[Space] Steampunk Firefly Idea

[Daigoro]

Originally Posted by Humabout

3. The further an orbit is from the primary, the lower the orbital energy, ergo the lower the orbital speed (an object's mass is independant of its location).

Yep, that makes sense. Except I'm still having trouble reconciling that with how speeding up takes you to a higher orbit, as in a Hohmann transfer which involves applying positive thrust to achieve a higher orbital energy. Wikipedia describes it this way-

Originally Posted by https://en.wikipedia.org/wiki/Hohmann_transfer_orbit#Explanation

The transfer...is initiated by firing the spacecraft's engine in order to accelerate it so that it will follow the elliptical orbit; this adds energy to the spacecraft's orbit. When the spacecraft has reached its destination orbit, its orbital speed (and hence its orbital energy) must be increased again in order to change the elliptic orbit to the larger circular one.

(Simplistic highlighting mine.)
So looking at the delta-V would mean looking at how much velocity you add here, but when you look at the orbital speeds of moons in the lower and higher orbit, the higher orbit is slower, as you say, which means you have to subtract velocity. Now that could mean that a rocket in a circular orbit can be moving faster than a moon in the same orbit, and it has to slow down to match it in that orbit, but for a circular orbit radius and velocity are proportional, so being in a particular orbit implies a specific velocity. Obviously I'm missing something, not sure what it is- I'm guessing it's about angular velocity versus tangential speed or something.

Originally Posted by Humabout

4. No form of lightsail or magsail provides anywhere neeeear enough acceleration to attempt a Hohhman transfer. Those will require a slow spiraling brachistochrone transfer (for more jnfo, please see Halfway to Anywhere).

I was basing that on this bit of text-

Originally Posted by https://en.wikipedia.org/wiki/Magnetic_sail#Inside_a_planetary_magnetosphere

By varying the magnetic sail's field strength over the course of its orbit, a magnetic sail can give itself a "perigee kick" raising the altitude of its orbit's apogee.

Repeating this process with each orbit can drive the magnetic sail's apogee higher and higher, until the magnetic sail is able to leave the planetary magnetosphere and catch the solar wind.

So, not talking about a 2 burn Hohmann transfer, but a low thrust orbital transfer. If technically a Hohmann is only a 2 impulse manoeuvre, then apologies. I was using it to mean matching an elliptical orbit's periapsis and apoapsis between circular orbits, regardless of how many impulses are used.

A brachistochrone transfer would require a constant thrust, which might be possible in a Jovian-style magnetosphere. I was imagining that tapping a high energy plasma torus in a lower orbit for a "perigee kick" might be more desirable, but proving this true would need a lot of computation. You'd also need to know the field strength of the magnetosphere along your orbit, the energy in the plasma torus, and how well your magsail can capture those and translate them to the desired kinetic energies.

Originally Posted by thrash

Remember, we're not talking about the Jovian system per se, except by analogy:

Yep, I was just giving it as an example of moons having magnetospheres not being unreasonable.

[Daigoro]

Originally Posted by thrash

Multiple moons offer the possibility of using very low energy transfer orbits to get around -- on the order of hundreds of meters per second delta-V, rather than multiple kilometers per second. See, e.g., Ross and Scheeres 2007.

Okay, I've actually skim-read that paper now. To try to summarise, there are points in a 3-body system, such as Jupiter-moon-rocket, where the rocket can apply small impulses to be captured by the moon, or to escape the moon's gravity and go back into orbit around the primary. Essentially, it means setting the periapsis to land in the moon's L2 Lagrange point.

Originally Posted by thrash

Humabout is correct about Hohmann orbits, but in a complex dynamical system (such as a gas giant with multiple moons) there are more efficient options than simple constant-thrust orbits. The math is difficult, however, and depends on the specifics of the system.

Indeed the math is difficult. For example, the paper you quoted simplifies a 67 body problem with individual orbital inclinations to a 3 body problem in a circular plane. It would be useful for mission control, provided with exact orbital parameters for all bodies in the system and the ability to precisely calculate trajectories and impulse patterns, to carefully plan out a computerised low energy transfer mission months ahead.

For this low-tech setting though, there'd need to be some more empirical knowledge- pilots have mapped a number of transfer points where they can cross into a moon's orbit, for example. If they're Lagrange points, they could set up flashing buoys there which shouldn't move out of position.

