Lt. Commander
Join Date: Dec 2007
Posts: 120
# 21
03-28-2012, 09:57 AM
Quote:
Originally Posted by AAA2513
The best aproach would be to accelerate continously Reaching relativistic speeds within a few months
Hmmmmmmmm -scratches chin-
Average weight of human, 80 kg.
Let's assume each passenger will need 120 kg of support (renewable food, medicine, etc).
Let's also assume we have 100 humans to start a small colony.

We now have a 20,000 kg spacecraft, which is pretty much the absolute minimum. The kinetic energy of such a ship going at 0.95 c is gonna be ....
KE = m c^2 [(1-v^2/c^2)^(-1/2) - 1] ≈ 2.2 m c^2
≈ 4 * 10^21 Joules

For reference, the *world* energy production in 2008 was 4.7*10^20 Joules.

So it would take the total output of the world's electricity 10 years to accelerate this craft. Double that, actually, since the propellant coming out the back would have to have the same energy going the other way.
Lt. Commander
Join Date: Dec 2007
Posts: 120
# 22
03-28-2012, 09:59 AM
Quote:
Originally Posted by ThePsycoticVulcan
What's the point of that??
The point of it is that it's feasible within modern technology - we have the propulsion and food production technologies on smaller scales and have proven they can be scaled, ultimately we're only really missing the orbital construction capacity to build something of that size.

Cryogenics, however, hasn't even been established as possible on humans. Only a few vertebrates have been frozen and revived, and only over short periods of storage with low success rates and drastically reduced life expectancy afterward. Even all our cryogenically stored corpses are just expensive caskets - reviews of the facilities have shown that even if it were possible, their technique causes more irreparable damage than standard embalming does, and isn't even done properly most of the time.



Quote:
I saw on a Stephen Hawking documentary that if the ship reached 99% of light speed, the time dilation would be so that the crew would only experience a day for every year of travel. So 600 years = 600 days for the crew.
Like most of his stuff that wasn't written for the scientific journals, it's a bit simplified. At 99% of c, you'll experience one day for every seven days of travel. Doing a little playing with the calculator in my last post, you get to the 1 day=1 year point at roughly 99.999621% of light speed. (factor 363.21670029572545 - almost close enough to account for leap years even).
Lt. Commander
Join Date: Dec 2007
Posts: 120
# 23
03-28-2012, 10:00 AM
Well a generational ship could be done with today's technology, or near future technology. We're not that close to relativistic travel or inertial dampers. Or cryostasis chambers for that matter.
Lt. Commander
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Posts: 120
# 24
03-28-2012, 10:01 AM
Quote:
Originally Posted by ThePsycoticVulcan
Not if we have inertial dampeners.
The best aproach would be an electron beam emitted from the vessel to place a negative charge on any dust particles which would allow the particles to be deflected magnetically.
Lt. Commander
Join Date: Dec 2007
Posts: 120
# 25
03-28-2012, 10:04 AM
Quote:
Originally Posted by hort_wort View Post
Hmmmmmmmm -scratches chin-
Average weight of human, 80 kg.
Let's assume each passenger will need 120 kg of support (renewable food, medicine, etc).
Let's also assume we have 100 humans to start a small colony.

We now have a 20,000 kg spacecraft, which is pretty much the absolute minimum. The kinetic energy of such a ship going at 0.95 c is gonna be ....
KE = m c^2 [(1-v^2/c^2)^(-1/2) - 1] ≈ 2.2 m c^2
≈ 4 * 10^21 Joules

For reference, the *world* energy production in 2008 was 4.7*10^20 Joules.

So it would take the total output of the world's electricity 10 years to accelerate this craft. Double that, actually, since the propellant coming out the back would have to have the same energy going the other way.
Utilizing the gravitational fields of stars starting with our very own sun to accerate and decelerate could reduce energy consumption considerably. Imaging the speed one could obtain over the severl months of droping in for close pass of our sun from a distance out past Pluto.
Lt. Commander
Join Date: Dec 2007
Posts: 120
# 26
03-28-2012, 10:11 AM
Quote:
Originally Posted by hevach View Post
Like most of his stuff that wasn't written for the scientific journals, it's a bit simplified. At 99% of c, you'll experience one day for every seven days of travel. Doing a little playing with the calculator in my last post, you get to the 1 day=1 year point at roughly 99.999621% of light speed. (factor 363.21670029572545 - almost close enough to account for leap years even).
A bit simplified? A week is a lot shorter than a year!
Lt. Commander
Join Date: Dec 2007
Posts: 120
# 27
03-28-2012, 10:11 AM
Gravitational slingshot only works if the object you're using is also moving in the approximate direction you're trying to go, and you need to start from outside it's gravitational influence (these two combine so that you're "falling" much longer than you're "rising" after perigee so you get a net gain). If you start from an orbit around the object, getting back out to that distance will rob you of what speed you gained. That's why probes don't slingshot out of earth orbit, they use boosters and slingshot on a second pass.
Lt. Commander
Join Date: Dec 2007
Posts: 120
# 28
03-28-2012, 10:19 AM
Another issue is one of fuel for propulsion. If you are using rocketry (since we haven't discovered subspace-distortion-based impulse drive or inertial damping), then to get up to near-light speeds, the only fuel that would work is antimatter. Fusion would not work, as the energy released would be about three hundred times less than for antimatter, which would result in needing impractically large (and heavy) fuel tanks.
Lt. Commander
Join Date: Dec 2007
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# 29
03-28-2012, 10:25 AM
Quote:
Originally Posted by ChibiClari View Post
Another issue is one of fuel for propulsion. If you are using rocketry (since we haven't discovered subspace-distortion-based impulse drive or inertial damping), then to get up to near-light speeds, the only fuel that would work is antimatter. Fusion would not work, as the energy released would be about three hundred times less than for antimatter, which would result in needing impractically large (and heavy) fuel tanks.
Antimatter isn't necessary... and for that matter, not feasible with current technology, or our current understanding of the stuff. We haven't found a way to produce it without expending more energy than it'll release in annihilation (not to mention the fantastic expense involved), we haven't managed to store multiple particles at once, or single particles for more than a few seconds, and don't even have a viable theoretical means to do so.

Acceleration doesn't need to be done all at once. Because of the distance involved and the need to be accelerating or decelerating for most of it to attain minimum travel time, sustainability is more important than output. Ion engines exist and are ideal for this kind of use, and you wouldn't even need something as exotic as fusion to power one.
Lt. Commander
Join Date: Dec 2007
Posts: 120
# 30
03-28-2012, 10:28 AM
Quote:
Originally Posted by hevach View Post
Gravitational slingshot only works if the object you're using is also moving in the approximate direction you're trying to go, and you need to start from outside it's gravitational influence (these two combine so that you're "falling" much longer than you're "rising" after perigee so you get a net gain). If you start from an orbit around the object, getting back out to that distance will rob you of what speed you gained. That's why probes don't slingshot out of earth orbit, they use boosters and slingshot on a second pass.
Fair enough.

I would have to go back and recheck some things, but as I recall a fusion ram jet becomes viable at about .5c and .5 c can be obtained by using an Orion project typr of propulsion system.

Or forget the ramjet and go Orion Project all the way. A nuclear warhaed in the 1 megaton range can release about 4.184 petajoules, which is about 4.184x10^15, and there are much higher yielding devices than that.
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