Various questions about cyclers, Saturn, and propulsion

[SamCashion]

QUESTIONS:
I'm in is conducting a study outlining my plan (using only currently-existing technology, and is based only on reputable science, no flaky stuff) to replace fossil fuels without evoking ideological contention.
Right now my friends and I are making a video exploring the possibility of mining Saturn's atmosphere for Helium-3. That project isn't what my questions are about, so let's not discuss that here. If you'd like to, we could discuss that in a more appropriate forum - I just wanted to give some context.

I have several questions to ask and I need more or less unambiguous answers. However, I realize many answers will have to be approximations, given the varying distances between the various celestial bodies over time. It would be great to get some consensus on here, as I'd like to be as accurate as possible. I really don't want to be a burden on anyone, so if you may point me in the direction of credible science (or a simulator workable within a browser, speculatively) that could explain why the answers are what they are, I would be much obliged. I appreciate any answers you may put forward. If you could cite some relevant scientific documentation readily accessible online, I would appreciate that a lot too.

Here are some of my questions. I am dividing them into sections. If you know how to answer these questions but do not want to take the time to do so, it would be great if you could please tell me how to find the answers out for myself, or, in the case of the more mathematical questions, which equations and figures I need to utilize. Thanks you.

EARTH-SATURN CYCLER

• Could it be possible to create an Earth-Saturn cycler, a spacecraft that would complete its trajectories in whole-number multiples of the synodic period between Earth and Saturn? Note: this is derived from the hypothetical but already possible with extant technology Earth-Mars cycler. We needn't bother discussing the toll the long haul would take on human travelers in the cycler, as I only need this to be unmanned. (if not, tell me why)
• How long (a range of distance) would it take such an Earth-Saturn cycler to get from Earth to Saturn and vice-versa? (This assumes the answer to the previous question is "yes," so you don't need to answer it if you answered "no" to the first)

SATURN

• What will the range of distance between Earth & Saturn be over the next 150 years?

DISTANCE & TIME RANGE for different PROPULSION METHODS on MANNED MISSION that invokes SATURNIAN ORBIT INSERTION for two months

• How much time would it take a large-ish manned vessel to get to Saturn with modern-day propulsion?
• How long would it take with VASIMR?
- Is VASIMR a serious investment, and is there any likelihood that it will work?

GRAVITATIONAL ASSIST

• Is it possible for the manned mission and/or the unmanned cycler to use a gravity-assist maneuver with Venus, the Sun?
• Would it be desirable to do so to conserve fuel and speed the mission up? (I am only concerned that it would take a lot of fuel to slow the vessel down when it needs to enter Saturnian orbit)
• How exactly would this affect the length of either mission, assuming the mission(s) use only extant propulsion methods?
• How exactly would this affect the length of either mission, assuming the mission(s) use VASIMR?

Happy holidays, everybody!
(and please no speculative answers, either answer with links or knowledgeable explanation) <3

[Van Rijn]

QUESTIONS:
I'm in is conducting a study outlining my plan (using only currently-existing technology, and is based only on reputable science, no flaky stuff) to replace fossil fuels without evoking ideological contention.
Right now my friends and I are making a video exploring the possibility of mining Saturn's atmosphere for Helium-3. That project isn't what my questions are about, so let's not discuss that here.

Well, I won't get into detail here, but if you're going to mention that, you should be aware that there is no currently existing technology to use Helium-3. If the technology did exist, Uranus would be a much better choice for mining than Saturn, as the escape velocity is much lower, but it still has helium.

Given those issues the rest of the questions in your post seem moot.

[SamCashion]

From what I can tell, they're on the verge of being able to use it and the problem was obtaining Helium-3. But I am apparently wrong, so I am glad that you brought those two things to my attention! These questions still apply in the quest to obtain a decent amount of helium 3, but substitute Saturn for Uranus. Is there a good study you could point me in the direction of talking about the pros & cons of Saturn vs. Uranus for Helium-3 mining? I've been able to at least gather that it would be better to mine from a gas giant than our moon. Would that be a fair assessment?

