lazerator
Aerospace
- May 18, 2003
- 3
First off we should all be very weary when someone attempts to use Thermodynamics to explain a dynamic process. Thermodynamics at it's heart is not dynamic at all. It concerns itself with end states caring little about how any particle got to where it is. That being said Thermodynamics cannot tell us anything about momentum transfer. To understand that we must look at the problem from a dynamic situation. First, we have a sail of given mass that will be hit with particles of zero mass but finite momentum, the photons. So the laws the govern the process will be electromagnetic. So clearly the amount of force the sail experiences will be a function of both intensity and wavelength. The total force can then be obtained by integrating over the wavelengths.
As to effectiveness, the photons will exchange momentum with the sail through the processes of absoption and emission. So it comes down to a discussion of electromagnetic interactions. This is clearly beyond the scope of thermodynamic principles. Furthermore, the pressure exerted is a kinetmaic pressure (force per unit area) rather than a measurement of fluid energy.
A quick review of Jackson (3rd Edition) problem 6.11 shows the basic ideas of what is occuring. However, the implicit assumption that the sail is a perfect absorber and furthermore emits no electromagnetic energy is in question. I also believe there is an assuption that no electromagnetic energy is converted to thermal motions in the sail.
As a final note, even if only 1% of the flux of solar EM energy is converted into motional energy you still gain an order of magnitude higher acceleration that by using proton momentum exchange.
As to effectiveness, the photons will exchange momentum with the sail through the processes of absoption and emission. So it comes down to a discussion of electromagnetic interactions. This is clearly beyond the scope of thermodynamic principles. Furthermore, the pressure exerted is a kinetmaic pressure (force per unit area) rather than a measurement of fluid energy.
A quick review of Jackson (3rd Edition) problem 6.11 shows the basic ideas of what is occuring. However, the implicit assumption that the sail is a perfect absorber and furthermore emits no electromagnetic energy is in question. I also believe there is an assuption that no electromagnetic energy is converted to thermal motions in the sail.
As a final note, even if only 1% of the flux of solar EM energy is converted into motional energy you still gain an order of magnitude higher acceleration that by using proton momentum exchange.
