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Post by chriso on Jun 6, 2014 3:52:47 GMT
I thought we were talking about just getting around in general. And sorry if I went off on a tangent when I didn't need to. No insult to your intelligence. So what sort of comparison are you thinking of? Not sure how lateral efficiency in sailboats is measured. Some sort of thrust comparison? Honestly, I am not entirely sure how a sail works when moving parallel to the wind. Someone willing to tell me if it operate as a thrust redirector, generates lift, or does something else? hmm... just found out there is such a thing as a "lightfoil", which is the optical equivalent of an airfoil: en.wikipedia.org/wiki/Optical_lift. Wonder where it fits into all of this
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Post by the light works on Jun 6, 2014 4:07:07 GMT
I thought we were talking about just getting around in general. And sorry if I went off on a tangent when I didn't need to. No insult to your intelligence. So what sort of comparison are you thinking of? Not sure how lateral efficiency in sailboats is measured. Some sort of thrust comparison? Honestly, I am not entirely sure how a sail works when moving parallel to the wind. Someone willing to tell me if it operate as a thrust redirector, generates lift, or does something else? hmm... just found out there is such a thing as a "lightfoil", which is the optical equivalent of an airfoil: en.wikipedia.org/wiki/Optical_lift. Wonder where it fits into all of this there's where the problem was. a sailboat on a tack essentially becomes a wedge. the sail generates force through a combination of catching air, deflecting air, and acting as an airfoil - exact proportions depend on the particular sail, and the angle of attack. but tacking involves pairing the angled thrust of the sail with the angled resistance of the keel. so you have a force trying to push the boat sideways, and another force trying to stop the boat from going sideways - and as a result the boat gets squirted on a vector to the combination of thrust and resistance. on a downwind run, the sail is mostly just catching wind. on a beam reach (90 degrees to the wind) it operates mostly by deflecting air, and close hauled (sailing as directly into the wind as it can) it operates mostly as an airfoil. most sailboats develop their highest speed on a beam reach, or slightly closer into the wind - often their hull speed can exceed the windspeed. obviously I don't expect a solar sailer to exceed the speed of light - but could it accelerate to a higher final velocity in exiting the solar system by doing this?
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Post by chriso on Jun 6, 2014 4:44:54 GMT
en.wikipedia.org/wiki/Points_of_sail(Thanks, the phrase "beam reach" got me something) So looks like a sailboat is oriented effectively perpendicular to the wind in an ideal situation. It looks like beam reach would probably be most comparable to a solar sail: a case of momentum exchange and redirection. I would probably measure performance in terms of thrust needed to get out of the system. Escape velocity is the same no matter what the situation. And I think yes, it should end up taking less thrust to get out of the system "sailing" to the side then it would going straight. If I did my research right, it looks like it comes down to gravity. When you are thrusting straight out, you are fighting gravity directly, and that "steals" part of your thrust. When you are simply increasing your speed in the direction of your orbit, you are not fighting gravity but letting the change in orbit "lift" you higher. The net effect being more of the thrust ends up being useful when you simply increase your speed. Which incidentally would explain how a solar sail can move into a system off photon pressure. And how one could sail a sailboat upwind. Does that make sense? Still trying to grasp it myself, I am afraid. Have not covered orbital mechanics to a large degree, so I am working this out using research and applications of laws. I might of missed something. I have to say, I am enjoying this conversation. Never considered that gravity could be acting as a keel, but it looks like that's exactly what happens.
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Post by silverdragon on Jun 6, 2014 6:57:29 GMT
The maths are giving Watson a headache.....
To answer my "Plan", I wouldnt be starting from a standing start. I would intend to aim at a slingshot around the sun, and deploy sails to create an increase in speed as I did that. Therefore, the initial slingshot would deal with all trajectory problems from "launch".... Just how much weight gravity and speed increase would have to be planned before launch?...
