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Reythia Profile
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Knight of Honor

Registered: 11-2005
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Re: "We Aimed for the Stars... Until We Stopped"


If I recall (and while I know trajectories, I'm not an expert in the subject of propulsion, so correct me if I'm wrong), there are three main problems with nuclear pulse propulsion.

1.) The Technical: The shielding from the nuclear radiation has not been fully tested, and no one really knows what to do with the leftover nuclear material when done. The first of these is probably fixable within a reasonable amount of time (and may already have been through preliminary testing, for all I know), so it's not much of an issue. The later is more serious. Then again, if you're out in space already, there's always the possibility of flinging the "leftovers" into the sun where they won't do any harm. So, at least once you're off Earth, that's not too much of a problem either.

2.) The Sociopolitical: Right now, we have to struggle mightily even to use the small nuclear "batteries" that the Voyager probes had for use on unmanned missions. There's no technical reason for this, any more than there's a technical reason against building nuclear power plants. But there's a social -- and thus political -- impression that anything with the word "nuclear" in it is inherently unsafe. Until this changes, I don't see us powering manned missions with nuclear power.

3.) The Practical: Though plans and theories about some form of NPP have been around for 40-50 years, as far as I know, no one has actually built a working model. At least, not a full-sized one. This isn't to say it'd be impossible to do, but it's a note that it would be difficult to do the first time, and that it almost certainly wouldn't work right the first time. (Murphy's law holds for engineers as much as anyone else!)

In short, NPP is an interesting concept and deserves more technical study, but we have excellent practical reasons not to want to do the full testing HERE. Moreover, between those practical reasons and an overwhelming fear of anything nuclear, it's hard to imagine any significant testbeds being built at the moment.

Finally, I have one question: does anyone know what would happen to a manned spacecraft moving at 0.1c if it hit a micrometeor? I mean a little tiny grain of something, not a giant rock. These cause headaches for the ISS and slightly larger ones can break or damage satellites, I know. I can only think that moving so much faster would make the damage worse (yea for inertia and momentum!). Does anyone know if we have the design capability to withstand multiple small punctures at 0.1c or even a significant fraction thereof?

(This, by the way, has been a topic of discussion when talking about much slower manned missions to Mars, too.)

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2/5/2009, 6:58 pm Link to this post Email Reythia   PM Reythia AIM MSN
 
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Shepherd

Registered: 03-2006
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Re: "We Aimed for the Stars... Until We Stopped"


Damage wise micrometeorites at that speed could be extremely destructive, the closest I can think of and it falls far short is imagining the damage a railgun can inflict. Of course speeds are much higher and mass much lower, still, typically what objects like that do is they tend to just shoot straight through the entire unit and come out the other side, blowing a small hole through it. So an easy solution I guess, if that assumption is correct is to build some redundancy in your designs. There are some ideas for fully withstanding micrometeorites as well, but they tend to be very mass heavy. Luckily in interstellar space your chance of running in to one is rather minimal (even for the entire trips duration), so it hasn't really been one of the larger focuses of research, most just take it as a small extra risk factor.

On a side note, the fear of NPP drives, has led to research in a sub field of it, typically the fusion pulse propulsion and antimatter catlysed/induced pulse propulsion systems. Which avoid the typical nuclear dangers by not using actual nuclear weapons, but using far more sophisticated and light solutions. Major advantages are ofcourse no nuclear proliferation threats, generally not the typical radiation dangers and being able to efficiently propel considerably smaller crafts. (A problem with early NPP as nukes don't tend to come under 0.1 kt yields and at that yield is very inefficient)
The big drawback of these methods is, is that they require much much much more sophisticated technology to get to work, which is why they are for now still theoretical drives which should work in principle, but have never had real world testing. Also developing such a drive system is probably pretty expensive and time consuming, so you'd need some fairly dedicated research to the problem. And well yeah, I'm not sure who'd be giving funding like that to any such theoretical drive system any time soon. There isn't enough money for a lot of other good ideas either after all, so something a bit out there like this isn't going anywhere fast at all.
2/5/2009, 8:44 pm Link to this post Email QS2   PM QS2
 
BaneBlade Profile
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Shepherd

Registered: 02-2006
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Re: "We Aimed for the Stars... Until We Stopped"


quote:

Reythia wrote:

  Does anyone know if we have the design capability to withstand multiple small punctures at 0.1c or even a significant fraction thereof?




I did some quick conversions because most of my engineering data deals with projectiles in feet per second or meters per second...and ignored some factors such as composition of the projectile, it's velocity and speed... and determined that .1c is really fast! 3/4" inch of exterior face tempered steel will fail when impacted by another piece of steel roughly the size of your pinky moving under 3000 fps. When you get into millions of feet per second...

I reckon at that kind of velocity even modern composite or reactive armoring is going to be insufficient, and they do not handle multiple impacts in the same place very well.

Of course you're talking about scales and velocities that are far beyond my ken.

