[danielbarla]

IIRC, that assumes we don't want to decelerate. Adding the fuel to decelerate also increases the fuel needed for the original acceleration itself, which then quickly becomes less feasible. In any case, I'm guessing they were referring to "current, tried and tested" technologies.

Edit: I'm also having a slight chuckle at the estimated costs presented. Somehow, I'd think that just getting 40,000,000 t of spaceship into orbit would cost a bit more than 1 year of US GNP.

[InclinedPlane]

To decelerate you'd use a magsail or an m2p2 type system. This is vastly more mass efficient than simply thrusting in the reverse direction.

[wcoenen]

The mass added by the "fuel" of the orion nuclear pulse drive is negligible: a nuclear bomb has very little mass compared to the impulse boost you can extract from it.

You could literally lift a city-sized space ship to orbit on the cheap with this technology. The problem is the nuclear fall-out for those left on the ground.

[jasonwatkinspdx]

It's not that simple. You are vastly underestimating how much scaleup is required to sustain that acceleration up until you get a reasonable trip time.

Here's the summary from NASA:

"Here are four examples [large graphic][1] of what it would take to send a canister about the size of a Shuttle payload (or a school bus) past our nearest neighboring star...and allowing 900 years for it to make this journey.

Well....If you use chemical engines like those that are on the Shuttle, well..., sorry, there isn’t enough mass in the universe to supply the rocket propellant you’d need.

So let’s step up to next possibilities, nuclear rockets with a predicted performance that’s 10 to 20 times better!

Well...it’s still not looking all that good. For a fission rocket you would need a BILLION SUPERTANKER size propellant tanks to get you there, and even with fusion rockets you would still need a THOUSAND SUPERTANKERS!

Even if we look at the best conceivable performance that we could engineer based on today’s knowledge, say an Ion engine or an antimatter rocket whose performance was 100 times better that the shuttle engines, we would need about ten railway tanker sized propellant tanks.

That doesn’t sound too bad, until you consider that we didn’t bring along any propellant to let us stop when we get to the other star system...or if we want to get there quicker than 9 centuries.

Once you add the desire to actually stop at your destination, or if you want to get there sooner, you’re back at the incredible supertanker situation again, even for our best conceivable rockets.

In conclusion, we’d really like to have a form of propulsion that doesn’t need any propellant! This implies the need to find some way to modify gravitational or inertial forces or to find some means to push against the very structure of spacetime itself."

The rest of the website[2] has some good information, even if the organization is a bit disjoint.

[1]: http://www.nasa.gov/centers/glenn/images/content/84509main_w...

[2]: http://www.nasa.gov/centers/glenn/technology/warp/ipspaper.h...

[wcoenen]

Your references are about rocket designs, which have limits to how fast they can expel reaction mass before they blow themselves up. Orion isn't really a rocket as it does the violent reaction outside of the spaceship. With Orion, the limit is how fast the pusher plate can be cooled or how much momentum can be handled by the shock absorbers.

Granted, even with fusion bombs there is still a limit to how much energy and momentum you can extract from a certain mass, so the same concerns of diminishing returns apply. But according to Dyson's numbers, a 133 year trip to Alpha Centauri might be possible[1].

Even discounting interstellar travel, a trip time of only a few weeks to Mars with current technology sure sounds interesting to me.

[1] http://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsi...

[jasonwatkinspdx]

Orion most definitely is still an action/reaction rocket. It works via radiation and particle pressure against the pusher plate. While its thrust is high, its overall efficiency is low because the majority of the fission energy result is not contained enough to influence forward motion of the spacecraft. High thrust is pointless for interstellar travel. High isp is essential. This is why fission or antimatter powered ion drives beat Orion. Orion is great for interplanetary however.

If you read the through the material on that site you will see they are familiar with Orion. While the public oriented site glosses over details, the companion site will provide you with papers with rigorous detail.

There is no escaping it: for practical interstellar travel, we must develop massless drive or some form of spacetime manipulation that changes the mass requirements.

[danielbarla]

As a "fuel", yes, it is pretty efficient. That said, the "optimistic" version of the spaceship (representing the upper bounds of that specific technology), still came in at 400,000 tons, after a sort of miniaturisation. That's a lot of spaceship, and as far as I know, none of those calculations cater for launching from the planet's surface. If nothing else, that would require additional fuel, and it's not negligable, because each ton of extra fuel is pushing the 400,000 tons already loaded. It's a game of (greatly) diminishing returns.

[chii]

if the launch system was all on another planet or moon, then this wouldn't be a problem.

I can imagine you'd ride a rocket to the moon, and then the real spaceship is launched this way for inter planetary travel. It'd be like the old days of ocean liners, you stay on it for months on end, and a ship flies out may be once every year or so.