Packages of Energy
To achieve the greater and greater velocities required to send a given mass farther and farther out into space, higher thrust must be exerted or it must be applied for a longer period of time.
The principles of physics tell us the longer thrust is exerted, the faster a rocket will go. For example, providing its mass does not change, a vehicle starting from rest under constant net thrust will be going 100 times faster after 100 seconds than after one second.
For rockets, mass does not remain the same; it decreases. As each kilogram of propellant is burned, the mass of the ascending vehicle becomes one kilogram less, and large rocket engines may burn hundreds of tons of propellant in seconds. As a result, the same thrust has much less mass to accelerate. The decrease in weight allows an increase in acceleration.
This principle is used in the concept of “staging.” A space launch vehicle usually consists of a number of sections known as stages. After the first rocket stage accelerates the entire vehicle and finally burns out, its tanks and motors are discarded because they are no longer needed and add unnecessary weight that slows acceleration. The same happens in the second and subsequent stages. The payload is finally all that remains, having been accelerated to the necessary velocity.
The Titan IV heavylift space launch vehicle looks like three rockets joined together side by side. On the outside are huge solid-fuel motors with a combined thrust of more than 3 million pounds. When these burn out, they fall away from the central core, which then continues the mission. The middle part is itself a two- or three-stage vehicle, but liquid-fueled.
Before the Titan IV booster, a smaller version of the Titan family (Titan III) was used on the Viking spacecraft, which was used to ”soft land” instruments on Mars. The Viking spacecraft was mounted on a Centaur rocket, which in turn was mounted on top of a Titan III booster. In this case the solid motors burned and dropped away; then each stage of the Titan III core burned and dropped away until the Centaur and its Viking Mars spacecraft continued alone. At the proper moment, Centaur’s own engines were started and imparted the additional thrust to give the necessary velocity for the spacecraft to continue to Mars.
In addition to mass, other factors affect velocities necessary for space operations. The higher a rocket goes from Earth, the thinner the atmosphere becomes, decreasing the amount of friction or drag imparted on the rocket.
In space there is very little if any atmosphere and essentially no friction or drag. The pull of gravity becomes less the farther you move away from Earth’s center. This effect is not significant for satellites orbiting a few hundred miles above Earth, but it becomes important if the spacecraft is to be sent thousands of miles into spacce.
Packaging the energy of a rocket vehicle into stages that can be discarded as they burn out has been the secret of launching into space. The number of stages may vary from two to five or even more.