New Mars Forums

I don't think you would necessarily WANT to plunge from a vacuum into normal air pressure. Accelerating in the air would build up the surface-effect buffer that you'd want to protect yourself from the static air, and establish aerodynamic stability as you accelerated, rather than having to deal with a discontinuity in air pressures.

From a purely kinetic viewpoint, of KE = 1/2 mv^2, moving through air at 1mB versus Earth's 1Barr, 5000m/s would give you an equivalent of 500m/s at sea level on Earth, that is, about 1100 mph. Of course, you'd be outside the atmosphere in a few seconds, anyway. So, I think the Martian mass driver would be do-able.

Someone mentioned that "you'd be outside the Mars atmosphere in 150 miles". I think that's very high. Earth, for example, drops down to less than 1 mb at 50 miles. I'm sure there's a pressure gradient for Mars posted on the net somewhere.

The drag problem is quite complex for a number of reasons, one of them having to deal with supersonic shockwave calcs. I'm sure someone has already researched supersonic drag characteristics for spheres, it's just a matter of digging it up, but I truly don't expect this to be a problem.

On another note; 300,000 G's in a rail gun? Are you serious? The best I'd heard to date was SSI getting 100G's out of a mag rail. I'd think just about ANYTHING would disintegrate at 300,000 G's. Are you (Turbo) sure about those numbers?

Interesting point concerning "sharp bits and boulders" in the lava tubes, but I still believe you'd need less equipment (mass to ship) than the bulldozer you'd need to move Mars-dirt (I almost said "earth") on top of whatever other structure you want to take along. Argueably, you could even use a spray-foam sealant to cover the rough edges before installing an inflatable bag, or just forget the bag entirely and do the entire interior with foam sealant.

And yes, I was referring to light pipes, not fiber, and you can do that sort of stuff very, very low tech. I was going to suggest something more like 4 m^2 of collector per meter of flora, since the light is so much weaker out there. The only nasty problem I see is having to dust off the mirrors (or inflatable parabolics) every few days. If one was to use heavy mirrors manufactured on-site and  mounted solid right in the ground, winds would, at least, be less of a concern.

Cronin's book, As It Is On Mars, attacks the question of concrete making on Mars, which is apparently a lot more difficult than most of us would like to think. Clay for concrete comes from rainfall-based effects, so it might be in short supply.

I'm not sure inflatable structures are such a good idea, either, since (as someone else pointed out, and Cronin points out) the pressure delta is humongous. Cronin suggests very, very heavy glass plates for the roof. Something to counter the 14 PSI pressure pushing against the ceiling.

My recommendation is to use lava tubes (suggested by John Lewis for Moon structures) since there's obviously been a lot of volcanic activity in the past. Line the interior with inflated plastic and cap the end(s), and you've got tons of protective material around you. Pipe in light from solar collectors; these could be low-pressure inflatables or manufactured on-site mirrors made with glass and metal, or just metal. Lots of protection, and very little to none heavy machinery required. Of course, there is no *guaranty* that there are lava tubes there, but I'd bet my life on it. And, I'll be they're huge, thanks to the low G's.

An interesting subject, getting people to "donate" to the Mars effort. I see a vision of someone going door to door trying to collect money together for a Mars mission. Humorous, of course, since most people justifiably see the space industry as heedlessly throwing huge amounts of money around to do trivial things, thanks to NASAs huge bureaucracy and safety issues. Given that the common populace is most likely never going to donate "a dollar each", which wouldn't even pay for 1 NASA launch, the donation process is going to be limited to the high-stakes venture capital, advertising, media, entertainment, and rich philanthropists. I don't see any good way for this to ever become anything but an elitist effort. I think the Mars community should therefore be actively targeting the spectrum of ultra-wealthy supporters.

Radio waves are, of course, actually a transfer of energy, otherwise your car radio wouldn't work as it needs a signal of some sort to amplify. Microwave energy, beamed to Earth from a Solar Power Satellite, can be of a tight enough beam that fringe effects of the beam wouldn't hurt anything, in fact, the beam density can be low enough that you could walk through it without killing yourself or even substantially hurting yourself. Beaming from the moon sounds pretty dumb, though. Beam divergence would make the beam nearly unusable by the time it got this far. The studies I've seen are for geosync orbits, which is still pretty far away.

But then, I've always thought SPS was a dumb idea. No matter how efficient you can make it, you can take that same efficiency and employ it on Earth's surface to collect sunlight without any launch costs and with easy access for repairs and replacements. Granted, you might only be functional for 8 hours a day, average, but who cares? The cost per watt to build the thing is a hundred times cheaper; a 100W solar panel costs $500 and weighs 23 pounds. Let's say you could make it out of exotic materials that only weigh a pound, it still costs $10,000 to get to orbit, and THEN you have to build a receiving station anyway, possible at the same cost the ground-based solar panels would have cost.

But, besides the fact that it's totally uneconomical, it would also increase the incident energy arriving on Earth, where ground-based panels wouldn't, and ultimately the conversion of the microwaves into usable energy would result in heat. A brand new mechanism to ensure global warming, as if oil burning wasn't enough.

