Could we build a starship using current technology?

[David Johansen]

No

But, that's assuming something new and groundbreaking doesn't come along in the next couple years.

Highly unlikely, mind you.

But really you absolutely need to have reactionless (which is to say impossible) drives to do 1 g for any astronomically significant period of time.

However, we should be able to build a solar powered soft launch plasma packet accelerator and a separate ship for it to push. Essentially you fire the fuel out of a cannon at a shield / sail so the ship doesn't have to carry its fuel. The resources required are considerable and the cannon facility itself has its limits. The cannon magnetically accelerates ionized matter like the stuff in a plasma TV rather than the hot death ray plasma of science fiction.

It's not 1G for a year by any means but it's probably the best bet at present. You probably do gravitic (use the target star's gravity to slow you down) or high atmosphere braking using the same sheild at the other end.

[generic mook]

What if you spent many trillions of dollars, and built a HUGE ion thruster in space? I mean, the size of New York City... and used it to propel a small ship.

[Mailanka]

Originally Posted by Dustin

For comparison, a proposed unmanned interstellar probe design (Project Longshot) using nuclear pulse propulsion and relatively few advances on current technology compared to other designs achieves an acceleration of .001g (.01 m/s2) after 70 years of flight, and twice that after 100 years of flight (when it reaches Alpha Centauri B).

There's no way that graph is correct. For one thing, an orion drive is going to accelerate you much faster than 0.01g. We have chemical rockets that accelerate you faster than that. Blowing nuclear bombs behind you (what a nuclear pulse drive is) will certainly kick you forward faster than that. Second, you'll never get to alpha centauri in 100 years at a speed of 0.01 m/s after seventy years.

You can actually read up on the Orion Drive in GURPS Spaceships. One engine gives 2g acceleration and each fuel tank (at TL 8) gives 3 mps, which is really, really good compared to everything else we can use. The main reason we don't use them is that people are very skeptical of the safety of blowing up tons of radioactive, weapons-grade bombs to get off the planet (and even if you get it up into orbit with more conventional means and then start blasting it around, what if the rocket crashes on the way up? We won't have people seriously considering an Orion Drive until we can construct it entirely in space with radioactive material collected IN space, and even then, there's a substantial anti-nuclear sentiment that has people freezing up at the very mention of the idea).

Also, Alpha Centauri is not THAT far away. Yes, it's very very very far away, but not insurmountable with modern technology. It's just that there's not much reason to spend billions of dollars building a ship powered by weapons of mass destruction that will get there in back in a couple of centuries. What would you do with something like that? What's the point?

[johndallman]

Originally Posted by generic mook

What if you spent many trillions of dollars, and built a HUGE ion thruster in space? I mean, the size of New York City... and used it to propel a small ship.

An ion drive needs to be mounted on the vehicle. Since the exhaust is charged particles, they repel each other, so the beam spreads out drastically and has no useful range. A fixed laser can propel a separate vehicle, but we don't know how to build one big enough to be useful.

[Anthony]

Originally Posted by Mailanka

There's no way that graph is correct. For one thing, an orion drive is going to accelerate you much faster than 0.01g. We have chemical rockets that accelerate you faster than that.

Bear in mind that all the Orion drives at that link have an exhaust velocity of 120,000 m/s or lower, other than Orion MAX, which is just the hard limit on theoretical performance for Orion and is most certainly not near future. A viable interstellar drive requires at least 3,000,000 m/s (0.01c), and increasing exhaust velocity decreases thrust.

[reverse_atomic_roger]

Have a look at the 100 year spaceship project http://en.m.wikipedia.org/wiki/100_Year_Starship

Its a DARPA project, and they reckon its going to take about 100 years to develop the tech for a plausable interstellar space ship.

That said, most of the stuff they plan to have is only a small stretch ahead of what we had now, so if you were going a bit cinematic it'd probably be fine.

[Fred Brackin]

Originally Posted by generic mook

What if you spent many trillions of dollars, and built a HUGE ion thruster in space? I mean, the size of New York City... and used it to propel a small ship.

Efficiency does not increase with size.

If you're talking about a ship that's 1% payload and the rest given to drive and fuel it still won't be anything like what you're hoping for.

If you used Spaceships and had 5% payload, 5% drive,5% power plant and 85% fuel you'd get something like 91.8 miles per second of Delta-
V.

This would allow you to leave the solar system but since you can only use half the Delta-V to accel and have to have the other half to decal you'll be travelling through interstellar space at only 45 miles per second.

At that speed it would take you over 78,000 years to get to Alpha Centauri.

[malloyd]

Originally Posted by scc

I calculated how fast a ship would be going after accelerating at 1 G for a year once, without accounting for time dilation, and if I remember correctly you're going at ~.99C

That's about right. The speed of light is just about a G-year (353 G-days).

Even playing a little loose with what you measure where, this is impossible now, or with any conceivable future drive. You'd have to turn something like 96% of your starting mass into energy to do this, even with a perfect mass to energy conversion system - which violates some nuclear force conservation laws.

[malloyd]

Originally Posted by Mailanka

There's no way that graph is correct. For one thing, an orion drive is going to accelerate you much faster than 0.01g.

Why? In fact if you read some of the original papers, this is a pretty reasonable number for the starship design versions. There's a tradeoff, for any given amount of energy (say that produced by one nuclear blast) you can use it to toss a large mass out fairly slowly (gives high thrust (lots of momentum) but low specific impulse (uses lots of reaction mass)) or a small mass out quickly (for low thrust but high specific impulse).

We have chemical rockets that accelerate you faster than that. Blowing nuclear bombs behind you (what a nuclear pulse drive is) will certainly kick you forward faster than that.

Depends on how big your pusher plate is and how good the shock absorbers are.

Second, you'll never get to alpha centauri in 100 years at a speed of 0.01 m/s after seventy years.?

That's an acceleration, not a speed. If you have managed 0.01 m/s for 70 years you'd be at 7.4% of c. You didn't of course, that's why it specifies at 70 years, you started off with a lower acceleration than that, but still.

Edit: Something I think a lot of people miss in starship discussions. In this context a high acceleration is usually a *bad* thing. It is almost certain to mean you are using way too much fuel for the velocity increase you are getting out of it.

[Dustin]

Originally Posted by Mailanka

There's no way that graph is correct. For one thing, an orion drive is going to accelerate you much faster than 0.01g. We have chemical rockets that accelerate you faster than that. Blowing nuclear bombs behind you (what a nuclear pulse drive is) will certainly kick you forward faster than that. Second, you'll never get to alpha centauri in 100 years at a speed of 0.01 m/s after seventy years.

Typical Orion designs are for operation within the solar system, and seem to be optimized for acceleration (to reduce travel times). Longshot looks to me like what you might need to build if there are no gas stations for 250,000 AU and you're bringing all your fuel with you.

Also, it's not moving at a speed of 0.01 m/s after seventy years, it's accelerating at a speed of 0.01 m/s2 after seventy years, with acceleration continuing to increase thereafter. At peak velocity, this design is pulling about .05c. I'm not an engineer or physicist, but I'm not seeing any reason to doubt that acceleration profile for an interstellar-capable design.