Colonizing the Solar System

[CraigD]

Originally Posted by Eclipse Now

What was the Kim Stanley Robinson propulsion system using materials gathered from the gas giants themselves? Are we saying that harvesting the gas from Jupiter is now out because the equipment required to do that would be "nuked" by radiation?

No.

I’d describe the vicinity of Jupiter is better described as “high wear”, rather than “out”, for spacecraft equipment. A case in point, the Galileo spacecraft, which was designed in anticipation of about 1/3 the radiation it actually encountered, operated successfully there for 7 years, from Dec 1995 to Jan 2002, and well enough to respond to commands and deorbit into Jupiter’s atmosphere Sep 2003.

Originally Posted by Eclipse Now

I'm not sure about Earth's Van Allen belts affecting the space elevator, unless there were people required to stay in those Belts. How high are the belts? Surely not high enough to affect the station at the end of the 36k km long elevator?

The inner Van Allen belt, which contains the highest energy particles, extends from roughly 700,000 to 10,000,000 m altitude, so is well below geostationary orbit at about 36,000,000 m (GEO).

Though many descriptions of space elevators assume them to be about 36,000,000 m long, with a massive countermass “station” slightly above GEO, and attached to the ground, it’s worth noting, I think, that this isn’t the only possible design, or even necessarily the best. The advantages of having the bottom of the elevator above the ground, at an altitude easy to reach but above the thickest part of the atmosphere (say, 30,000 m altitude) has been discussed in previous threads. The advantage of having it extend far above GEO is that an object (eg: a spacecraft) released at that altitude would have more speed than required for orbit. For instance, at about 10,000,000 above GEO, an object released would escape the Earth. At about 51,000,000 above GEO, an object released at the right time would be given the extra about 2929 m/s needed for a transfer orbit to Mars. Extending above GEO is easier than extending below it, as the acceleration away from GEO is smaller than for extensions below. At 10,000,000 above GEO, it’s about 0.14 m/s/s (about 0.014 g), at 51,000,000, about 0.45 m/s/s.

To have 1 g (about 9.8 m/s/s) acceleration, the elevator would have to extend 1,800,000,000 m above GEO, 50 times the distance from GEO to the Earth’s surface, and almost 5 times the distance to the Moon. At about 499,000,000 m, about at the Moon’s orbit, the acceleration’s about 2.65 m/s/s (0.27 g), and an object released there would exit the solar system.

Originally Posted by Eclipse Now

But in the KSR books the Martian space elevator came down …

That was a great moment in storytelling, but also a rather horrible idea when considering the potential impact on earth.

Another common depiction of space elevators is that they are very massive. However, calculation like those in “Space elevator cable mass & size calculating program” suggest that only a small difference in material strength results in space elevators being either impossible (as is the case with ordinary high strength steel), massive (as with Kevlar, with a mass of about 3.5e15 kg and a maximum effective circular cross section diameter of about 520 m), or nearly microscopic (as with hypothetical graphite “buckytube” material, with a mass of about 26200 kg and an effective diameter less than 0.001 m).

If materials much stronger than present days one are used to build a space elevator – which, given the difficulties bordering on impossibility otherwise, is IMHO likely to be the case if they’re every built - it’s quite possible the resulting structures will actually be lighter than air. In this case, broken cables, of which there are likely to be many, would be cause for concern about a bizarre kind of air pollution, rather than a catastrophic surface impact. Depending on its specific characteristics, this could more hazardous than a massive surface strike – imagine the effect on a jet engine of sucking in a hair-thin, kilometers long, super-strong piece of lighter than air broken space elevator!

[Eclipse Now]

Originally Posted by CraigD

If materials much stronger than present days one are used to build a space elevator – which, given the difficulties bordering on impossibility otherwise, is IMHO likely to be the case if they’re every built - it’s quite possible the resulting structures will actually be lighter than air. In this case, broken cables, of which there are likely to be many, would be cause for concern about a bizarre kind of air pollution, rather than a catastrophic surface impact. Depending on its specific characteristics, this could more hazardous than a massive surface strike – imagine the effect on a jet engine of sucking in a hair-thin, kilometers long, super-strong piece of lighter than air broken space elevator!

Another movie for Bruce Willis? Die Hard in a space elevator, where bringing down the space elevator and pointing it into the escaping bad guy's jet is the only way to save the day? OK, I think I need to see Star Trek to get this out of my system....

[Moontanman]

Originally Posted by Eclipse Now

Another movie for Bruce Willis? Die Hard in a space elevator, where bringing down the space elevator and pointing it into the escaping bad guy's jet is the only way to save the day? OK, I think I need to see Star Trek to get this out of my system....

