Most likely candidate for future spaceship propulsion

[CraigD]

Originally Posted by Moontanman

A space elevator has huge potential but lots of bugs to work out.

Even including the words huge and lots, this is likely an understatement.

Originally Posted by Moontanman

I like the idea of a space elevator as well but i wonder about it's flaws. Like it's fragility, an errant aircraft or even more likely a purposeful aircraft could knock it out easily and then all the man years of construction and mega tons of materials would be lost.

I suspect that if and when the challenges of making a space elevator are met, aircraft strikes will not be counted as among the hard ones. Given the strength of the materials required to make a space elevator, aircraft – which are already pretty fragile compared to most manmade structures – would be like tissue paper against steel in a collision. Also, note that only a small fraction of the total structure of a space elevator would be at altitudes aircraft can reach.

It’s also worth noting that, although common in popular science fictional and speculative descriptions of them, it’s not a strict requirement that space elevators reach all the way to the ground. There’re numerous advantages to not reaching the ground or anchoring them, but rather having their terminals at high altitudes (10 km or more), to which passengers and cargo must be transported via fairly conventional aircraft. (Imagine what it’d be like to land a 747 on a “runway in the sky” at 12,000 m altitude! )

Originally Posted by Moontanman

I've also read that a trip up the elevator would take weeks or even months, I've looked but I can't find where I read that.

You don’t really need a reference to figure this out.

geostationary orbit is 35,786 km above the Earth’s surface. Divide by your best guess at the speed some sort of climbing elevator car could safely achieve. The fastest conventional elevators have speeds of about 10 m/s, giving a travel time of about 41 days. A fast train can go 50 m/s or so, for a travel time of about a week. Since most of the trip would be in a near vacuum, much greater speeds might be possible, but so much depends on the detailed engineering of the system that it’s hard to guess if high speeds are practical, or worth their cost and risk.

Originally Posted by Gardamorg

Space elevators would be great if achieved, no, they would be monumental.

I think nearly everyone agrees on this

Originally Posted by Gardamorg

This is because a self sustaining fusion reactor is THE greatest propulsion method, and a craft can be built large enough to fit a large fusion reactor on board if it doesn't have to escape earth.

Being able to put huge masses into orbit at costs potentially as low as surface shipping would almost certainly revolutionize spacecraft construction, but we should be wary of calling any energy source THE greatest.

As the ieee.org article Gardamorg links emphasizes, despite decades of research, nobody’s really confident that self-sustaining fusion power will be possible anytime in the near future, or even if it can be accomplished, how much net power it will produce.

Also, as the wikipedia article “energy density” describes, the theoretical maximum energy/fuel mass ratio for fusion is only from 3 to 20 time that of fission, and about 1/140 to 1/300 times that to the absolutely greatest fuel, matter-antimatter.

OK, I take it back, it seem pretty safe to name the greatest potential energy source, so long as it’s antimatter.

It’s also important to note that even having a tremendous source of energy doesn’t solve all your propulsion problems. Rockets need reaction mass, even if only in the form of the relativistic mass of photons, and the less reaction mass available, the more energy is required to produce the same thrust. The wikipedia article “nuclear photonic rocket”, while not primarily about such a system, has some applicable discussion.

[Roadam]

A while back I come upon an article in wikipedia about the launch loop. …

Moderation note: This and three related post have been moved to the thread “The launch loop”, because it’s a separate subject, deserving of its own thread.

[Gardamorg]

Originally Posted by CraigD

As the ieee.org article Gardamorg links emphasizes, despite decades of research, nobody’s really confident that self-sustaining fusion power will be possible anytime in the near future, or even if it can be accomplished, how much net power it will produce.

Also, as the wikipedia article “energy density” describes, the theoretical maximum energy/fuel mass ratio for fusion is only from 3 to 20 time that of fission, and about 1/140 to 1/300 times that to the absolutely greatest fuel, matter-antimatter.

OK, I take it back, it seem pretty safe to name the greatest potential energy source, so long as it’s antimatter.

But unlike Antimatter, after a certain point in time, a large enough self sustaining fusion reactor doesn't require any fuel, and it's energy production will eventually achieve greater energy than Antimatter Annihilation, and it will continue to achieve even greater amounts of energy after that.

[CraigD]

Originally Posted by Gardamorg

But unlike Antimatter, after a certain point in time, a large enough self sustaining fusion reactor doesn't require any fuel, and it's energy production will eventually achieve greater energy than Antimatter Annihilation, and it will continue to achieve even greater amounts of energy after that.

Wherever did you get these idea, Gardamorg?!

Nuclear fusion produces energy by transforming elements with low atomic numbers (eg: hydrogen, atomic number 1) into ones with higher atomic numbers (eg: helium, atomic number 2) The slight difference in mass between the beginning and end product (eg: ) results in energy, according to the famous old equation , where in this case is the difference in mass between the beginning and end products.

A fusion reactor needs fuel, in the form of low atomic number elements. What’s meant by “self sustaining” is not that the reactor doesn’t need fuel, but that it doesn’t need an outside source of energy to run, either more energy than it produces (an “under unity” reactor) or less (an “over unity” reactor, the goal of fusion power research). So far, all man-made fusion reactions have either been under unity (various experimental “controlled fusion” reactors, or fusion bombs, which produce much more energy than that of the explosives and fission bombs used to cause the fusion reaction, but are very destructive and difficult to harness for such things as electric power generation or spacecraft propulsion.

