HPR Stability Issues

The Rocketry Forum

Help Support The Rocketry Forum:

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

Neilw

Simulates with KSP
Joined
Aug 19, 2014
Messages
81
Reaction score
0
Hello,

I was wondering about what kind of angle-of-attack was permissible for a rocket leaving the launch rod (this is obviously a high wind scenario). Right now, my simulations show a rocket moving at around 100 fps at the moment it leaves the rod.

As well, I was wondering if a rocket with 3.5 cal stability is safe. So far, the simulations I have say it is, but I just want to be careful.

Thanks!
 
When a rocket leaves the rod (barring unusual circumstances), it should have an angle of attack of 0. At 100 fps at the end of the rail, you shouldn't have a problem. Take my answer with this cave of giant salt crystals.
images
 
ummmmmmm

All I got was this terribly long link.

The problem I'm getting, is that it's leaving the rod in a 14 mph wind. This means that it has an AOA of ~10 deg right off the bat
 
ummmmmmm

All I got was this terribly long link.

The problem I'm getting, is that it's leaving the rod in a 14 mph wind. This means that it has an AOA of ~10 deg right off the bat

There is not enough information. A lot depends on the angular inertia of your rocket. A long heavy rocket that doesn't change direction easily will have more latitude than a short light nimble rocket. For example I launched a SLOW rocket this weekend in 15mph wind at a thrust to weight ratio of about 3 to 1. It flew straight as an arrow off the pad because is was a long on the heavy side rocket. It probably left the rail at 30 ft/s.
 
This article appears related to the question....

https://www.apogeerockets.com/Wind_Caused_Instability

Thanks. That article was pretty helpful.

How would I find the maximum permissible angle of attack using OpenRocket? I already tried using the component analysis tool and I got some strange results. It said the rocket would be unstable at a velocity of 100fps and and AoA of 7 deg. However, in the actual simulation, the AOA off the rod was 11 deg (speed ~100 fps, wind 14 Mph) and there was no instability.
 
The angle of the rod is not the same as angle of attack. A rocket comes off the rod at zero angle of attack (plus some amount due to ambient wind). To get an angle of attack requires an angle of yaw through the air, so to get a 7 degree angle of attack coming off the rod/rail you'd need to mount the launch lug or rail buttons 7 degrees off of the longitudinal axis of the rocket.

Sent from my iPhone using Rocketry Forum
 
Last edited:
How would I find the maximum permissible angle of attack using OpenRocket?
If you enter a rod length and wind speed and the simulation is stable, then you should be OK.
 
The angle of the rod is not the same as angle of attack. A rocket comes off the rod at zero angle of attack (plus some amount due to ambient wind). To get an angle of attack requires an angle of yaw through the air, so to get a 7 degree angle of attack coming off the rod/rail you'd need to mount the launch lug or rail buttons 7 degrees off of the longitudinal axis of the rocket.

Sent from my iPhone using Rocketry Forum

I apologize if my description was unclear. I meant that the total angle of attack (due to rocket velocity AND wind) at the point where the rocket leaves the rod is 11 deg. the angle of the rod itself is 0.
 
Hello,

I was wondering about what kind of angle-of-attack was permissible for a rocket leaving the launch rod (this is obviously a high wind scenario). Right now, my simulations show a rocket moving at around 100 fps at the moment it leaves the rod.

As well, I was wondering if a rocket with 3.5 cal stability is safe. So far, the simulations I have say it is, but I just want to be careful.

Thanks!

If you are asking about the launch rod or rail angle from the vertical position, it is 20 degrees for Tripoli. However I am not sure if you are talking about "weather cocking", a phenomenon caused by aerodynamic forces on the rocket body and fins. All things equal, the rocket is what it is, the rail is what it is and the wind is what it's going to be, within reason you can mitigate this effect by selecting the appropriate motor. What is the rocket and what motor where you considering?
 
If you are asking about the launch rod or rail angle from the vertical position, it is 20 degrees for Tripoli. However I am not sure if you are talking about "weather cocking", a phenomenon caused by aerodynamic forces on the rocket body and fins. All things equal, the rocket is what it is, the rail is what it is and the wind is what it's going to be, within reason you can mitigate this effect by selecting the appropriate motor. What is the rocket and what motor where you considering?

I'll just post the OpenRocket file for your benefit.

