In his book A Traveler’s Guide To The Stars, Les Johnson says a 10^7 kg ship would require 4.5 x 10^21 joules of energy to reach 0.1c. How much would be needed to achieve the 0.7c of the Venture Star in Avatar? Would it even be possible in the first place?
The total energy of a moving object is E = \gamma m c^2. Of that, m c^2 is just the object’s rest energy that it’d have anyway, so we want \gamma - 1.
The formula for that is \gamma = \frac {1}{\sqrt{1-\frac{v^2}{c^2}}}. For v = 0.7 c, that’s \gamma = \frac {1}{\sqrt{1-0.49}}, or about \gamma - 1 = 0.4. Plugging in the m and c^2, that gives us a kinetic energy of 3.6*10^23 joules.
Important to note here that that’s only a lower bound, and not a particularly tight one, since that’s just the kinetic energy of the ship, and every known method for getting a spaceship up to speed involves dumping a bunch of energy into something else, as well (such as the exhaust of the rocket engines).
Now, this clearly isn’t practical without some amazing wonder-tech far beyond our horizon, such as efficient direct mass-energy conversion. But if we’re willing to dismiss practicality and just ask about what’s possible, then sure, any sub-light speed is possible. You just have to start with a ridiculously high ratio of fuel to payload.
Off the top of my head, I think the amount of mass turned into energy during hydrogen fusion is about 0.7% - so you can see to carry enough energy to get to that speed, your ship would as Chronos says “have to start with a ridiculously high ratio of fuel to payload.”
One design suggested an iceberg of pure hydrogen (better yet, deuterium) as both a sheild in front of the ship and slowly consumed fuel. Ignoring that when you flip over and decelerate toward sthe destination, it’s not really a shield is it? Or, ignoring how you maintain a frozen lump of hyrogen. Details, details… Hand-waving SF authors also posit processes that simply change 100% of mass to energy, but other than anti-matter (stored in an extremely good container) there’s nothing that really does that.
Larry Niven suggested giant magnetic scoops that suck in the atoms of hydrogen free-floating in the interstellar space, but IIRC newer studies have suggested that would not be productive ship propulsion, even if we could induce fusion.
You misspelled Robert Bussard.
The thing about a Bussard Ramjet is that your space craft already has to be moving really fast before you can even begin to collect enough hydrogen. How you get up to that speed is the issue.
I think talking about fuel and payload is narrowing the question somewhat. There are ways that you could push a ship without carrying fuel, with light, with ramjets, whatever.
Right now none of these ways seem practical (particularly ramjets) but I’m just saying…we’re talking about far future tech already, why limit it to a BFR - big f-ing rocket?
Assuming your ramjet is based on fusion, the drag limits you to 12% of c. And propulsion from a laser at a launch site has its major inefficiencies, too, it’s just presumed that, by leaving most of the infrastructure at home, you can harvest enough energy to make up for the inefficiencies.
Even antimatter isn’t really 100%, in practice, since (assuming you’re talking about baryonic matter, not just positrons) most of the energy ends up in neutrinos, and we have no way of harvesting neutrino energy. Conceivably someone might find some way to make the neutrinos be emitted preferentially in one direction, but nobody’s come up with one yet.
can you give me a sense of how much energy is 3.6*10^23 joules?
Does this include deceleration at the other end of the trip?
86 million megatons. That’s very roughly the energy of one Hiroshima bomb for each human on Earth.
Realistically? No, I can’t. Any comparison I give to anything else is going to involve other numbers that are also too large for human comprehension.
All it is is the kinetic energy of a ship of that mass moving at that speed. There are many, many practical considerations that I left out that make the real numbers much worse. Decelerating at the end of the trip isn’t even at the head of that list.
Oh, that’s giving up too easily. How about this:
Humanity uses up about 580 million terajoules annually, or 5.8 x 10^20 joules.
So, roughly 1600 years of total worldwide energy production would be required to get the required energy.
Another one: A Saturn V rocket’s fuel amounted to about 8.4x10^12 joules. So the fuel required to get to .7C would be the equivalent of roughly 64 trillion Saturn V rockets.
≈ 0.72 × estimated energy released by the Chicxulub meteor impact ( ≈ 5×10^23 J )
≈ 9.2 × 2003 estimated energy in world’s total fossil fuel reserves ( ≈ 3.9×10^22 J )
≈ 14 × 2003 estimated energy in world’s coal reserves ( ≈ 2.6×10^22 J )
This suggests that if you have a propulsion system capable of reaching a decent fraction of c, a ramscoop could be used as a brake on the deceleration leg, possibly reducing the fuel mass ration somewhat.
The Venture Star in Avatar was designed to be accelerated by laser beam rather than by a rocket, and the energy required to perform this acceleration would presumably been collected by solar power systems in orbit near the Sun. So if we assume a highly efficient power collection system and a highly efficient laser propulsion system, we would need to collect at least 1600 times as much energy as generated on Earth today in order to accelerate this ship for a year and reach 0.7c.
How big would such a series of collectors be? Well, the amount of power collected by an array of collectors as large as the Earth (in Earth’s orbit) each second would be 173 petajoules. In a year that is 543 zettajoules, about ten times as much as necessary to accelerate this ship for a year.
But I expect that the inefficiencies involved would be very high indeed, especially since the lasers would need to accelerate the ship until it was nearly a light-year away. So we are probably talking about an array of solar power collectors hundreds as times as large as the Earth. This could be improved significantly if the collectors were much nearer to the Sun, but we are still talking about planetary-scale megastructures.
I think that this may perhaps be feasible one day in the far, far future, rather than in the relatively near future depicted in Avatar. We might one day build huge power collectors in orbit near Mercury using Mercurian resources, for example.
But consider that a civilisation that could collect hundreds of times as much energy as incident on Earth today would surely be a post-scarcity utopia, and what possible motive would such a civilisation have to send ships to a distant solar system where there is no support structure to maintain the colonists in such profligate luxury.
From a practical viewpoint, it would no doubt be easier to travel to Alpha Centauri at a much slower speed, and be resigned to a much longer trip. 0.7c is a pipe-dream for the foreseeable (and maybe even the unforseeable) future.
See, this is the kind of thing I’m talking about. To compare it to something “normal” like a Saturn V, you need a factor of 64 trillion. Which is just another big number. It’s no more informative than if I had said “360 sextillion times the energy of something that has an energy of one joule”, because “trillion” and “sextillion” are, to the human mind, both processed as just “big number”.
Well, I don’t know. For someone who wasn’t sure just how big 10-^23 is, giving some kind of comparison is helpful, rather than just saying, “Bigger than you can imagine.”
Even if you can’t get your head around trillions of Saturn V’s, at least that helps you ballpark it, in case you were thinking that ‘incredibly large’ just meant maybe ten or a hundred or a thousand Saturn V’s or something, which is at least in the realm of ‘massive engineering project’ feassibility. Trillions of Saturn V’s means there’s nothing we can possibly do to make this happen with anything like current tech.
From Avatar 2 the reason is some aliens on the planet produce a substance that dramatically extends a human’s lifespan…basically the fountain of youth (albeit not a fountain).
If you could collect 0.1% of the sun’s energy output, it would take about… one second to collect that much energy. Trivial for a civilization that is even approaching Kardashev Type 2.