Talk:Space fountain

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1 Minor Rewrite of Design Section?
2 Paul Birch hyperlink
3 Efficiency?
4 Tower stability
5 I have a photocopy of the original Starbridge papers
6 Pellets

[edit] Minor Rewrite of Design Section?

"The projectiles reach the bottom of the tower with almost the same speed that they had when they were launched, losing a small amount of energy due to inefficiencies in the electromagnetic accelerators and decelerators in the tower."

Anyone feel like doing the math to prove only a minor loss? Pellet is launched, upward force applied to tower to slow pellet with drag, store energy for later. Use a large magnet to expel the last of pellet's kinetic energy into upward force for the tower. How can the downward acceleration on the pellet (and thus upwards on the tower) provide a power even close to equal at the bottom again? Won't you have massive inefficients in the pellet to electricity and then electricity to pellet transfers? Even with the use of superconductors? If not, this should be explained in a clearer fashion.--Talroth 04:34, 2 December 2006 (UTC)

[edit] Paul Birch hyperlink

The Paul Birch hyperlink is to basketball player Paul Birch. This is clearly the wrong Paul Birch.

[edit] Efficiency?

If a space fountain needs a devices that shoots pellets straight up to the end of the tower, wouldn't it be easier to just use that device and no tower to lift cargo into space? What's the benefit of "adding a tower" to the pellet shooting machine? Is it efficient? How fast would those pellets have to be at the start for the whole process to work and some reasonable height? Paede 11:58, 10 Apr 2005 (UTC)

In theory, the starting speed is as low as you like (for as short a tower as you like). In practice, the bending magnets or what-have-you will likely assume a certain speed range for pellets being deflected, meaning that to inject new pellets into the system, you'll need a gun adjacent to the main deflecter that accelerates them to a few km/sec, to replace pellets that have been lost from an already-working system. This makes initial startup a bit trickier (you might do it by slowly populating the loop with full-speed pellets until the momentum transfer is enough to lift the tower top off of the construction framework). As for why you'd have cargo ride the stream instead of being the stream itself, there are several reasons. If pellet speed his higher than escape velocity (allowing an arbitrarily high platform), the accelerating forces felt when initially injecting the pellet and when deflecting at each end are immense (think "thousands of gravities"). While you could launch raw materials that way, you probably wouldn't want to launch most other types of cargo. Riding the stream, cargo can be launched at any acceleration by varying the strength with which it couples to the stream. Individual pellets are also likely small, to make injection more practical. Cargo riding the beam can be heavier; while it needs the same amount of energy imparted to reach a given height or a given final velocity, this energy can be put in over a longer time, for lower power required. Lastly, if the platform is relatively low (e.g., outside the atmosphere, but far below geosynch or possibly even LEO), the pellet stream can move at a speed well below orbital speed (the platform isn't in orbit; it's stationary relative to the surface, supported by the pressure required to deflect the stream). Launching cargo on this trajectory would just cause it to fall back to earth. A platform at or below LEO altitudes would be useful for tourism, observatories, or even another launcher (using fountains as support pylons for a Lofstrom launch loop's housing, or for a very long fragile-cargo magnetic accelerator). In short, the accelerator mechanisms are only good for one type of cargo being sent in one direction (small pellets that can withstand thousands of gravities of acceleration, being launched nearly vertically at or above escape velocity), while a fountain allows many different types of cargo to be sent on different types of path, depending on how the fountain is configured and what's on top. --Christopher Thomas 13:35, 10 Apr 2005 (UTC)

Also, the tower provides a tube of vacuum for the pellets to travel through so there's no problems with air resistance. Launching cargo straight from Earth's surface would require it to be encased in a lot of heat shielding, since it would go up like a meteor in reverse, and would produce powerful sonic booms that could have a nasty environmental impact. You'd also need a rocket on board to circularize the package's orbit, which could get expensive if you're launching thousands of packages (a low-orbit space elevator would need to provide transverse thrust for packages leaving it too, but this could be done with reusable tugs or with an accelerator on board the top station. Bryan 16:40, 10 Apr 2005 (UTC)

Thanks for your replies. I understand the sonic boom problem, but this could be already be solved with a 30km or 40km high tower - above it, there is virtually no air resistance anyway. My problem still remains: A pellet-shooting machine that can shoot stuff thousands of km up is an invention that is quite difficult to achieve. Isn't the whole concept of "Space Fountains" flawed because the pellet-shooting machine would make 95% of the encountered difficulties, and once you've got a working pellet shooting machine, there is no need for the surrounding tower above 20km? This is still science, not pure fiction, right? Paede 03:35, 12 Apr 2005 (UTC)

Check my reply again. A gun-type launcher has strong limits on what you can do with it, that are relaxed if you instead put a platform at the top and more slowly lift objects to to the platform by coupling them to the fast-moving stream. This is why the idea of a space fountain (or launch loop or space elevator, for that matter) is proposed. You correctly note that you don't need a physical shroud around the fountain once you're above the atmosphere, but that has little relation to the question of why you'd use a fountain in the first place.

