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COM Shifting

Adjusting the Centre of Mass to Aid Manoeuvrability
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In outfitting a space-craft, a number of thrusters need to be applied in order for the craft to manoeuvre.

Traditionally, pairs of thrusters are set up about an axis in order to impart spin, and central thrusters added to apply directional thrust. To combat different loading configurations, each thruster may have a certain amount of directional adjustment.

To aid manoeuvrability, this idea is to install a central structure within the craft (or rather, to build a craft about this structure) that contains weights that can be moved about along axis of direction. The structure should have at least 3 lengths, each at 90 degrees to one another, stretching into the x,y and z axis. In or perhaps instead, a circular configuration might be used to provide a different set of properties.

Now when a manoeuvre is performed, the weights within the structure can be shifted in order to provide the maximum turning force for a single or pair of thrusters.

Control is assisted by computer, so that the spacefarer only requires traditional flight controls.

Since weight is an important factor in space travel, the weights themselves should serve a dual purpose, perhaps them also being employed as fuel, oxygen, or some other storage. Using lighter materials can be offset (for turning purposes) by using longer dimensional lengths.

In addition to aiding turning and manoeuvrability, the COM shifting principal can also be applied to assist in balancing ships of a non-uniform shape.

N.B. The Other:Spacecraft:Propulsion category was chosen due to this idea being related to the movement of spacecraft.

zen_tom, Aug 11 2004

Foam Hulls http://www.halfbake...Hulled_20Spacecraft
Shameless link [zen_tom, Oct 05 2004]

Using Tethers in Space to Control Center of Mass http://www.sciam.co...FA-89FB83414B7F0000
.. COM control for artificial gravity and use of tether for propulsion .. [bpilot, Oct 05 2004]

www.gyroscope.com : Gyroscopic Orientation Module http://www.gyroscop...m/d.asp?product=CMG
"Rotating a spacecraft in space can only be achieved in two ways. Firstly thrusters (small rocket motors) that are positioned at points around the spacecraft can be briefly fired to rotate the spacecraft. This has the drawback of using some propellant which will eventually be used up. This clearly needs careful planning as any extra weight on a spacecraft by its very nature is inhibitively expensive. It also means that the thrusters are used when only necessary as it will reduce the spacecraft’s life (assuming it can’t be refuelled) or at huge cost (if it can be refuelled). The second solution is to use a Control Moment Gyroscope (CMG). This is one or more gyroscopes that are forcibly rotated using an electric motor. The resulting gyroscope torque rotates the spacecraft. Up to 3 axes can be controlled this way. Extra gyroscopes can be used for redundancy. The benefits are clear; Provided the spacecraft has electrical power, by solar power for example, then the aircraft can rotate. With a CMG this can be gone on a regular basis or simply used consistently to keep the spacecraft pointing in the same directional. " [zen_tom, Oct 31 2013]


       I don't understand how altering the centre of gravity will aid manoeuvrability or balance.   

       Are there any required assertions that you haven't stated? eg, that thrusters are too weak for ideal manoeuvring, and/or too strong for ideal balancing? And that swifter rotational manoeuvres are desirable?
benjamin, Aug 11 2004

       Center of mass, surely.
half, Aug 11 2004

       [half] yes you're quite right - edited name/description accordingly (from gravity to mass). and [benjamin] yes, I suppose the core requirement here is for swift rotational movements - or rather providing an improved performance for such moves. No, it's not a huge breakthrough, craft have been rotating succesfully for years, rather, just a possible improvement.
zen_tom, Aug 11 2004


       One thing I'd like to point out is that moving a weight around is conceptually identical to moving the thruster around. In both cases, there are two objects that are being moved in relation to each other with the thruster being attached to one of them. In your case, the object with the thruster attached is much more massive than the other object. In the case were we would say that the thruster is being moved, the object with the thruster is much smaller than the other object.   

       This is probably a good way to look at the problem. It could make a lot of sense to mount the fuel tanks on the same object as the thruster to avoid flexible fuel lines. It may even be desireable to have the crew compartments on that object as well. The only limitation is that as the object with the thruster gets more massive, the distance that the other object must move to get the same effect gets larger. At the same time, the required strength of this connection is less as the object without the thruster gets smaller.
scad mientist, Aug 11 2004

       [contracts] the purpose is primarily to help with orientation - inertial trajectory really shouldn't be affected. It's the adjustment of the centre of mass coupled with the position of the thrusters that allows you to, for example, spin about face, or perform some other manoeuvre (I hate spelling that word) with slightly more agility than might otherwise be possible.   

       In other words, providing agility and grace of movement as an alternative to the 2001 Blue Danube/lander style 'gently does it' imagery of space-flight is the main goal of the idea.   

       [scad m] Yes, you could consider having the thrusters themselves able to slide along an extended axis - and the effect would be the same. Using both (by having the centre of mass moved along one axis, while the thruster moves to the opposite end of that axis) would provide the maximum benefit.
zen_tom, Aug 11 2004

       If a single thruster is used with the thrust vector offset from the CG, the thruster will exert a torque on the structure dependent on the distance from the CG, as well as a lateral thrust in the direction of the thrust vector. This lateral component is usually undesireable, so a pair of thrusters is used, so that the lateral components cancel each other, and the ship experiences only a net torque. When two thrusters are used in this manner, the position of the CG is not an issue. Only the relative offsets of the pair of thrusters has any effect.
Freefall, Aug 11 2004

       What about using large mobile weights on a spacecraft to change its angular 'heading' without using thrusters at all?
supercat, Aug 12 2004

       See link for an excellent Scientific American article on use of tethers in spacecraft for artificial gravity, power generation, propulsion and orbit control. The artificial gravity application is most directly connected to adjustment of COM.
bpilot, Aug 12 2004

       I can't see this working with the linear channels. You mention circular channels for the counter masses to run through (which implies a spherical spacecraft) and that might work, but you'll consume the same amount of energy.   

       Also, thrusters can alter trajectory. These counter masses will only allow you to change orientation. This probably wouldn't matter to the Hubble, but probably wouldn't be practical for a spacecraft.
phoenix, Aug 12 2004

       [bpilot] I couldn't access your link without subscribing to SA.   

       Also, there appear to be two (or more?) schools of thought forming here:   

       The traditionalists, like me, who are considering using COM shifting, coupled with thruster use, to increase the agility of a space-craft.   

       And a bold new school, where no thrusters are used, but the ship is able to shift itself into different orientations by moving mass around - a bit like trying to spin on an office chair without touching the floor. Is this right? Or have I missunderstood something?
zen_tom, Aug 13 2004

       Big revolving masses are indeed used to reorientate spacecraft without the need for external thrusters.
david_scothern, Aug 13 2004

       //to keep the spacecraft pointing in the same directional.   

       Does that actually matter? It`s not like they`re doing line dancing up there, or there`ll be (significantly) higher drag?
not_morrison_rm, Oct 31 2013


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