Propulsion Technology

[xouper]

Sundog: <span style="background-color: #ffc">The only time I ever object</span> to ideas of space colonization is when it is clear that they are being voiced as answers to our problems here on Earth. As in, "Yes, Earth is overpopulated, but when we have cheap space travel all those problems will go away."

The only time? Even though I never made such a claim, you objected to my comment about space colonization anyway (in another thread). Or, if you think that's the claim I made, then I respectfully avow you are mistaken.

 

[daver]

I believe the same thing Sundog does. While it seems unlikely, or abhorrent, to think of 'leaving everything' it is really the only way LONG range travel/colonization can take place (as we understand physics and the universe today).

In terms of propulsion, if we look at the ion drive (viable for inter-solar system jaunts), it works on the principal of slow, but steady and consistent acceleration, right? What if it was to slowly, but CONSTANTLY accelerate? Give it a HUGE fuel supply, and in essense, let it burn. You get it into orbit, at orbital speed. If it takes one week to double that speed (or 4 weeks for that matter) so be it. After 3 months, it is going to be clipping along pretty darn fast, right? Lets say it doubles in speed every week, starting at orbital speed (forgive me, as that number illudes me at the moment). How long would it take to reach near-light speed? In this case, it isn't a matter of new propulsion, but almost strickly fuel concerns.

DEEP had a fairly small fuel tank compared to solid or liquid fuel systems, so it seems it is far more efficient, right?

Doubling your speed every week isn't the way acceleration works, even Newtonian acceleration. Given Newtonian acceleration, if it takes one week to gain 10 km/sec, it takes another week to gain another 10 km/sec.

Ion drives are more likely to be measured in milli-g's, or maybe km/sec/day. That's not too bad for interplanetary travel. You're still talking months to years to get anywhere. After three months of accelerating, you'd be going at about .5 au/week. So maybe nine months to Jupiter, two years to Pluto.

However, for interstellar drive, it sucks. 10%c at milli-g accelerations is going to take on the order of a century. Getting up to 87%c (where time dilation makes shiptime half of earthtime) takes an enormous amount of energy--if the energy could be supplied from outside the ship, it would require a mass equal to that of the ship to be completely converted into energy and applied to the ship. If the ship had to carry its fuel with it, it would require even more mass. I don't feel like rederiving the relativistic rocket equations; you might be able to get to 87%c with an anti-matter drive at a reasonable fuel fraction--i expect someone could google the answers if they wanted.

Fast interplanetary drives are going to require enormous amounts of energy--maybe a nuclear salt rocket, maybe some Orion variant, maybe fusion. Anti-matter would be great.

Anti-matter is about your only choice for interstellar drives that get to their destination within their crew's lifetimes.

 

[chance]

To me it seems likely that a drive capable of 10% c is not beyond the realms of science fiction, therefore the next problem is the human cargo. Hibernation would have to be achieved, or better grow your body (modified) once at the destination.

 

[rwald]

Or if you travel fast enough, time dialation will take care of it for you...

Personally, I like the idea of a ship accelerating at 1 g for the first half of the journey, and then in the middle turning around and decelerating at 1 g. I think that gets you to .999992c in 6 years and 3 weeks of ship's time.

 

[Crossbow]

Keep in mind that as far as space exploration goes, even if it takes a near luminal ship several decades to reach another star, this is only as measured in the earth's frame of reference. In the ship's frame of reference (and therefore the crew's frame of reference) it will take less time.

Yes that is true, however that would mean that for anyone who went on such a trip by the time they came back to Earth, then everyone who was not on a similar voyage would be dead.

As such, I expect that it would be quite difficult to find people willing to sign up for such a mission.

 

[rwald]

Anyone coming off a really bad relationship.

I see a movie about this, starring Bruce Willis.

 

[rockoon]

Yes that is true, however that would mean that for anyone who went on such a trip by the time they came back to Earth, then everyone who was not on a similar voyage would be dead.

As such, I expect that it would be quite difficult to find people willing to sign up for such a mission.

Where do I sign?

 

[Wile E. Coyote]

It refreshing to see the level-headed skepticism going on in this thread. The last time I saw a thread about the feasability of distant space travel, it was in reaction to the Straight Dope column, Why do we travel in space?. The reaction is at http://boards.straightdope.com/sdmb/showthread.php?s=&threadid=167867. Most of the people who responded said that Cecil lacked vision. I say he could see the plain truth perfectly well.

I do not feel that space travel is a waste of time. I am just being realistic in that it will not be like Star Trek and that it will require the same kind of pioneering spirit that early settlers of the new world were known for.

Firstly, I truly believe that death is not inevitable. Science will eventually find a way to preserve human life forever, and then star travel that takes a thousand years will not seem so bad.

Secondly, I envision a "slow" ship that is extremely large, has a spin-induced artificial gravity, and is self sufficient. The ship would have a lot of satellites that would venture away from the ship to obtain resources, and the interior of the ship would be an enormous cylinder in which towns and forests and gardens would be available.

This also assumes that virtual reality comes to the point where it is indistinguishable from reality. Without this, I think a lot of people would become insane.

 

[Sundog]

The only time? Even though I never made such a claim, you objected to my comment about space colonization anyway (in another thread). Or, if you think that's the claim I made, then I respectfully avow you are mistaken.

I stand corrected!

