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Propulsion

Unless I'm missing something, you can't tack into a solar wind, so solar sails only work outbound.

Most of the ideas presented so far seem more useful to me for long term, out of system flights, but I don't know if any are fast enough that we would actually use them for that. It will be a very long time, I think, before we think seriously of sending manned missions out fo the solar system in slower than light craft, if ever.

For tooling around the solar system, acceleration is more important than efficiency, as fuel should be relatively cheap, and the premium is on time spent accelerating. If we're going to regularly go to Mars or the Moon, or maybe Jupiter and Saturn, we need ships that get there fast.

And I don't think we want to use a series of nuclear blasts to lift objects out of Earth orbit. Same for massively powerful lasers. Too dangerous close in. I think chemical rockets are going to be the propulsion of manned flight, at least, for a long time to come.

At least, until we develop something with a lot more thrust.
 
Michael Redman said:
Unless I'm missing something, you can't tack into a solar wind, so solar sails only work outbound.
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You don't really need to, though, because you have gravity. It's slow, but so are solar sails.
 
Michael Redman said:
Unless I'm missing something, you can't tack into a solar wind, so solar sails only work outbound.

Most of the ideas presented so far seem more useful to me for long term, out of system flights, but I don't know if any are fast enough that we would actually use them for that. It will be a very long time, I think, before we think seriously of sending manned missions out fo the solar system in slower than light craft, if ever.

For tooling around the solar system, acceleration is more important than efficiency, as fuel should be relatively cheap, and the premium is on time spent accelerating. If we're going to regularly go to Mars or the Moon, or maybe Jupiter and Saturn, we need ships that get there fast.

And I don't think we want to use a series of nuclear blasts to lift objects out of Earth orbit. Same for massively powerful lasers. Too dangerous close in. I think chemical rockets are going to be the propulsion of manned flight, at least, for a long time to come.

At least, until we develop something with a lot more thrust.
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If we're going to be traveling to Jupiter's moons regularly, we're not going to launch from Earth. It would be better (as far as I know, which isn't much) to launch from the moon, as you could then use nukes without worrying about the pollution and radiation. Not to mention the much lower gravity.
 
I saw a documentary describing how a solar sail would be used, Surprisingly you head for the Sun first, using conventional rockets, when you get close to the sun (but not that close) the Sail is unfurled. The thrust is a product of the sail area and the mass of the payload, but some truly impressive speeds can be obtained that exceed any chemical rocket.
 
Agammamon said:
That's why you use a laser instead, and as new laser tech becomes availabel you can just keep adding ne emitters without needing to remove/replace old ones (though of course you can still do that). They don't even have to be particularly powerful or efficient.

While thrust from the Sun is steadily decreasing the advantage is still that most of your ship is payload and since (presumably) you're not in a hurry the low acceleration isn't a problem.
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You've probably read "The Mote in God's Eye". The aliens launch a solar sail ship to another star with a laser. The laser is so bright that it is visible to the humans living in the destination solar system. When it gets halfway there it turns around and uses the destination star as a brake. Took years of course. But maybe not a bad way to get a probe to Alpha Centauri.
 
Soapy Sam said:
Anyone remember E.E. "Doc" Smith? "Bolts of quasi solid lightning, blasting through the luminiferous ether".

They don't do that any more.
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I remember that. I do a quick reread of some of those books every few years just to see all that wonderful language. Fabulous battles of beams and rays ripping and tearing through things.

The propulsion system in those books is an inertialess drive. The ships are made to have zero mass, so a small force propels them along more than c. Problem solved. :D
 
bewareofdogmas said:
If we could tinker with the Higgs Field we could have massless ships?
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Maybe I'm wrong, but I thought that hadn't even been proven that it existed yet. Or am I thinking of the Higgs Boson?
 
Maybe I'm wrong, but I thought that hadn't even been proven that it existed yet. Or am I thinking of the Higgs Boson?-sorgoth


I think they are the same thing.:confused:
 
I can't see how a Solar Sail could be at all fast or effecient. First off, you steadily DECREASE thrust (if you can call it that) the further from the Sun you get, thus losing speed.
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You just lose acceleration - your speed still increases.

Once past the Termination Shock of our solar system, you get the OPPOSITE effect, in that all the ambient 'wind' from the rest of the galaxy's stars are pushing against you.
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But since the galaxy's stars are completely surrounding you, the "wind" from all directions would cancel out.