Don't count on it the new Star Trek is cheesy beyond belief! the directors and writers should be sent to Rura Penthe and left there to rot!

[Eclipse Now]

Hi all,
anyone read through this page? Extremely optimistic stuff about our ability to get out and explore the universe....
p2pnet news Blog Archive Want to help build a space engine?

[CraigD]

Originally Posted by Eclipse Now

anyone read through this page? Extremely optimistic stuff about our ability to get out and explore the universe....

p2pnet news Blog Archive Want to help build a space engine? is certainly optimistic, but – with all due respect to Michael Thomas, who appears to be a distinguished computer engineer – even though many of its links are broken beyond even recovery via archive.org, preventing me from reading all the offered descriptions, I’m pretty sure it’s also unrealistic, and based on an inadequate understanding of basic physics and limited knowledge of spaceflight science fiction and non-fiction.

Thomas’s claim

My Linear Particle Accelerator (LINAC) atom smasher concept for electron particle propulsion is a unique proprietary invention developed over the last ten years and takes space propulsion in a direction never thought of by any scientist or organization in history

notwithstanding, the idea of using a linear accelerator or cyclotron to accelerate the reaction mass of a rocket to relativistic speed is not a new one, appearing in at least occasional science fiction stories no later than the 1960s. It’s a fairly obvious approach to the fundamental rocketry problem of maximizing specific impulse (), a measure of the mass that must be expelled from a rocket (reaction mass) to change it’s momentum: by accelerating the reaction mass to a large fraction of the speed of light (c), then applying force to accelerate it more, the reaction mass is effectively greater when used than when stored as propellant, multiplying the rocket’s , and potentially increasing the amount of useful payload a rocket of a given starting mass can give a given change in velocity (delta V).

A problem with such a system, however, is that accelerating the reaction mass to nearly c requires a lot of energy, and, assuming the source of this energy must be carried by the rocket ship, is also equivalent to mass, so even assuming 100% efficiency (a tremendous overstatement), at some point (which I’ve not yet managed to calculate) increasing the propellant speed results in an increase of the mass of propellant + fuel. The machinery needed for an accelerator also threatens to be prohibitively massive compared to the propellant mass it accelerates – the SLAC, for instance, at about 2 miles (3.2 km) long, and even stripped to its essential components, must mass tens of thousands of tons, yet produces miniscule rocket thrust. So the idea that high propellant speed alone is a panacea, or silver bullet solution, to the problems of rocketry is simply wrong.

This is not to say that using particle-accelerator techniques in rocket motors is a bad idea, or an unexplored one. The VASIMIR motor, which has been under development since 1979 and is expected to be flown experimentally in space in the next few years is a prominent and promising example of such an approach.

This is also not to say that the goal of a spacecraft that can sustain acceleration of 1 g (about 10 m/s/s) is anything but awe-inspiring and worthwhile. However, IMHO, approaching this goal by searching for a radically different rocket motor is not radical enough. I believe it requires completely re-imagining the nature of a spacecraft, abandoning the idea of a self contained vessel reminiscent of an old-fashioned ocean ship, in favor of a system consisting of massive fixed (in solar orbit) facilities in which the payload/crew containing vessel is many orders of magnitude smaller than the total system. The “laser-powered light sail ship Promethius” described in Robert Forward’s 1985 novel Rocheworld is as good a speculative description of such as system as I’ve encountered, though it describes a much more modest performance of 0.01 g acceleration and 0.1 g deceleration for a maximum speed of 0.2 c on a 20 year trip covering 2 light years.

[Eclipse Now]

Just putting aside the fuel & acceleration technical arguments for a moment, what was with his calculations regarding time to Andromeda only taking 29 years? Was that relativistic ship time, while to the outside universe 2 million years has passed?

(He wasn't claiming some kind of Faster than light travel was he? )

[Jay-qu]

No, you are correct. In ship time it may only take 29 years, but to the (relatively) stationary observer it would take >2.5million years

[Eclipse Now]

Woah! "Planet of the Apes" time... hey, great way to test global warming theory. Take off in a ship, buzz around the interstellar neighbourhood for a few weeks, and come back a few hundred years later. Anyone read Orson Scott Card's "Enders Game" series where one had to basically say goodbye to everyone you knew and loved if going on a "trip"?

[Jay-qu]

Yep, basically its a one way time machine. I am sure if such a technology existed there would be many willing to throw some money in the bank and come back in a few hundred earth years..

[Eclipse Now]

As they say on Slashdot: What could possibly go wrong?