An idea for a stardrive (sort of)

[RJM Corbet]

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.

 

[Vertigo]

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.

There are only a few confusing cases where the fuel and the reaction mass are the same thing. This is the case with chemical rockets such as the Space Shuttle and the Saturn 5, which is how the misconception started in the first place.

Automobiles, airplanes, and boats are sizable vehicles with relatively small fuel tanks. Not so rockets. An incredibly powerful rocket might approach having half its mass composed of reaction mass and the other half structure, hull plates, crew members, and everything else. But it is more likely that 75% of the mass will be reaction mass. Or worse. Most rockets are huge propellant tanks with a rocket engine stuck on the tail and a tiny crew habitat stuck on the top.

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

 

[Metryq]

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?

 

[Vertigo]

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.

 

[RJM Corbet]

...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 ...

 

[Vertigo]

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

 

[RJM Corbet]

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 ...

 

[chrispenycate]

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...

 

[RJM Corbet]

Makes sense...

 

[Nik]

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 !!

 

[Vertigo]

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.

 

[Ursa major]

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).

 

[Abernovo]

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.

 

[Vertigo]

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!

 

[Ursa major]

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 )

 

[Vertigo]

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.

 

[Ursa major]

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.

 

[Vertigo]

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