Using Concrete to build space ships...

>
>Val and Frank, I'm still uncertain as to the advantages of this
>concrete spaceship you're envisioning. You guys seem to be ignoring
>the rocket equation, or are under the impression that it only
>applies to launches from Earth. Even if you launch from orbit, you
>need fuel. You need more fuel at the beginning to propel the fuel
>itself. The only thing that is different is that you don't need to
>achieve as high a delta-v.
>
>Find 8 asteroids. Ideally, each should be at Earth perigee 687 days
>apart. That asteroid will return to Earth perigee about 28 years
>later.
>
>Send out some mining equipment and rocket engines. You can mine the
>ice for water, ammonia and methane and make hypergolic rocket fuel.
>Alternatively, attach a mass driver and chew up the asteroid and
>fire off the pebbles for propulsion.

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?

>
>Over the years, bring those asteroids into a ideal resonance orbit
>between Mars and Earth.
>
>The attraction of this approach is that the fuel for achieving the
>resonance orbit is the asteroid itself.

See above.

>
>The concrete spaceship idea means you have to first go get the
>material to make basalt or concrete. Then build the ship. Then
>launch it to a resonant orbit.
>
>You're fixating on the construction but ignoring the issue of
>achieving orbit.
You'll see that for a Mars orbit, you'll need to achieve a delta-v
>of 4.7 km/sec. With hypergolic fuels like UDMH and Hydrazine you'll
>avoid the need for cryogenics and achieve an ISP of, let's say, 300.
>
>Thus your fuel will be 75% of the total mass at launch, FROM LOW
>EARTH ORBIT.
>
>Your concrete ship, 200,000 tons, will need 600,000 tons of fuel.
>Where are you going to get that much fuel?
>
>You may say that you'll use a high efficiency Ion Engine with a high
>ISP. OK. Where will you get 56,400 tons of Xenon? From Earth. You'll
>have to launch it from Earth and now you're once again confronted
>with the Rocket Equation. Your Xenon payload, on a rocket powered
>with LO2/H2 will be only 10% of the GLOW (GROSS LIFT-OFF WEIGHT) at
>best.

No, not ignoring. I realize that the issues of getting into a desirable
orbit, the construction point, and the mass are all intimately bound.
Therefore, I was counting on people such as yourself to point out problems
and supply lots of links (Tango, you're always good for that!).

As far as concrete, I never thought it was a good idea for several reasons.
One, as Chrales mentioned, it's not friendly when it comes to taking damage.
Two, as you mentioned, it adds several more layers of complexity to the
whole construction program. Three, it would be much more difficult to
maintain for the reasons that Charles mentioned. And so on. That is why I
advocated the idea of using plain old regolith, instead.

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?

I never thought ion propulsion would be suitable for the masses involved,
and you did quite a number on the conventional. Yet Thor mentioned Orion
type proplulsion, and I would add in Nerva as well. These could help bring
the whole thing into a more doable realm. And of course, don't forget the
trilithium option... :-)

>
>I just don't see the benefit of this concrete spaceship idea. For
>the same amount of mass you're going to have to launch to build the
>thing, you can send payload packages to a number of asteroids and
>over decades bring their orbits into resonance and use the asteroids
>themselves for fuel. Then send up equipment to actually outfit them
>for occupation and you'll have a lot of the building materials on
>hand, right there in the asteroid. No need to ship that from distant
>asteroids.
>
>What am I missing that makes the concrete ship idea so appealing?

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.

Having already mentioned the things that I view as advantageous concerning a
constructed cycler in a previous post, I won't waste your time reiterating
them. 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.

Considering the difficulties involved in getting say, 53000 tons of Xenon
into orbit, well, all great things are at first. The whole idea of the
cycler is to pay those costs up front, then have a relatively easy and safe
transit to and from Mars, with minimal costs. I am very advers to
piece-mealing things, and the bigger and more important they are, the more
adverse I am. Why not get the whole thing going (Earth/Mars Transits),
instead of one shot wonders?

By-the-way, thank you for the links. Some look very, very interesting.
Unfortunately I won't be able to give them the time they deserve until this
weekend, so excuse me if I stated anything that would be easily answered by
following all of them :-)

Val
>
>TangoMan
>

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