Interesting, but I never learned greek 
Not exactly. After you chuck the nuke out in front of the ship, it will be moving relative to the ship, so actually, there will be NO difference if its in the flight path or behind it.
Hmmmmm… That does make a certain amount of sense. I’m not entirely convinced, however. When you detonate a bomb behind you, you are accelerating away from the blast (riding the shock wave, if you will). When you detonate a bomb in front of you, however, you may slow down but you are definitely not travelling away from the blast (you are still travelling forward, just not as rapidly). This would be different, of course, if a single blast were enough to stop you in your tracks and cause you to reverse direction, but we are talking about using multiple blasts to slow you down.
Of course, I could be wrong…
Barry
BARRY–
The two situations are precisely symmetrical. Replace the nuclear blast with a baseball pitcher (in a spacesuit).
Case #1: you eject the pitcher with a certain given force, in the aftwards direction. So he’s behind the ship. He pitches a ball towards the ship (which will rebound against its antibaseball shield). He pitches with his full muscular force, and the ball has a certain velocity relative to his hand when it leaves his hand, which we will call H1. But because the pitcher is moving steadily away from his target aftwards, with a velocity of P1, you have to subtract the latter from the former to get the velocity of the ball relative to the ship, S1. Thus: S1= H1 - P1
Case #2: Now you eject the pitcher with a certain given force, in the FOREwards direction. So he’s in front of the ship. He pitches a ball towards the ship (which will rebound against its FOREWARD antibaseball shield). He pitches with his full muscular force, and the ball has a certain velocity relative to his hand when it leaves his hand, which we will call H2. But because the pitcher is moving steadily away from his target FOREwards, with a velocity of P2, you have to subtract the latter from the former to get the velocity of the ball relative to the ship, S2. Thus: S2= H2 - P2
But H1 = H2 ; and P1 = P2 ; therefore S1 = S2.
By Newtonian/Galilean kinetics, it is as if the ship were standing still. The speedup and slowdown forces are exactly the same, just oppositely directed.
If the ship is engaged in a continuous acceleration, some details change, but not in the way you are thinking. The push-from-behind will be somewhat diminished (lessening the resultant accelerative increment, forward-directed) while the push-from-ahead will be somewhat increased, because the ship is accelerating forward to meet the ball, producing a more forceful rebound (increasing the resultant accelerative increment, aftward-directed). In terms of the overall energy added to, or subtracted from, the ship, these effects cancel, and the energies are equal.
QUESTION TO ALL:
Let’s say we have an Orion-type vehicle, using nuclear explosions to accelerate and decelerate the ship. It has been mentioned that the ship increases in mass as it accelerates, and leading some to conclude that the blasts will be less and less efficient. But isn’t the mass of the fissile (or fusile) fuel increasing in the same proportion? Doesn’t that mean that the energy-output from the explosions will increase correspondingly?
AND AN OBSERVATION:
Does everybody understand that you don’t have to have a fixed laser and a reflecting “sail” to produce acceleration? The laser produces its own thrust, acting upon itself–like the recoil of a gun. The most efficient way to use lasers for propulsion is to take them with you–attach them to the ship and aim them backwards–“laser rockets” in other words.
It’s purely an engineering problem to assemble a big enough laser array to propel itself, and its power source (a nuclear reactor), and its payload at a rate of 1 G. Given the funding and the will, we could do it with present technology.
Another thing to think about if you’re going to be using an Orion, is that it’s mass will be decreasing (slightly, of course) as it chucks a nuke out the back. While I doubt that those decreases will counteract the relativistic mass increases, there will be a drop in mass and over time those elements will add up.
Because non of this stuff has been invented yet (or has been invented but is nowhere near practical for what you describe - ie railguns)
I thought that the ion engine testbed Deep Space 1 (launched in 1998) was the fastest thing that humans have ever launched. If I am forgetting something, please remind me what it is.
