Metaphysics vs. Atheism

[sphenisc]

Isn't it possible to set up fuel dumps on the way loosely analogous to the truck in the desert puzzle?

http://mathforum.org/library/drmath/view/55689.html

Obviously its got the added complexity of 'coasting' fuel dumps. But there's no need for return trips for the automated placement of fuel dumps. Of course on the way back you're going to be meeting fuel dumps going at some speed!

The most fuel efficient method is left as an exercise for the reader.

 

[Hellbound]

Hmmm.

US energy use is equivalemt to about 800 tons (roughly) of mass yearly, which is about 725,000 kg. So that makes it 9.86x10^-20 percent of the moon's weight yearly.

 

[sphenisc]

How much hydrogen etc. can you pick up in the interstellar medium?

 

[Kaarjuus]

To put the amount of hydrogen needed in better perspective, according to this, there is roughly 1,4x10^21 litres of water in the world. As hydrogen composes 1/9 of the mass of a water molecule, we would need to convert a ninth of the world's water to fuel.

Edit: spelling mistakes.

 

[Hellbound]

How much hydrogen etc. can you pick up in the interstellar medium?

Not a lot, but now you're introducing two more problems. First, adding the weight of equipment needed to scoop the hydrogen. Second, you're adding to the drag of the spacecraft in the interstellar medium, which means more energy is needed.

IIRC the interstellar medium has a density of somethig around 2 to 3 molecules per m³, but that's recall and I have no idea to the accuracy of that figure.

Ah, check the Wikipedia entry for Bussard Ramjet.

Interstellar Space contains an average of 10^-21 kg of mass per cubic meter of space. This means that the ramjet scoop must sweep 10¹⁸ cubic meters of space to collect one gram of ions per second.

Also gives some info on drag force.

 

[drkitten]

Using Fusion, I'll take 650 trillion Joules to mean 6.5x10¹⁴ J.

Choosing a 'cruise' velocity of 0.5c (which gives a reasonable time dilation), I get 0.5(1.5x10⁸m/s)²(10000kg) = 1.125x10²⁰ J. Dividing gives approx. 173,000 kg, or am I wrong?

\

No, you're not wrong. You were fusing the hydrogen -- I was simply burning it in the calculation you cited. Technically I screwed up because I didn't bring along any oxygen to go with it, about eight times as much by weight.

The reason that I mentioned burning hydrogen is because it's about the best/densest chemical fuel we currently know of (and I think we have reason to believe that it's simply the best there is). If you want to start talking about non-chemical power (e.g. fission, fusion, total conversion, &c), then there's the basic problem that we don't know how to make thrust from any of them. So you can get a lot more energy out of fusing hydrogen than out of burning it, but it's not clear how we can make a fusion-based reaction drive.

 

[CaveDave]

... some of those might have inferred with our cultural and/or biological development. ...

[Speeling Police]
You surely intended interfered, didn't you?
[Speeling Police]

Cheers,
Dave

 

[Hellbound]

\

No, you're not wrong. You were fusing the hydrogen -- I was simply burning it in the calculation you cited. Technically I screwed up because I didn't bring along any oxygen to go with it, about eight times as much by weight.

The reason that I mentioned burning hydrogen is because it's about the best/densest chemical fuel we currently know of (and I think we have reason to believe that it's simply the best there is). If you want to start talking about non-chemical power (e.g. fission, fusion, total conversion, &c), then there's the basic problem that we don't know how to make thrust from any of them. So you can get a lot more energy out of fusing hydrogen than out of burning it, but it's not clear how we can make a fusion-based reaction drive.

I think the assumption is simple thermal reactions. i.e.-a fission reaction, using the heat from it to expel a reaction mass (which may be the fission by-products themselves). Or fusion, allowing an opening in the fusion chamber to expel the helium/wahtever that's created (which is heated by it's own fusion). THis, of course, assumes we are able to control and confine fusion within the near future, as well as pass the break-even point.

Antimatter is the easiest (if one ignores containment problems), simply dump your antimatter into the chamber with some hydrogen. THe antimatter reaction heats the hydrogen as your reaction mass. IT also has an advantage in that for longer trips, instead of increasing your reaction mass, you simply increase the antimatter amount and inject more at a time, increasing the specific impulse of the exhaust (so the same amount of hydrogen ejects faster, producing more thrust...or a smaller amount of hydrogen ejects faster, producing the same thrust).

