Dirt Cheap Torchships?

[lwcamp]

Originally Posted by (E)

Then the are a whole pile of weapon based applications for the same tech, what happens if you fire a beam weapon through a visser wormhole?

If it is a laser - you just get a laser coming out of the other end. Photons are their own antiparticles.

If it is a particle beam, you will get a slight increase in energy on target. Since particle beams useful in space combat will be going so close to the speed of light that almost all the energy is in the form of the kinetic energies of the particles, the extra energy from annihilation will be small. Wither it is negligible or worth the extra complications of sending the beam through a wormhole depends on your tech assumptions. For 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.

If it is some other kind of beam weapon - well, it depends on the beam weapon type.

Luke

[lwcamp]

Originally Posted by acrosome

A question for the physicists: Would there be any issues with one of these Visser-wormhole antimatter rockets flying through one of these interstellar (Lorentzian?) wormholes?

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.

Originally Posted by acrosome

Would the engine have to be shut down and the Visser wormhole allowed to collapse to pass through, and then need to be re-initiated on the other side?

I can't think of why.

Luke

[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?

[Phoenix_Dragon]

Originally Posted by lwcamp

The Atomic Rockets site has a page devoted to torch ships that covers the waste heat problem
http://www.projectrho.com/public_htm...torchships.php
In this case, you will need to shield the systems from gamma, pion, and muon radiation rather than x-rays and neutrons, but the concept is the same.

That was one of the sources I was thinking of, too. They've also got a good write-up of antimatter as a fuel here.

Basically, it looks like if you're going to be getting 1g-plus acceleration, you have to use a magnetic nozzle (As a physical one would vaporize under the heat). It also looks like only about 66% of the annihilation energy will be charged particles that can be guided by the nozzle, while the other 34% just go wherever they want. So you've got a huge amount of energy radiating out. And a small portion of that energy is in gamma radiation. Looks like about 0.5% of the annihilation energy will be gamma rays. Unfortunately, that means that small portion is really a huge amount of gamma rays. I... don't feel like going through all the calculations there to find out just how much you'd get from an engine of a particular size (It's late and I'm close to nodding off, sorry!), but it appears to be a huge amount. You'd want that engine far away from any habitable parts of the ship, and a large amount of shield material between the crew and the engine. And even then, I think your radiation shield would be soaking up a good amount of heat that has to be radiated away.

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.

Originally Posted by Ulzgoroth

Is that really a problem for a rocket? You can use your reaction mass as coolant before feeding it into the reaction chamber, and then it gets thrown out and stops being your problem. It's certainly a thing for a power plant, though I wouldn't think any more so than any other sort of antimatter power plant.

The link lwcamp gave gives a good explanation of why such open-cycle cooling wouldn't work. Short version is, it works for chemical rockets because they have a very high propellant flow rate expelled at low velocity. A torch drive (High thrust, high delta-v, like this proposed drive) is the opposite: a very low propellant flow rate expelled at extremely high velocity. There just isn't enough propellant to take all the heat (And if you dump more propellant to carry away more head, you decrease exhaust velocity until you're somewhere back to chemical rocket levels of delta-v).

[lwcamp]

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?

Because if you make a wormhole (rather than just finding one), both ends of the wormhole will be right next to each other. Physics is local.

Luke

[lwcamp]

Originally Posted by Phoenix_Dragon

That was one of the sources I was thinking of, too. They've also got a good write-up of antimatter as a fuel here.

Awesome!

Originally Posted by Phoenix_Dragon

Basically, it looks like if you're going to be getting 1g-plus acceleration, you have to use a magnetic nozzle (As a physical one would vaporize under the heat). It also looks like only about 66% of the annihilation energy will be charged particles that can be guided by the nozzle, while the other 34% just go wherever they want. So you've got a huge amount of energy radiating out. And a small portion of that energy is in gamma radiation. Looks like about 0.5% of the annihilation energy will be gamma rays. Unfortunately, that means that small portion is really a huge amount of gamma rays. I... don't feel like going through all the calculations there to find out just how much you'd get from an engine of a particular size (It's late and I'm close to nodding off, sorry!), but it appears to be a huge amount. You'd want that engine far away from any habitable parts of the ship, and a large amount of shield material between the crew and the engine. And even then, I think your radiation shield would be soaking up a good amount of heat that has to be radiated away.

