Tuesday, July 5, 2011

Stopping an Interstellar Freight Train

So you're travelling to Alpha Centauri at 10% the speed of light and the two stars are shining out of your viewscreen. Congratulations, you avoided life support failure, the crew going crazy, or the ship's computer going crazy and killing the crew in their hibernation pods. Now how do you stop? I put this blog post up because I was inspired by the Icarus Project's post on the same subject, and I've been giving this a great deal of thought over the years.

The bad news is that it's going to take all the energy you put into getting up to your velocity into slowing down. You could use your rocket engines (assuming they still work) to slow down, but that means your initial fuel load goes up exponentially and the spacecraft at launch would have been the size of a mountain. (Launching this is going to make the Apollo project look like the Bobsy Twins and their kindly uncle building ships in their back yard*). The alternative is to simply cut your initial velocity in half and use the other half of your delta V (total velocity capacity) to brake at the other end. But it means getting there slowly, and slow means that taxpayers and politicians are less likely to fund such a venture (and the mission is already going to take decades).

Obviously, you would ditch as much of your unneeded payload as possible (but, light years from anywhere, every scrap of material is valuable). Radiation shielding, habitats, soil, empty fuel tanks, all jettisoned except for landers, science payload and your colony equipment. Oh, and colonists, all crammed into a tiny habitat, or camping out in the landers. The high-efficiency motor itself used to get up to this speed is likely to be massive; if one could drop it overboard if there was some other way to break, one probably would. Still, perhaps a fuel reserve could be used to slow the spacecraft a little bit before saying goodbye to the motor. A 10 or 20% reduction in velocity might make all the difference - this must be weighed up against the risk of the motor not working after all these years in vacuum and hard radiation, and already thousands of hours of operation.

The good news is that there are ways to slow down, albeit each with their own limitations. The most viable way to slow down would be with a Magsail. This uses the interstellar medium, which although is a huge vacuum, actually has one or two ions per cubic centimetre. Travelling at such high velocities, this actually translates into a very, very thin mass flow. Running a current through a huge superconducting loop hundreds of kilometres in diameter would tug on these stray ions with a magnetic field, transferring the ship's momentum to the them and slowing the ship down. A very elegant solution which works better when decelerating from higher velocities. Perhaps it would be better to hang onto the engine and only fire it when the ship's velocity got down to ~0.001c, or in the region of a few hundred kilometres per second.

The second no-fuel alternative is to use a solar sail. A BIG solar sail. But to slow down from 0.1c, even a mere 50 tonne payload would require a sail 1000km in diameter, and using an efficient inflated sail made of beryllium diving right in. Making the sail out of super-high strength, high temperature carbon nanotube technology doesn't help that much. Where it does get better though is that when the slower clip you can come in at, the less of a problem it is. The diameter appears to scale with the incoming velocity; so at 0.01c the sail diameter is only 94km. And of course, the sail mass is proportional to the inverse square of the diameter, so a 94km sail only weighs 1% of what a 1000km sail would be. Interested readers should consult Pat Galea's article on this. Alpha Centauri, however, does offer the chance to use two suns to slow down - albeit not in line with one another, making it a lot more difficult to brake at one and then brake at the other. A gravitational slingshot might help, though - calculated to shave off velocity instead of boosting it.

Aerobraking is how planetary landers shed all their nasty orbital velocity, but could we do that to shave off a few percent off of our incoming velocity? Planetary atmospheric deceleration is basically impossible for even a tiny fraction of lightspeed; the probe would almost surely incinerate. At very high velocities, radiative flux heating rather than conductive heating dominates. The radiative flux from plasma sheath surrounding the probe would also incinerate the sidewalls. Also, since this is a huge spacecraft, aerobraking is problematic even for regular atmospheric entry, due to surface area to volume constraints. Heatshields are heavy. Aerobraking also requires a spacecraft to be moving at a speed that is still slow enough to allow the planet's gravity to pull it close in an arc, getting more braking effect. Decelerating from even ~0.001c would create something like a Tunguska explosion on the surface of the planet. Even then, a survivable deceleration spike (say a couple of hundred G's if everybody's vitrified) would only be for a few seconds and shave off a few dozen kps, because, travelling in a straight line at even low sublight means you only dip into the atmosphere for a very short distance. Space shuttles and capsules can afford to decelerate slowly because they travel thousands of kilometres because they are travelling slow enough that they are curving around the planet as they do so. At such high speeds, a planet's gravity wouldn't curve the spacecraft's trajectory at all.

So aerobraking is a no-go. Our spacecraft would disappear in a Tunguska like explosion. So what now? We've got all this mass sitting at (basically) zero velocity for us to slow down, but we're going too fast and planet atmospheres are too small. I considered trying to punch out a "corridor" of atmosphere with projectiles, but I think that would dissipate too fast. Plus, it would be rather catastrophic for the planet's environment, doing some rather drastic remodeling. Not what you want if you want to study it, especially if there's the possibility of life.

This is a somewhat insane proposal that involves using your environment and whatever you have lying about to your advantage. The idea is precisely that your spacecraft is a flying bomb; a snotty tissue tossed out the window at a passing planet would obliterate a city when travelling at even these "low" sublight speeds.

