In the last post, we removed all the Center of Gravity (CG) and mass or weight overrides on the simulation of our rocket.
Now we have to find the CG and total weight of the actual model, so we can get a more accurate simulation to use. This will help us figure out what motors we want to use the in the rocket on a given launch day, so that we can reach a maximum altitude on nice calm days, and keep the altitude lower on windier days, to lessen the chance we'll lose our awesome rocket.
An accurate simulation will also tell us how long a delay grain we need for those motors. For example, do you want to use C6-3 (3-second delay), C6-5 (5-seconds) or C6-7 (7 seconds) motors? The difference is important.
Even though I've been focusing these posts on my particular rocket - the Quest Magnum Sport Loader, which I recently had to shorten to cut off some damage - these principles can be applied to any model rocket.
We'll need the following things for this last step:
- The rocket, including the recovery system (parachute, streamer, etc.)
- A length of string, or something on which to balance the rocket, such as the back of a chair
- A digital scale
- Recovery wadding
- A tape measure
- A pencil or pieces of masking tape to mark locations on the rocket
You'll also need your OpenRocket or Rocksim simulation open on your computer.
Once we've found the true weight of the built rocket, and the true CG, we will override those elements in our simulation. Then we'll get an accurate representation not only of its caliber of stability, but we'll be able to run flight simulations with a reasonable degree of accuracy, which will aid us in picking the right motors for a particular flight.
First, we'll weigh the rocket to find the true mass or weight of the finished model. Our simulation says the rocket weighs 101 grams without motors. That is the sum of all the parts in the OpenRocket design file. OpenRocket will insert these automatically when you build a design file. It takes an assumed density for a particular material - say, balsa fins - and will calculate a mass or weight based on the part's dimensions.
In reality, the density and weight of items like balsa fins and other parts will vary. Fins, especially, will vary in density and final weight. There are dense balsa woods and soft, lightweight balsa woods, and different rocket kits will have different qualities of balsa. Even the weight of the individual fins in a single kit will vary slightly.
But even if all the parts in our design file were accurate, OpenRocket doesn't take a couple of important things in rocket construction into account - glue and paint. Those things do add mass, and will change the final weight of the built rocket.
So, to get an accurate simulation, we do need a digital scale. It doesn't have to be fancy or expensive - a decent digital kitchen scale will do. And even though I'm working with the metric system here, you can also work with the Imperial scale (pounds and ounces) if you happen to have a scale which uses that system. The main thing is that the scale be accurate and sensitive enough to detect small changes in mass.
I have two digital scales, and they both work really well. Both of them were very reasonably priced.
The scale above left is a small metric scale which can measure to an accuracy of 0.1 gram. That's very sensitive, and perfect for small rockets and parts. It has a maximum capacity of 600 grams, a little over 1.3 pound. It cost less than $10 on Amazon.com.
The scale to the right can weigh in Imperial units, detecting pounds, ounces and tenths of ounces, or in metric units, accurate down to a single gram. It has a capacity of 110 pounds and a larger plate, meaning it can be used for larger rockets - which is great when you graduate to building and flying high power rockets. And it was only about $25 on Amazon. I was surprised to find such a good scale for such a reasonable price!
Since I'm going for maximum accuracy and working with a small enough rocket, I'm going to use the small metric scale.
What we need to find is the weight of the rocket itself, without the motors. Once we add motors to our simulation, the weight in OpenRocket will change, as will the CG, as we will see below.
You do need the parachute installed in the rocket, and, though it might surprise you, you should also install the recovery wadding. Why, you might ask? Aren't we trying to find the empty weight of the rocket?
Well, yes. But when running simulations, OpenRocket also doesn't take recovery wadding into account. You will always fly with the stuff (unless you are using a rocket with an ejection baffle, a device permanently installed in the rocket which protects the recovery system from the heat of injection charges), so you should assume it's part of the simulation.
Recovery wadding doesn't weigh much, so if you forget to install it, it probably won't make a big difference. But, for accuracy's sake, it's a best practice to consider wadding a part of the recovery system, and weigh it with the rocket.
Turn on the scale, let it boot up, and when it reads "0.0," carefully place the rocket on the scale.
The Magnum Sport Loader weighs 106.6 grams. The simulation of the rocket states that it's 101 grams. That's pretty close. Does that mean that the glue and spray paint on this rocket weigh 6.6 grams? Hard to say, since I didn't weigh the individual parts of the rocket as I was building it, as I tend to do now.
