I think that that's just an ordinary flywheel. At small sizes I think they're better because they can sustain higher rim accelerations.

I think 100% of spacecraft in a decaying orbit are going to perform a near miss or worse with an orbital ring- as the apogee decays, it's certain to go through an altitude where the ring is, and because an orbital ring encircles the Earth, it's always going to be there. It seems to be worse than the Space Elevator in that regard.

And its worse the heavier the reentering object is- heavy objects have a higher ballistic coefficient, and hence lose less altitude per orbit, and hence are more likely to strike the ring.

Mmm. The problem is, that when something gets hit by something in orbit, essentially both the hitter and the hittee both explode, and generate a large shower of fragments that are both more or less in the original orbits, plus or minus some delta-v due to the explosion.

So the fragments of the projectile will tend to continue forward and, while many will reenter, many will not, and are likely to impact the spokes, or after the orbital planes have been perturbed by standard orbital mechanics, will impact the orbital ring from a different direction at very high speed causing a fresh shower.

Basically, you've got a runaway space debris situation from the orbital ring itself back onto itself, Kessler syndrome.

I'm not sure that they're practical though. The problem with orbital rings is that they're in orbit, so they're subject to space junk and micrometeorites.

Launch loops are low enough down in the atmosphere that while space junk and meteorites could hit one, the debris has only got one chance to do so, whereas with orbital rings it can just keep going around until it gets lucky.

If I've done my maths right, the radius goes up by the square of the velocity (acc = v^2/r, rearrange r = v^2/acc)

But the energy density per unit mass goes up by the square of the velocity too (0.5 m v^2)

So the total energy in the ring actually goes up with the fourth power of velocity and the mass goes up on a square law (the mass per unit circumference is constant).

Oops that should be square of the velocity, but the general argument is still correct.

It's a bit of a shame that the energy density isn't higher,  it doesn't even seem to compete with batteries, never mind gasoline at laptop sizes for example.

Still, for stationary use it might work; and as you scale the radius up it becomes much better I think.

Well, the g-force on the rotor in the deflection coils goes as the square root of the velocity (centripetal force). As the speeds rise, for any give acceleration in the deflection coils, the size of the ring needs to increase.

So the limit on the acceleration is the deflection coils, which in turn is limited by how intense the magnetic field can be made, which is limited to about 1-10 Tesla (ish) (~1 T is permanent magnets, ~10 T is superconducting magnets which may be expensive to run, or very heavily cooled electromagnets, but they're of little use here).

Net upshot is that a storage ring needs to be pretty big to get decent velocities/energy storage; km scale.

But you can demonstrate the basic technology at meter scale; but just not the high velocities.

You could.

Lofstrom chose to do it that way because the low level sheath needs to be thicker to carry the full atmospheric pressure, and he says this makes it more awkward to also carry payloads along it.

Yes, that's Lofstrom's plan, there's an elevator up the side of the launch loop to the 'knee' where the cable flattens out, which is separate from the rotor.

Another thing that might work/help is to maintain the rotor at a different voltage to the sheath; that way after they've been ionised by impacting the rotor they will end up being moved to the rotor or to the sheath and collisions will stop.

So, plate the rotor with nickel and then boron on top? Sounds easy. :-)

Where's out? Out is out; into the air.

There's no obvious limit as to how fast or often you can pump molecules out, you can do it every few centimeters if you want. Just a scoop into a ventilation duct is all that is needed; the molecules hit the scoop, and yeah, maybe erode it a little, and then guide the rebounding molecules into the duct, where it meets lots of other joyous molecules bounding along to a vacuum pump that dumps them all out. As they travel down the duct they will cool right off.

The less molecules there are permitted in the sheath gap, the less erosion there is; the less erosion there is, the less molecules there are.

It seems like it would be a very efficient system, and while the temperature of the plasma would be very high, by controlling the number of molecules involved, erosion should be eminently controllable.

But clearly, the launch loop will need periodic maintenance; once a year or so or maybe a lot longer.

Show me hot gas moving sideways at 7.8km/s and I'll show you some gas heading for a scoop to take it all out, never to return.

So on the contrary, I think you can use the rotor as a stupidly efficient turbomolecular pump to *pump* gas out of the structure, and, possibly with a secondary pump to lower the pressure even further, you can maintain unbelievably low pressures; there has never been a pump as efficient as this one!

And since erosion is a strong function of pressure, I don't see how this can be an issue at *all*. :-)

So you're claiming that more energy would come out than goes into the entire loop in the first place???

Well, nobody is denying that some erosion won't occur; but I think you are kind of exaggerating the scale of any potential problem rather ;-)

Some existing ion drives *do* accelerate to higher velocities than that, and do indeed suffer erosion over time (a few months). Nobody is saying that such erosion can't occur, or won't occur, but with a launch loop you are deliberately *reducing* the pressure, whereas with an ion drive you are deliberately *increasing* the pressure. Any erosion or heating effect is going to be a positive function of pressure.

You have to put the temporary sheath back on, by the look of it- there's a high altitude vacuum sheath, but that would presumably implode at low altitude. So you would either have to dump the rotor and then you can let the air into the high altitude sheath on the way down, or else put the low-level sheath back on as you slow it all down as you say.