[QUOTE=retina;315743]It is probably worth pointing out that sending a probe now (or in 15 years) won't be worthwhile if it takes 40ka+ before any useful return on investement is expected. This is simply because in, say, 50 or 100 years some new technology will be available that will shorten the transit time to ~20ka and make the first probe a useless piece of space junk. This applies to the new 20ka probe also, as a newer technology will come around in an additional 50 to 100 years after that to reduce the travel time to 10ka, etc. It is not until you reach a point where the anticipated advance in technology is less than the anticipated travel time that it becomes viable to actually send a probe.

I suddenly got [url=http://mersenneforum.org/showthread.php?p=142866#post142866]Déjà vu[/url].[/QUOTE]

for a manned mission lifespan also comes into play, if you want to go both ways 43.7 years doubles to 97.4 years assuming 20 years old when the mission took off and subtracting the 15 years means I'd have to pick a captain that's less than 6 years old at the beginning of the whole process not to need them to break the world record in age to land back home safely. of course this timeline depends on achieving an average speed of 10% of the speed of light.

[QUOTE=science_man_88;315746]for a manned mission lifespan also comes into play, if you want to go both ways 43.7 years doubles to 97.4 years assuming 20 years old when the mission took off and subtracting the 15 years means I'd have to pick a captain that's less than 6 years old at the beginning of the whole process not to need them to break the world record in age to land back home safely. of course this timeline depends on achieving an average speed of 10% of the speed of light.[/QUOTE]

It'd be exciting at first but after a while, an astronaut would get so bored! The spaceship would become like a prison.

Imagine having a lifetime's supply of food and water on board.

[QUOTE=gd_barnes;315757]It'd be exciting at first but after a while, an astronaut would get so bored! The spaceship would become like a prison.

Imagine having a lifetime's supply of food and water on board.[/QUOTE]

It would be boring and prison-like. Such travelers had better hope for much better Virtual Reality tech. This is just one of the themes explored in many Sci-Fi stories, especially those concerned with "generational ships." That is, ships taking so long to get anywhere that human generations will pass during transit. Also of concern in such scenarios is the question of mutations and maintaining some form of government.

Food and oxygen, on the other hand, really would have to come from some form of plant life, most likely hydroponics on a large scale. This begs the question of how one keeps the lights on to grow plants.

[QUOTE=Andi47;315717]I wouldn't consider things like hammer and anvil or an axe as "moving parts" in the same sense as moving parts in a satellite or space probe: Within a probe there are things like motors, antennae or panels to be deployed, cameras with color-filters to be switched, and so on - in other words, the machine parts are changing it's position to each other, need lubrication and so on.
On the other hand, with a hammer or an axe, no part of the tool is changing its position relative to other parts of the tool, no lubrication is needed, and nothing can get stuck. OK, an axe can get blunt when used, i.e. hit against a tree or such, but still no parts get "stuck".[/QUOTE]As you wish.

Each of my examples contain mechanical structures which wear out as a consequence of movement when in use. If you would like an example which occasionally gets stuck when used, thereby rendering it inoperable, consider an arrow head.

[QUOTE]It'd be exciting at first but after a while, an astronaut would get so bored! The spaceship would become like a prison.[/QUOTE]You obviously have not watched Rocketman, one of the best space movies ever made!

[url]http://www.imdb.com/title/tt0120029/[/url]

:judge:

[QUOTE=xilman;315497]Indeed.

However, in 1961 we could calculate that the journey time to the moon would be of the order of a month, that almost real-time communications would be possible throughout the mission and that the energy requirements per kilogram of payload would be only a few times that which had already been demonstrated.

For your mission, the journey time will be at least a decade (itself not unreasonable, cf Voyager). Real time communication becomes infeasible long before the mission leaves the inner solar system.

If the journey time is to be less than a century, and I really doubt that anyone these days would want to embark on such a speculative project with a payback time of more than that --- especially as you posit that a mere 15 years would cause a fatal loss of interest --- the probe needs to travel at approximately 0.1c. That speed is 30,000 km/s or something over a thousand times the delta-v needed to get to the moon and back. Kinetic energy scales as the square of the velocity, so well over a million times the energy per kilogram is needed as for a lunar mission. Simple chemical thermodynamics tells you that chemical rockets won't provide that unless the fuel/payload mass ratio is prohibitively outside your $10G budget.

With technology which can be built in the next 15 years, that would appear to leave sails, nuclear thermal rockets, nuclear electric rockets, gravity assists and electromagnetic rail guns (*). The first three have very low accelerations at present technology levels; if they are to be used for long periods you either need extortionately large supplies of reaction mass for the rockets or (near) earth-based beamed power for sails to be useful much beyond a few astronomical units. There is no way that terawatt average-power lasers are going to be built and fielded in 15 years. I've already pointed out that gravity assists aren't going to get you much faster than a coasting speed of around 100km/s.

