Tuesday, March 17, 2015

The Mars conversation I'd like to be having

As a bit of an antidote for all the accumulated bile of the last few Mars One posts, I'd like to recommend this IEEE paper from Ian McNab. I'll admit it lost me on the curves a few times, but on the whole it's remarkably readable.

From the introduction:
In the past 40 years, mankind has ventured into space using well-established rocket technology involving liquid fuels and/or solid propellants. This approach has the advantage for astronauts and fragile payloads that the rocket starts slowly from the surface of the Earth with its full fuel load, and, as the fuel is burned off, the altitude and speed increase. In addition to minimizing the aerodynamic and aerothermal loads, this provides relatively modest accelerations—maximum values of a few “gees” are used for human passengers. Because only a small fraction of the initial mass reaches orbit, rockets of substantial size are required to place tens of tons into near-Earth orbit. Offsetting these remarkable successes is the very high cost of burning chemical fuel with a modest efficiency in a rocket engine to get out of the Earth’s gravitational well. Present estimates are that it costs $20 000 to get one kilogram of material into orbit. Unless alternatives can be found, it seems likely that mankind’s ventures into space will be limited to a few adventures that can only be undertaken by wealthy nations—the science-fiction writer’s dream of colonizing the planets and stars may be unaffordable.

Proposed solutions fall into four general categories: better rocket propellants; the space elevator; gun launch from the Earth’s surface; and laser launch. Although these options will not be discussed in detail, a few comments are appropriate. First, there appear to be no acceptable alternative rocket propellants that can offer substantial improvements compared with present choices. Second, although the space elevator seems to have great promise as a concept for the future, its practical realization awaits the development of a material that is strong enough to be able to carry its own weight (and that of the payloads it will lift) from the Earth’s surface to geosynchronous orbit. Third, estimates indicate that to launch payloads of less than a ton with a laser would require multigigawatt lasers far larger than any presently in existence
[Having concluded that gun launches are currently the most viable option, McNab starts drilling down into the details.] 

If the launcher is sufficiently long, the acceleration can be reduced to a level that is compatible with present component technology, although the acceleration forces will not allow people or fragile payloads to be launched with feasible launcher lengths. Guns may therefore be limited to launching robust packages such as food, water, fuel, and replaceable components. This may be an important support function for the International Space Station (ISS) or other missions

A disadvantage of gun launch is that the launch package has to leave the gun barrel at a very high velocity ( 7500 m/s) through the Earth’s atmosphere, leading to a very high aerothermal load on the projectile. The reentry vehicle community has successfully developed techniques to overcome this situation (when traveling in the reverse direction), and it seems possible that similar techniques can resolve this problem, either through the use of refractory or ablative nose materials or by evaporative cooling techniques. The mass of coolant required for this appears to be acceptable, as discussed below. The second concern for a gun is the size of the package that can be launched. Unless a very large gun can be built, the payload launched into orbit per launch will be a few hundred kilograms, which will require a large number of launches per year. For example, to provide 500 tons/year to orbit would require 2000 launches/year—a little over five per day on average. An infrastructure in space for handling this traffic and distributing the payloads will have to be created. Issues to be addressed will include decisions on handling or recycling the nonpayload components that reach orbit.
I don't want to get into whether or not we should be spending more on space exploration, and I certainly don't want to argue the merits of this proposal (that topic would take me out of my depth almost immediately). For now, I want to stay meta and discuss the discussion.

Let's think about the question of why so many reputable news organizations are devoting so much coverage to Mars One and so little to other, better aerospace stories such as this one.

What do I mean by better?

For starters, this is a credible proposal from a well-established authority published in an IEEE journal, and, based on my experience, it's an idea that other engineers in the field are not willing to dismiss out of hand; they may not consider it practical, but they do take it seriously. (For example, JPL was looking into using orbiting railguns to launch small interplanetary probes as far back as 1988.)

And we really are talking about a game-changer here. If McNab's estimates hold up, we're talking about reducing launch costs by considerably more than an order of magnitude. Even if we factor in the need to use traditional rockets for people and other delicate cargo, that cost reduction is still enough to shift the underlying economics of all space-based enterprises, ranging from asteroid mining to tourism to, yes, interplanetary colonies.

Finally, rail guns are cool. All Mars One has to offer is cheesy artist conceptions. With railguns you get video like this:

This is, of course, just one example. There are any number of fascinating stories about aerospace research. Why do they go unnoticed while Mars One continues to make the cut? Here are my guesses:

1. Bullshit does not count against you. As Elmo Keep spelled out in painful detail, every aspect of this story collapses under inspection, but even after Keep's expose, the stories kept coming;

2. People love a bargain (i.e. there's a sucker born every minute). I've noticed a number of cases recently where an unrealistically low price seems to make proposals more newsworthy (this example jumps to mind);

3. Everybody loves a messiah (even a Galtian one). Entrepreneurs and market forces are also easy pitches these days, while stories of public action and collective sacrifice fall out of favor. Of course, even in the Sixties, space was a tough sell (even as we were sending men to the moon, people were suggesting that the money would have been better spent down here), but now even the suggestion that we as a society would take on something expensive and challenging seems oddly quaint.


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