The launch loop

[Roadam]

Moderation note: The first four post of this thread originally appeared in the poll thread “Most likely candidate for future spaceship propulsion”. They were moved because it’s a separate subject, deserving of its own thread.

A while back I come upon an article in wikipedia about the launch loop. Its a 4k km long metal loop of the thickness of about 5 cm and covered by a sheet magnetically suspended over it. The idea is to have two stations at the each end of a loop, which would accelerate the metal via linear magnetic accelerators. The acceleration would be somewhat upwards so that the biggest stretch of the loop would be about 80km above. For the stability, a number of cables would anchor the loop onto the ground.
If someone would want to accelerate the spacecraft all you would need to do is to "sit" onto the cable and press a magnetic brake onto it. As the cable would be moving at big speed it wouldnt slow down much, but rather you would accelerate.
Thats the short info on it.

As the author claims it is more or less possible to build it at the current level of technology It would be impossible for me not to be enthusiastic about it.

Launch loop - Wikipedia, the free encyclopedia

[Roadam]

What do you guys think about launch loop? Or is this thread just about moving things trough space.

[Moontanman]

Originally Posted by Roadam

What do you guys think about launch loop? Or is this thread just about moving things trough space.

I'm interested lets hear about it.

[Roadam]

I already did wrote something about it

Originally Posted by Roadam

A while back I come upon an article in wikipedia about the launch loop. Its a 4k km long metal loop of the thickness of about 5 cm and covered by a sheet magnetically suspended over it. The idea is to have two stations at the each end of a loop, which would accelerate the metal via linear magnetic accelerators. The acceleration would be somewhat upwards so that the biggest stretch of the loop would be about 80km above. For the stability, a number of cables would anchor the loop onto the ground.
If someone would want to accelerate the spacecraft all you would need to do is to "sit" onto the cable and press a magnetic brake onto it. As the cable would be moving at big speed it wouldnt slow down much, but rather you would accelerate.
Thats the short info on it.

As the author claims it is more or less possible to build it at the current level of technology It would be impossible for me not to be enthusiastic about it.

Launch loop - Wikipedia, the free encyclopedia

http://www.launchloop.com/isdc2002loop.pdf
Launch Loop

I think its a very cheap access to orbit. From there, you are already halfway to anywhere. It doesn't take as long to get there either as its much faster than any space elevator proposed.
Author estimates costs at about 10 to 30 billion dollars. Without research that is. The whole shuttle program cost 150B up to now.
The problems he mentions are controlling harmonic motions of the loop and persuading people to build it.

[CraigD]

I think the launch loop is a very cool idea, promising a somewhat inexpensive system (Similar in cost to the Space Shuttle program) capable of launching unprecedented manned or unmanned masses into space.

The main problem I can see with it is whether its most basic component – a 5 cm iron belt/rotor moving at 14000 m/s (about mach 41) inside a magnetic bearing sheath – can work. IMHO, the potential of such a rotor/sheath/bearing system to fail via some difficult to predict manner – oscillation, kinking, etc – as well as some basic mechanical issues such as frictional heating and heat dissipation, are critical issues that must be addressed, and are difficult to address. While the basic mechanics of the system are simple, the details in this case are, as the saying goes, full of devils.

Before anyone can have much of an informed opinion on the details of the proposed system, I think some proof-of-concept engineering prototyping needs to be completed – actually fabricate a few hundred meters of the system’s rotor/sheath/bearing, and accelerate the rotor to a few hundred m/s (sub mach 1). Until such a milestone can be achieved, the idea is so highly speculative that sweating other details – such as how to make a 80 km high elevator to hoist spacecraft to the loop’s launch station – is, I think, premature.

Being premature doesn’t stop considering the details from being fun. Assuming the basic rotor/sheath/bearing is feasible, an interesting consequences of basic geometry is that the endpoint will need to move a combined distance of from 30 to 50 km (depending on the slope of the loop between the endpoints and its 80 km high launch section) while the system is being spun up. Assuming it’s deployed in the open ocean, this suggests that the base “turnaround” stations will need to be huge floating platforms that can be fairly rapidly moved via a series of anchored cables as the loop lifts off the ocean surface as it exceeds about 8000 m/s or so (the orbital speed of an orbit at the Earth’s surface is about 7905 m/s), this in addition to the many stabilization cables that will need to be adjusted as the system rises to operational height. It’s an amazing feat to imagine.

[Roadam]

According to the author. Both stations would probably be stationary. Maybe using islands for support.
There is no need for any elevators as space ships would just sit on the cable at one of the stations and turn on the magnetic brakes.

But yeah, Bearings and control are tricky.

[freeztar]

How much force would average human passengers experience with such a system? Would humans even be able to survive such an acceleration?

[Roadam]

Originally Posted by freeztar

How much force would average human passengers experience with such a system? Would humans even be able to survive such an acceleration?