[Fusile? Haven’t heard of that type of fuel 
] Only if you have an engine that is 100% efficient at turning mass into energy. If your engine turns 50% of the fuel mass into energy, then the other 50% is mass that you’ve accelerated just so you can throw it away. As the mission proceeds, the amount of energy that you spend accelerating fuel that you’re throwing away increases. Also, keep in mind that in SR, velocities don’t add linearly. If your engine is providing constant acceleration in your reference frame, then in Earth’s reference frame it will be providing steadily decreasing acceleration, due to relativistic effects.
Yes, of course you can take the laser with you, but that defeats the purpose. The whole point is that with a laser, you can not only leave it at home, but you can leave the fuel there too. You can eliminate the extra mass of the whole laser/power source/fuel section of the ship, and you don’t have to worry about the accelerating-fuel-you’re-throwing-away problem I discussed earlier.
PS: there’s no such word as “foreward”
Hmmmm… Another question; when people talk about ‘critical mass’ of an isotope, do they actually mean ‘critical molarity’? - it’s all to do with the probability of particles colliding within a given volume, isn’t it? - what I am asking is… the increased mass due to travelling at a significant percentage of c - that increased mass wouldn’t cause our onboard bombs to go critical, would it?
It’s even closer than that: Alpha Centauri is only 4.3 light-years away.
If the sun goes “nova”, then every astronomy textbook and peer-reviewed journal article written about novae in the last century will have to be thrown out.
The only novae we’ve ever observed involve a white dwarf sucking material off of (“accreting” from) another star that it’s in close orbit around. As material accretes onto the white dwarf’s surface, it piles on top of earlier accretions, packing itself tighter and tighter under its own weight, until the pressure and temperature get so high that thermonuclear fusion takes place. This burst of thermoculear activity “burns up” the white dwarf’s outer layers very very rapidly, releasing an enormous amount of energy. That’s what we see when we see a “nova.” (This should not be confused with a supernova, which is a different phenomenon entirely.)
The sun isn’t a white dwarf, and doesn’t have a white dwarf companion, so it will never ever go nova. The sun is also too small to go supernova. It will, however, bloat up into a red giant at the end of its main-sequence lifetime, which we currently estimate will happen in about 5 billion years or so. However, even if the sun started turning into a red giant tomorrow, we wouldn’t have to worry for a while. It takes tens of thousands of years for a main-sequence star to become a red giant, not merely a few hundred years.
The atoms within a piece of radioactive material are atrest with respect to one another, so as far as measurements taken onboard the spacecraft are concerned, they wouldn’t increase in mass. So it’s a moot point.
With an Orion spacecraft, this isn’t really an issue, because it’ll be hard as heck to get it up to relativistic speeds to begin with. Most realistic Orion plans call for a spacecraft that will “cruise” at 0.1c, which is barely at the lowermost detectable limit of relativistic effects. If you wanted your Orion space ship to go 0.5c, you’d have to carry nearly 80 times your spacecraft’s own weight in thermonuclear bombs (and if you wanted to slow back down to a stop after you’d achieved 0.5c, you’d have to carry over 6000 times your spaceship’s own weight in thermonuclear bombs!).
Tracer:
It ain’t that bad. You forgot that we can slingshot around the Sun and Jupiter.
Sadly, I don’t think that we’d get much of a boost from that. A thread I started on the Bad Astronomy Message Board discusses slingshots for interstellar travel, briefly.
Suppose that it becomes possible to cryogenically freeze someone and revive them relatively easily. I don’t mean we put them into a state of hibernation, but we find a way to freeze them, without ice crystals destroying their cells, to the point that they are literally a “humansicle,” and later revive them with no ill-effects. Would the “humansicles” be able to withstand higher gees than a live human? Or would there be problems with their bodies shattering under high gee acceleration?
TUCKERFAN, high G’s will almost certainly be the least of one’s concerns on an interstellar mission. Except in the case of a continously accellerating ramscoop-type ship, most interstellar journeys will involve coasting for years, decades or centuries. There will be engineering trade-offs that determine the optimum accelleration, but it will probably be very low, like a planetary mission done with ion engines.