NASA has done testing on thermal designs, using nuclear power to heat reaction mass. Look here under Thermal Rockets for a brief bit on it. Thermal engines seem capable of producing specific impulses 2 to 4 times as much as chemical fuels. Ion rockets have low thrust and power, but high efficiency and very high specific impulse.

There are possibilities for higher-energy chemicla fuels, as well, on that same page: Hydrogen-Flourine, Hydrogen/Ozone, and Beryllium/oxygen all have higher potential specific impulses than hydrogen/oxygen. All have drawbacks, though (acidic, explosive, and toxic, respectively).

 

[Hellbound]

Interestingly, the same site I linked to earlier gives a nice graph describing how your delta-V (desired change in speed measured in km/s) relates to the percentage of vehicle that must be fuel. The exhaust velocity of the engines figures into it, as well. So with antimatter, assuming an exhaust velocity of the speed of light, and a 1G max acceleration, we can figure out total deltaV assuming a constant acceleration out and back. 1G constant will reach c before our halfway point (well, not actually, but as close a percentage as makes not much difference, about 96% at half the distance). So best-case scenario we'd need 85% of our mass in fuel (Delta-V to c, then deccelerate again, so total delta-V is 2c (well, about 1.92c, but close enough).

 

[Dave1001]

To put the amount of hydrogen needed in better perspective, according to this, there is roughly 1,4x10^21 litres of water in the world. As hydrogen composes 1/9 of the mass of a water molecule, we would need to convert a ninth of the world's water to fuel.

Edit: spelling mistakes.

Well, clearly the earth's water would probably not be our target source of hydrogen fuel. Where else in the solar system would be good sources of hydrogen fuel? If we're limited to water, I suspect the Earth only has a tiny fraction of the water we need.

 

[Dave1001]

According to these stats, world oil production in 2005 was 84,361,000 barrels per day. The weight of oil depends on its source, but for about 8 barrels per tonne, this makes 3848970625000 kilograms per year. So, about 3.85×10^12 kilograms. The weight of the moon is about 7,35×10^22 kilograms. So the moon has about 2×10^10 times greater mass than the oil produced in 2005 in the whole world. So a lot more than the hundredth of one percent.

I don't know to what degree we're comparing apples and oranges here. Your numbers don't seem reconcilable with the numbers others have posted that the trip would take a few percentage point of the energy Americans expend in a year.

It seems counterintuitive to me that accelerating and decelerating a space ship that only weighs 10 tons and its fuel such that the passengers experience .1G to 1 G of force for a limited number of years would expend so much more energy that the earth's human population currently consumes in a year. Am I missing something? For starters, how long would the trip take from the perspective of observers from Earth? Huntsman? I presume something less than a gazillion years?

 

[Hellbound]

I don't know to what degree we're comparing apples and oranges here. Your numbers don't seem reconcilable with the numbers others have posted that the trip would take a few percentage point of the energy Americans expend in a year.

It seems counterintuitive to me that accelerating and decelerating a space ship that only weighs 10 tons and its fuel such that the passengers experience .1G to 1 G of force for a limited number of years would expend so much more energy that the earth's human population currently consumes in a year. Am I missing something? For starters, how long would the trip take from the perspective of observers from Earth? Huntsman? I presume something less than a gazillion years?

You're missing my corrections, in that the ship would weigh 10 tons for the payload only. The actual ship would weigh in at almost 400 tons, most of that fuel mass. It's corrected in my first post above, which I mentioned the first time you asked about it. This changes the figure to about 40% of U.S. energy use.

However, my calculations assume a few things that are unrealistic:

1. A 100% matter-to-energy conversion
2. A 100% efficient use of turning that energy into thrust.

Both of these are absolutely unsupportable for final figures. At a minimum, you have to double the energy requirement just to create the antimatter (again, assuming no energy is lost, you get 50% efficiency in manufacturing antimatter). Then, I did not account fo any reaction mass or any way for the energy release by the matter/antimatter reaction to be converted to thrust. A large portion of this will be released as gamma radiation, which can't be effectively used for thrust. Plus, the heating of the reaction chamber and surroundings, light released, etc, etc, etc. My estimate of 50% was likely far off before, with a bit more thought.

