Self Amplifying Precession

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MarkStull

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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,
 
Mark,
I have 2 suggestions for you. 1.) Perhaps the slapping in the launch tube is caused by the fact that the rocket is initially leaning to one side. This causes the thrust vector to be off center thus driving the nosecone into the tube wall and either rebounding off the wall or causing the ring fin to bind against the wall. Either of these scenarios would cause the rocket to attempt to right itself by changing its vector to the opposite direction. 2.) A suggestion on how to keep the rocket centered in the tube. Why not try a sabot (pronounced "say-bo"... I know, I'm anal retentive)? A sabot is used by the US Military to allow projectiles (ie tank rounds) to fly faster. Basically, you would attach 2 to 4 fall-away sections to the rocket in front of the fins. When the rocket leaves the launch tube, these sections fall harmlessly away. The sections would make the front of the rocket the same diameter as the ring fin and thus keep the rocket centered. The sections could be made from styrofoam or some other lightweight, cheap material.
 
Actually, to be technically correct, the sabot in this case is the rocket itself. ;)
 
Based on the rocket specs you posted, Rsim says about 80 ft/s when it leaves a 72" guide. This was with the US version of the CTI G engine, and of course doesn't take into account the effects of the launch tube.

FYI, the Arcas was a tube launched sounding rocket. It used plastic spacers to keep the rocket centered. These fell away as the rocket left the tube. It also used a closed breech with a piston to trap the exhaust gasses.
 
Straight from Webster:

sabot: a thrust-transmitting carrier that positions a missile in a gun barrel or launching tube and that prevents the escape of gas ahead of the missile.

I'm not sure if I understood the last post about the rocket being the sabot. Wouldn't the styrofoam be the sabot? The rocket would be the missile in the above definition.

The M1A2 uses this type of kenetic round. The actual round that comes out of the main gun is actually a dart about 50mm round I believe. The sabot wraps around this dart so that it can travel down the 120mm barrel and also provide the above benefits. Like Mark said this sabot falls away the minute it leaves the barrel and the projectile is left flying straight into the enemy tank.
 
Originally posted by Milo
I'm not sure if I understood the last post about the rocket being the sabot. Wouldn't the styrofoam be the sabot? The rocket would be the missile in the above definition.
I could be wrong but, my understanding was always that the dart was referred to as the sabot.
 
I guess the piston I mentioned in my post is really a sabot. Previously, I would have said the spacers were sabots, thanks for the clarification Milo!
 
The act of "Sabotage" was actually started by disgruntled factory workers in Europe (Holland and Belgium) who threw their wooden shoes into moving machinery in order to gum up the works. And the word Sabot means shoe.

I kid you not.
 
Originally posted by MarkABrown
I could be wrong but, my understanding was always that the dart was referred to as the sabot.

I've always understood it is the sabot that falls away leaving the projectile to fly toward the target. Actually these rounds are known as APFSDS rounds (Armor-Piercing, Fin-Stabilized, Discarding Sabot)

Found this site: https://www.primextech.com/

Need some tank munitions?

More links with diagrams:
https://www.fas.org/man/dod-101/sys/land/m829a1.htm
https://www.fas.org/man/dod-101/sys/land/m865.htm
 
Dear Mr. Brown,

Thank you for your suggestions. I always test the fit of my rockets in the tube, making sure they can't bind when they lean.

I have considered fall away sections, but can't figure a way that I really like. Our final rockets will be about 4' tall. If the fall away sections rest on the tops of the fins, that puts them down fairly low on the rocket. When the rocket tries to lean, it will tend to bind. On the rocking buoy, side forces will often be present. These sections would have to be really tall so they don't bind.

They could rest down against something higher on the rocket. There is a taper around the middle of the rocket that they could rest against, but with 1,000 newtons of thrust, the taper would try to pry the sections apart, making them bind. The tapered tops of the fins would tend to do that too.

Another concern about fall away sections is bio-degradability. We can't have chunks of styrofoam floating ashore somewhere. Everything has to sink in the ocean, and most everything should corrode away to nothing. If we make the sections out of a water soluable material, they would tend to lose their structural integrity from condensation.

One idea that I can't rule out, if it turns out that we can't let the rockets lean: Use a long needle nose that goes all the way up to the launch tube cover... where it is held centered until the nose cone breaks through. Carbon fiber rod would work well for this needle nose. Unfortunately, this may add a fair amount of expense and complexity.

Don't you just love these kinds of problems that make you really have to think { %-:>).
 
Originally posted by Milo
Actually these rounds are known as APFSDS rounds (Armor-Piercing, Fin-Stabilized, Discarding Sabot)
I stand corrected. ;) Thanks, for clearing that up for me.
 
We decided to keep the rockets centered in the launch tubes with three small fins near the middle of the rockets. These launch support fins can be made to either stay on the rocket, or fall away as soon as they exit the launch tube. So I am adding these fins to all my test rockets now.
 
I've started testing rockets with launch support fins. They try to fly, like little canard wings. This makes the rockets have a slight corkscrew as they roll, if there is any fin misallignment. I'd use three small wires instead of the fins whenever they need to be more than an inch long. It is nice, not having to worry about the rockets coming out of the launch tube half cocked. Every launch begins straight and true now.
 
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