Hopefully Interesting

For an author it's important to get things that the reader might find hard to swallow out there and dealt with as soon as possible.  I didn't really enjoy Neal Stephenson's last book, REAMDE , past the halfway point because too many improbably occurrences had piled up and my suspension of disbelief didn't recover.  By contrast his newest novel, Seveneves , seems to be doing an excellent job of getting the improbable stuff dealt with quickly and I've been enjoying the book without any hangups.  I've only gotten through chapter 7, acknowledged, but I've got a feeling I'll continue to enjoy this one. But of course the second improbable of the two things gives me a chance to talk about some physics I find interesting so I'm going to dissect what I think Stephenson gets wrong.  Not because I think the author is a bad person or wrote a bad book but just because I think the physics is nifty and reading this prompted me to share it. The basic setup of th...

See also parts  I ,  II ,  III ,  IV , and  V . Chemical rockets would be really nice if they were just a bit more energetic.  The energy that goes into pushing the propellant out comes from the propellant itself, so there's stuff you can do to minimize the transmission of heat from the propellant to the rest of your engine.  That helps you get around some of the problems of rockets where you  heat up the propellant  from outside.  And the fact that you're still basically using heat means you don't have to suffer the efficiency losses that happen turning heat to electricity when you use an  electric rocket .  But what sort of reactions are there that we might cause in our propellant that are higher energy than chemical reactions?  I think you've all seen the post title and know that I'm about to say "nuclear." Now I should make sure to say that unlike the other categories I've mentioned nobody is actively working...

See also parts  I ,  II ,  III ,  IV ,  and  VI . We've covered a bunch of ways of moving ships around in space by shoving stuff out their backs.  But there are some ways of moving around in outer space that actually don't involve the rocket equation at all.  When you fly in a plane on Earth you can push around all that nice air that surrounds you in your environment in order to fly.  Well, you can if you have a plane.  It's very convenient in terms of not having to carry around huge amounts of fuel.  There isn't any air in space but that doesn't mean that space is entirely featureless either.  There are basically three things I know of that you can push off against in order to go places in space: the light of the sun, the solar wind, and planetary magnetic fields. The principle behind solar sails is pretty simple.  You still have sunlight in space and it's very bright too, at least within Earth's orbit.  By Einst...

See also parts  I ,  II ,  III ,   V , and  VI . Here we talk about applying some sort of external heat source to your propellant so it expands a bunch and then shoots out of the rocket.  Technically chemical rockets are thermal rockets too but I'm trying not to be pedantic here.   There are lots of things you can use as your source of heat.  NASA has done a lot of studies with using a nuclear reactor to heat some propellant directly.  If you've got a big parabolic mirror you can focus sunlight on your engine and heat it that way.  If you've got a friend nearby with a big laser they can use it the same way. The constraints for these sorts of rockets aren't the same as for the last two sorts of rockets.  You don't have a fixed ratio of energy to propellant that chemical rockets have so the  v e  isn't fixed that way.  Energy is a concern as with  electric rockets  but not as large a one since its much ...

See also parts  I ,  II ,  IV ,  V , and  VI . So besides burning stuff, how else can we make our rockets move?  Well, one probe that's been in the news a lot recently is Dawn  which recently went into orbit around Ceres and showed us those two funny bright spots on it.  One of the nifty things about Dawn is that it uses an ion drive for propulsion. Ion drives, just one sort of electric drive out there, work by using electric fields to accelerate atoms very quickly out the back of the rocket.  Since they decouple the energy used to accelerate the propellant from the propellant itself there really isn't any firm limit on how fast the propellant goes besides how much electrical power you have available to shove them.  A typical ion thruster might have a  v e  42,000 m/s, almost 10 times higher than the best you can get from a chemical rocket. Unfortunately that electrical power is a bit of a sticking point.  It has to ...

See also parts  I ,   III ,  IV ,  V , and  VI . The most common sort of rocket and the ones we're all familiar with from seeing them on TV are the ones that work by burning stuff.  That is, they work by combining two different chemicals that react to produce the energy that propels the byproduct of the reaction out the back of the rocket.  Since the fuel that provides the energy and the propellant that is ejected for the momentum are the same thing this means that you're always using more or less the same amount of energy for the same amount of propellant.  This means that the v e , the velocity of the exhaust, is always going to be more or less constant depending on sort of chemicals you're using with some variation depending on inefficiencies in the engine. Many chemical rockets use cryogenic fuels - substances that are normally gasses but which have been cooled down enough that they liquefy and can be put in reasonably sized fuel tanks. ...

See also parts II , III , IV , V , and VI . I've been on a bit of a space propulsion kick recently between getting a copy of  High Frontier , rediscovering  Atomic Rockets , doing an archive binge on  Selenian Boondocks , and generally following all the excitement around  SpaceX .  I thought I'd write up a few blog posts on what I think of as the interesting bits.  Be warned that there'll be a bit of math involved but I'll try to keep it from getting out of hand. So, lets say you want to  go to space today .  Maybe you even want to head out to the Moon or Mars or Ceres.  When you travel on the Earth in a car or plane or boat the hard part is energy you spend is going to be in used to overcome the resistance of whatever medium you're traveling in.  You get up to some maximum speed and have to exert your motor constantly to stay at that speed.  When you want to stop you simply stop applying force and maybe apply a brake as well, n...