View attachment arcturus.ork

(Note: the launch rod angle is 0. :p)
 
Last edited:
With a stability of 3.5 it is easily affected by wind. No matter how fast your rocket is going at departure of the rod. Chances are it might still weather cock into the wind depending how strong the wind is. For sure a nice calm day should get you a straighter launch.


Alexander Solis

TRA - Level 1
Mariah 54 - CTI RedLightning- I-100 - 6,345 Feet
 
agreed. 3.5 is a LOT of stability and will likely weathercock. For my level 1 I flew a vulconate on a H180. To reduce the stability I actually wrapped the aeropack adapter with 8 ounces of sheet lead - got a nice straight flight in a sorta windy day. The adapter with lead still is in my range box for windy days...
 
Becoming unstable and weathercocking are opposites -- interesting that the OP is worried about the former yet others look at the rocket and see the latter.

The purpose of Barrowman in creating improved stability analysis was to avoid or reduce the tendency to overstability and weathercocking. When a rocket leaves the guide in a crosswind, resulting in an angle of attack to the air that results in a momentary instability, this is not the same as actual instability. At worse, it should anti-weathercock (I've had it happen at least 3 times, twice using those F15s). A rocket will rapidly begin moving with the wind, so if steady the problem should correct itself rapidly unless it results in some kind of oscillation or runaway behavior. Oscillation that damps itself out can often be seen in Open Rocket plots.

For that file, I'm getting 16 degrees angle of attack for the CP to move forward to the CG. I don't know how the OP got 8 deg. unless that's with some stability margin left. It would be permissible for 16 deg. or even higher to occur for an instant. It is desirable that stability reduces in these conditions, reducing weathercocking. I've had rockets launch down in the 30 fps range in 15 MPH winds with no detectable weathercocking of any kind.

The 100 fps rod velocity is certainly in excess of rule of thumb minimums such as 30 fps, 30 MPH, 50 fps. Of course for a rocket going 10,000 ft. that's a good thing.
 
Becoming unstable and weathercocking are opposites -- interesting that the OP is worried about the former yet others look at the rocket and see the latter.

The purpose of Barrowman in creating improved stability analysis was to avoid or reduce the tendency to overstability and weathercocking. When a rocket leaves the guide in a crosswind, resulting in an angle of attack to the air that results in a momentary instability, this is not the same as actual instability. At worse, it should anti-weathercock (I've had it happen at least 3 times, twice using those F15s). A rocket will rapidly begin moving with the wind, so if steady the problem should correct itself rapidly unless it results in some kind of oscillation or runaway behavior. Oscillation that damps itself out can often be seen in Open Rocket plots.

For that file, I'm getting 16 degrees angle of attack for the CP to move forward to the CG. I don't know how the OP got 8 deg. unless that's with some stability margin left. It would be permissible for 16 deg. or even higher to occur for an instant. It is desirable that stability reduces in these conditions, reducing weathercocking. I've had rockets launch down in the 30 fps range in 15 MPH winds with no detectable weathercocking of any kind.

The 100 fps rod velocity is certainly in excess of rule of thumb minimums such as 30 fps, 30 MPH, 50 fps. Of course for a rocket going 10,000 ft. that's a good thing.

My worry wasn't so much the rest of the flight as it was the moment the rocket left the rod. Right now I may be using a 14 MPH wind, but I know that the winds at the launch site (Green river, Utah) can sometimes go past 30 MPH. Obviously we are going to try for minimum wind conditions, but the launch window is still narrow (this is a competitive setting). As well, I was wondering if there was some kind of "rule of thumb" for situations like this.

As for safety at high caliber stability for the rest of the flight, I was more worried about velocity at deployment and I should have clarified that.

As for the 7 deg angle of attack, I went back and found that I had gotten that from an alternate model that I was trying with forward swept fins. Sorry about that. My caffeine levels were probably too low :p
 
Last edited:
My worry wasn't so much the rest of the flight as it was the moment the rocket left the rod. Right now I may be using a 14 MPH wind, but I know that the winds at the launch site (Green river, Utah) can sometimes go past 30 MPH.
Obviously you can't go past 20 MPH and stay within the safety code rules.

That said, Openrocket is doing a 6DOF simulation of what really happens when the rocket leaves the rail (though I don't think it accounts for effects when one button is off the rail and the other isn't, and it doesn't account for fin misalignment, thrust misalignment, off-axis CG, etc.) If the simulation is stable at a given wind speed then that's as good as you are going to get as far as proving the real flight will be stable. You can look at the reported AOA and rocket orientation to see how wiggly the flight will be.
 
Back
Top