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Post by silverdragon on Jun 6, 2014 6:59:56 GMT
Question.... With enough speed, how close to the flame of the sun could a craft pass without actually catching fire?... Its just damn great sail and all that, and a very hot ball of plasma heated gas?....
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Post by Cybermortis on Jun 6, 2014 10:30:03 GMT
No (real) space craft I know of 'tacks' regardless of its propulsion system. They speed up and slow down to alter orbit around the sun and hence their course. So their flight path is ultimately circular.
Even craft that have been sent out of out solar system followed this type of course. One of the reasons for doing this is that it allows you to pass by planetary bodies and use the slingshot effect to change course or further increase speed without having to use a lot of fuel to do so.
Yes, solar sails free you from having to carry a lot of fuel. But they are also not capable of the kind of acceleration you can get with other systems. Their acceleration is gradual, but constant, and therefore you'd want to use slingshot effects where possible and probably start off as close to the sun as you can manage as well as keeping the craft in as tight an orbit for as long as possible so it gets more thrust and works up a higher speed before it starts heading further out where the amount of thrust it gets starts to decline.
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Post by Cybermortis on Jun 6, 2014 10:40:11 GMT
Question.... With enough speed, how close to the flame of the sun could a craft pass without actually catching fire?... Its just damn great sail and all that, and a very hot ball of plasma heated gas?.... Depends on what the sail is made of. It's rather like asking how long a piece of string is. Best guess would be approximately the orbit of Venus with modern materials - we can get closer than this with probes but they can be built using thicker and heavier heat-resistant materials than you could use for a solar sail. You'd also have to factor in the additional stress placed on the sail and craft from the comparatively large amount of thrust that close in.
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Post by the light works on Jun 6, 2014 13:56:32 GMT
en.wikipedia.org/wiki/Points_of_sail(Thanks, the phrase "beam reach" got me something) So looks like a sailboat is oriented effectively perpendicular to the wind in an ideal situation. It looks like beam reach would probably be most comparable to a solar sail: a case of momentum exchange and redirection. I would probably measure performance in terms of thrust needed to get out of the system. Escape velocity is the same no matter what the situation. And I think yes, it should end up taking less thrust to get out of the system "sailing" to the side then it would going straight. If I did my research right, it looks like it comes down to gravity. When you are thrusting straight out, you are fighting gravity directly, and that "steals" part of your thrust. When you are simply increasing your speed in the direction of your orbit, you are not fighting gravity but letting the change in orbit "lift" you higher. The net effect being more of the thrust ends up being useful when you simply increase your speed. Which incidentally would explain how a solar sail can move into a system off photon pressure. And how one could sail a sailboat upwind. Does that make sense? Still trying to grasp it myself, I am afraid. Have not covered orbital mechanics to a large degree, so I am working this out using research and applications of laws. I might of missed something. I have to say, I am enjoying this conversation. Never considered that gravity could be acting as a keel, but it looks like that's exactly what happens. however, the catch is, in the solar system, the sun is always upwind regardless of where you are in your orbit - and you want to go downwind. in sailing, the best time you can make downwind is to crowd on as much sail as you possibly can and run directly downwind. doing this, you can effectively reach almost wind speed. if you are on a reach, you can get greater velocity - but you are also traveling a greater distance to reach your downwind target. In deference to one of the old members on the discovery boards - it is possible to make a craft that goes downwind faster than the wind - but that would require adding some sort of reactive drive, and I'm not sure you can do it as easily with a spacecraft as with a cart.
