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From a cost effectiveness point of view NONE of the conventional rocket vehicle concepts make very much sense.

I have lost the gist of this discussion.  Are we talking about launching a rocket from a balloon on suborbital flights?  A suborbital flight is basically a cop-out.  As for using a balloon to launch rockets, we would be attempting to engineer a very complex system whose only function would be to save a little bit of mass on the lower stage of the rocket.  Lower stages can be built cheaply (a la big dumb booster).  There isn't really any economic advantage to launching from a balloon.

If we are serious about achieving large-scale access to space with present day technology, I would suggets that we reopen discussion on ground-launched nuclear powered spacecraft (Orion), using small and ultra-clean nuclear fusion bombs for propulsion.  The radioactivity released by each launch would be small, resulting in less than one death globally.  The spacecraft would mass anything from ten thousand to millions of tonnes and could carry many thousands or even millions of people into low earth orbit and beyond.

The only thing preventing the construction and use of Orion ships are political difficulties, the technical issues are more or less solved.  If you want largescale access to space, this is the way to achieve it with technology available today.

No.  I simply don't think that policing the world will be succesful or in US/Western interests and I don't think the cost in money and lives justifies the end.

Do you seriously believe that the Iranian people want the US to invade their country and enforce 'regime change' upon them?  They will resent the intrusion and will resent the US for having attacked them.  Many Iranians and Americans will die in the process and the end result, likely as not, will not be pleasing or at all what you had intended or hoped for.  Invade the Iranians if you think that US national security is seriously threatened, but do not kid yourself that you are actibg in their best interests.

I do not believe that attempting to police the world against every country that elects/is inflicted with an undemocratic regime will be (a) effective and (b) in US/UK/Western interests.  In all cases, many native peoples (and US soldiers) end up dying, the US/West generally fails to achieve the sort of compliant pro-western and democratic solution that it had intended and a wave of anti-western resentment remains for decades afterwards.

But it is not our fight.  Policing the world to the extent that you suggest means devoting resources to the military on a scale that we cannot possibly maintain for more than a little while longer.

We need to take care of our people first and foremost.  They are the ones that vote for us, the ones that we have an electoral mandate for and the ones that would have to lay down their lives defending Sudan.  I simply do not see why they should be expected to sacrifice their lives and their cash for a fight that does not concern them.

The 21st centuru is going to be a time when the US is desperately short of resources and money, with a sliding economy and sliding geopolitical dominance in the face of India and china.  Maintaining dominance means first and foremost investing in new technologies that can be used to grow the economy.  Every dollar that is spent defending far away desert states which do not concern you is a dollar that you won't be able to invest in Fusion research, or the space programme or advanced rail systems or genetic engineering....etc.

The idea of trying to control the world by military force reminds me of the last days of the British empire.  By the early 1900s, it was obvious that Germany and the US would eclipsed the UK economically.  The response of Britain was unparralled military expenditure, attempting to control the world through force of arms, in order to make up for lost economic influence.  This actually hastened Britain's decline by sucking up resources that could have been invested in new technologies and new consumer products.

Iran's oil reserves are significant, but are only a small proportion of total Middle east reserves.  And Iranian production has already peaked, which goes some way to explaining why these people feel that they need nuclear power and why they are so desperate to keep oil prices high.

All the more reason for the wetst to develop technologies that lessen our dependance on oil.  Oil's sprinciple value is as an easily refined feedstock for transport fuels.  If most of our transport switches to electric, the price of oil will decline, the incomes of these desert states will crash and they will no longer be able to fund their supposed weapons programme.  Again, the solution needs to be technological rather than military.

I do not see that Putin has gone out of his way to make an enemy of the US.  The fact is that the US and Russia are geopolitical rivals.  But I could just as well ask:
'Why does the US set itself up as an adversary of China, Iran and Russia, when France, Switzerland, Netherlands, etc, seem happy to live in peace with these people?  What makes the US think that it needs to control the rest of the world by force of arms?'  'how is it that China/Japan both manage to live on good terms with Iran, Russia, etc, wheras the US is locked in a continuous cold war with the aforementioned states'?

I do not think that it is the responsibility of the Western world to enforce parliamentary democracy on the rest of the world.  I also question our ability to successfully do so.  If I understand your point correctly, the western world must have absolute control over the politics of the rest of the world to prevent some rogue state from aquiring nukes and smuggling them into our cities.  By that line of logic, US intervention in Iraq, Korea and Vietnam must have made the world a much safer place.

I do not suggest that we should ignore blatant security threats, but to attempt to control every corner of the world that happens to have a dictatorial government would be impossible and would actually generate adversaries that would otherwise not have become a threat to US interests.  It is like taking a shotgun into a swamp rather puting up mosquito nets.

