Rocket Stability

welcome back to lab rat scientific today's episode I want to talk about the stability of thin stabilized rockets but before I begin let's take a look at a fin-stabilized rocket used by NASA you that was a NASA signing rocket it uses fins to make sure the rocket flies in the proper direction it has no guidance system like the more expensive rockets like the Falcon Heavy now this is also a thing stabilized rocket you might think of it as a model rocket now you probably can't afford a million dollar sounding rocket but I suspect you can

afford a few model rockets to do some experiments with so let's talk about what I mean by a stable rocket now this is the air flow represented by these arrows and it indicates that the airflow is moving across the board now a stable rocket lorries want to keep its nose pointed in the airflow it might oscillate up and down a little bit but it always flies in the proper direction now an unstable rocket will have a tendency to want to fly sideways or you can fly backwards which is disastrous for rocket flights now let's talk

about some of the basic compounds of a rocket first of all we have the fins we didn't have a cylindrical body tube and a nose cone before you get in the details you want a strip this rocket down to its basic parts now let's start off with just a cylinder so let's do an experiment to see how the cylinder moves through the air by itself here's my test article to show neutral stability of a cylindrical body tube we've got a PVC pipe with a string hanging the center of gravity so if I swing this we

can see what the aerodynamic characteristics of a cylindrical body are but you should have noticed from that test was the body tube was flipping around and cartwheeling what I was trying to do in general however was trying to align itself perpendicular to the airflow here the airflow moving this way and one to line sideways the airflow not good for rocket flight so we're we due to a cylinder you try to make it more stable well the first step is to add some fins we place those fins as far as possible on the back of the

rocket so what happens is the air flow creating force on the back into the rocket pitching it around so no whose tends to point into the airflow now as I mentioned earlier the Rockets picking up and down during flight what's going on there this is the rocket pitches up the fins get an hang of attack here's the air flow here the fin it creates an angle of attack which produces my lifting force which brings the rocket back towards the airflow now the rocket will tend to overshoot when that happens be getting negative angle of attack

and that creates your lifting force downward again correcting the rocket so it's oscillating back and forth trying to get its nose point in the airflow so you have thin lift now that's not a very efficient configuration for this rocket we don't want a blunt end on our rocket so the next thing we do is place a nose cone to make the rocket more air an image problem with that however is that that nose also produces lift it's a curved surface in the airflow and it creates a lifting force and here's the rocket attempt to pick

up because of that force it actually becomes less stable and wants to go all the way around so the trick is to make sure that my fins lifting force is greater than the nose lifting force and thus will have a stable flight now there are two things that come into play for stability it's the center of pressure which is based on the aerodynamic forces and center of gravity now I can think of the center of pressure there's a balancing point between the nose and tail lift that tells us we can centralize that force in one

position on the right somewhere along the body to and the other fact we have to consider is the center of gravity the rocket the center of gravity is simply the balancing point and I can show that on this model here hanging from a string it balances outs that is the center of gravity now will lift a center of gravity represented by the pivot point here on my model on the board that's actually quite accurate because the actual rocket will pivot around the center of gravity now as I mentioned earlier the fin lifts in a nose

lift will create a center of pressure location and that's located somewhere on the body and I'm gonna assume it's gonna be right there for now now for stability we need to make sure that the center of pressure is behind the center of gravity and so we can identify that as stability is equal to the center of pressure measured from the nose - the center of gravity from the nose and that difference is how much stability my rocket has and we identified that as the static margin now ideally we want at least one body diameter diameter

between the center of gravity and center pressure and that will create a stable rocket configuration so let's try to do some demonstrations to see if we can find out where the center of pressure is on a model rocket okay so let's take a look at the shadow method for approximating the location of the center of pressure on a model rocket I've got my test article here and I place it onto a sheet of corrugated cardboard and then traced around the outside of the model essentially identifying the shadow of my test article and then cut out

the cardboard to create my shadow model now to determine the location of the center pressure I want to balance the model and that balancing point is identified as the location of the center of lateral area of this model which is also approximating the center of pressure location so now what I can do is take my test article and run some tests to see how accurate this position is here's my test article I'll be using for my swing tests it's made out of model rocket parts including a plastic nose a paper tube and balsa wood fins

I've also built a movable collar that I can adjust the a pivot point during my swing tests whoever is necessary to conduct the experiment I'm doing I've also got a weight system I can place in the back end to help adjust the center of gravity of the model I can also place weights up front to move the CG forward if necessary so now what I want to do is transfer the center of pressure location from the shadow model onto Maxwell test article so I'll do that very simply by placing my shadow model next to the

model rocket and transfer the center pressure location on the model next I want to do is put weights in the front in the back of the rocket to be able to get my collar to be right on top of the center of pressure location now if this is actually the center of pressure location the rocket will fly sideways or act lines stable when I do a spin test if the center of pressure is actually behind that this test article should fly stable as I do my spin test in this next step I need to move

the center of gravity of my model to be directly on top of the shadow model center of pressure to do that I'm using my trusted pennies and I found out through trial and error there's stack of five pennies mounted on the back end of the rocket will allow me to shift the center of gravity to the desired position I feel like in the back you can see where I've taped them in there nothing sophisticated and now you'll see that I have a balanced model to where the center of gravity is co-located with the center of

pressure so now what I can do is go outside and do my swing test to see if this configuration is stable so what did that swing test tell me since the rocket flew stable that tells me that Senator pressure must be somewhere behind the center of gravity location where my pivot point is with my string so the shadow model is giving me a center pressure here other methods will tell me it's somewhere else but what this does tell me though is that the shadow method is a very conservative estimate as long as I have my

center of gravity up here forward of my center of pressure the rocket will be stable if the shadow method says my center pressure is at this location but reality it's further back here I can't lose if I put my center of gravity in front of that balancing point or the shadow method CP location so again the shadow method produces a very conservative location for center of pressure approximation the location of the center of pressure can be calculated theoretically but you need to know aerodynamic properties of the nose cone in the fins so it can be

kind of cumbersome to do however luckily these days there's software that can do that for you the software I like to use is called open rocket it can be downloaded free from the internet let's go ahead and do some experiments to see how well the open rocket software calculates the center of pressure location this is the center of pressure location calculated by the open rocket software and this is the center of pressure location determined by the shadow method you'll see there's a pretty big discrepancy between these two points up to about two inches so what

do I do now is to transfer the center of pressure location from open rocket to my model rocket and then bounce the rocket so like move my collar to that Center pressure point and do another spin test to see what happens this is the rebalanced configuration to test out the center of pressure location generated by the open rocket software let's go ahead and go outside and give it a spin test okay Annette's been test we saw that the rocket was flying backwards that indicates either that the rocket was neutrally stable meaning to send him gravity

was right on top with center pressure were actually unstable which meant the center of gravity was behind the center pressure now I want to do is to move the center of gravity forward a little bit does he fight and get the rocket to be stable again and that will tell me roughly where my actual center of pressure is here's the final balanced configuration I've moved the center of gravity about 1/2 inch forward let's go outside and see what the spin test tells us okay and that's been test I could get this rocket configuration to be

stable flying nose-first the previous test always wanted to fly fins first indicating it was unstable what that tells me is my center pressure is actually between these two points pretty close to where the open rocket simulation says it should be in conclusion it looks like a shadow method didn't do a very good job at predicting the location of centre pressure however open rocket seems have done a real good job which gives me confidence to use that software on future rocket designs well I hope you learned a little bit about rocket stability and hope to see

you next time on lab rat scientific