Thanks for confirming Stephen, your maths look ok to me; What you call "waste" heat can be turned into useful backwards radiation with a proper reflector can't it? I even wonder if a thermonuclear engine producing just heat could not be turned into an efficient photon drive. P.S. It seems it could, according to http://en.wikipedia.org/wiki/Nuclear_photonic_rocket
Regarding long-range deadliness of the light beam I imagine it can be avoided by making it sufficiently uncollimated while keeping cos phi losses at an acceptable level. Besides I don't think the problem is photon-specific, any high-energy beam will be dangerous. Michel ----- Original Message ----- From: "Stephen A. Lawrence" <[EMAIL PROTECTED]> To: <[email protected]> Sent: Saturday, January 06, 2007 3:54 PM Subject: Re: [Vo]: Cold fusion powered rockets > > > Michel Jullian wrote: >> Aren't photon rockets supposed to be the most efficient of all? > > Right. From the point of view of someone on the rocket, for ordinary > fuel, when a piece of fuel of mass dm is ejected, the momentum gained is > > dP = v_e dm > > so > > dP/dm = v_e > > For a photon rocket, if we use "v" for "nu" = frequency and "l" for > "lambda" = wavelength, then the momentum and energy of one photon is > > dP = h/l > > dE = h v > > and the mass-equivalent of the energy of that photon (which is the mass > the ship actually "loses" when the photon shoots out the exhaust) is > > dm = dE/c^2 = hv/c^2 > > So for a photon rocket, > > dP/dm = c^2 dP/dE = c > > and, of course, for ordinary fuel v_e < c so the photon rocket's always > more efficient, in terms of the amount of momentum gained for a given > amount of reaction mass consumed. > > But that doesn't take account of the amount of waste heat you generate > making the photons, nor does it take account of the fixed mass of the > equipment you need to carry to make the photons. Chemical rockets are a > lot simpler than high powered lasers. > > And speaking of lasers, you don't want to stand in back of a high-thrust > photon rocket -- not even far, far in back of it! Its "exhaust" is > likely to be a multi-gigawatt laser. > > >> >> Michel >> >> ----- Original Message ----- >> From: "Robin van Spaandonk" <[EMAIL PROTECTED]> >> To: <[email protected]> >> Sent: Friday, January 05, 2007 9:46 PM >> Subject: Re: [Vo]: Cold fusion powered rockets >> >> >>> In reply to Jed Rothwell's message of Thu, 04 Jan 2007 15:56:22 -0500: >>> Hi, >>> [snip] >>>> None. But suppose only CF is made practical, and the temperature is >>>> limited to, say, 1000 deg C. I suppose that would call for a two- or >>>> three-stage approach, starting with heat to electricity. >>>> >>> Not necessarily. It is becoming evident that when CF actually occurs, alpha >>> particles are the usual nuclear product. It may become possible to arrange >>> for >>> them to be ejected directly rather than undergoing any form of conversion at >>> all. Multi-MeV alphas would yield a very interesting specific impulse, and >>> also >>> be far and away the most efficient way of utilizing the fuel. >>> >>> Regards, >>> >>> Robin van Spaandonk >>> >>> http://users.bigpond.net.au/rvanspaa/ >>> >>> Competition provides the motivation, >>> Cooperation provides the means. >>> >> >
