Comment by ufmace | Hacker News Reader

Comment by ufmace | Hacker News Reader

I've only skimmed the article so far, and flipped through a few other pages on the site. What bothers me here is that they're spending a lot of words and brainpower on a particular drive system for a theoretical starship - Fusion - and I don't see anything demonstrating how, even if they can build a fusion drive with the specified power output that can run for 15 years at full power and meet various other engineering criteria, this will actually be good enough to propel a reasonable-sized starship to another star system.

I found this[0] chart at a NASA site a while ago purporting to say that, given the limitations of reaction-based rockets, a fusion rocket is still nowhere near good enough to reach even the closest star system in a reasonable time. The mass to drive a Shuttle cargo bay sized canister past Alpha Centauri in 900 years using Fusion power is supposedly 1000 supertankers of propellant. That's the closest star system, and doesn't count stopping, much less getting there in under a millennium. Even anti-matter, the best reaction-based rocket we can conceive of, is supposedly wildly impractical.

Given that, I'd like to see some quality calculations on exactly how they plan to get around the reaction mass problem before it makes sense to worry about stuff like materials in high radiation environments, and I don't see anything like that on their site. They seem to just kinda hand-wave it away, as far as I can tell. It seems likely that to build a practical starship, we'll need a propulsion system far beyond anything we can imagine today. Well, more realistically, calculations by people who seem to be qualified to perform them, since I'm probably not smart or well-versed enough to really understand it.

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

>a fusion rocket is still nowhere near good enough to reach even the closest star system in a reasonable time

If by "reasonable time" you mean 40 years to get there at 10% of lightspeed. At those speeds the big limitation starts to be the ability to reject heat into space rather than the power source itself. The difference between fission and fusion is also only a factor of 2, so you could get there with fission in 80-100 years.

Though we're talking about long times, keep in mind that there is a vast swath of unexplored space in the Kuiper belt and beyond, where we may find many interesting icy worlds. Beyond 750au or so it's also possible to use the Sun as a gravitational lens, a good spot to send telescopes of all shapes.

The point is, what is the mass ratio of a plausible fusion rocket that can accelerate to 0.1c and then deaccelerate back down to stop?

The mass ratio of the Space Shuttle is 16 according to wikipedia[0]. As far as I know, all orbital rockets that we've built have mass ratios similar to this. That's higher than we'd like, but reasonably practical to build.

According to the linked chart, the propellant mass required to get a canister the size of the space shuttle payload to Alpha Centauri without stopping in 900 years using a fusion rocket is 10^11kg. I'm not sure what mass they're using for that shuttle payload canister, but let's say that it's the Wikipedia-claimed shuttle payload to LEO[1] of about 25,000kg. That gives us a mass ratio of about 4000000 by my calculation. Uhh yeah, you wanna build a spaceship with a mass ratio of 4000000? I tried plugging that in versus the mass of a Nimitz-class carrier, and I'm getting the mass of a smallish moon.

And that still doesn't count stopping at the end, or getting anywhere near 0.1c. Sounds like if you want to propel a reasonable-sized starship at the speeds you said, you're gonna need roughly a gas giant worth of fuel. If you know of way around that, please let me know.

[0] http://www.wikiwand.com/en/Mass_ratio [1] http://www.wikiwand.com/en/Space_Shuttle

>If you know of way around that, please let me know.

Beamed power has been proposed to get around this problem. Keep your oversized powerplant at home as much as possible. This still requires a monumental infrastructure, but once it's built it's reusable. More importantly, beamed power would be very useful inside the solar system.

I don't see how it does. The problem is not power, but momentum. You can beam power, but where does your reaction mass come from? Producing the required levels of power is relatively practical, as these things go, but reaction mass is subject to the nasty exponential growth of the rocket equation.

If you're talking something like solar sail off of a laser, the numbers I'm finding[0] say 300MW per N of thrust. I don't think you'll be travelling to the stars anytime soon on that.

[0] http://www.projectrho.com/public_html/rocket/reactionlessdri...

Reaction mass is less of an issue than you might think.

A particle accelerator could get reaction mass to 99.999%c. The energy requierments would be insane though.

Reolitically, there is going to be a tradeoff between effecency and exit velocity but for 10%c it's really just a question of energy.

I think you're talking about relativistic mass, which is irrelevant here. When it comes to rocket propulsion, reaction mass is everything. See the Rocket Equation[0]

By the law of Momentum, you cannot change the velocity of an object without pushing against, and changing the velocity, of another object. Vehicles on Earth mostly push against the planet, and the water and air on it. In space, there is nothing, so you must bring your own mass to push against if you want to change your velocity. But this sets up a rather inconvenient exponential situation, because the mass you expel to accelerate now must accelerate not only your vehicle, but also all of the rest of the mass that you're going to expel later to accelerate further.

It's why chemical rockets traveling to Earth orbit have about 20x more propellant mass than the mass that's going to actually reach orbit. The situation gets exponentially worse the higher you want to take the final craft velocity, which is why all interplanetary missions have gotten around mainly on gravitational slingshots. Increasing the rocket exhaust velocity helps, but only in a linear fashion. When it comes to reaching reasonable interstellar velocities, all the numbers I've seen say that it's wildly impractical even with fusion or anti-matter level exhaust velocities. Like mass ratios in the neighborhood of 4,000,000, as I calculated above, which is still nowhere near fast enough.

[0] http://www.wikiwand.com/en/Tsiolkovsky_rocket_equation

No, a particle accelerator accelerates particles close to the speed of light. The large hadron collider gets protons to 7TeV which = 99.9999991%c They also gain mass from this but if your spaceship only needs to hit .1c then that's more or less irrelevant.

Ion thrusters basically are low energy particle accelerators already. The problem, aside from energy consumption, is scaling up the thrust. You can't just cram twice as many particles through your particle accelerator without scaling up the mass of the thruster itself, or decreasing the exit velocity. And it does you very little good if your rocket has the fuel to accelerate to .1 c, if it takes you 10,000 years to do so.

What about project orion and using nuclear weapons to propel the ship once in space + a mass driver to put the ship itself up there. Humans can go up via traditional means.

I'd note that another critical, showstopping problem is shielding against the interstellar medium when moving at relativistic speeds.

http://en.wikipedia.org/wiki/Interstellar_travel#Interstella...

http://www.academia.edu/4692178/Speed_kills_Highly_relativis...

http://arxiv.org/ftp/physics/papers/0610/0610030.pdf

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