MOre likely efficincies are about 10%. So, you have 10%*50%, which means the total process is about 5% efficient. So, that 40% of the U.S energy budget translates to about 800% of the U.S. budget, which is about 30% more than the world energy budget for a year.

And we haven't built the ship, the antimatter production facility, or anything else.

To add more, currently the best proposed efficiency for an antimatter production line is 1%.

There's a LOT of problems to be overcome before we're anywhere close to this.

 

[Dave1001]

You're missing my corrections, in that the ship would weigh 10 tons for the payload only. The actual ship would weigh in at almost 400 tons, most of that fuel mass. It's corrected in my first post above, which I mentioned the first time you asked about it. This changes the figure to about 40% of U.S. energy use.

However, my calculations assume a few things that are unrealistic:

1. A 100% matter-to-energy conversion
2. A 100% efficient use of turning that energy into thrust.

Both of these are absolutely unsupportable for final figures. At a minimum, you have to double the energy requirement just to create the antimatter (again, assuming no energy is lost, you get 50% efficiency in manufacturing antimatter). Then, I did not account fo any reaction mass or any way for the energy release by the matter/antimatter reaction to be converted to thrust. A large portion of this will be released as gamma radiation, which can't be effectively used for thrust. Plus, the heating of the reaction chamber and surroundings, light released, etc, etc, etc. My estimate of 50% was likely far off before, with a bit more thought.

MOre likely efficincies are about 10%. So, you have 10%*50%, which means the total process is about 5% efficient. So, that 40% of the U.S energy budget translates to about 800% of the U.S. budget, which is about 30% more than the world energy budget for a year.

And we haven't built the ship, the antimatter production facility, or anything else.

To add more, currently the best proposed efficiency for an antimatter production line is 1%.

There's a LOT of problems to be overcome before we're anywhere close to this.

I saw your corrections although thanks for spelling it all out clearly here. My responses were to people that described using non-antimatter fuel like hydrogen fuel. Is antimatter a speculative fuel? If so, let's keep it to fuels that we know for a fact are usable in 2006.

I assume that that doesn't affect energy efficiency calculations of 10% efficiency, which is sounds like your saying is what's known to be realistic with technology in 2006.

But, I assume that using non-antimatter fuel affects the amount of fuel that will have to be transported, meaning that a lot, lot more fuel would have to be transported. How much hydrogen fuel would have to be transported for example? Where would we get if from? And do you think it could be done with today's technology? If not, how much of a leap would have to be made within a few centuries?

I guess, in short, could you redo these calculations, but using a known usable fuel source rather than antimatter? Could you give your assessment on whether we have sufficient quantities of this fuel source in our solar system that regular interstellar trips could be fueled by it? How much it would affect the entire mass of the space vessel throughout the trip? And what percentage of global energy use in 2006 terms would be needed to be expended to fuel a round trip? Thanks

 

[Jimbo07]

There's a LOT of problems to be overcome before we're anywhere close to this.

Columbus died in 1506, according to Wikipedia, still believing he had reached Asia. That is, by 1506, he was still underestimating the magnitude of his own problem.

In 2006, we're still crawling into orbit on experimental spacecraft, awed by still further distances that boggle the imagination. 500 years and we stand on an ocean vastly different than any previously imagined.

Now, if his ships had a top speed of between 5 and 15 knots, I'll generously say they went 25 km/h. The space station orbits at about 27,000 km/h, or roughly 1000 times the speed of ships 500 years ago, in a vastly different environment.

A 1000-fold increase over that would be, say, 25 M km/h, or 6.94x10⁶ m/s gives a little more than 0.2c or 500 years from now, 1000 times the speed of ships today, in a vastly different environment (interstellar space).

Now sometimes, no matter what resources you throw at a problem, you encounter physical limits. A good example is transistor scaling. However, there is usually someone willing to look at workarounds, such as increasing die size if transistor density isn't improving, different semiconductor and interconnect technologies, etc.

...

drkitten and I have crossed on this before (specifically, FTL at the time). My two wild analogies are a little glib, but I maintain that impossible is a very strong word. Impossible is radically different from, "There's a LOT of problems." In fact, impossible doesn't even mean the same as a LOT LOT of problems. You'd have to have a googolplex of problems without even the wild imaginings of a solution before you got anywhere near impossible.

