von who?

Does Nuclear Pulse Propulsion—see previous post—solve the problem of travel between the stars? The Dyson paper proposes exploding H bombs behind a spacecraft at the rate of one every three seconds for ten days to achieve a speed of 10,000 km per second. At that speed, a ship would still take best part of 150 years to travel between the Sun and Proxima Centauri, the nearest star, a distance of four and a third lightyears. Even a one-way journey would require as much again propulsion to slow down at the other end, and the mass of this would have to be accelerated on the outward journey, nearly tripling the size of the craft and its cost. The legion questions regarding the practicalities of multi-generational crew survival in a very limited living area over such a long period are too obvious to need detailing.
 
It seems then that even nuclear pulse propulsion is impractical for the transport of biological species between the stars. But what about robots? The argument goes something like this: an advanced civilization would soon realize that interstellar travel is only possible for machines. Wishing to explore the galaxy, announce their presence, and proclaim their achievements, the civilization would conclude that the only way to do this would be to send out thousands of self-replicating ‘von Neumann’ machines on one-way journeys directed to different parts of the galaxy. 
 
A von Neumann machine can build an exact copy of itself; in this case, a robot spacecraft capable of travelling to the stars. It uses built-in instructions and the raw materials that it finds on the planets it where it lands. The complete machine includes all the robotic paraphernalia needed for finding, mining and refining the minerals and other materials required, as well as setting up factories to manufacture the parts and fuel needed for the spaceship and new robot crew. It must also be able to construct a spaceship capable of lifting off from the planet and travelling to the nearest stars. It would be programmed to continue manufacturing copies of itself, each one to be directed to a different nearby star. Given enough time, and if such an advanced civilization really did exist, the Galaxy should be full of von Neumann machines. 
 
Since Earth is not knee-deep in von Neumann machines, we can conclude that there are no advanced civilizations in the galaxy… Can’t we?

Note added 18th April

Anyone with a basic understanding of Newton’s laws of motion will realize that there is a fatal error in my reasoning, detailed above, concerning the deceleration ‘fuel’ needed. In Dyson’s calculations, the mass of 300,000 H-bombs is 300,000 tons; the mass of the ship on its own is a mere 150,000 tons. Total mass, 450,000 tons. At the end of the acceleration period when the ship is travelling at 10,000 km/sec, the bombs have been expended and the mass of the ship is only 150,000 tons. 
 
But, to slow down at the other end, we need to add to payload of the ship the mass of another 300,000 H-bombs needed for deceleration. That increases the mass to be initially accelerated by a factor of three. Thus not 300,000, but 900,000 H-bombs would be needed for the initial acceleration. The total nuclear weapon inventory required, is 1,200,000 Hydrogen bombs.

Furthermore, blasting the components of the starship into Earth orbit for assembly in freefall, would need the equivalent of more than 9,600 boosters the size of the Saturn V used in the Apollo programme. Common sense dictates that such a venture is financially and practically absurd.