Space propulsion

[Crayz9000]

What you'd use, instead, would be small parasite craft. Call them shuttles or boats or pinnaces or landing craft, whatever. You'd ideally make them lifting bodies, so you can get them into the atmosphere and landed while expending a minimum of fuel to do so. They would be powered by some sort of fancy chemical rocket or jet engine/rocket hybrid with chemical/gas thrusters for orientation in space.

A good model to base such a lifting body on would probably be the "Dyna-Soar" project.

[Patrick Degan]

The other problem with it is that the warp can only propagate at c. Sure, once you've got it established from A to B, you'll get there really really fast. But you'll be waiting an awfully long time for that transitway to open.

Er, no. I'm not sure what you're referring to, but if you're thinking of gravitational waves, then it's a result of either linearized GTR or a shortwave approximation. In general, large disturbances won't have to be particularly wavelike in the first place because GTR is not linear, and there's a substantial difficulty of even defining their speed unless there is a particularly nice background. That's not something unique to gravitational waves--even if one wants to do something simple like comparing momentum four-vectors "here" vs. "there", then one runs into the problem of these vectors being in different tangent spaces, with no globally consistent (coordinate and path independent) way of transporting them.

At this point, I couldn't say what sense that was explained either. The objection was in one of Lawrence Krauss' books (I believe), by which he was pointing out that even if you had the requisite energy and exotic matter to make an Alcubierrie drive work, you still couldn't simply take the next starliner to Rigel and expect to get back for that Tuesday appointment after your trip. Unfortunately, as both of Prof. Krauss' books that I had were lost in the New Orleans flood, I cannot now refer back to the passage in question.

[Admiral Valdemar]

At that point, lone hydrogen impacts will do substantial damage to the ship and the cosmic background radiation will turn into a blaze from the front. The Lorentz gamma is γ ≅ 2e4, 5e8, 1e13 for one, two, and three decades, respectively. At three decades, a hydrogen atom would impact with the kinetic energy of 2e3J, if disbelief in the fact that the ship ever made it that far is suspended.

That is why by the time we get to that point, we'd have conical hulls with such a dense armour that such impacts would be suffered for a few years before the shield needed replacing. That, or a combination of such a shield with a liquid droplet element too, like the cooling system proposed in the Charles Pellegrino and Jim Powell "Valkyrie" interstellar craft. I'm confident that by the time we get to starships that can cross such distances, we'll have the powerplant and engine to go with the hull or EM shield to help deflect hydrogen and other particle impacts.

At least the bubble we're in works to our advantage in this respect with less hydrogen to bug us.

[Major Maxillary]

ANd what of using the tenth dimension to travel?

[Kuroneko]

At this point, I couldn't say what sense that was explained either. The objection was in one of Lawrence Krauss' books (I believe), by which he was pointing out that even if you had the requisite energy and exotic matter to make an Alcubierrie drive work, you still couldn't simply take the next starliner to Rigel and expect to get back for that Tuesday appointment after your trip.

That's not correct. The Alcubierre metric is explicitly constructed so that it is Minkowski outside a small radius. Since the bubble is so localized, it's simply a matter of assembling it, assuming of course that's possible in the first place. Are you sure he wasn't considering some proposal to construct Kransikov tubes instead? That would make a bit more sense.

That is why by the time we get to that point, we'd have conical hulls with such a dense armour that such impacts would be suffered for a few years before the shield needed replacing.

At the rather low density of ρ = 1H/cm³, at t ≅ 10a, the hydrogen impacts on the ship average at about 7e5W/m², in addition to the 2e4K cosmic background radiation (from the front, since it will no longer be isotropic). This is not at all realistic for any hull materials except unobtanium.

I'm confident that by the time we get to starships that can cross such distances, we'll have the powerplant and engine to go with the hull or EM shield to help deflect hydrogen and other particle impacts.

How would that work against neutral particles, e.g., cold interstellar matter?

[Arrow]

That's not correct. The Alcubierre metric is explicitly constructed so that it is Minkowski outside a small radius. Since the bubble is so localized, it's simply a matter of assembling it, assuming of course that's possible in the first place. Are you sure he wasn't considering some proposal to construct Kransikov tubes instead? That would make a bit more sense.

