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Spaceship Games and Propulsion
Name: Ondøej
Status: student
Age: N/A
Location: N/A
Country: N/A
Date: N/A
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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Update: June 2012
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