Repairing and Enhancing the Quest Magnum Sport Loader - Part 2: Verifying Stability

Click here for Part 1.

In Part 1, I cut the damaged portion of the Quest Magnum Sport Loader off, shortening the airframe by about 1 3/4 inch. The rocket now looks good, despite being shorter, and because I drilled some static ports - vent holes for a barometric altimeter - into the fat payload section, I can now launch the rocket with an altimeter on board, which will tell me how high it flies.

Now, I have to make sure the rocket is safe to fly. Just looking at the shortened rocket, it looks fine. If I didn't know it had been altered from its original design and build, I would assume it was perfectly safe to fly.

The repaired, shortened, Sport Loader

But I don't want to guess or assume - I want to make sure. That requires a few easy steps.

If you've read my posts on stability, or if you've read the Handbook of Model Rocketry by G. Harry Stine and Bill Stine, then you know that rocket stability depends on the relationship of the Center of Gravity (CG) and the Center of Pressure (CP). In short, the Center of Gravity must be forward - closer to the nose of the rocket - of the Center of Pressure. How do we know where the CG and CP are? How far forward does the CG have to be from the CP? And if the CG is too close to the CP - or, even worse, behind the CP, how do we fix that?

The picture above is a simulation of the rocket, as seen in OpenRocket, free, downloadable rocket simulation/design software. In the picture, the blue and white circle represents the CG, and the red circle represents the CP. As you can see, the CG is ahead of the CP, which means that the rocket is stable.

We'll come back to rocket simulation software in a bit. For now, let's concentrate on the actual, physical rocket in question: The shortened Sport Loader.

Finding the Center of Gravity is simple, so let's start with that.

Another term sometimes used to describe the Center of Gravity is its balance point. This is because when you've found the CG, the rocket will balance perfectly. (Center of Gravity is also sometimes referred to as Center of Mass, because that's really what it is.)

To find the CG of a rocket, simply balance the rocket. You can balance it on your finger, or the back of a chair, or anything you have handy. My preferred method is to use a loop of thick string.

I used to use the back of a chair, but it was hard to balance the rocket perfectly. Sometimes it would roll, and I had to make the tiniest little adjustments so it wouldn't fall off. Sometimes it tries to roll off my finger as well. The string holds the rocket in place, so you can really see where it balances.

Above, you can see that I've found the CG of the rocket - but this is the CG of the rocket with no motors installed. I'm going to mark that spot, and plug it into my simulation file later.

The forward edge of the blue tape marks the CG of the Sport Loader with no motors installed.
I'll explain why the rocket's nose is against the wall in a bit.

But for determining stability, we need to find the CG of the rocket from the moment it lifts off. That means we need to find out where the CG is when the motors are installed. Since this is a two-motor cluster rocket, the two motors should shift the CG significantly backwards.

To do this, you load the motor(s) into the rocket, as well as the parachute and recovery wadding - anything that goes into the flying rocket goes in when checking stability.

Once again, I balanced the rocket to find the new CG.

You can see the CG has shifted aftward dramatically. I'll mark that spot with blue tape as well, and compare them.

In this photo, the nose is beyond the left hand side of the picture. You can see the
difference in CG when the rocket is loaded with motors, compared with when it is empty.

* * *

Now that we've found the CG for the rocket, with everything loaded in it for flight, I'm going to interrupt this post for a moment, and I figure this is the right place to do it. If I don't, then after reading this whole post, someone is sure to write in the comments, "Just do a swing test."

What is a swing test? It's a simple (and fun) test to check whether a model rocket will be stable in flight.

To do a swing test - sometimes known as a string test, you'll need the loaded rocket and a long piece of string. The longer the string, the better, so long as you can actually get the rocket swinging. At least six feet long is recommended.

Simply find the CG, as above, with the loop of string - the string I'm using here is a strong piece of Kevlar, but any sturdy string, such as kite string, will do. What you then do is tape the string in place to the underside of the rocket, so that the loop will not slip, and the rocket will stay in place.

Go outside, hold the string in one hand above your head, and hold the rocket out at arm's length, with the nose pointed forward. Make sure nobody is standing too close to you. Begin turning your body in the direction of the nose and give the rocket a little push with your hand to get it started. Swing the rocket around and around, letting the string out to its full length. If the rocket flies nose first, you know it's going to be stable in flight. It might take a few tries to get it to fly right, but if it does, you can skip the whole CG/CP measurement if you want to.