At any rate, I need to change the weight of the rocket in the simulation, so we'll turn to our design in OpenRocket.
Up near the top left of the screen, in the design elements window, select Stage.
Double-click on Stage, or press the Edit button to the right. The following dialog box will pop up.
Check the Override mass box, and type in the weight of your rocket.
And now, as I write this, I learn something new: OpenRocket will not allow me to enter a mass of 106.6 grams. It rounds up to 107 grams. If I were to enter 106.4g, it would round down. So it's accurate to within a half a gram, but no closer. This will do just fine.
Now that we have an accurate weight for the rocket, it's time to find the true Center of Gravity. You can leave the Stage configuration dialog box open for now.
A quick and easy way to find the approximate CG for your rocket is to balance it on your finger. That's fine for flying out in the field - say, if you're trying a new, heavier motor and you just want to make sure the CG doesn't move too far back when you install it in the rocket. But I want to mark the CG on the rocket, so I'm going to balance mine. You can use the back of a chair or some other sturdy object with a straight thin edge. But I found it helpful to use a loop of string.
Here, I have the rocket balanced on the tube cutting jig you saw in Part 1 of this series.
Problem is, it took me a lot of adjusting the rocket back and forth by tiny increments to get it to balance like this, and it tried to roll off. Then, when I went to mark the CG, it fell off the jig.
With a loop of string, the rocket won't roll around, and once you find the CG, you can keep the string in the same spot on the rocket until you grab a pencil or piece of tape to mark the spot.
Make a simple loop in the string - here, I'm using a piece of Kevlar shock cord - and slip it over the rocket. Again, you need everything installed in the rocket except the motors. This includes the parachute and recovery wadding.
Find the spot where the rocket balances.
This is the CG of your unloaded rocket. You can mark the spot with a pencil or piece of tape. Since I didn't want to put a pencil mark on the paint, used low-tack painter's tape.
|I grab the loop of Kevlar string to hold it in place on the rocket...|
|...and carefully line a piece of tape up with the Kevlar. The nose cone is to the left of the frame|
of this picture, so the leading edge of the tape represents the CG of the unloaded rocket.
CG and CP are measured as a distance from the tip of the nose cone. You might just grab a cloth tape measure and place it at the tip of the rocket and then measure from there.
But this is a little like measuring your height by placing a tape measure on top of your head, and measuring around the curvature of your head down to the ground - it's going to make you seem taller than you actually are. If you measure along the curvature of the nose cone, you're going to get a false measurement.
You need to measure straight back. The way I do this is by placing the nose cone against a flat, vertical surface, such as a wall, and measuring from there back to the place.
Here, I have the rocket sitting on a cradle, to keep it horizontal to the table, and placed against a metal file box.
Then, I place the tip of a metal tape measure flush against the metal box and measure straight back to my CG mark on the rocket. As mentioned in a previous post, I always switch to the metric system when doing measurements like this, because it's simpler - everything is divisible by 10.
The CG of my rocket is 29cm (or 290mm) from the tip of the nose cone.
For comparison's sake, here's the measurement I got by measuring along the curvature of the nose cone with the cloth tape measure:
|The blue tape is holding the tape measure in place just so I can take a picture.|
Is this a big deal? Will it make much difference? Am I being too fussy here?
Well, maybe it won't make much difference how you measure the CG and CP locations on many rockets. But for the simulation, I'm trying to be as accurate as I can. And if you're measuring the rocket to check its stability margin, you want to measure as accurately as possible. Remember that the minimum margin of stability is 1 caliber - the diameter of the rocket itself. The CG needs to be forward of the CP by at least 1 caliber. If you have a rocket which is right at that 1-caliber margin, you want to make sure you get a good measurement. It might make the difference between having to add weight to the nose cone or not.
Alright, so we have our real-life CG location - 29cm, in my case. Let's put that into the sim. Go back to the Stage configuration dialog box in OpenRocket. Click the check box marked Override center of gravity, and input the actual CG location.
Now the blue and white CG symbol on the rocket design will move. In the case of the Magnum Sport Loader, it moves from here:
The bulk of the work is now completed. We just have a few more minor details to adjust, then we can run some simulations.
Click here for Part 5
Like my Facebook page for blog updates and extra stuff!
Follow me on Twitter @rocketn00b.