

A Plumber’s Guide to Starships Part 4 – Materials in High Radiation Environments - user_235711
http://www.icarusinterstellar.org/a-plumbers-guide-to-starships-part-4-materials-in-high-radiation-environments/

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ufmace
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...](http://www.nasa.gov/centers/glenn/images/content/84509main_warp06.gif)

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iwwr
>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.

~~~
ufmace
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](http://www.wikiwand.com/en/Mass_ratio)
[1]
[http://www.wikiwand.com/en/Space_Shuttle](http://www.wikiwand.com/en/Space_Shuttle)

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iwwr
>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.

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ufmace
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...](http://www.projectrho.com/public_html/rocket/reactionlessdrive.php)

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Retric
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.

~~~
ufmace
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](http://www.wikiwand.com/en/Tsiolkovsky_rocket_equation)

~~~
Retric
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.

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FiatLuxDave
I think I said this in the last thread about interstellar spacecraft, but
rockets are probably the wrong paradigm for interstellar travel. People are
right to point out that the amount of energy and mass ratios needed are huge.
However, if instead you leave your reaction mass behind, and use reaction mass
from where you arrive, then this is much less of an issue.

The classic example of a system which does something like this is the Bussard
ramjet. Another would be beamed power or beamed propulsion, ala the Moties
from Niven's _The Mote in Gods Eye_ or Leik Myrabo's work. My personal
favorite idea is using electromagnetic interactions to accelerate using the
Sun itself as the reaction mass, and then doing the same process in reverse at
the arrival star to generate power at the other end. The power could be put in
using a beamed system or other system where the infrastructure would stay in
this star system. But the energy could then be used at the other end to either
power needs for exploration or colonization, or to re-accelerate the ship for
another trip to another star.

To go travel to another close star in tens of years, you will need to have
around 10% of your mass energy in kinetic energy (a Lorentz factor of around
1.1). So, as a first approximation, if energy alone were the issue and a
reaction mass the size of a sun were used, an amount of energy equal to the US
energy consumption in 2008 could accelerate about 30 tons to 10% of c (see
[http://en.wikipedia.org/wiki/Orders_of_magnitude_%28energy%2...](http://en.wikipedia.org/wiki/Orders_of_magnitude_%28energy%29)).
This is big, but not out of the range of human capabilities. And if the energy
were recyclable many times, then this amount of energy would be essentially
the largest investment in the creation of a true reusable starship.

This is interesting, because it gives us a good minimum cost for such a
starship. If we assume a typical cost of energy of 10 cents per kWh, and a 60
ton vehicle (around the size of a Space Shuttle orbiter) going 10% of c, this
gives us $750 billion for the cost of energy alone. This is about six times
the inflation adjusted cost of the Apollo program.

