Are We There Yet?

Okay.  So, once we are on Mars, we know basically what we need to do.  Set up electromagnetic deflectors, build glass domes with anti-UV coatings, fertilize and compost like there's no tomorrow, live in caves until we can build some apartment buildings that spin to roughly simulate gravity, set up nuclear reactors (hopefully the less messy, fusion kind), distill and heat and pressurize ourselves an atmosphere, bring some farmers along, and set up shop.  But, one thing we haven't talked about is how to actually get there.

Until now, in my blog, I've been assuming that I'm dictator of the United States, and I can with a word allocate a trillion dollars of American resources to the greatest project in the history of mankind.  I've been acting as if money was no object.  However, as anyone who even slightly follows American politics can tell you, money is very much an object.  The whole presidential election this year seems to be centered on how to fix the budget crisis without ruining the economy or selling our souls to China.  So, what is the cheapest, most efficient way to get out of this world?

First, we should look at our options.  And our first option is the old standard: liquid hydrogen and liquid oxygen bi-propellant rockets.  These have a very high efficiency and specific impulse (about 455, which is excellent), and since it is stored cold, it can be used to cool the engines before fueling them.  Plus, the reaction product is just water, so there is no notable pollution from this type of rocket.  There are issues, however, and the most important is thrust to weight ration.  Hydrogen is the lightest element, which is good in that exhaust pressure is based on the number of molecules shot out, and that high pressure usually means increased exhaust velocity, and therefore a faster spaceship.  However, the downside of lightweight fuel is that force is mass*velocity, so while velocity is high, mass is low, and the thrust to weight ratio is not the best.  Also, with liquid propellants one needs to have a lot of pumps and tanks and accouterments, all of which are heavy.  Still, of all rocket fuels, this is probably the best all-around fuel that exists, and it is used in the majority of launches currently.  Other fuels tend to come with either many more problems or much less power.  It's cheap-ish already, and since we can make it from water via electrolysis, and we have oceans full of water, it's not like we will run out any time soon.

The real problem with chemical rockets, though, is that they don't have enough power.  One ends up spending a lot of fuel to carry other fuel.  For example, with the Saturn V rocket, the biggest, most powerful rocket built to date, over 95% of the liftoff mass was fuel, entirely burned away.  At liftoff, it weighed 2.8 million kilos, but could only carry 45000 kilos to lunar orbit, and what came back was a tiny little pod!  This is the reason for stages on rockets: any weight you can get rid of during flight is weight your engines don't have to push anymore, which improves efficiency.  It's actually cheaper to throw away rocket parts than to keep them on the ship!

So, what we really need is something that explodes so powerfully that we only need to run the engines for a minute or two before you are in orbit.  What's the most powerful explosion you can think of?  A nuclear bomb, perhaps?  Try 50 of them.  The principle behind nuclear pulse propulsion is simple: build a rocket with an explosion catcher on the back end, drop a nuke behind it every few seconds, and ride the shock waves into space.  This may seem crazy, but in terms of thrust to weight ratio and required fuel supply this is probably the most efficient imaginable way to shoot something into space.  The concept has been around since the 1950s project Orion, but was never carried to the test phase, because, well, it is a pretty hard sell. (Okay, here's what we'll do: we'll put you into a rocket, and then explode the most powerful weapon ever created by man right underneath you....What?  You don't want to go?)  After the 60s, nuclear treaties prohibited the use of nuclear explosives above ground, effectively making these rockets illegal.  But, with a potential specific impulse of 6000, there are some who want to bring them back.

The downsides are the (obvious) danger, the expense of weapons-grade fissile material, and the fact that it will shower the radiation of fifty nuclear bombs down onto the populace every time the system is used.  The last is the reason I don't much favor this idea; it's not much use colonizing a new world if in the process we ruin the one we have.

What other options are there?  Well, there's space elevators, but honestly, I consider those to be a pipe dream.  The mechanics work fine, and the physics is sound, but what do you make it out of?  The current hopefuls are carbon nanotubes, and graphene ribbons, which can reach the required strength to support their own weight, but have never been made more than a few centimeters long before microscopic imperfections drastically reduce strength causing failure.  If we can get from a few centimeters to 50000 kilometers, then it can work, but will still be almost as expensive as using rockets, in addition to being very vulnerable to space debris and meteoroids that orbit outside the atmosphere, and the fact that people on the elevator move SLOW.  This will limit daily freight to orbit, and also means that people spend a lot of time in the Van Allen radiation belts, which is dangerous.

