Slashdot Mirror
Magnetic Space Launches
DiZNoG writes "This CNN article discusses NASA experimenting with the idea of using Mag-Lev technology to launch payloads into space. Mentioned in the article is that the U.S. Navy is working on the technology for it's aircraft carriers to launch fighters. Unfortunately the NASA project is horribly underfunded ($30,000) for research. Cool technology, let's hope that the Navy research gets us a step closer to not burning all that Oxygen and Hydrogen to get to space...
Well, I don't think the *cost* of energy (in terms of dollars) really is the issue here. It is the amount of onboard fuel which displaces the amount of cargo you can take into orbit. And since fuel has weight, the more fuel you add, the more fuel you need to achieve orbit. So, earth-based electricity vs. vehicle based fuel really would be a plus.
let's hope that the Navy research gets us a step closer to not burning all that Oxygen and Hydrogen to get to space...
Yes, we must reduce emissions of deadly Dihydrogen Monoxide! It's already filling our rivers, streams and oceans, and has been found even in the ice of Antarctica! The time to act is now, people! Before our wells are full of this dangerous chemical!
[O]nce you pay for the construction all that is left is electricity and maintence.
The same could be said for New York City. The devil is in the details, my friend. Folks thought the Shuttle would open up cheap access to space, since we'd get to reuse the orbiters. Ha ha.
[To avoid dangerously high acceleration, manned flights] will be launched off of a gradually ascending slope spanning a couple of kilometers.
Sorry, but that's still way too short. To achieve a minimum orbital velocity in a 2-kilometer run, you'd have to accelerate at a little more than 1500 gees. Splat.
Even with a 100-kilometer ramp, you'd be dealing with an average acceleration greater than 31 gees. It appears that, as far as space projects go, this will only ever be useful as an initial-stage boost, or for boosting raw materials into space for orbital construction projects.
Of course, it would still make a nice high-tech catapult for lobbing massive conventional weapons hundreds of miles, but of course no one in the Pentagon is thinking of THAT possibility...
I'm not positive, but I'm pretty sure that no material has the tensile strength to hold its own weight all the way to the moon. If you held a 5 foot string, it weighs practically nothing. If you dug a 100 mile hold and held a 100 mile string that was dangling down it it would rip your arm off. If you suspended it from something stronger than you, the string would just break under its own weight.
Plus you can't anchor a string to the earth and the moon. The earth rotates much faster than the moon orbits. If you attached it to just the earth it would only line up with the moon once a day, and it would be going so fast as it passed it you would be smashed into the moon. By the same token if you attached it to the moon, it would fly around the earth every 24 hours, meaning it would be blazingly fast, about 350 mph. Bad rope burn if you try to grab it.
However, it might be possible to build a 'string' that is strong enough to simply lead into orbit. Anchor one end to the earth, and the other to a large mass slightly outside geosync orbit, which is still way way closer than the moon. Then you can climb the string all the way to the mass and be flung away from the earth. At any rate we still don't have strong enough string. Yet.
Jherico
What can the average user can do to ensure his security? "Nothing, you're screwed"
The advantage here would be that you dont need to burn fuel to make the fuel move. You dont need to add extra weight to get started. Im not an expert, but i assume that the basic idea would be gather speed (not even necessarily vertically to begin with), and then launch it vertically. It needs to be vertical to escape the drag of the atmosphere as quickly as possible.
dominionrd.blogspot.com - Restaurants on
Yup. And for a more manageable 10g, you'd need a 315km run to reach geosynchronous velocity. Of course, you'd also burn to a crisp in the atmosphere ;-)
The advantage of railgun / rocket sled launches is in getting you some of the way up to orbital velocity, but there's still a good long way to go. Basically, you can't reach orbital velocity while still inside the atmosphere, so you have to carry a bunch of fuel up with you whichever way you cut it.
Here's some handy dandy info for those who want to have a play with the numbers and have forgotten their Newtonian stuff:
Geosynchronous orbit is at 42,245m, which requires an orbital velocity of 7869m/s. Gravity is 9.81m/s^2
Distance = half of acceleration times time squared (s = 0.5 * a * t^2) and velocity equals acceleration times time, so time equals velocity divided by acceleration (v = a * t, t = v / a)
If you know the speed that you want and the acceleration that you can tolerate, this gives you:
s = 0.5 * v^2 / a (e.g. for 7869m/s and 98.1m/s^2, s = 0.5 * 7869 * 7869 / 98.1 = 315602m = 315km)
Or, if you know the distance you have and speed that you want, and want to know the acceleration you need:
a = 0.5 * v^2 / s (e.g. for 7869m/s and a 2km run, a = 0.5 * 7869 * 7869 / 2000 = 15480 m/s^2 or about 1578g!)
