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Aswad -> RE: I want the WORLD to go to Mars....not just the United States.... (5/8/2013 1:19:43 AM)
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ORIGINAL: shallowdeep
Thanks for the insight into your figure's origin, Aswad.
You're welcome. As I said, back of envelope.
I should add: off the top of my head (just as the idea was). For instance, I forgot LHC was reusing a preexisting tunnel.
As for the tunnel aspect, I agree that the cost of building such a tunnel in the Mt. Everest region is expensive, though it is possibly worth it for the launch height, shaving off more than a third of the atmospheric density. Otherwise, I would have suggested Norway, where you could get a polar orbit and ca. 2km launch height without disturbing the local population at all, and where there is ample infrastructure to work from.
Atmospheric density is arguably one of the hardest things to deal with. Technology for the maglev, you can do R&D on the ground, without any dangerous surprises, but testing for atmosphere handling is going to be a lot harder, and normal atmospheric pressure is quite different from the reduced one at high altitudes. Though, on the ground, you can also do a number of experiments on reducing the atmospheric density temporarily (e.g. superheating).
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Taking a gun approach might give significant advantages, but I guess I wouldn't personally classify it as a "current" technology.
If by current you mean "minor variation on something we've already done", then no, it's not a current technology.
If you mean, "currently deliverable", then yes, it's a matter of deciding to do it and proceeding to do so.
My sense of current is the latter. It's a lot of hard work, and it may not be the optimal choice, but we know we can do it, and that it'll work. We can't "just build it", of course: there will be R&D involved in building it. But we know what R&D we need to do, and we know how to go about that. Surprises may happen in any project, but there's no major unknowns, no places to stop and scratch our heads or the like, and with proper scheduling, we can get it done in the alotted time. It's a question of getting the job done, not a question of being able to. Hence, possible with current technology.
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Maglev trains are probably a closer fit in many ways.
Closer, agreed.
But a maglev train operates at a speed that is of an entirely different order of magnitude, in continuous operation, with the train itself containing a significant portion of the active electronics. A maglev launcher would, by contrast, use a huge amount of energy per segment for a very limited amount of time to accelerate that much mass to those speeds. Of course, you could go for smaller payloads to simplify construction, but to fully take the step into the early space age, I think we'll need to aim for the kiloton range.
Where the comparison with the LHC enters the picture is one of the secondary reasons for using a linear launcher: you're going to want distributed energy storage, sensors and logic to get sufficiently accurate transverse deflection to position it and angle it, because you don't want to hit the atmosphere at the wrong angle, relative to what the hull has been designed for, particularly not with passengers that may be buffered against the impact along one axis only (a long gas compression shaft inside the module, for instance, with temporary eddy current damping along the same axis, will sink a lot of impact energy).
You could also make a comparison to coil guns, but those really haven't been explored in enough detail to be meaningful.
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Because the LHC was able to make use of existing infrastructure, the price probably doesn't accurately reflect costs for the sort of civil engineering undertaking that would be involved in 1600 km of evacuated track construction – perhaps with a terminus atop a remote equatorial mountain, no less.
Agreed. As noted, I forgot the reuse of existing infrastructure.
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As another point of comparison, just replacing a bridge span can cost around $6.3 billion for 3.5 km.
Constructing the Hardanger bridge, by comparison, costs less than half a billion per kilometer. Of course, we're talking guesstimates here in any case, but I maintain you wouldn't even get close to the cost of the Second Gulf War, though I'll concede the half a trillion total cost guesstimate may have been somewhat optimistic.
EDIT: The Gotthard Base Tunnel under the alps is 50km long, with 150km worth of road, and costs about $10 billion. It's bigger and drills through solid mountain, but it's also shorter. In the world of guesstimates, one might call $100 billion a reasonable guesstimate for the bulk portion of the launch tunnel. The Channel Tunnel and Seikan Tunnel both seem to support the feasibility and the cost ballpark.
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In contrast, maglev trains that begin to approach the performance characteristics needed haven't gone beyond the most theoretical of design stages, let alone been built previously.
This is another reason for choosing to stick to 1g acceleration in my suggestion. Vacuumized systems can maintain a fairly high speed, but high acceleration would involve an amount of energy that would be an unreasonable risk in terms of how far we would be pushing the envelope. Even so, the maglev launcher isn't going to be the most demanding thing to develop.
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There would still be a significant amount of basic engineering research required; it's not the sort of project where design and construction could start right away.
With 1600km worth of tunnel required before you can start installing the high tech, you can absolutely start construction right away. It's a question of scheduling, both parallell and series subprojects. Research, yes, but not in the sense of "see whether it can be done".
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And big, somewhat-vaguely-defined systems seem to have a tendency to end up costing well more than projected…
This is more a capability and maturity issue in project management culture worldwide than a question of being able to put reasonably good bounds on the figures involved. Plus, we'll need to do something along these lines eventually, and thus the question is one of managing the costs along the way, providing budget makers with reasonable data about what money will be required when, and so forth.
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Even with the heating issues aside, just the sudden deceleration from drag forces when transitioning from an evacuated tube to atmosphere, even at a rarefied Everest-like elevation, seems like another major challenge – one that might preclude use for any cargo sensitive to high acceleration, including people.
As noted, gas compression springs can take a lot of the initial impact, and temporarily superheating a column of air can significantly reduce the density. Due to the possibility of a very long projectile, one can also distribute the stresses over more surface, akin to very low drag bullets (though, granted, this isn't comparable).
But, yes, this will be one of the hardest parts to deal with, in terms of R&D cost and effort.
If you have a good solution for anchoring a hydrogen balloon setup or the like, I would suggest suspending an extension tube with the ionic barrier at the end, such that the impact can be avoided (at 50km exit, you'd be meeting negligible air resistance). Something akin to a spaceshaft, in essence. The anchoring problem is difficult, but not unmanageable, so it's kind of a question of which of the problems will be easiest to solve.
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The inverted reentry heat problem, where the greatest air density would be faced when traveling at maximum speed rather than at minimum speed, seems like it might require some sort of novel heat management.
Ablative heat shielding seems like the most sensible solution. Something that doesn't conduct enough heat to the interior, instead evaporating into the airstream, and ejecting the heated shield once outside the atmosphere, so that it'll take stored heat with it and burn up on reentry while the payload settles in space.
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To avoid those issues, most of the proposed maglev launch designs I've seen floated involve somehow getting the launch tube's exit above 20 km… but that starts getting even further beyond current capabilities.
See above comments on lighter than air suspension rigs.
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Using a maglev track to assist the launch of conventional rockets might be more feasible in the near term. Due to the exponential nature of mass requirements for propellant, getting a rocket up to even low Mach numbers before ignition would provide substantial savings. I know NASA was doing some research into that at one point.
Yes, you could absolutely do something similar to the approach I suggested to launch a scramjet, with a much shorter track. Possibly a second shaft on the inside of the concrete structure for the part running up the mountain, so you could start getting utility from the effort earlier by using this second shaft to do assisted launching.
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That's the thing; I'm not aware of anyone serious having proposed something like that and I have some difficulty believing anyone would.
I haven't read it, but I saw mention of it on Slashdot a short while back, and assume that's what he was referring to.
IWYW,
— Aswad.
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