SPS & Beamed Energy Propulsion

SPS - Reply to J. P. Kosky
I would like to take issue with your comments. Surely it is the space
tether that is just `theoretically possible' and then only if
unbelievably vast quantities of materials like super-carbon fibres
can be produced and some nation on the equator allows it, not to
mention the cost and a dozen other `small' problems, like
creating a
potentian hazard for every other non-GEO satellite ever launched.
Even if achievable, it would also be so vulnerable to attack that I
for one wouldn't be putting any money on it.
I really don't see any possibility of such a massive
infrastructure
program ever getting off the ground. It may be possible in the very
distant future, but only if built from space once moon bases and
asteroid mining is developed to provide the raw materials. That puts
a space tether at the end of the list, not the beginning. In 50
years impossible more like 150, and I consider myself an
optimist. For the present the space tether like fusion is
fanciful vapourware and the problem is, we need a solution that we
can work on NOW.
Beamed Energy propulsion (BEP) however, is not just
`theoretically
possible'. The basic technology is proven science and a high
percent
of it is virtually off-the-shelf if we go with masers.
I am sure someone will correct me if I'm wrong but as I
understand
the technology, BEP relies on the focussing of an energy beam of
sufficient heat and energy/density to ionise a reactant, including
possibly air. That energy beam could be a laser, a maser, x-rays
(very unlikely but through a QNR reactor, still possible for unmanned
flight) or even basic un-tuned microwaves. The basis is the rapid
ionisation of a reactant and the higher the temperature, the greater
the efficiency
Even solar energy could be used and has indeed been investigated by
both MSFC and Phillips US air force labs for an orbit transfer role.
Each potential energy source has its pros and cons and none should be
dismissed out of hand.
The primary criteria are that the beam is able to sustain
temperatures of around 6000C or above. Physically, containment at
those temperatures is the main technical problem but not an insoluble
one.

The figures published by NASA (and Myrabo) regarding the Lightcraft
are not absolutely clear on the details of the power source so I
stand to be corrected. It is recorded that the tests used a 10kW
pulsed IR laser but do not record its efficiency. I assume they are
following convention and the 10kW is the INPUT power to the laser, in
which case the OUTPUT beam,(reflected by the typical laser efficiency
of 2%-5%) is around 200w-500w, at best. If this is so then, by my
observation of videos and flight time, the 50gram Lightcraft
accelerated at around 2g's on 500 watts of laser beam. (Even if
the
laser supplied 10kW to the craft, it still only equates to 200kW/kg.)
If the above assumption is correct then simple math suggests this
equates to 1kg of thrust/per 10kW of energy received at the craft, or
better. It would be reasonable to assume that scaling up will greatly
reduce heat losses and therefore provide greater efficiency.
Certainly acceleration and energy efficiency must be factored in but
present estimates (of 1Mw/kg, to orbit) are commonly based on a
lasers' poor efficiency and are therefore totally unrealistic.
Masers
have a range to (or from) GEO, are at least 15x as efficient as
lasers, require far less high-tech infrastructure both in energy
delivery and collection and in craft systems. Microwaves also have
far better penetration of pollution and light cloud.
Microwaves and even solar energy could also have a special role in
the first stage of lift.
At present BEP has been proven using only a single laser but any
scaled-up version would almost certainly go to masers, the same
energy type that would be used by SPS, to beam energy to the ground.
Assuming a knowledge of SPS potential, now consider.
SPS anticipates receiving perhaps 5Gw over an area (say) 5km wide, an
average E/D (over 20km2) of 400w/m2. However, the energy profile is
always going to be very much higher (10x?) at the centre of the
footprint than at the edges. Assuming 4kW/m2, an entirely realistic
figure, this energy could be reflected beneath a craft placed on a
suitably raised platform (using simple metal mirrors) or it could be
channelled through waveguides.
You also suggest that chemical processes have a long way to go in
possible improvement and that beamed energy is anyway going to be
based on chemical energy. I believe, in both cases, you are way out
of court. In space, beamed energy systems have already demonstrated a
(minimum) 3x-10x greater efficiency than the best chemical rocket
ever built and because of the exponential nature of this
technologies, will therefore use 1000x-2500x less fuel.
Within the atmosphere, the potential advantages are even greater. BEP
has already proved it can `theoretically' achieve infinity in
a fuel
mass/payload rating. And the evidence for this is in the can. The
first Lightcraft carried absolutely no fuel at all none
zilch
zero.
As to the basic energy source, as previously mentioned, the MSFC and
Phillips have already been investigating the use of direct solar, and
most `electric' thrusters use solar cells. There is also
near-term
potential for a solar/microwave transducer, after all, it is exactly
what SPS is based on. The only chemical BEP needs, is the reactant,
and that could be almost anything, including air and water.
Myrabo has proved the concepts involved in supplying a BEP craft from
above and below. Using his `air spike' a direct lift to a GEO
would
seem the goal to aim for, powered by solar-based energy. Ultimately,
anything less will mean failure but, as proposed, there would seem to
be a comparatively simple path to follow, in easy stages, with
minimal risk and incredible, unlimited potential.
CCMcM