If you were blocking sigs, you wouldn't have to read this.
This makes me feel REALLY old, but the EML technology research has been going on for over 20 years. I recall the 1990 High School CX debate topic very well and spent most of the year debating EML launchers (prototyped on Sandia National Labs railgun). We spent the summer in the library in New Mexico visiting Sandia and UNM to research our cases. They were already launching coffee can-sized payloads at that time.
Some of the EML experiments from the late 80s and early 90s were visited at a 95 IEEE pulsed power conference: here. Of course, it's been a HOT topic since pre-85, when the first IEEE pulsed power conference was held.
We've been at the brink of maglev space launches for the alst 20 decades. Maybe it'll happen tomorrow. Probably not. There's basically no money in this sort of solution for defense contractors, so it generally languishes in congressional committees when it comes time to fund...
Oh well. It would be cheaper, cleaner, safer, and a whole helluva lot more fun at parties... but the same issues applied 20 years ago as today: it doesn't get funded b/c it's a public works-type solution to space. There's no money for Lockheed in something like that.
> I'm either 5'11" (say, roughly 6') tall or 180.34cm. Now, which of those gives you a better mental picture of how tall I am?
The one you are more used to of course. That doesn't make it better in any objective sense.
I'm about 190cm (say, a handswidth under 2m), or 0.009 furlongs, or 0.3 rods, or 0.09 chains. Which of those gives a better mental picture?
Incidentally are you really 5'11" to within 1/200th of an inch? If not, the apparent accuracy of the ".34" you quote is completely bogus.
> Does metric even have "dry volume" measurements?
Yes of course. Cubic metres. Same as wet volume, since a volume doesn't actually change depending whether its contents are wet or dry. The dimensions of volume are length^3, so the SI unit for volume is (unit for length)^3.
rant
Few people here seem to understand the crucial issue. A couple do, but their posts haven't been modded up... here's another try.
You don't build a magrail to give your spacecraft orbital velocity. Of course that's silly, for the reasons given above. You use it to give you some small PART of your velocity. This is extremely beneficial.
The crucial insight is that each bit of fuel you use for some stage of the flight needs to be lifted be even more fuel in the previous stage. Think backwards from orbit and it will make sense.
Say you have a 100-kilo satellite you want to accelerate at a constant rate for some period of time. For the last second of your flight, you need to burn, say, 10 kilos of fuel. That means the second before that, you need enough fuel to accelerate 110 kilos, 100 Kg of spacecraft plus the 10 Kg of fuel you'll need in the next second. So you'll need 11 kilos of fuel for the second-to-last second of acceleration. The second before that, you need 12.1 kilos. and before that, about 15 kilos. If you know anything about exponentials, you can then imagine how much fuel you need for the FIRST few seconds of the flight.
(This is not actually quite how spacecraft usually work, but it illustrates the general point nicely)
Over 90% of the fuel you are carrying is used just to lift the rest of the fuel that is burned later on, and a huge fraction of it is burned in just the first few seconds. And of course each kilo of fuel you carry requires a larger spacecraft to hold it, which in turn weighs more, which in turn requires even more fuel. So, if you can use a 10km or 100km rail to get your first few seconds of acceleration, you save a huge amount of fuel. This means a smaller spacecraft, which in turn means even LESS fuel carried.
The power burned by the railgun/mass driver/maglev whatever may actually be more expensive in raw form than rocket fuel (i.e. kerosene, in Russian rockets, which is less expensive per joule than electricity. US rockets use liquid hydrogen, which costs a bundle because you have to use vast amounts of electricity to cool it.), but it doesn't exponentially increase in magnitude as you head down the rail, because it's transmitted through wires rather than carried as mass in the spacecraft. Every second, you only need the same amount of electricity you used the previous second.
The same is true of chemical-powered ram and shock cannons, where fuel filling a cylindrical pipe is combusted behind the accelerating spacecraft travelling through the pipe. (not recommended for human payloads).
Furthermore, if your spacecraft has wings, this may give you yet another benefit. The shuttle has wings, but launches straight up, meaning for the ascent they are just dead weight requiring a huge, exponentially-scaled mass of fuel to lift. But on an almost-horizontal launching system, the wings can provide lift, and thereby actually be useful on the ascent stage. This of course is made easier if the vehicle already has significant velocity before it even lights its engines.
This whole system may not be a panacea; I'm skeptical too. But it probably is worth looking into, because it may help and doesn't require any technologies that don't yet exist. (unlike skyhooks/beanstalks or other strangenesses)
I stole this sig from someone cleverer than me.