As a child, there was a book which fascinated me. It wasn't mine (at least at first), and as it turned out didn't belong to the library where I encountered it. It was the property of a remarkable old woman, who, seeing that I was enamored with it, eventually gave it to me as a gift.

It was written by a Chicago Tribune reporter, Guy Murchie, and entitled "Music of the Spheres."


The first edition published in hardcover in 1961 bore this simple title, the second, paperback edition, published in 1967 was broken into two volumes had the titles expanded to: "Music of the Spheres: The Material Universe from Atom to Quasar, Simply Explained; VOLUME I, The Macrocosm: Planets, Stars, Galaxies, Cosmology" and "Music of the Spheres: The Material Universe From Atom to Quasar, Simply Explained (Volume II: The Microcosm: Matter, Atoms, Waves, Radiation, Relativity)." I much prefer the shorter title.

Murchie was a fascinating writer, and he distilled many complex concepts down into simple forms.  What I remember from this book was his description of how a rocket gets into orbit. It's a concept many people simply fail to grasp: The "gravity turn."

A rocket may launch vertically, but in order to achieve orbit, it must travel parallel to the ground. However, when the rocket is sitting still on the ground, it and the ground it is sitting on are both in motion. Take the distance a point on the equator travels in a day; 40,075 km, and divide it by period of a day; 23hours, 55 minutes, 48 seconds. This gives you the rotation speed of the Earth at the equator; 465 meters/sec (1,675 km/hour). Traveling straight up into space wastes this velocity, and would require that we expend far more energy than necessary. Guy Murchie explained this to me when I was seven years old. A rocket standing on the launch pad is already moving, and we need to take full advantage of that fact. The sooner we can add this motion to the acceleration provided by our engines, the better. The reason for this was explained by German rocket pioneer Hermann Oberth, and we call it the "Oberth Effect." A rocket is more efficient when its thrust is applied in the direction it is already moving. We add the potential energy of the rocket's fuel to the kinetic energy of its motion, to get the total, or "specific energy." The closer our thrust vector is to our velocity vector, the more efficient our engines become! The faster our velocity when we apply thrust, the more efficient our engines are.

Thus, when launching into space, we want to do so as near to the equator as we can manage. On the equator would be perfect, and the further away we are from that, the less advantage the Earth's rotation provides.

This, boys and girls, is rocket science. No, it's not easy, but you need to have at least a passing familiarity with it, because damn it, we're a spacefaring civilization. We should behave like it.

Above, I've described two reasons why we need to perform a "gravity turn." First, we need to take advantage of the Earth's rotation. Second, we need to take advantage of the Oberth Effect. Here's how you perform a gravity turn. Immediately upon launch, or as soon as you've achieved sufficient forward velocity to do so (but before you're moving so fast that turning becomes difficult), you perform a turn of only a few degrees. Gravity will take over, and gradually pull the nose of your rocket down toward the horizon. This is why we call it a "gravity turn." There are details. There are always details, but that's the basics: Turn a few degrees, and let gravity do the work.

The gravity turn is also known as a "zero-lift" turn. This is the third reason why we perform a gravity turn. The Earth has an atmosphere. Traveling through the atmosphere in anything other than a smooth, continuous ballistic arc, will generate lift and place stresses on our rocket. The more stress we place on our rocket, the heavier we'll have to make it in order to resist that stress. The heavier we make our rocket, the more fuel we need to get it to orbit. The more fuel we need, the larger we need to build the engines... It's a vicious cycle. In order to keep our rocket lightweight and efficient, and not tear itself to pieces, we need to perform only very slight turns while in the atmosphere or no turns at all.

There's a whole lot math involved which I've entirely left out here and I'm avoiding the full discussion of aerodynamic forces. I should mention "max Q," the point at which the rocket achieves its maximum aerodynamic stress. It is a key milestone in the launch and ascent of any space vehicle. After max Q, aerodynamic stress on the vehicle declines until the vehicle reaches space, where there is no atmosphere, and thus no aerodynamic forces. The gravity turn minimizes max Q.

Finally, I should note that "in space" and "orbit" are two very different things. The gravity turn gets us to space but once there, we're not yet in orbit.  For that, we need to discuss our first orbital maneuver, orbital insertion. That will be part two of this article.