WIP Discussion (Part 1!)

[Mjolnir]

As far as I'm aware, in the vacuum of space, an exhaust plume will look exactly the same regardless of orientation or current velocity. In the comic, the plumes sometimes bend to indicate change in trajectory, but I think that's more visual convenience than strictly realistic.

Like a big water sprinkler...but as high as the exhaust velocity is going to be, probably only visible on sensors at a scale where the ship is a pinprick.

One would have to be extremely close to the sun in order for solar wind to have a significant impact on the drive plume of a ship. At 1 AU, the pressure is measured in nanopascals.

And the particles which actually make a drive plume visible are extremely tiny and light.

As any tail of a comet will show you, the pressure of the solar wind at 2 AU is more than sufficient to blow such things away.

The drive plume of a ship accelerating at 40 g is not going to behave the same as a cloud of ionized gas being stripped away from a chunk of slowly sublimating ice.

[Absalom]

I've been looking through NASA articles about plasma, fusion, ion & other thrusters for their future spacecraft, does anyone have any opinions on which is going to be the thruster of the future? Anyone else geeking out over all of this amazing sci-fi space ship tech becoming reality?

Different drives for different purposes. Ion/Hall Effect/etc for distances & long-term station keeping. Plasma for high-thrust tasks, but only if you have a major power source: they're a decent choice if we need Earth-orbit space tugs. Fusion is firmly in the realm of future-tech: non-self-sustaining is just a revision of plasma, and self-sustaining is not possible with current technology. The EmDrive could be an interesting alternative to the Ion drive category, but the tests have always been on the edge of detection.

Has anyone seen any technological breakthroughs concerning artificial gravity, inertial dampeners, or radiation shielding? You know, the technologies that would allow us to explore the solar system unhindered by the current hurdles.

For charged particles you can always play with magnetic fields, but otherwise your best bet is hydrocarbons (or water, if you don't mind hideously expensive launches). Artificial gravity and inertial dampeners will hopefully become available via manipulation of the Higgs field, but that's so far beyond future-tech that it's daydream-tech.

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).

Fusion is an energy creation process, not a drive system. The ions, plasma, ... can be created using fusion...

"Leaky" fusion is a suggested form of plasma drive.

[Arioch]

Fusion is an energy creation process, not a drive system. The ions, plasma, ... can be created using fusion...

"Leaky" fusion is a suggested form of plasma drive.

That's the drive used by the Moties. "They're just enclosing the hydrogen, fusing it and blasting it out. A plasma bottle."

[Krulle]

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 1g when the ship has reached one of our closest neighbours.

That's the drive used by the Moties. "They're just enclosing the hydrogen, fusing it and blasting it out. A plasma bottle."

Oh, great... :p Another book I need to get my hands on and read.
Thanks! (and I mean that sarcastically and honestly at the same time. I love reading SciFi, but have absolutely no time currently for private hobbies like reading a good book).

I also need to read it as the Moties sound a bit like the Thraddash from Star Control 2/ The Ur-Quan Masters.

[Sweforce]

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 1g when the ship has reached one of our closest neighbours.

That's the drive used by the Moties. "They're just enclosing the hydrogen, fusing it and blasting it out. A plasma bottle."

Oh, great... :p Another book I need to get my hands on and read.
Thanks! (and I mean that sarcastically and honestly at the same time. I love reading SciFi, but have absolutely no time currently for private hobbies like reading a good book).

I understood it that photon drives are weak NOW and still useful. I doubt that there isn't possible to make powerful versions as well.

With FTL, at least for space travel stories I would point to the Alcubierre drive. In the Outsider verse the Alcubierre could represent humanities first attempt to reach the stars first theorised in the late 20th century and eventually a research test rigs could be made. For a story I would go for a more then a century old Alcubierre test rig somewhere, operational but not to a useful degree other then another tool for the scientists in their research. From time to time they add some new parts to it and remove some old junk but it is essentially the same experiment worked on when time and funding permits. What they lack is enough exotic materials and and antimatter to build and fuel a upscaled experiment. Both manufacturable but in nowhere near the quantities needed and the invention of the jump drive killed most of the founding anyway.

