Designing a fictional FTL drive

[Batman]

So-how, exactly, does it convert heat []that never touches it[/i]? Unless your Phlebotnium array is sitting right next to the sun and about a lightsecond across, you're not going to collect any significant fraction of the energy the star radiates. and if it is, unless Phlebotnium is the cheapest and most abundant substance in the universe, it would be far more efficient to simply go with fusion reactors (and would probably still be even then).

[Sleck]

I believe that Simon_jester proposed that a star's gravity well plays a part in the drive's function - I see nothing wrong with this and suggest that while the heat and light is used as an energy source (but can be replaced by fusion, though an an entire fusion reactor would be significantly harder to manufacture and install than a simple tech solution you got from Merlin that fits on your outer surface and absorbs EM radiation), the gravity is what enables travel in the first place, though of course it is up to The Disintegrator as to what he decides on.

[Batman]

I think you still don't quite grasp how incredibly little of the energy the star pours out actually touches your ship. A 1km x 1km square gets you, at perfect efficiency, sitting right on top of the 'surface' of the sun, 6.311E13W. A fusion reactor can do that by burning a whopping 360 grams of hydrogen a second.
As I already admitted the gravity theory works for me, and I've offered another (if admittedly whacky) alternative. My one and only gripe is that baring a technobabble direct tap, having to be close to a star for power collection simply doesn't work.
BTW, S_J-random number, educated guess, or what? 'Surface' gravity for our sun is not quite 28 g.

[Tethis]

I'm brand new to this board, but designing a plausablle FTL drive is something to which I've been giving a lot of thought recently for my own project. (sorry Einstein)

I think the op is on the right course. Decide what you want it to do first, THEN decide on any technobabble you want to surround it with (and maybe don't bother putting most of the technobabble in the story). Basically, I think the advise in places like Atomic Rockets is really sound.

Sounds like you're heading towards a fairly classic approach -- use FTL to get to the edge of the system, then STL to move around in-system.

A few questions to think about.
1. Does it take more fuel to move around within a system at STL speeds or to make the jump between systems at FTL speeds.

2. Do you want a "jump" style FTL like Traveller, Mote in Gods Eye, or Battlestar Galactica or a warp style drive like Star Trek. Since you say you want STL battles I"m guessing you'll probably want a "jump" style FTL.

If an enemy has to bring his fleet in at the edge of a star system and trudge to the target at STL speeds, while under observation all the way, it's likely that any battles will be far from any planet. Is that a problem?

Perhaps the biggest problem I see with that approach is your desire to do that and also avoid WMDs. It turns out it's the STL drive that's the problem and not the FTL drive if you want to get in from the edge of the system in a reasonable time. Suppose your STL drive is a 1-g torch drive (you can accelerate at 1-g until the cows come home) and you have to cover 40 AU to get from the edge of the star system to the planet you want to invade/trade with/whatever. Doing a really quick back of the envelope calculation, it looks like it'll take you a bit over 18 days to cover the distance accelerating at 1-g half the way and decelerating half the way. You can cut that in half if you're willing to go at 4-g but without some sort of inertial compensation the crew may not be good for much when they arrive.

So far so good. 18 days may not be a problem in a story.

Unfortunately, this brings up an ugly issue if you want to avoid weapons of mass destruction. Such a drive will have a total delta-v for that trip in from the edge of the solar system of a bit over 15,000 km/sec!!! NASA would kill for such a drive. If that same ship just accelerated all the way to the planet and impacted it (didn't slow down) and if it massed 100 tons, it would hit with an energy of around 2691 megatons. By contrast, the biggest nuclear bomb ever detonated was 57 megatons.You may be heading into dinosaur killer range with that. You definately have a WMD.

You could just ignore the problem I suppose. After all, that worked for Larry Niven in his Known Space series. By contrast, in his Mote in God's Eye which had torch ships he mentions that the warship could slag a planet so Niven wasn't avoiding WMDs but worked his story around them.

So if you want to avoid weapons of mass destruction, you may need to avoid torch drives for your STL. That means you need to reduce the power of your STL drive (FTL drive can still be anything since it's fantasy regardless). This means you have to settle for a less capable STL drive with FAR lower delta-v. This means that if you still jump in at the edge of the system it will take you FAR longer to get to the action. If you're not willing to wait months perhaps the best course would be to jump in closer to the planet, but still too far away to detonate warheads on top of cities and far enough away to have interesting space battles.

The old Traveller RPG game had a good solution. You couldn't jump in closer then 100 planetary diameters of any large (i.e. has significant gravitation) body. (I THINK it was 100 diameters, it's been years since I played that game). This still gave you plenty of room for space battles without the issues of a torch drive that can take you across a solar system in a couple weeks. However, it DOES mean the defenders will have to be on their toes. If an invasion force jumps in to attack Earth (diameter 12756 km) it will have to jump in at a distance of 1,275,600 km. Coming in at 1-g (again accelerating half the way, decelerating half the way) they'll take a bit over 6 hours to get to Earth. You can play with the distance at which they jump in to get something you like (if you increase that distance by 4, you increase the time required by 2). In any event, I think that a six hour warning should give plenty of time for an interesting space battle.

Taking the above example, something which covers that distance at 1-g would have a total delta-v of 223 km/sec which is MUCH more reasonable although still way beyond anything NASA can manage today. It's still a lot of energy though. A 100 ton ship which just accelerated all the way and impacted would hit with a force of about 600 kilotons. But it beats the previous by several orders of magnitude.

If you do that I'd suggest you not rip off Traveller TOO blatently. But you want things jumping in much closer to the planet.

I've got a lot more thoughts on this subject but I'll leave this for now.

[jwl]

I'd probably start by having a look at some of the papers that came out during the FTL neutrino error that happened last year. If you assume FTL is possible, those might be some of the best guesses to how.

[Simon_Jester]

BTW, S_J-random number, educated guess, or what? 'Surface' gravity for our sun is not quite 28 g.

I could have sat down and done the math quite easily, have in the past, forgot the answer, and picked something I figured sounded about right.

[Batman]

BTW, S_J-random number, educated guess, or what? 'Surface' gravity for our sun is not quite 28 g.

I could have sat down and done the math quite easily, have in the past, forgot the answer, and picked something I figured sounded about right.

Just idle curiousity, because for a random number it would have been astoundingly close