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Interstellar travel is hard and complicated, yet I want to have a plausible interstellar propulsion system for my setting. Rockets are out because of the rocket equation and the horrible top speeds even if antimatter is used. Laser-sails are out because they can't slow down. Ramscoops are out because while their fuel collection rises with their velocity, drag rises with the third power of their velocity, limiting them to a top speed of 0.1c.

My idea was to combine the Ramscoop with the Laser-sail to get a system cable of reaching 0.9c with a reasonable mass ratio.

The spacecraft has a reactor, a habit, a cargo section and a large structure made of superconducting electromagnets. The structure is formed like a trumpet and protected with reflective shields, so it remains cold enough to work. It protects the spacecraft from interstellar gas, acts as a funnel for fuel and ejection mass and as a pusher plate.

The second part of the propulsion system is an array of powerful lasers in the home system. They push laser sails up to a significant fraction of lightspeed. These sails are shot at the spacecraft. As the ultra thin sails encounter the magnetic field, they break apart and turn into plasma. Encountering a ten tesla plus magnetic field at relativistic velocities tends to do that to matter. The plasma transfers its momentum and then some to the spacecraft as it is deflected.

The spacecraft could even slow down if the path ahead had been seeded with slower light-sails. It would just have to turn around. These slower light-sails would have to be deployed years before the spacecraft launches. While the spacecraft couldn't slow down with this method below the velocity of the breaking sails, its magnetic field would create even more drag to decelerate it. As soon as it is slow enough, the spacecraft is ditched and a small fusion or antimatter propelled craft with the crew, mining equipment and a self-replicating factory is deployed. These guys continue to set up basic asteroid mining and industry. Their goal is to build the same laser facilities the home system used to accelerate the propulsion sails. Once both sides can deploy the sails, a reliable travel corridor has been established.

One idea I had was to augment the sails with fusion fuel. The mag-sail would funnel the metal-carbon-fusion-fuel mixture down itself. At the deepest and tightest point of the mag-sail one would compress the plasma with stronger magnetic fields and a laser firing squad. Magneto-inertial fusion, basically. I'm not sure if this is practical, not practical or only practical during certain parts of the journey. The parts of the journey where I think such a fusion augmentation would be most useful are the final breaking phase and the end of the acceleration phase. In both instances the relative velocity between sails and mag-sail is so low that little energy is gained.

The fusion fuel that will be used is critical. The options are, assuming that both the CNO Cycle and proton-proton fusion are impractical, D+D, T+D, D+He3, He3+He3 an p+B11. D+He3 and He3+He3 are problematic as helium can't be combined with the sail in an easy manner. Any fuel tank would be destroyed by the interstellar medium. The sail itself will be more resistant to damage. p+B11 is great as both elements be connected to the sail as a layer of paint and it is neutron free. The issue is that it is 500 times harder to ignite than D+T. D+D and D+T are both neutronic, but can be ignited relatively easily and can be added to the sail as paint as well. D+T has the additional issue that the tritium will slowly disappear. No fuel stands out, all have their issues. p+B11 is probably most reasonable option assuming advanced technology.

Is the interstellar propulsion system described above realistic and reasonable? What issues might it face and how could one fix them?

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  • $\begingroup$ It seems workable. But once you have established yourself at both sides, you can use lasers directly. But if the inpact fusion part works, it might still be worth it. $\endgroup$ – b.Lorenz Jan 12 at 21:48
  • $\begingroup$ @Bitterdreggs. Tesla is meant to disctibe the strength of the spacecrafts magnetic sail. The incident energy from lasers on the sails would be measured in watts per square meter - then extrapolated to newtons per square meter, but that wasn't what I wanted to disctibe there. $\endgroup$ – TheDyingOfLight Jan 12 at 22:14
  • $\begingroup$ @b.Lorenz I've considered pure laser approaches, but they have issues. For once the lasers are (optimistically) diffrection limited, so they won't be useful beyond a certain point. While more energy is certainly an answer, the sails would make the system significantly more efficient. $\endgroup$ – TheDyingOfLight Jan 12 at 22:16
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    $\begingroup$ This idea has appeared in numerous science fiction stories. Niven has used it. I think Pournelle. I think it may have appeared in a Heinlein story but not absolutely sure. Variants have appeared in other stories. Also, you can slow down using inter stellar gas, though it might be hard on your sail. $\endgroup$ – puppetsock Jan 13 at 14:45
  • $\begingroup$ spelling: braking, not breaking. Also "habitat" not "habit". $\endgroup$ – Peter Cordes Jan 13 at 20:43
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What you're basically describing here is not a million miles away from the Sail Beam concept by the late Jordin Kare. That idea seems basically sound, and is my personal favourite kind of fast, plausible, hard-science starship drive. There are a few variations on it floating around, but the underlying idea of using a stream of small laser-driven lightsails to impart momentum to a starship is common to all.

Encountering a ten tesla plus magnetic field at relativistic velocities tends to do that to matter.

I'm not sure it does. It doesn't really matter though, as you can vapourise incoming sails with a puff of gas or a laser. If you're fielding multiple incoming sails in a row, then you can convert some of the energy of the plasma cloud released by the first disintegrating sail to electrical power to zap the next sail.

The spacecraft could even slow down if the path ahead had been seeded with slower lightsails. It would just have to turn around.

Make the spacecraft either dumbell or donut-shaped. Don't even need to turn around, then. Turning around at high sublight speeds is a terrible idea, because your crew and electronics will be very thoroughly dosed with high energy radiation, unless you're carrying a frankly unreasonable amount of shielding, or doing really weird and probably risky repositioning-reassembly manoevers. Anyway, not worth it. Make your ship work in both directions.

While the spacecraft couldn't slow down with this method below the velocity of the breaking sails...

To be pedantic, it isn't the velocity of the sails that matters, but the relative velocity they have with the starship. Whilst there will be a bit of a dead spot where the relative velocity is zero (when you'd be using your magnetic fields as a brake, as you observed) once you fall below the velocity of the sail beam you can carry on braking using sail-plasma just fine.

It also allows you to fly back towards the sail beam source, if you're prepared to let the sails pass through the magnetic field before ionising them. Relativistic needle-threading is probably not an exercise for those of a nervous disposition, though.

One idea I had was to augment the sails with fusion fuel

That sounds a lot like a combination of sailbeam with older ideas like MagOrion and fusion runways (the latter also a Jordin Kare idea), and it probably isn't worth it. If you're capable of driving your sails to any reasonable percentage of the speed of light (lets say 30%) then there's no benefit to doing any fusion. The amount of additional energy you can extract isn't that great, in part because the exhaust velocity of the fusion products is just too low and in part because there just so much oomph already available in the kinetic energy of the sails.

Although fusion would be pointless for your idea, I will note that ignition of even quite "hot" fusion recipes becomes a lot easier when your ship is moving really fast and the fuel pellets are not (or vice versa). The fusion runway idea suggested that smacking fuel pellets together at >200km/s would be enough to ignite fusion (though other sources apparently wanted more like 3500km/s, though both are well within your operating parameters) . It may be that this is a useful way to extract energy from sails when their kinetic energy relative to the ship is too low.

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