Could we make a sail that only absorbs photons travelling in one direction, so we always get accelerated in the direction we're pointing?

David
 
How can you use nuclear explosions as propulsion without them having an, er, detrimental effect on the fabric of your ship?
 
richardm said:
How can you use nuclear explosions as propulsion without them having an, er, detrimental effect on the fabric of your ship?
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In an atmosphere most of the damage from a nuke comes from the energy of the explosion pushing things like the air and earth around causing shockwaves and such. In space its just a big release of energy. The particles push on a special plate and move the ship along.
 
Holy geez, people. Haven't any of you read any Robert L Forward? http://www.aeiveos.com/~bradbury/Authors/Engineering/Forward-RL/

You use a light sail, but mount a huge-ass laser somewhere in orbit to power it. You want to get back "down" once you're out in the system? You just mount another big-ass laser out there to push you back! The fuel isn't carried on board, nor is the laser. All you need is the sail.

Edited to add:

BTW, just read a piece of an interview. It was Forward that gave Pournelle and Niven the idea to use lightsails in "Mote".

Also, where are you planning on launching this Orion spacecraft? (Niven & Pournelle used one in Footfall, didn't they?)
 
The WIkipedia has a good section on propulsion: http://en.wikipedia.org/wiki/Spacecraft_propulsion

I've just skimmed over the preceeding; if you have any questions on advanced propulsion, that's a good place to start.

Solar sails are good for going in and out. You wouldn't normally orient your sails normal to the sun, instead you'd angle them so that the net thrust is either more or less against your current velocity vector or with it; this will cause you to spiral in or out. Solar sails are never going to be particularly fast, and the acceleration, even given optimistic assumptions, is probably never going to rival even ion propulsion. They do have the advantage of not using much fuel. I don't know, maybe they'll be good for shipping home stuff from the Mercury mines.

There's a variant of solar sails that uses the solar wind instead of light pressure. It looks to be easier to implement than solar sails.

Ion propulsion, as has been mentioned, has high Isp but very low thrust. It's useful for situations like station keeping for terrestrial satellites, but probably not that useful for interplanetary travel unless you're planning on going out past Mars (or in past Venus) (chemical rockets could get you there much faster for not that much more fuel). Unfortunately, solar arrays aren't particularly useful past Jupiter, so you'd need nuclear power for most of the fun missions. And not those wimpy radio-isotope thermal generators--you need full-blown fission reactors, at least in the megawatt range, preferably higher.

Orion has been mentioned, and its more elegant offspring, Medusa. High thrust, high Isp--just the thing for manned exploration. The nuclear salt water rocket is another high thrust high Isp drive (it's pretty much the atomic drive of the 50's SF stories--tons of thrust, capable of taking you from Venus to Jupiter in a matter of weeks, radioactive as hell).

Astronomy hasn't been very kind to the Bussard ramjet--the interstellar hydrogen density in our vicinity is quite low--ramjet sails are probably impractically large.

I worked out (probably sometime in high school) the various exhaust velocities resulting from 100% efficient fusion drives. 4H->He4 (the only reaction that makes sense for a Bussard ram) gives an exhaust velocity just shy of 12%c (this isn't the top speed of a Bussard, by the way--if you could figure out how to fuse the incoming hydrogen without accelerating it to the same speed as your starship you could go faster. I've been calling this a Bussard Scramjet, just for giggles). There are various other proposals for a Bussard, including carrying anti-matter onboard your ship and using a ramscoop to collect the hydrogen to react it with.
 
I like the idea of massive rail guns. They're useful for small packages (maybe if we start mining somewhere). If the planet you're launching from has no atmosphere (Moon, Mercury, Callisto) you can use a very long electromagnetic coil and a launching bucket. The magnets pull the bucket along and turn off as it goes by, accelerating the bucket to escape velocity. The bucket is then slowed down by more magnets and the package shoots out of the coil. This can be calibrated to sufficient precision that you can put another spacecraft in stationary orbit to catch the packages as they come out. With this sort of system, you can launch several kilograms of material per second in a continuous manner, far cheaper than is possible with any other sort of propulsion.

The one drawback is the necessity of a lack of atmosphere. Maybe if we built a giant tube 30 km straight up on Earth and pumped all the air out of it? Is that even feasible?
 
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