Stability at mach+ speeds

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Yeah i tried that now and either the results didnt get any better than the sim i had now or it was just impractical.

I think you need to look at this from the other end.

You should post your OR file and your results to verify your methodology and put yourself open to scrutiny by the community. There are some very knowledgeable folks here when it comes to this specific topic, and I am not one of them, but I can follow advice and regurgitate what others know.

However if at the end of the day your methodology and numbers hold up but the results are not in your favour, then you need to adjust your approach.
 
Sim it and plot stability vs time. Do not let the stability go under two, or at least not by much.

If you fly with such a slight margin as you are planning you are flirting with going unstable and losing your rocket.

Transonic is hard to predict due to compressibility issues, and you are spending a chunk of the flight there.

Your altitude will not be drastically different.

As you get to Mach 1.5 plus the Cp can move a bunch.
So far i have 1.6 cal at top speed. Moving at to 2 at top speeds makes me lose about 100 ft in altitude (which isnt that much).
 
The ork file was in post 21. And sorry, I meant to post my ork file earlier but I clicked on the write up by accident.
 
Below is a stability vs. time plot, with the Mach number thrown in. The calibers don't look bad as far as margin, of course a bit more margin at launch would be nice, but then you are going for a record. There is another guy on TRF, Coleman AKA RocketHunter. Coleman has built a few of these bare min rockets or in his case "flying cases", but the design and objectives are similar. You might want to reach out to him and pick his brain.

 
Below is a stability vs. time plot, with the Mach number thrown in. The calibers don't look bad as far as margin, of course a bit more margin at launch would be nice, but then you are going for a record. There is another guy on TRF, Coleman AKA RocketHunter. Coleman has built a few of these bare min rockets or in his case "flying cases", but the design and objectives are similar. You might want to reach out to him and pick his brain.

Yeah, i did see a couple of his threads. One i find interesting is his 29mm MD (https://www.rocketryforum.com/showt...-quot-Altitude-Seeker-quot-16-000-amp-mach-2). It's basically the same design as my MD, small fins (with the same design as mine), alot of nose weight, short tube, and long NC. He did ensure that the stability margin stayed over 2 over the entire flight (but his was supposed to reach Mach 2) but then again, i dont think he launched his so i dont know the results.
 
Well with 10-20 mph winds you are likely to have some weather cocking. The G150 seems to be a nice high thrust motor so you should be up and stable pretty quickly, mitigating your weather cocking considerably for the first boost phase of the flight. As with most flights it is very likely to turn into the wind once the velocity drops off during your coast phase. However I wouldn't be concerned with the stability at that stage as it will just improve.

That sounds like just continuing to angle over. Once a rocket is substantially free of the ground, it is not moving at an angle to the air and the only thing that can cause weathercocking would be a change of wind velocity (e.g. if the wind speed decreases, it will "weathercock" downwind). In a sense, if wind speed was always constant anyways, weathercocking should be viewed as not movement of the air, but of the surface of the Earth and the launcher relative to the air.

As to whether the rocket will weathercock at launch, that will depend on the stability margin at lower speeds, not the mach speed stability. So it would be desirable for the CPs to be more the same (or reverse the normal movement of CP forward), if there is any way that's possible. It *is* a fast rocket though, so weathercocking is hardly the biggest issue. Another consideration would be if the CP moves forward at high angles of attack, it might not be as overstable as you think.
 
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Well to get over 11,000 ft in the sims, i had to add 54 grams of nose weight and reduce the fin area to a very small size. The rocket's top speed is mach 1.5 (and stability at top speed is 1.6 cal) and so i was wondering if it still could go unstable due to the small size of the fins or end up getting less altitude then is given. I've attached the new ORK file here.

View attachment 314693
 
That sounds like just continuing to angle over. Once a rocket is substantially free of the ground, it is not moving at an angle to the air and the only thing that can cause weathercocking would be a change of wind velocity (e.g. if the wind speed decreases, it will "weathercock" downwind). In a sense, if wind speed was always constant anyways, weathercocking should be viewed as not movement of the air, but of the surface of the Earth and the launcher relative to the air.

As to whether the rocket will weathercock at launch, that will depend on the stability margin at lower speeds, not the mach speed stability. So it would be desirable for the CPs to be more the same (or reverse the normal movement of CP forward), if there is any way that's possible. It *is* a fast rocket though, so weathercocking is hardly the biggest issue. Another consideration would be if the CP moves forward at high angles of attack, it might not be as overstable as you think.

Your first paragraph isn't correct. A rocket that has started to weathercock will continue to arc over, even if the wind is at a steady velocity, until it's flying directly into the wind or an ejection event happens. Given a long enough flight a weathercocking rocket will reach horizontal and then, continue turning downward. A weathercocking rocket would never turn downwind; it will always seek to fly directly into the wind, just as a real weathercock will always turn into the wind.


Steve Shannon
 
^^-- The continuing to arc over is close to if it came straight off a launch rod at that angle, it's not additional weathercocking. That's most true though with a lightweight rocket, a flying motor would have a lot more angular momentum, but that's not weathercocking either at that point. Problem is the wind sets off all kinds off problems and I find it unlikely the OP can beat a record if his launch conditions are substantially worse than those of the existing record.

