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.