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Post by the light works on Jun 6, 2014 14:11:10 GMT
No (real) space craft I know of 'tacks' regardless of its propulsion system. They speed up and slow down to alter orbit around the sun and hence their course. So their flight path is ultimately circular. Even craft that have been sent out of out solar system followed this type of course. One of the reasons for doing this is that it allows you to pass by planetary bodies and use the slingshot effect to change course or further increase speed without having to use a lot of fuel to do so. Yes, solar sails free you from having to carry a lot of fuel. But they are also not capable of the kind of acceleration you can get with other systems. Their acceleration is gradual, but constant, and therefore you'd want to use slingshot effects where possible and probably start off as close to the sun as you can manage as well as keeping the craft in as tight an orbit for as long as possible so it gets more thrust and works up a higher speed before it starts heading further out where the amount of thrust it gets starts to decline. It is also the case that no real space craft has been tasked with going to a specific place far outside our solar system. the analogy I can come up with - which is inherently flawed, but I think will serve to illustrate the point - is the rovers we have sent to Mars. I my understanding is that we launch them into orbit long enough for them to get oriented, but from there, they blast on a more or less straight path to mars (less because they are also orbiting the sun, etc) rather than simply orbiting Earth in wider and wider orbits until they intersect with mars. so given that you will eventually redirect to leave Sol orbit at a tangent; how sharp a tangent is the best option for minimizing total transit time? addendum: also, every real spacecraft that I know of uses thrust based propulsion systems. that makes them more like a motorboat than a sailboat, and no motorboat that I know of tacks, either.
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Post by Cybermortis on Jun 6, 2014 14:13:55 GMT
No, you just alter the sails to slow your speed which results in a shallower orbit around the sun. Unlike a sailing ship you don't need (and practically speaking can't) simply change your direction. Remember that we are not talking about a vessel that is moving at 10-15 mph but something that is moving at several thousand miles per second. It doesn't matter what type of propulsion system you are using, you are not stopping and changing direction any time soon.
Solar sails would, of course, allow a ship to actually stop since their reverse thrust isn't dependant on fuel supplies. But the time needed to do this to reverse course would be so large that it would be faster to continue on the original heading and slow down into a 'lower' orbit that matches your intended target. Also worth noting that heading for, say Earth, counter to its orbit means hitting it at 67,000 miles per hour plus whatever speed you already have. So if you want to get to another planet and into orbit (or just use the slingshot effect) you really want to creep up on it from behind.
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Post by the light works on Jun 6, 2014 14:31:08 GMT
No, you just alter the sails to slow your speed which results in a shallower orbit around the sun. Unlike a sailing ship you don't need (and practically speaking can't) simply change your direction. Remember that we are not talking about a vessel that is moving at 10-15 mph but something that is moving at several thousand miles per second. It doesn't matter what type of propulsion system you are using, you are not stopping and changing direction any time soon. Solar sails would, of course, allow a ship to actually stop since their reverse thrust isn't dependant on fuel supplies. But the time needed to do this to reverse course would be so large that it would be faster to continue on the original heading and slow down into a 'lower' orbit that matches your intended target. Also worth noting that heading for, say Earth, counter to its orbit means hitting it at 67,000 miles per hour plus whatever speed you already have. So if you want to get to another planet and into orbit (or just use the slingshot effect) you really want to creep up on it from behind. but we are also talking about an "ocean" that is billions of miles across. certainly the solar sailor "enterprise" would not have the same turning radius as my Hobie cat - but it is also covering a much greater distance. I think you are still trying to address leaving the solar system on solar wind powered craft with physics required for orbital maneuvering in thrust driven ships. if you are trying to achieve interstellar travel and you get to Earth, you are going the wrong way. the strategy of even insystem travel should be different between a solar wind driven vessel and a thrust driven vessel. with a solar wind driven vessel, you have an outside force that is opposite solar gravity. by this, it is theoretically possible to achieve a path that is a straight line outwards from the sun - though, as you point out, that is kind of wasteful in that you already have orbital momentum from launching from an orbiting body. in theory, if you want to maintain a thrust driven vessel in earth's orbit around the sun (assuming classroom physics is in play) it will have to be going faster than a solar sail driven vessel of the same ballistic properties - because the thrust driven vessel is a pure ballistic craft, while the sail driven craft is also catching the solar wind which will want to widen its orbit.
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Post by Cybermortis on Jun 7, 2014 11:54:38 GMT
Well, not really.