EDIT:
Just wanted to note that I am now asking these supplemental questions in a hypothetical way, as if the technology to use Helium-3 were available. I truly appreciate you informing me that the technology is not yet available.

[Trakar]

...
EARTH-SATURN CYCLER

• Could it be possible to create an Earth-Saturn cycler, a spacecraft that would complete its trajectories in whole-number multiples of the synodic period between Earth and Saturn? Note: this is derived from the hypothetical but already possible with extant technology Earth-Mars cycler. We needn't bother discussing the toll the long haul would take on human travelers in the cycler, as I only need this to be unmanned. (if not, tell me why)
• How long (a range of distance) would it take such an Earth-Saturn cycler to get from Earth to Saturn and vice-versa? (This assumes the answer to the previous question is "yes," so you don't need to answer it if you answered "no" to the first)
...

Why have an unmanned cycler travelling the elipse between Earth orbit and Saturn orbit?

What advantages do you see this structure providing?

[a1call]

• Is it possible for the manned mission and/or the unmanned cycler to use a gravity-assist maneuver with Venus, the Sun?

See the 2 entries in the same thread:

http://www.bautforum.com/showthread....13#post1493713

http://www.bautforum.com/showthread....49#post1494149

[Jens]

Why have an unmanned cycler travelling the elipse between Earth orbit and Saturn orbit?
What advantages do you see this structure providing?

I assume he is hoping to not have to decelerate the craft, and I'm guessing he's thinking of the system where planes come down on lakes and load up water without stopping and then take off again to throw the water on forest fires.

[SamCashion]

Why have an unmanned cycler travelling the elipse between Earth orbit and Saturn orbit?

What advantages do you see this structure providing?

:/ That shouldn't be relevant to the question.

I don't want to get into that because that's a completely different conversation.

[Trakar]

I assume he is hoping to not have to decelerate the craft, and I'm guessing he's thinking of the system where planes come down on lakes and load up water without stopping and then take off again to throw the water on forest fires.

I see no advantage for this in a cycler, you still must accelerate the mass at either end of the trip to cycler velocity in order to connect to the cycler (or decelerate from cycler velocity to planetary orbital velocities), once the mass is accelerated what need is there for the cycler? The only potential advantage a cycler offers is a larger habitat for human voyagers on months (years, decades) long trips and for the heavy power systems for such trips. These would not have to be accelerated and decelerated at each end of the trip. But at first look, an unmanned cycler isn't going to be usable as an atmospheric harvestor (no free lunches), nor is it functional or useful for transporting cargo to or from Saturn for the simple reason that once you have accelerated to match velocities with the cycler, the cycler is redundant.

[SamCashion]

See the 2 entries in the same thread:

http://www.bautforum.com/showthread....13#post1493713

http://www.bautforum.com/showthread....49#post1494149

Those posts seem to be about solar sails, not gravitational assistance.

[a1call]

Those posts seem to be about solar sails, not gravitational assistance.

It's a combination of gravity of the sun and solar sail using its light.
The point is both gravity and light dissipate proportionally to square of the distance (disregarding gravitational anomalies at great distances).
So you accelerate towards the sun (with sails down) using its immense gravity and then once at the maximum possible velocity, you employ the sails to cancel out the sun's gravity and keep going on the other side without decelerating.

[Van Rijn]

From what I can tell, they're on the verge of being able to use it and the problem was obtaining Helium-3. But I am apparently wrong, so I am glad that you brought those two things to my attention!

Unfortunately, it is fairly common to see an article about the possibility of He3 mining that doesn't bother to mention we can't use it. The D-He3 reaction is substantially harder to achieve than the D-T reaction and we don't even have useful D-T reactors yet. Nor is it clear that He3 will ever be very important. The key advantage (lower neutron production in a D-He3 reactor) would be nice, but neutrons aren't all that hard to deal with.

These questions still apply in the quest to obtain a decent amount of helium 3, but substitute Saturn for Uranus. Is there a good study you could point me in the direction of talking about the pros & cons of Saturn vs. Uranus for Helium-3 mining?