As for electromagnetic rail guns, do you really think that we can build a linear motor long enough and high power enough to accelerate even a kilogram to 0.1c in the next 15 years? Assume that the payload can withstand 10,000 g acceleration. To a reasonable approximation, the boost phase to 0.1c will take 0.1 / 10,000 years (using the handy approximation that c = 1 g-year). That comes something over five minutes, during which time the probe will travel about an astronomical unit. Again I ask, do you really think that we can build a linear motor long enough in the next 15 years?

Now examine the energy requirements for a 1kg probe (how is such a thing going to be able to send results back?). Neglecting relativistic increase of mass (down in the 1% noise at this scale of approximation), the kinetic energy is 0.5 * (3e7)^2 Joules, or around 0.5PJ. In more common units, this is close to 100 kilotons of TNT. The kinetic energy alone of a tiny 1kg probe is markedly larger than that of all the Apollo missions put together.

Using known physics, only fusion rockets show any hope of reaching the nearby stars in less than a century. Unfortunately, fusion rockets aren't ready to be flown in a 15 year time scale.

Slightly further down stream might be ramjets, but they're even higher technology than "standard" fusion rockets. First you need to get them up to speed (see earlier problems) and then you need to do something with the material collected. For a start the ISM is principally hydrogen, much harder to fuse than D/He3. Even if you use it only as reaction mass accelerated by an on-board power supply, the ISM is very dilute and there are very severe problems with inlet stagnation at the throat of the collector. Building the collector and the PSU to drive it (and accelerate anything collected if H fusion is too hard) is going to take a serious mass budget; all that mass has to be accelerated to high velocity before it starts being useful.


(*)Well, a few others. Magnetic tethers, for instance. Unfortunately the galactic magnetic field isn't strong enough to give the accelerations needed.[/QUOTE]


according to V = V[SUB]i[/SUB]+a*t 10,000 g would take over 305 seconds to get to the needed speed for a manned mission and then the average wouldn't end up being that speed to do it in 1 second we'd need 3,057,032.3 g and since forces this high can only ( if ever) be tolerated for milliseconds you need to scale it again to 3,057,032,300 g just to survive ( again if ever).

[QUOTE=science_man_88;315788]according to V = V[SUB]i[/SUB]+a*t 10,000 g would take over 305 seconds to get to the needed speed for a manned mission and then the average wouldn't end up being that speed to do it in 1 second we'd need 3,057,032.3 g and since forces this high can only ( if ever) be tolerated for milliseconds you need to scale it again to 3,057,032,300 g just to survive ( again if ever).[/QUOTE]Why did you feel the need to translate my "something over five minutes" to the ludicrously precise values you give for an acceleration I gave only to a single significant figure?

Very little macroscopic material, other than electron-degenerate or neutron-degenerate matter can withstand mega-g accelerations even for milliseconds without catastrophic deformations. I'm not sure, but rather doubt, that even electron-degenerate matter can withstand giga-g fields. Corrections welcomed.

Needless to say, 10,000 g for five minutes is not survivable and even 100g is difficult for the human body to survive for as long as a millisecond. It's been done, by people falling off a cliff onto sand, but it is not a comfortable experience and I'd recommend writing your will and having a fully equipped ambulance standing by before attempting it.


Paul

[QUOTE=xilman;315793]Why did you feel the need to translate my "something over five minutes" to the ludicrously precise values you give for an acceleration I gave only to a single significant figure?

Very little macroscopic material, other than electron-degenerate or neutron-degenerate matter can withstand mega-g accelerations even for milliseconds without catastrophic deformations. I'm not sure, but rather doubt, that even electron-degenerate matter can withstand giga-g fields. Corrections welcomed.

Needless to say, 10,000 g for five minutes is not survivable and even 100g is difficult for the human body to survive for as long as a millisecond. It's been done, by people falling off a cliff onto sand, but it is not a comfortable experience and I'd recommend writing your will and having a fully equipped ambulance standing by before attempting it.


Paul[/QUOTE]

because I thought different accelerations in d=vi*t+.5*a*t^2 gave different distances and therefore could in theory make it more possible to build something of the given distance.

[QUOTE=xilman;315793]Very little macroscopic material, other than electron-degenerate or neutron-degenerate matter can withstand mega-g accelerations even for milliseconds without catastrophic deformations. I'm not sure, but rather doubt, that even electron-degenerate matter can withstand giga-g fields. Corrections welcomed.[/QUOTE]LHC: (7 TeV / (20 minutes * c))/proton mass = 190 mega-g

[url=http://en.wikipedia.org/wiki/Plasma_acceleration]Plasma accelerator[/url] (42 GeV / 85 cm)/electron mass = 890 exa-g

Correction welcome if I got the equations or numbers wrong.

[QUOTE=retina;315798]LHC: (7 TeV / (20 minutes * c))/proton mass = 190 mega-g

[url=http://en.wikipedia.org/wiki/Plasma_acceleration]Plasma accelerator[/url] (42 GeV / 85 cm)/electron mass = 890 exa-g[/QUOTE]Consistent with my qualifier "macroscopic"

[QUOTE=xilman;315799]Consistent with my qualifier "macroscopic"[/QUOTE]Yes. I wasn't making a correction to your claim. I just thought it was interesting to see what sort of g forces things might be undergoing on a daily basis.