What to do if rocket is overstable?

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Bat-mite

Rocketeer in MD
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My RW X-Celerator came with a lightweight, plastic, glassed nose cone. With it, my stability was exactly one cal. However, due to a shock cord mishap, I destroyed it. I replaced it with a FW FG nose cone with aluminum tip. It looks great, and is much more durable.

However, in so doing, I obviously increased the weight at the front end significantly. Now the stability is about 2.5 to 3 cal.

My two flights with the original NC were straight up and awesome. My one flight with the FG NC weathercocked hard and only achieved about half of its expected altitude.

Any ideas on how to bring the CG aft a little bit? I know this is the exact opposite of most people's problem!

Thanks.
 
True, but we only have a 4000' waiver at our summer field. I certainly don't want to make the rocket shorter, and I can't add a fin to push the CP forward.
 
You may need to add weight to the tail. Try stick-on weights for RC airplanes. When you have the CG where you want it, perform a little minor surgery and epoxy the weight inside the fin can.
 
True, but we only have a 4000' waiver at our summer field. I certainly don't want to make the rocket shorter, and I can't add a fin to push the CP forward.

Seriously? 4000 ft. is hardly HPR territory. You could still fly an AT J540 Redline, (77 fps) mine sims to 3660 ft. with that motor. Or really rip it off the pad with the CTI J1520 Vmax, (121 fps). Bet it won't weathercock with that motor!
I had one flight with a K490 Green that weathercocked in 15 mph winds, but that should be expected. Now my X-Celerator flies on Redline or Red lightning loads. The K750 brings it off the rod at 86 fps. All of these calculations are from Thrustcurve, with a 12 lb. rocket.
 
Curious, what kind of wind have you launched in, and what's the sim'ed off-rail velocity? I did over a dozen launches on a rocket that ran 5-6 cal overstable, and it was never anything but straight-up even in light winds, with altitudes within ~20% of what OR predicted except in cases where ejection was very early. In fact what always amused me about this rocket was almost every launch has somebody remarking on how straight it was on my videos (it's a GLR Firestorm 54, so I credit the Acme fin can for that). It seems to me you would need pretty strong winds or a rather slow launch for 2.5-3 cal to make a big difference. So perhaps just choose motors with a higher initial thrust? I didn't go by any guidelines other than what ThrustCurve or OpenRocket considered safe launch velocities, lift-off weights were in the 4-5 lb range and I flew it on a few I280DM's (5.79 cal), 2 I500T's (5.9 cal), an I140W (6.42 cal), I218R (5.87 cal), I161W (6.11 cal), I245G (5.94 cal), I357T (5.65 cal), I287SS (5.23 cal), I255RL (5.68 cal). The first flights were on an I215R and I65W, the rocket configuration was different those flights (more weight, but all ahead of the Cp so it was probably even more overstable), I wasn't taking as good notes back then so I don't have Cp vs. Cg data in my flight log. Off-rod velocities per OR were all above 50 ft/s though some only barely (the I65W was probably less, but I don't have the log data to back that up. That was still a straight-up flight, but the drogue charge never fired so it was mostly a straight-down flight also until the main fired. :p ).

So from my perspective I don't see overstability as anything that necessarily needs to be fixed, and I never even decided not to launch it due to wind, though I did consider higher off-rail velocities if the winds were higher.
 
I have seen weathercocking due to insufficiently stiff launcher whipping the rocket. I have seen weathercocking due to loss of a launch lug. I've seen weathercocking due to a loose fitting rail guide coming out of the rail slot when in the wrong size slot. I have seen weathercocking due to insufficient velocity at the end of the launch rail for the wind conditions. I don't consider 2.5 calibers of stability that big a deal. It sounds fine to me.

Gerald
 
I have seen weathercocking due to insufficiently stiff launcher whipping the rocket. I have seen weathercocking due to loss of a launch lug. I've seen weathercocking due to a loose fitting rail guide coming out of the rail slot when in the wrong size slot. I have seen weathercocking due to insufficient velocity at the end of the launch rail for the wind conditions. I don't consider 2.5 calibers of stability that big a deal. It sounds fine to me.

Gerald

+1

Yeah 2.5 to 3 cals is not that big of a deal. If you don't like it then just (as others have suggested) put in a bigger or faster burning motor.
 
It's not overstable, especially in wind when the CP moves aft because the airframe actually counts. When you had the weather-cocking, what was the windspeed, the motor, the weight of the rocket, and the angle of the launch rod?

More than likely the motor selected was underpowered for the wind as the minimum thrust to weight ration should exceed the wind speed in mph if you do not want weather cocking. Also did you angle the rod towards the wind to reduce the winddrift? This exacerbates weather-cocking because the rocket will always turn into the apparent wind and generate a lower apogee. The correct approach is to angle the rod a few degrees downwind (not more the 5 degrees under most circumstances) and the rocket will weather cock towards the vertical and will accelerate enough to maintain vertical flight. You will have a longer recovery walk, but you will reach the expected apogee. The final approach is not to launch the rocket if the wind is too high.

