Re: Feasible Space Elevators

I haven't ignored it. For a Rotovator design, you hook on and ride half a rotation and then let go. The forces are purely radial in that case. If you are climbing from the tip of rotation to the center, you do see a Coriolis effect, but let's look at the numbers.

Assume you have a maglev-type track that lifts a payload from the tip to center in 3 hours (200 km/s for a ~600 km radius), and the tip velocity is 2400 m/s. Therefore you much transfer 0.22 m/s angular velocity to the structure. The radial tension in the structure is 9.8 m/s at the tip, so you produce 2.2% deflection from purely radial while climbing. This is a manageable deflection of the cable structure.

Re: Feasible Space Elevators

You clearly don't understand orbital mechanics. Of course grabbing a sub-orbital cargo and accelerating it to higher orbit will lower the Rotovator orbit. That I why I mentioned on-board propulsion to make up the kinetic energy lost.

That allows you to substitute low efficiency rocket (on an unassisted rocket) for high efficiency electric thrusters (on the Rotovator), saving you 90% of the propellant consumption, which turns into more payload.

I spent a career in Boeing's space systems division, and am writing a book on the subject ( http://en.wikibooks.org/wiki/Space_Transport_and_Engineering_Methods ) so please credit me with having a clue what I am talking about.

Re: Zero velocity?

Actually, Cayambe in Ecuador ( http://en.wikipedia.org/wiki/Cayambe_%28volcano%29 ) is a better choice. Its the highest point on the Equator and has decent slopes for ground accelerators.

Feasible Space Elevators

Trying to span the depth of the Earth's gravity well with a single structure is terribly non-optimal from an engineering standpoint for a number of reasons. Smaller elevators, however, are quite feasible with off the shelf carbon fiber.

Imagine a rotating cable with a tip velocity of 2400 m/s, and a comfortable 1 gravity at the tip. The radius then works out to 587 km. You don't want the tip to enter the atmosphere, so you set the orbit of the center to 750 km, and the lowest point is then 163 km. The center then orbits at 7.48 km/s, and the tip is moving 2.4 km/s less or 5.08 km/s. Subract the Earth's rotation and you have a velocity of 4.61 km/s relative to the ground.

Your launch vehicle now has a much less challenging job than getting all the way to orbit by itself. It merely needs to reach a landing platform at the tip of the cable. To return to Earth, it has much less velocity to dissipate, so the re-entry heating is much less. Cargo heading to higher orbit merely rides for half a rotation, then lets go. It now is moving at 2.4 km/s above orbit velocity, which puts you in a high transfer orbit.

The load on the cable varies linearly from center to tips, therefore is equal to a cable under 1 g half the length, or 293 km. Good carbon fiber has a breaking length at 1 gravity of 360 km. We want a decent margin of safety, so only load the cable to 40% of ultimate strength. This requires tapering the cable from center to tip, as each point has to support the payload + cable outside that radius, but the taper ratio is not severe, about 7:1 in area.

Because this design is 40 times shorter, it is much less exposed to meteor and debris damage, but they still are a risk. Therefore you build the cable out of something like 21 strands, of which 7 are spares, and cross-connect them every 5 km. So when the inevitable impact happens, you only have to replace the one 5 km segment, which is 0.02% of the total structure. The "single cable" illustrations like the one in this article are just terribly unrealistic from a safety standpoint.

This type of rotating space elevator is called a Rotovator, and can start being useful while under construction, so you don't have to build it all at once. As the length increases, and tip velocity goes up, the launch vehicle needs less velocity, and can therefore carry more payload. If some of the payload is more cable for the Rotovator, the increased cargo on later flights "pays back" the payload spent on launching the cable. This payback time in cargo mass can be relatively short.

You can build a second Rotovator in high orbit, which captures payload sent up by the first one in low orbit, and forwards them to interplanetary trajectories. Since kinetic energy is not free, you need onboard electric thrusters to maintain the Rotovator orbits, but those have ten times higher efficiency than conventional rockets, so you still come out very much ahead on net cargo.

Re: In troubled times...

> I'm not sure I would rank bitcoins as safer than euro stored in a bank backed by a deposit guarantee, the ecb

Cyprus *had* a deposit guarantee, they just threatened to bypass it by taxing deposits. And it was the ECB who was encouraging this theft of depositor funds. The sudden interest in bitcoin is from other people in Europe wondering which other banks might decide to renege on their guarantees.