As for building the gun, light gas guns that presently exist can already launch projectiles at the required speeds. See space gun, supergun, and light gas gun for more information. In practice, something resembling a coilgun would be more likely to be used as an injector, due to higher rate of fire. The main engineering challenge for building a space fountain is likely to be the bending magnets that redirect the stream of pellets at the ends of the loop, not the pellet injector.--Christopher Thomas 19:49, 12 Apr 2005 (UTC)

Thanks, that clarifies everything! Paede 00:44, 13 Apr 2005 (UTC)

[edit] Tower stability

How do you deal with the Coriolis effect? Won't this have the effect of slowly toppling the tower? njh Fri May 6 08:13 2005 (UTC)

I'd have to check the papers for the official compensation mechanisms, but as far as I can see, as long as the toppling time is much larger than the time it takes the pellets to make one cycle, you can apply a lateral force to the pellet stream to keep the station stabilized, and apply a corrective force to the stream at the ground station. The limit is that you can't deflect the stream too much without having to change the geometry of the ground station. In practice, you'd probably set up the stream geometry to let you compensate for known forces from the start (possibly resulting in widely-separated base stations for outgoing and incoming streams, depending on the tower configuration). The time scale for correction for ground-to-geosync is about 3 hours (more if your pellets are close to minimum speed, less if they aren't). Intermediate-level platforms shorten this timescale. If someone who's read the original papers on the fountain can describe how the authors planned to stabilize it, that would be very useful. --Christopher Thomas 13:15, 6 May 2005 (UTC)

Update - I've worked through some of the problem specifics, and it looks like you could stabilize a tower using just the top platform and the base station. The particle streams aren't fired pointing directly at the tower and the base station, instead following hyperbolic or elliptical arcs (free-fall paths influenced by Earth's mass, above or below escape velocity, respectively). In a non-rotating reference frame, the upwards stream looks like it's fired on a hyperbolic path starting at the ground station's position at time t₁, ending at the platform's position as of time t₂. The downward arc is similar, starting at the platform's position as of time t₂ and ending at the ground station's position as of time t₃ (different from the station's position at time t₁). In a rotating reference frame in which the ground station and platform appear stationary, it looks like you're firing projectiles at an angle that exactly compensates for the warping caused by the Coriolis pseudoforce. There are many possible arcs that can connect the ground station and the platform, using different particle speeds; by varying the angle and speed, you can apply force to the platform in any desired direction in the plane of Earth's rotation. If you use two loops instead of one, you can correct in three dimensions (though the two ground stations need to have substantially different latitudes for this). This is mainly important if you're trying to stabilize a platform that's not quite in the equatorial plane. This was a fun problem to work through; thanks for bringing it to my attention :). --Christopher Thomas 22:12, 7 May 2005 (UTC)

There is a simpler solution. Lets assume that we have a cable that can guide these high velocity particles. Say the particles are magnetic and that the cable sheath uses the "null flux" guiding mechanism that is used in some maglev trains. The high speed particles means losses are low. The beam will then apply magnetic pressure on the cable walls thus guiding the beam and cable, and the cable is also under tension for stability reasons. But we can also have the return cable, that is the cable with the beam travelling in the opposite direction, mechanically attached to the other cable. So if we consider a small length of "rope" we have two sections of guide cable with the beams travelling in the opposite directions. In this case the Coriolis force will cancel out since in each cable the force from one beam is the same but in the opposite direction to the other beam.Delt0r 12:49, 27 September 2005 (UTC)

[edit] I have a photocopy of the original Starbridge papers

Keith Lofstrom [1] here, inventor of the launch loop [2]. Seeing this article reminded me that I have a few file boxes of papers from the 70's and 80's, when all this stuff was new and a lot of enthusiasts were working on it. I can provide lots more information.

I have a bad photocopy of Rod Hyde et al's papers, the original 30 Jan 1981 paper and the 2 April 1982 addendum. I have quite a few other papers from the era, too. I am heading out of town for a week, but when I get back I can scan them and put the images up on the launch loop server.

