Question about time dilation

[Ariphaos]

Rockets don't magically stop working when you have a delta V that exceeds the exhaust velocity, they just get much less efficient.

A rocket does not magically get less efficient because they are moving faster in some frame of reference. All factors removed, a rocket's efficiency is constant.

I have done the math and (if I got all the numbers right) a rocket with an exhaust velocity of 58,900 km/s (the upper limit for AM-fusion)

The upper limit is entirely dependent on the amount of matter that the AM reactor, however it is designed, can focus a unit of reactant energy into. This is, in theory, limited by c, with the caveat that a minimum of 55% of the reaction energy is in neutrinos.

Fusion is limited by firing off its helium ash and thus has a maximum of 31,000 km/s. You are basically adding dumbtech for absolutely no gain.

could get away with a mass ratio of around 200 for a speed of .5 c.

Mass ratio of 200!

It's not great but it's enough to send a small payload (a few hundred tons) to a nearby star within a human lifespan.

Compared to a mass driver or light sail design, which can send a few hundred trillion tonnes. And is not a flying bomb.

Yes, I acknowledge that is an optimistic estimate, and yes there are better systems, but it has the virtue of being cheap in the short term.

No it isn't. Right now it could cost no less than constructing a mass driver or light sail.

In the near term an AM-fusion is a much lower bar to shoot for than a pure AM rocket, which will require expensive infrastructure investment (an Asimov Array).

If your AM fusion rocket has an exhaust velocity of .2 c, then by quadrupling the amount of antimatter generated, you can do away with the fusion entirely and save yourself a few thousand tonnes of fuel.

On the other hand, a mass driver or light sail system would be capable of firing a ship at .2c, which could then decelerate at the target star system on its own, using a pure fusion drive. The light sail requires a smaller infrastructure (and the same infrastructure required for the mass driver), but the mass driver is a one shot investment, and doesn't require obscene levels of precision until you get to actual interstellar migration.

Your own math, for your AM-fusion hybrid, requires a mass ratio of 200 to get a .05 c speed boost over a pure fusion design in combination with a light sail or mass driver.

The light sail described blows the simplified design and mass driver away, of course, but it's less politic-proof.

Not to mention the virtually unlimited speed; delta V limited only by how long you care to keep the laser trained on the ship. The only really big liability is the limited range.

There is an interstellar and interplanetary speed limit imposed by the rarefied medium. This varies of course but it's on the order of .95 to .999 of c depending on where you are. There is also an accuracy issue at extreme distances, as the light sail may be forced to twist or drift when at extreme velocities.

[Junghalli]

A rocket does not magically get less efficient because they are moving faster in some frame of reference. All factors removed, a rocket's efficiency is constant.

I mean it's generally the point where the skyrocketing mass ratio "death spiral" starts.

The upper limit is entirely dependent on the amount of matter that the AM reactor, however it is designed, can focus a unit of reactant energy into. This is, in theory, limited by c, with the caveat that a minimum of 55% of the reaction energy is in neutrinos.

I meant the upper limit given for the fusion-antimatter hybrid mode of the Valkyrie, which is what I was going off.

Fusion is limited by firing off its helium ash and thus has a maximum of 31,000 km/s.

The thrust is created by directing a spray of 'heavy elements'4, created as a byproduct of the fusion reaction. These 'heavy elements' leave the reaction travelling at 12 - 20% of light speed. They are then captured by a magnetic coil and forced into leaving the craft in the same direction, thus providing thrust.

It doesn't give a reference for the higher-end figure though, you could very well be right.

Mass ratio of 200!

That's actually not bad for a .5 c rocket. Relativistic rocketry is a bitch.

Compared to a mass driver or light sail design, which can send a few hundred trillion tonnes. And is not a flying bomb.

A light sail is unquestionably a far better alternative, and is the design I would back now if that article Winston posted is anywhere close to accurate. I'm unacquainted with this proposed mass driver system you speak of.

There is an interstellar and interplanetary speed limit imposed by the rarefied medium. This varies of course but it's on the order of .95 to .999 of c depending on where you are.

I'd say that's plenty good enough for us. A drive system that would theoretically let a ship get up to .95 c with a 3000 ton payload and no extra cost vs. a speed of .5 c except that of firing the laser for a longer period pretty much blows the shit out of everything else. Practically the actual speed may be lower but really, anything past .9 c is probably into diminishing returns territory unless your goal is a time dilation factor huge enough you won't need suspended animation or you're building an RKKV.

[Paolo]

Mass ratio of 200!

