my level 2 certification, and the flight wasn't that great. It seems to kinda corkscrew on the ascent as if it were having stability issues, but I had 4 caliber stability on that rocket. It flew straight last August on an I285, and a J285 is not that different. What happened? Any ideas?

Here are some videos:

Unsuccessful level 2 cert this June:


http://www.youtube.com/watch?v=myDPygvEBGU

Successful level 1 cert last August (second flight):


http://www.youtube.com/watch?v=rwz37a046Y8

[dixontj93060]

Didn't quite hear the LCO... Did you add weight to keep the flight near 1 mile altitude versus the 6K feet?

[rocdoc]

Your stability was the issue, but not because the rocket was unstable. Rather, it was overstable. When a rocket has a margin as large as yours, it has a tendency to cone, or spin around the nosecone. It appears to me in the flight that the aft end of the rocket was rotating wildly around a relatively stable nosecone.

Many fliers just worry about having the center of gravity at least a caliber forward of the center of pressure. You demonstrated that there is a limit at which it is too far forward. My L1 flight (a Giant Leap Crossbow with the dual deployment [read -- longer] kit) flew similarly, though not to the same extent. Adding a larger motor makes the problem worse, not better, especially if some of the motor's mass is also forward of the CP.

One of the things you could have done to salvage the squirrelly flight would be to use your electronics for drogue ejection as well. The lower altitude would not have bitten you in the butt like it did here. I know this is controversial, but I fly dual deployment with redundant electronics on every project (including my L1 and L2 certs) because I never trust motor eject. Give me a plugged motor any day.

Sorry to hear about the failed cert. Hope things weren't too badly damaged. And heck, if it's a total loss, you just have to build a new rocket. Dang!

[cjl]

Your stability was the issue, but not because the rocket was unstable. Rather, it was overstable. When a rocket has a margin as large as yours, it has a tendency to cone, or spin around the nosecone. It appears to me in the flight that the aft end of the rocket was rotating wildly around a relatively stable nosecone.

Many fliers just worry about having the center of gravity at least a caliber forward of the center of pressure. You demonstrated that there is a limit at which it is too far forward. My L1 flight (a Giant Leap Crossbow with the dual deployment [read -- longer] kit) flew similarly, though not to the same extent. Adding a larger motor makes the problem worse, not better, especially if some of the motor's mass is also forward of the CP.

One of the things you could have done to salvage the squirrelly flight would be to use your electronics for drogue ejection as well. The lower altitude would not have bitten you in the butt like it did here. I know this is controversial, but I fly dual deployment with redundant electronics on every project (including my L1 and L2 certs) because I never trust motor eject. Give me a plugged motor any day.

Sorry to hear about the failed cert. Hope things weren't too badly damaged. And heck, if it's a total loss, you just have to build a new rocket. Dang!

A very common, and very wrong explanation. Overstable rockets tend to weathercock more, and they respond faster to things like wind shear, but they do not inherently have any tendency to cone or act as the rocket in the above video does.

What can cause coning is fin misalignment, but I'd be surprised if that were the case here, since the rocket coned so violently in the J285 flight, but not substantially at all in the I285 flight. Another possibility is misaligned thrust - what did the nozzle look like when you got the rocket back? I don't think this was the case, but it's at least worth looking into.

Finally, was it windy that day? I notice your rocket has fairly small fins, and it's possible that if your rocket has a fairly large moment of inertia and relatively little damping that it is simply underdamped. That would cause it to oscillate around a stable flight path, especially on a windy day. That would be solved by going to slightly larger fins. Was your rocket a kit, or scratch built?

[Eric1]

My first instict is weathercocking, In the video you can see a Mean machine swaying in th surface wind. My bet is wind shear at altitude is the witch at work here. I agree with CJL you could use larger Fins! Better Luck next time!!


Peace Love and Happiness
Eric
BaddAzzRocketry.com

[RoyAtl]

A very common, and very wrong explanation. Overstable rockets tend to weathercock more, and they respond faster to things like wind shear, but they do not inherently have any tendency to cone or act as the rocket in the above video does.

What can cause coning is fin misalignment, but I'd be surprised if that were the case here, since the rocket coned so violently in the J285 flight, but not substantially at all in the I285 flight. Another possibility is misaligned thrust - what did the nozzle look like when you got the rocket back? I don't think this was the case, but it's at least worth looking into.

Finally, was it windy that day? I notice your rocket has fairly small fins, and it's possible that if your rocket has a fairly large moment of inertia and relatively little damping that it is simply underdamped. That would cause it to oscillate around a stable flight path, especially on a windy day. That would be solved by going to slightly larger fins. Was your rocket a kit, or scratch built?

