rocket always kicks to side after leaving launch rail

[rocketdoctor99]

I have a mid power rocket that Im flying on F20 engines, I simulate it in open rocket and have a stability at just under 2. It fly strait on calm days but last weekend winds were averaging at 8 mph and higher and everytime it left the rail it would kick to the side. could this be caused by my CG being too high. the rail buttons are both on bottom tube is there a standard for placing rail buttons?

 

[Nytrunner]

High winds can catch a rocket and make it weathercock if theres a higher separation between the CG and CP. Especially on slower motors like the F20

Hiw long and wide is your rocket?

 

[neil_w]

What is the rocket, and what is the stability margin? What you're describing *sounds like* routine weathercocking, where a rocket will turn into the wind off the rod. Normal.

Possible ways to reduce this are (a) reduce the stability margin (be careful!), (b) fly in calmer conditions, or (c) fly a higher thrust motor that will give the rocket more speed off the launch rail.

[ninja'd by Nytrunner]

 

[GalantVR41062]

Any pictures or video? What's the rocket? Size, weight, number of fins? Also what's the launch pad setup you are using?

On 3 fin rockets I started setting up my pad so one fin of a 3 fin rocket would be into the wind, I hope this helps not to spin the rocket off the pad.

~John

 

[neil_w]

On 3 fin rockets I started setting up my pad so one fin of a 3 fin rocket would be into the wind, I hope this helps not to spin the rocket off the pad.

Have you found that that actually works?

 

[Buckeye]

More information needed. I assume "kick" means a short-duration, high-amplitude event. Weather-cocking is more gradual and arcing, in my view.

Longer effective rail length always helps.

 

[Bat-mite]

You need more thrust. The rocket coming off the rail/rod has not reached a high enough velocity for the aerodynamic forces to kick in.

Run a sim and you'll likely see "Insufficient velocity off the rail". Make sure you put in accurate wind speeds.

 

[rocketdoctor99]

An update to this added a much bigger motor F67 from F20. This is a TARC rocket so had to add some more weights to keep altitude to 800 feet, but model showed stabilization at 1.3. The first lauch off a 4 foot rail the rocket again kind of kicked to the side, when we flew from an 8ft rail it flew much straighter.

 

[caveduck]

When a rocket leaves the guidance of the launch rail, there is a sudden onset of rotation into the relative wind. The angle of attack can be surprisingly large - the effective wind direction is just the vector sum of the velocity along the rail and the crosswind. If the rocket is at 50 fps and the crosswind is 9 fps (about 6 mph), the AOA is about 10 degrees. That is really a lot for TARC. At our site we require TARC rockets to use 6 foot rails and don't allow motors under ~F30.

There is a separate issue about flexing of the launch pad due to motor thrust on the blast deflector. If the deflector is directly mounted to the pad (as they usually are), when the motor starts up the initial force of the motor is absorbed almost entirely by the deflector, applying force to the launcher that is proportional to the initial thrust and dependent also on the angle of the deflector. A lot of leg systems including jawstands and almost all fabricated "spider" stands are pretty springy; they will deflect from the initial thrust and then snap back as the rocket gets higher up the rail and the thrust is no longer directly hitting the deflector. This can cause a noticeable "tipoff" even with no wind.

 

[Alan15578]

When a rocket leaves the guidance of the launch rail, there is a sudden onset of rotation into the relative wind. The angle of attack can be surprisingly large - the effective wind direction is just the vector sum of the velocity along the rail and the crosswind. If the rocket is at 50 fps and the crosswind is 9 fps (about 6 mph), the AOA is about 10 degrees. That is really a lot for TARC. At our site we require TARC rockets to use 6 foot rails and don't allow motors under ~F30.

There is a separate issue about flexing of the launch pad due to motor thrust on the blast deflector. If the deflector is directly mounted to the pad (as they usually are), when the motor starts up the initial force of the motor is absorbed almost entirely by the deflector, applying force to the launcher that is proportional to the initial thrust and dependent also on the angle of the deflector. A lot of leg systems including jawstands and almost all fabricated "spider" stands are pretty springy; they will deflect from the initial thrust and then snap back as the rocket gets higher up the rail and the thrust is no longer directly hitting the deflector. This can cause a noticeable "tipoff" even with no wind.

When you launch a rocket it first adjusts to the wind reference frame, so it could be observed to jump to 8 mph horizontally with the wind, but since it is statically stable it will also tend to rotate into the wind, that could be interpreted as a jump in angle. While the rocket is on the launcher it is constrained from "jumping" into the wind reference frame. The transition can be quite complicated, and can include launch rod or rail whip. Furthermore, If you are launching a heavy rocket form a light weight commercial launcher, the loaded launcher may hold steady on a smooth surface, but as soon as as the the motor fires the weight of the rocket no longer keeps the launcher fixed to the ground through friction, and it can slide across the ground with the wind before the rocket leaves the guide rail. Regardless of the specific details of the launcher dynamics, it is to your advantage to use the longest and most rigid launcher available, even if you have to provide your own.

 

[Viperfixr]

I mentioned the same thing about a rocket I have to Crazy Jim recently. He said to look at the rail button alignment. Darned if he wasn't right--they were off. I can't wait to fly the rocket with re-aligned rail buttons and see if the off-vertical launch problems go away (I bet they will).

 

[cavecentral]

Also check igniter leads are tied to the pad and you are not carrying them up the rail and pulling the rocket. I made that mistake a few times.

 

[heada]

Also check igniter leads are tied to the pad and you are not carrying them up the rail and pulling the rocket. I made that mistake a few times.

I made the mistake of twisting the leads onto the motor retainer once. On that same flight, I wrapped the controller leads around the leg of the pad. Needless to say, it was a bad move and made for a very "interesting" launch.

 

[samb]

In general, a longer launch guide (rod or rail) is a good thing. What about the fins on this puppy ? Size, shape, span etc. would have some bearing on how much air they catch. With your original stability margin just under 2 I might see if slightly smaller or reshaped fins, maybe a little swept back, might work without the added weight. I know it's prime time for TARC test flights now. Good Luck !

[edit] pictures ?

 

[BABAR]

Was wondering about separation distance of the rail buttons, although if you have sufficient velocity shouldn’t be an issue

A side wind might push the nose to the opposite side once the forward button clears the rod (basically swiveling around the single remaining rear rail button.). This might explain an abnormal trajectory beginning the instant it leaves the rail. Is the direction of tilt always right or left, with the rail slot facing you (rather then toward you or away from you, again in reference to rail slot facing you)?

Make sure the rocket smoothly slides up and down and OFF the rail, as previously mentioned a misaligned button or something “catching” might throw your rocket off.

 

[Voyager1]

An update to this added a much bigger motor F67 from F20. This is a TARC rocket so had to add some more weights to keep altitude to 800 feet, but model showed stabilization at 1.3. The first lauch off a 4 foot rail the rocket again kind of kicked to the side, when we flew from an 8ft rail it flew much straighter.

I believe you have answered your own question here. Use a longer rod or rail to ensure a higher unguided launch velocity.

 

[58pan]

could this be caused by my CG being too high. the rail buttons are both on bottom tube is there a standard for placing rail buttons?

Usually the bottom button is at the bottom of the rocket, usually screwed into the bottom centering ring. The top button should be at or just above the center of gravity. But they usually are screwed into the top centering ring.

 

[UncleJoe]

Without a pic, I'd recommend adding 10% more fin area.