Disadvantage of overstability of a rocket

Hi everyone!

As we already now, the static margin above the 2.00 is called "overstable" for a rocket. Can you tell any disadvantage of overstability, if the main objective of the rocket is only climbing to specific altitude. All comments are welcome!

Thanks!

Overstable rockets tend to "weathercock" or arc upwind as they go up, and the higher they go the more they arc over, the higher the wind speed the quicker the arcing.

They also have a tendency to "cone" quite a bit, requiring more impulse to achieve the desired altitude.

rharshberger said:

Overstable rockets tend to "weathercock" or arc upwind as they go up, and the higher they go the more they arc over, the higher the wind speed the quicker the arcing.

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... and the arcing means that there will still be a horizontal motion (possibly significant) even at apogee, causing increased stress on the laundry

The above comments are totally correct. In my limited experience, weathercocking is the biggest issue. On the other hand, while 1 to 2 is optimum, it is possible to fly successfully with substantially higher ratios, particularly if winds are light, the motor has ample thrust (avoid "long burn" motors), and the launch rod or rail is lengthy.

The stability margin of 1-2 calipers is for sub sonic speeds, the center of pressure moves as velocity increases.

Supersonic rockets need more calipers of stability. Also when talking larger motors in lighter rockets watch the stability margin as the motor burns and burnout weight.

I have seen coning on my scratch built BT55 body tube rocket that was 48" tall but a bt60 body tube with the same fins and length was a nice straight flier.

I use thrust curve for estimating all the vital velocity's and adjust the numbers to match actual flight data with my rockets.

Having more stability margin can account for errors in the CP, then you can adjust the CG after test flights if you see adverse flight characteristics.

~John

I like using two calipers, in three locations each to get an accurate average reading.

In my opinion overstability is a minor issue that gets way too much attention. My 6" diameter 10 ft Formula 150 has 5 cal of stability which supposedly should make it extremely "overstable" but it has never had a problem with weathercocking, even when I flew it in 20 mph wind at LDRS. On windy days the big problem isn't weathercocking due to overstability, it's getting dragged across the field by the chute after it's back on the ground.

Same here, two of my most-flown rockets (38mm MD and 54mm MD) regularly fly with stability ratings between 5 and 9 calibers, and I have never had a bad flight with either of them that had anything to do with weathercocking (even in decent winds) or apogee arcing issues (the 54mm was flown mostly motor-eject, and that had its own set of issues but only due to very early or late deployments). The only time I've ever had a rocket cone was one that was right at 1.5 calipers, but I think it had more to do with the camera I had taped near the top of that one. I have always ensured a good speed off the rail, 50ft/s minimum has always been my target, though some might have been closer to 45 in lighter winds.

DavidMcCann said:

I like using two calipers, in three locations each to get an accurate average reading.

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Hmm, I also like using two calipers to make sure I have two calibers of stability. One caliper is mechanical the other electronic. For calibers, I prefer 9mm and .45.

Can never be two, er, too careful.


Tony

Caliper= measuring device, Calibre/Caliber=measurement.....

rharshberger said:

Caliper= measuring device, Calibre/Caliber=measurement.....

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Excalibur=Excelsior

dhbarr said:

Excalibur=Excelsior

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Excalibur=sword....

Nathan said:

On windy days the big problem isn't weathercocking due to overstability, it's getting dragged across the field by the chute after it's back on the ground.

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I shudder at the thought of one of your paint jobs getting "dragged across the ground " -
I love grass fields-

That's why I love colored FG... I quit painting rockets a few years ago. The last paint job that I did lasted about 10 seconds after landing... once the W got ahold of the chute.

Nathan said:

In my opinion overstability is a minor issue that gets way too much attention. My 6" diameter 10 ft Formula 150 has 5 cal of stability which supposedly should make it extremely "overstable" but it has never had a problem with weathercocking, even when I flew it in 20 mph wind at LDRS. On windy days the big problem isn't weathercocking due to overstability, it's getting dragged across the field by the chute after it's back on the ground.

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I always thought the relationship between overstability and weathercocking was simply that overstable rockets likely have more weight in the nose than is necessary, thus the rocket takes off slower than it otherwise would with nose weight closer to the minimum necessary.

I fly over-stable rockets all the time. If the rocket leaves the rail significantly faster than any winds, the flight will be straight as an arrow. If the rocket leaves the pad slowly in significant winds, the fins will be at an angle of attack, and will turn the rocket up wind.

Essentially, the fins on any rocket will try to minimize the relative wind angle, that is, the angle the air passes over the fins with the rocket's velocity factored in. On an under-stable rocket, the fins aren't up to the task. On an over-stable rocket, the fins are more effective than necessary, and will over compensate for the wind.

Never tilt a rocket into the wind. That’s especially true for an overstable rocket.

