# An idea for a stardrive (sort of)



## Vertigo (Jan 1, 2012)

OK I've been kicking around an idea for a sort of stardrive. It doesn't need much new technology but it does need some. However I have one or two problems with it and wondered if any good people could help or shoot it down in flames if I've missed anything obvious.

We are not talking FTL, so journeys are still going to take a long time. One of the biggest problems is how to carry enough fuel to accelerate a spaceship up to close to the speed of light and decelerate again. Let's say we want a cruising speed of .8 lightspeed which would give around 65 years to travel 50 light years. Now, one way around the fuel carrying problem would be a ram scoop style of drive (my personal favourite actually). However there is considerable doubt whether the accelerating force would actually be greater than the decelerating drag caused by the ram scoop itself. So I started thinking of alternatives...

How about *not* carrying the fuel with you? In fact the starship itself would only carry limited manoeuvring fuel. Instead it would be accelerated up to cruising speed and decelerated at the other end by a fixed mass driver system either within the star system or just outside it. "Ok" I hear you say "sure, but those kind of accelerations would leave humans a reddish pulp spread all over the bulkheads." But, and here comes the new technology, what if all the crew are in deep freeze? In deep freeze their bodies would be solid and capable of withstanding much higher accelerations. Question; I wonder just how high an acceleration a frozen body could take (if appropriately supported) before internal bits started snapping? Maybe, before freezing, all internal cavities are filled with some substance that will freeze to great strength and be easily flushed out aftwards. Freezing also has the side benefit that the crew could remain in deep freeze throughout the flight, allowing the ship to be very small and only have to provide power to maintain the crew cryo systems. Even that shouldn't require much, with no conscious crew the ship could be allowed to drop to the ambient temperature of the space it travels through, also reducing the energy needed to maintain the cryo systems.

So far so good (maybe) but now the problems begin. Let's suppose we can generate (and our ship and cryo systems can manage) an acceleration of 1000g. Thats a heck of a lot, I know, but just go with it for now! At 1000g the mass driver acceleration would have to be maintained for 6000 million km in order to reach a speed of .8 lightspeed. That's roughly the distance from the sun to Pluto! Anything less than .8 lightspeed starts making the journey simply too long. So, for example, an acceleration of 500g and a cruising speed of .6 lightspeed takes the journey time up to over 80 years and the acceleration stage would still be 4500 million km. Not much better really.

So here's the real question; do you think it would be possible to construct a mass driver that is circular, round which you are accelerated until you reach your required speed before being relased off on your journey? Would the centripetal forces be too much to control magnetically? Maybe the mass driver could encircle a (small?) planet allowing the planet's gravitational field to assist in that control?

Oh incidentally another mass driver would be needed to decelerate the ship, storing the energy generated by that deceleration ready for the next launch. OK the very first journey would take much longer, with maybe a light or magnetic sail for deceleration, before the destination mass driver is built.

So any thoughts?


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## Ursa major (Jan 1, 2012)

Setting aside the freezing - speaking from total ignorance on the matter, I wonder if that (and the subsequent defrosting) might not do as much damage as the acceleration(s)....

Simply for safety's sake, the ship ought to have some means of decelerating - in case it misses the "catcher" at the far end - in which case, it ought to have a ram scoop, one that both collects matter for deceleration _and_ makes use of the drag.


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## RJM Corbet (Jan 1, 2012)

An ion drive, for instance, builds up acceleration incrementally until the vessel is traveling pretty fast, although nowhere near light speed, of course. I don't know if fuel weight is going to really be much of an issue, when you're talking about anti-matter engines, etc. It's going to be minimal, for 65 years. A thimbleful.

As you say, the problem is acceleration and deceleration G forces ...


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## Vertigo (Jan 1, 2012)

Well yes, as I said the cryo bit is the one peice of technology that we do not yet have and I am assuming we will develop the means of freezing and thawing people successfully.

Re the deceleration. I had considered the idea of an emergency sail brake. However it can't do both things. You can have a scoop and use it's drag (this is how most ramscoops decelerate) but there's no point keeping the gathered fuel (except possibly for later manoeuvring). You can't really have an engine blasting out particles and the scoop collecting them both on the same end of the ship. It is interesting to note that a ramscoop ship would not actually turnover when switching to deceleration; it would simply stop the accelerating engines and rely on the drag. However this would take an awfully long time because you wouldn't know you'd missed until you had reached the target system which you would then overshoot by maybe 25 lightyears or more (in my example) before you could slow down and turn around; then you've got to get back again. Also you can't just decelerate to a standtill and then accelerate back because ramscoops can only operate at high speeds so your turn around would actually have to be a huge circle.