It's also a fun test to do, because you get to see kind of what the rocket would look like in flight, but from a close vantage point, which you never get to do when the rocket is actually launched.

Here's a good video with a swing test in it.

So, why not "just" do a swing test? There are a few reasons. For one, maybe you don't have the room. As I write this, it's winter, so it's cold outside, and I don't have a lot of space outside that isn't populated by cars, houses or shrubbery.

Another reason is that some rockets do not pass the swing test, yet are perfectly stable. I'll elaborate on this point when I write Part 3 of my stability series, but I'll just briefly say that long rockets or rockets with large fins sometimes have a hard time flying straight on a swing test. And larger rockets are harder to get swinging at a fast enough speed for the swing test to work, especially without hitting something.

Yet another reason for going beyond swing testing your rocket is that perhaps you're interested in understanding the rocket's stability in a more theoretical way. Calculating the CG/CP relationship is interesting - it's the theoretical part of rocketry. Then, swing testing or launching the rocket once you've determined stability is the experimental part. That's fun science! I find the swing test useful as a way of double-checking my work. A failed swing test prompts me to take careful measurements, to see if I really do need to modify the rocket.

If your rocket passes the swing test, it will be stable. If it doesn't, then you need to go on to the next step.

* * *

Now we need to find the Center of Pressure - CP.

The easiest and most reliable way to do this today is with rocket simulation/design software. There are several kinds, but the most common are RockSim and OpenRocket.

RockSim is sold by Apogee Components. It's a fine software, quite sophisticated, and a lot of rocketeers swear by it. If you get serious about rocketry - especially scratch building and high power rocketry - it's a good investment. But it does cost over $123. You can download a free three-month trial of RockSim if you're curious about it.

OpenRocket is free to download and use. It uses Java, so you'll need to have that installed on your computer. It lacks some of the bells and whistles of RockSim, but it's a good place to start for beginners, and some rocketeers simply stick with it, because they don't necessarily need the few extras RockSim provides. One of my fellow club members, an experienced rocketeer who flies lots of high power rockets, uses both software to run simulations before he launches. As he puts it, "Sometimes it's good to have a second opinion."

Center of Pressure is determined by the shapes and sizes of all the external components. It has to do with the relative surface areas of all the parts. As such, it doesn't matter whether your nose cone is heavy or light, or what materials your fins are made of - just what size things are and where they are placed on the rocket. Most specifically, what puts the CP toward the aft of the rocket, where it should be, is the fins.

If you're very meticulous about measuring all parts of your rocket, getting the shapes and angles of the fins correct, figuring out the exact length-to-diameter ratio of your nose cone, etc., you can create a pretty good simulation of your rocket on your own. But if you are using a kit, such as from Estes or Quest, you can very likely find a good sim file somewhere online of your very rocket.

Since the Sport Loader is sold by Apogee Components, I was in luck. Apogee has kindly put free RockSim files of most of the rockets they sell on their website. I use OpenRocket, but I can still open and modify RockSim files with it. The two software are mostly compatible.

I open the Sport Loader RockSim file, and here's what I see.

You can see that the CG is well forward of the CP. In the top right corner of this image, you see the following information:

Stability:2.48 cal
CG:35.6 cm
CP:47.7 cm

This is the stability information for the original rocket without motors. CG and CP locations are measured from the tip of the nose cone. So, in this instance, the CG is 35.6 centimeters from the tip of the nose cone, and the CP is 47.7 centimeters from the tip of the nose.

Incidentally, when taking stability measurements, I always use the metric system. It's much easier, since everything is divided by 10 (how do you measure 15.655 inches with a ruler?), and millimeters are small enough units that if you get a measurement to within less than a millimeter of accuracy, you're probably going to be just fine.

"Cal" is short for caliber, and it refers to the distance between the CG and CP. Caliber is based on the diameter of the rocket itself - more precisely, on the diameter of the widest part of the rocket, which, in this case, is the payload section. This distance between the CG and CP is called its static margin of stability.