Steve Shannon said:

Never tilt a rocket into the wind. That’s especially true for an overstable rocket.

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THIS

For high altitude birds, I have purposefully tilted them with the wind with great results.

rfjustin said:

THIS

For high altitude birds, I have purposefully tilted them with the wind with great results.

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Exactly; vertical or with the wind.

Steve Shannon said:

Never tilt a rocket into the wind. That’s especially true for an overstable rocket.

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Unless into the wind is away from the crowd/flight line. If that's true, check where rockets are landing/flying to try to keep them from arcing over or landing in the crowd.

Andy Greene said:

I shudder at the thought of one of your paint jobs getting "dragged across the ground " -
I love grass fields-

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So does he.

Andy Greene said:

I shudder at the thought of one of your paint jobs getting "dragged across the ground " -
I love grass fields-

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I'm going to take some new pictures of my Formula 150 to show how it looks before and after it got dragged a quarter mile across Higgs farm last year.

Steve Shannon said:

Never tilt a rocket into the wind. That’s especially true for an overstable rocket.

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Could you elaborate on that please? I understand not to tilt an overstable rocket into the wind, but really it should never be done? If so, a lot of people are doing it wrong, tilting into the wind so it blows back to the pad after ejection.

billdz said:

Could you elaborate on that please? I understand not to tilt an overstable rocket into the wind, but really it should never be done? If so, a lot of people are doing it wrong, tilting into the wind so it blows back to the pad after ejection.

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I have known lots of people who try to do just that, but it’s gonna turn towards the wind anyway. The result at best is a rocket popping the chute with a horizontal velocity. Worst case is a cruise missile. The worst cruise missiles I’ve seen were people trying to ensure they didn’t have to walk too far on certification flights.
I set them vertical or even downwind.
When the wind is going directly away from the flight line I set them angled to one side or the other, but I never tilt directly into the wind (maybe I should say no more than a degree or two).

Steve Shannon said:

I have known lots of people who try to do just that, but it’s gonna turn towards the wind anyway. The result at best is a rocket popping the chute with a horizontal velocity. Worst case is a cruise missile. The worst cruise missiles I’ve seen were people trying to ensure they didn’t have to walk too far on certification flights.
I set them vertical or even downwind.
When the wind is going directly away from the flight line I set them angled to one side or the other, but I never tilt directly into the wind (maybe I should say no more than a degree or two).

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I assisted with an SLI flight recently, and that rocket was pretty overstable. We had some wind for the flight, and I tilted the rocket maybe 3 degrees downwind. No one was happy with that arrangement (but I was and I had the only vote). The rocket weathercocked straight up, and then arced over into the wind. Landed not far from the pad. Pretty clear how this worked from the video.



Jim

Here is what happened when I thought I could out smart the wind on my first L1 cert flight attempt a few years back. I just figured I would tilt it into the wind and it would come back to me but instead it flew as far uprange as it did vertical and deployed the chute while doing 60+mph and flying horizontal. Oh yeah and to top it all off it landed in a creek. Needless to say it was a humbling beginning for me in this hobby.

Andrew

AndrewW said:

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Here is what happened when I thought I could out smart the wind on my first L1 cert flight attempt a few years back. I just figured I would tilt it into the wind and it would come back to me but instead it flew as far uprange as it did vertical and deployed the chute while doing 60+mph and flying horizontal. Oh yeah and to top it all off it landed in a creek. Needless to say it was a humbling beginning for me in this hobby.

Andrew

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That is exactly what I’ve seen.
When I certify people I walk out to the pad with them. Like Jim said they question it when I prevent them from tilting into the wind. I don’t think they really believe me when I tell them about the risk. Everyone has to learn the lesson somehow. I was fortunate enough to see someone else do it once on his L2 flight. We found the remains of that rocket a few years ago, more than ten years later. That was an expensive lesson for him; he did everything else just right. He dropped out of rocketry shortly afterwards.

Steve Shannon said:

That is exactly what I’ve seen.
When I certify people I walk out to the pad with them. Like Jim said they question it when I prevent them from tilting into the wind. I don’t think they really believe me when I tell them about the risk. Everyone has to learn the lesson somehow. I was fortunate enough to see someone else do it once on his L2 flight. We found the remains of that rocket a few years ago, more than ten years later. That was an expensive lesson for him; he did everything else just right. He dropped out of rocketry shortly afterwards.

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Ironically it was this “valuable” lesson that got me hooked on the hobby. It started me down the road of really understanding what happened and what is really going on when we fly these things. I have always been intrigued by what appears to be exceedingly simple things that in reality are very complex. If was as simple as stuffing a motor in a length of tube with fins and a nose cone and lighting it off I would have probably lost interest a long time ago.

Clarification please! Are you all saying straight up or with the wind for only over-stable rockets?