Hopefully Interesting

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 easier to turn an energy source into heat than it is to

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 come from somewhere and that somewhere is going to be re

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.  Take the fuel used by the Sp

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, neither of which require fuel. Sp

So I was sort of busy the last two weekends with  Intercon  and  PAX  being back to back.  Hence this is a bit late. We got  a lot of snow .   Seriously.   Enough that people considered  Extreme measures .  We might be going over the all time winter record this weekend if we get just a bit more, we'll see. This kickstarter  (oh look, I guess it failed in the time since I found it) strikes me as an excellent idea in principle but not one that's going to be successful as a Kickstarter.  The idea is that for the serves that provide us our internet one of the biggest costs is the electricity consumed by the data center.  And the biggest drain on electricity isn't what the servers consume but what's required to power the air conditioning for the data center.  But if you spread out all those servers and put them in homes in places where they aren't running the AC all the time then the cooling cost basically disappears.  Then the owner of the server just reimburses the