In order for a rocket to remain stable in flight, the static margin must be at least 1 caliber. The reason for this is that the Center of Pressure can move forward, depending on the flight conditions. If the CG moves too close to the CP, the rocket will be neutrally stable, and if it goes forward of the CG, the rocket will be unstable, and will flail around in flight.

With the Sport Loader, we have a rocket whose widest diameter is 4.9 centimeters, or 49 millimeters. That means that in order to maintain a minimum static margin of 1 caliber, the CG must be at least 49 millimeters forward of the CP. Anything less than 1 caliber stability is referred to as marginally stable, and is not safe enough for flight. In the image above, the CP is 12.1 cm forward of the CP, giving us a static margin of 2.48 caliber.

Of course, rockets do not fly without motors, so let's see what the static margin is with the motors installed.

The CG has shifted aftward to 42.1 cm from the tip of the nose, and we still have a safe static margin of 1.14.

Well, that's great for our original rocket, but what about our newly-shortened, repaired rocket? We'll need to re-measure that.

I want to make my simulation as accurate to the real rocket as possible, so I need to measure the new, shorter length of the tube.

Looks like I've cut the body tube down to about 33.2 centimeters in length. Why don't I just change the design in OpenRocket and see if the rocket is stable? That should work, right?

Let's take a look.

Whoa, what?? I take off an inch and three quarters, and suddenly the rocket has a stability margin of 0.627? The rocket looks like it should be stable, so why is it now marginal?

Here's a clue - the Sport Loader with a very short, stubby tube:

I've shortened the body tube to about 7 centimeters or so, and now the CG is behind the rocket!

Of course, this doesn't make sense. But I'm showing it to you, because it took me some thinking to figure out what the problem was. OpenRocket doesn't come with instructions, and for about a year, there were two important functions I was completely unaware of: the ability to override both the mass of the rocket, and the rocket's Center of Gravity. That's what's going on here: the CG in the RockSim file has been overridden.

The design elements box in OpenRocket. The CG symbol means that the
Center of Gravity has been overridden. The little weight symbol
means that the mass has been overridden.

This is because when you build a rocket in simulation software, it calculates roughly where the CG will be by finding the CG of the individual components. But it doesn't take into account certain key components - namely, the kind and amount of glue or epoxy you use to build the rocket, and the weight and thickness of paint. Often, a person will build a simulation of a rocket, then compare the calculated CG with the CG of the real, built rocket, then override the CG in the simulation. This will help you to get better simulations.

The fact is, no matter how accurate a simulation may be, you still need to verify the measurements on the actual rocket, when making repairs or modifications. So, while simulations can be useful, for now, let's get back to the real rocket. What we need to do is find, measure, and mark the CP on the rocket we have.

The simulation tells us that our new Center of Pressure is located 43.8 cm from the tip of the nose cone. You could take a cloth tape measure, put it on the tip of the nose cone, and measure from there. But to be truly accurate, you need to measure straight back from where the tip of the nose cone. That's why I placed the nose cone against the wall in the pictures above. I'm going to measure from the wall, which will be on the same plane as the tip of the nose, back to the CG and CP.

It's kind of like when you go to the doctor and they measure your height - they use a straightedge to find the top of your head.

So, from the wall, where the tip of the nose cone is placed, I used a rigid, metal tape measure to mark a spot on the rocket 43.8 cm from the tip of the nose cone.

I've already marked and measured the CG for the rocket, both with no motors, and with motors installed.

With the motors (and parachute and recovery wadding) installed, the real CG of the rocket is about 36.1 cm from the nose.

That's a difference of about 7.7 cm.

The rocket is about 4.9 cm in diameter at its widest point - the payload section. The static margin is larger than 1 caliber - I have now verified that the rocket will be stable, and is safe to fly.

So, why did I bother checking the CG on the simulation, instead of just using it to find the CP, which is all I needed to do here? And why did I mark the CP of the rocket with no motors installed, if I only needed to find the CP with the motors in to check stability?

The answer is that I wanted to be able to run accurate simulations of my rocket in the future. We'll talk briefly about that in the next post.

For now, though, if you alter a rocket in any way, through repair, or through upgrading a kit to take a larger motor - which I have done, and it's been a lot of fun - take these steps, and you can be sure your rocket will be stable.

Or, just do a swing test!

Click here for Part 3

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