Repairing and Enhancing the Quest Magnum Sport Loader - Part 3: Fixing the Sim Part A

The shortened simulation of the rocket shows stability to be marginal - only 0.627 caliber. Below, we fix that.

Click here for Part 1

This is kind of an addendum to the previous two posts on repairing the Quest Magnum Sport Loader and then verifying its flightworthiness. The actual model is finished, stable, and ready to fly. Now, I want to fix the simulation in OpenRocket to reflect the actual model.

Why do I want to do this? A couple of reasons. The first is curiosity: I want to see how accurate OpenRocket is in finding the correct Center of Gravity (CG) when I load different motors. Sometimes I look at the CG on a RockSim or OpenRocket file, and wonder can that really be right?

But more importantly, I want to be able to run flight simulations before I actually go out to launch the rocket. Depending on weather conditions - wind, mainly - I might decide to put different motors in the rocket. On a particularly breezy day, a high flight increases the likelihood that I will lose one of the models I'm proudest of. And if I fly an altimeter in the payload compartment, that could be an expensive loss. Running a flight simulation will give me a rough idea of how high the rocket will fly with different motors. I can then make a more informed decision whether to put A, B or C motors into the rocket, depending on how high I am willing to let the rocket fly on launch day.

I can then also compare the simulation to the data from actual flights, recorded on the altimeter. I can see how close to the predicted altitude the rocket actually flew. Then I can look back at the simulation and figure out how I might make the predicted altitude more accurate. That's part of what makes rocketry interesting. There's more to it than just watching something go up and come down.

I think I should mention that as good as rocket simulators are, they're almost never 100% accurate when it comes to altitude prediction. There are just too many variables to account for in the physical flying environment. But they're pretty good, and much better than guessing. And, as I said before, I'm not a master at OpenRocket, but I know enough for a beginner to be reasonably accurate.

Let's get started.

First thing, I start OpenRocket, and open the original RockSim file, downloaded from the Apogee Components website. This is what I see.

This original design file has not been altered to reflect the shorter, repaired rocket. Before I do any of that, I'm going to "Save as" an OpenRocket file. OpenRocket and RockSim are mostly compatible, but not 100%. Sometimes, if you make a change to a RockSim file in OpenRocket, then save all your changes and close the file, you will find when you go back to the file, some of the changes you made have not been saved. Saving the file as an OpenRocket (.ork) file will fix this problem.

Up in the design box, we see a number of things we'll want to change.

Every weight symbol means that the mass, or weight, of something has been overridden. Every blue and white circle (the CG symbol) means that the CG of something has been changed. These are on several components, but in fact they don't matter as much as the overrides on the part called Stage. That refers to the entire rocket (or to the upper stage, if this were a two-stage rocket). Overriding the mass and CG on Stage overrides everything else below it.

We're going to get rid of all the overridden data for this simulation and input our own.

The rocket's mass has been overridden in the simulation. With no motors, it is said to weigh 99.2 grams.

When you remove part of a rocket in real life (in this case, we removed nearly 2 inches of the airframe when we repaired the rocket), the mass will change a bit. But in the last post, you saw that simply shortening the airframe in the design file wasn't enough - because the CG had been overridden to 35.6 cm, no matter how much I shorten the tube, the value won't change.

Obviously the Center of Gravity can't be behind the rocket!

That's the same for the mass. In fact, since the override on Stage outranks any other override on any component, I could make each of the fins weigh 20 pounds, and the simulator will still say that the rocket weighs 99.2 grams.

First thing I'll do is remove the mass override for Stage. Double-clicking on Stage on the design components box brings up a dialog box.

Simply remove the check mark in the Override mass box, and the mass of the rocket will equal the combined mass of all the parts.

Suddenly, the rocket is said to have a mass of 281 grams! That's a huge difference.

The CG, however, remains where it is - 35.6cm from the tip of the nose cone. We'll remove that override as well.

Incidentally, you can use either grams or pounds and ounces. I am sticking with grams for now, because I have a digital scale which uses metric and is accurate to 0.1 gram, and I'll use that to complete the simulation here.

Let's get rid of the CG override.

Suddenly, the CG jumps way forward, to 16.9cm from the tip of the nose cone, in the lower third of the payload section.

And here we see why the rocket weighs so much. There are "mass components" in the payload section, representing the payload this rocket is designed to carry - in this case, raw eggs.

Oddly, though the rocket only has a capacity for two eggs, there are three of them represented here. They are called "Egg1," "Egg2," and "Unspecified," but they all seem to represent eggs, because they all weigh about the same - 63 grams.

Two of them are hard to see, because they have been assigned a length of 0 by the person who built this simulation. One of them is located exactly where the nose cone meets the black coupler attaching it to the payload section. You can see it more clearly if I highlight it.

Raw eggs are heavy - about 2 ounces a piece. And three of them won't even fit in this rocket. Besides, we want to get an idea of what the rocket will do with no egg payload at all. If we want to fly one or two eggs, we can add these back to our simulation.

I select each egg in the design elements box and delete them. The CG has now shifted aftward, and the total mass of the rocket is now said to be 98.7 grams.

Now we're getting closer. I'm going to make a few more alterations to the simulation before turning to the physical rocket to take my measurements.

First, I've decided to fly the rocket with only one parachute. The rocket is designed to separate at apogee and come down on two chutes - the payload comes off and descends on its own parachute. I have decided I don't like this for a couple of reasons - I want to keep the parts of the rocket together, and I'm not confident the chute for the rocket booster will always come out of the airframe without the weight of the nose cone pulling on it. So I'm going to delete one of the parachutes, as well as the mass overrides and CG overrides of any components which have them. I'm also adding a shock cord, because the rocket obviously has one, despite the fact that it's not present in the design file. Finally, I'll shorten the airframe to match the repaired rocket I have.

I'll go back to the simulation and override a few things once I've weighed and measured the physical rocket itself. For now, though, I have a simulation file which is of accurate length to match the model I have on hand, with a CG said to be 24.8cm from the tip of the nose, and a mass of 101 grams with no motors installed.

What I need to find out to have a reasonably accurate simulation - and one which will tell me if the rocket is stable - are two things: The actual weight or mass of the completed rocket (with no motors installed) and the actual Center of Gravity of the completed rocket (again, with no motors installed). I can then input that information into the simulation. Finally, I'll find the CG of the rocket with motors installed, and see how accurate the CG in the simulation is with virtual motors installed in the simulation.

We'll do that next time.

Click here for Part 4

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