stability of rocket

[rocket trike]

I design a rocket in Rock sim. The Barrowman and rocksim methods say that the rocket is stable. The cardboard cutout methods said it is not stable. So I did the swing test where you find the CG and hook a string to it and swing it around your head. When I did this there was no wabble but the rocket flies backwards. So is this asying the the rocket is unstable and I should add nose weight to the nose cone?
If anyone could give me help i would appericate it.

Rocket trike

 

[astronboy]

I have had this happen sometimes. Is the rocket very large or very small? Is it a standard design? IE: Just nose, bt, and fins? Sometimes 'creative' designs do not swing test well.

I would suggest swing testing the rocket again, but try geting it started in the right direction. It sometimes is better if you have a helper to hand launch the rocket while you start swinging the string.

 

[airforce]

Trike,
Experiment, add a little weight and test the result. I've often found the old nose weight addition always seems to help.

 

[Micromeister]

Trike:
Astronboy, is correct, many complex designs or odd configurations just will not swing test. Many of the Estes models we fly everyday will not swing test. However, Models flying backwards during test are unstable. Airforce is also on the right track, I have had great success with unruly or sensitive designs by adding a little nose weight and retesting. Always better safe than sorry.
would you post a pic of the design, we many be able to determine why she won't swing.
Hope this helps a little

 

[powderburner]

By the time you make the string long enough to get a 'good' swing (large diam, smooth motion, decent velocity) then it is SO long that you have a hard time starting or stopping the test without dragging the model rocket on the ground.
Swing tests can be very quirky. If it swing tests OK then it probably IS stable, but if the swing test fails you are left with a rather ambiguous result (it ain't always unstable just because the swing test didn't work).
The recommendation above (to add a little nose weight) is an excellent one. Use hobby clay. After a few successful flights you can start removing small dabs of clay ballast for further testing.

 

[rocket trike]

Ok here is a picture of the rocket. Also I have more info on what is going on.

The rocket is made out of a BT55 body tube. The rocket is 18.243in long and weighs 1.4oz with out motor and 2.3oz with motor.

Here is what rock sim is saying about the rocket
Barrowman method CG 12.918 CP 15.770 static margin 2.18
Rock sim method CG 12.918 CP 15.324 static Margin 1.84
cardboard cutout method CG 12.918 CP 11.177 static marg -1.33

Iwas am cofussed about why the CP is so different from the barrowman and rock sim method from the cardboard cutout method?

Rocket Trike

 

[Matt]

kool fin design man

 

[jflis]

I am not familiar with RockSIM, but others may be able to answer this question...

is it possible that RockSIM is placing "imaginary" fin area in that space between the fin tips? If so, that would have a great effect on the CP that would not show up with the barrowman equations nor the swing method.

just a thought
jim

 

[n3tjm]

The Paper cutout method is not 100% reliable either. It works fine for the basic fins and a nosecone method, but anything more complex, you have pure guess work. Just to let you know, swing tests never have worked for me. I trust Rocksim's calculations, and so far all my models have flown fine.

 

[astronboy]

Hey... where is the pic? It looks like Matt and Jim could see it, but I cannot download it. Does anyone else have this problem?



I really want to see this design!!

Henway, I use Rocsim a lot. Usually, I find that the Rocsim CP method often shows a rocket more stable than barrowman. I do not even bother to use the cardboard cutout methodas I consider it too much of a "guesstimation".

 

[Micromeister]

