Monday, April 4, 2016

Playing With the Flight Simulation

Now that we've run a first simulation using Estes C6-5 motors, let's try a couple things. What if I'd built the rocket without streamlining the fins? What might the altitude difference be?
I'll go back to the rocket design and set all the fin cross sections to "square," then run another simulation. With six streamlined fins, the simulation predicts an altitude of 936 feet. What about with six square fins?

According to OpenRocket, the increased drag from square fins over streamlined fins has cost us altitude - we've gone from 936 feet down to 692 feet - a significant difference!

Is this accurate? It's hard to say. Some people claim that rocket simulators overestimate altitude predictions somewhat, or that they overestimate the value of airfoil-shaped fins. I can't make a claim about it without testing it myself. But there will surely be at least some difference in altitude.

OK, let's go back to the simulation which matches the rocket we're using here. With streamlined fins, I should be able to expect an altitude of somewhere over 900 feet on two C6-5 motors. What if I'm flying on a smaller field with trees on the edge, or it's kind of windy? What if I want to fly the rocket, but not go so high?

Simply create another configuration in the Motors & configuration tab. Let's try the three most common 18mm motors - the A8, the B6 and the C6, and compare them.

I can highlight all three simulations and run them at once.

Looks like I can keep the rocket to around 400 feet on the B motors for a small field or windy day, and even lower with the A motors - 140 feet - for an even smaller field, or maybe a simple demonstration of the rocket. Liftoff velocity appears safe enough for all three configurations, and I accurately guessed which delay times I'd need to select for each motor combination.

Let's try one more thing. Let's compare the Estes C6-5 motor, which we've been using for all the simulations up to this point, with the Quest Aerospace C6-5 motor.

As you may have read, the C in this motor designation refers to the total impulse of the motor, between 5.01 Newton-seconds and 10 Newton-seconds. The 6 refers to average thrust, measured in Newtons. So, a C6 motor is supposed to have an average thrust of 6 Newtons, and a total impulse of up to 10 Newton-seconds. Click here for a refresher on motor basics.

However, despite what you may have read, an Estes C6 motor does not have an average thrust of 6 Newtons. Its average thrust is about 4.7 Newtons. The average thrust of the Quest motor is even lower - 3.5 Newtons. Both motors have a total impulse of 8.8 Newton-seconds.

This leads to something very interesting. Since total impulse is approximately equal to average thrust multiplied by burn time of the motor, the lower-thrust Quest motor should burn for a longer time period. And, in fact, it does. The Estes motor burns for 1.9-2ish seconds, while the Quest motor burns for 2.5 seconds - 25% longer! Both motors impart the same amount of force to the rocket - 8.8 Newtons. The higher-thrust Estes motors make the rocket fly faster.

And here's the really interesting part. There are two forces keeping a rocket from flying upwards forever: Gravity and drag. Gravity is a constant. Drag is influenced by a number of things, but especially by the velocity of the rocket. Drag increases as a square of velocity. So, if you double a rocket's velocity, drag increases four times. If you triple the rocket's velocity, drag increases nine times!

With the right combination of optimal mass, lower thrust, and longer burn time, often the lower-thrust motor will take a rocket to a higher altitude than it's higher-thrust counterpart of the same total impulse.

Let's test this out in the simulation.
On the Motors & configuration tab, I'll create two configurations, one for the Quest motors, and one for the Estes. To avoid confusion, I'll click on the Rename configuration button and type in the correct brand name of the motors I'm using for each configuration.

Going back to the Flight simulations page, I run both simulations at the same time. Here are the results:

As you can see, the Quest motors take the rocket higher, breaking 1,000 feet in altitude. Optimum delay for both flights is just over 5.5 seconds, so C6-5 motors will work well regardless of which brand we select.

Let's unpack the information here.

On the Quest motors, the rocket leaves the launch rod traveling at 48.9 mph, 17.7 mph faster than on the Estes motors, with which it leaves the rod at 31.2 mph. At this point, the rocket with Quest motors is experiencing much more drag than the rocket with Estes motors. But the story isn't over - the motors are still burning, and the flight has just begun.

We've already established that the Estes motors have a higher average thrust than the Quest motors, so why is the rocket with the Quest motors traveling so much faster?

The answer lies in the thrust curves* of the individual motors.

Here is the thrust curve for the Estes C6 motor:

As you can see, in under a quarter of a second, the thrust peaks at nearly 12 Newtons, then settles back to a lower-level thrust of under 5 Newtons for the rest of the burn. This initial, peak thrust is pretty common in black powder model rocket motors - a peak early in the burn, followed by a lower thrust for the rest of the burn - and has to do with the surface area of the propellant being burned at a given moment.

For comparison, here's the thrust curve for the Quest motors:

Here, we can see a dramatic difference. The initial thrust peaks at over 22.5 Newtons - much higher initial thrust than that of the Estes motors. After the peak, the thrust reduces to a much lower level, but for a much longer burn time.

We've seen that the rocket with Quest motors leaves the rod at much higher velocity, which means higher drag, and but that the average thrust is much lower. Why does the Quest rocket go higher?

Let's look at the flight simulation plot for both flights from OpenRocket.

