Henry Spencer, "Beyond Chemical Rockets: Overview and Near-Term Options"

Henry Spencer  started off saying chemical rockets have limits.  There is only so much energy you can get out of the chemical bonds.
Henry says it is important to consider what it means to be better than chemical rockets.  He showed a couple equations.  Henry walked through the consequences of the equation.  You can get delta V of about 10 km / second from chemical rockets.  This is barely useful for getting around the solar system.  It would be nice to get higher delta V. 
Chemical rockets work OK for getting around in the Earth Moon system.  But the inner solar system is a thousand times bigger.  Henry listed four basic approaches:

1) More energetic fuels?   Not any good candidates.
2) Non-rocket: environment:
Solar sails are good for slow difficult missions.  Could create a small army of them to explore the asteroid belt.  There are a few other areas we could use solar sails. 
Planet assists – can get a gravity assists, but there are small windows when can use them.
Planet atmospheres – could use for breaking.
3) Non-rocket: outside help
Rotating tethers – Some possibilities.
Can use lasers to provide power, or to drive solar sails. Issues with pushing solar sails of thermal absorption and needing a lot of power.
Microwave beaming – need a big infrastructure, on both ends.
4) Separate energy and mass
Henry showed another set of equations.  Where do you get the power?  Solar cells is an option.  Nuclear is a possibility.  For now this is a government monopoly so can forget it.  Handling all the energy is a serious problem.  He went through a number of additional options.

There are several issues with testing this on earth.  Need a good vacumn testing environment.