Re: Airborne lasers

From: William Mook (william.mook_at_mokindustries.com)
Date: 12/18/04 

Date: 18 Dec 2004 13:44:45 -0800

Scott Lowther wrote:
> William Mook wrote:
>
> > You are saying it is flatly impossible to cancel sound of moving
gases!
>
>
> Realistically? Practically?

Yes.

> For a
>
> hypersonic exhaust flow
> of high mass flow rate
> in the lower atmosphere? Yes.

Of course if any one of those three conditions don't apply, you have
something quiet enough for broadscale use, which was my only point at
the outset. Lots of stuff flying through the skies without troubling
anyone propelled by laser beams from space.

At lift off and landing you'd have subsonic flows at a large range of
practical mass flow rates in the lower atmosphere. As you ascended,
the exhaust velocities would increase in speed to transonic,
supersonic, and ultimately hypersonic - as specific impulse improved.
Of course, noise on the ground is the limiting factor in this analysis
- but that falls off with the square of altitude.

> Because once the gasses are away
> from they vehicle, they are no longer controllable; the flow becomes
> turbulent and cahotic, and the simple sound of the air getting
smacked
> out of the way of the exhaust flow will be louder'n hell.

Depends on the details as you pointed out with your three conditions
above. All you're really saying is you won't use hypersonic flows at
lift off and landing.

Surely you've heard of Fluid Dynamics? If you control the details
across the normal area to a flow tube you can do quite a bit - despite
the problems you suggest would occur.

http://www.pdas.com/panrefs.htm

> > Remember, you can control the exhaust speed of each of the
> > microengines too! :)
>
>
> Yes. You can make the system very quiet if you make the exhaustr
speed
> subsonic. of course, your thrust approximates zero at that point, but
hey.

Yes, subsonic flow has the capacity to be uniquely quiet. However,
supersonic and hypersonic flows have some interesting low-noise
solutions as well.

But, let's cheerfully accept your condition and see if thrust
approximates zero, hey?

Let's look at the details;

http://www.grc.nasa.gov/WWW/K-12/airplane/thrsteq.html

Worst case assumes no pressure assists across the propulsion area - and
no winglike or teardrop shapes that multiply these flow pressures up to
10x or more;

F = mdot * Vexhaust

Well at 22C the speed of sound in dry air is 345 m/sec. So, for each
Newton of thrust the engine produces with an exhaust speed of say 330
m/sec you have;

mdot = F/Vexhaust = 1/330 kg/sec

Now Myrabo's test rockets are really jets. This just shows that you
can use a wide range of working fluids in laser energized propulsion
systems - including air.

So, mass flow rates aren't really a problem since in jet mode you have
lots of air to work with at take off and landing. At higher altitudes
you have distance from the ground, and lower air densities helping
deaden sound on the ground.

Now, air density on the ground is 1.225 kg/m3 - so, that means at 330
m/sec flow rate each square meter of area has 330 x 1.225 = 404.25
kg/sec flowing through it.

This is your mdot per square meter of a laser jet with a subsonic
exhaust speed (each point in the mass flow area under detailed control
as described earlier across the flow tube) so,

F = 404.25 * 330 = 133.4 kN per m2 --> 13.5 metric tons per m2

This is just an ROM estimate, real fluid analysis will come up with a
different figure - but the ROM estimate indicates 10 tonnes per sq
meter are possible. For objects 1 sq meter in area and massing 10 kg -
clearly subsonic flows are quite sufficient to lift them and carry them
on their journey silently. For objects 10 sq meter in area and massing
1 tonne (about the size and weight of an automobile) - we can also lift
them and toss them upward quite silently as well.

> >
> > Surely, someone as brilliant as you can see the implication of this

>
>
> The implications of the implausible? Yes. Another crappy movie like
"The
> Core."

Non-sequitous response. Clearly if you control the mass flow rate,
velocity and direction among a million points per square meter of flow
tube area, you can control many aspects of the flow field and the
resulting noise levels.

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