Bob
 
True, but we only have a 4000' waiver at our summer field. I certainly don't want to make the rocket shorter, and I can't add a fin to push the CP forward.

Technically our waiver for the Sod Farm is 10k, actual flying ceiling is determined by the B.O.D. when the range is opened for the day depending on weather and wind. Simple solution is bring lots of different rockets. And if the ceiling/waiver is high enough fly it on a bigger motor, you can, that way you do not have to modify to fly. We were open to 5k on Sunday.
 
The apparent consensus is that you're not excessively stable, and I would tend to agree. But if you really want to reduce that static margin, how about forward fins, as Phill suggested? You stated you "can't add a fin to push the CP forward," but why not.

Here's a thought. Add a section of body tube just long enough to glue a coupler into the bottom and accept the nose cone shoulder in the front. Place small fins on that. Secure it to the main body tube with something stronger than shear pins but removable, like screws or these, and then you can fly the rocket with or without it. Of course I'm leaving out lots of details you'll have to work out about attachment, recovery deployment, etc.
 
Try higher-thrust shorter-burn motors. Overstability only becomes a problem a lower velocities, if you pound it into the air you'll be fine.
 
Or you could go back to the N/C that it came with:

https://rocketrywarehouse.com/product_info.php?products_id=227

I really like my X-Celerator just like it came from RW, Seldom attend a launch without it.

2-22-2014wayneandsharonandrapiditty.jpg


You have a good selection of alternatives now, keep us posted on what you decide on.
 
Will, I'm right there with you. Wind was about 8 MPH. I was surprised to hear my altimeter only beeping out ~1900 feet on a J360. The heavier nose is the only thing different from the earlier flights.
 
I replied to Will before I read everyone else's replies. Sounds like next launch will be on a higher thrust motor. And Bob K may be onto something. I think I did angle upwind a bit.

Thanks for all the ideas.

The reason I'm sticking with the new NC is because it is so easy to stick a tracker in there. The hollow NC that it comes with has to be modified to put the tracker in, and I did that, and then I broke it. I don't want to have to do the mods again. Also, I once dropped the original NC on my basement (tile) floor and chipped the heck out of it. So I like FG a little better.
 
Has anyone ever intentionally added weight to the rear of a rocket to alter the stability? Or for any other reason?

One of the factors that affects the rocket's likelihood to turn is it's "moment of inertia." And moving weight to both ends of the rocket, away from the center would give it greater angular momentum. IOW, the same exact rocket, with the CG and CP at exactly the same spot would have less likelihood to turn if the mass were distributed to the tips than if it were concentrated at the CG. Has anyone ever intentionally tried to alter the moment of inertia by adding weight to the tail, or any other means?
 
I've got a rocket with 4-5 calibers of stability depending on the motor and flies straight as an arrow. No intention of adding any tail weight.
 
John,,
That's far too low of an over-stability # to be of concern..
The rocket's flyin great...
If you'd like just use higher initial impulse.....

Teddy
 
Think I found the problem. I ran the sims on Thrust Curve, and the motor I used failed due to insufficient velocity off the pad: 37 fps. So I can see where I can blame the motor choice. However, at Red Glare, where we had incredibly stiff winds (16 MPH+), I launched it on a motor that gave me 38 fps, and it went straight up.

I guess between the change in NC and the 1 MPH less off the pad, I got a nice turn into the wind. I will be going with a higher impulse motor next time.
 
I'm not a fan of launching on motors that don't provide at least 50 fps off the rail and usually try to find something closer to 60 fps. YMMV.
 
Think I found the problem. I ran the sims on Thrust Curve, and the motor I used failed due to insufficient velocity off the pad: 37 fps. So I can see where I can blame the motor choice. However, at Red Glare, where we had incredibly stiff winds (16 MPH+), I launched it on a motor that gave me 38 fps, and it went straight up.

I guess between the change in NC and the 1 MPH less off the pad, I got a nice turn into the wind. I will be going with a higher impulse motor next time.

In Thrust We Trust!

It's funny how higher thrust is considered the cure-all for both overstable and understable rockets.

On long, overstable rockets that tend to weathercock into the wind, high thrust is supposed to get the speed up, so the wind is less of a component of the overall airspeed vector.

On short, stubby, understable rockets, the problem is not weathercocking, but a possibility of unstable tumbling. The high thrust is supposed to get the speed up so that the rocket can make the most of the minimal margin. And base drag can start to become a stabilizing factor at higher speeds too.

In either case, you can get an errant flight if there isn't enough initial kick off the rail, but for totally different reasons. You don't necessarily need a lot of impulse, but you do want thrust.
 
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