[idlewords]

It's not just about propulsion, but radiation. The radiation environment in space is awful and you need thousands of tons of shielding to effectively screen a crew from cosmic rays. Moreover, because of secondary radiation, partial shielding from energetic cosmic rays is worse than none.

A unique property of nuclear pulse propulsion is that the engineering gets easier the more massive the spacecraft. That leaves lots of room for radiation shielding and consumables, and makes it the only halfway practical choice for multi-year missions into the outer solar system.

[SECProto]

> That leaves lots of room for radiation shielding and consumables, and makes it the only halfway practical choice for multi-year missions into the outer solar system.

I disagree, one of the types I referenced is cyclers, which are specifically good for the same kind of reason. NERVA is another alternative that becomes similarly easier as you scale it up. All three types would need in-space assembly, so no big issues with any of them that way.

Don't get me wrong, Orion is cool (and the only way to conceivably run an interstellar mission with known technology/physics), but it isn't necessary (or even desirable imo) for interplanetary missions, at least as far as saturn anyway.

[idlewords]

It requires something like several hundred g/cm^2 of polyethylene for adequate GCR shielding. So back of the envelope, if the crew compartment is a cube 10 meters on a side, it requires 500g x 6,000,000 cm^2 = 3,306 tons of plastic to shield. I'm not aware of any non-nuclear-pulse design that can carry around that kind of heft. Just how big are the cyclers you mention allowed to get?

[SECProto]

It requires the shielding no matter what the transportation method is. If one wanted to build a cycler, conceptually it is somewhat similar to staging a lunar mission with rendezvous in low earth orbit. The construction methodology would be something like the following:

1. Build the base structure of cycler in earth orbit, then transfer it into the orbit you want (i.e. earth-jupiter intersecting). This would be structure, engines, shielding. This might require a couple dozen nuclear thermal boosters, or maybe it has to be phased over two launch periods and assembled in the cycling orbit

2. Assemble in earth orbit everything you want for a manned mission (food, water, landing craft, astronauts)

3. Rendezvouz that material with the cycler on its next orbit (I don't know the timing for earth-jupiter - maybe 12 years after step 1). This is a comparatively small effort, as the shielding and habitable space was already taken care of with step 1

4. live your life for the year or whatever the transfer is

5. Take your landing craft from the cycler to Europa surface

Different crews can hop on the original cycler by repeating steps 2 to 5. Conceptually you would want multiple of these cyclers coming and going, so that a crew can arrive on Europa on cycler A and return on Cycler G, only spending 5 years on Europa instead of 12. Their advantage is that the big heavy mass (sheilding) only has to be launched once, and can be used repeatedly, indefinitely.

[sigilis]

You would assemble such a craft in orbit. Some of the proposed designs for nuclear pulse propulsion conceive of a spacecraft of truly astounding proportions. The wikipedia article on Project Orion mentions an Alpha Centauri bound design that would be 20km in diameter and 10Mt in mass.

However, now that I think about it, cyclers would only need to use the NPP engines (bombs) to get into their cycling orbits, so you might be able to get away with accelerating them while unmanned. You could design such a craft with much less radiation shielding, but still include enough to prevent cosmic rays from killing (or more accurately, increasing their cancer risk) the passengers giving such a craft a significantly larger fraction of usable mass for other purposes. Initially accelerating a Mars cycler with unmanned landers with supplies and materials for an initial colony with humans to follow could be an interesting design.

All of this is more expensive than anyone is willing to pay for, even if there was an appetite for the nuclear proliferation risk of making and then putting a thousand compact bombs on a spacecraft. Not even mentioning the risk of the bomb carrier rockets exploding and causing a 100x Broken Arrow situation.

It is for this reason that I must urge the government to immediately fund the writing of several novels about how this can go wrong and right for me to read.