The loop is 2000km long just because of that. 3g at trans lunar orbit velocity launch -> 14 km/s. 1.5g at insertion into low orbit. I guess 8 minutes at 3g acceleration wouldn't be too harsh.

I am thinking about ways to make this thing more redundant. Some kind of a self dampening, more stiffness...

Also I am not yet sure how does author achieve levitation, as ferromagnetic metals are usually attractive.

[CraigD]

Originally Posted by Roadam

There is no need for any elevators as space ships would just sit on the cable at one of the stations and turn on the magnetic brakes.

Now that you mention it, it does seem that rather than what Lofstrom described in his 1985 paper, “THE LAUNCH LOOP: A LOW COST EARTH-TO-HIGH-ORBIT LAUNCH SYSTEM”

The vehicles are brought up these cables rather than up the west incline to simplify the spacing controllers on the incline. Other benefits of this approach are minimized incline weight, shorter upward transit times, and less likelihood of sheath damage.

it would be better to use the sloping sections of the loop as elevators, using the same magnetic induction brake that the to-be-launched ship uses on the main track.

Lofstrom’s reasons for proposing an elevator appear to center around two features: the necessary differences between the lower, sloping sheath, which must be much thicker in order to maintain a vacuum and be weatherproof, and the upper, which is in near vacuum at 80,000 m altitude, so need only provide magnetic bearing to keep the rotor in a smooth path; and not wanting to put load on rotor in its sloping section, which would be difficult to manage for the systems many guide cables.

I don’t understand why a load of the launch ship at 1 g (9.8 m/s/s) on different part of the sloping section from a vehicle moving under 300 m/s (we’d likely want to avoid going super-sonic in any substantial atmosphere) is less troublesome than a 1 g load on a fixed point (the west station), followed by the force of accelerating the launched ship at a bit over 3 g (30 m/s/s) on the upper track for a bit over 6 minutes to a final speed of up to escape velocity (11400 m/s) or more.

Perhaps Lofstrom’s main reason is the “shorter upward transit times ” he mentions. I get a travel time of about 1 hour traverse nearly 1000 km at a barely subsonic speed of 300 m/s, vs the 80 km vertical elevator trip, at a reasonable elevator speed of 10 m/s, of a bit over 2 hours. Loftstrom might plan a slower sloping section speed, or a faster elevator.

A major difference between the system described in Lofstrom’s 1985 paper and this 2002 slideshow (from which the wikipedia article appears to get its graphics and description) is that the 1985 proposes a flat, ribbon-like iron rotor, while by 2002, it’s proposing one with a circular cross section.

Originally Posted by Roadam

Also I am not yet sure how does author achieve levitation, as ferromagnetic metals are usually attractive.

That part, I understand.

The rotor is magnetized. The sheath is conductive. When the magnetic field of a particular section moves past a section of the conductive sheath, it induces a current, causing that section to have a magnetic field. Their magnetic polarities, it turns out, are in opposite directions, resulting in mutual repulsion.

It’s similar to how an Inductrack maglev train works, except that the rotor doesn’t have its magnets arranged into the more optimal pattern of a Halbach array. This is OK, as it results only in the levitating force being lower for a given rotor speed. In a Inductrack maglev train, this results in a higher “liftoff” speed at which the train cars begin levitating, a bad thing. The rotor in launch loop, however, move at a very high speed, so will induce a lot of current and repulsive magnetic fields in the sheath’s conductor, even with its magnetic fields less-than optimally arranged.

Given all of our uncertainties and guesswork, it might be worth our while to contact Keith Lofstrom. If his personal webpage is any indication, he seems an approachable, techish person. I imagine he’s interested in promoting discussion of the launch loop concept, and wouldn’t mind helping.

If nobody else wants the honor, I’ll email him an invitation at the address give on his personal webpage.

[Roadam]

Be my guest.
Its good having more experienced people in here. That way some of us who are still younger could learn alot.

I somehow guessed that the rotor is just a normal metallic wire. I think it would be tricky to magnetize it in more than one direction though.

Lofstrom's proposal has westward and eastward parts of the loop separated by several hundred meters of altitude. That way the lower one is supposed to be a stable platform for correcting the upper one trough different instruments. But as both ground stations are supposed to be accelerators they would both be a source of disturbances and I think that eastward part of the loop wouldn't be much more stable than westward one.

So why not linking them together by a frame? Rotors magnetization wouldn't be very strong and magnetic strength would be falling by several orders of magnitude in respect to the distance. So I guess two magnetized cables separated by several meters of zigzag frame wouldn't influence each other much.
So if both tracks would be connected it is possible to control left and right swings with repeated sections of crossed actuators.

As for moving up or down, if the rotor is moving at more than orbital speed it would continuously want to go up. So if too light, the upper section of the loop would actually want to pull ground sections up. If I am not mistaken.