The first published paper in the general area of momentum storage launchers (my term) or dynamic structures (Bob Forward's term) would be the Roger Arnold and Don Kingsbury articles about The Spaceport in the November and December 1979 issue of the Analog Science Fiction Magazine. This is an orbiting mass driver that captures payloads. Immediately upon seeing the November Analog, I went up to visit Roger Arnold in Kent, Washington - he was working at Boeing at the time. I had been working on a long "flying cable" (tether) system (I presented it at the Orycon 1 science fiction convention in 1979), and Roger's rather elaborate (and high gee) capture system intrigued me. We had a bit of an argument about "frozen spinach launchers", but parted the best of friends. I wonder where he is now?

The launch loop was born out of the flying cable - the problem with all these systems is stability and control, and I decided I needed a rigid fixed surface to work against for lateral stability. For example - if you are aiming flying rings at a cm-accuracy target 40,000 Km away, and transit time is an 5000 seconds, you need to measure lateral and rotational velocities within 2 micrometers/second (about one part in 2E-10 compared to breech velocity). If you are measuring between two points a kilometer apart, you need to measure displacements of 200 nanometers, and know with exteme precision where the datums are on every individual ring. Of course, with rings you need to control 6 velocities and 6 positions to this positional accuracy. And the rings that miss ... Yikes!

So, assuming a planet as a source of basic stability and reaction mass, and a grid of laser interferometers to make up for the fact that planets are actually rather mushy things (tides, earthquakes, and all that), and realizing that there were dozens of other problems with vertical launchers, I went horizontal.

The first thing I came up with was something like Ken Brakke's Skyrail (L5 News, July 1982) or Paul Birch's Orbital Ring (JBIS, Nov 1982) but I abandoned that in mid-1980 as having most of the same problems, as well as being impossible to build from the surface. Since I only needed a 2000Km launch path for 3-gee-to-escape, I chopped off the useless extra 38,000Km and made a closed loop. Hence, the launch loop. It evolved from there, and resulted in a submission to the AIAA Advanced Space Propulsion competition in April 1981. That was followed by a short paper in the November 1981 American Astronautical Society Reader's Forum, another short non-technical paper in the August 1992 L5 News (Ken and I learned about each other from this, and this let Rod Hyde know about both of us), and a presentation at the April 1993 L5 Conference in Houston. I presented just after Eric Drexler presented his first public paper on nanotechnology. We were both mobbed by enthusiastic fans in the hallway for hours after the presentations. That night, Eric and I watched the Soviet Mir space station pass overhead from the roof of the hotel, and discussed Ricardo's Law of Comparative Advantage in a world of big, slow humans and quick, cheap nanointelligences. I also met Bob Forward there, which began a long friendship. Robert Heinlein came to my room party ...

In May 1983, I met with Ken and Paul at the 6th Princeton Space Manufacturing Conference. None of our papers were accepted, but we took over a classroom for a couple of days afterwards to formalize some analysis conventions and do some calculus together. There we explored the "two stream instability", with Paul becoming convinced that we could never make it work. I argued that with damned good measurement ("many wavelengths of light"), and smart active control ("only 14 millimeters per microsecond"), and frequent (100 Km apart) cables to the ground to shed correction forces, taking out the instabilities was quite doable (what Bob Forward called "A mere engineering detail").

I had been working with Stan Schmidt at Analog since September 1982 on yet another short non-technical paper, which appeared in the December 1983 Analog magazine (my first paid sale of writing). This connected me to science fiction author Dean Ing, which connected me to Leik Myrabo, which finally got me a slot at the July 1985 AIAA Advanced Propulsion conference, where I could finally publish some damned equations!

This was all like pushing rope - work was getting busy (I was managing a design team), and I was getting approximately zero outside help. I came to the depressing conclusion that this wasn't going to happen in my lifetime unless I earned enough money to build the damned thing myself. Note, as of July 2005, I haven't made the money thing happen yet. Sigh.

Awesome! Thanks for all this detail, I'll try to work as much of it as I can into this and related articles (I wrote the original version of this based solely on Forward's article in Indistinguishable From Magic). Since Wikipedia's got a policy against original research that this first-hand account might fall under, would it be possible for you to copy and paste some of this information on the history of the concept's development onto your launch loop page? That would provide an external source that I could reference.

In return, I promise to pledge a portion of the vast profits I'll no doubt one day earn from my editing work here on Wikipedia to the construction of a space fountain. :) Bryan 03:05, 20 July 2005 (UTC)