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Comment by Eupolemos | Hacker News Reader

I must admit, I don't understand this often repeated notion of "cruising speed" in interstellar space.

To me, it feels like some people think of crossing the void between stars as crossing the the Atlantic in a bottle, trying to blow my mind with how long the crossing will take, even if that bottle is given an absolutely amazing initial throw.

For interstellar travel to make sense, there _must_ be a 'cruising acceleration', not speed. The notion isn't that strange - after all, cruising speed in a ship on earth requires a constant engine power, which in space would mean constant acceleration.

IIRC, a comfortable cruising acceleration of 1G would make it a 4 year trip (6 as experienced by Earth). We just need a whole lot of efficient fuel and an interstellar propeller :P

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How do you think a ship is going to power a continuous acceleration? 1G for 4 years is... I don't really know, but it's a heck of a lot of energy even if you've somehow beaten Tsiolkovsy's rocket equation. You don't get to hand-wave this away in a remotely serious conversation. There's also the small problem of the interstellar medium wrecking your hull at high velocity, if you ever get there.

Well, the hull problem is there at 1% or 15% of c or whatever is reached. You don't get to wave how you get to the 1% of c speed away either, so let's not set double standards; interstellar transport for humans _is_ all scifi-land.

IMO, we simply can't get there with chemical rockets as we know them. Continuous acceleration in space is just any kind of engine that keeps running and provides thrust. We're not talking cars here.

The estimation of the energy needed is easy to make sense of - what does it take to have a rocket (or more likely, some other engine) hover over the ground for 4 years using some kind of thrust?

When we have that capability, we can pretty much do non-suicidal interstellar transport. But it requires new technology and for me, that constitutes sci-fi. Doesn't mean a conversation can't be remotely serious though, IMHO.

But the point I actually wanted to make was, that whenever people talk of cruise-speed in interstellar transport, it seems to me like they haven't grasped the scope of interstellar travel with an intent of conquering it. Rather, they are implying it just can't be done - without arguing for it.

The term 'cruise-speed' itself becomes an annoying passive-aggressive attack on the notion of interstellar transport. That was my original reason for posting.

Maybe replace it with "coasting speed" and see if that feels better? It's a valid strategy. We have proposals for drive systems that get to .01c where the physics sort of works. I'm not aware of any realistic proposals that can supply 1g for any length of time. Maybe a ramjet, but last I heard those work better as brakes because, again, interstellar medium. Drag is another reason "cruising speed" might be a coherent concept even under thrust.

You can cheat a bit by making use of laser propulsion[0] to propel a rocket that contains the necessary fuel for deceleration.

To almost completely eliminate fuel requirements in subsequent trips you would bring the self-replicating equipment necessary to scale up the industrial capacity to build an complimentary laser system in the destination star system. This would allow you to propel ships on return voyages and slow down ships coming to the planet.

The downside of course is that you've now built a death laser in each star system that could be used to wreak havoc. But then again in this scenario you've also built self-replicating space bots that can turn asteroids into stuff or planets into asteroids into stuff.

[0] https://en.wikipedia.org/wiki/Laser_propulsion

>... that contains the necessary fuel for deceleration.

Is it not possible to plot an interstellar course that utilizes gravity for deceleration?

Wait - didn't Newton say that a body will not accelerate nor change direction unless a force enacts upon it? Once you set an initial velocity in void space with no air, you should never decelerate unless you accelerate in the direction inverse from your motion.

Humans are very fragile to sudden acceleration. Unless we assume really out there, indistinguishable-from-magic tech, manned spaceflight cannot "set an initial velocity" suddenly, nor a sudden breaking at the end. To get to any other star in years, not centuries, the only way to go is to accelerate on the whole first half of the way, and then decelerate on the remaining half.

Yes, that was indeed implied for the line "For interstellar travel to make sense..." to make sense :P

I don't mean to be annoying, but I don't think I understand why you ask?

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