Hi, everyone. I have been watching this board for a few months now since I accidentally stumbled on a link to it in the credits of a Msting I found on Web Site #9. I have a task that could use the help of all of your collective nitpicking talents, and hopefully, provide a new and interesting topic to bicker about.

Ok, here is the deal. I am working on a science fiction book that involves a large spaceship. The ship, built in orbit, is probably 100 times the size of an aircraft carrier and was essentially designed to be a biosphere. It’s stocked with raw materials, factories, farms, solar panels and redundant nuclear power plants. The plan would be for the ship to serve several generations of people while it putted a long to god knows where.

The propulsion system is a set of engines that provide a very small, but constant thrust. Using F=ma, we see that this will yield a very small, but constant acceleration. My idea is, over years the ship builds up speed until it approaches the speed of light. I am not sure if any one knows what really happens when you get close to the speed of light, so to make matters simple, my thought was to have the crew cut the engines at 0.7c or something like that. In the absence of any other forces, the ship would remain at that speed using no power forever.

So there is my basic design. Everyone please rip it apart. What am I missing? Are there other forces in space then I am ignoring? I know gravitational forces are present, but if we plot a course to avoid any close contact to any large masses, with they be a problem? With our very small acceleration, inertia should not be a problem, correct? We can’t exactly stop on a dime, either. Actually, it could take years to stop the ship since we only have those same engines to provide a reversal force. What about space debris? With kinetic energy = mv^2 and my v being 0.7c, a piece of dust is going to feel like a cruise missile. Any ways around that with out resorting to Star Trek?

Debate anyway, loyal nitpickers

Clarke solved it by having an meteroid shield in front. This was essentially a huge ice plate.

Oh, and your concept of speeding up to .7c and cutting engines is reasonable. Boulle used it in the original Planet of the Apes novel. Why .7c? Why not more?

Just out of curiosity, what's your propulsion mechanism?

Another factoid, assuming a constant acceleration of 1g, it would only take about 1 year to get up to about .9c

v = at => t = v/a
v = .9c = 270,000,000 m/s
a = 1g = 10m/s^2
t = 2.7E8/10 = 2.7E7 = 27,000,000 s

Now, there are about 33 million seconds in a year, so you'd reach .9c in just under a year, given 1g. That time scales linearly with your constant acceleration (i.e. .5g accel = 2 year, etc...)

I kind of just pulled 0.7c out of the air. I think it’s left over in my head from Physics III in college. I could be mixed up, but I seem to remember 0.7c being the point where things got kind of hairy with relativity.

I was planning on using something similar to the rocket boosters the space shuttles use. Not sure what I would use for fuel, since it would be kind of hard to just “gas up” out in space. I did have another crazy idea using electromagnets and alternating current to create a constant opposing field in one direction, but I am not sure how that would work. Again, though, the thrust force can be very, very small since we are in no hurry. Like you said 1g gets us up to speed in a year.

.7c is where you get a gamma factor of roughly 2. That is, lengths contract, mass increases, time slows down by a factor of two.

The precise value is slightly more than .7071c
(sqrt(2)/2 * c).

Whoops! Sorry. .7c gives you a gamma of 1.4 (specifically sqrt(2)).

To get a gamma of 2, you need .866c

gamma = 1/sqrt(1 - v^2/c^2) =>
gamma^2 = 1/(1-v^2/c^2) =>
(to figure out v for a gamma of 2)
2^2 = 1/(1-v^2/c^2) => 4 = 1/(1-v^2/c^2) =>
1-v^2/c^2 = 1/4 =>
v^2/c^2 = 3/4 = .75 => v = sqrt(.75)c = .866c

One thing I noticed was a few errors in spelling; I hope you'll be meticulous about that when you get the book finished; it's annoying. Usually I don't say anything, but if you're sending it to a publisher, they really nitpick. And why did you say "book" instead of "novel"?
Robert L. Forward has some neat ideas in his books, both science fiction and science fact. One of his books is called "Future Magic".
Ion drive would be a good choice, since the ship can just grab the abundant ions from the interplanetary/interstellar medium. The acceleration would be very slow at first, but constant.
The Journal of the British Interplanetary Society often has ideas of a science-fictional nature, although they often have advanced math to back up their meticulously thought-out ideas.
One article earlier this year listed several different types of wormholes which might exist in the extreme conditions surrounding some stars. The author used some terms I didn't understand, so I tracked down his email address, sent him a note asking him to clarify, and he did kindly clarify it a little. (I can't be more specific--I don't have his email note at hand, and it's rather involved.)
Good luck.

