NASASpaceFlight.com Forum
General Discussion => Q&A Section => Topic started by: cube on 12/21/2022 09:49 am
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Hello,
If during atmospheric re-entry a capsule (with a shifted center of mass to produce lift) does not produce any rcs thrust to orient itself, will the capsule naturally roll? and why?
Thank you!
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Hello,
If during atmospheric re-entry a capsule (with a shifted center of mass to produce lift) does not produce any rcs thrust to orient itself, will the capsule naturally roll? and why?
Thank you!
No, because there is no force to cause the roll.
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Are you sure? because i read an explanation someone said and he explained that the capsule (with offset center of mass) would naturally position itself with its center of mass (heavier side) down if no rcs thrust was done because the capsule without force applied by the RCS will position itself according to its aerodynamic focus, therefore according to the distribution of the masses on board.
I'm having trouble understanding this and would love to hear your thoughts on this?
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Are you asking about roll in the sense of rotation around the axis of symmetry? Or about tumbling?
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Are you sure? because i read an explanation someone said and he explained that the capsule (with offset center of mass) would naturally position itself with its center of mass (heavier side) down if no rcs thrust was done because the capsule without force applied by the RCS will position itself according to its aerodynamic focus, therefore according to the distribution of the masses on board.
I'm having trouble understanding this and would love to hear your thoughts on this?
If the capsule is heavier on one side, the heavier side will end up on the bottom with no other forces acting to offset it, due to gravity. If the capsule is aerodynamically axisymmetric then that orientation will be stable (as there will be no aerodynamic upsetting forces).
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I would say yes, in a realistic scenario it will always start to roll. The spacecraft is never perfectly symmetric. All the tiles and the small gaps with fillers are never perfectly symmetric. Wind is never in perfect standstill. So it will always start to roll.
Now, when it starts rolling (and that is more of a wild guess from me, based on the aerodynamics of curve balls) it will push more air to one side, causing more forces to act. I'm thinking of that one Falcon 9 which started rolling while trying to land and the RCS got depleted. So it will rather amplify roll. And with nothing to dampen it...
If the center of mass is of center, I would still assume it will start to oscillate.
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Need to know speed range - hypersonic, supersonic, trans-sonic or subsonic - and shape to answer this correctly.
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I would say yes, in a realistic scenario it will always start to roll. The spacecraft is never perfectly symmetric. All the tiles and the small gaps with fillers are never perfectly symmetric. Wind is never in perfect standstill. So it will always start to roll.
not really
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I would say yes, in a realistic scenario it will always start to roll. The spacecraft is never perfectly symmetric. All the tiles and the small gaps with fillers are never perfectly symmetric. Wind is never in perfect standstill. So it will always start to roll.
not really
The forces have to be large enough to overcome the spacecraft's angular inertia or it won't start rolling.
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Are you asking about roll in the sense of rotation around the axis of symmetry? Or about tumbling?
Yes I am asking about roll.
If the capsule is heavier on one side, the heavier side will end up on the bottom with no other forces acting to offset it, due to gravity. If the capsule is aerodynamically axisymmetric then that orientation will be stable (as there will be no aerodynamic upsetting forces).
During atmospheric re-entry how can gravity force the capsule to be placed with the heaviest side down (and therefore the lift vector facing down)? Why did gravity attract the heavy side more than the light side?
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Are you asking about roll in the sense of rotation around the axis of symmetry? Or about tumbling?
Yes I am asking about roll.
If the capsule is heavier on one side, the heavier side will end up on the bottom with no other forces acting to offset it, due to gravity. If the capsule is aerodynamically axisymmetric then that orientation will be stable (as there will be no aerodynamic upsetting forces).
During atmospheric re-entry how can gravity force the capsule to be placed with the heaviest side down (and therefore the lift vector facing down)? Why did gravity attract the heavy side more than the light side?
just as a dart flies with weight forward. the center of pressure offset is going to aligned it.
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Are you asking about roll in the sense of rotation around the axis of symmetry? Or about tumbling?
Yes I am asking about roll.
If the capsule is heavier on one side, the heavier side will end up on the bottom with no other forces acting to offset it, due to gravity. If the capsule is aerodynamically axisymmetric then that orientation will be stable (as there will be no aerodynamic upsetting forces).
During atmospheric re-entry how can gravity force the capsule to be placed with the heaviest side down (and therefore the lift vector facing down)? Why did gravity attract the heavy side more than the light side?
Because entry is not microgravity/freefall, so objects will be affected by gravity as normal. The lift vector will point up, with all capsules designed to lifting entry (e.g. Apollo).
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It seems to me that gravity accelerates mass always at the same intensity regardless of the amount of mass so I don't see how gravity attracts the heavy side more than the light side?
If this is true that would mean that even in orbit the capsule would naturally orient its heaviest side towards the Earth?
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It seems to me that gravity accelerates mass always at the same intensity regardless of the amount of mass so I don't see how gravity attracts the heavy side more than the light side?
If this is true that would mean that even in orbit the capsule would naturally orient its heaviest side towards the Earth?
Jim already answered your question but you should watch this..and it's not "gravity attracting the heavy side".
https://www.youtube.com/watch?v=MTKHqfloB7Q
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The formulation " heavy side down" is wrong. Gravity does indeed act through the center of gravity which is the same as the center of mass which the capsule rotates around and it can therefore not affect roll*.
A better formulation would be that the "lifty side points up". If the center of lift is offset from the center of mass the lift will induce a torque unless the gravity vector, center of gravity, center of lift and lift vector all align.
*A gravity gradient can induce torque (i.e. gravity stabilized satellites) but these forces are insignificant in this context.