Friday, July 17, 2015

Rocket Stability - or - What Happened To Homer's Rocket? (Part 1)

A few months ago, because of a recommendation from Chad, I read Homer Hickam's book Rocket Boys. This was adapted into the film October Sky, starring Jake Gyllenhaal.

In 1957, after the launch of Sputnik, Hickam became obsessed with building his own rockets. But model rocketry hadn't really been invented yet - in fact, the hobby got its start that same year - and a lot of kids tried building their own rockets at home, sometimes with tragic consequences. Cooking up propellant isn't something kids should do in the kitchen. Nobody should do it, unless they really know what they're doing, and most people don't.

If you've read the book, or seen the movie, you know Homer's first few rocket flights didn't go well. The first "flight" blew up his mother's fence. The second didn't blow up - but it flew out of control.

Since his rockets were made of metal, he was really lucky he didn't kill anybody. This is why we make model rockets out of paper, balsa and plastic. Their destructibility is a safety feature, in case something goes wrong in flight.

But what happened to Homer's rocket? Why did it fly all over the place, rather than straight up?

Reading the book was fascinating, and since I'd been studying rocketry for several months, it was fun to see how Hickam came up with a lot of innovations that we see in modern model rocketry today - electric ignition systems, tracking altitude from the ground, using a launch rod and launch lugs - all on his own. Model rocketry was invented in 1957, and it didn't get really popular until the 1960s, so a lot of young rocketeers did not have the benefit of its innovations or safety features, but many of them discovered better practices along the way.

I'd seen the movie years ago. But when I read the book, I thought, I bet I know why that happened. Actually, the reason why could have been one of several things.

There are a number of things to consider when designing, building and launching a rocket to ensure a stable flight. If you are merely building a kit, you probably don't have to worry about it that much. Most model rocket kits are well designed, and designed to be stable.

But if you'd like to design your own rockets - and it isn't that hard, as long as you understand a few simple concepts - it is very important to know something about stability. Also, sometimes we like to hack a rocket kit so it will take a larger, more powerful motor than the kit was designed for. I did this with the Estes Cosmic Explorer, because I love the way the rocket flies with standard motors so much, and I wanted to see it go higher.

My two Estes Cosmic Explorers - the stock kit build on the left, which accepts up to a
C motor and flies to about 600 feet, and the upgrade on the right, which accepts a much
more powerful E motor, and can top 1800 feet.

A larger motor is heavier, and so stability issues come into play - this is another time it's important to understand the basics of rocket stability. I was able to upgrade the Cosmic Explorer with confidence, knowing that it would have a stable flight, because I understood the basics of rocket stability.

Stability is covered in depth in G. Harry Stine's Handbook of Model Rocketry, which I highly recommend reading.

But I want to cover some of the basics here.

What keeps a rocket stable in flight? The answer might seem obvious: the fins. But simply slapping a set of fins on a rocket is not enough. You need to understand why the fins work, and what might prevent them from working.

The two most important concepts to understand with regard to rocket stability are the following: Center of gravity and center of pressure.

Center of Gravity

Every object has a center of gravity. This is also sometimes called the center of mass. It's a theoretical point somewhere on, inside, (or sometimes outside) the body of the object around which the mass is equal in any direction. This is also known as its balance point, because if you can balance an object - a stick, for example - on your finger or the back of a chair (or whatever), you've found its approximate center of gravity.

Any object in space, whether it's in the vacuum of outer space, or tumbling through the atmosphere, will rotate around its center of gravity. If you flip a stick in the air, it will rotate exactly around its balance point. A gymnast doing a somersault rotates around his or her center of gravity.

Remember how I said that sometimes the center of gravity is located outside the body of an object? That's how a boomerang operates. It rotates around its center of gravity, which is located somewhere in the air between its two arms.

A rocket will also rotate around its center of gravity. Keeping that rotation under control is what stability is all about. Although gravity is pulling equally on all parts of a rocket, from the tip of the nose cone to the end of the motor hook, it acts through the center of gravity.

In rocketry, the center of gravity is often abbreviated CG.

Center of Pressure

 The center of pressure is the average location of pressure variation on an object. It's another theoretical point on a rocket - this time, the theoretical center of all the aerodynamic forces operating on the rocket. It is determined by the total surface area of the rocket, and in a way, it's similar to the center of gravity; it's the point where all the aerodynamic forces are in balance. The surface area in front of the center of pressure is equal to that of the surface area behind the center of pressure. Much like the center of gravity, air pressure acts on all parts of a rocket equally, but because the forces are balanced before and behind the center of pressure, we say that aerodynamic forces act through the center of pressure.

But the center of pressure is tricky, because while the center of gravity of an object mostly stays put*, the center of pressure can move around, as we'll see.

In rocketry, the center of pressure is often abbreviated CP. CG and CP are very important concepts, so keep them in your mind.

*(On a model rocket, the center of gravity does move forward slightly during flight. More below.)

The center of gravity is usually indicated with a blue and white checked circle, and the center of pressure is indicated with a red dot, often with a red circle around it, as in the picture below. Notice where the CG and CP are in relation to one another.

Click here for Part 2.

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  1. Actually, Homer and his Rocket Boys were pretty much the norm in the early days of amateur rocketry in the 50s and early 60s. Back then, they were building metal rockets using "homebrew" propellant. It was easy to get hold of chemicals to make rocket fuel from in those days. Unlike today, there were no restrictions on many hazardous substances, so you could buy them at drug stores, hardware stores, and garden shops. And there were many accidents, some of them serious, too, because there were many young people trying to make rockets who really had little or no idea just what they were doing. While modern model rocketry was invented in 1957, it took years for people across the country to find out about it. I first learned about it at a summer camp I attended in 1962. But even then model rocket supplies were only available by mail order. It wasn't until about '64 that you could buy them in hobby shops. in the part of New York where I grew up.

  2. Daniel,
    I recently found your blog and continue to enjoy reading past entries as I catch-up to the current entry. Great explanations and illustrations! I too love the movie October Sky!

    1. Thanks, Scott! I appreciate knowing someone is getting some useful information from the blog.