It doesn't require a study - Uranus has the lowest escape velocity of the Gas & Ice giants. Crossing the solar system is not nearly as big a deal as the actual extraction and getting it into space where it can be transferred across the system.

I've been able to at least gather that it would be better to mine from a gas giant than our moon. Would that be a fair assessment?

Perhaps in the very long run, a few centuries from now, if there is ever a significant demand for He3. Currently, the most practical way to get He3 is to produce tritium, and let it decay. It isn't clear that lunar extraction would be practical, but it would be a lot easier to build facilities on the moon rather than the hardware for gas or ice giant extraction.

If you're looking at space based energy production, the most likely near term option would be solar power satellites. That's still a long shot because of the cost and practical difficulties, though.

[SamCashion]

Thanks Van Rijn, that clears a lot up. I'm glad I didn't go further with this and end up looking really silly.

[cjameshuff]

From what I can tell, they're on the verge of being able to use it and the problem was obtaining Helium-3. But I am apparently wrong, so I am glad that you brought those two things to my attention! These questions still apply in the quest to obtain a decent amount of helium 3, but substitute Saturn for Uranus. Is there a good study you could point me in the direction of talking about the pros & cons of Saturn vs. Uranus for Helium-3 mining? I've been able to at least gather that it would be better to mine from a gas giant than our moon. Would that be a fair assessment?

If showstopping problems or massive delays don't crop up and things get funded, we could have D-T fusion power plants in a matter of a couple decades (the DEMO followup to ITER). This is deuterium-tritium fusion, though, which does not use He-3. Aneutronic fusion using He-3 is much more difficult, and it's almost certainly easier to deal with neutrons and produce tritium than to go get He-3 from such distant sources for use in such reactors.

Assuming that's not an issue though, Uranus would have a major disadvantage in sheer amount of travel time required...about 3 decades round trip for cycler orbits. Saturn's quite far enough. I don't think the escape velocity is really an obstacle: you'll have no shortage of propellant, and can have tugs slowly haul payload into a high eccentricity orbit using high efficiency engines with the final small delta-v to be performed when the cycler comes by, or send it off directly to the destination with a relatively small burn from a higher thrust engine, taking advantage of the Oberth effect from the deep gravity well. Jupiter really looks like a much better place to get helium...higher atmospheric concentration of helium, much shorter trip times to destinations in the inner system, and relative ease of reaching such destinations by using Jupiter's gravity well to your benefit.

I see no advantage for this in a cycler, you still must accelerate the mass at either end of the trip to cycler velocity in order to connect to the cycler (or decelerate from cycler velocity to planetary orbital velocities), once the mass is accelerated what need is there for the cycler? The only potential advantage a cycler offers is a larger habitat for human voyagers on months (years, decades) long trips and for the heavy power systems for such trips. These would not have to be accelerated and decelerated at each end of the trip. But at first look, an unmanned cycler isn't going to be usable as an atmospheric harvestor (no free lunches), nor is it functional or useful for transporting cargo to or from Saturn for the simple reason that once you have accelerated to match velocities with the cycler, the cycler is redundant.

An unmanned cycler still allows you to spend more of your momentum change on accelerating payload. Consider one with a nuclear electric power system for orbital maintenance (and shifting its orbit to account for the imperfect match of the actual solar system to cycler orbits), as well as a tether or beam propulsion system for accelerating payloads being picked up or depositing them at their destinations. The cycler's mass could act as a momentum bank to prevent large velocity changes during pickup/dropoff and giving the high propellant efficiency engine time to make it up while between destinations. The cycler itself does the job of accelerating the payload without itself accelerating by any great degree, and it carries a high efficiency engine to do what accelerations it requires.

You're most likely better off with stationary tether/beam propulsion stations at each end, though. The cycler approach keeps a lot of hardware in transit for the majority of the time, only being of use for two brief periods per orbit. Dedicated launching/receiving stations/craft could keep punching out a constant stream of ballistic cargo packages, instead of sitting unused most of the time. And again, waste H2 and He-4 would make for a steady supply of propellant.

