Friday, November 5, 2010
DARPA and 100-Year Starships: Not so Fast
Recently there was a big flurry of stuff in the news about DARPA commissioning a "100 Year Starship Study." Tossed in amongst this was a variety of other stuff, like microwave beam-powered launch and sending humans on one-way trips to Mars. Naturally the media got this all backward and took it to mean that NASA was planning to send humans to Mars by 2030 - but leave them there. Which is rubbish, as at the moment NASA has no plan and no direction except to visit a NEO in the 2030s. Woo hoo.
The fact of the matter is, as the Tau Zero blog, Centauri Dreams confirms, is that DARPA is merely interested in setting up the business case for an organisation with developing deep-space flight as its goal (and presumably interstellar flight). It recognises that there are huge hurdles to overcome, and that the technological windfall might be very great indeed. Consider hibernation - which would be of enormous benefit in keeping critical patients alive until they can be treated.
Unfortunately, there's not even a hint of anybody starting something up seriously.
As the content of this (poorly maintained) blog attests to, I'm quite interested in interstellar travel and colonisation. I'd recently read Ark by the eminent Stephen Baxter, which dealt with an interstellar mission spurred on by a global flood (deep subterranean aquifers suddenly releasing their contents). I was so inspired by the description of the ship that I went back to the drawing boards of my own designs and started looking up papers on the subject. Now, Faster-Than-Light travel is right out - simply because nobody has any idea on how it might eventually come to light (no pun intended of course). And then there's the problem of how we might be able to power it - something like the total energy output of a galaxy to move one piddly ship? Then there's all the other problems - how do you make the warp bubble, the warp bubble destabilising, etc. etc.
Which brings us back to regular Slower-Than-Light travel, the capabilities of which are not very spectacular. It turns out that to get anywhere at a reasonable pace, heck, even to launch a decent-sized colony ship at Voyager II velocities, a prodigious amount of energy is required. How much?
Marc G. Millis of the Tau Zero Foundation has an interesting article on the arXiv preprint server, "Energy, incessant obsolescence, and the first interstellar missions", is quite revealing - and depressing. It puts into perspective the enormous energy requirements even modest STL requires. It's a lot of energy. Let's put it into perspective how much:
ex) and you get e2. Plug that into the calculator and you get 7.389. What this means is that your spacecraft is now 6/7 fuel load and 1/7 everything else. Basically a flying fuel tank. What it also means is that for your spacecraft to go at, say, 3% the speed of light, your exhaust must travel at 3% the speed of light (c). That's hard. Chemical rockets manage something like 0.001% c.
The second side of the equation is where it really stings. See that vex2? Multiply 3% c (9 million metres per second) by 3% c (another 9 million metres per second) and you wind up with energy levels in the range of kilotonnes and megatonnes per kilogramme. So, it pays to go bigger and slower. Doubling your exhaust velocity (and hence top speed) quadruples your energy requirement, whereas doubling your mass only doubles the energy. Putting a satellite in orbit requires something in the range of a WWII conventional bombing raid in terms of energy. Sending a teeny probe to Alpha Centauri at 3% the speed of light requires lots more energy. How much more? OK, let's break into the equation.
1/2m = 500kg (for a 1 tonne probe).
We'll assume the probe's delta V is the same as its exhaust - 2.7. Less the 1 = 1.7.
vex2 = 8.1 x 1013
Putting it all together:
500 x 1.7 x 8.1 x 1013 = 6.885 x 1017 Joules. About 150 megatonnes yield (metric tonnes). By comparison, the highest nuclear weapon yield is in the region of 5.2 megatonnes per tonne. That's roughly 1000 Hiroshimas to get one tonne to Alpha Centauri in about 135 years. Double the delta v, however, and the energy requirement only goes up by 3.7. (the actual kinetic energy requirement is still the same). So it's somewhat more efficient to have a lower exhaust velocity and more fuel.
Millis compares energy used by space missions today with total world energy output, and compares energy growth with the energy required for an Alpha Centauri mission. It's rather depressing. Only by 2500 are we able to send such a probe to Alpha Centauri. And sending a 25 000 tonne colony ship on a millennia long journey to the stars requires more energy for a long trip than to actually send it on its way! (Best case scenario 2200). All of this dovetails with other authors' projections, economic, spacecraft top speed, etc. which puts the first possible interstellar colony ship in two centuries' time. Bummer. But it at least gives us some other numbers to play with, which I'll (hopefully) blog about over the next couple of weeks.
What it does show, though, is that big generation ships are likely to be available earlier than unmanned probes. And that a "space ark" is possible perhaps in one or two hundred years, if we throw enough money at it (which may only happen in case of impending global disaster). The (somewhat) silvery lining is that Millis considered the proportion of energy spent on spaceflight as a constant. With mass commercialisation, that number may well go up as a percentage. Much the way the energy spent on flight has gone from zero to becoming a major global warming problem...