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Name: Ondøej
Status: student
Age: N/A
Location: N/A
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Question:
Hi, I was always wondering about various design of the spaceships in movies and games. Especially about the propulsion. Because many ships have asymmetric design there. My question is how is the center of the mass at the ship affects the engine. For example if the ship have a shape of a triangle and none of the side is the same size and the density of the material is the same everywhere on the ship. Does the position of the engine matters? I mean if I place one big engine at the end of the one side will it cause the ship to spin or will it start moving forward? Does the center of the mass of the ship affects it? Does the ship have to be symmetric in shape or it can have one huge engine on one end of the side and small one at the other?


Replies:
Hi Ondøej

You seem to suspect that some of the space ship designs you have seen in the movies, etc., may have some stability problems. I would have to agree, since these "designs" are not generally based on any scientific principles. There is one basic rule that is required if a rocket of other space vehicle is to travel in a stable fashion. Imagine that all the engines were replaced by a single engine whose thrust and direction resulted in exactly the same performance as all the engines in the spacecraft. In order to achieve stable performance, the new engine's direction of thrust must pass through the spacecraft's center of mass. The more formal way to say this, is that the vector sum of all engines' thrust (the combined effect of their magnitude of thrust and their individual directions of thrust) must pass through the spacecraft's center of mass.

It is possible to have (let us say) one engine that has twice the thrust of a second one. But in this case, the two engines must be positioned on opposite sides of the center of mass, and the weaker one must be placed twice as far away from the center of mass than the stronger one is on the other side. The resultant thrust will be the total thrust of both engines together, and the direction of thrust will pass through the center of mass. If the resultant thrust does not pass though the center of mass, a "moment" will be created that will cause the spacecraft to spin or tumble.

Regards,

Bob Wilson


Movie makers are poor spaceship designers. To accelerate a space ship without also spinning it, the line of the engines' thrusts must pass through the ship's center of mass. This is a difficult thing to get right especially if you want cool looking ships like a Borg Cube (Star Trek Next Generation).

Some kind of thrust vectoring (bouncing the exhaust against a plate attached to the spaceship) could allow a space ship's line of thrust to not pass through the center of mass, but that would use up precious thrust and make the ship slower to accelerate.

Of course, if writers avoid the crew ending up as grease spots on a rear bulkhead during acceleration by inventing the "Inertial Compensator" (original Star Trek TV show), who is to say any design might not work?

An interesting and fun question! I hope this helps.

R. W. "Bob" Avakian
Instructor
B.S. Earth Sciences; M.S. Geophysics
Oklahoma State Univ. Inst. of Technology


For a rigid body, the resultant thrust force of all engines must always act through the center of mass of the body Or else the body will start spinning.

Here is an interesting presentation of the concept of Center of Mass:

http://en.wikipedia.org/wiki/Center_of_mass

Sincere regards,

Mike Stewart


Hi Ondoej,

You are correct, if a force (such as a rocket or other propulsion) acts on a spaceship at an angle relative to the center of mass, it will cause the space ship to rotate. However, you can adjust for an angular force using additional propulsion sources (additional rockets for steering, etc.). Alternatively, you can use multiple propulsion sources, you can reposition the location of the propulsion, or you can aim/direct the propulsion to counteract rotation.

Hope this helps,

Burr Zimmerman



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