Recent testing of a launch tube rocket launch has revealed a strong possibility that the rocket may sometimes slap back and forth inside the tube. This is my latest memo to the rocket sonde project team, as described in "Water Waiver".
I am more than a little concerned about angular velocity as the rocket exits the tube. I have no clear picture as to why the rocket slapped the inside of the launch tube so hard. The circular fin of Tandem 6C, fit the inside of the launch tube with just 2 or 3 mm clearance on each side. So the only part of the rocket that could move horizontally very much was the top. It was a shorter skinnier rocket than most, so the rocket could lean maybe 5 degrees any direction in the tube.
I do not know what force would be present that would make the top of the rocket bounce back and forth, or circle in there. The rocket is leaning against one side at ignition. The inside of the launch tube is still smooth and kind of slick, because it is fairly new. I can only guess that the rocket must have slapped back and forth in some kind of self amplifying manner, to achieve the side force necessary to pop the engine out of place.
Catherine and I had a circular fin rocket crack a radial support fin lose at Lethbridge. The rocket still went vertical, but in a corkscrew. I repaired it overnight, and it flew perfect the next day. It was obvious that the fin cracked lose inside the launch tube, because the corkscrewing began instantly at the top of the launch tube. At the time, I had no explanation as to why the fin cracked lose. Now, I'm thinking that this same intense slapping occured.
The bad wobble of the first flight at Harrison Lake probably happend because of this phenomenon.
There are four things about this slapping that I don't like: It could damage the sonde or other rocket parts; It could cause the rocket to aim poorly; It could slow the rocket as it tries to accellerate in the launch tube; It could cause the rocket exceed the stall angle of the circular fin in the first second of flight, causing instability.
This last possibility could explain some of the inconsistancy I've noticed, trying to determine the center of pressure. Since the circular fin has no wing tip, it really tries to fly. If the stall angle is exceeded on a circular fin, much less stability is provided to the rocket. My test of the rocket with the extremely tall circular fin proved to me that broadside stability will not necessarily prevent a rocket from tumbling. I have the impression that stability varries with the angle of attack of circular fins in a very non-linear manner, once a portion of the circular fin begins to stall. This would be fairly easy to determine in a wind tunnel, but impossible for me to test here.
I tend to build my rockets plenty strong. If I had anticipated hard slapping in the launch tube, I would have put a ring around the bottom of the engine, like I did with Tandem 4C (which hasn't flown yet). It is no problem adding one. And it could even help stability a bit.
I have spent a lot of time thinking of different ways to keep the top of the rocket centered. The rocket that we used to send the sonde up at Lethbridge was the only one I made that incorporated a centering system. I used three wires sticking radially out of the bottom of the nose cone. But we felt that that method would add a lot of drag... especially when supersonic. I have yet to think of any other way that I really like.
I'm glad this happened here. Obviously, more testing is in order. This testing in Texas could really help us anticipate some potential problems.
There is one problem I've forgotten how to solve, that I'm sure you know the answer: With the rocket accellerating at the maximum the sonde can reliably withstand... 30 Gs I think. How fast will the rocket be going when the fins exit the launch tube, with a 2 meter long launch tube? It would seem that this simple calculation will give us a good idea how much cross wind is acceptable. The rocket will need to be going several times the wind speed to prevent weathercocking.
Cheers,
I am more than a little concerned about angular velocity as the rocket exits the tube. I have no clear picture as to why the rocket slapped the inside of the launch tube so hard. The circular fin of Tandem 6C, fit the inside of the launch tube with just 2 or 3 mm clearance on each side. So the only part of the rocket that could move horizontally very much was the top. It was a shorter skinnier rocket than most, so the rocket could lean maybe 5 degrees any direction in the tube.
I do not know what force would be present that would make the top of the rocket bounce back and forth, or circle in there. The rocket is leaning against one side at ignition. The inside of the launch tube is still smooth and kind of slick, because it is fairly new. I can only guess that the rocket must have slapped back and forth in some kind of self amplifying manner, to achieve the side force necessary to pop the engine out of place.
Catherine and I had a circular fin rocket crack a radial support fin lose at Lethbridge. The rocket still went vertical, but in a corkscrew. I repaired it overnight, and it flew perfect the next day. It was obvious that the fin cracked lose inside the launch tube, because the corkscrewing began instantly at the top of the launch tube. At the time, I had no explanation as to why the fin cracked lose. Now, I'm thinking that this same intense slapping occured.
The bad wobble of the first flight at Harrison Lake probably happend because of this phenomenon.
There are four things about this slapping that I don't like: It could damage the sonde or other rocket parts; It could cause the rocket to aim poorly; It could slow the rocket as it tries to accellerate in the launch tube; It could cause the rocket exceed the stall angle of the circular fin in the first second of flight, causing instability.
This last possibility could explain some of the inconsistancy I've noticed, trying to determine the center of pressure. Since the circular fin has no wing tip, it really tries to fly. If the stall angle is exceeded on a circular fin, much less stability is provided to the rocket. My test of the rocket with the extremely tall circular fin proved to me that broadside stability will not necessarily prevent a rocket from tumbling. I have the impression that stability varries with the angle of attack of circular fins in a very non-linear manner, once a portion of the circular fin begins to stall. This would be fairly easy to determine in a wind tunnel, but impossible for me to test here.
I tend to build my rockets plenty strong. If I had anticipated hard slapping in the launch tube, I would have put a ring around the bottom of the engine, like I did with Tandem 4C (which hasn't flown yet). It is no problem adding one. And it could even help stability a bit.
I have spent a lot of time thinking of different ways to keep the top of the rocket centered. The rocket that we used to send the sonde up at Lethbridge was the only one I made that incorporated a centering system. I used three wires sticking radially out of the bottom of the nose cone. But we felt that that method would add a lot of drag... especially when supersonic. I have yet to think of any other way that I really like.
I'm glad this happened here. Obviously, more testing is in order. This testing in Texas could really help us anticipate some potential problems.
There is one problem I've forgotten how to solve, that I'm sure you know the answer: With the rocket accellerating at the maximum the sonde can reliably withstand... 30 Gs I think. How fast will the rocket be going when the fins exit the launch tube, with a 2 meter long launch tube? It would seem that this simple calculation will give us a good idea how much cross wind is acceptable. The rocket will need to be going several times the wind speed to prevent weathercocking.
Cheers,