However the ship would have manoeuvring thrusters and beacons could be set up for guiding it in. I suspect the bottom line is that you probably couldn't afford to miss! Apart from anything else you want to keep the mass of the ship as low as possible otherwise the mass driver power requirements would become really silly and having a ramscoop engine means you now have to include a significant mass of engine to what would otherwise be little more than a capsule.



RJM Corbet said:


> An ion drive, for instance, builds up acceleration incrementally until the vessel is traveling pretty fast, although nowhere near light speed, of course. I don't know if fuel weight is going to really be much of an issue, when you're talking about anti-matter engines, etc. It's going to be minimal, for 65 years. A thimbleful.
> 
> As you say, the problem is acceleration and deceleration G forces ...


 
As I said at the start I'm trying to avoid the problem of additional new tech and anti-matter drives are really only pure speculation at the moment. But unless I'm much mistaken you are still talking about huge amounts of fuel, even with anti-matter and to make the journey time reasonable you have to accelerate to close to the speed of light. The whole point of this is to come up with a way to avoid carrying your fuel.


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## RJM Corbet (Jan 1, 2012)

Well as you know, Vertigo, the latest Mars rover has its own plutonium power plant that, theoretically, will keep it moving for quite some time. The weight isn't a major issue, and it avoids the problem of the solar panels dusting up.

So do you need to worry about all the scoops and things? All you need is an effective internal power source ...


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## Ursa major (Jan 1, 2012)

Once you know that the ship has to be able to decelerate by itself (if only because trying to catch it when it's travelling at .8 light speed seems a tad optimistic, to say the least), you would probably need a process whereby the ship had a small amount of fuel, greatly augmented this during the flight (with some attendant drag), and then use the total for the onboard engine to decelerate enough to enter, and "manoeuvre" within, the target system.


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## Nik (Jan 1, 2012)

IIRC, floating crew in liquid would cushion them against a lot of 'g' forces. Still would not be pretty...

Uh, Bussard ram-scoop might not work hereabouts because Sol & Co are deep in 'Local Bubble'.

One notion I've seen suggested would be to catapult fuel pellets ahead of the ram-scoop's predicted path. A coil-gun in long orbit, perhaps at either Saturn Lagrange position might serve. Snag is you need to 'populate' a chunk of the route before launching your star-ship along it. The acceleration phase is easy, but slowing at the other end is hard. You may need a robotic precursor mission to set up the braking pathway...

A laser-riding star-wisp seems about the fastest route, but you're talking compressed data as payload, nothing recognisable as crew...

Reaches the point where you must as Einstein to 'look away now' and postulate some short-cut.


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## RJM Corbet (Jan 1, 2012)

Vertigo said:


> ... As I said at the start I'm trying to avoid the problem of additional new tech and anti-matter drives are really only pure speculation at the moment. But unless I'm much mistaken you are still talking about huge amounts of fuel, even with anti-matter and to make the journey time reasonable you have to accelerate to close to the speed of light. The whole point of this is to come up with a way to avoid carrying your fuel.



They're already proposing 0.2 lightspeed with projected present technology. I think the jump to 0.8 will require new technology ...


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## Abernovo (Jan 1, 2012)

A (planetary-circumference?) mass driver MIGHT work to get it started, I suppose, but I agree it's probably necessary to carry some fuel source, if for nothing else, to slow it down. Small target, damage/destruction if catches even slightly wrong, lost in space if misses completely.

As to cryo-, ice under pressure cracks internally. If bodies followed suit, you might end up with solid forms at the other end, but no longer functioning.


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## chrispenycate (Jan 1, 2012)

The circular cyclotron type accelerator seems possible -- you'd only be able to accelerate one ship at a time though. Which is a pity, because you could use much lower accelerations for longer periods of time -- say, a gee and a half for three years, , so you don't need to freeze your passengers (the critical, "can I apply enough lateral force to keep this craft on a circular track which happens to coincide with the mechanism for generating the force" region is when the speed is maximum).

However, hitting a what? Kilometer square target ( the mouth of the accelerator#) from several light years distance, even with continuous correction, and precision beacons, doesn't seem like something one could rely on.