You are thinking as if the best way to get out of a solar system is to head in a straight line. This is certainly the shortest route, but it isn't the best way even for conventional propulsive systems.
Heading out of the solar system in a straight line allows gravity to act on the craft, which slows it down. This means that you'd need a launch system that makes the Saturn V rockets look like fire-crackers to ensure that you'd have enough velocity to escape the suns gravity and leave the system for good.
From the practical point of view it is easier and more efficient to use physics to your advantage to get 'free' thrust.
First off you are launching from Earth, which is not a stationary object but one that is hurtling though space at 67,000 miles per hour. You use this to your advantage by speeding up in the direction Earth is moving - automatic free speed boost. Since all but one of the planets in our solar system are orbiting in the same direction this in turn means that you are automatically heading in the right direction to approach planets from 'behind', which allows you to use the slingshot manoeuvre to pick up additional speed to offset the 'drag' caused by gravity. (Coming up from 'behind' also gives you far more time to spot any errors in your calculations and correct them before you slam into several trillion tons of rock at a combined speed of several hundred miles per second)
This principle remains the same regardless of what the propulsive system is, because you are getting something for nothing. Even solar sails have limited 'fuel' because the amount of thrust provided by the sails is directly related to the distance from the sun. In this context even solar sailed vessels would not want to just head straight away from the sun, because it means that they will quickly run out of 'fuel'. It makes much more sense to keep them orbiting as close to the sun as is practical for as long as possible in order to build up as much speed as is needed. Then to use slingshot manoeuvres to further increase speed for no cost.
As counter-intuitive as it seems at first glance this is in fact the faster way, since the velocity of the craft once it leaves the solar system would be significantly higher than a conventional craft.
Yes, a solar powered ship heading straight out of the solar system will still have a higher velocity than a comparable rocket powered craft following the exact same course, since the former will be accelerating (or at least getting some thrust) for most of its voyage while the latter will not. But the difference in interstellar terms is effectively zero - it took the Voyager Probes some 35 years to get out of the solar system. A 'sailing' craft following the exact same course would probably have cut a year or so off that at best. However a sailing craft that was allowed to spend (say) a year closer to the sun building up speed before heading out might well have cut several years off the flight time - even when the time it spent doing nothing is taken into account.
So in space the 'fastest' route is not automatically the shortest route, no matter what type of propulsion you happen to be using.
If you had a reactionless drive system* this situation would of course be different. But we don't, and have no idea if such a thing is even possible.
(*For those who don't know a reaction-less drive is one that violates the laws of momentum by moving an object without imparting inertia on that object. If the drive is turned off the craft will stop, rather than continue on the same course at the same speed. Star Treks War Drive is a reaction-less drive, since if it is turned off the ship stops moving. The impulse engines in the Trek universe also seem to be reaction-less**, based on the fact that nine times out of ten any ship that looses impulse power stops moving - most noticeably the USS Excelsior in Star Trek III which looses power outside a Star Base (due to Scotty's sabotage) and grinds to a stop. There is also a similar scene in ST IV in which Star Fleet prepares to launch ships from the same Star Base to intercept the probe heading for Earth only for all the ships and the station to loose power. The ships that were heading for the doors stop, rather than continuing on their course and running straight into the still closed doors.)
(**This, incidentally, is not supported by any of the technical guides or on screen dialogue for any of the shows. All of which indicate that the impulse engines and thrusters are at heart just very powerful - and presumably efficient - versions of current day rocket/jet designs. Which does raise the interesting question as to how the thrusters could work in reverse as Picard once ordered in the episode 'Relics'...)
