/sci/ - Science & Math

[Anonymous No.6969613]

I was thinking about the physics of maneuvering high mass objects in space the other day, and ran into a problem I couldn't quite understand.

Say a spacecraft is an x meter long, cylindrical, tube-like structure with an engine pointed out one end. Further, that the other end of the cylindrical ship has a protective dome with a central antennae; the antennae points directly where the ship is going, as the engine is pointed in the opposite direction.

Now, figure we have an object of high mass and irregular shape. Would asymmetry in the mass distribution with respect to the point of propulsion induce a spin in the object, or would it simply travel linearly but with it's "front" slightly angled to compensate for the uneven mass?

I hope that makes sense, I really don't know how to better phrase the question as my knowledge of physics is pretty basic.

The logistics of space maneuvering are fascinating, and future human craft will inevitably have methods for it.

[Anonymous No.6969618]

the object will start to spin - yes
center of gravity is important in spaceflight.

[Anonymous No.6969627]

>>6969618
Ironic

[Anonymous No.6969638]

>>6969613
those spaceships all look like guns

[Anonymous No.6970069]

>>6969618

MAY start to spin

You can launch an irregular shape without spin, so long as the center of thrust goes through the center of mass.

And for that matter, you can spin a highly regular shape by applying thrust off-center.

[Anonymous No.6971583]

>>6970069

ksp is a great tool for demonstrating this. It makes putting a shuttle-clone into orbit a whole heap harder, because the engines are still pointed to fire through the COM when the extenal fuel tank is there. However, after its dropped the COM moves. Thats why the real shuttle used its orbital manevouring system (those two smaller rockets under the shrouds to the side of the tail plane) for orbital insertion

[Anonymous No.6971592]

>>6970069
this.
Also, if the thrust was not through center of mass, it would probably work to have the thrust at a certain angle in respect to the aircraft to compensate for spin, though this would not be as efficient as having the thrust parallell to center of mass.

[Anonymous No.6972556]

>>6970069
Not OP here.

To produce spin, there would have to be other forces acting on the body, producing a moment about the centre of mass.

In a truly zero g environment, you could apply thrust that does not go through the centre of mass and produce resultant movement directly in the direction of thrust.

[Anonymous No.6972667]

Eve online ships. Don't worry much about this. The problem is usually with small objects that don't have any power to change direction.

[Anonymous No.6972697]

>>6972556
If it doesn't go through the center of mass, you generate a torque, which causes the thing to spin.

[Anonymous No.6972726]

>>6969618
>>6969627
>MFW

[Anonymous No.6972757]

You should think about the vector rather than just the point of thrust. In general a thrust will cause both rotation and linear motion. The rotation is around the centre of mass, and the linear motion is in the direction given by the point of thrust thtough the centre of mass. Only a rotation or only a linear motion can be achieved by thrusting either through the line from point of thrust to centre of mass, or perpindicular to it as necessary. 5 mins in photoshop gave the attached image, where I've drawn these three cases.

If you think about the us shuttle, it is asymmeytric, and has angled engines on the orbiter, so that it doesn't rotate and only moves forwards. Despite what you expect, the shuttle with the external tank moves diagonally, not along the direction it's pointing.

Any asymettric craft will do the same, and end up moving linearly in a different direction to that which you might expect.

[Anonymous No.6973212]

>>6972556

Not so.

[Anonymous No.6973254]

>>6972556
Others have already said it, but if it's not threw the centre of mass are torque is created to to the delay in acceleration of the centre of mass to the area initially under acceleration.

[Anonymous No.6973515]

>>6972757
The comment I'd like to add is objects do not need to have an even density and the center of mass does not need to be in the center of volume.

[Anonymous No.6973522]

Anyone has the story about the big spacecraft where the captain writes about all the shit going wrong?
>people going crazy on the kilometer long bridge
>water system clogs in the middle due to its own gravity
>each section of the ship becomes it's own country
It was a fun read.

[Anonymous No.6973659]

>>6973522

[Anonymous No.6973661]

>>6969627
The correct term is moment of inertia

[Anonymous No.6973693]

Follow up question; when is a bond strong enough for two objects with two centres of mass to turn into a singular object with the centers of mass turning into one with a new position. Does the size of the center of mass become exactly twice as big as the sum of the two old centers?

[Anonymous No.6973704]

>>6973693
the center of mass is a point

[Anonymous No.6973709]

>>6973704
it's a sphere bcus a sphere has less tension -_-

[Anonymous No.6973835]

>>6973709
oh boy.

[Anonymous No.6974265]

>>6973709

[Anonymous No.6974391]

>>6973254
Providing the link between the thrust and centre of mass was stiff enough, there would be negligible torque on the craft.

From an engineering stand point, it wouldn't be cause for concern. A large spacecraft is probably going to be pretty stiff.

[Anonymous No.6974435]

>>6974391
if off-CoM thrust doesn't produce torque, what the fuck are these for?

[Anonymous No.6974441]

>>6974435
Deorbiting.

[Anonymous No.6974454]

>>6974441
nope, that's the OMS engines' job (note that they are angled towards the CoM)

[Anonymous No.6974455]

>>6974454
forgot my image

[Anonymous No.6974456]

>>6974454
My bad.

[Anonymous No.6976407]

>>6974435
Pretty sure I'm not being retarded, but I'm willing to admit I'm wrong. Please explain this to me:

Assuming open space, away from any large masses.

Assuming the only action on the mass is applied thrust of known magnitude and direction.

In the free-body force diagram we have pic related.

ASIDE from the delay in acceleration between the thrust centre and the centre of mass (i.e. assuming the sticks are of v. high stiffness), without any external forces being applied, what produces the torque about the C.O.M.?

[Anonymous No.6976414]

>>6969613
>>6969618
>>6972757
>the object will start to spin - yes
>posts pics of Eve ships

OP confirmed spending all his time in Eve online spinning ships

[Anonymous No.6976563]

>>6976407
Think of it this way:
You place a cylindrical object on an air-hockey table. Assuming the object does not move around from the air current, we have essentially eliminated friction. Now, if you apply a forward force at one end of the cylinder (not the COM), it will rotate about it's COM. The object will move in a straight line, however, when you apply a force sideways (through the entire cylinder), the force acts through the COM, and pushes the object in a straight line.

Basically, it doesn't matter if you're in space or not, you will always get a moment when you apply a force at a distance from the axis of rotation. IE: X direction force applied at a Y distance causes a Z rotation.

[Anonymous No.6976589]

https://www.youtube.com/watch?v=lSnU_LKi5vs

[Anonymous No.6976839]

>>6976407
>what produces the torque about the C.O.M.?

That would be the Thrust and the Arm

Torque is literally Force times Distance from center of mass. If the thrust is, say, 10 N and the arm is 1 m long, you've got 10 Nm of torque.

[Anonymous No.6976961]

>>6976563
>>6976839
So what you're saying is that the inertial mass of the object basically forms a pivot about which a moment of magnitude M=F x r is produced? (providing the thrust applied is at a tangent to the of a circle radius r, centred on the COM)

I understand. Thank you.