Re: Modest Proposal - Common Interplanetary Booster

Silicon solar cells are 22% efficient (I'm thinking now of Swanson's
Sunpower back junction cells) at most, and in space solar energy has a
density of 0.137 watts per sq cm and produces 0.03 watts electrical
per cm2, a dichroic mirror reflects away all wavelengths longer than
bandgap energy, with the cell radiating away 0.068 watts per sq cm as
heat.

0.068 W/cm2 = 5.67e-12 * T^4 = 331K

That's about 58C ... 136F which is warm, but operable. Cutting off
shorter wavelengths cools temps further, but at a cost of cutting
power output.

Now, consider that solar cell thicknesses can be as little at 275 um
thick - massing 64 milligrams per cm2.

Using the 0.03 watts/electrical per cm2 this is 469 kW/tonne.

Now there are added things, like the film mentioned, foils to conduct
electricity, mounts structure etc., etc., etc., - but the solar cells
can be thinned by chemical and mechanical polishing - and so forth -
this is a good estimate of what's possible.

Now consider a bandgap matched laser illuminating the solar panel at
6.75 watts/cm2 being converte with 99% efficiency. With a optical
bandpass filter that admits only the laser wavelength, the same 0.068
W/cm2 is radiated away, so the same operating temperature. The same
64 milligrams per cm2 - but now there's a whopping 6.68 watts/cm2 of
electrical power!!! That's 104.4 MW/tonne

A resistojet rocket with 1,000 sec Isp requires 44.5 MW/tonne thrust
A plasma arc rocket with 2,000 sec Isp requires 89.0 MW/tonne thrust.
An ion rocket with 5,000 sec Isp requires 222.5 MW/tonne thrust.

Now consider that in order to create an ionized stream you need to
make a plasma, and to make a plasma you need to make a hot gas.

Consider that with 104.4 MW/tonne you and create a laser electric
rocket that has three modes of operation;

A resistojet mode that heats cryogenic hydrogen to high temperatures
and produces 2.34 thrust to weight.

A plasma arc mode that heats a high temperature gas and produces 1.17
thrust to weight.

An ion rocket that takes a plasma and accelerates it and produces 0.47
thrust to weight.

A chemical rocket has a 70:1 thrust to weight typically, and to
produce 1.3 gees at lift off it comprises 1.85% total lift off
weight. With a 8% structural fraction - the engine is 25% of the
empty vehicle weight.

Limiting our engine mass to 35% of the total vehicle weight - we have
the following gee forces possible with this improved system;

resistojet mode - 0.819 gees - sufficient for an upper stage during
ascent.
plasmajet mode - 0.409 gees - sufficient for an upper stage late
ascent
ion mode -0.164 gees - again very late upper stage and into orbit.

With 1/6th gee you can almost do a lunar landing at 5,000 sec Isp.
Which is pretty amazing. Transfer times aren't going to be a problem
with this system.

Modifying the central stage with this sort of system means deploying a
wing - how large does the wing have to be? Well if the stage mass is
9.7 million pounds and gee force is 0.819 - then we have 3608 metric
tons of thrust

That's 160.5 GW.

This is a little less than the 220 GW powersat I've spoken of
elsewhere.

At 6.68 watts/cm2 - 66.8 kW/m2 - 66.8 GW/km2 - A square on each side
1.1 km!!

Quite a sizeable wing!

and not doable.

Increasing intensities 1 million times reduces wing sizes to 1 meter
area - which is far more reasonable size. The problem now is
temperature.

For the laser, 1,000,000x is possible, and for the PV too if it can be
kept cool. Another advantage is that the mass is reuced by a factor
of 1,000,000 - 104.4 TW/tonne so, 160.5 GW engine has a power plant
that masses only 1.53 kg!!!!

The power plant is now way way down there - and we can reduce engine
size to that of a chemical engine, and launch from the surface!! In
fact increasing the power levels to 802.5 GW - increases the mass of
the power plant to only 7.65 kg.

The problem is heating the PV cell.

One way to do that is to pass the propellant through the PV wing - to
keep it cool.

But, when we do that some argue we are making the PV a heat exchanger,
and we might as well do that directly. because the first two methods,
the resistojet and arcjet, can be done by DIRECT LASER ACTION.

So, we can see that going 1 million times the intensity above - which
is perfectly doable - provides a means to reduce the wings to meter in
size this way and up our thrust to weight while increasing thrust and
power levels and whatnot.

The ion rocket can also be done this way, using the cooling action of
the propellant moving through the PV wing to keep it cool during
operation, and then use the tremendous electrical output to operate
really intense arcjet and ion rocket - using laser action itself to do
direct heating - with maybe some resistojet assistance.

At this point we dispense with stages and turn EACH of our launch
elements into super massive cruisers.

With a 50 km/sec exhaust velocity launched with a 1.2 TW beam from the
surface of Earth - we have to increase our thrust to produce 1.3 gees
at lift off - and now our 9.7 million pound vehicle requires only 2.4
million pounds of propellant, and 1.3 million pounds of structure -
providing a lift capacity of 6.0 million pounds to GEO and return.
Lifting 5 million pounds to the moon and returning it. Lifting 5
million to Mars and returning.

ALL WITH THE SAME AIRFRAME AND AVIONICS

Merely replacing the Aerospike engine, with a laser/thermoelectric
engine.

The 'power pads' are so small and lightweight, the base of the vehicle
is replete with them. They attach to the skin of the vehicle, and on
its base, and propellant is passed automatically through the hot spot,
regardless of how the vehicle is oriented. The intense laser beam is
also redirected from the ground during launch.

So, for every single launcher described here, that puts 600,000 pounds
into GEO -and 280,000 pounds to the moon or mars, with the erection of
the powersat network, and the perfection of the thermoelectric laser
engine, we now have THREE vehicles capable of putting up 6,000,000
pounds to GEO and 5,000,000 pounds to Mars!!!

I mentioned these elswhere in other posts, but perhaps it wasn't
clear.

A fleet of a three dozen vehicles lifting 15 million pounds across the
solar system each year becomes a fleet of 108 vehicles lifting 780
million pounds across the solar system each year.

This by only continuing power plant construction and solving a few
open issues with cooling PV cells operating under intense
conditions.

Clearly the most efficient growth path in performance with the least
R&D and investment dollars.

The next step is to take this technology, and put it on smaller
vehicles using highly reliable MEMs rocket arrays

http://www.nsti.org/procs/Nanotech2006v3/3/W51.03
http://www.me.berkeley.edu/mrcl/rockets.html

using the same approach...

A 1/2 ton vehicle carrying four adults - requires no more than 200 MW
when pulling a little less than 2 gees - and requires 84 kg of
propellant - a little more than 1 cubic meter of liquid hydrogen - to
attain orbit.

In this application, it may be possible to use superheated steam that
decomposes by thermolysis - and then ionize both the oxygen and the
hydrogen - but there are open issues. Even so, being able to use
water - reduces propellant volume to 84 liters - about the size of an
automobile gas tank today!!!

The age of the daily driver rocket will certainly have arrived.


I have related here the thought process that went into my decisions to
proceed as I have - I believe these are the best decisions by far.
.

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