TJ

Actually, slowing down is pretty easy. As a baseline, imagine a solar sail in orbit around the sun, outside the influence of Earth. If the solar sail is perpendicular to the path of the solar wind, the whole force of the solar wind will be bent on pushing the sail away from the sun, and its orbit will slowly spiral outward. Now, tilt the solar sail at a steep angle relative to the sun. If you've tilted it the right way, the solar wind will provide a small vector pushing out, but a very large vector slowing down your orbit. Slower orbits spiral inward (assuming continuous thrust. A pulsed thrust would merely put you in an elliptical orbit).

Well, I have to give bradguth credit on one (and only one) point; I just checked here;

http://www-star.stanford.edu/projects/mgs/profile.html

and noticed that at an elevation of about 55 km, Venus's atmosphere sports an Earthly temperature of about 20C and a pressure of about 1 Barr. And, thanks to a horrendous pressure gradient there, it'd be pretty easy to live there. Of course, down on the surface it would kill you. I'd heard the whole argument about hydrogen being dissociated from water from solar energy, and the readings that indicate there's not any water at all there, but based on the temperature and pressure gradient, I have to say that's a little surprising. It wouldn't shock me to find out that there IS a lot of water there and that there was something wrong with our reading technique, but I find that hard to believe.

I tend to doubt most of Brad's other conjectures, though. If there's any carbon-based life on Venus, it better be floating in the atmosphere already. Plus, he should realize that anthropomorphizing plate tectonic features is a pretty useless effort. OTOH, I'd love to see someone drop a blimp in there and take some photos, if there's enough useful light down there. It might be pitch black under all that cloud cover.

And even if there IS a mile-long bridge, that doesn't mean it ain't natural, Brad. We have some nice natural bridges here on Earth, too, despite the higher gravity. I wouldn't put too much of my soul into interpreting NASA radar images; the detail isn't *that* good.

The difference between the CO2 plan and the Water plan, is, of course, you need to dig the water out using some digging device, one that would have to act autonomously and probably weigh quite a bit, versus a CO2 air pump that weighs next to nothing. I don't think there's much water vapor in the atmosphere, is there?

Tom

I looked at his site, and for the life of me I can't find anything related to propulsion; it's all about storage devices for harddrives. I found one link that goes to a Pop Sci article about flying saucers run by nukes for propulsion, but not a word about him in there. Where on his sight does he propose his concepts?

TJ

Free space wattage is 1400Watts per square meter in Earth-orbit. The really nice solar photovoltaics they have can pull only about 20% efficiency, so you're looking at a collection of roughly 280 Watts per square meter as usuable electricity. Scale that up to whatever size you want.

Another option is to use an inflatable solar collector with reflective surfaces and heating a working fluid and running a turbine. The efficiency on this is much, much higher and ultimately weighs less overall, but involves using parts that break down. I think dependability is a factor, here. Don't have solid figures for this, though.

TJ

Hi, CM. Brilliant idea, using space junk to fab a new vehicle, especially for the nukes. I happen to know for a fact that there are complete satellites in orbit, unused, fuel and all, that are "pet rocks" due to launch screw-ups. If someone could salvage those, it'd be a boon for everyone concerned. Some of them can't communicate with the ground at all; the main power buses never even turned on.

Of course, delta-V from satellite to satellite might kill you. A 60 degree change of inclination is paramount to a full 7.5kps delta-V change, might as well launch from Earth. Best to find an orbit with as many salvagable satellites in or near it to as possible so no inclination changes are required.

On the other hand, if you're going from an equitorial orbit to a North/South inclination, you can use charged tethers to electrically change your orbital  inclination, which would mean reducing your refueling requirements. And, you can use the equitorial bulge to drift your orbit to get the ascending nodes aligned, so that's a freebie. I don't know if the charged tether technique is one-way or not; does anyone know if it can bring you back to equatorial?

Canth, you bring up a valid point; I once heard that it would cost less to outfit and send up an expended shuttle booster tank than it would cost to clean and outfit one that was up there for "free".  That, of course, is due in part to the mass of the components that go inside, and the cost of EVA's, so the point is arguable.  Still, the nice thing about salvaging nukes is that you don't have to okay their launch with the environmentalists. And, the MAIN reason it would cost so much to salvage anything is that we don't already have a presence up there doing it. I think you could make money just by deorbiting stuff for others.

I just about died when they deorbited the Mir. No matter how much it sucked, the solar panels were still putting out lots of juice; and, of course, every pound of mass up there cost $10K to get there. What a waste to bring ANY of it back down.

TJ

The ACE spacecraft (http://www.ips.gov.au/asfc/current/bzspeed.html) measured the solar wind at 630km/s (it varies a lot), which is about 1/500th light speed, a lot higher than quoted. Also, you could increase your vehicle speed beyond the speed of the solar wind by tilting your solar sail at a steep angle with respect to the wind (opinion, not fact).