[Van Rijn]

Assuming that's not an issue though, Uranus would have a major disadvantage in sheer amount of travel time required...about 3 decades round trip for cycler orbits. Saturn's quite far enough. I don't think the escape velocity is really an obstacle: you'll have no shortage of propellant, and can have tugs slowly haul payload into a high eccentricity orbit using high efficiency engines with the final small delta-v to be performed when the cycler comes by, or send it off directly to the destination with a relatively small burn from a higher thrust engine, taking advantage of the Oberth effect from the deep gravity well. Jupiter really looks like a much better place to get helium...higher atmospheric concentration of helium, much shorter trip times to destinations in the inner system, and relative ease of reaching such destinations by using Jupiter's gravity well to your benefit.

But how do you get the He3 into orbit? I'd think that would be hard enough with Uranus, let alone Saturn, and Jupiter would be worse.

I don't see the travel time being much more of an issue for Uranus than Saturn. If people went there, it would be with the expectation of living there decades at least, and (relatively) fast transport would have to be a priority. He3 would be transported by unmanned vehicles. They might or might not be fast - the mass of He3 wouldn't be that great, so they could probably get away with sending it relatively high velocity.

[Trakar]

:/ That shouldn't be relevant to the question.

I don't want to get into that because that's a completely different conversation.

Asking about reasons for a priori mission structure hardware that seems both unneccesary and a complication rather than a compliment to mission goals should be relevent to all technical questions about such a mission proposal. If the only reason for it is as an plot-device for a story, then it indeed has a purpose, but I am unaware of any sound technological or economic reason for using an unmanned cycler for this type of mission role.

[Trakar]

...
An unmanned cycler still allows you to spend more of your momentum change on accelerating payload...

How so?
I'm not seeing what you are describing.
In most of the cycler systems I am familiar with, the cycler itself is doing little or no momentum change aside from whatever minor adjustments/corrections which need to be accomplished to account for multibody calculation complexities. Payload accelerations/decelerations are accomplished prior to docking with, or subsequent to undocking from, the cycler.

Consider one with a nuclear electric power system for orbital maintenance (and shifting its orbit to account for the imperfect match of the actual solar system to cycler orbits), as well as a tether or beam propulsion system for accelerating payloads being picked up or depositing them at their destinations. The cycler's mass could act as a momentum bank to prevent large velocity changes during pickup/dropoff and giving the high propellant efficiency engine time to make it up while between destinations.

Seems like an unnecessary step, why not just fling the payloads straight from the Saturn orbital refinery/depot to the Earth orbital or vicinity depot/distribution center? Then you can pretty much target and shoot continuously without trying to target a cycler which will then carry it to near Earth vicinity and then try to target an Earth orbital or vicinity target.

The cycler itself does the job of accelerating the payload without itself accelerating by any great degree, and it carries a high efficiency engine to do what accelerations it requires.

so you are talking about a set of orbiting tether launchers shooting payloads at, and catching payloads from, tethers cycling between the two? still sounds like a bunch of third wheels to me.

You're most likely better off with stationary tether/beam propulsion stations at each end, though. The cycler approach keeps a lot of hardware in transit for the majority of the time, only being of use for two brief periods per orbit. Dedicated launching/receiving stations/craft could keep punching out a constant stream of ballistic cargo packages, instead of sitting unused most of the time.

Ah!
my thoughts presactly!

And again, waste H2 and He-4 would make for a steady supply of propellant.

I suppose you could make the cycler your refinery, but it would seem to be better to refine the payload before you send it to Earth, or the cycler.

[Shaula]

http://www.whydomath.org/node/space/references.html may have some interesting links on it for you. You probably have come across this before but it strikes me as as a good minimum thrust solution to the problem.

[SamCashion]

;_; While I'm quite disappointed, I must accept the truth.
It seems to me that my inquiries about cyclers to Saturn are irrelevant, knowing now that not only are we decades away from fusion, Helium-3 isn't even a prerequisite for it.

Trakar, thanks for enlightening me. As for what I was inquiring about as far as cyclers, I'm no longer as curious cause of the fusion stuff.