I'd cut the engines at 0.8c because it make the relativistic time compression math easy at 2 years out of the ship equals 1 year in it. (Well except for any other ship the "outside" people may be on, etc.) Of course it has been a long time since college...

As I said above, it's actually .866c (sqrt(3)/2) to get a factor of 2.

I stand corrected. Thanks ScottN. It really has been a long time since college.

Steven,

What errors in spelling did you find?

Also, I thought ION engines were just in Star Trek and Star Wars. Does that actually work? What happens, do you just charge particles and let them shoot off a charged plate to get the "equal, but opposite reaction" thing? Wouldn't you eventually run out of material?

I read some theories on a solar sail, but it seemed like a bad idea. You take a sheet of gold and hammer it as thin as possible, and then hang it in front of your ship. Then photons bounce off of the sail, pushing it forward. A neat idea, but it seems hardly practical for the long term.

Jason,

The NASA probe Deep Space 1 was an ion drive. And you've described it properly. The thing about an ion drive is that you can refuel as you go (at least in a solar system), as there's plenty of material out there.

Solar sails don't have to be gold... you could use a polymer sail as well.

I'm sorry, I must have been in a dour mood when I posted that. People on *other* discussion boards are awfully careless about their spelling and grammar, and I guess I wanted to warn you to be more careful than them. I can't find any spelling errors in what you wrote. I was being overzealous or something, I guess.

Here is the web site of Wil McCarthy, a science and SF writer with a lot of interesting ideas. One new topic is programmable matter, with artificial atoms which could hypothetically have whatever amazing new properties people wanted.

http://www.sff.net/people/WMcCarth/

And his science articles:

http://www.sff.net/people/WMcCarth/fact.htm

I'll go stop nitpicking now and do something else.

Jason,
A solar sail needs to be miles across to move anything substantial. A solar sail would be used to move something from Earth orbit to the orbit of a superior planet, taking a good bit of time to get there; it probably wouldn't do for an INTERSTELLAR voyage since the available light would decline as it kept getting farther from Sol. Ion engines are very efficient in terms of fuel consumption, and would work as long as you have adequate propellant to ionize and adequate power to ionize it with; however, every article on the subject I've seen says that ion engines can only provide an acceleration of a FRACTION of a gee. For any kind of manned interstellar mission, you would probably need some sort of PLASMA rocket motor, powered by some sort of nuclear FUSION reactor. I suggest that you read up on "Bussard ramjets", which would refuel on-the-go from particles electromagnetically scooped out of interstellar space. Also, you might want to read some of the postings on "space.com" for more information and ideas. Good luck, and let us know how you're coming along from time to time.

ion engines can only provide an acceleration of a FRACTION of a gee

Remember... v = at.

let a = .01g or .1m/s². (OK, it's 0.098, but close enough).

That means, after one day (86400 seconds), you are now travelling at 86400 * .01 m/s, or 864 m/s, or roughly 3000 km/h. After one month, you would be travelling at 100,000 km/h. Of course, that's assuming a 0.01g acceleration. If you were accelerating at just .1g, you'd be travelling at a million km/h after a month. It adds up fast. For interstellar distances, you could accelerate for a long period of time.

Reposted from PM (This seemed like a good place for it...):

Please take a moment of silence today, tomorrow, and Sunday.

Today is the 37th anniversay of the Apollo 1 capsule fire.
Tomorrow is the 18th anniversary of the Challenger explosion.
Sunday is the first anniversay of the Columbia breakup.

Any chance they'll take a moment at the Superbowl to remember?

ScottN,

If the halftime show was U2 I'd lay money on it. (If not a moment of silence, something like their 9/11 tribute even if the names stopped at "E")

Unfortunately I doubt the weird ones sister has that much class.

Hey Scott! I was just flipping through next week's TV Guide and I found that as part of the pre-game show, there will be a performance by an 80 person choir in honor of the crew of Columbia. So yeah, there will be a tribute to one of the space tragedies you've cited. I plan to record it.

Go Carolina!

Unfortunatly, because of Janet Jackson's little stunt, few will remember the astronaut memorial part of the show.

I saw Bono's thing. It was nice, but my thought was a moment of silence would have been classier. Then I remembered. This is the Super Bowl. Classy is a word they don't know.

Frankly the game was so good I don't think Janet Jackson's nipple will be long rembered either.

(Personally I found the "tasteful" Cialis ads more disturbing than her proving she had no class.)

Back on topic:

For relativistic effects easy to calculate use 0.8c when time dilation is 50%, if I remember.

Blue it's 0.866c (see above). Specifically it's (sqrt(3)/2)c.

ScottN,

Thanks. It's been years since college.