I wasn't referring to you. The objection WAS related, though, as I felt you were making the same error (in my view) as others regarding availability of resources.

 

[Dr. Imago]

Or, should we even bother to explore/colonize space?

I think what's fascinating is that we, the current height of evolutionary achievement on the Earth, feel compelled to explore space and move to other planets as a long-term goal. We regularly visit this topic in our literature, and never cease to seem to be fascinated by the stars.

Yet, we are literally loaded with bacteria, latent viruses, and other microorganisms. So, it seems that if we accomplish this goal, it could possibly be argued that really the bacteria have simply figured out a quite ingenious way to get themselves to other places in the galaxy.



-TT

 

[clusterm2]

Also note that there is a limit to the amount of acceleration the human body can survive. I think we will end up being very practical and stick with an acceleration of 10 meters/sec or there abouts. This acceleration will simulate earth gravity nicely. It will only take a year or so to get near the speed of light.

Yes yes yes but when we have our inertia-less drives it won't be a problem. We just need to get the inertia bit out of the drives then Bobs your uncle, simple.

 

[sorgoth]

Yet, we are literally loaded with bacteria, latent viruses, and other microorganisms. So, it seems that if we accomplish this goal, it could possibly be argued that really the bacteria have simply figured out a quite ingenious way to get themselves to other places in the galaxy.

Heh. Maybe bacteria will become the true colonisers of space, remaining in places where all humans have long left...

Yes yes yes but when we have our inertia-less drives it won't be a problem. We just need to get the inertia bit out of the drives then Bobs your uncle, simple.

...Inertia-less drives? Yeah, right. Making an artificial wormhole would be easier than that.

 

[rwald]

But don't they have inertial dampeners in Star Trek? Why can't we make them in reality?

 

[xouper]

rwald: But don't they have inertial dampeners in Star Trek? Why can't we make them in reality?

We can't? Heard on a recent commercial flight from Detroit to Tampa:

"This is your captain. We are now at a cruising altitude of 35,000 feet and there are reports of mildly turbulent air ahead, so we will be turning on the intertial dampers, and you will again be free to move about the cabin. Thank you for flying Enterprise™ Airlines."

 

[daver]

Or if you travel fast enough, time dialation will take care of it for you...

Personally, I like the idea of a ship accelerating at 1 g for the first half of the journey, and then in the middle turning around and decelerating at 1 g. I think that gets you to .999992c in 6 years and 3 weeks of ship's time.

Well, Bussard rams have been discredited, so there's no longer any feasible mechanism for reaching that speed.

 

[rwald]

Remind me, what are Bussard rams?

 

[TexasBEAST]

Remind me, what are Bussard rams?

They call them Bussard "scoops" in Star Trek, right?

Ship-mounted collectors of space debris for use as fuel, theoretically allowing you not to have to carry everything onboard the ship from the very beginning.

Unfortunately, reality is there's just not enough particulate density out there to really be of much benefit. But still enough to blow a Columbia-esque hole in your hull.

---

Question: Antimatter anhilation is the greatest energy yield reaction yet known right?

Q: The greatest problem with action-reaction systems now is the mass of fuel required to be carried, right? The duration of the trip and time paradoxes and all that, those are uncomfortable to think about, but perfectly doable. Just the problem of how do you carry enough fuel?

Q: If a warp drive were built, would it really even be "propulsion" anymore? Or "drive"? Seems to me that it would be an environmental control device, rather than a ship propulsion device. Your nuclear reactor "impulse drive" would be "propulsion"; your "warp reactor" would not.

 

[rwald]

I'll try to answer your questions, TexasBEAST...

Q1: Well, with a matter-antimatter drive, you convert 100% of the mass of the fuel into energy. Therefore, I'm pretty sure that it's the greatest energy yield reaction. Of course, that doesn't account for the storage containment system, a system to ensure that all the energy goes into propulsion (instead of some of the gamma rays being absorbed by the ship), etc. But I suspect that you're already aware of this.

Q2: The other problem is just making enough fuel. Currently, antimatter is made one atom at a time, which isn't the most efficient way to create fuel. So, these are the two main problems from a practical stand point.

Q3: I guess it would depend on your definition of "propulsion." If by "propulsion" you mean ejecting stuff out behind you so that you accelerate forwards, than no, a warp drive is not "propulsion." However, if you defined propulsion more loosely, than warp drive could be seen as propulsion. In the end, it doesn't really matter.

 

[Walter Wayne]

Q: The greatest problem with action-reaction systems now is the mass of fuel required to be carried, right? The duration of the trip and time paradoxes and all that, those are uncomfortable to think about, but perfectly doable. Just the problem of how do you carry enough fuel?

This doesn't just apply to action-reaction systems. Assume 100% matter-energy conversion, at that all energy is turned to kinetic energy, and accelerating to any appreciable speed requires enormous amounts of fuel. So perhaps generational ships are much more reasonable than using time dilation to fix everything.

Walt

P.S. If my calcs are right (big if), to get to .999992c (as mentioned by rwald's in a post above) one requires the fuel to about 250 times the mass of the remaining ship and cargo.

 

[LucyR]

Doubling your speed every week isn't the way acceleration works, even Newtonian acceleration.

I suggest that you take a few steps back and consider the difference between constant and non-constant acceleration.

Lucy.