h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Be carefull not to bring it in contact with normal money, or all will go up in smoke

I would have thought the main advantage of the Ion engine was quite obvoius - as I understand it it doesnt need fuel. The acceleration is crap but top speed is almost limitless.

actually it does need fuel, in the form of gas to ionise and emit.

however the amount of fuel is massively smaller than any other existing reaction mass based technology.

also, you say the acelleration is crap, but this misses the point of how it (and a number of other systems like it) work. instead of putting a lot of energy very inefficiently into an initial thrust, and then coasting most of the rest of the way (like they did in the appolo moon program) you instead put in a tiny amount of thrust, but leave it running for the length of time your fuel supply allows an your journey demands.

because it has a continuous acceleration model, it does indeed provide for a massive top speed.

however because it has to operate continuously, the thrust must by necessity be small. this makes it impractical for launching from a planet for example, but ideal for launching from orbit. (along with similar techniques like solar sailing).

Well, obviously it has to expel something to make it go foward.
The problem with solar sails is you cant go toward stars - only away.

Now if only we could invent a space rudder.

Surely if you get close enough to a star, gravity is going to start pulling the solar sail in anyway, and I guess you could use inertia for the rest of the way.

Depends at what velocity you approach the star.
Without a rudder, keel or daggerboard there is very little control over your direction - you judt drift with the solar wind.

Gravity will be no problem, just as it did not hinder you leaving our own sun.
When you approach another star, the ship may turn around safely and start breaking by the new solar wind, but more probably the sail will be blown backwards while the payload merrily goes on, ending up being wrapped in the sail and destroying the whole.

I seem to think NASA worked out that a physical sail would be far to heavy, so they devised a theory involving an electromagnetic field instaed.

You could turn around in interstellar space. Then, by the time you approached the next star, you would be ready for breaking.

You could turn around in interstellar space. Then, by the time you approached the next star, you would be ready for breaking.

actually, that is not as big a problem as you would think.

solar sailing is like ordinary sailing, in that you can't go directly against the wind, but you can use it to tack.

this means that for all intents and purposes, you can go in any direction you like, by changing the vectors of your orbit.

Ah, but in ordinary sailing you have a rudder & daggerboard, which push against the water. This is what stops you from going backwards. Without these wonderful devices you would only be able to go downwind. What could they push against in space?

with solar sailing, the solar wind "pushes" against the sail. this has a small, but measurable pressure, which you can use to modify your vector.

effectively, you are using this light pressure as thrust.

by carefull choices of when and how to use this pressure, youcan change your orbit. as you probably know,you continue on your current orbit until a force is applied, and this force is the light pressure acting against the solar sail.

byvetoring your sail (like you do with ordinary sailing), you can affect the direction this thrust occurs in.

" by vectoring your sail (like you do with ordinary sailing), you can affect the direction this thrust occurs in."

No you can't, unless you can use the gravitational pull of a nearby body.

F

In normal sailing you rely on the force of water on the dagger board (or even on the hull) to allow you to go upwind. I can't see that working in a vacuum.

wrong fathom. over at http://www.ugcs.caltech.edu/~diedrich/solarsails/intro/sailing.txt it says

"putting the sail at normal incidence to the sun is not the best
configuration. It is better to angle the sail in such a way as to maximize the component of the thrust which is parallel to the direction of travel. This turns out to be when the angle between the sun and the perpendicular to the sail is about 35.3 degrees. In this configuration the spacecraft is being pushed along the direction of travel, and so it climbs the gravity well. In the counter-intuitive realm of orbital mechanics, the spacecraft slows down the whole time it is climbing the well.

Well, if the only important thing is the component of the thrust along the velocity vector, it can clearly be aligned the other way to oppose the velocity vector. This pushes against the direction of travel, dropping the sail down the gravity well, causing it to speed up the whole time. A solar sail, contrary to popular belief, can travel sunward just as easily as it can travel anti-sunward."

I think caltech is a good enough source to verify my statement, especially as it points out that this can be done only using the sun.

My reading of that is that gravity takes on the role of daggerboard/etc. Not exactly the same, but it could be used I guess.

Hello Xyroth, I'll check out the link when I get the chance but the important aspect here is the gravity well. Caltech seem to have assumed the solar sail operates like a para-wing and can be steered in the solar wind so can be used to cause the orbit to decay. I still think there's something about this that doesn't look right so perhaps I should ask the guys at Caltech if they're sure ...

Nonetheless a solar sail can't tack like a boat which is where the argument arose.

"Second star on the left and keep on 'till morning."

Thanks for the link.

F

Actually NO..I hate her guts ever since she overfed my poor little goldfish"Brian"while I was on holiday in Scarborough.She's not even on my Christmas card list now.....

couldn't resist