Project Orion is kind of ludicrously inefficient, making it largely useless for interstellar travel. A nuclear powered ION engine can achieve a much higher exhaust velocity and replace that giant pusher plate with useful propellant.

It’s one of those designs that trades something considered largely meaningless, total engine weight, for something considered far more important exhaust velocity. This always ends up being a far worse trade off than simple models suggest.

PS: It could have very high acceleration before running out of fuel, but if the trip is going to take decades either way that’s not very important.

What good does a high exhaust velocity if your fuel stays all inside the ship?

As long as we are talking hypothetical designs, you can use spent nuclear fuel as reaction mass an ION engine just fine. That said, weight wise it’s fairly trivial so you could just eject it.

Project Orion is detonating nuclear weapons which consist of far more than just fuel, they also need high explosives etc, however much of the fuels energy is wasted both from low yield bombs being less efficient but also simply radiating energy in all directions. Nuclear powered ION engines however would be ejecting far more of their reaction mass of say xenon per KG of fuel that leftover fuel is basically irrelevant. In other words it’s a more efficient nuclear reaction, a vastly more efficient engine, and a much lower mass fraction spent on the physical engine.

This gets more extreme when you consider a crewed mission would need lots of power during the trip so they would want/need to build redundant nuclear reactors anyway.

> Nuclear powered ION engines however would be ejecting far more of their reaction mass of say xenon per KG of fuel that leftover fuel is basically irrelevant.

Wait, if that is true, then the ION engine has a lower ISP than direct explosions, not higher. Unless the reactor is orders of magnitude more efficient than the bombs.

Nuclear ION reactors are orders of magnitude more efficient. I think you’re being confused by not actually modeling what’s going on.

Smaller nuclear detonations are less efficient in that they directly liberate less energy from their fuel. A nuclear explosion in space also radiates most of it’s energy as X-Rays which on earth you heat the atmosphere but in space mostly just do nothing. So their directly turning less nuclear energy into motion. Further, a smaller percentage of that force is then collected by the pusher plate compared to the Bell nozzle at the bottom of a traditional rocket. https://en.wikipedia.org/wiki/Bell_nozzle

Now as nuclear is so much more energy dense than chemical energy on paper they look better. But compare ISP’s and you see just how much energy is lost.

All of that can be simplified by just looking at ION thrusters ISP and vs the ISP of hypothetical Orion rockets. At that point it’s just a question of what percentage of mass is reaction mass vs the reactor + fuel and it turns out you can get reaction mass well over 80% making at least 60% more efficient before considering the pusher plate assembly’s mass vs physical ION engine’s mass.

PS: And that’s when looking at existing ION engine designs. Hypothetically, you could eject matter at a large fraction of C using energy giving orders of magnitude higher ISP, at the cost of lower energy efficiency etc.

There is another option that doesn't involve "scary" nuclear.

https://en.wikipedia.org/wiki/Breakthrough_Starshot

Check out Hardtack Teak. It was an exo-atmospheric nuclear test above Johnston Atoll. The blast was visible from Honolulu and a large glowing red cloud drifted over the island chain all the way from Johnston Atoll. There are areas of the zone underneath the explosion which are still hot to this day. And that was just one of the tests that went right. There were plenty of failures too, both in atmosphere and on the launchpad, which spread nuclear material and toxic fuel all over the place. High-altitude nuclear explosions are definitely a threat to humanity, no doubt about it.

Well, you don't necessarily need to launch from Earth - starting an Orion mission from say high lunar orbit should be perfectly safe.

Maybe. Personally, I would want to see A LOT of math and paperwork first before I listen to anyone who uses "perfectly safe" and "nuclear explosion" in the same sentence. I'm still hoping we can figure out some other form of space travel which doesn't involve really, really, really big explosions.

Nuclear Salt Water Rocket rides on one smaller continual nuclear explosion: https://en.m.wikipedia.org/wiki/Nuclear_salt-water_rocket

I think for Nuclear Propulsion we do need to start building in Space or on the Lunar Surface.

Another interesting idea is to propel micro crafts with lasers. I think there was an article on HN about that.

Micro craft with light sails propelled by lasers have been discussed quite a bit. My favourite instance of it is in Charlie Stross's novel Accelerando.

At risk of contaminating future lunar bases.

I'm pretty sure the moon is already being bombarded with all sorts of nasty radiation.

Ha. Excellent point.

I would guess that radioactive particles on the surface would be different that radioactive bombardment from space. You could shield your shelter from space, but if the soil had radioactive particles in it, then you couldn’t bring it in your shelter. For that matter, you would also have to shield the floor, or at least remove a layer of soil. You might have to do that anyway to build a foundation.

If it isn’t obvious, I don’t know what I am talking about.