Still this may by why the historians ships use reaction less drives.

[Mjolnir]

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 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. And I don't know how to parse the bit about acceleration being near nonexistent when it's 1 g.

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).

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.

Once you have something at the destination, you've got other options for braking. The first thing you deliver should probably be an automated factory to set up laser stations or similar infrastructure for braking later spacecraft.

[Krulle]

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.

And I don't know how to parse the bit about acceleration being near nonexistent when it's 1 g.

I forgot (or accidentally deleted) the "at the start of the mission" after "acceleration".
Due to the mass of the fuel, the ships start with 0,000something g until so much fuel has been burned that the mass has been reduced that the engine can actually achieve 1g.
The output of the engine is the same, but the mass it has to accelerate with the output is getting less and less over time, so the acceleration the engine can achieve increases.
Current engine technology, including forecasts in engine efficiency increases, don't foresee us coming into practical ranges within a few decades. If you make the engine large enough to achieve 1g at the start, with the tons of fuel you're hauling around, the engine is mostly dead weight towards the end of the journey (meaning you'll need even more fuel than forecasted to brake your ship, or you'll need to ditch 99,99999% of your engines). If the engine is made to achieve 1g at the end of the journey, the engine is too small to actually make the ship leave the solar system at the start of the voyage (i.e. you won't reach escape velocity of the solar system).

It all comes down to one thing: without FTL we will be confined to the solar system, except for "we pull together because we want Humanity to reach for the stars and thus we pay for it together" missions, which will even then likely be confined to highly automated science missions and possibly one token colonisation mission, as the drain on Earth's economy will be gigantic, without any foreseeable return of investment.

We might be lucky enough that our grandkids are alive when earnest mission planning starts.... (I dream of a different timetable, though).

[Arioch]

That's the drive used by the Moties. "They're just enclosing the hydrogen, fusing it and blasting it out. A plasma bottle."

Oh, great... :p Another book I need to get my hands on and read. Thanks! (and I mean that sarcastically and honestly at the same time. I love reading SciFi, but have absolutely no time currently for private hobbies like reading a good book).

I also need to read it as the Moties sound a bit like the Thraddash from Star Control 2/ The Ur-Quan Masters.

It's one of the true classics of science fiction -- a must-read.

[icekatze]

hi hi

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.

[Mjolnir]

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.

[icekatze]

hi hi

I think that if there is a living crew onboard, and the lifespan of the crew is somehow a concern regardless of the length of the voyage, that having a booster stage would be preferable to building a second spaceship that can overtake and match velocities with the primary ship in order to deliver the crew to the primary ship at a later point in its voyage.

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.

[Mr.Tucker]

I will shamelesly quote myself from an earlier disscussion, since I'm a lazy-ass :

Every drive has a potential niche. What space systems would look like in case of near-term human expansion and space exploration is a menagerie of electrical and other propulsion concepts : simple stuff like arcjets for stationkeeping, multi-use systems like VASIMR or NTR for orbital tugs, high Isp for long voyages,etc. My expectation would be that cargo craft would use different types of sail designs (being propelantless is a big advantage), and that very long range mission probes (like, say, to the solar focal point) would use a combination of such drives (launch with whatever, accelerate with electric, slow down with magsail. Just an example). Fusion is still some time away in my opinion (could be wrong).

FTL is, in my humble opinion, probably possible. How it would be possible is however, hard to gleam at our present tech level. I remember reading somewhere that the ratio between a journey to Mars and a journey to Proxima is the same as between Columbus' journey to america and a journey to Mars (80 milion to one about). So a journey to proxima is as difficult to us as a journey to Mars would be to Columbus. Doesn't mean it's impossible,just that we may simply not be advanced enough.
Non-FTL methods we can envision are drives that rely on beamed power. Like Robert Forward's Starwhisp. 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.

