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# What is the fastest a space ship could travel in space?

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Showing 1-24 of 24 posts in this discussion
Initial post: Dec 23, 2012 7:04:38 AM PST
What is the fastest a (living) human being could travel in space? Inside our solar system? Between star systems? Robotic interstellar probes?

What is the most optimistic (somewhat) realistic high head theories? At least what could possibly possible from the most optimistic viewpoint? Without interdimensional timewarps and deformed freaks tripping on sandworm birth fluid. Unless that is a possibility.

Will the Earth get so crappy that humans be willing to go into a "frozen" stasis for millions of years just for the chance of a new home?

In reply to an earlier post on Dec 24, 2012 5:26:07 AM PST
The ultimate limiting factor is the velocity of light. But there is a practical limit, considerably lower, which arises from the physics of spacecraft propulsion, and this is on the order of a tenth of that. We know, from Newton's laws, that there is no way of propelling a spacecraft except by ejecting reaction mass. The momentum imparted to the craft by this is the product of the ejected mass, times the velocity of that ejection. Since the mass that can be carried is limited, the exhaust velocity must be as high as possible. But the energy required to do this goes as the square of the velocity, so the energy requirement becomes monstrous. Only a nuclear fusion reactor can possibly supply the required energy -- and a half century of research on these has yet to produce one that works.

For further discussion of this, look for references to "specific impulse". This, abbreviated Isp, is defined as the amount of time that you can get a pound of thrust by ejecting a pound of combusted fuel. (Typical values for chemical rocket engines top out at about 350 seconds.)

In reply to an earlier post on Dec 24, 2012 2:55:32 PM PST
Charlie T. says:
An ion or photon drive would get close to c, but at very low acceleration.

In reply to an earlier post on Dec 24, 2012 3:21:39 PM PST
Re Charlie T., above: You are ignoring the energy needed for the power source. Certainly, you can spit reaction mass out at close to the speed of light, but you have to have a power plant that can do so with limited fuel mass.

I did a breadboard design for a fusion-powered spacecraft. Gross mass, 100 tons; payload, 30 tons, fuel, 30 tons of deuterium and lithium; vehicle mass, 40 tons (engine mass, 10 tons). To get something approaching a 1 G acceleration, the machine would have to run at a power level of a terawatt -- roughly equivalent to the entire electric generating capacity of the United States. Such a vehicle could never operate from the earth's surface -- it would vaporize the spaceport and much of the surrounding countryside.

In reply to an earlier post on Dec 24, 2012 10:51:32 PM PST
Ashwood says:
Robert A. Saunders says: We know, from Newton's laws, that there is no way of propelling a spacecraft except by ejecting reaction mass.

Ash : You can also have friends give you a push.

http://en.wikipedia.org/wiki/Beam-powered_propulsion
http://en.wikipedia.org/wiki/Light_sail

Posted on Dec 24, 2012 10:53:21 PM PST
Ehkzu says:
There has also been research into space sails using photons from the primary as the "wind. Of course you couldn't deploy such sails until you'd gotten out of a planet's gravity well, almost certainly from using reaction mass.

One other factor limiting speed in space is that if there's a biological crew, even if you had a power source that could pour it on you wouldn't want the crew to reach your destination in the form of a bloody paste on the aft bulkhead (assuming the spaceship does a 180 on its orientation at the midway point, decelerating as vigorously as it accelerated.

I think the fastest thing we've sent up is Voyager I, isn't it? Didn't they say it would take about 80K years to reach the distance of the nearest solar system, Alpha Centauri?

In reply to an earlier post on Dec 24, 2012 11:02:51 PM PST
Re Ashwood, 12-24 10:51 PM: "You can also have friends give you a push." Neither beam-powered propulsion nor light sails can work at interstellar distances. Both might be useful in near-earth work -- but only for interplanetary probes.