I've seen a case, and had video, where a slow rocket could not only be seen angling into the wind, but additionally pushed sideways by the wind as soon as it came off the rod. By shortly after launch, regardless of how much weathercocking occurred, the entire rocket is moving mostly sideways with the air as a unit in addition to moving through that air. The only way weathercocking happens after that point is if the wind increases OR decreases (or changes direction, more complex same thing). If the wind decreases, the relative wind direction change to the rocket will be opposite of the original wind direction. It would take a complete wind halt or reversal to cause a very noticeable effect, but easily could be substantial compared to the what remains later in the flight of the original angular momentum, rendering that insignificant.
 
^^-- The continuing to arc over is close to if it came straight off a launch rod at that angle, it's not additional weathercocking. That's most true though with a lightweight rocket, a flying motor would have a lot more angular momentum, but that's not weathercocking either at that point. Problem is the wind sets off all kinds off problems and I find it unlikely the OP can beat a record if his launch conditions are substantially worse than those of the existing record.

I've seen a case, and had video, where a slow rocket could not only be seen angling into the wind, but additionally pushed sideways by the wind as soon as it came off the rod. By shortly after launch, regardless of how much weathercocking occurred, the entire rocket is moving mostly sideways with the air as a unit in addition to moving through that air. The only way weathercocking happens after that point is if the wind increases OR decreases (or changes direction, more complex same thing). If the wind decreases, the relative wind direction change to the rocket will be opposite of the original wind direction. It would take a complete wind halt or reversal to cause a very noticeable effect, but easily could be substantial compared to the what remains later in the flight of the original angular momentum, rendering that insignificant.

I think we're looking at this from two different frames of reference. Mine is the rocket itself. As long as wind flows past the rocket and the Cp is aft of the Cg, there will be a torque on the rocket, proportional to the aerodynamic force and the length of the lever arm, the distance between Cp and Cg. This torque is commonly called a restoring or restorative force and turns the rocket towards the source of the wind. It will only stop turning about its Cg if the air velocity relative to the rocket fins and normal to them drops to zero. Heavy, fast rocket are less affected than slow light rockets. Over stability of course is simply a lengthening of the lever arm. A rocket that is spinning of course is stabilized in a way that is much less apt to be affected by weathercocking.
I agree that a rocket body can translate with respect to the surface of the earth immediately after takeoff especially. I especially see that with very lightweight model rockets. If the wind speed relative to the ground is greater than the airspeed of the rocket it certainly could be pointed one direction and moving the other direction with respect to the earth. Similarly, if the rocket translates at the same speed as the wind there will be no normal force acting through the Cp and the rocket will not weathercock until the wind changes velocity as you say.


Steve Shannon
 
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I think we're looking at this from two different frames of reference. Mine is the rocket itself. As long as wind flows past the rocket and the Cp is aft of the Cg, there will be a torque on the rocket, proportional to the aerodynamic force and the length of the lever arm, the distance between Cp and Cg. This torque is commonly called a restoring or restorative force and turns the rocket towards the source of the wind. It will only stop turning about its Cg if the air velocity relative to the rocket fins and normal to them drops to zero. Heavy, fast rocket are less affected than slow light rockets. Over stability of course is simply a lengthening of the lever arm. A rocket that is spinning of course is stabilized in a way that is much less apt to be affected by weathercocking.
I agree that a rocket body can translate with respect to the surface of the earth immediately after takeoff especially. I especially see that with very lightweight model rockets. If the wind speed relative to the ground is greater than the airspeed of the rocket it certainly could be pointed one direction and moving the other direction with respect to the earth. Similarly, if the rocket translates at the same speed as the wind there will be no normal force acting through the Cp and the rocket will not weathercock until the wind changes velocity as you say.


Steve Shannon

The wind causes the rocket to rotate around its CG as a fulcrum, but only if aero forces are all at one point and the mass at another point will the movement be fully rotational. Moment of inertia causes the fin force to have a chance to begin moving the mass downwind, and the wind force is not entirely on the fins, airframe aero forces become significant with angle of attack. So while a rocket can rotate a lot about the CG, the CG also doesn't go in a straight line, I'm saying it takes on a component moving downwind -- although it also has a horizontal term caused by engine thrust due to angle which will be to windward if it weathercocks. This is leading me to realize that methods that control weathercocking have a disproportionately high effect in reducing it, because weathercocking only has a limited time to occur. If a rocket can somehow resist all effects of the wind to angle it, and is fired exactly vertical in a constant breeze, it will be moving almost exactly downwind and upward within a short time (unless the miracle that causes the non-weathercocking also causes it to ignore the air entirely, in which case there will be bigger problems).

Things that reduce weathercocking include a high moment of inertia (e.g. weight in the ends) and controlled CP shift with angle of attack, along of course with limited static stability. Increasing aerodynamic surfaces beyond needed for stability can increase damping factor.
 
I agree with all that. In the first sentence the dependent clause is satisfied by the theoretical concepts of Cp and Cg.
Your comments about the moment of inertia are exactly right. Small models with low mass and a larger stability margin will be much more quickly affected by weathercocking than more massive longer ones with more conservative stability margins.


Steve Shannon
 
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