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## Metryq (Jan 1, 2012)

Don't forget Project Rho as a resource. The "realistic designs" page includes an analysis of the ship seen in the movie _Avatar_, which sounds very similar to your specifications (carrying as little fuel as possible, about 0.70c). Other technologies, including ramjets and derivatives, are detailed on other pages of the site.


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## Vertigo (Jan 1, 2012)

Metryq said:


> Don't forget Project Rho as a resource. The "realistic designs".


 
I refer to them a lot, but I must admit I'd missed that page on the Avatar ship. However I must admit that having to rely on someone firing a laser at you for years seems a little optimistic. What if they decide it's too expensive and stop. Also a truly enormously powerful laser would be required to give and acceleration of 1.5g, as the article says we are not talking about a little satellite in LEO. The article does comment that the mass of the starship is "conspicuous by its absence".



RJM Corbet said:


> Well as you know, Vertigo, the latest Mars rover has its own plutonium power plant that, theoretically, will keep it moving for quite some time. The weight isn't a major issue, and it avoids the problem of the solar panels dusting up.
> 
> So do you need to worry about all the scoops and things? All you need is an effective internal power source ...


 
I think the thing you need to remember here is that in space you need two types of fuel. On Mars the energy generated by a tiny nuclear power plant can be easily converted to a a moving force with simple electrical motors. On a space ship a relatively small amount of nuclear or anti-matter fuel would be sufficient for generating the necessary _*energy*_. But energy is not enough in itself; to power a starship you must convert that energy to a force and with nothing else to push against you must expell matter in an exhaust to generate the necessary forward thrust. You will need an *awful* lot of matter (the second type of fuel) to do this which is the main principle of the ramscoop, gather that matter as you go along.

There is also the problem of mass. The more mass your spaceship has the more exhasut fuel required to give the necessary thrust. Then the more fuel you carry the more you need to carry to move that fuel and the whole thing grows logarithmically. Well I don't know if it's logarithmic but it's certainly not linear.

I'm not sure hitting your target would be that difficult. As I said you would carry some manoeuvring fuel and you would be constantly tweaking your course as you go along. Put beacons sufficiently far out to give you time to make alterations or have a beam that you would "ride down" and I think that part is completely possible. You would not need to be that accurate on launch. It would probably be nigh on impossible to hit a target that size over that distance without later alterations. Just think about the levels of accuracy that we achieve today with our interplanetary probes and they rely mostly on precision at launch.

Chrispy - I think I would still want some sort of cryo mechanism. Partly because the whole journey is going to be very long anyway but mainly because adding the mass of life support, living space and food would, I suspect, make the energy required to get you up to speed prohibitive. If there is no conscious crew and the main drive is not actually on board then the size and mass of the ship could really be very small. I do like the idea of a circular cyclotron with lower acceleration after all what is an extra two or three years against the overall journey time of maybe 60-70 years. I'm still concerned though by the centripetal force. As your speed starts mounting... 

just going to go off and do some calculations.


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## Vertigo (Jan 1, 2012)

OK done some calculations for the cyclotron approach and it's quite scary!
The centripetal force is given by the following equation:

F = m * v^2 / r

So lets say your cyclotron is the size of geostationary orbit around Earth. I'm not saying that's where you would put it but the bigger it is the smaller the centripetal force so lets make it big! So the radius of geostationary orbit is around 40,000 km or 40,000,000 m. Lets look at the force when you reach .5 of lightspeed, that is around 150,000,000 m/s. Finally let's say your mass is around 1000kg; optimistic for a starship but it gives a starting point.

This gives a centripetal force of 562,500,000,000 Newtons (I think I've got all the units right?). Could such a centripetal force be controlled in a cyclotron? And how?

Bear in mind your ship mass would probably be greater than 1000kg, your cyclotron would probably for practical reasons have to be a lot smaller and we are really looking for 70-80% of lightspeed for your final velocity. All of which would only increase the centripetal force to be managed.

I am becoming less optimistic about this approach


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## Ursa major (Jan 1, 2012)

I still think you're being far too optimistic about the far-end procedures. The ship may have a certain amount of manoeuvrability, but it is travelling at 0.8c (240,000 km/s), which must affect all sorts of things, such as the time between observing something and reaching it, and various comms issues, all of which must impact - possibly literally - the catching process and equipment.

