Question about stability

Wildman Rocketry
the rocketry show
eggtimer rocketry
The Rocketry Forum

Help Support The Rocketry Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.
Montaro

Montaro

Well-Known Member
Joined
Aug 17, 2011
Messages
250
Reaction score
0
1 week ago i flew my MR7 prototype, it works fine, but went i review the video i noted some tail side moves and i could saw the smoke doing a "S" trackway.

1.- I supouse that is not a stable fligth isn't it ?
2.- how can i correct that ?

Using Open Rocket with this rocket design the Stability are:
B6-4: 1.63 cal
C6-5: 1.42 cal

[YOUTUBE]vBJVfY0HTcA[/YOUTUBE]

Thanks!

View attachment MR7.ork
 
so i could be completely wrong on this - so someone please correct me - but it looks like the rocket is pushing into the launch rod, and then the 's' curve looks like 'rod whip' or that the end of the rod is pushing off the end of the rod. so perhaps your launch lug is too high. try putting a second launch lug it may help - but i could be completely wrong.

pg
 
A couple of thoughts:

A longer launch rod would help. What you have isn't very long. As mentioned above, lug placement may be an issue if it is too far away from the CG at launch.

Also, you are launching off the road, so you are in a "pocket" of sorts where wind is slowed down by the local brush on the sides of the road, contrasted to a full open field. In those circumstances, winds near the launch pad are hard to nail down unless you are flying balloons at select altitudes to get some information on how they are behaving. Wind speed (and even direction) will change with altitude.

Or it could have been a combination of the two.

Regardless, a 4 foot (1.2 meter) or longer launch rod should help.

Greg
 
I think you are OK on stability but you may be seeing a combination of "rod whip" and "tip-off" along with weathercocking. If the rocket was truly unstable I doubt you would have gotten that much altitude out of it.

Plus, when you shoot launch video from a location real close to the pad, you don't get a real accurate look at what the rocket is doing as it goes up since you have to "pan" rapidly to keep the rocket in the field of view.

My guess is you may have had some kind of cross wind, plus you may have had some "rod whip" as the rocket went up the launch rod, then as soon as it was in free flight, the wind caught the fins and pushed it onto a slightly angled ascent track resulting in a somewhat crooked flight.

Plus it is possible one or more fins might be slightly misaligned -- this would also push the rocket into an angled ascent path.

As others have suggested your best option may be to use a longer launch rod -- 4 feet or maybe 1.5 meters. That will allow the rocket to get more speed before it leaves the rod and be somewhat more resistant to weathercocking.
 
Last edited:
I really appraised you comments.

I will start adding another small launch lug near to CG. and then i'll look for a bigger launch pad.

Thanks again
 
Last edited:
Looks like you're in Joshua Tree/ yucca Valley, eh? You're pretty close to lucerne dry lake- there's gonna be a great launch there OCT 8-9. You should go and bring your rocket.

Have you tried launching it a second time and observed the results??? Was there wind? If it's windy enough, the rocket will actually head INTO the wind, because the wind pushes the TAIL FINS, which points the rocket into the wind. It's either the fins, lugs or rod.
 
To me, the flight looks fine at first, and the wiggling only starts later. Skip forward to about 1:20 in the video, when the slow-motion replay starts. Right off the rod, the rocket is as straight as a lamp-post. At 1:22, it tips slightly to the left but is still going pretty straight. At 1:24 it swerves back to the right and by 1:25 there's a visible zig-zag in the smoke trail. The rocket continues to zig-zag at 1:28 and beyond. Also worth checking is the pad camera footage starting at about 1:37 and repeated in slow motion at about 1:48. The rod does wobble a bit but the rocket flies straight; it does not start zig-zagging until it is well clear of the rod.

The rocket appears to be a Mercury Redstone, with small fins and probably a heavy nose. That combination is indeed prone to zig-zag a bit once it reaches high velocity. The small fins don't produce much correcting effect until the rocket is significantly off course, and then they produce a lot. The rocket swings back into line, swings past in line, and repeats the process in the other direction.

A longer rod may make the rocket more resistant to weather-cocking, but since the effect described above happens at high velocity, a longer rod won't have much effect on that. Bigger fins will, but then it won't be a scale Mercury Redstone any more. I'd say that, although it isn't perfectly straight, that flight is safe enough in that the rocket goes near enough straight up - it's not as if the rocket tips hard over shortly after launch and goes on a low trajectory.
 
Nothing looks wrong about those flights to me.

If you have *any* wind, a stable rocket will slightly react to it as it leaves the rod, then it will over correct itself, and do this a handful of times before it settles into a straight flight. An over-stable rocket will react too much to it.

