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I've heard that the larger the pusher plate of an Orion drive, the more efficient it is. Eg: a 20 m Orion drive would be far better than a 6.6 m one.

Is this true? If so, why?

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Source

Are these 6.6 m designs as good as a larger-diameter one?

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    $\begingroup$ The larger the plate the more force gets transferred by the explosion. $\endgroup$
    – GdD
    Feb 6 '20 at 9:33
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    $\begingroup$ I'm reasonably certain (but not currently certain enough to cobble together an answer!) that the efficiency refers to the bombs. Big Teller-Ulam devices give you more bang for your buck, and to make use of them effectively you need big pusher plates. They don't scale well the other way. $\endgroup$ Feb 6 '20 at 11:14
  • $\begingroup$ @GdD depends on how well collimated the cloud of propellant is. A bigger plate would let you use the same pulse unit from further away so that the stuff would have cooled more by the time it hit the plate, so maybe you can make some tradeoffs on plate resistance or something? $\endgroup$ Feb 6 '20 at 11:16
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Roughly speaking, you want to maximise your pusher plate size, but without making it bigger than is needed for your propulsion units.

The idea is that you chuck a propulsion unit out the back of your ship, it travels to some standoff distance and goes bang in such a way that it projects a cloud of energetic plasma towards your pusher plate. You want as much of that plasma as possible to bounce off your pusher plate. If your pusher plate is too small (or your means of collimating nuclear explosions isn't good enough), obviously some of that plasma will escape around the edges and be wasted.

There are three ways you could fix this. Firstly, you could increase the amount by which you collimate the explosion from your propulsion units. This may be beyond your engineering abilities. Secondly you could reduce the standoff distance between the pusher plate and the propulsion unit when it goes bang.

Both these approaches share the problem of a lot of bang being bounced off a comparatively small area. In the case of the reduced standoff distance, you have the additional problem that the cloud of propellant is also somewhat hotter, having had less time to cool down radiatively on its journey between the detonation point and the pusher plate. These issues increase the rate at which the pusher plate is ablated by each nuclear pulse, reducing the lifetime of the drive system. The reduced standoff distance will also require heavier radiation shielding to protect the crew and ship.

The third approach is to use smaller propulsion units. The problem there is that nukes just don't scale down very well, both in terms of their minimum size and the amount of bang you get for a given size, mass and price-point.

Are these 6.6 m designs as good as a larger-diameter one?

Probably not.

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