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Post by the light works on Jun 7, 2014 15:00:54 GMT
to address the Star Trek questions, first: I am currently watching the first year of TNG,and at that point, at least, Picard's orders are consistent with naval helm orders - in which he reverses thrust fora rapid stop, or orders engines stopped and the ship coasts to a stop. as or revering thrust, I have not seen that episode recently enough to remember, but I suspect "thrusters in reverse" is just the same as a deceleration burn in a spaceship. it uses different engines than forward thrust does, or uses engines that may be reoriented. (they also use inertial dampeners, but those only act on what is inside of the ship and is part of their artificial gravity system. they somehow work to simulate 1G at "ship's down" regardless of what acceleration the ship is under - excpt when the ship is getting hit with something, which either overcomes the dampeners ability to compensate rapidly enough, or actually throws the dampeners off balance, so they compensate wrong) this is followed in a side story based on TOS in which Kirk managed to retake control of a ship by sabotaging the inertial dampeners during a turn, which had the effect of pressing him against a bulkhead he had braced against; and throwing the hijackers against the wall, as they had no warning. that done, back to the intricacies of navigating gravity wells: I think you are still trying to use solar sails like conventional rockets. this may be valid if solar sails will operate efficiently as a light foil. don't forget that except for reflection, and light striking your solar sail will be coming from one direction: down. therefore, you have two options in setting sail: ride the light up, or sideslip. the closest terrestrial navigation question we have to navigating in a gravity well with solar sails would be to sail up a river with the wind blowing upstream. I'm trying hard to remember, but I think I HAVE done that - which is to say I know I have sailed upstream in a river, and I think it was downwind, because I had to pass a railroad bridge, which was closed for a train, and I didn't want to find myself too tall for the bridge and trapped against it by the following wind once I figured it out. (it is difficult to estimate mast clearances from the bottom of the mast) I found a nice article talking about planning interplanetary travel. - again, this is designed around thrust based propulsion rather than solar sail propulsion, but it illustrates my point about when we send ships from earth to mars, there is a point they stop basing their trajectory on earth orbit, and transition to basing their trajectory on solar orbit. in the same way, when we aim for interstellar travel, there will be a point at which we stop doing solar orbit and start doing galactic orbit. www.philsrockets.org.uk/interplanetary.pdfI also found a couple papers on solar sail trajectories: www.intrance.org/paper/200408_providence_dachwald_sail.pdfand www.engr.uconn.edu/~cassenti/SpacePropulsion/OptimumSolarSail.pdfthe dachwald model looks an awful lot to me like a series of maneuvers calculated to allow the sail craft to pass as close to the sun as possible without melting it down, to get as much initial thrust as possible, and then allowing it to depart on a hyperbolic path. in those particular models, the goal is to end up in orbit around a planet that is still in orbit around the sun, but it still involves transitioning from an orbital path around the sun to a path that is significantly away from the sun. so again - to summarize - by my understanding of the papers: for interstellar travel by rocket, you would begin in earth orbit, accelerate out of earth orbit, putting you in solar orbit, and then accelerate out of solar orbit, possibly using planets for slingshot maneuvers to assist in accelerating out of solar orbit, putting you in galactic orbit. for interstellar travel by solar sail, you would begin in earth orbit, accelerate out of earth orbit, putting you in solar orbit, then DEcelerate to maximize your acceleration from the solar sail by maximizing your proximity to your power source - and at that point, you want to make your transition to thinking in terms of galactic orbit, and using the sun for your slingshot maneuver. the critical difference between the two schools of thought is that with a rocket, your thrust is equal regardless of orientation. if your motor can deliver 10G of thrust, you can deliver thrust in any direction relative to the sun's gravity. with a solar sail, your primary thrust is directly away from the sun's gravity, and while you can modify the thrust by changing your angle of sail, it also reduces your total available thrust. I.E. if you want to thrust directly towards the sun, you will essentially have zero thrust. while the falloff ratio will undoubtedly not be exactly proportional, if you want to thrust at 90 degrees to the sun, assuming your maximum acceleration is 10 Gs, you would only be able to achieve 5 Gs of acceleration. It might be a better analogy to ask how your trajectory would be different between launching a ship to mars by rocketing it from the moon, and launching it to mars using a tractor/pressor beam to push it from Earth.
