Using Concrete to build space ships...

--- In ssi_list@... "Valens Agnitio"

> I am not sure I like the idea of firing off innumerable pebbles
> close to transit orbits... Figure what happens over a period of
> years and firing off pebbels every few seconds... Space may be
> vast, but we're talking about pretty specific paths. I wonder: How
> would that affect the risk factors, especially if we do it on
> eight asteroids?

Mike has quoted some studies that purport to show that the
probabilities of impact are quite small. The point is that space is
indeed far larger than we're used to referencing.

However, low probability doesn't mean impossibility. If one wants to
avoid the littering of the cycler orbit with such debris, all one
has to do is to launch the pebbles at a velocity of 40+ km/sec and
they'll achieve solar escape velocity. Let the Alpha Centuarians
deal with the debris in 32,000 years. :)

Don't neglect the very viable possibility of mining frozen ices such
as water, methane and ammonia, and turning them into rocket fuels.
No pebble problem.

> Using LEO as a construction point doesn't make sense, as you
> clearly pointed out. Certainly there are other locations or orbits
> that will allow us to significantly reduce delivery costs of
> materials and launch (boost?). Along with appropriate timing and
> some gravitational assists, perhaps the propellant costs can be
> made managable?

Where will the propellant come from? If Xenon or Argon, it's most
likely Earth, unless you intend to scoop it from the Martian
atmosphere, but that's a whole other story.

If nuclear, then you've got the whole launch safety issue. Further,
you still need to provide hydrogen as a reaction mass. Now you've
got cryogenic storage issues.

If you're looking at a ORION type nuclear propulsion scheme, then
you've got the issue of getting all of the nuclear bombs into orbit.

If it is chemical propulsion, then you're either launching from
Earth or mining the ices at other NEOs, transporting the gases to
your cycler ship, then you've got the rocket equation limitations to
deal with as you propell the ship up to 4.3 km/sec delta-v.

Better to simply use material that is already moving at the required
delta-v in an approximate orbit. Send out a small construction ship
with life-support for the crews multi-year mission. Send out
construction equipment. The crew rides alongside the asteroid as it
journeys on the cycler orbit. They construct the cycler from the
asteroid material. After their work-term is completed they head back
to Earth on their ship and another crew makes a rendevouz with the
cycler and starts their work.

Look, cyclers won't be the first order of business with respect to
Mars missions. There'll have to be a compelling reason to send lots
or people and materiel to Mars. The expense of doing so via
traditional means may become burdensome. That's when cycler
construction, and the long construction times, and multiple
platforms becomes worthy of consideration.

Yes, cyclers will make sense once there is a need for them. Further,
the necessary infrastructure will be available as well.

I think that they will be constructed en-route because that saves
the expense of hauling mass to the construction site and also save
the expense of accelerating the mass to the cycler orbit. Those are
two HUGE benefits that more than make up for the inconveience of
constructing the cycler on a multi-year journey away from the Earth.
In fact, you could combine the construction phase with a Mars
mission so that people or materiel are dropped off at Mars and the
crew deadheads back to Earth.

> One other thing... How close would that bring asteroids to being
> possible impactors on Earth? I ask in case at some point everyone
> gets bored with space and the orbits are not maintained.

The whole point of developing a resonance orbit is that the cycler
comes close to Earth, but never impacts it. Further, the attraction
of the cycler orbit is that it shouldn't need much maintenance.
That's why you need 8 or so cyclers.

Play around with the NASA orbit simulator I linked to. You'll see
that when an asteroid reaches perigee at Earth, and then later
approaches Mars, that it doesn't immediately intersect Earth again
as it crosses Earth's orbit. Earth may be 150 days behind in its
orbit. This cycler may make another 7 journeys between Earth *orbit*
and Mars *orbit* before it makes perigee at Earth again.

This concept is different than a spaceship where you fire your
engines in order to trim the orbit and can boost to align yourself
on an orbital path. Now you've got to carry a lot of fuel in order
to make the course corrections.

Cyclers just cycle. They shouldn't need the corrections. Therefore,
their orbits shouldn't need much, if any, maintenance.

> However, consider that for the energy and time needed to move one
> asteroid of a trillion tons into a suitable orbit, how many
> constructs could we get going? I'll bet it's more than a few. And
> they're much more disposable than the asteroid would be, and
> therefore easier to replace or modify.

First off, find yourself asteroids that have desirable orbits
already. Then send some probes and do some sampling. If all checks
out then send crew and equipment. If the asteroid is too massive,
then cleave it into a portion that you can use. Explosives, spinning
it, or solar baking it might be the techniques used to break it
apart.

Now take the piece that you want to use, and start moving it into
the cycler orbit and leave the remains moving in the resultant orbit.

Now you're just moving the piece that you want. The crew returns to
Earth while the new orbit is pursued.

Once the orbit is achieved, send another crew to start construction.

> Why not get the whole thing going (Earth/Mars Transits),
> instead of one shot wonders?

Because the necessary infrastructure isn't in orbit to achieve the
more ambitious construction program.

Even when there is a need for cyclers, which won't be before Mars is
a viable proposition, it will still be energy intensive to haul all
your material to one location, fabricate the cycler and then
accelerate it into orbit.

TangoMan