[Space] Steampunk Firefly Idea

[scc]

The Mag Sail thing is interesting, maybe not for surface to orbit work, but for getting around once your in orbit, very nice. What I'll probably do for travel time is 1d-3, if the number is negative wait that number of weeks for magnetic null zone to clear, if 0 re-roll

[Daigoro]

I was Googling around for some more info on magsailing around inside Jupiter's magnetosphere, and there's suprisingly little out there. Atomic Rockets talks about magsails, but they're mainly focused on boring interstellar trips.

There was this one website however...
http://www.sjgames.com/gurps/Rolepla.../MagSails.html

[thrash]

Didn't know that was openly available -- excellent. Note that the magsails in GURPS Spaceships are at the 50 tons/ton of thrust level. If the magnetic liftoff stunt interests, you might want to use a superscience version capable of 0.01G per system (5 tons/ton of thrust).

[Daigoro]

Originally Posted by thrash

If the moons are habitable, they probably require fairly significant magnetic fields of their own to shield from particles streaming off the gas giant.

Ganymede does, though it doesn't appear to be that strong.

A sufficiently strong magnetic field and correspondingly excellent magsails would allow launch from the moons' surfaces, although only at their magnetic poles

Note that the magnetic poles can be at fairly low latitudes, so they could be in relatively temperate areas. Is it possible that they could stay fixed relative to the primary it's orbiting, while the moon's surface rotates under them? This would mean that there could be a band of launch sites on the surface in different countries- although I might be overestimating the thrust available from a magsail here.

Another possibility, going the superscience steampunk route, is that launch bases are built with massive powerful electromagnetic field generators, allowing magsail ships to be launched from the surface. Bases might have to be located near mountaintops and would look like heavy industrial sites. There might even be environmental fears that too many of these magnetic launch sites may disrupt the moon's natural magnetic field.

Originally Posted by scc

What I'll probably do for travel time is 1d-3, if the number is negative wait that number of weeks for magnetic null zone to clear, if 0 re-roll

To be honest, this is probably the simplest solution. I don't know about a "null zone" though- maybe disruption from solar storms, the nearby passage of Io with its associated ion and plasma emissions, or magnetic storms from the gas giant instead.

I was thinking though about looking at a typical magsail journey. Once out of the moon's orbit, on a trip from one moon to another further out (like Europa to Ganymede), the ship first deploys sail to drag against the gas giant's magnetic field to start a Hohman transfer. This moves the ship from a higher circular orbit to a lower one, by passing through an elliptical arc with its periapsis on the lower orbit. The initial target orbit is in Io's high energy plasma torus, where the magsail can quickly pick up velocity on each pass to raise its apoapsis to the orbit of the higher destination moon. Once the right apoapsis is achieved, further acceleration impulses would be needed at the higher orbit to bring the craft into a circular orbit, or if the correction synchronisation is chosen the craft could slip directly into orbit around the destination moon. It seems that Europa at least also has a plasma torus, so this would also help with gaining orbital velocity. (All according to my imperfect understanding- corrections welcome.)

I'd imagine a magsail pilot sitting at controls with a number of compass globes showing the direction of magnetic field lines from sensors around the periphery of the ship, with one hand on the voltage control for the magsail and another on its tilt mechanism. He'd have charts and almanacs for the day's orbits and conjunctions of the major moons, and a telescope trained on the magsail at the end of its mile-long tether.

Action would be concentrated in short bursts at various periapsis and apoapsis passes, followed by long periods of waiting through the remainder of the orbits. Each orbit may take hours or days (comparable to the orbital periods of the moons?) so indeed passing from one moon to another could take a number of days.

Yep, you could play all that out, but it's probably better to simply roll dice for how long the journey takes.

[scc]

Originally Posted by Daigoro

To be honest, this is probably the simplest solution. I don't know about a "null zone" though- maybe disruption from solar storms, the nearby passage of Io with its associated ion and plasma emissions, or magnetic storms from the gas giant instead.

The magnetic force lines of gas giant's are frequently described as tangled, the idea here was because of that they planet they where leaving from wasn't currently on a line of force, or if it was it wasn't useful

Originally Posted by Daigoro

I was thinking though about looking at a typical magsail journey. Once out of the moon's orbit, on a trip from one moon to another further out (like Europa to Ganymede), the ship first deploys sail to drag against the gas giant's magnetic field to start a Hohman transfer. This moves the ship from a higher circular orbit to a lower one, by passing through an elliptical arc with its periapsis on the lower orbit. The initial target orbit is in Io's high energy plasma torus, where the magsail can quickly pick up velocity on each pass to raise its apoapsis to the orbit of the higher destination moon. Once the right apoapsis is achieved, further acceleration impulses would be needed at the higher orbit to bring the craft into a circular orbit, or if the correction synchronisation is chosen the craft could slip directly into orbit around the destination moon. It seems that Europa at least also has a plasma torus, so this would also help with gaining orbital velocity. (All according to my imperfect understanding- corrections welcome.)