A perfectly straight flight is desired, of course, but a slightly wobbly flight is not bad, it just means the fins are working as promised.

A slightly spiraling flight ( or 'coning', caused by pitch/roll coupling) is, to me, the best type of undesirable flight.
 
jeffgeraci said:
Looks like you're in Joshua Tree/ yucca Valley, eh? You're pretty close to lucerne dry lake- there's gonna be a great launch there OCT 8-9. You should go and bring your rocket.

Have you tried launching it a second time and observed the results??? Was there wind? If it's windy enough, the rocket will actually head INTO the wind, because the wind pushes the TAIL FINS, which points the rocket into the wind. It's either the fins, lugs or rod.
Click to expand...

I'm in monterrey, mexico. so is too far :(, this hobby is not popular in mexico, that is the reason i'm asking because the only flight i saw it are my :D. I should look more the wind contition before launch and try to learn about it.

adrian said:
To me, the flight looks fine at first, and the wiggling only starts later. Skip forward to about 1:20 in the video, when the slow-motion replay starts. Right off the rod, the rocket is as straight as a lamp-post. At 1:22, it tips slightly to the left but is still going pretty straight. At 1:24 it swerves back to the right and by 1:25 there's a visible zig-zag in the smoke trail. The rocket continues to zig-zag at 1:28 and beyond. Also worth checking is the pad camera footage starting at about 1:37 and repeated in slow motion at about 1:48. The rod does wobble a bit but the rocket flies straight; it does not start zig-zagging until it is well clear of the rod.

The rocket appears to be a Mercury Redstone, with small fins and probably a heavy nose. That combination is indeed prone to zig-zag a bit once it reaches high velocity. The small fins don't produce much correcting effect until the rocket is significantly off course, and then they produce a lot. The rocket swings back into line, swings past in line, and repeats the process in the other direction.

A longer rod may make the rocket more resistant to weather-cocking, but since the effect described above happens at high velocity, a longer rod won't have much effect on that. Bigger fins will, but then it won't be a scale Mercury Redstone any more. I'd say that, although it isn't perfectly straight, that flight is safe enough in that the rocket goes near enough straight up - it's not as if the rocket tips hard over shortly after launch and goes on a low trajectory.
Click to expand...


Yes, make sense, indeed i added the weight in the nose, could be best if i put more weight but not too close to the nose?, Yes i think the same the fins are to small but as you said is i tring to maintain the scale, indeed this is just a prototype, the next step is do it but with high details,

RoyAtl said:
Nothing looks wrong about those flights to me.

If you have *any* wind, a stable rocket will slightly react to it as it leaves the rod, then it will over correct itself, and do this a handful of times before it settles into a straight flight. An over-stable rocket will react too much to it.

A perfectly straight flight is desired, of course, but a slightly wobbly flight is not bad, it just means the fins are working as promised.

A slightly spiraling flight ( or 'coning', caused by pitch/roll coupling) is, to me, the best type of undesirable flight.
Click to expand...

This is my first model, the second version, the first version was failer, so i'm very happy with the result, but when i sow it in the video i note that, so i decided to ask because i'm newbie and i didn't know is this is stable or not .


Thank you everybody, for you comment are very helpful !!! :clap:
 
I wouldn't worry. If I had a penny for every one of my scratch-builds which flew as straight as that first time, I'd have quite a few pennies.
Instead of about 6d.
 
Montaro said:
Yes, make sense, indeed i added the weight in the nose, could be best if i put more weight but not too close to the nose?,
Click to expand...
No. You want the weight as far forward as possible for maximum effect. Putting more weight further back just makes the rocket heavier without doing anything useful for stability. You have put the weight exactly where it should go.

But, as others have said, the flight is fine. Don't worry about it. Basically, to answer your original first question - yes, that is a stable flight!

Yes i think the same the fins are to small but as you said is i tring to maintain the scale, indeed this is just a prototype, the next step is do it but with high details
Click to expand...
Good idea - check that the rocket flies first before putting too much work into it, so the work isn't wasted if it crashes. It didn't. Now I look forward to seeing the detailed version!
 

Similar threads

Skipper_52
Old man new builder
Replies
3
Views
422
Skipper_52
Skipper_52
JSW
Tail heavy LOC Warlock - Review my stability analysis please?
Replies
12
Views
1K
ROCKETDAWG
ROCKETDAWG
atestani
Fineness and Stability Margin in OpenRocket
Replies
17
Views
615
neil_w
neil_w
M
[Question] 2 Stage Rocket (over)Stability
Replies
22
Views
2K
Matthew Tan
M
Azamiryou
Stability change in first quarter second of flight
Replies
9
Views
568
Azamiryou
Azamiryou

Latest posts

Back
Top