[wolfekeeper]

If the top of the fountain is not close to geosynchronous orbit, your satellite will need a rocket to give it orbital speed after it detaches from the top of the fountain, so you don't gain that much by putting it there.

Yes, but the launch loop variety gives the vehicle enough lateral speed to reach escape velocity without any rockets.

[briligg (OP)]

looking at the power issues you could use a satalite in space to provide the energy for it. AS you said yourself it would need multiple energy sourses just incase one failed. Although if you used a satalite to provide it with energy that might have implications on the design for the fountain as you would have to connect it to the fountain somehow.  

I have toyed with that thought. Maybe if the fountain was within the Arctic Circle, and was built to 200km, which seems to be a popular figure, it would have a constant view of the sun. Maybe, with new-fangled thin film solar cells, a pretty large area of solar collectors could be attached to the tower itself, providing some power. Probably not enough to maintain the tower, but maybe enough for a short-term emergency reserve. If satellites that collect solar energy were then placed in an orbit that would pass over the tower frequently, maybe they could beam their collected energy to a collector on the tower while in range, several times a day.

An interesting thing to note here is that under 'near-term applications', the wikipedia article mentions that it could be useful for energy storage, somewhat similar to flywheels.
http://en.wikipedia.org/wiki/Space_fountain#Near-term_applications
So i guess that means that the energy in the system can fluctuate. I need to find where i read that this could be made possible by making the tower partially telescoping - if it can extend some percentage at the joints between sections, everywhere there is a coil gun / drag device setup, the tower could rise in response to increased in the power supplied to it, and sink when the power is reduced again. If those expanding sections are above the atmosphere, you don't need to be concerned about maintaining the vacuum in the tubes where the pellets travel, so that wouldn't be an issue.

Or, you could put the fountain near the equator, and put a satellite above it in geosynchronous orbit, that would beam it solar energy constantly by laser. Schemes for solar satellites for energy always beam the energy to earth, usually as microwaves, because the beam has to make it through the atmosphere and not damage anything stuck in its path. But beaming to the top of a space fountain wouldn't have to consider stuff like that. Instead, the issue would be making a collector large and sturdy enough to absorb the energy in the laser beam, instead of melting.

At any rate, these are things you could only do once the fountain is built. While erecting it, you would need to supply it with energy from a plant on the ground. That is when the big energy input is needed, during the phase when the pellet stream is initially being powered up. The power would be slowly increased over time, but a huge amount is needed.

This is maybe one reason why space fountains aren't discussed more often. Probably, you would need to build several nuclear reactors dedicated purely to powering the fountain. For safety reasons, should there ever be a total failure and the fountain crash to earth, you would also need an empty space for it to fall into. There aren't an awful lot of places where people would be comfortable with nuclear reactors nearby, much less ones connected to a space fountain. Northern Canada sounds like a good option to me. If the idea of putting it in permanent view of the sun makes sense, there are a number of big, empty islands in Canada's Arctic Ocean that would make great sites.

[briligg (OP)]

If the top of the fountain is not close to geosynchronous orbit, your satellite will need a rocket to give it orbital speed after it detaches from the top of the fountain, so you don't gain that much by putting it there.

Yes, but the launch loop variety gives the vehicle enough lateral speed to reach escape velocity without any rockets.

I have read that a launch loop may be an easier project in the short term, although it would require even more space. Also, most proposals do have rockets providing the final bit of acceleration to put the ships in orbit. That way, the orbit can be adjusted to make it stable. Also, there is an issue of how the rods in the loop heat up in response to the resistance created by the ships launched, and i guess rockets reduce that somewhat.
http://en.wikipedia.org/wiki/Launch_loop

But would rocket assistance for a fountain launch need to be a lot more, to get into earth orbits? Would a launch loop or a fountain be better for destinations beyond earth?

[wolfekeeper]

The technology to do this more or less exists, but the costings on this are currently running at about $20k/m.

It sounds a lot, but it's not much more than the cost of a motorway or railway to go the same distance.

[briligg (OP)]

Just to expand on that a bit - from Wikipedia:

Lofstrom estimates that an initial loop costing roughly $10 billion with a one-year payback could launch 40,000 metric tons per year, and cut launch costs to $300/kg, or for $30 billion, with a larger power generation capacity, the loop would be capable of launching 6 million metric tons per year, and given a five-year payback period, the costs for accessing space with a launch loop could be as low as $3/kg.

I wonder if the $10 billion version could be upgraded to the $30 billion version by enlarging its power station.

One advantage space fountains have is that small-scale versions have possible commercial applications, allowing the technology to be developed at a lower risk to the investors. Also from Wikipedia:

A very small-scale fountain tower could be used for constructing tall antenna masts rapidly, perhaps for news events and military operations. A larger and more permanent fountain tower could be ten or twenty kilometers tall, allowing one facility to provide radio and television broadcasts to enormous areas such as the steppes of Asia. Fountain towers might also prove to be an economical alternative to communication satellites for point-to-point television and FM radio communication between the various islands of some of the smaller nations in the Pacific Ocean. An elevator and observation platform could also be added as a tourist attraction.

[wolfekeeper]

Yes, I believe it can be upgraded like that.

[briligg (OP)]

My question now is about the the different launch trajectories of each system. Launch loops are a horizontal launch, space fountains are a vertical launch. If a vehicle is accelerated up the height of the fountain and flung outward into space, how much energy does it take to then turn it into a stable earth orbit? Could you have rockets kick in at its apogee to give it the orbital speed it needs?

The thing is, space fountains appeal to me more because they are more efficient. The energy lost due to waste heat is huge in the case of launch loops.

btw, this thread now shows up in position 16 in google if you search for 'space fountains'. sheesh.

[wolfekeeper]

Going above about 80kms makes the structure susceptible to meteorites and space junk. You need a long structure anyway, because otherwise the accelerations need to become too high to be survivable; you end up with a spam launcher (people get spammed).

Note also that launch loops are not inherently more inefficient than space fountains; in fact it's pretty much the other way around, the higher the vertical aspect of the structure, the more the structure weighs; these types of structures hang down from the top, and the tensile members have to be tapered; as the height increases you need more and more taper, and for any given capacity, the weight of the tension members increases.

That means that you get more waste heat from the small fraction of stabilisation power to handle the magnetic instabilities associated with Earnshaw's theorem to carry these extra loads.

And in any case you need lots of sideways speed to reach stable orbit.

[briligg (OP)]

Note also that launch loops are not inherently more inefficient than space fountains; in fact it's pretty much the other way around, the higher the vertical aspect of the structure, the more the structure weighs; these types of structures hang down from the top, and the tensile members have to be tapered; as the height increases you need more and more taper, and for any given capacity, the weight of the tension members increases.

That means that you get more waste heat from the small fraction of stabilisation power to handle the magnetic instabilities associated with Earnshaw's theorem to carry these extra loads.

Huh, that's new information, and quite over my head, i'm afraid. I waded through the Wikipedia article on Earnshaw's theorem, but i don't have the math. One question though - are you considering that all the weight is supported at the top? Because a large portion is supported by the coil guns and drag devices on the sides.

[briligg (OP)]

Btw - here is a great site on launch loops, way better than anything i've found for space fountains.
http://launchloop.com/LaunchLoop

[briligg (OP)]

And now, Saudi Arabia is about to spend the $30 billion necessary for the really good launch loop - on one really huge skyscraper a mile high.
http://gizmodo.com/#!5790505/saudi-arabia-to-be-permanent-member-of-mile+high-club

sigh...

[wolfekeeper]

These types of dynamic structures are not very good at heavy lifting, but they could look down on those more traditional structures from above easily enough.