But that side effect may equal or exceed the ion thrust. The photons are either reflected or absorbed, giving two values for light pressure (reality is a mix based on percentage reflectivity). Some of the absorbed ones can be electricity rather than thermal impact energy, but the impulse is still there.
The electricity can power an ion drive, but I think the 1st Law ensures the ion thrust can't exceed the photon thrust on the system. The photon thrust is the direction of the Poynting vector for absorbed photons. By Snell's law, some thrust vectoring can be obtained for the reflected photons, and up to twice the pressure since they leave at the speed of light too. However the "lightship" would have a much smaller range of navigational opportunity than a sailing vessel with a keel and the ability to resist sideways thrust. I doubt it can tack to sunward, it would be more like a square-rigged ship. ________________________________ From: "[email protected]" <[email protected]> To: John M. Steele <[email protected]>; U.S. Metric Association <[email protected]> Sent: Mon, June 20, 2011 1:00:14 PM Subject: Re: [USMA:50686] Solar Electric Propulsion Yes, Ions are the masses which impart the propulsion thrust, The much less massive electrons need to be discarded to maintain charge neutrality. The forces of solar photons are a side effect in the application of ion drives. Although the velocities of the ions in the propulsion jet stream are very high, as close to c, (3 x 10^8 m/s), as possible, they are still well below the speed of light. How are high are they in proposed (and already flown) ion drives? Let's search the Internet to find out. EAM ---- Original message ---- >Date: Mon, 20 Jun 2011 09:15:04 -0700 (PDT) >From: "John M. Steele" <[email protected]> >Subject: Re: [USMA:50686] Solar Electric Propulsion >To: [email protected], "U.S. Metric Association" <[email protected]> > > Ignoring relativity, the physical interpretation is > the velocity at which you can throw fuel mass > "overboard." If it is some kind of "ion drive" I > suppose you can't ignore relativity. > > The photons from the sun also act with direct force > on the solar panels. Past studies have looked at > solar sails, etc. Converting some of the energy to > an ion drive could be a way to "vector" the thrust > from the light pressure and provide more control. > > ------------------------------------------------ > > From: "[email protected]" <[email protected]> > To: U.S. Metric Association <[email protected]> > Sent: Mon, June 20, 2011 9:50:32 AM > Subject: [USMA:50686] Solar Electric Propulsion > The NASA John Glenn Center in Cleveland, Ohio is > requesting Proposals > for a demonstration of Solar Electric Propulsion, a > method of slowly thrusting > probes along spiral trajectories into the outer > regions of our solar system. > > The "specific impulse" (m/s) required for such > missions can only be achieved by electric propulsion > (not by chemical rockets) fueled by solar or nuclear > energy. > > Specific Impulse (thrust times time; divided by the > mass ejected by the rocket motor) (newtons x > seconds per kilogram) i.e. (kg x m/s^2) x s per kg = > m/s. > > In outdated studies, some engineers equated mass > with weight and got the less > acceptable result that pounds times seconds divided > by pounds equals seconds. > > Let's see if the winning proposal(s) can correctly > distinguish mass and weight? > > Gene Mechtly