A minor nitpick - 1/3 of those pions that are created by the proton-antiproton annihilatio will immediately decay into extremely energetic gamma rays. So the 0.5% of the energy coming as gamma rays from positron-electron annihilation will be joined by about 33% of the energy coming as gamma rays from proton-antiproton annihilation.

The electron-positron annihilation gamma rays are fairly benign as far as gamma rays go. Sure, they will deliver a lot of heat and ionizing radiation, but they're not too hard to block at only 511 keV and they will mostly interact via photoabsorption so they won't have too much scattering. The pion decay gammas, on the other hand, will be so energetic that they can participate in photo-nuclear interactions. These will have the annoying properties that they can turn your heat and radiation shields radioactive and slowly transmute and disintegrate the matter in them as they blast nuclei apart. It will take quite a high fluence of them - but then, with a torch ship you will have a high flux to begin with. i haven't run the numbers yet to figure out how long you've got before you need to replace your heat shields.

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.

Yup. That sounds about right. You can also have a tractor configuration - with the main body of the spacecraft towed behind the engines on a very long cable - like kilometers long if necessary.

Luke

[thrash]

Originally Posted by acrosome

So, is this at all plausible technobabble?

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.

[Phoenix_Dragon]

Originally Posted by lwcamp

A minor nitpick - 1/3 of those pions that are created by the proton-antiproton annihilatio will immediately decay into extremely energetic gamma rays. So the 0.5% of the energy coming as gamma rays from positron-electron annihilation will be joined by about 33% of the energy coming as gamma rays from proton-antiproton annihilation.

Ooooh yeah. Forgot about that, heh. And thanks for the writeup, too! :)

You can also have a tractor configuration - with the main body of the spacecraft towed behind the engines on a very long cable - like kilometers long if necessary.

Yerg. I'm reeealy leery of puller designs. An engine's efficiency as a engine is directly proportional to its efficiency as a weapon and all that. Even at a few kilometers, I imagine the exhaust would be thoroughly deadly. You'd have to have the engines very widely spread to give the main craft clear of the drive plumes, you'd have to take extra care any time you need to turn your vector of thrust (So you don't turn the main craft into a pendulum swinging into the exhaust)... heck, you'd have to be very careful to start at extremely low accelerations to carefully bring the cable up to tension before applying full thrust so you don't snap the cable or whip the main craft around. And if some technical issue crops up (Even something as simple as a momentary fuel flow hiccup), the whole thing could turn off-axis and burn through your ship.

[Ulzgoroth]

Originally Posted by Phoenix_Dragon

The link lwcamp gave gives a good explanation of why such open-cycle cooling wouldn't work. Short version is, it works for chemical rockets because they have a very high propellant flow rate expelled at low velocity. A torch drive (High thrust, high delta-v, like this proposed drive) is the opposite: a very low propellant flow rate expelled at extremely high velocity. There just isn't enough propellant to take all the heat (And if you dump more propellant to carry away more head, you decrease exhaust velocity until you're somewhere back to chemical rocket levels of delta-v).

Torch engines are not a very low propellant flow rate - that distinction has to go to the high-energy non-torch rockets that have orders of magnitude lower thrust at the same delta-V values.

However, it does look like for instance the referenced 'antimatter plasma', at spaceships torch stats, might have a flow rate of only 23 grams per second (for 1.5 tons worth of rocket) which probably is too low to be achieve much cooling.

[Anthony]

Originally Posted by Ulzgoroth

Torch engines are not a very low propellant flow rate

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.