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What is drag in aircraft, rockets or spacecraft like compared to drag for these on earth? How is the air/wind different?
Question Date: 2018-01-08
Answer 1:

This is a good question. I assume here you want to compare the drag on Earth and in space. The short answer is the total drag will be much smaller.

The drag in aircraft is complicated and consists of many contributions that resists the motion of aircraft. I will talk about a few type of drags:

1) Parasitic drag. It is the resistance force the aircraft experience when it is moving inside the medium, which is air in this case. There are several contributing factors that determine the drag force: the form of the aircraft, relative velocity, the density of the medium (air).

a) form drag: If you have a really bad design with a large cross section along the motion direction, you will certainly experience stronger resistance because of the air, which means larger form drag. This part will not depend on where you are, in space or on Earth.

b) skin friction: when the aircraft moves inside the medium, it will experience the friction. This type of drag will depend on the relative velocity. The higher the relative velocity, the stronger skin friction will be. If the wind is blowing in the same direction, the drag due to the sink friction will certainly be smaller and vice versa. Of course, the skin friction will also depend on the air density. Think about that, if the density is close to zero, as in space, little or no friction will exist. So in short, the type of drag will be much smaller in space with almost zero air density.

2) Lift induced drag: If an airplane wants to fly and stay in air, you need the wings (and other parts) to create the lift to counter the gravity, otherwise it would be extremely difficulty for an airplane to stay in air. The lift will also project part of its force against the moving direction. Such force is called lift induced drag. This type of drag will be inversely depend on the air density in a reasonably range. However, this type of force will be much smaller when the relative velocity is much larger. Assuming everything is the same, you would expect larger lift induced drag. But in reality, since on Earth you need large speed to create lift to counter the gravity, lift induced drag is not that big. While in space the gravity is much smaller, thus it does not require that amount of lift to counter the gravity, therefore, I would say the lift induced drag is also much smaller in space.

3) Wave drag: This is the drag we have when the aircraft travels at the transonic and supersonic speed due to the presence of shock waves. In space, the air density is close to zero. No shock waves will come into existence because the lack of air. This this type of drag will also be much smaller in space.

In summary, the total drag in space will be much smaller. M

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