[Mjolnir]

hi hi

I think that if there is a living crew onboard, and the lifespan of the crew is somehow a concern regardless of the length of the voyage, that having a booster stage would be preferable to building a second spaceship that can overtake and match velocities with the primary ship in order to deliver the crew to the primary ship at a later point in its voyage.

What are you going to save? A few months? Years? At a hundredth of a g average acceleration, it would only take 5 years to reach 5% c. When you've got a century or two of travel ahead of you, do you really care?

I am assuming the burns will be a small fraction of the overall trip duration. Otherwise is wasteful...delta-v applied toward the middle of the mission makes relatively little difference in trip duration. You want your average velocity close to your peak velocity, or you're just wasting costly delta-v in getting to that peak, only to start shedding it soon after. Long burns also extend the amount of time equipment has to be active.

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.

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.

[Absalom]

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 1g 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.

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.

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".

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.

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.

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.

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.

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.

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).

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.

[icekatze]

hi hi

What are you going to save? A few months? Years? At a hundredth of a g average acceleration, it would only take 5 years to reach 5% c. When you've got a century or two of travel ahead of you, do you really care.

I think you missed the point I was trying to make. My response was entirely in regards to the hypothetical situation already outlined, of needing to send the crew to the ship while it was already en route. If you have a century or two of travel ahead of you, then completing the mission within the lifespans of the crew will not be possible, and once they board the ship, they're not coming back.

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.

Building the ship in a higher orbit is not substantially different than using boosters. Conceptually speaking, it is using boosters, as each and every one of the construction vehicles is going apply Delta V to a piece of the ship and then detach itself.

The Oberth effect is unlikely to be meaningfully helpful for this type of mission.

The Oberth effect has nothing to do with my point. My point was this: If you are starting with a roughly circular orbit of about 400 kilometers (orbital speed about 7.7 km/s), you'll need to add about 3.5 km/s of velocity in order to reach escape velocity. But you can't just point your ship's nose in the direction you want to go and start accelerating non-stop, unless you have enough thrust to escape before your perigee intersects the atmosphere. You'd have to stop thrusting for large portions of the orbit in order to prevent lowering the perigee dangerously, unless you've got something massive to keep changing the ship's trajectory for days at a time.

The original vessel being described in the thought experiment had a starting acceleration of, at most, .00098 meter's per second per second. That is going to take at least 41 days and some change to reach escape velocity if it is thrusting constantly. If you have to wait to thrust until the ship is approaching the prograde part of its orbit, then it could take significantly longer. Which could be an issue if you are concerned about the lifespan of the crew.

[Mjolnir]

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.

I see nothing at all to gain here. The chasing ship is going to have to support the crew comfortably for years. They could just spend a couple more years on the ship itself, in greater comfort and with the opportunity to actually do something with the place they're going to be spending the rest of their lives. Sending the crew afterward seems completely pointless, and only adds another vehicle and more propellant that ends up not actually speeding up the interstellar trip by even the slightest amount.

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".

Just don't drop a singularity...

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.

Braking against the solar wind of the star? That could help, but seems unlikely to get you all the way down to orbital velocities without requiring deceleration to start so far out you end up adding lifetimes to the trip time.

Now, if you send a minimal automated mining and construction package ahead of you to set up a magbeam braking system...

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.

The most hydrogen-rich hydrocarbon is methane, which is 3/4 carbon by mass.

The densest way to store hydrogen for this kind of purpose is probably cryogenic liquid or solid. The cooling systems aren't going to have to work very hard when you're out in interstellar space.

[Mjolnir]

hi hi

What are you going to save? A few months? Years? At a hundredth of a g average acceleration, it would only take 5 years to reach 5% c. When you've got a century or two of travel ahead of you, do you really care.

I think you missed the point I was trying to make. My response was entirely in regards to the hypothetical situation already outlined, of needing to send the crew to the ship while it was already en route. If you have a century or two of travel ahead of you, then completing the mission within the lifespans of the crew will not be possible, and once they board the ship, they're not coming back.