I am glad to know that tossing nuclear waste at good ole Sol is reasonable. I used it once in a story and always wondered if I was crazy.

Last edited by BaneBlade, 2/5/2009, 8:56 pm


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2/5/2009, 8:55 pm Link to this post Email BaneBlade   PM BaneBlade
 
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Knight of Honor

Registered: 11-2005
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Re: "We Aimed for the Stars... Until We Stopped"


And on a tangent to the subject...

...my satellite (GRACE -- the Gravity Recovery and Climate Experiment) seems to have just been hit by a micrometeor! Not kidding. We just got the level 1B data today (or maybe yesterday) and realized that something funny happened on Jan 24th. As best we can tell, we got a sudden spike of external acceleration from absolutely nothing. Here are some of the pictures (linear and angular accelerations based on our super-sensitive accelerometers):

Image
Image

Fortunately, there seems to have been no damage and not even any interruption in our science data collection. (Since we know the acceleration, we can subtract it during our state calculations. We have to do this all the time for things like atmospheric drag, which is why the accelerometers are there in the first place).

Anyhow, micrometeors DO happen! (Though admittedly, much more often in LEO than in open space!)

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Apprentice

Registered: 09-2007
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Re: "We Aimed for the Stars... Until We Stopped"


quote:

Reythia wrote:

In short, NPP is an interesting concept and deserves more technical study, but we have excellent practical reasons not to want to do the full testing HERE. Moreover, between those practical reasons and an overwhelming fear of anything nuclear, it's hard to imagine any significant testbeds being built at the moment.



Well, no, not here. Wouldn't it make more sense to build it in space, use a conventional drive to get it very far away (say about 300,000 miles), then test the NPP anyway?

And of course you're right; it isn't going to happen. I'm just noting that it should be within our current technological capabilities to do it, although the need to build it in space is another factor that might make it extremely difficult.

Now, keep in mind that the reason I say this is because Carl Sagan said it's very "practical from an engineering point of view." You're all contributing a lot of information that he did not have in his book, for whatever reason (like maybe he didn't know...?) that sheds more light on the subject.

Last edited by Michael58, 2/6/2009, 2:40 am


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Squire

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Re: "We Aimed for the Stars... Until We Stopped"


quote:

Reythia wrote:
Finally, I have one question: does anyone know what would happen to a manned spacecraft moving at 0.1c if it hit a micrometeor?



Kinetic energy = 0.5MV^2

Say a 0.2 gram grain of dust (look at a diamond ring to see how big 0.2g is -- it's one carat) at 3*10^7 metres per second (10% speed of light).

0.5 * 0.0002 * 30000000 * 30000000 = 90,000 megajoules.

That's roughly equivalent to 20 tons of TNT exploding. That's actually pretty small. A tiny faction of the Hiroshima bomb or the Tunguska explosion, for example.

Still, I wouldn't want to be too close to it.

Except... it's not going to give up all that energy to the spaceship. It's going to give up a tiny amount, enough to punch a 1-carat sized hole in the hull, barely slowing down, and keep going. (Plus make an exit hole, etc.) No matter how fast it is going, its small diameter is going to limit the amount of damage it can do because it's not wide enough to make a wide hole.

Gun designers hit the same problem. As they made rifle bullets faster, they should have been able to also make them smaller because they would still have the same kinetic energy and therefore the same killing power. If you can shrink a 7.26 (mm diameter) bullet to 5.56 and still deliver the same kinetic energy, that's a good thing because your soldiers can carry more of a smaller bullet. But they found the new, fast 5.56 rounds were not doing the same damage to targets, because they made a smaller hole in a person and they were fast enough to come right out the other side again. That's why the NATO 5.56 round is designed to lose stability and tumble after it hits a person. Tumbling slows it down so there's more chance of it staying in the body instead of flying cleanly out again, and also the tumbling action means it imacts more flesh on the way through, thus simulating the killing power of a slower but fatter bullet like the old 7.62.

So speed isn't everything. It's what you do with it.

Though there is also the factor of supersonic shock waves from faster bullets, which are bad for flesh, but I don't know how they would affect a metal space ship hull.



Last edited by David Meadows, 2/6/2009, 2:12 pm


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BaneBlade Profile
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Shepherd

Registered: 02-2006
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Re: "We Aimed for the Stars... Until We Stopped"


Reythia: That's really cool. I sat around and played with that data until my head was full of questions. emoticon Thanks!

David: Your example using bullets is right for the most part. In fact I'm pretty impressed at how much you know. emoticon

There are only a few things that were wrong or slightly inaccurate. I deal with these kinds of ballistics constantly, as opposed to questions about objects moving at 0.1 c! emoticon There are too many factors with those kinds of speed that boggle my mind. I actually started to wonder if the projectile entered the cabin, would the heat from the friction between the projectile and the cabin atmosphere be enough to ignite the O2, at those immense speeds. LOL

If you're interested in some clarifications on external and terminal ballistics regarding bullets I'll start a new thread in the private forums, as it's pretty off topic for this thread, and I'm not sure how much of that information I want on an open forum. I have alot of links to research data, and xls spreadsheets for calculating ballistics in those ranges. I actually used some of those spreadsheets combined with engineering data available at work on steel failure for the previous example.