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Post by Cybermortis on Jun 7, 2014 16:15:48 GMT
The inertial dampeners work to counteract the effects of inertia on the ship and crew as it moves or accelerates. (As one of the tech guys on TNG put it, this is the system that stops the crew being turned into 'chunky salsa' on the back wall every time they engaged the warp drive.) Gravity on the ship is provided by an entirely different system, generators located under the decks that are part of the life support systems.
The orders given in regards ship movement are consistent with an ocean ship, but not a spacecraft. An ocean going vessel can go in reverse because it can change the direction in which the propellers rotate. You can't do the same thing with rocket/jet engines, at best you can redirect the direction of thrust to some degree - but never directly backwards as you can do with a propeller.
To be fair the thrusters on the Enterprise are located at the edges of the saucer section and in the case of the Enterprise D on the edges of the stardrive section. Both places would allow for the thrusters to be angled back far enough to allow counter-thrust. (The order in question was, as I recall, Starboard thrusters full, port thrusters full reverse)
The impulse engines are an entirely different matter, especially on the Enterprise D where the main impulse engines are (helpfully) placed at the base of the goose neck and have a large cowl covering the top part. It looks cool and would no doubt work well enough for providing forward thrust. However this placement also badly limits how far you could redirect the thrust without damaging the ship in the process. 'Full reverse' from the engines should incinerate the lower part of the goose-neck, too far to port or starboard would do much the same to the warp pylons if not the nacelles themselves and angling thrust downwards is going to melt a hole right through the engineering section and the warp core. The secondary impulse engines, which are located on the rear edge of the saucer section, could be redirected mostly forward to give reverse thrust because of their physical location. But they too are recessed into thick cowling's, and also run into the problem that you couldn't angle the thrust too far downwards without venting them straight at the warp nacelles - which is probably not very healthy for the warp engines.
So the design looks cool, but in reality 'reverse thrust' is either going to result in a lot of repair work or require using the thrusters to turn the ship around so the impulse engines can start slowing the ship without risking serious damage in the process. (The only explanation as to why they don't need to do this, assuming that the engines are reaction drives, is that the forward thrusters are capable of stopping the ship on their own...which then begs the question as to why you'd need a huge impulse engine in the first place....)
The only major Trek ship I can think of that possesses impulse engines that would give a large amount of thrust-direction without damaging the ship in the process is the Intrepid class (USS Voyager). The engines on that class are located at the base of the warp pylons and could be redirected in quite a large arc without being aimed at the ship.
Nope. Effective increase in speed heading directly away from the sun would be the total thrust of the engine minus the force of gravity acting on the craft. Hence trying to accelerate directly away from the sun, regardless of propulsion, is less efficient than accelerating around the sun.
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Post by the light works on Jun 7, 2014 16:59:05 GMT
you mean that if I take a rocket with a 2G motor, and try to thrust directly away from earth, it will deduct 1 G from the power of the rocket, which means that it will be unable to overcome earth's gravity at all? no? then maybe you should consider that it is time to accept that I do understand some of that which I am speaking. a rocket capable of delivering 10 Gs of acceleration will deliver 10 Gs of acceleration, whether there is gravity applying negative acceleration or not. this is airplane on a conveyor belt stuff here - rocket engine acceleration being defined as that force the engine imparts on the rocket, not counting any acceleration imposed by any outside force.
it is time for you to actually LOOK at the documents I cited instead of just keeping on keeping on with your "the only way to increase your distance from the sun is to keep orbiting faster so you are flung away by centripedal force" posture.
if you are going to get to Alpha Centauri at any point in the lifetime of the universe, then at some point in your journey, you have to stop going in circles around Sol. both my fire engines at my home station are rather gutless when they are cold, and my driveway is steep enough that if I stop in the driveway, I may or may not have enough power to get out of the driveway. this can be helped by getting a running start pulling out of the station, but if I don't actually go up the driveway, then the running start gets me nowhere.