[Humabout]

Originally Posted by thrash

... in a two-body system, which this is not.

If you are referring to coasting, that maneuver can be used to pick up speed regardless of transfer orbit if you hit the right launch window (which don't work with any wonderful frequency, and that will mess with regular resupply missions needed for colonization). If not, you're limited to brachistochrone transfers. For more information or to calculate these things based on information available from the system generation rules in Space or to generate figures usable with Spaceships, please refer to Halfway to Anywhere. If you want to generate full porkchop plots and sort out your transfers from there, here's a thread with lots of good info and a calculator. If you don't want to fuss with numbers, make ships move at the speed of plot - a speed that is always worth considering!

Originally Posted by thrash

Still not talking about the Jovian system per se.

Very good! I was expanding on to previous comments regarding the Jovian system. Even if you would rather speak of other gas giants of which we have measurements, Saturn has a sizable magnetosphere, also. And it's expected that Uranus and Neptune will follow suit with proportional magnetospheres, too; although, theirs may be significantly weaker depending on how much metallic hydrogen is present in the planets.

Originally Posted by thrash

The OP specified moons of a gas giant in the habitable zone. These are highly unlikely to be icy bodies, since they would have a distressing tendency to melt. Differentiated rocky bodies, with an iron core, could certainly sustain a higher magnetic field. Moreover, to be habitable as specified they would have to be protected from radiation; therefore, they have significant magnetic fields.

Again, I was speaking about existing bodies orbiting an existing gas giant. Not fictitious moons five times the size of Mars. Such moons would likely have iron cores, and if those cores contain a molten component, they will have a measurable magnetosphere - after all, all of Mercury, Venus, and Mars have dead cores. Of these, Venus seems to show some degree of a magnetosphere induced by the interaction of the solar wind and the ionosphere, but this is extremely weak and does virtually nothing to protect the planet from cosmic radiation.

Originally Posted by thrash

A more interesting point is radiation protection for ships in transit and stations (either fixed or free-flying) beyond the protection of the habitable moons' magnetic fields.

I again refer to existing gas giants as a point of reference. The magnetosphere of Jupiter is incredibly strong (10 times the strength of Earth's and 18,000 times the moment). To really put it in perspective, Jupiter's field strength is about 428,000 times the strength of the Sun's at Earth's orbit. This leads me to suspect that even a moon with an iron core won't have a field strength that can protect against the gas giant's planetary radiation belts.

For comparison, if Earth generated the same field that Jupiter does, the Van Allen Belt would extend more than three times the distance from the Earth to the Moon. A body half the size of Earth would have an even

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Since this conversation is derailing the thread, I'm only going to further respond to comments related to the OP's post.
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Originally Posted by thrash

It's relatively easy to give the ships magnetic radiation shields as a collateral drive benefit. For stations, either allow limited "force fields" just for radiation shielding, or equip them with undersized drives just for the shielding benefit. Either way, their orbits would deviate slightly from pure gravitational -- but there's something majestic about visiting a station that sails, however slowly. A station-based drive field might also be configured to facilitate close-in maneuvering, providing a buffer for arrival and a push-off for departure.

Energy shields kind of edges away from the steampunk aesthetic pretty fast, though. I'm not against sailing space stations, though. I'd just say the ether is real and they sail using that. Check out Spaceships 7 for a lot of awesome ideas for that sort of thing.

Originally Posted by thrash

Mini-magnetospheric plasma propulsion (M2P2) might be a better option than the superconductor loops in the Pyramid article, on the strength that they (a) don't require miles-wide physical structures and (b) they provide the radiation shielding automatically, as long as they are in use.

An interesting thought, but to get the kind of thrust you'd need, you still need a massive electromagnetic field that would lethal to human life. Although going with my suggestion to use science as it was understood in the Victoria Era, that could be ignored...but then again, so could planetary radiation belts. And really, you could fly through space on ether sails, too. Honestly, this is the sort of thing I'd pursue to get that steampunk feel. Also, FYI, superconductor loops are from Spaceships, p. 25.

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In conclusion, all of these references are to give perspective on the realistic depiction of such a system as modern science explains it today, regardless of how the OP chooses to portray it. Personally, I think this detracts from the steampunk vibe and would look up the Victorian understanding of such things (remember Mars had water and Venus was a jungle planet back then, too). I would personally pick and choose carefully to craft the vibe you want and go from there. It was a time of rapid scientific advancement, and you could decide any number of competing theories are correct, as it suits your story.