How would sending another ship chasing after the first, accelerating to a higher speed to catch up and then decelerating early to rendezvous with it, ever change this?

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.

Building the ship in a higher orbit is not substantially different than using boosters. Conceptually speaking, it is using boosters, as each and every one of the construction vehicles is going apply Delta V to a piece of the ship and then detach itself.

No, it's nothing like using boosters. The problem brought up was orbital debris, not the delta-v involved in departure from LEO, and components and raw materials being pushed around by transport craft can be navigated around orbital debris more readily and with less risk than an interstellar starship using its main drives.

The Oberth effect is unlikely to be meaningfully helpful for this type of mission.

The Oberth effect has nothing to do with my point. My point was this: If you are starting with a roughly circular orbit of about 400 kilometers (orbital speed about 7.7 km/s), you'll need to add about 3.5 km/s of velocity in order to reach escape velocity. But you can't just point your ship's nose in the direction you want to go and start accelerating non-stop, unless you have enough thrust to escape before your perigee intersects the atmosphere. You'd have to stop thrusting for large portions of the orbit in order to prevent lowering the perigee dangerously, unless you've got something massive to keep changing the ship's trajectory for days at a time.

The original vessel being described in the thought experiment had a starting acceleration of, at most, .00098 meter's per second per second. That is going to take at least 41 days and some change to reach escape velocity if it is thrusting constantly. If you have to wait to thrust until the ship is approaching the prograde part of its orbit, then it could take significantly longer. Which could be an issue if you are concerned about the lifespan of the crew.

First, why would the ship ever have to stop thrusting? You seem to be assuming it can't alter the direction of its thrust for some reason.
Second, really, who cares about 41 days on an interstellar trip? Who cares about an extra year? What difference could it possibly make? I'm not seeing anything at all to motivate the additional cost, risk, and complexity of a chaser craft that only exists to deliver a crew that could just as well have been on the spacecraft from the start.

[icekatze]

hi hi

How would sending another ship chasing after the first, accelerating to a higher speed to catch up and then decelerating early to rendezvous with it, ever change this? ...who cares about 41 days on an interstellar trip? Who cares about an extra year? What difference could it possibly make?

I don't know, you tell me. I was attempting to examine the conclusions of the premises as stated without accepting the premises themselves. In discussion and argumentation, this is often called "for the sake of argument," because sometimes it is useful to address one issue at a time. And I suggest again that you have actually missed the point I was trying to make, in spite of my efforts to clarify.

You seem to be assuming it can't alter the direction of its thrust for some reason.

There is a difference between being unable to do something, and being unable to do something without devoting any additional mass to the action.

[Krulle]

It's more a psychological issue.
How would you feel being on a spaceship that has started to accelerate away from earth, and you're still nowhere near Jupiter after some years?

You would have preferred to stay those years on Earth.
Compared to the total mission cost, the extra start of a chaser ship is negligible.
And depending on the sling shots to leave the solar system, it may well be suitable to start from Earth in the direction the interstellar ship is flying, and let it catch you (you fly ahead, while it comes from slingshot sun past earth).
You could even refuel the ship, exchange crew (which would have to leave the interstellar ship after slingshot from Sun passing Earth), and you have an additional set of things you missed as spare parts and found out you'll need while doing the sling shots, while the mounting crew did the shakedown flight of the ship (which became the slingshot) and finalised last elements.

You have to think of the astronauts on board of the ship, for them the first years close to Earth will matter whether aboard or still on Earth preparing there.

Anyway, for now it is a very theoretical discussion.
Thanks for humouring me.

[Absalom]

There's also the possibility of starting from the far side of the solar system, or starting the engine before construction begins on the habitat section. Maybe you decide to change out all of the electronics at the same time, or decide to only introduce some biosphere elements at that time because they can't be sustained earlier, or something else yet. You can come up with any number of reasons if you try.