What Makes a Rocket Go
The force which propels rockets, called thrust, has often been demonstrated with an ordinary toy balloon. If you suddenly release a blown-up balloon, the air inside will rush out its open neck. Obeying Sir Isaac Newton`s law which states that every action sets up an equal and opposite reaction, the rushing air creates a reaction force – thrust – which drives the balloon in the opposite direction.
Just as a balloon is thrust forward by expelling air, a rocket is thrust forward by expelling particles – usually in the form of a gas – from its nozzle. The greater the flow rate through the nozzle, the greater the forward thrust.
Notice that the forward motion of the balloon is not caused by the air expelled pushing against the atmosphere, the way an airplane propeller does. Even if there were no atmosphere to push against, the balloon would still zip around the room. In fact, it would even go a little farther and faster, since it wouldn`t have to push its way through the air.
Thrust is measured in pounds. In order to launch a rocket from the ground, the number of pounds of its thrust must be greater than the number of pounds it weighs. It`s like a tug-of-war between Earth`s gravity and the rocket engine`s thrust: if the engine can push up (thrust) more than gravity is pulling down (weight), the rocket will move up.
How rapidly the rocket moves up depends on how much greater its thrust is, compared with its weight. Rocket engineers call this the thrust-to-weight ratio.
The Saturn V moon rocket, for example, together with the Apollo spacecraft, weighted about 6 million pounds. The thrust of the first stage engines was 7, 5 million pounds, so the engines could win the tug-of-war and the whole vehicle lifted off. The thrust-t-weight ratio in this case was 7, 5 (million pounds of thrust) to 6 (million pounds of weight). This is the same as 5 to 4, or we could say 1.25. That means that the thrust is 1.25 times the weight.
Now you can see that if the thrust were 2 or 3 times the weight it would move even faster. Well, that is what happens as the rocket starts to move. The thrust remains the same, but since the engines are burning up propellants very fast (in the case of the first stage of the Saturn V, 15 tons every second), the weight is getting smaller all the time. That is why the rocket moves slowly at first but accelerates very fast and is usually out of sight in two or three minutes.
Another important factor is specific impulse, which we usually writ with the symbol Isp. To the rocket engineer, this means the same kind of thing as miles per gallon; it tells him how much thrust he can get from a pound of propellants. For example, if he says that a certain type of engine will give him an Isp of 300 seconds, he means that I pound of propellants will generate 300 pounds of thrust for 1 second, or 1 pound for 300 seconds, or some other combination of thrust and time that can be multiplied together to make 300. Naturally he wants to get the most thrust he can from each pound of propellant weight, so he likes the Isp to be as high as possible.
One more very important thing that is considered in rocket design as the mass ration. That simply means the total weight of the rocket loaded with propellants compared with the weight left after the propellants have all burned up. We like to have the mass ration as high as possible, because that means we get more payload placed in orbit, or we can place the same payload in a higher orbit. In order to improve the mass ration, rocket engineers often design rockets with more than one stage, because in this way they can throw away part of the weight when they don`t need it any more. When the first stage has burned up all of its propellants, it is separated and allowed to fall back to the ground. The the casing, tanks, engines, and so on for this stage don`t have to be carried up any higher. Most rocket vehicles have two or three stages.
An ordinary balloon can be used to demonstrate thrust. Photo: © Megan Jorgensen