Energy conservation does not necessarily limit the maximum thrust possible. The key is obtaining the highest *propulsion* "specific impulse" (m/s) possible. i.e. the highest "exhaust velocity" (m/s) that is possible.
Back to the Internet to find some realistic numbers for that velocity. EAM. ---- Original message ---- >Date: Mon, 20 Jun 2011 10:27:08 -0700 (PDT) >From: "John M. Steele" <[email protected]> >Subject: Re: [USMA:50686] Solar Electric Propulsion >To: [email protected], "U.S. Metric Association" <[email protected]> > > 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