I'd imagine a magsail pilot sitting at controls with a number of compass globes showing the direction of magnetic field lines from sensors around the periphery of the ship, with one hand on the voltage control for the magsail and another on its tilt mechanism. He'd have charts and almanacs for the day's orbits and conjunctions of the major moons, and a telescope trained on the magsail at the end of its mile-long tether.

Action would be concentrated in short bursts at various periapsis and apoapsis passes, followed by long periods of waiting through the remainder of the orbits. Each orbit may take hours or days (comparable to the orbital periods of the moons?) so indeed passing from one moon to another could take a number of days.

Yep, you could play all that out, but it's probably better to simply roll dice for how long the journey takes.

Very interesting and very steampunk, but I wonder is it's practical. At the very least these things are probably use some sort of vacuum-tube based electronics because of EM interferrance

[Anaraxes]

Originally Posted by scc

these things are probably use some sort of vacuum-tube based electronics because of EM interferrance

For steampunk, I'd go with a mechanical analog computer, like the ones used for fire control on WW2 battleships (and retained on the Iowa class when the US Navy brought those back during the 80s). The spaceship is always going to need its flight computer, so lack of programmability isn't a problem.

[Humabout]

So much to respond to and typing on my phone, so I'll just do a couple of highlights.

1. You might want to look at an article called Halfway to Anywhere from Pyramid #3/79 - Space Atlas. That will give you everything you need to sort out delta-V requirements for moving between worlds. Remember that your primary is the gas giant, and all masses and distances are in relation to Earth (because of the precidents set by Space). The raw formulae are also presented if you prefer SI units.

2. Jupiter's magnetosphere so thoroughly irradiates the gallilean moons, minus Callisto, that people will suffer acute radiation syndrome and die in days if not hours.

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).

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).

5. Damned cool idea in general. Personally, I'd drop all TL7 stuff and look up the scientific assumptions of the Victorian period and treat them ALL as true. Base your spaceships on that. Make it "hard scifi" to what was known and you will better achieve the feel than trying to assume modern physics and chemistry - neither is very forgiving, especially when your only power sources are pressurized steam and gunpowder.

[thrash]

Originally Posted by Daigoro

Ganymede does, though it doesn't appear to be that strong.

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

Originally Posted by scc

The setting in a nut shell is K2 star with a gas giant at 0.62 AU with 3 habitable moons, all three moons will be about half the diameter of Earth, so quite small.

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. It might be worthwhile to calculate the brachistichrone orbit as Humabout suggests, then reduce the time on a successful Navigation(Space) roll -- say, 10% per point of success, up to maybe 50%.

[Humabout]

Originally Posted by thrash

It might be worthwhile to calculate the brachistichrone orbit as Humabout suggests, then reduce the time on a successful Navigation(Space) roll -- say, 10% per point of success, up to maybe 50%.

By definition, a brachistochrone transfer is the fastest transfer orbit possible. Rather, you should use it as a cap on how fast you can possibly reach a destination for any given acceleration. It would be reasonable, however, to increase travel time by any amount so long as it never exceeds a Hohhman transfer (the slowest transfer orbit). For low thrust transfers, I'd probably eyeball a minimum transfer duration at halfway between a brachistochrone and a Hohhman, just as a ballpark without any math behind it.

It's also worth mentioning that while Ganymede - and to a lesser extent, Europa - have magnetic moments, it's currently believed to be a result of a liquid ocean of saltwater moving beneath the frozen surface and is truly minuscule. IIRC, Europa's manages to shift the field lines of Jupiter's magnetosphere by all of 0.4% of a degree - which is how it was inferred.

[thrash]

Originally Posted by Humabout

By definition, a brachistochrone transfer is the fastest transfer orbit possible.

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

It's also worth mentioning that while Ganymede - and to a lesser extent, Europa - have magnetic moments, it's currently believed to be a result of a liquid ocean of saltwater moving beneath the frozen surface and is truly minuscule.

Still not talking about the Jovian system per se. 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.

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 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.

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.