If showstopping problems or massive delays don't crop up and things get funded, we could have D-T fusion power plants in a matter of a couple decades (the DEMO followup to ITER). This is deuterium-tritium fusion, though, which does not use He-3. Aneutronic fusion using He-3 is much more difficult, and it's almost certainly easier to deal with neutrons and produce tritium than to go get He-3 from such distant sources for use in such reactors.

Could we now discuss all of this business? If anyone knows more about this, do weigh in!

[Van Rijn]

I suppose you could make the cycler your refinery, but it would seem to be better to refine the payload before you send it to Earth, or the cycler.

Since the He3 is about 1 part in 10,000 of the He4, and there's more hydrogen in the atmosphere than helium, a floating atmospheric station is about the only way to go, but maintaining it, and transport to orbit would be a big problem.

[Van Rijn]

Could we now discuss all of this business? If anyone knows more about this, do weigh in!

If you want to discuss fusion reactor tech., you should probably start a thread in science and tech. One quick point: At this time, conventional (fission) reactors are really the only feasible nuclear power technology.

[SamCashion]

Alright, after doing some reading in the topic, I'll do that. Thanks. I suppose the remainder of this thread should be about cyclers, then.

[IsaacKuo]

Pretending for a moment that we really want lots of natural He3 (as opposed to much cheaper artificial He3), the best option might be to use a weird sort of Jupiter/Sun cycler. There are some asteroids like 2009 HC82 which are in a highly eccentric orbit. It would take relatively little delta-v to send such an asteroid into a sun-grazing orbit.

The basic flight plan would be to perform a thrust at perihelion, which sends the asteroid into a very fast Jupiter grazing escape trajectory. It takes relatively little delta-v near the Sun to produce a high velocity at Jupiter. That's a good thing, because grazing Jupiter's upper atmosphere will produce a heck of a lot of drag.

On the way to Jupiter, you dig a funnel shape into the asteroid leading to a butterfly valve "throat" and a huge "stomach" partially filled with LH2. This is used to scoop up a little bit of gas from Jupiter's upper atmosphere when it arrives at Jupiter. The scooped up gas is extremely hot with extremely low ambient pressure, which is why the LH2 is necessary. The LH2 will cool the incoming gas enough to let the stomach fill with a little bit of it despite the low ambient pressure. So, in the end, you get a tiny bit of He3...but plausibly more than if you invested your He3 mining resources in any other venture to gather natural He3. (But hey, if you really wanted massive amounts of He3 you could have stayed on Earth and just created it from Lithium.)

This process will erode the funnel intake like crazy, but that's okay. Anyway, if you aim things just right, your Jupiter grazing encounter will leave you with just enough speed to still escape Jupiter with about 5km/s v_inf. This v_inf cancels enough of Jupiter's orbital velocity to send the asteroid back down toward the Sun in a sun grazing elliptical orbit.

This new orbit's period is about 4 years, so you just coast for 2.5 orbits before aiming again for Jupiter. After those 2.5 orbits, you do another perihelion thrust to send you on another fast hyperbolic trajectory toward Jupiter.

The same strategy could be used to mine natural He3 from Uranus, but that would involve cycle times of 85 years rather than 12 years.

[whimsyfree]

Unfortunately, it is fairly common to see an article about the possibility of He3 mining that doesn't bother to mention we can't use it. The D-He3 reaction is substantially harder to achieve than the D-T reaction and we don't even have useful D-T reactors yet. Nor is it clear that He3 will ever be very important. The key advantage (lower neutron production in a D-He3 reactor) would be nice, but neutrons aren't all that hard to deal with.

The way every technical online forum (including Wikipedia) gets polluted with this He3 nonsense is depressing proof of just how disconnected from reality the space lobby is. D-He3 isn't even aneutronic because there is no way to prevent the D+D->n+He3 side reaction. To get aneutronic fusion from He3 you would need a straight He3 reactor, which faces much greater confinement problems than D-He3 which faces much greater confinement problems than D-T.

[cjameshuff]

But how do you get the He3 into orbit? I'd think that would be hard enough with Uranus, let alone Saturn, and Jupiter would be worse.

You scoop it up and use the hydrogen and He-4 as propellant to correct what the drag does to your orbit. And the longer travel time means slower response to shifts in demand and more hardware in transit for a given throughput...it matters.