[thrash]

Originally Posted by Humabout

If you are referring to coasting, that maneuver can be used to pick up speed regardless of transfer orbit if you hit the right launch window (which don't work with any wonderful frequency, and that will mess with regular resupply missions needed for colonization). If not, you're limited to brachistochrone transfers. For more information or to calculate these things based on information available from the system generation rules in Space or to generate figures usable with Spaceships, please refer to Halfway to Anywhere.

"Halfway to Anywhere" is an introduction to two-body orbital motion. The OP's setting, however, is a multi-body system, with many opportunities for clever tricks and shortcuts. For just one example, the orbital periods of the various habitable moons about the gas giant are likely on the order of days, not months or years. Some of them will be in resonant orbits, with periods that are simple multiples of one another. In this situation, launch windows to take advantage of gravity assists should occur nearly every day -- a very important fact for interlunar commerce.

Such moons would likely have iron cores, and if those cores contain a molten component, they will have a measurable magnetosphere...

The real-world gas giant moons are all thought to be heated by tidal forces, which would also apply to fictitious rocky moons. Being larger than Mars or Mercury would also argue for better internal heating from radioisotopic decay. The situation doesn't have to be probable, just plausible.

This leads me to suspect that even a moon with an iron core won't have a field strength that can protect against the gas giant's planetary radiation belts.

Yet, as you point out, the other gas giants have significantly smaller magnetic fields. A smaller or slower-rotating gas giant primary might still have a useful magnetic field but not so powerful that it renders its large, rocky moons uninhabitable. Again, plausible not probable. But this is an insight that may help inform the OP's world building.

An interesting thought, but to get the kind of thrust you'd need, you still need a massive electromagnetic field that would lethal to human life.

The engineers who proposed the M2P2 system for crewed spaceflight clearly don't agree.

[Humabout]

Originally Posted by thrash

"Halfway to Anywhere" is an introduction to two-body orbital motion.

One that covers coasting and calculating launch windows, actually... And again, if the OP wants to break out n-body solutions, I've already linked to one resource that can help him. Unfortunately, GURPS won't help him, as it's planetary design system doesn't provide the requisite information. Of course, it is an interesting modeling exercise.

Originally Posted by thrash

The real-world gas giant moons are all thought to be heated by tidal forces, which would also apply to fictitious rocky moons. Being larger than Mars or Mercury would also argue for better internal heating from radioisotopic decay. The situation doesn't have to be probable, just plausible.

If you're going for "whatever is plausible," just make stuff up. If you're looking for something likely, consider that the only rocky planet or dwarf planet known to have an active metallic core is Earth, and that includes both an extra-hot planet right next to the sun (it's getting heat in spades) and Venus, a near duplicate of Earth in size, but with approximately 20% more solar heating. But yes, if you search all of the roughly estimated 100 octilliion stars, such a system probably exists somewhere several times over.

Originally Posted by thrash

Yet, as you point out, the other gas giants have significantly smaller magnetic fields. A smaller or slower-rotating gas giant primary might still have a useful magnetic field but not so powerful that it renders its large, rocky moons uninhabitable. Again, plausible not probable. But this is an insight that may help inform the OP's world building.

I am ignoring smaller gas giants because they are far less likely to accumulate enough of an accretion disk to produce the size moons the OP is talking about, and to manage to capture three might-as-well-be-planets in stable orbits sounds suspiciously unlikely. Again given an estimated 100,000,000,000,000,000,000,000,000,000 stars in the universe, yeah, it's probably happened somewhere. He will probably need to rely on this justification to have the kind of solar system he's talking about anyway. So he is certainly justified in handwaving this however he likes. It's just more likely to be a Jovian-scale giant rather than a Neptunian one.

Originally Posted by thrash

The engineers who proposed the M2P2 system for crewed spaceflight clearly don't agree.

Those who propose concepts rarely naysay their ability to perform usefully. And if you'd like, I'll go pull responses from other engineers who think it's not worth even considering. This is common anytime new technology is proposed. Only building and testing one would truly settle the debate...or then again maybe not. So yeah, it may be a cool idea, but from a worldbuilding perspective, what I'd ask myself is, "Is this steampunk? Does it fit the setting? Does it suit my story's purposes?" If so, run with it and handwave any "realistic" nonsense that gets in the way. If not, ditch it even if it's perfectly reasonable (otherwise he'd be using Saturn Vs to launch his lunar expeditions).