I wouldn't let the failure of realizing the movie 2001 inform wild speculation about events 500 or more years from now...

ETA: Just to be clear, my love for interstellar travel is fuelled much more by science fiction than science fact, which I freely admit. I think there is an interesting enough set of problems with interplanetary travel, and should I be lucky enough to do any instrumentation work on any spacecraft that actually flies, I will consider myself to be more than fulfilled.

 

[Hellbound]

I saw your corrections although thanks for spelling it all out clearly here. My responses were to people that described using non-antimatter fuel like hydrogen fuel. Is antimatter a speculative fuel? If so, let's keep it to fuels that we know for a fact are usable in 2006.

I assume that that doesn't affect energy efficiency calculations of 10% efficiency, which is sounds like your saying is what's known to be realistic with technology in 2006.

But, I assume that using non-antimatter fuel affects the amount of fuel that will have to be transported, meaning that a lot, lot more fuel would have to be transported. How much hydrogen fuel would have to be transported for example? Where would we get if from? And do you think it could be done with today's technology? If not, how much of a leap would have to be made within a few centuries?

I guess, in short, could you redo these calculations, but using a known usable fuel source rather than antimatter? Could you give your assessment on whether we have sufficient quantities of this fuel source in our solar system that regular interstellar trips could be fueled by it? How much it would affect the entire mass of the space vessel throughout the trip? And what percentage of global energy use in 2006 terms would be needed to be expended to fuel a round trip? Thanks

The 10% figure was a more realistic efficiency for an antimatter drive.

Conventional fuels are in the 1% range, at best. I couldn't even begin to calculate the numbers on that, but it going to end up being several times the amount of energy the world produces in a year.

 

[Schneibster]

With slowships, you have two problems:
1. Energy source.
2. Reaction mass.
Only a nuclear reaction can provide the energy needed; chemical reactions produce too low an energy density to be practical (a tenth of the Moon's mass, or some such absurdity). Thus, it's fission or fusion, from what we know right now, and fusion is pretty woo; antimatter isn't even on the table yet. We might do something with antimatter, once we can make it, and it would certainly be more powerful than anything we're going to do with either fission or fusion, but I suspect we have quite a ways to go before we have practical fusion, I'll be surprised if we have anything up and running on a practical basis before mid-century, and antimatter looks a lot farther away than that.

I'd go so far as to predict that when (yes, when, not if) we first send something or someone to another solar system, and I think it will be late in this century, we'll use solar sails for a significant amount of the acceleration and braking at both ends, and slingshot around the star in question (whether that be the Sun on this end or the destination system's star on the other) to maximize the benefits of the reaction drive we will no doubt start with; something that we can burn and make a lot of acceleration really fast, and keep as much momentum as we can figure out how through the slingshot. After we're out of the slingshot, or perhaps merely after we've exhausted our initial push, an ion drive is quite likely, since it provides the highest efficiency of acceleration of the reaction mass; most likely at a very low thrust, like well under a tenth of a gravity, perhaps as low as a hundredth. You get a lot more bang for the buck accelerating charged particles using an electromagnetic field than you do heating them, unless you have a really good heat source, which we don't (even fusion won't do it; fusion makes more energy in charged particles than it does in heat). This takes care of both the energy source and reaction mass problems, but at the cost of a very long transit time. Someone do the math; I'm too lazy. The math on the solar sail will be interesting too, both in terms of size vs. thrust, and in terms of bundling it up when it stops being a winning proposition and then carrying it to the destination, then spreading it again, etc.

As far as FTL goes, as far as I can tell from a great deal of research, and I'll even confess to having begun with a bias toward it, it looks impossible. The problem is that if you travel from point A to point B at FTL speed, there will always be some possible observer for whom you would arrive before you left, and this violates mass/energy conservation because you would have to exist at both the source and destination at the same time. In other words, unless mass/energy conservation is wrong, FTL is impossible.

Now, if I had found instead, for instance, that it just would require relativity to be wrong, you know, Albert had a little brain fart, heh heh, then I might buy that; but mass/energy conservation? No way. Not gonna happen. It's just too basic to the way that reality appears to work. Sorry to pee in your cheerios.