So, translating for those with a rudimentary knowledge of physics, the Alcubierre warp bubble can move at FTL speeds?

Also, assuming a civilization has the technology to make an Alcubierre drive, which requires negative energy, would it also be correct to assume that they could build some sort of postive-negative energy disjunction drive that is effectively a reactionless space drive?

[Patrick Degan]

At this point, I couldn't say what sense that was explained either. The objection was in one of Lawrence Krauss' books (I believe), by which he was pointing out that even if you had the requisite energy and exotic matter to make an Alcubierrie drive work, you still couldn't simply take the next starliner to Rigel and expect to get back for that Tuesday appointment after your trip.

That's not correct. The Alcubierre metric is explicitly constructed so that it is Minkowski outside a small radius. Since the bubble is so localized, it's simply a matter of assembling it, assuming of course that's possible in the first place. Are you sure he wasn't considering some proposal to construct Kransikov tubes instead? That would make a bit more sense.

Again, I really could not say. It's been a few years since I read either book and now they are lost from my collection so I can't even double-check to be certain whether or not I read him right.

[Kuroneko]

So, translating for those with a rudimentary knowledge of physics, the Alcubierre warp bubble can move at FTL speeds?

Yes, but such bubbles don't appear to be physically possible or controllable even if possible.

Also, assuming a civilization has the technology to make an Alcubierre drive, which requires negative energy, would it also be correct to assume that they could build some sort of postive-negative energy disjunction drive that is effectively a reactionless space drive?

Given that the Alcubierre drive is itself a reactionless space drive, absolutely. However, the matter distribution in the FTL bubble is not physical, i.e., is not compatible with any known theory of matter.

[Knife]

The M2P2 system would be a good start, though like other solar sails and ion drives, it really doesn't have the acceleration one would want, though once up to speed, it could be very impressive.

Though, I guess you could use a plasma engine for a quick sprint, then dial it down for the micromagnetisphere.

[Admiral Valdemar]

At the rather low density of ρ = 1H/cm³, at t ≅ 10a, the hydrogen impacts on the ship average at about 7e5W/m², in addition to the 2e4K cosmic background radiation (from the front, since it will no longer be isotropic). This is not at all realistic for any hull materials except unobtanium.

The shield wouldn't take the full brunt, just be there for any unexpected larger grains that may impact over time. The hydrogen would be better used by ionising it and using it as reaction mass in something like a RAIR Bussard ramjet. You then at least don't need the mass of shielding you'd require if you were going head on into a few years worth of hydrogen.

How would that work against neutral particles, e.g., cold interstellar matter?

Ionising particles with a laser upfront would be the way to do that, though the idea of using a droplet shield would help too, perhaps even with regards to heat dissipation from whatever drive you use too. For larger obstacles, I would think having solar sails fly infront of the ship would help take care of all but the largest, highest energy chunks of matter.

If I had a crystal ball, it'd help, but these are ideas I've seen discussed on sci.physics and science and sci-fi boards around the web. We're a long way off even bothering to deal with such problems until we get a true interstellar drive anyway, given the megatonnes of fuel and reaction mass we'd need for even an AM torch vessel to reach another system.

[Gigaliel]

The other problem with it is that the warp can only propagate at c. Sure, once you've got it established from A to B, you'll get there really really fast. But you'll be waiting an awfully long time for that transitway to open.

Er, no. I'm not sure what you're referring to, but if you're thinking of gravitational waves, then it's a result of either linearized GTR or a shortwave approximation. In general, large disturbances won't have to be particularly wavelike in the first place because GTR is not linear, and there's a substantial difficulty of even defining their speed unless there is a particularly nice background. That's not something unique to gravitational waves--even if one wants to do something simple like comparing momentum four-vectors "here" vs. "there", then one runs into the problem of these vectors being in different tangent spaces, with no globally consistent (coordinate and path independent) way of transporting them.

I believe he is actually refering to Krasnikov tubes which also use exotic matter to warp spacetime, but only allow FTL travel on the return trip. Of course, the return trip also involves travelling 'back' in time so that Earth would only experience as much time as the astronaut did. Certain geometries still allow time travel, though.