The slightest errors in the velocity will change the time of arrival (affecting that perceived by the ship and that by the catcher differently). Yes, it's all maths and computers will be more powerful by then, but the tiniest error could lead to disaster.


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## Vertigo (Jan 1, 2012)

Well you might be right there Ursa but I feel one aspect of future technology that can be safely predicted is that our ability to do things at astonishing levels of precision will increase massively as it has done at a really astonishing rate since the Industrial Revolution. I actually think we are more likely to be able to hit that nail on the head than we are to successfully achieve working cryo technology.

That said if I was thinking of a story involving such a set up, a miss at the destination could make a very interesting plot element.


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## chrispenycate (Jan 1, 2012)

The structure of the ring would have to be extremely rigid (extremely difficult with something that size) or be feedback control dynamic (and remember lightspeed delay is not negligible; predictive distortion, and a huge risk of instability).

The accelerator coils would, I think, have to be rotateable in their force axis. When you consider that all the force runs through those bearings, any one of them failing causing a catastrophphic end to the flight. And probably the accelerator.

But a one tonne ship? They'd have to have a space station it the target system (probably right next to the decceleration coil, if you could aim that carefully) for the defreezing; there wouldn't be any life support at all. I had imagined a "small" ship as a thousand tonnes or so. So you'd need to set up an asteroid civilization first, and your planetfall (assuming you colonised a planet at all) could very well be by orbital tower rather than shuttle (even without reorbital capability, how much must a shuttle weigh?) and the origin ship would probably take at least centuries if not millennia before the --um, shuttle service? -- could be installed. Well in the future then.

You wouldn't want to launch from the inner system. Too much "atmospheric friction" from solar wind and system dust. So you build the ring in the asteroid belt, and fly it out to the Oort cloud, along with its support freezing station, and an impressive power supply.

As you push the ship in one direction, by conservation of rotary inertia the ring starts rotating in the opposite direction, so you have to project alternate ships/capsules in alternate directions so the thing never gets too much centrifugal strain at the rim (it's millions of times the mass of the capsules, sure, but they're being spat out at 80% of c). Actually the fact of ejecting all that mass outwards from the solar system will ultimately push the ring back towards the sun, but that'll be thousands of Earth orbits; somebody else's problem.


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## RJM Corbet (Jan 1, 2012)

Ursa touches on the relativistic problem of mass increasing with speed: the faster you're going the more push you're needing. I didn't think of that, so if that's the point you're making, sorry, yes, of course you're right about needing a huge amount of fuel to shift even 1000kg because at 0.8C it's going to weigh ... what?

In theory the _principle_ of incremental ion-drive type acceleration over a few years seems feasible, based on extending present technology, and it removes the g force problem in the acceleration phase. Targeting obviously must be  by extending present trajectory science and computer guidance technology, and deceleration can then also be gradual.

Freezing the occupants will definitely reduce their need to take a LOT of food along. That's a very valid point. Deals with the problem of 65 year cabin-fever too ...


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## chrispenycate (Jan 1, 2012)

Relativistic effects can be ignored at 80% c, or even ninety; you need ninety six, ninety eight before the mass increase or time dilation are measurable reasonably. At eighty, it'll be milligrams.

And I was never against hibernation, just din't want to rely on it to maintain human structural integrity.


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## RJM Corbet (Jan 2, 2012)

chrispenycate said:


> Relativistic effects can be ignored at 80% c, or even ninety; you need ninety six, ninety eight before the mass increase or time dilation are measurable reasonably. At eighty, it'll be milligrams.



Thanks, Crispen. Good to know that.



> _And I was never against hibernation, just din't want to rely on it to maintain human structural integrity._



I wish I could be a bear too, in winter ...


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## Metryq (Jan 2, 2012)

This whole concept sounds an awful lot like the big gun in _From The Earth To The Moon_.

I was thinking about how VLBA creates an artificially wide aperture for radio astronomy. Would it help to stretch an accelerator across the Solar system as a series of segments? The technology might first be developed as a regular service between planets, single send/receive stations at each planetary port. When the engineers were comfortable with it, multiple movable stators might be set up allowing one to aim at stars outside the ecliptic.


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## RJM Corbet (Jan 2, 2012)

Metryq said:


> This whole concept sounds an awful lot like the big gun in _From The Earth To The Moon_.
> 
> I was thinking about how VLBA creates an artificially wide aperture for radio astronomy. Would it help to stretch an accelerator across the Solar system as a series of segments? The technology might first be developed as a regular service between planets, single send/receive stations at each planetary port. When the engineers were comfortable with it, multiple movable stators might be set up allowing one to aim at stars outside the ecliptic.



