Dirt Cheap Torchships?

[Ulzgoroth]

Originally Posted by Anthony

They are very low propellant flow rate relative to their power level. Basically, if you use propellant as coolant, it reaches the same temperature as it would in a thermal rocket. Keeping it cool enough for conventional materials then caps Isp at around 15-20 km/sec for hydrogen, much less for other fuels.

This assertion seems to depend on a lot of assumptions (or calculations) about how much waste heat you're dealing with

[Anthony]

Originally Posted by Ulzgoroth

This assertion seems to depend on a lot of assumptions (or calculations) about how much waste heat you're dealing with

Well, yes, a thermal rocket does have some variance depending on how efficiently it converts heat to thrust, but it simply doesn't matter that much, as the temperature varies with the square of exhaust velocity.

Torch drives in Spaceships start at 15 mps/tank, corresponding to an exhaust velocity of 480 km/s. That's an energy of 115 GJ/kg. Hydrogen gas has a specific heat that's in the range 13-18 kJ/kg*K over the heat range of solids, and can be heated by maybe 2500K, so it can absorb on the order of 35 MJ/kg, meaning cooling it by fuel flow requires a 99.97% efficiency (even 20 km/s requires 85%).

[Ulzgoroth]

Originally Posted by Anthony

Well, yes, a thermal rocket does have some variance depending on how efficiently it converts heat to thrust, but it simply doesn't matter that much, as the temperature varies with the square of exhaust velocity.

Torch drives in Spaceships start at 15 mps/tank, corresponding to an exhaust velocity of 480 km/s. That's an energy of 115 GJ/kg. Hydrogen gas has a specific heat that's in the range 13-18 kJ/kg*K over the heat range of solids, and can be heated by maybe 2500K, so it can absorb on the order of 35 MJ/kg, meaning cooling it by fuel flow requires a 99.97% efficiency (even 20 km/s requires 85%).

How much waste heat the engine accumulates is not the same as how hot the reaction mass is.

[Anthony]

Originally Posted by Ulzgoroth

How much waste heat the engine accumulates is not the same as how hot the reaction mass is.

If you're using the reaction mass to cool the engine it is.

[Ulzgoroth]

Originally Posted by Anthony

If you're using the reaction mass to cool the engine it is.

Only if you're defining all heat production in the engine as 'waste'. Or assuming that you don't do anything with the reaction mass other than use it as coolant and expel it.

[Anthony]

Originally Posted by Ulzgoroth

Only if you're defining all heat production in the engine as 'waste'.

Well, yes, any heat that doesn't get converted into motion is waste. Not sure why that matters, though, we only need to worry about waste heat as non-waste heat is presumably doing something productive instead of causing problems, and efficiency is based on the portion of available energy that gets turned into something useful rather than wasted.

Originally Posted by Ulzgoroth

Or assuming that you don't do anything with the reaction mass other than use it as coolant and expel it.

I'm not sure what this has to do with anything. The reaction mass has known heat capacity; if the waste heat of the drive exceeds that, it's inadequate for cooling.

[Ulzgoroth]

Some of your replies still make no sense to me, but I seem to have gone off the rails in post #23. Your second paragraph in #22 seems solid to me on review (though I have no idea what the first was supposed to mean).

[Anthony]

Originally Posted by Ulzgoroth

Some of your replies still make no sense to me, but I seem to have gone off the rails in post #23. Your second paragraph in #22 seems solid to me on review (though I have no idea what the first was supposed to mean).

Just that there's a reasonable range in the actual amount of inefficiency, but since the energy per kg of reaction mass is second order in exhaust velocity, the efficiency required goes up very fast as exhaust velocity increases.

[acrosome]

Originally Posted by lwcamp

But, if you have small, light wormholes, there are other implications. Like - why do you need torch ships in the first place? Send one end of the wormhole to where you want to go, and then just step through.

Yes, that I anticipated. I was going to come up with handwavium to keep the wormholes in space. Maybe the Lorentzian ones are massive, like in the Orion's Arm setting. Or maybe the wormhole stabilization process needs flat space, or something. Because I want more conventional interface methods. And, yes, to keep the antimatter engines that spew radioactive death away from worlds.

Originally Posted by lwcamp

You can get a wormhole's mass down to probably something like the Planck mass, which will only release 500 kg TNT worth of explosive oomph (or maybe 100 kg TNT worth, depending on whether the base Planck energy or the reduced Planck energy is more relevant), so small wormholes could be kept on a planet in remote or highly reinforced locations - but they won't have enough mass reserve to send anyone through them and would mostly be used for communication, or transmitting power, or maybe making antimatter locally for a power plant. Now you can put the antimatter-making wormhole on your spacecraft. Just be aware that if it collapses, things will not end well for the spacecraft.