The Estes flight plot:

And the Quest flight plot:

We can see the vertical velocity - the blue line - increase until motor burnout occurs, at about 2 seconds for the Estes motor, and for about 2.5 seconds for the Quest motor. By the time motor burnout occurs, the Estes rocket has caught up with and surpassed the velocity of the Quest rocket, by about 9 miles per hour.

But look at where burnout occurs for each rocket - at around 310-325 feet for the Estes rocket, and around 400 feet for the Quest rocket. Once motor burnout occurs, the rocket will only slow down - and the Quest rocket has a head start of about 75 feet when coasting begins!

The Estes rocket is traveling faster, but can't catch up to the Quest rocket. Aerodynamic drag increases as a square of the velocity, so already the Estes rocket is experiencing more drag due to its increased velocity.

And the air gets thinner as altitude increases, so drag decreases as you go upward. How much difference in atmospheric density will the rocket experience in 75 feet? Well, not much, but there is an difference.

So the Quest rocket, while traveling 9 mph more slowly than the Estes rocket at motor burnout has a 75 foot head start at coasting, and experiences less drag due to being higher in the air and traveling more slowly.

Playing around with simulations like this is a good way to see how your rocket can reach higher altitudes. But maybe a difference of 80 feet in altitude isn't such a big deal to you. Well, there's another reason to try Quest motors. Longer burn launches are fun to watch! 2.5 seconds may not seem like a long time, but when it comes to model rocket motor burns, you really do notice a difference.

*Accurate thrust curves are hard to find. There were several thrust curves on for these two motors, and not all of them agreed with one another completely. I selected the two thrust curves to best illustrate the point here.

A Word About Payloads

The Quest Magnum Sport Loader is a payload-carrying rocket. It's specifically designed to loft 1-2 raw eggs. Earlier in this series, I modified the payload bay so I could fly the rocket with a barometric altimeter.

Static ports - tiny air holes drilled into the payload compartment - allow the air pressure inside
the rocket to match the air pressure outside so the altimeter can get an accurate reading.
If you do add a payload to your rocket, whether it's an egg, an altimeter, a camera, or a little toy astronaut, you will add mass. To get an accurate simulation, you'll want to repeat the steps with the payload installed.

The Jolly Logic Altimeter 2 adds 9.9 grams.

This 808 keychain camera - a common
payload - adds 14.4 grams.

This foam rubber padding, which I'll use to hold and protect
the altimeter, adds 2.1 grams.

The rocket with the altimeter and padding now weighs
118.4 grams. I'll need to adjust my sim for greater accuracy.

Eggs in particular are pretty heavy. Even if you're not terribly concerned about the accuracy of the altitude prediction, if you add a heavy payload, you want to run a new simulation with the new mass of the rocket, for safety. I know I can fly the Magnum Sport Loader with one egg on two C6-5 motors. But with two eggs, I might need to select a shorter delay time.

* * *

There's one more feature in OpenRocket I'll show you quickly - the different ways you can view the rocket design.

In the top left-hand corner of the bottom panel, where you see the rocket, you can select View Type.

We've mostly seen the rocket in Side view, which shows you the basic design in a 2-dimensional layout. You can also see the rocket from the back, by selecting Back view.

This feature can help you accurately place launch lugs, odd fins, cluster motor tubes and other items on the rocket, by sighting straight up from the aft end.

There are also three different 3-dimensional ways to view the rocket. 3D Figure and 3D Unfinished both show the rocket as a see-through but 3-dimensional image. The main difference is that in 3D Figure, all the components are color-coded. I suppose this is to help you distinguish individual parts more easily when looking at the rocket.

Finally, 3D finished will show you the completed rocket. The components default to natural colors (tan body tube and balsa fins, white plastic nose cones), but you can change the color to get a final idea of what the paint job might look like.

The 3D views all allow you to rotate the rocket, both vertically an horizontally, so you can get a good look at the whole thing. If you're designing your own rocket, it's good to be able to turn the thing around and look at it from all sides to decide whether you like the looks of it before you start building.

* * *

How accurate are these simulations? Will my rocket actually fly to an altitude close to the 945-odd feet predicted? And if the prediction is inaccurate, how can I improve it?

I don't yet know the answers to those questions, because I haven't tested it out yet. And despite the fact that it's a free, excellent tool for modern rocketeers, there are a few things OpenRocket doesn't take into account: additional drag caused by the static port holes, or the shoddy work I did on my airfoils, for example.

I'll return to this subject when I've had the chance to launch the rocket with an altimeter on board. We'll see how accurate the predicted altitude is, and we'll try to figure out what went wrong if the prediction is totally off. The next launch is scheduled for April 23.

Stay tuned.

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  1. I'll be interested in the flight comparisons.
    My experience with the Quest (Chinese made, tan nozzles) C6-5 engines hasn't been good. An example - A friend was flying a Dr. Zooch Saturn V. Boost seemed normal but altitude was lower than expected. It fell to the ground before the ejection went off. I've seen that in other BT-60 sized heavier, draggy models.
    In smaller diameter, lighter models the Quest C6-5s seem to be fine. A two engine cluster might be okay. I wonder about the extra weight of an egg payload.

    1. I'll make sure to keep the eggs out of it for the first try. Thanks for the heads up on that!