with a rocket, your rocket is delivering 10 Gs of acceleration - whether you are at the sun, or passing pluto. taking the long route gives you more runup time, and doesn't waste fuel trying to fight against gravity. a solar sail delivers its greatest thrust as close to the sun as it can survive, and by my understanding drops off exponentially. according to my understanding of the document on interstellar solar sail travel, if you don't build enough outward momentum, you will reach a point where you no longer have enough "wind" to escape solar orbit. - and at that time you will not have enough lateral acceleration from angling your sail to increase your orbital velocity.
but as long as you insist on treating a solar sail like a rocket, I am just wasting my time.
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Post by Cybermortis on Jun 7, 2014 19:23:43 GMT
I never said that.
I said that it was more efficient to orbit the solar system than attempting to just blast your way straight out of it, since this naturally puts you in an ideal situation to further increase velocity through the use of the sling-shot manoeuvre.
What I wasn't quite as clear about was why this was the case for solar-sailed craft.
Basically it comes down to the amount of thrust you can get out of the 'engine' over X amount of time. Rockets can provide high amounts of thrust, but only over short periods as they will run out of fuel - and you run into problems in that the more fuel you carry the greater the mass and the less thrust you get.
Solar sails in comparison provide only small amounts of thrust, but that thrust isn't dependant on fuel only on distance from the sun. So if the intent is for your solar craft to leave the solar system it makes sense to try and keep it as close to the sun as possible for as long as possible so it can build up speed. In theory if you could keep a solar sailed vessel within the solar system for long enough it could pick up far more speed over time than any other propulsion system we have or could build.
Much of course would depend on the size of the sail and the materials it is made of. Larger sails could provide more thrust, but also additional mass and run into problems in regards structural strength. Heat tolerance is also an issue, since this denotes how close to the sun you could get before you start running into problems. With a large sail that has a large amount of heat resistance and strength you could pick up more than enough speed fairly quickly, especially if you can slingshot around the sun at the same time. But as far as I'm aware current materials wouldn't allow this, so the more practical option is the 'multiple orbit' course further from the sun.
Again, this is relation to getting out of the solar system at a reasonable speed - that is at least equal to what could be managed with conventional propulsion systems. If you want to get to another star system even a comparatively small increase in speed could mean the difference from you being alive to see it happen, or your great grandchildren watching it from a retirement home.
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Post by the light works on Jun 7, 2014 20:18:15 GMT
I never said that. I said that it was more efficient to orbit the solar system than attempting to just blast your way straight out of it, since this naturally puts you in an ideal situation to further increase velocity through the use of the sling-shot manoeuvre. What I wasn't quite as clear about was why this was the case for solar-sailed craft. Basically it comes down to the amount of thrust you can get out of the 'engine' over X amount of time. Rockets can provide high amounts of thrust, but only over short periods as they will run out of fuel - and you run into problems in that the more fuel you carry the greater the mass and the less thrust you get. Solar sails in comparison provide only small amounts of thrust, but that thrust isn't dependant on fuel only on distance from the sun. So if the intent is for your solar craft to leave the solar system it makes sense to try and keep it as close to the sun as possible for as long as possible so it can build up speed. In theory if you could keep a solar sailed vessel within the solar system for long enough it could pick up far more speed over time than any other propulsion system we have or could build. Much of course would depend on the size of the sail and the materials it is made of. Larger sails could provide more thrust, but also additional mass and run into problems in regards structural strength. Heat tolerance is also an issue, since this denotes how close to the sun you could get before you start running into problems. With a large sail that has a large amount of heat resistance and strength you could pick up more than enough speed fairly quickly, especially if you can slingshot around the sun at the same time. But as far as I'm aware current materials wouldn't allow this, so the more practical option is the 'multiple orbit' course further from the sun. Again, this is relation to getting out of the solar system at a reasonable speed - that is at