Most options that intend to solve these problems do so by creating even more unrealistic technologies, like a fountain of pellets being constantly fired into space, on which we could hitch a ride.  Or, an ultra fast mag-lev train 2000 km long made of an iron tube in a sheath that hovers 80 km up in the air, and jumps the tracks when it gets going fast enough (actually, the physics for the hovering part is not the (main) problem, rather the energy supply: it takes a LOT, and if it ever turns off, 2000 km of iron bar will fall from 80 km up, and crush things.).

One idea that I like is basically to shoot stuff into space using really big guns, especially rail guns.  The main problems here are air resistance and g-force.  To shoot something reasonably aerodynamic into space, we need to get it up to 9 or 10 km/s ( 1.2 km/s higher than actual orbital velocity) to overcome drag.  To put it in perspective, if we accelerate that much in one second, we undergo about 1000 Gs of force, which will turn a human body to pulp, and we'll need a rail gun 5 km long.  This could be useful for solid freight, but anything with any empty space in it, like a lung, or a computer, would be obliterated.  Let's go for a much more gentle 5 Gs.  At 5 Gs, most humans will feel terrible, but they will live, and not suffer any lasting damage (For comparison, the most Gs on any roller coaster is 4.5 and only for a few instants).  To reach the same speed at 5 Gs will take 200 seconds, and require a launch tube (hardly a gun at this point) 1000 km long (200 sec, average speed 5 km/s, 200*5=1000km), which, by the end, is at an angle of roughly 60 degrees.  The cars will need their own rockets to direct themselves into a stable orbit after leaving the atmosphere, but fuel requirements would be minimal.

The gradient would need to be slow, or the curve upward will increase the G force to deadly levels, meaning that we are looking at a structure that starts deep underground and shoots out of a mountaintop.  To be efficient, the underground tube will need to be a vacuum, so that air resistance doesn't get in the way of acceleration, and the best place to put it is on the equator, from west to east.  Best place to build such a thing, then, is either Mt. Kenya in Kenya, or Kilimanjaro in Tanzania.  I favor Kenya for the relative political stability, and the fact that it is almost exactly on the equator, but Kilimanjaro is taller by about 700 meters, and the track could run nearly straight beneath the Serengeti plain for the full length, while in Kenya it would have to curve to avoid starting beneath Lake Victoria.  In both places, there are nature and wildlife concerns to be considered, but since the track would be very deep underground (at least 1000 meters), and the exit at an altitude where nearly nothing lives, and the power supply is 100% electric (read: we only need to pollute if we want to), I think that the benefit to humanity, along with the boon to the local economy, will outweigh the potential damage to nature.

However, after launch, we run into that air drag problem in a different way.  10 km/s is about Mach 30.  That's so fast that the cars would probably hit the air at the end of the tube like a wall.  They might be built to  withstand the hit, or be sufficiently aerodynamic that the impact will be minimal, but with aerodynamic shapes, friction heating will become a major problem.  Oddly, blunt surfaces are better at heat elimination, because they can make an air buffer that sends the heat around the ship rather than through it, while aerodynamic designs capture heat on the leading edges, destroying the structure.  However, a blunt car will need to go faster to escape the atmosphere, which means a longer track, more energy, and more friction which will probably eliminate the benefit.  So, the trick is to design a launch vehicle that can withstand the heat of 20 seconds in atmosphere, without being blown to bits.  One way is ablative coatings; basically, you coat the ship with hundreds of thin layers of something that will melt off while in flight, carrying heat away with them.  If this could ablative coating could be painted on, then the launch vehicles could be reused; just repaint them after they come down.  The other way is heat shielding like on the shuttles, but those ceramics are brittle, and I would be worried that impact with the air would shatter them, leading to catastrophic failure and death.  All you need is 20 seconds; after that, the car would be 120 km up or so, and outside the meaningful atmosphere, at which point the ship starts its on board thrusters.

But, all of that is a pipe dream.  Realistically speaking, a Mars colony in the next thirty years will use our current hydrogen-oxygen bi-propellant systems.  There's simply no way we can build anything else to do the job faster than that.  And, really, the limiting factors on rockets to present have been lack of demand that would make mass production sensible, because only NASA and satellite TV companies buy them.  If we decide to make space a real industry, not just a research project, then the costs will fall greatly, as they always do with mass production.  The kilogram cost to orbit is already down beneath $2500 for certain rockets, thanks to private space industry like SpaceX, where when it was just NASA, the price was over $10000/kg payload.  At $2500/kg, a trillion dollars will get us 400 million kilos into orbit.  If we subtract the cost of building whatever it is we send (say, 250 billion), then we should say 3/4th of that.  300 million kilos.... That's about the weight of a large skyscraper, which usually costs a lot less than 250 billion, but this one will need a lot of technology packed on board.

So.  300 million kilos (assuming that prices don't go down, which, realistically, they will).  Tune in next time to talk about a few of the things that will make up that weight!