That's actually not bad for a .5 c rocket.

It's not only not bad, it's better than a perfectly efficient rocket. A photon rocket that can reach 1.5e8 m/s requires a total to payload mass ratio of 243.

[Paolo]

Woops. I used specific impulse instead of exhaust velocity. Looks like the minimum mass ratio for a perfectly annihilating AM rocket is roughly 1.7.

[Ariphaos]

I mean it's generally the point where the skyrocketing mass ratio "death spiral" starts.

Indeed, and that's part of my point, here.

It doesn't give a reference for the higher-end figure though, you could very well be right.

It's pretty simple to work out for a given amount of energy = KE = .5 * M * v^2. Take the energy created by a gram of deuterium fusing into helium, and see how fast it could make that helium go.

Adding antimatter to the equation ups the amount of energy that you can apply. There is no real practical higher limit - one could contrive a system (obviously more on the fictional end of science fiction) where nearly the entire reaction of a proton-antiproton collision was channeled into the neutrinos produced, giving them each an energy of approximately 120 MeV. This has the effect of allowing an interesting drive that is not as much of a weapon as the drive is interesting (it's still a weapon, of course).

That's actually not bad for a .5 c rocket. Relativistic rocketry is a bitch. :P

It's a bad ratio for something -moving- through the ISM at .25 c

A light sail is unquestionably a far better alternative, and is the design I would back now if that article Winston posted is anywhere close to accurate. I'm unacquainted with this proposed mass driver system you speak of.

Just a giant AU-long solar-powered coilgun. It's one way unless you set up a similar system at the target world, but it's faster to start up and doesn't require a constant beam.

I'd say that's plenty good enough for us. A drive system that would theoretically let a ship get up to .95 c with a 3000 ton payload and no extra cost vs. a speed of .5 c except that of firing the laser for a longer period pretty much blows the shit out of everything else. Practically the actual speed may be lower but really, anything past .9 c is probably into diminishing returns territory unless your goal is a time dilation factor huge enough you won't need suspended animation or you're building an RKKV.

Do I need to go into why RKVs are made of stupid and fail again? -_-

I should write an article about them or something.

[Winston Blake]

Just a giant AU-long solar-powered coilgun. It's one way unless you set up a similar system at the target world, but it's faster to start up and doesn't require a constant beam.

That doesn't seem feasible. Forward's crewed light sail reaches 0.5c in 1.6 years. A quick non-relativistic calculation gives a lower limit of 1 531 AU for 0.5c at 5 g's. That's 34x the distance from here to Pluto, even assuming people could withstand that kind of acceleration for 35 days. Your 'giant' AU-long coilgun would squish the occupants with 7 655 g's. The long duration of a light sail's acceleration is an advantage.

[Ariphaos]

That doesn't seem feasible. Forward's crewed light sail reaches 0.5c in 1.6 years. A quick non-relativistic calculation gives a lower limit of 1 531 AU for 0.5c at 5 g's. That's 34x the distance from here to Pluto, even assuming people could withstand that kind of acceleration for 35 days. Your 'giant' AU-long coilgun would squish the occupants with 7 655 g's. The long duration of a light sail's acceleration is an advantage.

I use .2c, genetically engineered occupants, cooling and liquid suspension, but yes, a single AU isn't nearly long enough. Keep in mind we are talking about a civilization capable of doing this in the first place, even for normal humans there are tricks to handle excessive g forces with.

There's also a limit to how fast you can accelerate ferromagnetic materials with coilguns period, unfortunately I haven't been able to puzzle it out : /

[Junghalli]

Some interesting reading I found on the internet with respect to lightsails.

http://www.sff.net/people/Geoffrey.Land ... ail89.html
http://www.niac.usra.edu/files/studies/ ... Landis.pdf

Apparently performance can be dramatically improved by using a dielectric sail, instead of the aluminum sail Forward used in his proposal. The second link contains a calculation for a saphire (aluminum oxide) sail with, if I read correctly, 1200 times better performance. If I'm reading the paper correctly this would allow the laser to be reduced by a factor of 145 in power, so it would only need to be around 300 TW at minimum working from the power figure Forward gives, instead of 43 PW. Although 300 TW is still many times more power than the entire electrical grid of present-day Earth it makes the phased array considerably less of a pain in the ass. And it absolutely blows the shit out of Asimov Arrays for economy.

The first site suggest silicon carbide would be the best sail material, I don't know how it compares to saphire though.

I know the thread's been dead for a while but it was interesting and relevant and the thread was still on page 1.