I agree with cjl. FINS FINS FINS!!! You need bigger FINS!!

The exhaust flame does look a little skewed in the first fraction of the flight. That could be enough to push the angle of attack to more than can be countered by the small fins. The higher AoA also causes much more drag, slows the rocket down, lessens the altitude. Notice that after burnout, it settles down.

I would say double the fin area (at least), and lessen the nose mass if possible to get a two caliber margin.

[RoyAtl]

My first instict is weathercocking, In the video you can see a Mean machine swaying in th surface wind. My bet is wind shear at altitude is the witch at work here. I agree with CJL you could use larger Fins! Better Luck next time!!


Peace Love and Happiness
Eric
BaddAzzRocketry.com

I'm sure the wind didn't help, but it didn't cause this.

[Eric1]

I'm sure the wind didn't help, but it didn't cause this.

Agreed, After further viewing Wind shear alone is NOT the sole issue. I viewed by quickly double clicking mouse to get a stop action effect. Possiblly off centered nose weight in combination with smallish fins and Wind shear added up to this bad flight. Best I can tell all of these appear to be contributing factors. Once thrust is off center all you have is fin surface trying to correct. Interesting to try to break it down, DTH this is my perspective.

[TD Sky]

Wind shear? Even if its agreed that "it" is not causing the problem, isn't the term being used rather loosely in this thread? Shear is a large gradient of wind speed in a small distance...not just "wind". Wind shear can't normally be "observed" from the ground. It just looks windy to me.

[James Donald]

Assuming the motor nozzle is OK, you have a classic coning problem caused by rocket spin, over stability, and wind.

When the rocket spins any force that tries to change the rocket's direction will cause it to precess around the center of gravity. The wind acts on the center of pressure, which in your case is far behind the center of gravity. This gives more leverage to change the rocket's direction, causing the precession to have a large amplitude.

Additionally, a large moment of inertia caused by the most of the weight being in the nose and tail, away from the center of gravity will slow the correction of the rocket's direction by the aerodynamic forces. If this frequency is close to the precession frequency the result will be instability.

There must be some spin of the rocket involved. The question how much spin is required to cause this effect? I should probably do the math.

James Donald

[bobkrech]

You need to give us the specifics of your rocket: weight with motors, diameter, CG and CP from nose with those motors, fin position and dimensions, rod length and wind speed at time of launch. Without this information it is difficult to make a definitive assessment of what might have happened.

The comments about being overstablity are incorrect. An overstable rocket will turn into the wind and stay there. Your rocket did not do this. Its motions indicate that the fins are stalled, and not providing sufficient correction to the flight path. You launch in a windy area and this type of failure was apparent on the video of the prior failed flight to your successful L1 cert flight.

The rocket will most likely be less stable on a J285 than an I285 because the J285 weighs 595 g and is 367 mm long versus the I285 that weighs 510 grams and is 301 mm long, shifting the CG aft somewhat reducing the stability. Additionally, the thrust to weigh ratio of the rocket will decrease slightly due to the extra propellant weight, reducing the rod velocity.

In general, we can speculate what might have happened by looking at your rocket, your launch rail, and the wind conditions at your field. Your rocket is long and skinny with small fins. This generic form factor is susceptible to unstable flights in higher winds because the Cp moves forward and reduces the stability in high crosswinds and is greatly aggravated by small fin area. RS and other simple CP calculators do not estimate the forward shifting of the CP in a cross-wind cause by the airframe. An apparent zero wind stability margin of 4:1 can erode to zero in a stiff crosswind, especially if the launch rail is not sufficiently long to allow for a significantly higher leaving velocity that what is necessary for a calm day. This can be mitigated by increasing the rod velocity either by lengthening the rail length and/or increasing the T/W ratio of your rocket in wind so the rail velocity increases proportionally to the wind velocity. Both increase the airspeed and reduce the angle of attack of the fins when the rocket leaves the rail.

Bob

[dixontj93060]

As they used to say on Family Feud... Good answer, good answer...

You need to give us the specifics of your rocket: weight with motors, diameter, CG and CP from nose with those motors, fin position and dimensions, rod length and wind speed at time of launch. Without this information it is difficult to make a definitive assessment of what might have happened.

The comments about being overstablity are incorrect. An overstable rocket will turn into the wind and stay there. Your rocket did not do this. Its motions indicate that the fins are stalled, and not providing sufficient correction to the flight path. You launch in a windy area and this type of failure was apparent on the video of the prior failed flight to your successful L1 cert flight.

