Spaceship Design

[Jay-qu]

In an antimatter “steam” rocket motor, neutrons (and protons) are essential as the most massive particles in its exhaust.

 

Here’s a very simplified diagram of an antimatter steam rocket reaction (in glorious ASCII):

[input      ] [Reaction                                                             ][Exhaust]
|P- (the    |
|“antimatter|                  |many   |
|stream")   |>-            /->|photons|>--->|many   |>-------------->|Everything|   |hot, ionized |
               _anhilation_                 |photons|>->|e- (in  |>->|else (in  |>->|dissasociated|
|P+ (in H2O|>--/            ->|exotic   |>-/            |cool H2O|   |cool H2O  |   |H + O        |
|molecules)|                   |particles|               /            /
                                                       /            /
|the rest  |>------------------------------------------/------------/
|of the H2O|

So the annihilation produces, ultimately, photons, which heat the unannihilated water into plasma “steam”, which escapes the engine as exhaust, causing thrust.

exactly what I was getting at, they arnt useless they will provide us with an impulse :esmoking:

[Kayra]

so you think fast moving neutrons are useless to us?

 

I think that fast moving neutrons going in the right direction would be VERY useful to us.

 

Is there anyway to manipulate these things?

[CraigD]

You still haven't answered my question on the quantity of water needed to create sufficient thrust.

You’re correct – because it’s a hard question!

 

The higher the average speed of the exhaust, the less the total mass of water the antimatter “steam” motor needs to generate a constant 1 g of acceleration for an entire mission. In principle, a very strong motor vessel with a very small exhaust “hole” could have nearly any exhaust speed, allowing a very small mass of water to be expelled at nearly the speed of light. In practice, an exhaust speed of .01 c seems very ambitious. This would allow a 10^8 kg ship to operate for about 3 year at 1 g with about 5*10^7 kg – 50% of the ship’s total mass. Increasing the exhaust speed allows the constant acceleration time to be increased, and/or the water mass decreased, roughly proportionally to the increase in exhaust speed.

 

The engineering details of how to achieve .01 c or better – about 3 times those achieved by the best current experimental, magnetic-pinch plasma rockets, such as the VASIMR, are daunting, but who expected it to be easy?

[Jay-qu]

I dont know of any way know to manipulate them - but doesnt really matter, every action has an equal and opposite reaction, its useless to push them in one direction just so they create an impulse - net force will equal 0.

[Janus]

You’re correct – because it’s a hard question!

 

In practice, an exhaust speed of .01 c seems very ambitious. This would allow a 10^8 kg ship to operate for about 3 year at 1 g with about 5*10^7 kg – 50% of the ship’s total mass.

 

I'm sorry, but you have seriously under estimated the amount of reaction mass you would need.

 

To calculate the true value, we'll start with the relativistic rocket equations. (as a rocket operating at 1g for 3 years would reach relativistic speeds.)

 

The two of importance here are

 

[math]V = \tanh \left (\frac{at}{c} \right ) c[/math]

where V is the final velocity of the ship,

a = the acceleration

t = the time for which we accelerate.

 

and

 

[math]V = \tanh \left ( \ln(MR) \frac{V_e}{c} \right )c[/math]

 

where MR = the mass ratio of fully fueled ship to unfueled ship,

and

Ve = the exhaust velocity

 

Since both equations sovle for V, we can equate them.

 

[math]\tanh \left ( \ln(MR) \frac{V_e}{c} \right )c= \tanh \left (\frac{at}{c} \right ) c[/math]

 

simplifying, we get

 

[math] \ln(MR) V_e= at[/math]

 

if we solve for MR, we get:

 

[math]MR = e^{\frac{at}{V_e}}[/math]

 

plugging in the values of

a= 9.8 m/sec²

t = 94,608,000 sec (3600 sec/hr * 24 hr/day * 365 days/year *3)

Ve= 3,000,000 m/sec (.01 c)

 

And we get a mass ratio of

 

[math]MR = 1.66 x 10^{134}[/math]

 

IOW, it would take more than the mass of a google of suns to accelerate even 1 kg of mass at 1g for 3 years with an exhaust velocity of 0.01c!