Trike:
Like Jlfis, my first question is how did the program calculate the fin area, Cool design, but those thin trangular base fin tip areas may be the problem.
For informational purposes you chould know, all model rocket "John Q Pulblic" affordable computer programs. are at very best, a most helpful but only marginally accirate tools. Jim Barrowman a former Nasa Engineer and current consultant first condensed these formulas back in the early 70's. His original formulas for Model rocket (subsonic) stability were a brillant piece of work. But even here, Jim had to make simplifications and assumptions to allow us non-engineer's to use them with only basic math skills. I use these long hand calculations on very complex models to this day. While the computer programs have evolved and expanded exponentially over the past few years, All still must make many assumptions to limit the variables that allow them to be placed in a program that will "do the job" but still be in a package useable on todays computers without costing a fortune. I do not know what assumptions were used in roc-sim, I do know roc-sim simulations come out 10 to 15% higher than visually tracked and closed flights. Weather the sims program effects the cp/cg program is not clear. I do know the assumptions in the Barrowman "original" calculations. I can only guess that Tim & company used the same figures in Roc-sims version. That said, I personally would always go with the Barrowman method. I'm not knocking the program, just be aware there are short cuts that had to be taken to get it into an under 100 buck format so we can all afford to use it.
Doug, the cardboard cutout method, when actually done on cardboard, gives a very good laterial center of pressure location. If anything it always errs on the WAY overstable side. Cardboard cutout is sometimes the only way to get an idea of C.P. on really odd shapes and complex configurations (good examples my flying Pig and Easter basket). I have no idea how roc-sim "Computes" this method, in that respect you are correct I would never rely on this method in a computer simulation. but It is a very useful tool in its real world form.
Bottom line Trike, your model looks like it will fly, I'd really reinforce those fin tip spikes if you plan on flying it more than once.
Hope this helps

 

[Mike]

Originally posted by astronboy
Hey... where is the pic? It looks like Matt and Jim could see it, but I cannot download it. Does anyone else have this problem?

Astron, the pic's surronded by a lot of white area, use the scroll bars to scroll around untill you find the pic.

HTH

 

[powderburner]

Originally posted by Micromister
Doug, the cardboard cutout method, when actually done on cardboard, gives a very good laterial center of pressure location. If anything it always errs on the WAY overstable side.

Micromister is correct in saying that the cutout method very often gives excessively conservative answers. When you stop and think about it, if your rocket is turned around and presenting its SIDE to the oncoming airflow, you have more problems than stability. A good bunker would be advisable.

Estimating or calculating actual stability characteristics is quite complex, as you can probably imagine. Think of a rocket traveling in an idealized attitude (straight into the airflow) and picture the head-on view of the rocket. You see the leading edge of the fins and the nose cone (simplifying assumption being that there are no body transitions to larger diameters). Yeah, if you look really close, there is maybe a launch lug or some buttons, but they are small and I'm ignoring them.

Now picture a similar view, only the rocket has a small perturbation from the flight path. Say, a small angle like 4 or 6 degrees. The fins now become wings, at an angle of attack to the relative airflow that equals that small angle (the 4 or 6 degrees). The fins generate a lifting force, perpendicular to the body axis, that tends to push the tail of the vehicle back in line with the flight path. The stability of the model is directly affected by the ability of the fins to efficiently generate aerodynamic lift. Like, did you sand the edges square, or round, or did you put some semblance of a real airfoil on your fins?

At the same time, the nose cone and the rocket fore-body are also at that same angle of attack. Unfortunately, the sides of these surfaces that are AWAY from the flight path are the place where airflow must accelerate to get around, causing reduced dynamic pressure and tending to pull the nose farther away from the flight path.

Who wins? The fins or the nose? This is where all the complicated aerodynamics comes in and makes everything messy. Also, vehicle weights and especially longitudinal mass distribution affect the dynamic motion response, and the energy expended (making the rocket wobble back and forth) makes the stability analysis worse. Now add in varying atmospheric conditions, barometric pressures, humidities, launch altitude, and air densities, and you begin to get the idea that you could spend the rest of your natural life trying to mathematically analyze this problem. (Oops, I left out thrust misalignments, launcher tip-off dynamics, misalignment of fins and nose cones, effects of off-center parachute packing, .....)

The practical engineering approach is to load a wad of weight in the nose. Yes, performance is a little worse. So make a few flights and then remove part of the nose ballast and test again. This is simple, and simple is good. Besides, it's an excuse to launch the rocket again (if you need one).

Your design looks really great! Take your time deciding on the paint job and choose something that really plays up your unique design. Maybe paint it to look like the lower portions of the fins are for a booster that just happens to be closely nested to the upper stage?

 

[astronboy]

OK, Now I found it.... DUH!!

Nice design, but the small fin area may be the issue. I would add nose weight and string test again. Also add nose weight using ROCSIM and see how it affects your sim flights.

Let us know how you make out!!