Articles: Importance of Balance
Before a Mars airplane can fly over Mars, it has to get to Mars first. Since it can’t travel by itself, it has to be packed inside a container, or aeroshell. This aeroshell is put on a spacecraft that flies from Earth to Mars. A propulsion rocket carries the spacecraft out into space.
As the rocket blasts into space, everything onboard the spacecraft must be carefully packed to withstand the force of launch. This force must be strong enough to escape Earth’s gravity. Gravity is a force that is always pulling things down toward the center of the planet so they don’t go floating off into space. Being in a rocket is a bit like riding on a roller coaster. It takes off suddenly and goes very, very fast. You have to be strapped in and hold on tight, or you get bumped around. To escape the gravity of the Earth, a rocket must go almost 300 times faster than the fastest roller coaster ride, so making sure everything is safely packed is very important.
Once the rocket gets beyond the Earth’s atmosphere, there is no drag because there are very few gas molecules to fly through. The spacecraft lets go of the propulsion rocket, which is no longer needed. But it does use smaller rockets throughout its journey to help it navigate.
Once the spacecraft reaches Mars’ atmosphere, the aeroshell’s packing and balance become very important. When the aeroshell is released into Mars’ atmosphere, it will spin out of control if the airplane inside it was not packed to balance properly. This is because gravity pulls on an object’s center of mass, which is where the object is the heaviest.
You can find the center of mass of a ruler by putting the ruler on your finger so it balances. Your finger will be at the ruler’s center of mass. Now put an eraser on one end of the ruler. You’ll see that the other end of the ruler flips up. You either need to move your finger or put erasers on both ends to balance it properly.
It’s the same with a can of soda. If the can is either full or empty, you can roll it in a straight line. But if it’s only partly full, it will wobble and swerve when you try to roll it as the soda inside sloshes around and throws the can off balance.
The airplane must be carefully balanced when it is packed inside the aeroshell so that it falls safely toward Mars. One side of the aeroshell cannot be heavier than the other. Any other cargo onboard the airplane, including fuel and research instruments, will change the airplane’s center of gravity. These must all be loaded carefully so the airplane is stable when it leaves the aeroshell and starts flying. If the airplane is unbalanced, extra weights could be added to the inside of the aeroshell to help stabilize it.
There are several ways to pack the airplane inside the aeroshell. These include telescoping and inflating. The airplane can be designed like a telescope, so each part fits inside another. Or it could be designed so that it inflates like a balloon. When it gets into Mars’ atmosphere, its parts can quickly expand outward and it can start flying. One of the problems with this method is that the airplane may not expand completely. If just one part doesn’t expand, the airplane may not fly.
Another method is to fold the airplane up inside the aeroshell. However,
it is important to use as few folds as possible. The more folds you
have, the greater the risk that one of them might not unfold when the
airplane gets to Mars.
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Editor: Brian Day
NASA Official: Liza Coe
Last Updated: September 2005
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