In reply to an earlier post on Dec 24, 2012 11:09:16 PM PST
Re Ehkzu, 12-24 10:53 PM: "if there's a biological crew,.." Proposals for interstellar travel typically posit an acceleration of one G, which if sufficiently prolonged would get the job done. Indeed, higher accelerations increase the overall fuel expenditure.

Many years ago, Robert A. Heinlein wrote an article which was published in one of the SF magazines, noting that with a one-G acceleration, a trip to Mars would be something like an overnight junket, while a trip to Pluto would take only a couple of weeks. But he completely ignored the question of possible propulsion schemes for such -- which rather surprised me, as he was typically quite thorough with his science.

In reply to an earlier post on Dec 24, 2012 11:15:50 PM PST
Last edited by the author on Dec 25, 2012 1:34:24 AM PST
Ashwood says:
Robert A. Saunders says: Neither beam-powered propulsion nor light sails can work at interstellar distances. Both might be useful in near-earth work -- but only for interplanetary probes.

Ash : A body in motion tends to stay in motion, even over interstellar distances.

If you set up a relay of beam stations (on large slow ships) in the starting solar system, you can give your ship a big boost of acceleration that doesn't cost reaction mass/fuel and that boost stays with you the entire journey.

Once you get out of beam range, you can dump the solar sails and switch to normal engines, or if you are traveling to an established system there may be a beam relay there to help slow you down.

Just because you can't do interstellar travel using beam powered propulsion exclusively, doesn't mean that beam powered propulsion won't be useful for speeding up those trips.

In reply to an earlier post on Dec 25, 2012 5:04:16 AM PST
What do you think the first unmanned mission outside the solar system will be? (Other than the "accidental" Voyagers 1 and 2).

I'm guessing it might be to the nearest earth-like planet once it's been identified. We might have to plan for a mission in which those who launch it will never live to see any of the data it sends back.

Or maybe we'll have the rather funny spectacle of it being passed by successive generations of faster probes as technology improves.

In reply to an earlier post on Dec 25, 2012 10:56:23 AM PST
[Customers don't think this post adds to the discussion.]

In reply to an earlier post on Dec 25, 2012 1:41:33 PM PST
Re Ashwood, 12-24 11:15 PM: "If you set up a relay of beam stations ..." In principle, one could do this. Note that these stations would all be orbiting the sun, at different distances and thus different speeds. Consider the matter of obtaining an alignment of these such that all could be used for boost....

In reply to an earlier post on Dec 26, 2012 1:04:20 AM PST
Ashwood says:
Robert A. Saunders says:
Re Ashwood, 12-24 11:15 PM: "If you set up a relay of beam stations ..." In principle, one could do this. Note that these stations would all be orbiting the sun, at different distances and thus different speeds. Consider the matter of obtaining an alignment of these such that all could be used for boost....

Ash : You left out "(on large slow ships)" they wouldn't just be passively orbiting. They could use solar sails and engines to make sure everything lined up at the right time. Since they can take their time and slowly coast into place, it wouldn't require as much fuel to get them into position.

Posted on Dec 30, 2012 2:40:26 PM PST
D. Vicks says:
As someone said above Fusion Powered rockets are the best choice for now.

Posted on Dec 30, 2012 3:57:39 PM PST
John Donohue says:
Lawrence Krauss addressed this quite informatively in his The Physics of Star Trek. One obstacle to getting a practical rocket up to anything like relativistic speed is the need to carry fuel -- both to go and to return. The fastest thing we have put into space so far travels at about 50,000 mph with respect to the Earth's orbit -- that is, about 0.0000746 of the speed of light.

In reply to an earlier post on Dec 30, 2012 9:51:44 PM PST
Dave vicks wrote:
"As someone said above Fusion Powered rockets are the best choice for now. "
==========================
Thermonuclear fusion has not been mastered as we speak.

Rockets do not run on heat. Rockets need material impulse to conform to Newton's Third Law of motion.