 

[Kaarjuus]

I'd go so far as to predict that when (yes, when, not if) we first send something or someone to another solar system, and I think it will be late in this century

Mankind will have great trouble powering itself late in this century after oil supply dries up and you think we'll have enough power for interstellar flight?

Using a solar sail, if we even are able to develop one that works well, will take forever. This would require a self-contained little world, with hundreds of generations of people manning the craft. I enjoy science fiction immensely, but this is unrealistic.

 

[Dave1001]

Mankind will have great trouble powering itself late in this century after oil supply dries up and you think we'll have enough power for interstellar flight?

I think that's a bit pessimistic (not talking about the interstellar flight aspect), because we have so many other demonstrated sources of energy, we have so many demonstrated ways to increase energy use efficiency, etc. My understanding is that we primarily use oil for energy not because it's the only energy source in abundance to serve our needs, but because it's currently the cheapest to extract and use (and further, my impression is that it's currently the cheapest to extract and use due to existing oil-based infrastructure build-up).

 

[Dave1001]

The 10% figure was a more realistic efficiency for an antimatter drive.

Conventional fuels are in the 1% range, at best. I couldn't even begin to calculate the numbers on that, but it going to end up being several times the amount of energy the world produces in a year.

Too bad (that you don't think you could make a calculation with that) cause I suspect it will still seem on balance realistic. As for it taking several time the amount of energy the world currently produces in a year, that doesn't seem like it should be especially prohibitive within the next several hundred years. It's probably not fair to imagine that in 300 years, the amount of energy the world produces in a year will be a similar scale leap as from 1700 to the present. But I'm curious how our energy production has increasted from 1950 to the present, given that I think our energy technologies are basically the same now as they were then (but my sense is that our economy and energy production has increased by orders of magnitude).

 

[Dave1001]

With slowships, you have two problems:
1. Energy source.
2. Reaction mass.
Only a nuclear reaction can provide the energy needed; chemical reactions produce too low an energy density to be practical (a tenth of the Moon's mass, or some such absurdity). Thus, it's fission or fusion, from what we know right now, and fusion is pretty woo; antimatter isn't even on the table yet. We might do something with antimatter, once we can make it, and it would certainly be more powerful than anything we're going to do with either fission or fusion, but I suspect we have quite a ways to go before we have practical fusion, I'll be surprised if we have anything up and running on a practical basis before mid-century, and antimatter looks a lot farther away than that.

I'd go so far as to predict that when (yes, when, not if) we first send something or someone to another solar system, and I think it will be late in this century, we'll use solar sails for a significant amount of the acceleration and braking at both ends, and slingshot around the star in question (whether that be the Sun on this end or the destination system's star on the other) to maximize the benefits of the reaction drive we will no doubt start with; something that we can burn and make a lot of acceleration really fast, and keep as much momentum as we can figure out how through the slingshot. After we're out of the slingshot, or perhaps merely after we've exhausted our initial push, an ion drive is quite likely, since it provides the highest efficiency of acceleration of the reaction mass; most likely at a very low thrust, like well under a tenth of a gravity, perhaps as low as a hundredth. You get a lot more bang for the buck accelerating charged particles using an electromagnetic field than you do heating them, unless you have a really good heat source, which we don't (even fusion won't do it; fusion makes more energy in charged particles than it does in heat). This takes care of both the energy source and reaction mass problems, but at the cost of a very long transit time. Someone do the math; I'm too lazy. The math on the solar sail will be interesting too, both in terms of size vs. thrust, and in terms of bundling it up when it stops being a winning proposition and then carrying it to the destination, then spreading it again, etc.

Fascinating! Is all of the technology you discuss here currently available and proven? Fission (is that the same as currently used atomic power?) and the technology that a solar sail uses? ion drives accelerating charged particles using an electromagnetic field? In other words, would it be technically possible to launch such a mission with today's technology? If some of these technologies are still speculative, then could such a mission be launched without them? If it can be done with today's technology, and the only prohibitive factor is cost, then I agree with you that such manned adventures will be launched, although I would probably stick with the several hundred year window. That we have not returned to the moon in a manned mission in 30 years is an indication to me that it will be hard to predict the rate of manned mission envelope-pushing in the time frame of decades.