least equal to what could be managed with conventional propulsion systems. If you want to get to another star system even a comparatively small increase in speed could mean the difference from you being alive to see it happen, or your great grandchildren watching it from a retirement home. are you going to burn rockets to hold you closer to the sun longer so you can build up orbital speed? now we are back to the "a keel doesn't really work in space" comment. the only way you can "hold" your orbit close in a craft that is powered by force pushing directly away from the sun is to either apply opposing force, or by spilling the force you are powering your ship with. take yourself a smallish ball and a long spool of string. for test one, tie the ball to the end of the string, and then swing the ball around your head, slowly paying out string, until you can no longer maintain enough orbital velocity to keep the ball off the ground. mark the distance you were able to get the ball away from you. this is your model of getting a solar sail craft out of the solar system by increasing the speed of the orbit. for test two, untie the ball from the string, and throw it as hard as you can, on a level trajectory. again, mark the distance achieved. this is your interpretation of my model of direct departure from the solar system. for test 3, tie about a foot of string to the ball, swing it around your head once or twice, and then release the string. this is a closer approximation of what is actually the best model for departing the solar system on a solar sail vessel. the very critical factor involved is that with a rocket, the acceleration is limited by the amount of fuel you are willing to pour into itl whereas with a solar sail, the acceleration is limited by how far you have gotten from the sun; but the acceleration drops off exponentially while the gravitation drops off proportionately. this means that with a rocket, all getting closer to the sun means is you have to fight more gravity; and the more fuel you can get into the further orbit with you, the less gravity you have to fight. on the other hand, with a solar sail, the closer you get to the sun, the more rapidly it can accelerate, and the further out your orbit is, the less rapidly you can accelerate, until, as I said before, your gravity exceeds your ability to accelerate your spacecraft, and you have no other option but to return to a lower orbit and try again. in short: solar sails are not rocket science. trying to apply rocket science to solar sails will give you wrong answers.
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Post by the light works on Jun 7, 2014 20:24:23 GMT
furthermore: a "slingshot" maneuver around the sun gives you no gravitational acceleration in relation to the solar system. it just deflects your course in relation to the sun. however, done properly, it CAN improve your relative speed compared to the galactic core. - but as the first document I posted points out, a "slingshot" maneuver is less than a full orbit - because anything else involved redirecting ALL of your kinetic energy.
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Post by Cybermortis on Jun 7, 2014 20:49:30 GMT
The thrust you get from a solar sail is considerably smaller than what you get from a rocket.
Therefore, for a solar-sail to get up to the same sort of velocity as a rocket it has to apply that thrust for longer - ie ideally be as close to the sun as you can manage for as long as possible.
Sideways movement, 'leeway' of the solar variety, isn't a major issue and would be factored into your calculations - apart from anything else the orbit would increase as the speed increased anyway. The 'orbit' would therefore be a spiral one way or another, the trick is to get that spiral as tight as you can manage at least in the initial stages so the speed builds up as high as can be managed. Again, this is assuming that you are trying to head out of the solar system. If you just want to reach another planet within the solar system then you don't need - and probably wouldn't want - speeds that high.
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Post by the light works on Jun 7, 2014 20:50:39 GMT
from Wikipedia, J.D. Bernal, one of the more recent physicists involved in solar sail theory postulates that you want to fly out to, say, Neptune, then furl the sail (he said close hauled) and dive it past the sun for your gravitational assist. en.wikipedia.org/wiki/Solar_sailall in all, it seems a lot to me like a solar sail is going to be best for interplanetary work, and its best application for interplanetary travel is to boost your craft to high interplanetary orbit, before transferring to reaction drives. (probably rockets for the initial boost and ion drive once you pass the rockets' maximum efficient velocity. but, again, it appears that to the best of the physicists' knowledge, solar sailing only very loosely correlates to sailboating. they believe that a parachute style sail will collapse, and the craft will effectively not have a keel or rudder.
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