I think the concept is much more far fetched than working with an internal propulsion unit. Look at the technology involved in the _LHC_, accelerating protons only -- ramped up to accelerating 1000kg.

Or a rail-gun type thing? But the g force is going to be huge.


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## Vertigo (Jan 2, 2012)

Well my original thoughts were for a mass driver, much the same principle as a rail gun but no physical contact needed. The problem with a rail gun is that the electrical brushes will wear away the rails and this gets worse the higher the speeds involved. A mass driver works in a similar way but is contactless.

However the more I look at this the more impractical it gets. Not surprising really - better minds than mine will probably have thought of this idea and rejected it long ago 

It might still have a place in accelerating a ramjet up to the speed required to begin collecting fuel.

Incidentally RJM I think you are still missing the problem with fuel; it has nothing to do with relativity and time dilation. It's just a simple fact that you have to throw a lot of mass out the back to give you any kind of thrust. Here is an explanation from Project Rho:



> In a rocket, there is a difference between "fuel" and "reaction mass." Rockets use Newton's third law of Action and Reaction in order to move. Mass is violently thrown away in the form of the rocket's exhaust and the reaction accelerates the rocket forward. This mass is of course the "reaction mass." It is sometimes also called "remass" or "propellant."
> 
> The "fuel" is what is burned or whatever to generated the energy to expel the reaction mass. For example, in a classic atomic rocket, the fuel is the uranium-235 rods in the nuclear reactor, the reaction mass is the hydrogen gas heated in the reactor and expelled from the exhaust nozzle.
> 
> ...


Note "reaction" here does not refer to nuclear reaction but Newton's Third Law of Action and Reaction. The bottom line is that it seems to be generally accepted (with current science) that to try and get to another star in a reasonable time (less than a thousand years say) it would not be possible carrying your own reaction mass. Options include Bussard ramjets which carry their own fuel but collect reaction mass on route. However as Nik suggested we occupy an area of space known as a "bubble" where the interstellar particle content is extremely low. Another option is a fixed, system based laser combined with a light sail on the ship. 

As I say it was an idea but I just don't think it would work out in practice. Never mind - move on to the next one


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## Metryq (Jan 2, 2012)

Vertigo said:


> However as Nik suggested we occupy an area of space known as a "bubble" where the interstellar particle content is extremely low.



It's been ages since I read _The Mote In God's Eye_, but weren't the Moties cut off from star travel (hyperspace, or something like that) by having the misfortune to evolve in a "bad" section of space?


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## Vertigo (Jan 2, 2012)

You're right. I can't remember the details but in their case wasn't it something to do with their location close to a red giant sun or something. I seem to remember that it made FTL travel very dangerous for them and all their attempts had failed. They eventually managed to travel out of their system using a light sail if I recall correctly and then when they were nearing the human system they threw all their warrior caste people born on route out into space. Something along those lines.


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## RJM Corbet (Jan 2, 2012)

Vertigo said:


> ...Incidentally RJM I think you are still missing the problem with fuel; it has nothing to do with relativity and time dilation. It's just a simple fact that you have to throw a lot of mass out the back to give you any kind of thrust.



To accelerate, yes. But once you're out there it's frictionless and a small incremental acceleration meets no resistance. Then you take E=MC2 convert matter into energy, or anti-matter (which you say is too far-fetched) and you won't need a lot. You don't need continued propulsion: once you've achieved your speed you'll just keep going. Thrust is for escape velocity, and then to achieve traveling speed only, then you shut it down and keep moving at your 0.8C, with small inputs for course corrections, until it's time to start slowing down again, then you start needing (reverse) thrust again?

But I like your concept too: it's like a sailing ship, using cosmic winds ...


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## Vertigo (Jan 2, 2012)

That's very true if you go for the coasting approach, though it would still take a lot of reaction fuel to get to 80% c. Whilst cruising you would need no energy at all so your reactor could just tick over. There is actually a huge benefit to accelerating all the way to the mid point if your passengers are not in some sort of deep sleep. 80% is pretty close to the speed of light but as Chrispy says not close enough for really significant time dilation. If you push even closer to get the time dilation effect then it won't make much difference to the journey time as seen by the outside world - you can't do 50 light years in less than 50 years. However due to time dilation it can make a *huge* difference to the time experienced *on board* the ship so that you might only experience a 25 year journey to cover 50 light years.