I agree that shifting your life savings into bitcoin is a bad idea, it is too new and unstable for that. But being inherently transnational, it is a good way to move your money to a safe haven outside the control of the ECB and euro bankers. The risk in using it for funds transfer is much lower, since you would hold them for only a short period of time.

Google > ITU

This is why I would rather have Google as my Internet overlord than the ITU. They at least understand how the darned thing works.

Governments are a local monopoly on force. Since humans are self-interested, they will always try to leverage that monopoly into total control. Therefore you have to provide strong limits on government power, such as constitutional guarantees of certain rights and limits on their ability to meddle in things they should not.

If you want to make certain actions illegal (possession of child porn is the poster child (pun intended) for such actions), make the possession illegal. Don't go breaking use of networks, video cameras, studio rentals, and cash because they *might* be used for that purpose. They have other uses that are perfectly legitimate, so you end up punishing the bystanders and not just the criminals.

Re: Surely I am not the only...

Now I understand the origin of the HCF assembler command (Halt and Catch Fire). Thank you.

Re: Cost Benefit

When you air-launch, you never need to go vertical, and that accounts for a large part of the performance gain. When a conventional rocket takes off vertically, gravity opposes thrust and subtracts 1 gee from the net acceleration. When you are horizontal or near horizontal, gravity is perpendicular to thrust and does not slow you down.

Air-launch has an optimal flight angle early-on that is between horizontal and vertical, and the later parts of the flight are pretty much identical to other types of rocket. So it has less losses from fighting gravity in the early flight, and so better performance overall. The other gains you get are less drag loss and higher engine thrust, both from being at lower pressure, plus the actual altitude and velocity of the airplane.

The other part of the cost benefit besides performance is that an airplane has effectively an infinite operating life compared to one-use rocket stages. Thus the cost per flight of the airplane is much reduced. The cost of actual flying time (fuel and maintenance) will be ~$15,000/hr. This will be a tiny part of the total launch cost.

Re: Ummm

This is a math co-processor to assist the CPU to crunch numbers faster. It is not a graphics card, so it does not have monitor outputs. If you want a workstation that can do both heavy duty number crunching and real time graphics, you would need two cards - this one plus a graphics card.

Re: Isn't there [DMCA penalty]

It does not have to be a fine paid to the government. It can be a "processing fee" paid to the content host (YouTube in this case) if it turns out to be a false takedown. That compensates the host for their trouble. You might set up to allow a few mistaken takedowns, but by the third or later, you get charged for wasting everyone's time.

Re: Why?

It has the same purpose as any other form of money - to buy things with it. And it's usefulness depends on how easy that is and the range of things you can buy with it. It's still in beta though, the latest version of the bitcoin software is at version 0.62 I think. Unless you like playing with new software, you might want to wait a bit till they get the bugs worked out.

Re: It's unlikely but maybe the Private Sector could get there first...

Actually the first steps are more telescopes for finding asteroids, since we only know about 8% of the ones larger than 100 meters. Then prospector missions to visit and get samples back from the candidate ones you find. Looking at these asteroids from millions of km away, like we do from the ground, just does not tell you enough to determine which ones to mine and what you can get out of them. That pretty much follows Earth mining practice, first survey and take mineral samples back to the lab, then decide if you want to mine there.

Re: No gravity well - but asteroids are moving targets

No, you can exploit asteroid resources *in space*. Low orbit satellites need fuel so they don't fall down due to atmospheric drag, and high orbit satellites need fuel to get to high orbit in the first place. So the first mining product will likely be fuel for satellites. Anything else will come later and be gravy. If nothing else, the slag after extracting fuel can become radiation shielding, which you need for anywhere above low orbit.

Even in low orbit, you need some shielding to use modern electronics, because they are sensitive to particles from the radiation belts.

Re Gravity?

Gravity is proportional to mass, which at a fixed density goes as volume. Volume goes as the cube of the radius. Gravity is inversely proportional to the square of the distance from the center of the planet. So combining terms, gravity goes up linearly with radius, or as the cube root of mass.

In the case of a planet with 4.5 times the Earth's mass and the same density, then the surface gravity would be (4.5)^(1/3) = 1.65 times ours. Since the parent star is "metal poor" (astronomers call anything above Helium on the periodic table a metal), this planet is likely to have less iron core and heavy rock mantle, and more lightweight elements, We will have to wait for better telescopes to be sure what it's made of.