(And here I should note that I cheated and used a projectile with a high ballistic coefficient [because I don't have data for projectiles in a vacuum], a shape designed to penetrate, landing in a manner that imparts the greatest energy into the smallest area, striking at a perpendicular angle [more obtuse or acute is less likely to cause failure in the recieving medium], etc. Basically a worst case scenario, but then again with the speeds we're discussing most of that becomes superflous.)





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2/6/2009, 5:34 pm Link to this post Email BaneBlade   PM BaneBlade
 
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Squire

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Re: "We Aimed for the Stars... Until We Stopped"


quote:

BaneBlade wrote:
If you're interested in some clarifications on external and terminal ballistics regarding bullets I'll start a new thread in the private forums, as it's pretty off topic for this thread, and I'm not sure how much of that information I want on an open forum. I have alot of links to research data, and xls spreadsheets for calculating ballistics in those ranges. I actually used some of those spreadsheets combined with engineering data available at work on steel failure for the previous example.



I would be very interested emoticon But not if it would mean you releasing confidential data, as it's not fair to put you in that position...



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Squire

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Re: "We Aimed for the Stars... Until We Stopped"


Moving away from the science, to the science fiction...

Okay, so I've heard lasers aren't practical in space but if you were getting lightspeed or close to it from your ships and your ship was being chased by another ship couldn't you just jettison lots of tiny particles into its path? I mean, things like sand or something? At those speeds, with enough sand, wouldn't it essentially shred the oncoming spaceship?

Interesting thought...

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2/8/2009, 12:33 am Link to this post Email Blitzen   PM Blitzen
 
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Shepherd

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Re: "We Aimed for the Stars... Until We Stopped"


Ok.... Lasers not practical in space? Why ever not? Also posting the reply in the energy weapon thread might be more appropriate. emoticon
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BaneBlade Profile
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Shepherd

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Re: "We Aimed for the Stars... Until We Stopped"


quote:

David Meadows wrote:


I would be very interested emoticon But not if it would mean you releasing confidential data, as it's not fair to put you in that position...




Heh, I was thinking more along the lines of releasing personal information.

However I made the mistake of peeking at Wikipedia and found that they have a small section on Terminal Ballistics. Stopping Power

Take a peek at the section called "Other hypotheses of stopping power." You can see the issue is still being debated.

Another theory that the article doesn't mention is that it could be more effective for the round to exit the body. This provides two points for fluid loss, and exit wounds are generally larger than entry wounds. (Imagine the bullets energy expanding forward in a cone shape. The same effect can be seen on a chip on a windshield. The point of contact is smaller than the point of termination.) In addition this would create a longer wound channel than one which halts within the body.

I suppose the ideal bullet that would satisfy all of the varying hypotheses would be one that just barely exits the body, so that it imparts almost all of it's energy, while still creating an exit wound. At the same time we would want to give this hypothetical bullet a large diameter, a long width (for the purpose of increasing it's weight), and high velocity. In actual practice this is harder than it sounds, as we would be bound by the size and capacity of the case, the available propellants, and the need to impart good external ballistics (things like, low drag ratio, ballistic coefficient, etc. that affect range, accuracy, velocity etc.)

What the article does not cover specifically is why the newer 5.56x45 NATO bullets are designed to tumble.

"All FMJ bullets with tapered noses will tumble in flesh with enough velocity, because their center of gravity is aft of their length center--causing them to want to travel "tail first" in denser mediums (like water and tissue)." That includes 7.62 rounds.

The real goal was to create a round that would retain enough velocity that when it began tumbling it would fragment.

"If the rounds are moving fast enough when they yaw to about 90 degrees of their original trajectory the stress on the bullet from traveling sideways through a dense medium (tissue) will overcome the structural integrity of the bullet and it will start to break up."

This "breaking up" is fairly dramatic and the fragments tend to spread out in a cone shape, each creating it's own wound channel. Unfortunately bullets yaw and fragment only within certain ranges/velocities, with these newer bullets the designers were trying to create one which would yaw and fragment across an increased spectrum of ranges/velocities.

The why: The Hague Convention banned the use of expanding bullets (like hollow-points) in warfare. (I'm still not sure why. They are probably safer to bystanders and less likely to ricochet than FMJ's) NATO members therefore do not use hollowpoint bullets. However a bullet which tumbles and then fails is not bound by that treaty. You see semantics are still very important. emoticon

There you have it in short. I decided to stay away from the external ballistics, as it was even more off topic. Sadly that's the part I find more fascinating as it involves more math and physics. This way is a bit less technical, so everyone can read it.




Last edited by BaneBlade, 2/9/2009, 4:20 pm


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