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With even the most efficient chemical fuel we have to reach Alpha Centauri(closest star system to our own) in a human lifetime, you would need a fuel tank millions of lightyears across. So we need to get a lot faster should we want to escape out tiny little 200 AU(stopping at the termination shock)bubble that is our solar system. There are many ideas, but all of them contain a lot of sci-fi. Fission rockets, while not bad (principally) taking us around our SS, are too slow to get us lightyears away, also considering that they are quite inefficient(by universal standards, not by human standards). Fusion is at least several years away, and a lot longer before we can effectively use it as a propulsion method. And don't even get me started on antimatter and warp drives. Antimatter propulsion/Pion rockets, while proven to work, is a pipedream at best. We can barely make a few antiprotons in particle accelerators, and we would need kilograms of it, not to mention keeping it controlled until we want it to annihilate with matter. Warp drives are barely out of the realm of hypothesis, and we don't even know if they have any potential in principle, let alone in practice.

So far, what is the most practical design for a near light speed propulsion system, that could maybe put interstellar travel on the table for the first time?

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    $\begingroup$ There are plenty such devices in science fiction. There currently is no such device in science reality. $\endgroup$
    – David Hammen
    Aug 1, 2021 at 10:31
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    $\begingroup$ I think you have answered your own question here. $\endgroup$
    – GremlinWranger
    Aug 1, 2021 at 12:56
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    $\begingroup$ There aren't any. $\endgroup$
    – Organic Marble
    Aug 1, 2021 at 13:37
  • $\begingroup$ The very closest to actual practical application is..... Star Trek Warp Drive. As in, it is just as unlikely to be workable as any of the other schemes, and there are more pages of documentation on it than on any of the others. $\endgroup$
    – CuteKItty_pleaseStopBArking
    Aug 1, 2021 at 15:30
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    $\begingroup$ The only other way to answer this question would be to accept very, very, very relaxed versions of the word "near" in "near-C speeds". By effectively diving into the Sun, the Parker Solar Probe can achieve 1/1500th of lightspeed. $\endgroup$
    – CuteKItty_pleaseStopBArking
    Aug 1, 2021 at 15:34

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The Project Orion pulsed nuclear propulsion is probably as close to practical as can be with current technology (it was originally envisioned with 1950's technology). It could get to maybe 0.1c.

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  • $\begingroup$ But didn't project Orion entail detonating nuclear weapons behind your spacecraft? 1: how are you going to protect your spacecraft and 2: how are you going to slow down? $\endgroup$
    – Gregory
    Aug 1, 2021 at 20:04
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    $\begingroup$ Slow down by flipping the spacecraft around and detonating nukes in front of it. Protecting the spacecraft is "easy" -- you've got a great big metal plate with shock absorbers the size of skyscrapers. It's protecting everything else that's hard. $\endgroup$
    – Russell Borogove
    Aug 1, 2021 at 20:18
  • $\begingroup$ @Gregory, the suggested devices were (relatively) low yield (between .5 and 5 kilotons) and would have detonated at a bit of a distance from the ship. Orion would not literally have been pushed by nuclear explosions, those were just the energy source to turn a medium into plasma, which then pushed against the pusher plate. AFAIK nobody doubted the principle, the unsolved engineering challenge would be not to have anything in the payload compartment being crushed to bits by sudden jolts of acceleration. $\endgroup$
    – Eike Pierstorff
    Aug 2, 2021 at 9:40
  • $\begingroup$ Worth noting on Orion's (hypothetical) propulsion, it wasn't necessarily going to be as simple as being pushed along by the spherical shockwaves from low yield nukes. They looked into directed nuclear detonations: en.wikipedia.org/wiki/Casaba-Howitzer $\endgroup$
    – paulmrest
    Aug 2, 2021 at 22:02
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Orion is good technologically, but very impractical considering environmental regulations.

I suggest the best shot we have is Breakthrough starshot project: Very lightweight crafts with solar sails accelerated by lasers.

We need to develop new technologies and engineering, but it seems to be doable.

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  • $\begingroup$ Yeah, the issues with getting sufficient energy storage on board the spacecraft with fission or even fusion mean that a system powered by external beams projected from fixed stations has an enormous advantage. The laser systems are non-trivial, but are far more straightforward to scale up than antimatter-based options, which are pretty much the only other way you can get anywhere near $c$. $\endgroup$
    – Christopher James Huff
    Apr 26, 2022 at 16:02
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The problem with many of the conventional thrusters with extremely high isp is their TWR, and with fuel tanks for at least kilometres long, I can't imagine ion thrusters are going to cut it. As you said fission thrusters have a relatively low ISP as well as a poor thrust to weight, or at least thermal nuclear engines, which are limited by the melting temperature of the reactor core, however, the nuclear salt water rocket bypasses this problem by containing the fissile material in a salt, as plutonium chloride, and dissolving that in water or even some other reaction mass. this mixture is stored at far beyond critical mass, but is inhibited by boron lined fuel tanks and boron rods. As soon as the propellant leaves the boron lined fuel lines and enters the nozzle however it immediately undergoes fission, producing and ISP of 10,000 seconds and a TWR of 2000, making every other feasible engine look anaemic in all respects. To understand it intuitively, it combines the best of nuclear reactors and bombs, or like a revised Orion drive. This technology isn't totally out of the realm of possibility, with the only real hurdle being heat management, which could be solved by dumping lots of liquid hydrogen on the nozzle walls through film cooling.

This doesn't take into account the politics of nuclear energy however, which is always gloomy.

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  • $\begingroup$ This also doesn't take into account the psychology of nuclear energy, almost as gloomy as the politics. Even though a nuclear engine is much safer than traditional rockets, people don't know that, and I must admit, it sounds quite dangerous(even though it isn't) $\endgroup$
    – Gregory
    Aug 2, 2021 at 5:26
  • $\begingroup$ @Gregory for the astronauts or the public? I think for this question you almost have to completely disregard logistical challenges, excluding only the glaring one of R+D. But yes, convincing the public would just about put the fear of god in me. luckily this is hypothetical, for now at least. $\endgroup$
    – R. Hall
    Aug 2, 2021 at 6:19
  • $\begingroup$ Nuclear engines are not safer than traditional rockets, they are only vastly better. How were you planning on getting home, with that hot rocket, into our biosphere? We can build nuclear reactors, but not ones we can live next to for years. How would we even get you off such a craft? What if it hiccups, on the landing pad, en route, or coming home, with men raddled with radiation sickness at the helm? What shielding will you use? Political problems are hardly the primary ones. How will you even leave the ship without getting burned down by the engine? $\endgroup$
    – chiggsy
    Aug 10, 2021 at 4:59
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I think the best way would be using the Alcubierre drive. The thing about that is that a rocket would use negative matter to make it seem like it is travelling faster than light even though it isn't. You could even get to Alpha Centauri in under 4 years with this method. The only issue is that we don't know how to create negative matter but if it is done then this is probably the way of the future. More Info: https://en.wikipedia.org/wiki/Alcubierre_drive

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