An ideal fusion reactor could use the end product of one fusion reaction (eg: ) as fuel for another (eg: ). Once a fusion reaction has produced nickel (atomic number 56), it’s impossible to use the end product to generate more energy, as the next fusion reaction produces end products that mass slightly more than their beginning. Such reactions are seen only in stars, which are in a sense ideal fusion reactors.

[Roadam]

Antimatter would be great if we would know how to produce it efficiently. At the moment I think the number stands at about a few percent.

So if efficient conversion would be possible, one could build large power plant of some sort(solar, fusion, fission,... whatever). And use the energy to produce and store antimatter. Containers could then be loaded onto a starship and there you go, fast space travel.

[CraigD]

Originally Posted by Roadam

Antimatter would be great if we would know how to produce it efficiently. At the moment I think the number stands at about a few percent.

According to Robert Forward’s 1995 IMHO classic popular science book “Indistinguishable from Magic”, antimatter factories are about 0.000003% (1 in 60 million) efficient. Including the cost of the factory and its operation, the present cost of antimatter at about ten trillion dollars per milligram. The present production rate of antimatter is about (one trillionth of a gram per day).

Originally Posted by Roadam

So if efficient conversion would be possible, one could build large power plant of some sort(solar, fusion, fission,... whatever). And use the energy to produce and store antimatter. Containers could then be loaded onto a starship and there you go, fast space travel.

In the same chapter/essay (chapter 1), he cites a study he did for the US Air Force concluding that, if a factory were designed carefully to maximize production (present day factories are based on accelerator/collectors designed to provide the maximize useful scientific data), the efficiency for on-grid, Earth-based factories could be improved to about 0.01% (1 in 10 thousand). The resulting cost is about ten million dollars per milligram, though production rate would still be too low to meet present day spaceflight demand. Still, assuming a first-generation antimatter powered water steam rocket, such a cost would be about 1/10th the cost of current chemical rockets for routine tasks such as orbiting satellites and sending spacecraft to other planets.

To produce antimatter at sufficient rates for routine space travel, production would need to be on the order of 1 g/day. According to Forward, this would require space-based factories with the equivalent of a present-day photovoltaic panel 100 km by 100 km at 1 Earth’s orbit (AU) distant from the Sun (which would equate to about 1/7th the area at a distance of about 0.38 AUs, just inside the orbit of Mercury.

My views on the subject are pretty close to Forward’s.

[Gardamorg]

Originally Posted by CraigD

According to Robert Forward’s 1995 IMHO classic popular science book “Indistinguishable from Magic”, antimatter factories are about 0.000003% (1 in 60 million) efficient. Including the cost of the factory and its operation, the present cost of antimatter at about ten trillion dollars per milligram. The present production rate of antimatter is about (one trillionth of a gram per day). In the same chapter/essay (chapter 1), he cites a study he did for the US Air Force concluding that, if a factory were designed carefully to maximize production (present day factories are based on accelerator/collectors designed to provide the maximize useful scientific data), the efficiency for on-grid, Earth-based factories could be improved to about 0.01% (1 in 10 thousand). The resulting cost is about ten million dollars per milligram, though production rate would still be too low to meet present day spaceflight demand. Still, assuming a first-generation antimatter powered water steam rocket, such a cost would be about 1/10th the cost of current chemical rockets for routine tasks such as orbiting satellites and sending spacecraft to other planets.

To produce antimatter at sufficient rates for routine space travel, production would need to be on the order of 1 g/day. According to Forward, this would require space-based factories with the equivalent of a present-day photovoltaic panel 100 km by 100 km at 1 Earth’s orbit (AU) distant from the Sun (which would equate to about 1/7th the area at a distance of about 0.38 AUs, just inside the orbit of Mercury.

My views on the subject are pretty close to Forward’s.

The stations would be destroyed by mass coronal ejections, you don't put something that close to the sun and expect it to survive, unless you could build it really strong, and if it weren't made of photovoltaic material, than it wouldn't work.

[CraigD]

Originally Posted by Gardamorg

The [solar-powered antimatter factory] stations would be destroyed by mass coronal ejections, you don't put something that close to the sun and expect it to survive, unless you could build it really strong, and if it weren't made of photovoltaic material, than it wouldn't work.

What’s your source of this prediction, Gardamorg?

We orbited Mariner 10 within the orbit of Mercury in 1974. CMEs occur from about 0.5 to 6 times a day, and Mariner 10 didn’t fail due to them, but rather when it ran out of propellant in 1975.

There are certainly challenges to operating spacecraft near the Sun. As with all spacecraft, preventing and managing damage to delicate electronics is critical. Heat dissipation, always a problem with spacecraft that generate a lot of heat, is a problem even for those that don’t. Mariner 10 managed heat in part by angling its solar panels to control how much it absorbed. MESSENGER, which is currently making a series of Mercury flybys to adjust it trajectory to begin orbiting Mercury in 2011, uses both solar panel angling, and a reflective and insulating heat shield that keeps most of the spacecraft in shadow.

The same team that designed and built MESSENGER, JHU/APL, is planning a dedicated solar probe spacecraft, named rather unimaginatively Solar Probe, planned launch in 2015, that will have an orbit that brings it within 10 solar radii (about 6.6e6 km) of the Sun, about 10 times closer than Mercury.

A close orbiting solar-powered factory like I describe would need a more effective heat dissipation system than spacecraft that have or are planned to closely orbit the Sun. Radiators are the obvious and traditional approach. I’ve read about a few more novel ones.