[cjameshuff]

How so?
I'm not seeing what you are describing.
In most of the cycler systems I am familiar with, the cycler itself is doing little or no momentum change aside from whatever minor adjustments/corrections which need to be accomplished to account for multibody calculation complexities. Payload accelerations/decelerations are accomplished prior to docking with, or subsequent to undocking from, the cycler.

Yes.

Seems like an unnecessary step, why not just fling the payloads straight from the Saturn orbital refinery/depot to the Earth orbital or vicinity depot/distribution center? Then you can pretty much target and shoot continuously without trying to target a cycler which will then carry it to near Earth vicinity and then try to target an Earth orbital or vicinity target.

It could have a lower minimum cost. One cycler doing most of the work, instead of stations at each end. If you can build multiple cyclers, it's cheaper to build launching/receiving stations.

so you are talking about a set of orbiting tether launchers shooting payloads at, and catching payloads from, tethers cycling between the two? still sounds like a bunch of third wheels to me.

...no. By "The cycler itself does the job of accelerating the payload", I mean the cycler itself does the job of accelerating the payload. One cycler with a pickup/drop tether and a nuclear electric propulsion system to compensate for the momentum shifts of the pickup/drop operations and for the usual corrections to its orbit.

If you build two such cyclers, you can just station one at Earth (or wherever) and one at Jupiter and convert them into launch and receiving stations throwing constant streams of packages to each other...continuing to use them as cyclers doesn't make much sense. If you can only afford to build one cycler or one launch-receiving station (the two really being quite similar), the cycler makes some sense.

[Van Rijn]

You scoop it up and use the hydrogen and He-4 as propellant to correct what the drag does to your orbit. And the longer travel time means slower response to shifts in demand and more hardware in transit for a given throughput...it matters.

A ram with a nuclear thermal rocket?

That sounds incredibly inefficient. I can't imagine why you'd do that instead of using atmospheric stations.

[cjameshuff]

A ram with a nuclear thermal rocket?

Nuclear thermal or nuclear electric.

That sounds incredibly inefficient. I can't imagine why you'd do that instead of using atmospheric stations.

Well, because it's vastly easier, and seems almost certain to be more efficient than atmospheric stations...you can stay in orbit and use high-Isp engines to maintain your orbit, rather than climbing out from deep in an atmosphere that's made of just about the worst possible mix of gases for buoyant structures using high thrust engines that have far lower propellant efficiency. A transport from one of your atmospheric stations has a huge amount of atmosphere to plow through while burning upwards through a high gravity field before it can even start to accumulate orbital velocity, and this spacecraft has to throw away all of its orbital velocity to rendezvous with that station in the first place.

Consider the payload fraction you're likely to achieve doing that without throwing away the majority of the vehicle, and take that into account while comparing the relative efficiency...you're fueling and launching a huge booster with a tiny He-3 payload, I'm scooping atmosphere that's mostly H2+He-4 and extracting the He-3 later, using the waste gases as propellant to restore the scoop craft's orbit. You're using some kind of high thrust engine to climb to orbit from a dead stop, I'm using high efficiency nuclear electric or nuclear thermal to maintain my orbit and set up scooping passes.

[Trakar]

It could have a lower minimum cost. One cycler doing most of the work, instead of stations at each end. If you can build multiple cyclers, it's cheaper to build launching/receiving stations.

I think we are combating terminology distinctions.

Cycler - a platform that follows an orbit that intersects the orbits of two seperate destination planets.

tether-launcher - an orbiting rotating tether that picks up payloads being launched to orbit at one destination planet and flings them into an orbit that intersect the orbit of the other destination planet. These launchers can also act as "catchers" to snag incoming payloads from the other destination planet.

Now you seem to saying that a cycler should also be have a tether-launch/catch capacity and that adding many of these cyclers with tether-launch/catch capabilities would be cheaper and easier than simply flinging the payloads between the two seperate destination launcher/catchers?

