Quote from: HalfjackIt won't get you there in a useful amount of time.  The low exhaust velocity of the massively inefficient Orion system means your fuel-to-payload ratio is enormous for a steady acceleration/deceleration flight plan.  And by enormous I mean crazy huge and not just expensive.  Orion will get you to Mars but for the stars simple reaction drives aren't going to cut it unless you can get exhaust velocities into the tens of millions of meters per second.

I am talking generation ships. Orions start out massive and get bigger, so size isn't the problem. If you want to scoot over there and scoot back, it won't work. You blast up to escape velocity, cut the drive, and drift however long it takes, then decellerate when you get there. Without fusion, there's nothing else that would work at all. A fusion ship would get you faster, but that technology is beyond us, and may stay beyond us. 30 years ago it was a decade away, twenty years ago it was thirty years away, and now it's probably fifty...

The only thing that could run at constant accelleration between stars is a drive that doesn't carry it's own fuel, IOW a bussard ramjet, and according to the latest theories, they won't work.

-clash

Quote from: flyingmiceThe only thing that could run at constant accelleration between stars is a drive that doesn't carry it's own fuel, IOW a bussard ramjet, and according to the latest theories, they won't work.

Exhaust velocities for exhaust propelled by matter/anti-matter annihilation get the fuel to payload ratio into the thousands for 1G constant acceleration, and gives you four or five 9s near to c by turnaround.  Time dilation effects around there are on the order of a thousand or more to one, making your subjective trip length fairly small.  Of course, you need to spend a lot of energy convincing anyone to share a rocket with anti-matter.    Well that and we're still firmly in science fiction territory with that kind of technology.

So yeah, if you don't care how long it takes to get there you could probably go now.

Quote from: jeff37923I dunno. I bet we could cobble together a solar sail or a magsail probe to hump that 20 light years if there was enough interest.

Yup, and it would probably only take us a couple of hundred thousand years at current tech to get there.

Of course, give it 20 years and who knows what we can do?  Still, right now 20 light years could just as well be 20 million.

Quote from: HalfjackExhaust velocities for exhaust propelled by matter/anti-matter annihilation get the fuel to payload ratio into the thousands for 1G constant acceleration, and gives you four or five 9s near to c by turnaround.  Time dilation effects around there are on the order of a thousand or more to one, making your subjective trip length fairly small.  Of course, you need to spend a lot of energy convincing anyone to share a rocket with anti-matter.    Well that and we're still firmly in science fiction territory with that kind of technology.

So yeah, if you don't care how long it takes to get there you could probably go now.

Sure - M/AM is awesome, but you need fusion to make AM in quantity, and...

-clash

How about ion drive? Very very very slow acceleration, but you can thrust for a looooong time. And its pretty fuel efficient.

Quote from: WerekoalaVery very very slow acceleration, but you can thrust for a looooong time.

Ooh.  You're making me hot.

!i!

Quote from: WerekoalaHow about ion drive? Very very very slow acceleration, but you can thrust for a looooong time. And its pretty fuel efficient.

It's still limited by reaction mass -- you have to shoot *something* out the back.  Reaction drives are always limited by exhaust velocity from which we can determine the fuel to payload ratio for steady acceleration.  Ion drives have higher exhaust velocities than chemical rockets but still can't achieve steady acceleration without carrying unreasonable fuel loads -- you need exhaust velocities a couple of orders of magnitude higher.

Quote from: BalbinusYup, and it would probably only take us a couple of hundred thousand years at current tech to get there.

Of course, give it 20 years and who knows what we can do?  Still, right now 20 light years could just as well be 20 million.

I'm not sure if it would be a couple of hundred thousand years. Maybe more along the order of a few hundred to a few thousand depending on how the sail is launched (the best being a downward spiral towards the sun and then unfurling the sail at closest approach IIRC). I'm at work right now and don't have access to my books, or else I'd do some back-of-the-envelope style calculations to give a better ballpark figure. The Starflight Handbook has some good ideas on this score.

With a light sail system (solar won't be much use for most of the journey so you're going to need to shoot giant lasers at it from the moon or something) you're getting accelerations on the order of millimeters per second squared.  But it is constant and you don't carry fuel.  On the other hand, it's not entirely clear how you slow down for the second half of the voyage.

At 1mm/s^2 you get 5 light years in 224 years and you're doing about 2% of C when you get there.

What about the lightsail systems that use a laser on a base asteroid or somesuch pumping highly focused light to increase the acceleration rates (Mote in God's Eye used this for the Moties)?

Quote from: WerekoalaWhat about the lightsail systems that use a laser on a base asteroid or somesuch pumping highly focused light to increase the acceleration rates (Mote in God's Eye used this for the Moties)?

Dat's what I taid!

Quote from: WerekoalaWhat about the lightsail systems that use a laser on a base asteroid or somesuch pumping highly focused light to increase the acceleration rates (Mote in God's Eye used this for the Moties)?

Light pressure just doesn't impart all that much momentum before the energy levels start melting your sail.  That gives you a little more acceleration while your sail turns into reaction mass though.

Even if you can use this to get useful levels of acceleration, I don't quite get how you slow down.

Why bother slowing down? Make the first probe a flyby one with a mission profile similar to the Pioneer and Voyager probes.

Quote from: jeff37923Why bother slowing down? Make the first probe a flyby one with a mission profile similar to the Pioneer and Voyager probes.

Well I think the idea was to land humans on it, but they could wave at it as they hurtle past I guess.  And radio home their findings for us to get 20 years later.

Someone needs to get on the ball and develop reactionless thrusters.  Don't ask me where to start though -- I'm the idea guy.  Hyperspace jump drives would be handy too.

Quote from: HalfjackWell I think the idea was to land humans on it, but they could wave at it as they hurtle past I guess.  And radio home their findings for us to get 20 years later.

Oh, well, since you put it that way, having a human impact the planet at near-C could be called a "landing" I guess...