Towards the end of main engine deceleration, the spacecraft dispenses a number of small, independent solar sails. This zip on towards the target star while the main craft continues to large behind. Having no payload, they can decelerate to lower velocities by braking at the star. The main craft then arrives in the system, ditches its main engine and deploys its own solar sail. It brakes hard at the target star, bleeding off a large chunk of its remaining velocity and carries on past the star, heading towards the now near-stationary sails. The next phase is tricky and smacks of insanity. As it approaches each sail, a small railgun on board fires a pellet at each mini-sail in front, blowing it up. The explosion is like a small nuclear detonation, and the main craft's sail catches the debris, slowing it down almost like a parachute. Alternatively, the sails decelerate completely (if the closing velocity is slow enough) because they are so light, and then shoot back out towards the main craft at the same speed, getting a huge energy boost from the sun. If depending on thousands of mini-sails is too problematic, then one big sail could be released and then dole out its energy in the form of small pellets of ice, which the main craft could then ram into. The disadvantage is that not all of the available mass would be used up in the ramming. The mini-sails' total mass would be totally used up in the ramming explosions, whereas the mass of a larger sail wouldn't be used at all - it would just carry on out back towards Sol.

A brief understanding of this concept is that the energy for deceleration is only available at the target star for a short time, which gets even shorter the faster the probe is moving and thus requires much bigger sails. Using mini sails captures this energy and the main craft gathers it up in chunks (by basically ramming into them).

*I have no idea who these characters are, I only know Jerry Pournelle mentioned them. i.e. Something from before my time. I assume it's like the Hardy Brothers (which I also never read).

Friday, July 1, 2011

The Science of Mass Effect

What's a computer game and a propellantless drive doing in a blog about realistic interstellar flight with solar sails, fusion drives and the scuzzy facts of cosmic radiation? Well, I was thinking back about having played Mass Effect, and looking back at some discussion with some (smarter than me) folks who discuss and experiment with propellantless propulsion and advanced physics, I thought I'd try and show how some of this is cropping up in mainstream entertainment (what do you mean you never heard of Mass Effect?).

First off, Mass Effect, and its sequels. I'm sure a lot of you will have at least heard of it, and some played it. It's a combination of shooting game and role playing. While it's a fantastically well crafted and seriously fun game with some great acting, it's also really meaty in its science. Unlike Star Trek's "Particle of The Week" technobabble, the creators of Mass Effect carefully pondered what they could do with cutting-edge physics and technology. Ships, for example, have to have radiators or else they'll cook from their own internal heat in the vacuum of space. Weapons are variants on mass drivers and need to either cool down or deposit their waste heat into disposable heat sinks. Soldiers and everyday civilians are enhanced with cybernetic implants and nanotechnology.

Other topics are touched on, such as what happens when AI evolves and computing progress leads to Singularities. This is reminiscent of Sci-Fi authors like Peter F. Hamilton and Charles Stross. In fact, the game's fluff specifically mentions a couple of people I've had lively discussions with - space wargame designer Ken Burnside and the creator of the Atomic Rocket website, Winchell Chung. You can see a pair of recruits with their names being chewed out (with some mild profanity) here.

The most interesting aspect for me is the titular Mass Effect. In the game, the Mass Effect is generated by applying electrical charge to a chunk of Element Zero (just your average plot Unobtanium). There's a bit of waffle about how this Element Zero manipulates dark energy, the odd force causing the universe to expand. This Mass Effect is used to generate gravity, allow ships to accelerate at stupendous velocities, travel faster than light and to create force fields. In this video, everybody's favourite pop scientist, Michio Kaku, discusses the tech of Mass Effect.

What struck me is that the Mass Effect is an inertia-modifying effect. Which is exactly what the Mach Effect is, just without the Element Zero. It can be achieved by common or garden variety capacitors, so the theory goes, or anything else that fluctuates in internal energy quickly enough. I'll spare the long discussion, but it's really a logical outgrowth of Einstein's General Relativity to explain inertia - basically it's caused by the gravity of the rest of the universe pulling on an on object. Wiggle the object in the right way, and you can get those rubbery strings of gravity to work for you (note, nothing to do with String Theory). A bit like how a vibrating table can make an otherwise heavy object easy to push. Basically, the object's inertia is being lowered for a microsecond, and if you time the shove right, you can push it with less force. Interestingly, it appears that it would great gravity fields around it (because of all those stretched or relaxed gravity "strings"). Those gravity fields could give us artificial gravity generators, force fields, tractor beams and maybe even faster than light travel. Just like the Mass Effect universe.

The scientists (notably Dr. Woodward), engineers and Joe Averages (i.e. yours truly) who discuss the Mach Effect were talking about how to raise awareness of it, so instead of making a Doritos-and-Mountain Dew-fuelled looong email trying to explain all of this, I thought I'd put it here in the public eye, so to speak. A lot of promising physics concepts are familiar in Sci-Fi, or are otherwise making their way into the public consciousness thanks to the general curiosity of people surfing the net. Of course, Joe Average might look at you and go what? But those of us who watch Big Bang and have a vague clue about what Sheldon spouts may know. And maybe all that's needed to get promising revolutionary technologies off the experimental bench and into spacecraft...

When playing Mass Effect 2, I took a trip down to the engine room and saw the Mass Effect core vibrating... much like the way the Mach Effect devices would work (although you wouldn't necessarily *see* the vibrations, which would be in the mega to gigahertz range...). 3D artists, game programmers and designers are a smart bunch, and you often see unexpected references to some really intriguing ideas wrapped in a game or movie. I wonder if the inspiration for the Mass Effect was indeed the Mach Effect?