Good writing weekend!

[seawasp]

This weekend was a good one for writing; Kathleen and company were able to avoid major kid disasters or blowups, so I ended up with both Saturday and Sunday clear. I was in a Jason Wood mood on Saturday and wrote nearly 5,000 words in the first new Morgantown story since I finished Digital Knight back in early 2003; title is "Shadow of Fear". On Sunday, I went over 15,000 words on Threshold, the sequel to Boundary. Effectively I think this is over 20,000, as there's quite a few additions I expect from Eric.

Threshold is probably going to be the hardest of the three Boundary-universe books to write; we're developing the political, social, and personal universe equally in Threshold, and it's COMPLICATED. I was able to sort of fake out all of the stuff in Boundary, even though Eric seriously modified things; I can't pull that off on some of these, though. Eric will have to write those sections and, for once, I'll have to edit/smooth out his stuff to make it fit with mine. There's also a LOT of new science-related stuff I'm having to do -- people on Baen's Bar are seeing some of that, with me asking questions about nuclear reactors and superconductors and orbital calculations -- which isn't easy, even though I can (as an SF writer) decide to occasionally handwave away practical difficulties. Once Threshold is done, I'll have a good enough grasp of the entire universe to just write the next one, at least as a rough first draft. But Threshold itself is going to "be a pure bitch", as Maddie would say.

Comments

Yeah...

[seawasp]

... interplanetary -- I want basically trips from Mars to Ceres, and Ceres to Jupiter and/or Saturn. (then, ideally, some idea of transits within those sub-systems, but I could probably handwavey and BOTE those)

Also, how would one use a planet like Jupiter to slow down, rather than speed up, and how much of a reduction in velocity could you get? Say you were going faster than your target and had to slow up quick?

Re: Yeah...

[(Anonymous)]

I've been discussing similar moves with Travis Taylor and he suggests using aerobraking. Check '2010' for a fictional example, 'Mars Odyssey' for a real one.

-- Alex S.

Re: Yeah...

[seawasp]

Hmm. That'd depend on the geometry of the craft rather heavily. And its makeup. You wouldn't even TRY that with Nike, for instance, or at least I'd be surprised if someone could come up with a safe way to try that with such a vessel. you'd want something pretty compact, tough, and probably aerodynamic, yes?

Re: Yeah...

[kengr]

Both the slow down and speed up gravity assists tend to involve craft that are *not* yet in orbit around the body, but in a solar orbit.

The difference is whether you pass "ahead" or "behind" the planet as it orbits. It basically tries to "pull you along". Which produces a velocity change on the order of the differencer in velocities.

The *details* are the killer.

And it's not good for a *quick* velocity change.

But here's a thought. Given Jupiter's magnetic field, you might be able to deploy a mag-sail and use *that* for braking. That would avoid the problems with aerobraking, and help with radiation shielding.

Re: Yeah...

[seawasp]

The magbraking I thought of, and would be very interesting, but I can't find anything like sufficient detail on the magnetic field of the planets to determine whether it's actually practical to do, or whether you'll get no noticeable slowdown from the magnetic field until you're already IN the planet's atmosphere.

Re: Yeah...

[kengr]

Given that magsails are normally proposed to use the trapped filed in the solar wind, I think Jupiter's field is strong enough. :-)

Besides, you aim for a close pass, not straight at the planet!

Re: Yeah...

[james-nicoll.livejournal.com]

The forces that apply to magsails are minute, though. This means lots of time and space. I don't think you can halt a space ship at Jupiter this way (Annoying, I know I was in a thread about this but I cannot find it).

Re: Yeah...

[james-nicoll.livejournal.com]

Of course, you could exploit Jupiter's field in a different way: hang a conductive cable off Metis, then trade Metis' potential energy for power. You should be able to get 100,000+ terrawatt-years out of Metis before it deorbits and you could then apply the power to incoming space vessels, directly or otherwise.

Re: Yeah...

[seawasp]

Now that's fun. But it would seem perhaps somewhat difficult to pull off as a sort of improvised method unless one had thousands of miles of conductive cable just hanging around in the closet.

Re: Yeah...

[seawasp]

I rather thought not. Deploying a large magsail or equivalent while cutting through Jupiter's field would have SOME effect, but unless I can find a way to calculate it accurately I'm going to have to assume "not really very much".

Re: Yeah...

[james-nicoll.livejournal.com]

Aside from aerobraking (1), you can do a passive flyby, which can change your velocity thanks to the fact that your path by a given planet will be symetric with respect to the planet but not the star it orbits: the ship gains or looses energy wrt the star by a certain amount but the planet looses or gains in the same amount, opposite sign, so everything balances. You can also do an Oberth Maneuver, which I am very fond of.

Assume a rocket is falling from infinity past a massive object. The fuel in the tanks had a significant potential energy wrt to the object and if you do a burn at periapsis (closest approach), you can exploit this as follows:

delta vee (final) = [(Vesc+Vimpulse)^2 - (Vesc)^2]^1/2

Vesc is the escape velocity at periapsis, Vimpulse the change in velocity due to a rocket burn and delta vee (final) the delta vee you actually get.

This, in contradition to at least 40 years of SF, makes "gravity wells" a valuable resource, although not to the people in them. It's like Liars Poker, everyone else's gravity is useful.

