Krulle wrote:Absalom wrote:Krulle wrote:For long travel distances, if no FTL shortcut is found, photon drives will be the only viable solution. But they have a terrible low output, but by far the best propulsion/energy ratio.
Fusion is supposed to be decent if we can ever get a net-positive fusion reactor. There's also the old Orion Drive proposal, some variations of which are actually plasma/fusion drives themselves (MicroMagOrion, or something like that).
The problem is that the mass of the fuel is lost afterwards, and that we therefore need to put tremendous amounts of fuel on the ship. The distance (without FTL) is prohibitively large, the amount of fuel needed therefore tremendous, so much even, that most calculations I've seen so far suggest starting the ship, and sending the crew to it more than 10 years later, to catch up with the ship when it is somewhere near Jupiter for the first sling shot (or even catch it between Jupiter and the Sun for the big slingshot).
The amount of fuel will make the non-FTL interstellar ship so massive, that the acceleration will be near non-existant when it still has enough fuel to brake with 1
g when the ship has reached one of our closest neighbours.
If I was trying to launch an interstellar colonization mission with technology that we can currently be confident is possible, then I'd have two contrasting systems:
1) Near-cee probes. They'd be a solar sail, with a probe body contained at their center. I'd likely build a mirror array around Mercury to provide higher acceleration, but I haven't studied the subject enough to know if I would go all the way for a laser array in Mercury orbit. The first few would just go streaking through at full speed, but later ones might slow down, possibly even enough to stay in-system.
2) Assuming a colonization effort, I'd send a Daedalus. Maybe it would have Orion boosters, maybe it wouldn't. Assuming that the acceleration from everything else was going to be small enough, I would have the largest solar sail ever created by humanity connected as well. The ship, it should be noted, would be meaningfully larger than the actual Daedalus proposal: the ship would include a sufficiently large habitat structure to maintain a decently large biosphere. The actual drive would somewhat be separate from the habitat. The drive, sufficient support systems, and a worksite for the rest of the structure would be initially launched. After it had achieved sufficient speed, a separate Orion propelled ship would carry the crew & additional materials to the ship: due to the differing design criteria (indefinite sustenance for the Daedalus, though with a Daedalus's speed limitations presumable 100+ years, a few years max for the Orion) the crew vehicle should have a much higher acceleration, and thus the capability to catch up. Whether the Orion could make it back is a side issue: the entirety of it's crew can be the Daedalus crew if needed. Upon reaching the destination, the Daedalus will initially turn off the drive while leaving the Bussard scope on: it will act as a magnetic sail, thus slowing the ship better than should be expected from a solar sail (which doesn't mean that you shouldn't use one to slow down).
2b) If it's found to be appropriate, a second, unmanned Daedalus will be sent ahead of the first: one of the big hurdles of a Daedalus is that gathering the interstellar wind itself slows the ship, and most won't cross the path of the ship until it has long-since passed. By having a lower-mass ship perform the majority of the gathering action, a slightly higher maximum speed should be possible.
Mjolnir wrote:Krulle wrote:The problem is that the mass of the fuel is lost afterwards, and that we therefore need to put tremendous amounts of fuel on the ship. The distance (without FTL) is prohibitively large, the amount of fuel needed therefore tremendous, so much even, that most calculations I've seen so far suggest starting the ship, and sending the crew to it more than 10 years later, to catch up with the ship when it is somewhere near Jupiter for the first sling shot (or even catch it between Jupiter and the Sun for the big slingshot).
The amount of fuel will make the non-FTL interstellar ship so massive, that the acceleration will be near non-existant when it still has enough fuel to brake with 1g when the ship has reached one of our closest neighbours.
A Jupiter slingshot will add nothing significant to a ship on an interstellar trip...at most 13 km/s. Rendezvous with an outbound starship is probably not practical...the ship leaving later will need much more delta-v to catch up and then stop when it reaches the first ship.
I conditionally disagree. If the outbound ship has comparable specs to the crew ship then certainly it wouldn't be practical, but probably the ships will be fairly different. Both might use similar engines, but the crew transport is likely to be less the world trade center of the interstellar ship, and more a one or two story bungalow. The accelerations are likely to be very different, for the same reason that identical engines pushing 1 pound vs 10 pound loads will experience different accelerations.
Of course, in the case of my example, the weights would likely be more dissimilar, and the engines would likely be adjusted as well.