I wasn't too worried about that for this particular drive since I stipulated deep sleep for the passengers (reduce mass requirements) and it really doesn't make much difference if the crew/passengers deep sleep through 25 years or 55 years.

For example if you accelerated at 1g to 80%c then cruised, your acceleration and deceleration stages would take a little over a year and a little of half a light year in distance. Your overall journey time would be around 63 years and the apparent journey time for the crew would be around 39 years. On the other hand if you maintained an acceleration and deceleration of just .2g for the whole journey then your journey time would still be around 59 years (not much different) but the journey time experienced by the crew would be just over 24 years (a 15 year saving for the crew). If you could maintain a 1g accleration for the whole journey (serious reaction mass requirements) then the journey time would still be about 52 years but now the crew's experienced journey time would be just 7 years. You can really make that time dilation work for you! But the reaction mass will always be the problem with that approach.

To finish your sailing ship analogy off the only problem is that our particular area of the Galaxy would be akin to being becalmed due to the low particle density


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## RJM Corbet (Jan 2, 2012)

Yes I do get the point, at last, that it doesn't matter how much energy you have available, it must have mass to react against to provide thrust. Hmmm ...


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## chrispenycate (Jan 2, 2012)

With a perfect drive, ejecting reaction mass at light speed (as photons, preferably collimated) getting a ship up to 80% light speed would require ejecting 80% of the start mass of the vessel. Slowing it down at the other end would require 80% of what was left, assuming 100% efficiency (which I don't believe the third law of thermodynamics allows you, anyway). Yes, gravity slingshot can improve this a trifle (essentially transferring the momentum to the planet you're swinging round) but not by the order of magnitude needed to keep things practical.

You could use your Laithwaite linear accelerator (which is, incidentally, the origin of the railgun and the bullet train) to put packages of fuel where the manned craft could collect them, which would remove the acceleration force problem, if complicate navigation a bit.

The point is, it's not money for nothing; the recoil from whatever you are sending will ultimately make your accelerator start moving, and, unless you can find two destinations with diametrically opposite directions the momentum of the outgoing capsules and that of the deccelerated incoming capsules will combine to push the accelerator itself in the direction away from the target system, presumably toward the sun. With, considering its mass, near irresistable force.

I assume they'll have organised things so it can't possibly hit the Earth, but, by the time they can build this, the inner system will be cluttered with habitats, telescopes and greenhouses, none of which are designed to be towed out of the way of this huge construction, even drifting at centimetres per second...


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## RJM Corbet (Jan 2, 2012)

Makes sense...


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## Nik (Jan 3, 2012)

That's why my Convention' star-ships initially use a 'double bubble' to go FTL...

One Pole, null g,
Three Thrust,
Five fly,
Earth, Moon and Mars,
Nine go to the stars !!


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## Vertigo (Jan 3, 2012)

Nik - what is a "double bubble".

And yes, Chrispy, I think the scale of this thing is pretty much dooming the idea  (never mind I have other ideas tucked away ). If it was built linearly it would be just too big to be sensible and if it was a cyclatron approach I think the centripetal forces produced at even 50% light speed simply could not be contained.

That said it has rasied an interesting thought for me. As mentinioned earlier in the thread as you go above say 80% light speed the journey time does not get much shorter so cruising a good chunk of the journey would not matter. However going faster makes a big difference to the journey time experieced by the crew which also wouldn't matter if you had a working cryo system.

So it seems to me that if you have a working cryo system then their is no great advantage to pushing close to the speed of light. On the other hand if you haven't got cryo systems then you want to get as close to the speed of light as possible so the crew's journey time is as short as possible, even if the actual journey time is not much changed. That alone gives some food for thought on different approaches.


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## Ursa major (Jan 3, 2012)

There's another issue, one that has nothing to do with the means of propulsion as such.

Why would a few people** be spending so much time*** in a cryo system to go to another world, particularly one that's already been populated (so they're not explorers or colonists)?

They won't be doing so for any sort of trade we're used to seeing. (As time has gone by, people's need for instant gratification seems to have increased. Who's going to wait decades for a delivery?) They're not tourists; well, they might be, but I would guess that their numbers would be small, not enough to justify the expense. They won't be military or administrators, not in the sense that they're being sent on a tour of duty.