The rocket will most likely be less stable on a J285 than an I285 because the J285 weighs 595 g and is 367 mm long versus the I285 that weighs 510 grams and is 301 mm long, shifting the CG aft somewhat reducing the stability. Additionally, the thrust to weigh ratio of the rocket will decrease slightly due to the extra propellant weight, reducing the rod velocity.

In general, we can speculate what might have happened by looking at your rocket, your launch rail, and the wind conditions at your field. Your rocket is long and skinny with small fins. This generic form factor is susceptible to unstable flights in higher winds because the Cp moves forward and reduces the stability in high crosswinds and is greatly aggravated by small fin area. RS and other simple CP calculators do not estimate the forward shifting of the CP in a cross-wind cause by the airframe. An apparent zero wind stability margin of 4:1 can erode to zero in a stiff crosswind, especially if the launch rail is not sufficiently long to allow for a significantly higher leaving velocity that what is necessary for a calm day. This can be mitigated by increasing the rod velocity either by lengthening the rail length and/or increasing the T/W ratio of your rocket in wind so the rail velocity increases proportionally to the wind velocity. Both increase the airspeed and reduce the angle of attack of the fins when the rocket leaves the rail.

Bob

[jaz]

Great thread. This level of analysis really adds to the depth of the forum. Thanks all it always great to learn from everyone experiences.

[DTH Rocket]

Thanks everybody for the input. I've been working a lot lately and this is the first chance I had to look at this thread since I posted it.

I'm seeing a common theme here, possible nozzle issue, too small fins, and wind issues. Unfortunately I didn't inspect the nozzle closely before I threw it away, but I can say that there wasn't anything so unusual about it that I would notice it.

I agree that the fins are rather small for its length. But how that could get it to actually vortex like it did remains a mystery to me. I'll look into how the CP could move forward in the presence of a crosswind. I seem to remember a Peak-of-Flight article on how long rockets can sometimes go unstable.

Here are as many specs as I can think of:

Weight: 3.75 lbs
Length: 72 in
Diameter: 3 in
CG: 45 in
CP: 58 in
Margin: 4.3 calibers
Rail length: I THINK like 72 in
Average windspeed (guess): 5-7 mph

Thanks again everybody for all the ideas.

[rokit]

These are a few things that have caused some of my rockets to have flight profiles similar to yours:

How tight does the coupler fit, and how stiff is it? Is there any slop at the joint? If so, it could contribute to the flight profile from the videos. Same goes for the nosecone.

There is some rolling going on. How is the weight distributed in the rocket (think unbalanced washing machine)? I don't think the roll rate was high enough to be the sole cause, but might contribute some.

Is the stability margin from the sim, or did you check it before flight, verifying the CG was where it was supposed to be?

Are there any bulkheads or centering rings that keep the parachute and recovery harness from sliding aft? With a long rocket, even a light weight that shifts aft could affect stability.

Is the motor tube parallel to the axis of the rocket? I have a rocket where I made the centering rings myself. They fit inside a molded fiberglass tailcone, where the halves are mated with a strip of fiberglass. I had to shape the CRs to clear the strip. Long story short, the MMT ended up off by a few degrees, and that rocket flies similar to yours.

Mike

[DTH Rocket]

Hi rokit,

If anything, the coupler fittings were too tight. I had to do a lot of sanding before launch.

I'm 95% sure that the weight distribution was okay. With such a long rocket with such a high moment of inertia any small variance probably wouldn't affect it anyway.

Yeah, the margin is both from the sim, and I checked it at the field and it hadn't changed. 4.0-4.3

There are no bulkheads to prevent the parachute from sliding aft, and that is because the parachute and shock cords fill the entire bay. It can't slide in either direction.

Your last question made me curious, because I did make the centering rings myself. I even made the MMT myself. However, I'm looking at the rocket right now, and it looks like the motor is completely centered fore and aft, which means it must be running very parallel. But I wonder...

[n3tjm]

Nice looking rocket. Your fins do look small from the videos which probably had something to do with your issue. Here is another thought.

I cant tell by looking at the second video, but how much tube coupler is joining the two sections of the rocket? If this coupler is to short, the two sections will try to fold into slight V, increasing the pressure on one side of the rocket and veering it off coarse. My Ladyrobin is Prone to this very issue. On small I's the flight is nice and straight. On larger I motors and J's I seen it wander around. To counter this effect I made the tube coupler as tight as possible on both ends but not to hinder deployment. Last flight of the Ladyrobin was with a J415. You can tell from this video, it still tried to do it. The ground video show a slight veering towards the end of the burn. After seeing this, I decided not to fly it with a K the next day because I was affraid of some kind of failure.


http://www.youtube.com/watch?v=v6KK9HjPLh4