Mjolnir wrote:Unless you have the ability to convert most of your fuel to energy, the fact that the mass of expended fuel is ejected is a good thing.You don't want to keep hauling the end products of fusion around, and you may as well use the mass to produce thrust as you dump it overboard. This is the major problem with photon drives, there's no good way to fit the required energy aboard, apart from hauling huge amounts of antimatter around. Unless we work out a way to convert normal matter directly to energy with high efficiency, photon drives will require antimatter (or be limited to niche uses, such as things like fine positioning of components of an orbital interferometer).
Ultra-distant tech: black-hole photon drives. Theoretically 100% mass-to-energy conversion. I propose the name "Hawking drive".
Mjolnir wrote:The most practical form of photon propulsion is really the laser sail, which produces twice the thrust for a given amount of power, but more importantly, doesn't have to haul the power plant around. A high efficiency fusion drive would also have the performance required to reach nearby stars within a century or two. A combination using photon sail propulsion for departure and fusion for braking might be the most practical approach.
Or just a mag-sail, if you decided to use some or all of the fusion fuel in the acceleration stage.
Mjolnir wrote:icekatze wrote:Having a poor starting thrust is what staging is for. You have a booster stage at the start, and it decouples once it is spent.
That is the answer for most rockets. For an interstellar journey, you don't gain much...you're not really in a hurry to finish your burn, since you'll likely spend most of your time coasting anyway. The slow start might be unimpressive, but it's still momentum gained.
You might still need staging of some sort, but more for shedding excess structural mass that's no longer needed for carrying propellant and for shedding worn-out drive equipment, if you need anything that can't be repaired/rebuilt/recycled mid-flight.
Hmm. Maybe store the fuel as a hydrogen-heavy hydro-carbon, preferably solid & stable enough to be stored 'exposed', using the waste carbon from hydrogen removal as both propellant mass & ablative engine material? Probably not hydrogen-dense enough, though.
icekatze wrote:I would accept as an axiom that once a ship has reached escape velocity, thrust becomes vastly less important than efficiency, but while a spacecraft is still in orbit of Earth, having enough thrust to break away in a reasonable number of orbits can still be important. If not because of time considerations, then at least because of the probabilities of hitting space junk. Also, if a ship has an optimal escape window, then it is possible that spending a more significant amount of thrust during that window would be a good idea.
The Oberth effect is unlikely to be meaningfully helpful for this type of mission.
Mr.Tucker wrote:FTL is, in my humble opinion, probably possible. How it would be possible is however, hard to gleam at our present tech level.
I don't think that literal FTL is likely to be possible (unless we find a way to massively interfere with probability fields), but I expect it to be possible to "cheat" around it. After all, the very expansion of the universe + a thought experiment (supposing that you place two masses near each other, with a third connected to one of the initial two with a cable that won't snap & a generator, and the third is distant enough from the other two to measure cosmic inflation, will the generator produce energy?), suggests that inflation may have an involvement with energy, thus pointing to the possibility that spatial manipulation via energy field might be possible.
Mr.Tucker wrote:The only non-beamed power realistic concepts I've encountered so far (that have an actual shot at working IMHO) are the Daedalus probe, Peregrino's ''Valkyrie'' antimatter starship (that inspired the ISV ''Venture Star'' from Avatar) and the clever concept called the ''Firefly'' that came out of a study group under the Icarus Project.
I wouldn't want to actually use an anti-matter system if I could avoid it: seems to have anomalously high risk of failure to me (most fuels don't explode upon contact with their containers).
Firefly sounds interesting, but it might be best to be wary of where you point the X-Ray cannon that you've mistaken for an engine.
Mjolnir wrote:icekatze wrote:I would accept as an axiom that once a ship has reached escape velocity, thrust becomes vastly less important than efficiency, but while a spacecraft is still in orbit of Earth, having enough thrust to break away in a reasonable number of orbits can still be important. If not because of time considerations, then at least because of the probabilities of hitting space junk. Also, if a ship has an optimal escape window, then it is possible that spending a more significant amount of thrust during that window would be a good idea.
I doubt debris would really be a serious problem. If outgoing trajectories from LEO aren't safe, build it in higher orbit, perhaps at a Lagrangian point.
You probably should anyways. Materials from the Moon might be used in such a project, but I think it better to use lunar ores to construct asteroid mining equipment, and certainly you shouldn't use bulk materials from the Earth (at least, unless you're panicky and trying to evacuate the Earth).
Mjolnir wrote:As for optimal windows, you've got a mission delta-v of several percent of c at least. Earth orbits the sun at 0.0001c, the orbital mechanics are lost in rounding error.
If you use a Firefly then depending on the focus of the X-Ray beam, you might want to start from the L5 while Earth is moving towards Alpha Centauri so that you can increase your distance from the Earth when it first comes back into your exhaust cone.