So who are they? And how can their existence and need to travel cause society as a whole to fund their travels?

It strikes me that if FTL isn't possible, there may be generation ships, sent out to colonise other worlds, but there will be precious little inter-system travel. 




** - There wouldn't be many passengers on board the small sizes of ship being discussed.

*** - Even allowing for shortened "experienced time" (experienced by the ship, if not its passengers).


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## Abernovo (Jan 3, 2012)

I've had similar thoughts, that over long distances, one-way generation ships would be pretty much the only option on the table. There is another, less palatable, option, though. That is the creche ship idea, full of embryos. Again, this would require advances in cryogenic storage, but probably not as much as with whole bodies.

However, there are serious ethical questions to this option. You're sending embryos, so there is an issue of lack of consent. The embryos would be raised presumably by machines, bringing the Brave New World scenario into play, not to mention the question moral inculcation, possibly resulting in a Lord of the Flies scenario. Put together, that could be pretty unpleasant. And, you have the whole hot potato of the religious/political embryo ethics debate.

I'm not coming down on any side, but it has been put forward in the past as an option.


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## Vertigo (Jan 4, 2012)

I don't really like the embryo idea, though it may well be the most practical way to colonise distant planets. It feels too much like the behaviour of primitive animals that do not nurture their young. They just lay their eggs or whatever and leave them to it. I don't feel it is in our nature to operate that way except in dire necessity. Maybe some huge catastrophe is going to destroy our planet/sun/solar system and that is the only way to ensure survival of the human race or something along those lines.

Ursa: I can't really answer those questions as I don't have any particular scenario imagined for this. It really was just speculation on possible ways to achieve interstellar travel. Not such a good idea as it turns out but never mind!

That said, I do actually agree with all your points. Without an FTL drive that is comparatively cheap to operate (comparable with, say, ocean going cargo vessels) I don't see how trade, tourism or even warfare across star systems could ever be practical. The costs would be astronomical and what would you trade anyway. It's hard to imagine a product that would be worth shipping across interstellar space. Most star systems would have similar resources available to their occupants. Except maybe ones very low in heavier elements but then it is considered unllikely that complex life would ever evolve in such systems. The only motivation I can think of for undertaking such travel would be seeking knowledge - exploring. Which, as you pointed out, is not really relevant for systems already colonised.

My personal favourite scenario is one where an entire planetary population has taken to living in habitats after their planet becomes unlivable and then start exploring. In my WIP (a rather grand name for something that is no more than germ of an idea) a intelligent race has evolved on a planet with a star similar to out own. Only they evolved several billion years later in their star's lifecycle. As their star begins to expand into a red giant they are forced to leave their planet to live in converted asteroid habitats. Having made that move some (though not necessarily all) decide they might as well travel further. The motivation being knowledge and they effectively take their entire homes with them! And because of that there is no real hurry!


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## Ursa major (Jan 4, 2012)

I wasn't expecting an answer, if only because I'm not sure that there are many convincing ones to hand. And their absence doesn't prevent some authors writing successful sub-FTL-based space operas, so it doesn't seem to be an insurmountable barrier, provided the stories are engaging enough that the reader isn't tempted to speculate on the economic/financial underpinnings of the society to the extent that they notice that they're the least convincing parts of the set up (iffy technology notwithstanding).




(Sorry about the non-Twitter-friendly sentence length )


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## Vertigo (Jan 4, 2012)

That is quite some sentence Ursa  and I agree; it is the story that counts, so long as the technology isn't too ridiculous. And it can get so. 

My favourite example is the star drive from the Old Man's War series. As I recall it involves jumping from one parallel universe to another and jumping to a new location at the same time. The thing is that you don't actually come back to your original one; you stay, whilst your alternate makes the same jump into another nearby parallel. So it's not actually you that arrives at your destination in your original universe but an alternate you! Apparently not an issue as nearby parallels are so identical that you couldn't tell the difference.

Pretty far fetched but good enough to provide backing for some very good stories.


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## Ursa major (Jan 4, 2012)

Just the sort of sentence that would, were it in a novel, drive any of the readers who've not already closed the book to wonder if any of it makes any sense at all.


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## Vertigo (Jan 5, 2012)

Sorry Ursa I edited a chunk onto my last post which you probably didn't see - didn't think you'd be back so quick


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