Using Concrete to build space ships...

> Those 3 one km diameter solar sail carriers would probably have a
> lot less of mass than your small crew carrying space transporter.

So what? The ship is quick to get to destination, can be reused
multiple times, is versatile so that other missions are possible.
The amortization of the ship is plausable.

> Who is going to pay to get that into orbit and the fuel it will
> need to reach an asteroid probably out of earths magnetic shields?

I'm not claiming that they'll be free. You're the one making that
claim.

> The solar sails would need only little fuel.

OK. From a previous post, we know that we can get 1 Newton of thrust
from 212,750 m^2 of solar sail operating at optimum condition.

You're proposing 3 1-km diameter solar sails. Each such sail would
have 785,000 m^2 of sail. This would equal 3.7 Newtons of thrust.

Let's say that the asteroids to be mined are somewhere between Mars
and Earth. Their distance will vary between 250,000 km and
78,000,000 km as they progress through their orbits. Let's say that
we work with an average distance of 40,000,000 km for the cargo to
travel.

Recall that:
Force = mass*acceleration
Distance = 0.5acceleration*time^2

Each solar sail puts out 3.7 Newtons. You have 3 sails.
You need 200,000,000 kg of material.

Let's say each load is 1,000 kg.

Accel = Force/Mass = 3.7/1000 = 0.0037 m/s^2

Time = SQRROOT(2*Distance/accel) = SQRROOT(2*20,000,000,000 / 0.0037)

3,287,980 seconds to accelerate and the same to decelerate.

76 days for 3,000 kg of mass.

66,667 such journeys. This doesn't include the travel time back to
the asteroid without any cargo.

All told, you need 13,881 YEARS to haul that much mass with 3 solar
sails.

> Has the small manned transporter radiation shielding?

Sure, a water core like I've previously described.

> Wouldnt you say, that a space craft which can have a constant
> thrust of 7 Newton all the time if needed without using fuel at
> all could move cheap?

Sure, once you discount the other factors. Yes, it is cheap.

> >How much energy is required to maintain position at L1?
> It would be less than in an orbit I think.

The moon is in orbit around the Earth. How much energy does it take
to maintain its orbit?

L1 is not stable in all 3 axis.

> Solar sails can hold position in space, at least in the inner
> planets area. There is no need to determine energy requirements
> since they are able to hold position using free energy through
> photonic pressure.

Yes, Forward's statite concept makes use of this feature.

So, by making use of this effect are you telling me that you're
going to maintain stability of your infrastructure at L1 by use of
such statites?

> When you have a standstill you can point the craft in any direction
> you like just use the thrusters as long as needed to get it going.
> Could you do that with a fast moving mountain of asteroid?
> With that kind of inertia?

You sure could! Instead of pointing the asteroid as you would a
spacecraft (which you only do to get full effect from the rear
thrusters) you would re-align the mass driver or the attached
rockets to where they needed to be in order to get the thrust vector
that was desired.

> Probably it will be hard to change direction at all with such a
> huge mass.

Well, hard is a relative term. We are talking about building huge
vessels that cycle in an orbit between Earth and Mars afterall.

Look, no one is going to pick an asteroid that has a retrograde
motion. That would necessitate erasing all of its velocity and
reversing it. No thanks. Pass.

You would look for an asteroid that has a pretty complementary
orbit. Look to the tables and graphing links I provided earlier.
There are lots of candidate asteroids. Now it's simply a process of
choosing the ones that work the best.

> >I haven't seen any calculations that claim that you're going to
> > get over 4 km/sec boost in velocity. Can you provide some info
> > on this?

> I didnt do that calculations. Could you prove that it is not
> possible? Than go ahead.

Here is a thorough explanation of the physics of gravitational
slingshots:

http://www.mathpages.com/home/kmath114.htm

The quick and dirty number is that you can probably boost your
velocity by about 2.25x if everything works ideally.

So, you still need to get up to 1.9 km/sec to come out with 4.3
km/sec. Further you still need to carry that fuel. Further still,
you need to make all sorts of corrections on your 220,000 tonne
cycler in order to finalize its orbit. You got to carry the fuel,
baby.

> >Your 200,000 tonne cycler will have to increase it's velocity by
> >4.3km/sec to get to a Mars orbit.
> >
> >KE = 0.5 mv^2 ------> 0.5 * (200,000,000 kg)(4,300 m/s)^2 = 1.85
*
> >10^15 Joules
> >
> >Now let's find an asteroid of 4,000,000 tonnes mass. TWENTY TIMES
> >HEAVIER. It is moving at a velocity of 4 km/sec in a nearby
orbit.
> >It will need a delta-v of 0.3 km/sec in order to get into the
cycler
> >orbit.
> >
> >KE = 0.5 (4,000,000,000 kg)(300 m/s)^2 = 1.8 * 10^14 J.
> >
> >You'll need TEN TIMES more energy to get your cycler into the
proper
> >orbit than it would take to get a asteroid that is TWENTY TIMES
more
> >massive into the same orbit.

> You did not account for the energy won through the gravitational
> slingshot on the cyclers side.

OK, I'll pull data from the mass fraction graph in a previous post.

By using a gravity assist from the Earth, let's assume a best case
scenario, and you now only need to achieve 1.9 km/sec. That
translates into a Fuel mass fraction of about 48%. Let's say that
you also need to carry maneouvering fuel for orbital trimming
operations, how about another 7%. OK, so now 55% of your starting
mass is fuel, so your total mass is now 444,444 tonnes.

At 20 tonnes payload per launch from Earth, you're looking at 12,222
launches to supply the fuel.

Your Kinetic Energy is going to amount to 0.5(444,444,000 kg)(1,900
m/s)^2 for 8.02 * 10^14 Joules and that's with the slingshot effect
factored into the equation.

THAT'S STILL 4.5 X THE AMOUNT OF ENERGY NEEDED TO CHANGE THE
VELOCITY OF A 4,000,000 TONNE ASTEROID BY 300 m/s.

It just doesn't work to build this thing in Earth orbit the way
you're planning.

> You also just seem to state that no change of direction of your
> asteroid is needed. I doubt that. Just adding delta-v wont do the
> trick.

Why do you doubt that? Nobody will pick an asteroid with retrograde
motion.

> I am with you on this but am a bit uncomfortable with throwing
> rocks out for propulsion. But using water for steam propulsion or
> making fuel off it would be fine for me.
>
> Frank

Makes no never mind to me :) Whatever is the most technologically
easiest is fine. You'll probably have lot's of mass to exploit.

TangoMan