Mohamed F. El-Hewie

In reply to an earlier post on Dec 30, 2012 9:54:38 PM PST
Last edited by the author on Dec 30, 2012 10:04:45 PM PST
Doctor Who says:
Have you ever even bothered to learn what something is before posting about it? The rockets run by ejecting material either prepared from the reaction or directly a product of the reaction. They don't run on heat, at least not any more than the Apollo rockets.

http://en.wikipedia.org/wiki/Fusion_rocket

Posted on Dec 30, 2012 10:23:50 PM PST
D. Vicks says:
Thread reminds me of the Bowie song, Major Tom.

In reply to an earlier post on Dec 31, 2012 12:05:04 AM PST
Re Doctor Who, 12-30 9:54 PM: For a change, el-Phooie has a point. What we need is momentum, which is mass times velocity. With most rockets, that velocity derives from heat, as from a chemical reaction. But with the ion drive rockets currently being considered, they could operate at the local temperature, as the exhaust velocity is electromagnetically (rather than thermally) generated. It is true, of course, that one can define an equivalent temperature for a particular exhaust velocity using Boltzmann's equation, but there would be little point in doing so.

In reply to an earlier post on Dec 31, 2012 9:53:00 AM PST
Doctor Who (a.k.a. Physics Geek) wrote:
"Have you ever even bothered to learn what something is before posting about it? The rockets run by ejecting material either prepared from the reaction or directly a product of the reaction. They don't run on heat, at least not any more than the Apollo rockets."
=======================

Your expertise in getting into troubles is astonishing.

Mohamed F. El-Hewie

In reply to an earlier post on Dec 31, 2012 10:15:42 AM PST
Robert A. Saunders wrote:
"But with the ion drive rockets currently being considered, they could operate at the local temperature, as the exhaust velocity is electromagnetically (rather than thermally) generated. "
============================
Creating high EM potential-difference gap requires conversion of heat to electric energy, with heat the final product of thermonuclear fusion. No one ever succeeded in driving high voltage out of nuclear fusion without first capturing the kinetic energy of the byproducts of the DT fusion. There are no means of using those energetic byproducts particles as direct thrust plasma for rockets.

Getting from heat of nuclear fusion to EM energy for accelerating ions to plasma kinetic energy would never achieve net Specific Impulse to thrust the weight of the fusion reactor, the weight of EM transformers, on top of the space-probe.

Unless high voltage is generated without fusion reactor or massive iron cored transformers, plasma thrust will be limited to miniature maneuvers such as tweaking antennas, solar panels, and the like, but never to push the rocket en mass.

Keeping in mind the wisdom "never say never", an engineer is needed to optimize the balance between body masses and net impulses, with plasma ions immensely low in weight and require immense energy to produce out of neutral matter.

Mohamed F. El-Hewie

In reply to an earlier post on Dec 31, 2012 1:02:30 PM PST
Doctor Who says:
http://www.popsci.com/military-aviation-amp-space/article/2009-10/plasma-rockets-could-make-pit-stops-mars-and-beyond

It is already practical for use.

In reply to an earlier post on Dec 31, 2012 1:04:56 PM PST
Doctor Who says:
That is yours. Have you learned basic physics yet? Or to stop lying about your level of education? According to court records you have "the equivalent of a masters", not a PhD.

In reply to an earlier post on Dec 31, 2012 1:11:43 PM PST
Re Doctor Who, 12-31 10:15 PM: "It is already practical for use." True -- if you aren't in a hurry. Existing and contemplated ion drives have very low thrust, so to make an appreciable effect on an orbit requires a long time. I have heard figures for the Isp of these to be on the order of 60,000 seconds, which is a lot better than chemical rockets but nowhere good enough for a weekend junket to Mars.
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## This discussion

Discussion in:  Science forum
Participants:  10
Total posts:  24
Initial post:  Dec 23, 2012
Latest post:  Dec 31, 2012