The cycler seems like an unnecessary complication and added expense to a system that would work just fine with just a tether launcher/catcher at each end. A cycler that also launches and catches payload packages seems like an unnecessary complication that provides little to no inherent benefit to the system.

If you build two such cyclers, you can just station one at Earth (or wherever) and one at Jupiter and convert them into launch and receiving stations throwing constant streams of packages to each other...continuing to use them as cyclers doesn't make much sense.

A cycler, that does not cycle between the planets, isn't a cycler?!

But the rest of this (tether launcehers, catchers at both ends) makes perfect sense to me.

If you can only afford to build one cycler or one launch-receiving station (the two really being quite similar), the cycler makes some sense.

Still a bit confusing. If I can only build one tether system, I would choose to build it at Saturn to launch manueverable payloads back towards Earth. I still see no benefit of a cycler, which could not slow down enough to use a tether capture system to snatch significant payloads from Saturn orbit and seems like it would sacrifice a great deal of orbital momentum in any attempt to do so. If I could build two systems, I would put one at each destination. I just don't see the point where an unmanned cycler would become a valuable component,...however, like I stated initially, if we are talking about a manned ferry system, then the cycler begins to gain some attractive qualities.

[IsaacKuo]

Nuclear thermal or nuclear electric.

Nuclear thermal has insufficient exhaust velocity--only about 9km/s, far short of what's needed even for Uranus, much less Saturn or Jupiter.

That said, I just remembered another advanced propulsion possibility which I call aeroramjet. This is similar to a ramjet, but you use inert propellant instead of fuel. The aeroramjet simply has a funnel in the front and a nozzle in the rear. Instead of a combustion chamber is simply a cylindrical tunnel where the inert propellant is injected into the gas stream. An aeroramjet is not particularly efficient, but it can use unrefined dense propellant. Essentially, you are getting energy by sacrificing mass for speed.

You could start off with a Jupiter crossing asteroid, like I mentioned earlier, but then you mine inert propellant from it. You end up trading this propellant for atmospheric hydrogen/helium. In theory, you could trade one ton of inert propellant for two tons of atmospheric hydrogen/helium--in the unrealistic case of a 100% efficient aeroramjet (and assuming you start from escape velocity and end up in low orbit). More plausibly, you'd have to trade several tons of propellant for each ton of hydrogen/helium.

[cjameshuff]

Now you seem to saying that a cycler should also be have a tether-launch/catch capacity and that adding many of these cyclers with tether-launch/catch capabilities would be cheaper and easier than simply flinging the payloads between the two seperate destination launcher/catchers?

I'm saying almost the complete opposite. A cycler with tether launch/catch or beam propulsion capability can pick up payloads at one end of its orbit and drop them off at the other using the same equipment. This could provide a lower minimum cost than separate launcher and catcher stations, each of which would be very similar to such a cycler in investment. A second cycler is a waste of resources, however, because you could just as easily build a launcher/catcher pair and greatly increase throughput by putting many payloads in flight with the same equipment. But if a single cycler does what you need, you can set up another cycler between another pair of destinations...it's not a useless concept, just not one that scales up past a single cycler for a given set of destinations. (I'm assuming no refrigeration requirements...pressurized He-3, not liquid. The extra equipment for keeping it liquid might make the cycler approach scale past two cyclers.)

The cycler seems like an unnecessary complication and added expense to a system that would work just fine with just a tether launcher/catcher at each end. A cycler that also launches and catches payload packages seems like an unnecessary complication that provides little to no inherent benefit to the system.

Being about half the cost to set up seems like it could be a pretty major benefit.

Still a bit confusing. If I can only build one tether system, I would choose to build it at Saturn to launch manueverable payloads back towards Earth.

That only solves half the problem. You need a lot of delta-v capability on every single payload, or a catcher system. Earth specifically could use heat shields and aerobraking to handle much of the delta-v, other destinations aren't so fortunate.

I still see no benefit of a cycler, which could not slow down enough to use a tether capture system to snatch significant payloads from Saturn orbit

The cycler doesn't have to slow down...that's more or less the whole point of a cycler...and the tether requirements are the same for launching payloads to the destination or picking them up with a cycler already on that trajectory.