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I'd be curious to know what the OP's thoughts are in general. It strikes me that he probably doesn't care what some engineers think or even what an n-body system implies. But if he does, I'll gladly hit up some sources and pull more detailed information with proper references to papers. Otherwise, I'd like some creative direction so I can better give my uninvited advise :)

[thrash]

Originally Posted by Humabout

If you're looking for something likely, consider that the only rocky planet or dwarf planet known to have an active metallic core is Earth, and that includes both an extra-hot planet right next to the sun (it's getting heat in spades) and Venus, a near duplicate of Earth in size, but with approximately 20% more solar heating.

So 25% probability, with sample size n = 4?

Mercury and Venus both have exceptionally low rotational rates, which suggests that their examples should be considered with some care. Rocky moons, tidally bound but in orbits on the order of days, are in a different environment. As you yourself pointed out, the icy moons of Jupiter have small but measurable magnetic fields presumably derived from salt water oceans below their surfaces maintained by tidal heating.

Those who propose concepts rarely naysay their ability to perform usefully. And if you'd like, I'll go pull responses from other engineers who think it's not worth even considering.

<shrug> It's been peer-reviewed, with new results published off and on for the last 15 years. I don't see any of the nay-sayers in the published literature.

You are referring to M2P2 and not the so-called EM drive, though, aren't you? All of your linked threads were about the latter, which is a very different animal.

[Humabout]

Originally Posted by thrash

Mercury and Venus both have exceptionally low rotational rates, which suggests that their examples should be considered with some care. Rocky moons, tidally bound but in orbits on the order of days, are in a different environment. As you yourself pointed out, the icy moons of Jupiter have small but measurable magnetic fields presumably derived from salt water oceans below their surfaces maintained by tidal heating.

The difference between maintaining a temperature above 273 K to a depth of 75ish miles (or somewhere in that order of magnitude) of 975 miles and maintaining a core temperature above 1,811 K are two different things. Where I'd expect to see some interesting tidal effects would be on moons orbiting the a gas giant orbiting in the habitable zone of a red dwarf. The proximity of the two bodies should produce much larger tidal forces on those moons than a situation like Europa around Jupiter. Still not sure it's enough, though.

[Fred Brackin]

Originally Posted by thrash

...
A more interesting point is radiation protection for ships in transit and stations (either fixed or free-flying) beyond the protection of the habitable moons' magnetic fields. It's relatively easy to give the ships magnetic radiation shields as a collateral drive benefit. For stations, either allow limited "force fields" just for radiation shielding, or .

From what I remember of the relevant Transhuman Space playtests (and a few other sources) the radiation of Jupiter's magnetosphere is much less of an issue than appears to be assumed here.

It's al mid to low energy charged particles and in terms of Gurps radiation rules it multiplies PF by 20. Any probable ship hull (and perhaps especially something steampunkish made of iron or steel) should be able to stop those particles.

Likewise any atmosphere reaching Earth-like levels of breathability will stop such particles as well. The surface of some of the real world Galileans is dangerous to humans in thin-skinned suits or vehicles is because there is no significant atmosphere.

[Flyndaran]

Originally Posted by Humabout

The difference between maintaining a temperature above 273 K to a depth of 75ish miles (or somewhere in that order of magnitude) of 975 miles and maintaining a core temperature above 1,811 K are two different things. Where I'd expect to see some interesting tidal effects would be on moons orbiting the a gas giant orbiting in the habitable zone of a red dwarf. The proximity of the two bodies should produce much larger tidal forces on those moons than a situation like Europa around Jupiter. Still not sure it's enough, though.

That would create some horrific motion of the ocean, so to speak. Quakes on the order of bad disaster movies ala 2012.

[Humabout]

Originally Posted by Flyndaran

That would create some horrific motion of the ocean, so to speak. Quakes on the order of bad disaster movies ala 2012.

Oh, I said "interesting" for a reason. :p There would be all sorts tectonic activity, I'd think, too - volcanism, earthquakes, etc. I'm not sure it'd be truly uninhabitable, but it'd be difficult to colonize.