[Pyrotex]

I'm sorry, but you have seriously under estimated the amount of reaction mass you would need. ...IOW, it would take more than the mass of a google of suns to accelerate even 1 kg of mass at 1g for 3 years with an exhaust velocity of 0.01c!

And there's also the unstated fact that exhaust velocity is a direct function of engine temperature. EV of 0.01c would require temps in the millions if not billions of Kelvins. Ouch!

Ion engines get around this to some extent by making the total thrust so small that excess heat can be handled.

[Jay-qu]

slight *bump*

 

Seems everyone likes the Motley Cruiser.. :naughty:

 

So any other aspects of a ship that people want to discuss? How bout manouvering and shielding (weapons :naughty:)

[TheFaithfulStone]

Nanotech disassembler for weapons. None o' that directed energy crap. Just straight up, good old fashioned take you apart molecule by molecule.

 

TFS

[Jay-qu]

how? nanites that 'eat' things on a molecular level?

[CraigD]

How bout manouvering and shielding (weapons :))

Nearly any advanced rocket produces a very-high energy exhaust. In the event that such a ship ever needed an weapon, its exhaust should serve that purpose well, and at no performance cost or design effort.

 

Shielding – armor – seems a terribly difficult design challenge. In the “arms race”, the “break it” side seems always to maintain a lead over the “keep it together” side. I suspect that spacecraft requiring protection from conscious, hostile threats will need to rely on stealth, not armor. On the distance and velocity scales of advance spacecraft like we’ve been discussing, this doesn’t seem an insurmountable design challenge. Managing photon (light, radio, and especially heat) so that it doesn’t reach whatever intends you ill is the requirement, and though one that calls for a comprehensive and design effort expensive approach, is not beyond existing technologies. The details can be elaborate.

[GAHD]

Speaking of "arms" :eek2:

Dynamic Balance Controll of Multi-Arm Free-Floating Space Robots PDF

 

Damned interesting article that points out how having multiple "arms" on any space craft would be benificial to keeping it from loosing it's 'balance' in space.

 

Imagine an eight armed robot flailing around in space to keep it's orientation in relation to it's mothor ship.

[TheBigDog]

On a generation ship you need to have a fully self sufficient environment. You need this in every regard. Food, air, energy, water, education, maintenance, healthcare, recreation. Everything. So I ask, what is the minimum space required to sustain food for a single person? What is the minimum human population for creating and sustaining a diverse genetic population? Given these two numbers, what are the minimum dimensions for a Generation Ship's habitable space? Consider today's technology. We cannot hypersleep or use any other tricks. And the ship should have a service life that is almost unlimited to allow interstellar travel at non relativistic speeds.

 

Bill

[CraigD]

For all that the questions have been seriously addressed with millions of dollars and man-hours for about a generation, they remain interesting, and their answers still very much in flux. A good, if somewhat non-technical and perhaps overly romantic account of the state of the art as of the mid 1990s can be found in Kevin Kelly’s “Out of Control”’s chapter ”Closed Systems”, which cover’s the pioneering Russian work in the 1960s, through the flamboyant and controversial US “Biodomes” of the 1980s. Rather than blather volumes about this here, I’ll leave that to master-blatherer Kevin Kelly :) and just put in my take on the huge literature around the subject

So I ask, what is the minimum space required to sustain food for a single person?

The Biodome-ites tended to play around with a rough figure of ½ acre/person (2000 m^2 / person). The Russians managed, using a much more forced, energy intense (eg: they electrically sterilized the recycled air, force-grew the agriculture and atmospherics, etc), to keep 3 men in a sealed 4-room complex, using about 50 m^2/person. The extreme variability of these answers contributes to my summarizing the issue as “in flux”.

What is the minimum human population for creating and sustaining a diverse genetic population?

Unlike the eat-breath dynamics mentioned above, of which there’s been no remarkable progress in the past couple of decades, recent advances in practical genetics and reproductive medicine require, I believe, that this question be rephrased from “what is the minimum human population for …?” to “what is the minimum amount of genetic data needed … and what is the best way to store it?”

