An idea for a stardrive (sort of)

Discussion in 'Science / Nature' started by Vertigo, Jan 1, 2012.

  1.  
    Vertigo

    Vertigo Mad Mountain Man

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    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

    Ursa major Bearly Believable Staff Member

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    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

    RJM Corbet Never Sure

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    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

    Vertigo Mad Mountain Man

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    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.

    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.
    Last edited: Jan 1, 2012
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    RJM Corbet

    RJM Corbet Never Sure

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    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

    Ursa major Bearly Believable Staff Member

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    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.
    Last edited: Jan 1, 2012
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    Nik

    Nik Speaker to Cats

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    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

    RJM Corbet Never Sure

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    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

    Abernovo Accident-prone, allegedly

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    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

    chrispenycate resident pedantissimo

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    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

    Metryq Cave Painter

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    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

    Vertigo Mad Mountain Man

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    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".

    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.
  13.  
    Vertigo

    Vertigo Mad Mountain Man

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    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 :(
  14.  
    Ursa major

    Ursa major Bearly Believable Staff Member

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    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

    Vertigo Mad Mountain Man

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    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

    chrispenycate resident pedantissimo

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    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

    RJM Corbet Never Sure

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    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 ...
    Last edited: Jan 1, 2012
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    chrispenycate

    chrispenycate resident pedantissimo

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    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

    RJM Corbet Never Sure

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    Thanks, Crispen. Good to know that.

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

    Metryq Cave Painter

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    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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