The U/4 wasn’t a “lowball estimate”; it was bounding the upper range of what could be theoretically extractable (assuming a uniform spherical expansion). It does not account for any thermodynamic losses or loss of radiation capture. In the case of a nuclear detonation in the Earth’s atmosphere, those losses are recovered because most of the radiation that isn’t captured during the initial detonation is absorbed by the atmosphere (which is opaque to x-ray and gamma radiation over significant distances) and converted into the thermal pulse and overpressure shock wave. The same is not true in vacuum, where the only conversion of radiation to useful impulse is going to come from the materials in the bomblet. Converting all or even most of the radiative output of a nuclear device to useable impulse (e.g. as Dyson suggests by “complete burn up to helium”) is not even remotely plausible unless you are positing a device the size of a medium-sized asteroid. And the expansion of a nuclear explosion is hardly adiabatic; even ignoring internal losses due to self-interaction of charged plasma just the fact that it is both extremely high temperature and increasing in surface area as it expands means that it is losing energy like crazy. One of the limiting conditions on the actual design of an ORION-type spacecraft is the amount of heating on the pusher plate and the need to reradiate the heat limiting the frequency of pulses at very large yields. And unlike a rocket, where expansion of the gas in the nozzle is confined and used to add to the impulse—the (p[SUB]exit[/SUB] - p[SUB]ambient[/SUB])∙A[SUB]exit[/SUB] term in the rocket equation—a vessel using pulse propulsion gets impulse only by momentum transfer of the axial component of the gas that strikes the pusher plate, essentially regardless of temperature.
I know a lot of people think this, but in fact, the opposite is true. The fact is that in a free fall vacuum environment the lack of an effective acceleration field or drag means that every single movement needs to be opposed by an applied force. For astronauts, this requires tools with counterstroke or countertorque devices to apply any significant load. Even for simple deployable advices, precisely controlled operation is necessary to avoid imparting unrecoverable dynamics to the vehicle. For handling large structures, this requires fine control of very large impulses lest the structure become unstable or crash into something else. Orbital assembly (and other activities, such as transfer of fluids or bulk materials) is a very difficult and immature field.
If your perspective is that “rockets just aren’t that much more complex than a car,” I don’t know what to say. It’s not just the mechanical complexity and bulk of the hardware; the cost and effort is all of the integration and testing that has to be done to assure that the vast number of things on a rocket that can cause unrecoverable failure (and the consequences of losing a rocket); the difference between fueling a car (five minutes at a fuel pump) versus fueling a rocket (2-3 days to load propellants plus topping off cryogenic propellant); the maintenance and assurance on safety critical systems such as flight termination and ground support equipment; and of course all of the effort on the control side to launch and track the vehicle to assure that it has inserted into the proper orbit and doesn’t pose a hazard to other satellites. Launching a hundred rockets a year of, say, the Falcon 9 (essentially one per week from both Vandenberg and Cape Canaveral sites) would be an extremely impressive accomplishment. Launching 10 or 100 times that number is just beyond remote feasibility.
No, that’s not what I’m saying at all. There are plenty of things in space exploration and resource extraction that are plausible with reasonable developments of existing or nascent technology. There is a wide array of things we could have been doing today, or could be doing in the span of a couple of decades which would give us greater access to space resources and support a human presence in Earth orbit and beyond. But sending people to another star system in a human lifetime (and decelerating them and keeping them alive) is vastly beyond extant or practicable propulsion, energy, or habitation technology. I’m sorry that physics gets in the way of the fantasy, but that is the reality.
No, this is not correct. It isn’t just enough to get net output out of a fusion reaction; you have to extract enough in order to sustain the reaction, which depending on method and assumptions is an one to two orders of magnitude greater than unity, and then be able to withstand the thermal and neutron environment. Every time someone gets close to unity output out of inertial confinement fusion or a z-pinch, some pop sci rag grabs onto the story with the implication that controlled fusion power generation must be just around the corner. If it were actually that easy, it would already be done.
As for transferring human cognition to machine intelligence, propulsion systems powered by artificial black holes or antimatter, wormholes, warps drives, et cetera, these are tropes of science fiction for a reason; because as a literary conceit they allow for scenarios that are not practicable in the current state of technology. I do suspect that when and if we ultimately explore beyond the solar system, it will be by some proxy (possibly machine intelligence, or creatures designed for space habitation and long transits, or some other means as yet unknown), but unless there is some unexpected scientific breakthroughs it won’t happen in the next few decades. Even though science fiction and the increasingly visually convincing capabilities in CGI make it seem like anything can be done with just enough effort and gumption, the reality is that there are many things that are just beyond our current capabilities to do regardless of the amount of money or labor we throw at such problems.
Stranger