Using Concrete to build space ships...

>
>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. :)

Excellent... Another option I had not considered. That starts to make
asteroids sound a bit more attractive, especially if the process can be
automated to some degree.

>
>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.

True. But the more I think about it and the more Claudio talks about it, the
more I think that nuclear is the way to go.

>
>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.

I am not so sure that we need hydrogen for the nuke idea. It seems that
pretty much anything can be heated to plasma and accelerated out the back.
Why not use materials organic to the cycler itself?

As far as Orion, that would be a whole lot easier if it turns out there are
accessible deposits on the moon or an NEO. If not, the only real obstacle is
the hysterical Greenies. If they can be byppassed, then we just launch from
Earth.

>
>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.

Still curious about how much of that delta-v must be supplied by the
rockets, and how much we can cheat with gravity assists and an appropriate
building site. Here's an idea for you: Start the fully constructed skeleton
on the way to mars, and then launch buckets of regolith after it via .......
That would solve a huge amount of the fuel problem. A nice fusion of your
idea with mine, I'd say :-)

>
>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.

A good concept. The crew can build their own shielding from the asteroid
when they get there, and leave it for follow on crews.

>
>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.

I agree.
>
>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.

So, it will take a long time before it's a danger...

>
>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.

They will need corrections. I doubt that we can place them accurately enough
to obviate the need. In addition, there will be loads on and off, which you
yourself pointed out would change the orbits to a small degree.

>
>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.

Interesting ideas, and sound as well. However, suppose most of these things
are just rubble piles, and there aren't any that are suitably close to what
we need. Then building the cycler may be a necessity.

>
>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.

At some point, someone with the time and know how will have to identify some
canidate asteroids and do all the math. Then a comparison with the
construction method may reveal the more cost efficient way.

However, I recognize the asteroid method as a sound and potentially viable
way to approach the cycler idea. But it's nice to have an alternative though
out, isn't it?

Val

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