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Comment by JoeAltmaier | Hacker News Reader

I've always thought that Mars missions should 'drag a bag' behind an atomic rocket. The best shielding is the inverse-square law. Put the habitable part of the mission vehicle several miles from the fissionables! At the end of a tether.

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But directly behind the rocket is where most of the energy goes.

I'd also be curious how the tether would avoid getting cooked by the rocket exhaust. I think a long mast, putting the riders in front of a large mass of fuel, would be much simpler.

But you're right, it is a lot easier to design an ultra-light, super-strong tether than a giant mast.

Are we sure a feasible tether/mass could handle the impulse of a nuclear explosion?

Pretty sure he had this in mind: https://en.m.wikipedia.org/wiki/Nuclear_thermal_rocket

My father-in-law worked on that in the 60's (project Rover). It got defunded for some reason, or we might have been to Mars in the last century...

Just swing it in a circle? Lots of pendulum arrangements that don't put the meat in the path of the heat.

Two habitats with an engine in the middle spinning around like a top could provide gravity, and absorb the engine impulse. Under no acceleration the three bodies would be co planar, and under acceleration would deform into a cone shape.

That's a smart design! If you spin up to 0.5g and accelerate at 0.5g (which, while not enough for takeoff, is plenty for interplanetary travel) the tether angle would be 45 degrees, more than enough to get out of the exhaust plume.

Another free benefit would be that on a change of acceleration, angular momentum would be conserved. Rotation would increase or decrease to keep the acceleration felt by the occupants approximately constant!

Look at the design of the ship from '2001', it's like that for a reason [0]. No SF like hard SF!

[0] http://2001.wikia.com/wiki/Discovery_1

Unless you put in two exhausts, jutting out at an angle, adding up to forward thrust.

That is extremely wasteful, in an environment where every drop of rocket fuel is precious.

Its possible that the exhaust could be made to disperse without losing much thrust - like a defocussed electron beam. Just so long as its cool enough when it hits the habitat. Which could have an aluminum-foil shield or some such...

Yes that design has been proposed before. The mass savings from using a tensile structure can make up for the slight loss of thrust efficiency. There was a somewhat plausible design in Avatar.

http://james-camerons-avatar.wikia.com/wiki/Interstellar_Veh...

From what I've seen the Project Orion and later designs concluded that was the most efficient design. Imagine a parachute (hemisphere) with the place where the explosion happens in the center and the (heavily shielded) capsule behind it, so fully 50% of the explosion is providing forward thrust. You don't have a single tether passing through the heart of the exhaust, rather you've got several going around the outside (like the frame of a cone tent, with the capsule at the apex). This was apparently easier than designing a rigid "pusher plate" with the right shape to capture a lot of thrust.

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