Say the Oh God Don't Let Us Smash into Io and Explode into a Million Superheated Fragments is falling past Jupiter at 50 km/s. Jupiter has a high escape velocity (~60 km/s at the cloud tops) and the crew passes close enough to J that the Vesc is 55 km/s. If they do a 15 km/s burn at periapsis, their final delta vee would be about 50 km/s, bringing them to a dead halt wrt to Jupiter.

Of course, having neglected to take into account the fact that they probably wanted to be in orbit around Jupiter, not hanging very briefly in its sky, the OGDLSiIaEiaMSF then plummets out of space and into the upper atmosphere, where a combination of air pressure and rentry heat destroys the ship. Their math was solid, though.



1: Or Robert Forward's attempt to find a force stronger than gravity to couple to passing bodies, which involved a long bungee cord and a harpoon.

Re: Yeah...

[seawasp]

Aside from aerobraking (1), you can do a passive flyby, which can change your velocity thanks to the fact that your path by a given planet will be symetric with respect to the planet but not the star it orbits: the ship gains or looses energy wrt the star by a certain amount but the planet looses or gains in the same amount, opposite sign, so everything balances.

Hmm. How much delta-V could you get out of that? I'm thinking of Fast Ship chasing Slower Ship, needing to match up with Slower Ship, and not having some other way of slowing down much. Either aerobraking or using the flyby are pretty much its main options here. No rocket fuel available to dump in an Oberth.

Say the Oh God Don't Let Us Smash into Io and Explode into a Million Superheated Fragments is falling past Jupiter at 50 km/s.[...] their final delta vee would be about 50 km/s, bringing them to a dead halt wrt to Jupiter.

Of course, having neglected to take into account the fact that they probably wanted to be in orbit around Jupiter, not hanging very briefly in its sky, the OGDLSiIaEiaMSF then plummets out of space and into the upper atmosphere, where a combination of air pressure and rentry heat destroys the ship. Their math was solid, though.

I always love your examples, James. Though you left out the part where you say something like, "... plummets out of space and into the upper atmosphere, something I have always found much more amusing to watch than to do; the one time I did this, it caused several scars..."



1: Or Robert Forward's attempt to find a force stronger than gravity to couple to passing bodies, which involved a long bungee cord and a harpoon.

To what Forward story are you referring? Or were you just being snarky about Forward because it sounded amusing (which it did)?

Re: Yeah...

[james-nicoll.livejournal.com]

It's from SATURN RUKH, where the ship steals small amounts of momentum from a number of moons using a temporary physical connection along the lines of a harpoon and long cable.

The part I have a problem with is the impromtu connection at the surface of the moon. Otherwise, for low delta vees, the numbers aren't too bad:

Say the Queeqeg comes barreling by the TransSaturnian Station IMMOVABLE OBJECT at 10 km/s. A small ring on a cable is fired out to match veolcities with the ship's tow hook. Assuming that the ship can't take more than 10 gees, it will take about 100 seconds to stop the ship wrt to TST IO and about 500 km.

Depending on the design, you may have to deal with about half a megawatt of heat per kg of ship.

Re: Yeah...

[seawasp]

Hi, James. Going over some of your old (and useful) messages, I was revisiting this one, and had some other questions/thoughts. You can just reply via email if you don't feel like posting.

Aside from aerobraking (1), you can do a passive flyby, which can change your velocity thanks to the fact that your path by a given planet will be symetric with respect to the planet but not the star it orbits: the ship gains or looses energy wrt the star by a certain amount but the planet looses or gains in the same amount, opposite sign, so everything balances.

How much delta-vee can you get out of that? I mean, if I want to either speed up or slow down as I approach Jupiter, can I do a passive flyby to get me to do so by 1 km/s, 10, 50? And is there a limit to how fast I can be flying to begin with (i.e., if I'm booking along at 10,000kps, do I still get the little nudge?)

You can also do an Oberth Maneuver, which I am very fond of.

Assume a rocket is falling from infinity past a massive object. The fuel in the tanks had a significant potential energy wrt to the object and if you do a burn at periapsis (closest approach), you can exploit this as follows:

delta vee (final) = [(Vesc+Vimpulse)^2 - (Vesc)^2]^1/2

Vesc is the escape velocity at periapsis, Vimpulse the change in velocity due to a rocket burn and delta vee (final) the delta vee you actually get.

You use the equation below in this amusing demonstration:


Say the Oh God Don't Let Us Smash into Io and Explode into a Million Superheated Fragments is falling past Jupiter at 50 km/s. Jupiter has a high escape velocity (~60 km/s at the cloud tops) and the crew passes close enough to J that the Vesc is 55 km/s. If they do a 15 km/s burn at periapsis, their final delta vee would be about 50 km/s, bringing them to a dead halt wrt to Jupiter.

Of course, having neglected to take into account the fact that they probably wanted to be in orbit around Jupiter, not hanging very briefly in its sky, the OGDLSiIaEiaMSF then plummets out of space and into the upper atmosphere, where a combination of air pressure and rentry heat destroys the ship. Their math was solid, though.

Rereading this raised two other questions for me:

1) Is the equation the same for trying to SPEED UP as you approach Jupiter or wherever, and if so where are you doing the burn to ensure maximum increase rather than slowdown?

2) Your figures above give the OGDLUSiIaEiaMSF a speed to begin with of considerably less than escape velocity. If they didn't do a burn at all, would they have ended up plummeting anyway? It would seem to me that at the very least they'd be stuck in an orbit around Jupiter rather than continuing on to some happier locale. If not, how slow would they have to be going as they passed Jupiter in order to end up flambe instead of escaping?