In theory

[magnemoe]

18 hours ago, DerekL1963 said:

Well, it's theoretically a thing.  Experiments with using actually using tethers on orbit have not in general fared well.   If anyone's actually used one, I can't find any traces thereof.  (Lots of bold predictions, no actual data.)

Think they are used on some cubesats simply to increase orbital decay after end of life. 

[PB666]

15 hours ago, magnemoe said:

Think they are used on some cubesats simply to increase orbital decay after end of life. 

I think the Italians tried it, they tried to inflate a parachute in space, the problem in space is that the particles are not cohesive, consequently there is no static pressure (uhmm I thin that is why its called space).

[DerekL1963]

13 hours ago, PB666 said:

I think the Italians tried it, they tried to inflate a parachute in space, the problem in space is that the particles are not cohesive, consequently there is no static pressure (uhmm I thin that is why its called space).

Uhmm...  if there's no static pressure, then why do satellites deorbit due to drag in the first place?  (Answer: you don't need static pressure to create drag.)

 

On 12/23/2017 at 3:14 AM, magnemoe said:

Think they are used on some cubesats simply to increase orbital decay after end of life. 

They've been tried, no success (mostly due to tether deployment problems).

[PB666]

40 minutes ago, DerekL1963 said:

Uhmm...  if there's no static pressure, then why do satellites deorbit due to drag in the first place?  (Answer: you don't need static pressure to create drag.)
They've been tried, no success (mostly due to tether deployment problems).

They do deorbit over days and weeks, and in general it takes very little dV for a space craft at LEO to change its periapsis to the point the craft will enter the atmosphere. The earth is 6,000,000 meters in radius and you only need to change radius by 100,000 to 400,000, that is the change of a of only 0.83% (dv<100 m/s) if you divide that by a week  that an acceleration of 0.0001 m/s. If you were to consider space as air and that specific force = k*m*v2  where v = 7800 then the k*m proportion of the 10^-12  either the mass in space is rather ineffective at slowing things down or there is very little of it, we know the second is true, there is very little static pressure everything in space is in high relative motion, the forces that are created are not created by compact wholly elastic collisions but by molecules colliding at high speeds, speeds sufficient to create plasma and force chemical reactions on the surface.

Its all about boundary layers and fluid motion. This is all old hat, pretty much since the first attempts to pass Mach speed it was notice that fluids moving at high speeds do not behave as lower speeds. In the case of the nose of a rocket the boundary layer separates, that layer determines 'whats' fluid and 'whats' craft. The effective mass of the space craft increases, but more importantly the pressure on the nose cone increases because the surface that 'sees the flow of air' effectively increases. Two molecules are bound together because they are under the boundary layer, they cannot move laterally as fast as the vessel moves forward, they slow down relative to the outside and for a moment become bound to the ship. The important point here is that if you want to control the behavior of an object in fluid then one aims at flows that are laminar and boundary layers that can be relatively easily predicted. As speed increases the only real way to predict behavior is to use a Sears-Haack shape and a parachute is not that. I suppose you could have a very long and wide Sears-Haack shaped balloon hanging from a long tether, but that defeats the point of drag. Secondarily as the drag increased the object would want to fall below the space-craft, pulling it -Radially and very little retro, the craft would be pulled back wards and the balloon vertically. Thus as force increased the tether would be behind the space craft on its Z-axis, which the craft has already cleared of molecules, which means the drag rate would fall. The point about drag in space is that for reflections you want to create oblique angles relative to the prograde vector. However if you create too oblique of an angle you would loose control, therefore you need to both control the reflections (such as the rolling backwards of a heat shield) and have rigidity with respect to the space craft.

The proof of the pudding is in the eating, one way Space X handles space craft control flow at superMach speed is with grid fins, rigid structures that protrude. To create more drag in space you need more contact surface. If the crafts orientation is Z to the direction of travel then the drag is created by surfaces in the XY plane (IOW along the XY component of its surface projections), therefore to slow the craft down faster it would need more surface and surfaces that have at least 2 rigid components each. A simple solution is to have folded aluminum foil, thin as possible, or even plastic, sets of two poles on two hinges that are orientied in the Z direction when folded and that -Z relative to the space crafts direction of travel (dZ/dt by definition is always positive since the coordinate system is spacecraft specific). When deployed the poles expand, the tip of each pole (starting at -1z unit vector relative to base) would change dZ = dXY so that one set of poles would have moved 0.707x, 0.707z unit vectors and 0.707y and 0.707z. (this means Z relative to the hinges coordinates the end of the pole is -.293z), they tile back away from the direction of travel.  Four such sets would create a set of spoilers around the space craft. In this system as the craft turns the pressure on the leading foil would increase while on the lagging foil would. Another way is to create an inflatable but flattened donut around the space craft that is also at -Z relative to direction of travel and tiles backward.  

However by the time you have done this, even using carbon fiber, you have added more weight than the added rcs fuel to burn periapsis back to 6480000 meter where the crafts orbit would decay much faster. If you are so high in orbit where the RCS to burn back is too high, you are also too high up for increased drag to be effective, its effectiveness is the square of the speed times the density, both of which are lower in high orbit, so its best to RCS these space craft to a graveyard orbit and await a cleanup space craft. Grid fin like structures make sense in spaceXs situation because the craft does not have to reach all the way to orbit, the fins are light relative to the weight of fuel, it cannot use parachutes due to weight and durability and because fairly heavy grid fins will certainly encounter heavier atmosphere as part of the need to land. A deorbited spacecraft almost never needs to land.

There is a situation however where this might be useful. We can imagine clean up space craft, its ION driven and has solar panels, behind the craft is a wiremesh that can be expanded to allow more junk, in front of the craft is a hole that opens to allow stuff in. Lest assume that each bag is lightweight, so that the cleanup craft can carry several of these. Once the bag is full of space debris the space craft descends to its lowest possible safe altitude and releases the bag. Remember that lowest possible safe orbit, even for an empty ION drive spacecraft is not very low, since its typical accelerations are in the 0.0001 to 0.01 range.  By the time the space craft finishes that bag is heavier and considerably more dense that the original space craft, and you would not want the bag to collide with other objects because in doing so it would scatter debris. Embedded in the perimeter garbage enclosure you could have very thin-walled inflatable manifolds that swell up and increase the rate of drag, the manifold could be a flatten do-nut shape that is released from the outside of the cage. This then could lower the spacecrafts orbit so it is no longer a threat to other orbiting spacecraft; however, it would not be able to control its point of reentry. Thus at some appropriate point it might be useful to draw the bag back in to control where it lands (cause no one really wants a bunch of radioactive spent spacecraft to come flying down into their major metropolitan area).

Notice that in all of these designs, none employ tether, tethers lack the ability to govern control.

[DerekL1963]

4 hours ago, PB666 said:

[A great deal of irrelevant verbiage snipped]

That's all very nice...  But has nothing to do with anything I said or with the topic at hand.  Boundary layers and fluid motion only apply where the atmosphere is thick enough for those phenomena to occur - many, many miles below the altitudes under discussion.

 

4 hours ago, PB666 said:

Notice that in all of these designs, none employ tether, tethers lack the ability to govern control.

Which isn't a problem, because tethers aren't intended to provide control.

[mikegarrison]

Boundary layers and static pressure and such apply to fluids. Air acts like a fluid when there is enough of it, but up at LEO there isn't enough of it. Up there it acts like a bunch of particles instead of a fluid. So concepts like viscosity, boundary layers, static and total pressure, etc. don't really apply. Those are all mathematical constructions that describe the behavior of fluids.