Natural Science Forum / Physics / General Physics / July 2008

Is there air in the vacuum of space?

  0) What is the working fluid in vacuum?  Momentum transfer by
photons as such is laughable for spacecraft propulsion.

  1) Propulsion increases as the area of the driven receiver, r^2.
The payload of the craft increases as r^3.  It already stinks.

  2) Energy density sufficient to ionize, combust, or merely distort
the air (re mirages) ruins the process by defocusing, absorption, and
scattering.  Gonna use a phase-conjugate mirror, Bunkie?  Telescope
artificial star corrections have one-way negative feedback.  A
high-power beam is Star Warsy with positive feedback, too.

  3) The very best gas lasers are no more than 10% efficient.  Laser
chains like SHIVA are jokes.  Laser diode arrays can do better, but
not by enough to be process-believable.

Geeze. The writers of that web site need to do some proofreading.

Combusts? What? Nah.

As the text explains, it's the expansion of the plasma, not any
kind of combustion. Essentially it's a little ram jet. You heat
air in a pulse, that gives you a single push on the bottom of
the projectile. After the plasma expands and cools, air moves
back in due to atmospheric pressure. This step probably sets
the limit on pulse frequency.  And then you repeat the pulse.

I'd be guessing, but what I'd be guessing is that this scheme
would be drastically inefficient. I'd be surprised if anybody
ever bothers using this scheme for anything bigger than a
really pretty demo at a science museum.

Anyway, that scheme won't work without air. The pressure from
the laser directly is going to be microscopic. Remember your
relativity formula.  The energy in a light beam is

e = pc

where p is the momentum and c is the speed of light.

So, to get one Newton of force, the power in the beam
would have to be 3E8 Watts, or 300 MW. That's a honking
big laser.
Socks

DarkMatter <darkmatter34@yahoo.com> wrote in news:a7aca7a6-34a8-4a6b-bb61-
a93f542dd3b5@8g2000hse.googlegroups.com:

Yes

In sci.physics, DarkMatter
<darkmatter34@yahoo.com>
wrote
on Wed, 23 Jul 2008 09:02:24 -0700 (PDT)
<a7aca7a6-34a8-4a6b-bb61-a93f542dd3b5@8g2000hse.googlegroups.com>:

Yes, but the amount of energy required is frightful.
It takes 3 * 10^8 watts to generate 1 kg-m/s of momentum
shift.  However, the laser propulsion would be the absolute
best v_e of any rocket known to man, although the M_i/M_f
ratio is bound to be the absolute worst, absent discovery
of a better fuel source, in the Rocket Equation:

v_f = v_i + v_e * log(M_i/M_f)

If one assumes 1H1 + 5B11 = 3 2He4 + energy, one gets
1.007825032 + 11.009305 => 3 * 4.002603, or
a log(M_i/M_f) of at most 0.007759.  Chemical rockets
can get a log(M_i/M_f) of almost 3, if not even more.

An alternative is to simply discard the helium atoms;
since M_i/M_f is essentially the SR gamma in this case,
one gets a v_e of about 0.039 c.

v_f[laser] = 1 c * 0.007759 = 0.007759
v_f[helium] = 0.039 c * 3 = 0.117 c

Laser propulsion doesn't look all that viable.