 

I suspect that the answer to the first part of the rephrased question is “why be stingy?”, and to the second, almost certainly not “inside living, breathing, socially functioning human beings”.

 

With current medical technology, a population of millions can be stored as frozen egg and sperm and preserved for … no one really knows how long, but likely centuries. A “seed host” population as small as tens of individuals (all woman, for the first generations) could be used to populate a target world with a billion individuals with about the same genetic makeup as our current population in about a few centuries.

Given these two numbers, what are the minimum dimensions for a Generation Ship's habitable space? Consider today's technology.

Given the numbers I’m getting, the answer is “pretty damn small” – not larger than that of a large 20th century airliner.

We cannot hypersleep or use any other tricks. …

Of course, in reality, we can use any trick that are actually available when the engineering to be done.

 

It’s not far-fetched to expect that practical techniques in molecular biology will, with in the next couple of decades, permit DNA to be assembled from any data medium, nor artificial storage devices may significantly exceed the storage density of in vitro or frozen cells. When and if this occurs, the possible scale of “colony ships” could experience a jump similar to the size of computing hardware from 1960 to 2006.

 

An example of a potential colony ship using artificial-to-biological transfer technology might be a variant of MASER or LASER –propelled ”starwisp”, massing well less than a kilogram and traveling at .2 c.

 

I have a hunch that the same psychological and economic factors that have, in the past half century, produce a world containing millions of small computers, rather than the half-dozen or so massive ones knowledgeable mid-20th futurists expected, and proven the success of system based on economic return-on-investment models based on less than 10-year cycles, rather than the centuries turn-of-the-20th-century economists expected, is likely to have a similar effect on “manned” space travel. I expect the hardware of space colonization will be much smaller than all but a few pre-1980s futurists expected.

[Jay-qu]

I dont see much point in a generation ship. It would take a very long time in design and building and even longer on its voyage. It could take hundreds of years just to get to the next star system (and there might not even be anything there worth having a look at).

In the time taken to get such a long way, back here on earth we could have come up with a new method of space travel, one that will not take hundreds of years but perhaps only tens of years. Which would be able to 'overtake' the old generation ship and even though it set out years before the new ship would beat it there... This all hinges on us creating a new method of space travel though.

[TheBigDog]

I see no poinnt in waiting. When it can be done it will be done. If a ship can be overtaken by newer technology before it arrives so be it. From a species survival point of view it is an imperative that we propogate to other planets since we KNOW that our own planet will ultimatly lose its ability to support life.

 

Bill

[Jay-qu]

But it should at least be dont when it is feasible, sure there is probably off the shelf tech that could get us there, but it would be very expensive and take a lot longer than waiting for our tech to advance just that bit more.

[CraigD]

I dont see much point in a generation ship.

I see no poinnt in waiting. When it can be done it will be done. If a ship can be overtaken by newer technology before it arrives so be it. From a species survival point of view it is an imperative that we propogate to other planets since we KNOW that our own planet will ultimatly lose its ability to support life.

I agree with TBD.

 

If a generation ship is rendered obsolete and irrelevant by subsequent technology, all the better. But risk analysis leads us to consider a significant possibility that any implementation of a spaceflight technology may by our race’s last. Natural catastrophe, or self-annihilation, the end result when “all our eggs are in one basket/planet” is the same – a sadly premature end of our promising kind.

 

Here the question passes beyond engineering, and beyond risk analysis, into the realm of politics and psychology. How does one go about successfully promoting any space colonization program, let alone one as ambitious as a generation ship, when the political reality is that such programs are appear to be beyond the imagination of the individuals and institutions who can deliver the needed resources?

 

A lot of very bright and hard-working folks have considered this question, and come up with a myriad of answers. So far, none have successfully inspired government or private backing sufficient to produce credible progress. With the possible exception of China’s, government-backed space programs are in decline. The private space industry is a scene of small financial carnage, where Darwinian-like selection seems to be optimizing for the ability to deliver satellites to orbits from LEO to GEO only.

 

In a society increasingly dominated by disinterested financial investment, how does one promise return on investment for space-colonizing ventures?

 

How does one sell space :)