The technology for the main drive has already been in use for decades, its the same tech used for supercolliders but in a much larger scale.
As for the tracking system there already is an earth based tracking system that can track debri in earth orbit down to IIRC baseball sizes. This tech can be refined and improved and in space should be able to detech small enough particles.
Great improvments have already happened in this field, thier already testing a military weapon based on a railgun. On the moon, a railgun would not need to be very powerful. It probably wont need to be much more powerfull than the Superman ride at sixflags.
The AI tech is already here, it would just be a matter of time to do all the work neccesary to complete it.
Suspended animation tech is pregressing REALLY fast as well. IT won’t be much longer until it is working. Plus they wouldn’t need to kill and freeze the people, merely lower the temperture so the heart rate can go down to once per minute or so. Plus I said this would be after the first SOL orbit of the ship.
Finally, I figure it would take something on the order of 50-100 years to complete this project. Plus we have the technology to start the project right now, and all the other tech advances will come either from research or the actual construction of the Ship.
Tracer, notice that I said what if and IIRC in my post.
There’s a famous sci-fi novel about a huge ramjet-type spaceship that circumnavigates the entire universe (and even survives the destruction of the universe!). Indeed, it’s so famous that the name and author escape me.
It’s not very old, and I understood it to be based on sound scientific principle. You good folks are telling me it wasn’t? What was the author’s error?
PS–
As “aft” is the opposite of “fore,” then “aftward” means “toward the aft of the ship” and “foreward” means “toward the fore of the ship.” You’d rather use sternward and prowward?
I believe that the word “mass” in that context refers to rest mass. Physicists have changed what they mean by “mass” in response to relativity; mass used to be the force divided by the acceleration, but then Einstein showed that that varies with speed, so “mass” was redifined to be equal to what used to be called rest mass.
Furthermore, “critical mass” is the minimum that could cause a chain reaction; just because you have critical mass, that doesn’t mean that you will have a chain reaction. Putting the material in a configuration with high surface area or mixing in another material that absorbs neutrons are both ways to keep a critical mass from going into a chain reaction.
tracer
Well, assuming of course that they aren’t destroyed in the nova. 
Tuckerfan
One of the first things that starts to wrong when a human is put under high g’s is that the blood no longer circulates properly, so eliminating the need for blood circulation would help, at until the next bottleneck.
Scott Dickerson
I spoke with excess haste. I tried to look it up on dictionary.com, but it crashed Explorer. While MS Word doesn’t recognize the word, a second try at dictionary.com yields
So it is a word, but is obsolete and does not mean what you think it means.
It’s rather misleading to think of it as an “increase in mass”. That’s really only a useful illustration of how a given force has less effect at larger fractions of c.
Strictly, it is the unitless ratio gamma which increases, not the mass m, and this quantity is more a chracteristic of the “spacetime” a body is travelling in than the body itself. Therefore, the proposed consequences of an actual increase in mass (criticality threshold, more available energy from E=mc^2 etc.) would not occur.
As for the space laser, it would indeed work better on the ship itself since it would be sread out due to the Uncertainty Principle. Imagine looking full on along the beam, as though you were using it on a sniper rifle. At any point a photon’s position must be inside the beam (*ie.*its position is accurately known). However, this would mean that no photon had any component of its momentum perpendicular to the beam, and hence accurate knowledge of its momentum. Since both cannot be known to such accuracy simultaneously, it would spead out rendering it useless for targeting a minute fraction of an arc-second in the night sky.
I find the whole photon sail concept to be a little ridiculous. No one’s taking laws of motion into account, which would describe and equal and opposite accelerating force. Technicalities overcome, all you’re left with is a very lame and inefficent means of propulsion. folks aren’t thinking hard enough.
As for the second issue - yes, there’s a launch vehicle in testing using microwaves to boost from the ground. Pity any birds flying through the stream, thgouh (assuming they get higher thean 30’.