Hmm. I really don't want FTL communications or an ansible. I'll have to think about that.

Originally Posted by lwcamp

IFor example, if you just left the particle beam machinery, along with its power supply and cooling systems and technicians and janitorial service and other infrastructure, behind on a planet and just carry one end of the wormhole (with the other end parked right in front of the particle beam), then there's no real reason not to make it a non-orientable wormhole and get a bit of extra bang from your beam.

Hmm. I like to avoid having tiny drones that can fire battleship-eviscerating wormhole particle beams. I'll have to think about that, too.

Originally Posted by lwcamp

None that I have been able to identify. Physics is local (or at least the general relativity part of physics is) so the Visser wormhole would treat the space-time of the Lorentzian wormhole just like any other space-time and go along happily. The causality-isses with time lag and what not will be handled automatically with the small wormhole goes through the larger, so you shouldn't get any unexpected time loops that way.

Hmmm. Noted. :)

Originally Posted by doctorevilbrain

I'm surprised no one asked the obvious questions. How do you know that the wormhole is going to get to where you want it to go? How do you capture the antimatter and put it into the spaceship?

Both ends of the Visser wormhole are carried with the spaceship, and they are micrometers apart. It's basically a device to produce antimatter on the fly, as needed.

Originally Posted by Phoenix_Dragon

It's probably possible, but it would probably look very different from the usual image that comes to mind for a space ship. I'm picturing an engine held out on a loooong gantry holding it several tens of meters (Maybe even a hundred or more, if possible) from the main body of the ship, and that main body would probably want to be narrow to minimize shield weight and absorption area. And it'd probably still have to have some pretty good radiators.

Yes, I'm all over that, actually. Sort of like the ships in the Tens Worlds setting for Attack Vector: Tactical

Originally Posted by thrash

Although a Visserian wormhole can be created using an arbitrarily small about of exotic matter, it takes a fair amount of mass to expand it to usable size. Using the formula in Visser's book (Lorentzian Wormholes: From Einstein To Hawking), a traverseable wormhole with a throat radius of 500 nm and a mouth radius of 300 m needs a mass of 10^9 tonnes. Obviously, this would not be particularly useful as part of a spacecraft engine, but it might be worthwhile for creating anti-hydrogen in bulk.

Dammit. I hate it when real physics won't cooperate. I may ned more handwavium, there.

[lwcamp]

Originally Posted by thrash

Although a Visserian wormhole can be created using an arbitrarily small about of exotic matter, it takes a fair amount of mass to expand it to usable size. Using the formula in Visser's book (Lorentzian Wormholes: From Einstein To Hawking), a traverseable wormhole with a throat radius of 500 nm and a mouth radius of 300 m needs a mass of 10^9 tonnes. Obviously, this would not be particularly useful as part of a spacecraft engine, but it might be worthwhile for creating anti-hydrogen in bulk.

That book was from 1995, however, so if there's a more recent source with a different set of equations I would love to see a cite.

This gets kind of tricky. At the discontinuities of the edges that support the wormhole (and assuming negligible tension, and a planar shape), you require a linear energy density of -3.04E43 J/m, or -3.4e26 kg/m. That's a lot. But this wormhole is in flat space-time. This means it has no gravitational imprint on the surroundings. Someone floating next to it would feel no gravitational attraction. By the equivalence principle, this means that the wormhole mouth considered as a single object (rather than its constituent parts) has no net mass. Therefore, it would require no energy to create.

If you want to, you can envision that the negative mass of the wormhole is balanced by the positive energy in the gravitational field at the singular regions supporting the wormhole. This isn't a very exact way of thinking about it, since there is no way to localize the energy of the gravitational field or associate it with a specific energy density, but it is not too wrong.

Now a Visser wormhole is an idealization, an approximation that can probably never be realized. But it may be possible to get close enough to this ideal that the Visser wormhole makes a useful model. In practice, one might expect a wormhole to require some energy to create (and thus it will have some mass), even if for no other reason than to prevent it from collapsing when something goes through it. But although the components will have insane energy densities, there is no a priori reason to expect that the energy or mass of the entire mouth will be excessive (nor is there any reason to expect that they won't be - this is still an unknown area at our current level of understanding).

Luke