If the past 30 years has taught us anything, it should be that the western nations generally and the US in particular, simply do not have the power to control the internal affairs of the rest of the world.  Trying to do that requires that we be continuously ready for war, pumping vast amounts of resources into weaponry and our miltary and be prepared to sacrifice tens of thousands of lives for the 'freedom' of people in irelevant far away lands, who will not appreciate our intervention anyhow.  All of this draws resources away from worthy projects at home, which could genuinely improve US/British living standards.

As for electrically powering transport in the UK/Rest or the world, I would argue that sort of systems neccesary, require massive government investment or at least government coordination of private investment in order to take off.  You also have to have the right idea in the first place.  The US government is presently pumping millions of dollars into developmnet of fuel cell vehicles and hydrogen refuelling infrastructure which will niever be economic.

There has been much speculation as to how interstellar travel will eventually take place.  Faster than light travel looks impossible on the basis of all science that we presently understand.  Even exotic sublight transportation systems such as anti-matter, bussard ramjets and dark-matter engines are difficult to foresee, given the inefficiencies in the manufacture of anti-matter, the interstellar drag imposed by hydrogen ions on a bussard ramjet and the speculative nature of dark-matter.

On this basis, it must be assumed that the fastest forseeable interstellar transport systems will be fusion, solar sail and beamed energy.  This limits transport speeds to approximately 0.1C, making manned interstellar travel very diffucult, unless we are prepared to build extremely large and expensive multi-generational spacecraft.

The other option would be to send robotic craft to other starsystems which would automatically manufacture human beings and human habitats when arriving at the target solar system.  Such a ship could be thought of as a 'seeder' ship, carrying with it only the genetic codes of its crew and the instructions on how to build suitable infrastructure on arrival.  Upon arrival, the craft would locate a small carbonaceous asteroid and manufacture a habitat, along with all of the infrastructure that the first batch of human would need.  From there, the first batch of humans could completely colonise the target star system.

On this basis, taking miniaturisation of components to its limits, a starship need be no larger than a coffee cup and there is the possibility that with radical advances in nanotechnology, it might even be microscopic.  It should therefore be possible to mass produce litterally billions of starships, colonise the entire galaxy within about 10,000 years and go on to colonise other galaxies on a timescale of a few tens of millions of years.  This could be done without humanity ever leaving the Earth as a biological entity.

Some numbers for added clarity:

For a mass driver 100metres long and an exit speed of 10km/s, a mass driver with acceleration 50,000g is needed.

Total acceleration time = 0.02seconds.

Total energy required = 50MJ/Kg (probably closer to 60 including losses)

Total power required during acceleration = 2.5GW/Kg.

That's a lot of power pouring into a very small projectile. The odds are that it would melt within the barrel.  Transmitting that much power to the balloon using beamed propulsion would appear highly dubious to me.

An interesting set of points.  The habitable zone around a star would depend upon the size and properties of the planet, as well as sunlight levels.  If Earth or Venus were at Mars' distance from the sun, their thicker atmospheres would probably allow liquid water to exist in some locations.

A larger planet with more significant geothermal heating could maintain liquid oceans with little or a lot less sunlight.  It would also have a continuous stream of greenhouse gases entering its atmosphere.  Under these conditions, the habitable zone of our sun might stretch out to the asteroid belt.

The most serious near term effects of global climate change will be changes in rainfall distribution and generally rising temperatures in large arable food producing regions.  This will result in declining yields, which could have serious implications for global food prices, given that margin between supply and demand is already tight due to water shortages, rising salinity, declining topsoils, urbanisation, etc.

That is true.  But the device that we are proposing is only useful for launching bulk materials, so costs do need to be reasonably low: probably no greater than $100/Kg launched into low orbit.

The power supply would be by far the heaviets component.  I can think of a number of possibilities:

1) Beamed power by microwaves, with the surfcae of the balloon acting as a rectena type device;
2) A small, open cycle gas turbine;
3) A conducting tether;
4) Ultra-thin solar cells across the surfcae of the balloon;
5) a small unshielded high-temp nuclear reactor

So to put it crudely, the higher up you go, the more air you need to displace in order to support any given mass from the balloon.  This would appear to make balloons inpractical for reaching heights above 30-40km.

The next question is: how high do we need to go and how thin does the air need to be, in order to operate a mass driver capable of launching payloads to orbital speed?  At Earth's surface it is obviously not feasible.

Would it be feasible at 32km?

Air resistance = K x air density x V(squared).

At 32km: air density = 0.013 kg/m3 (1% Earths surface).

Given that meteorites are fire balls at this altitude, it is probably safe to say that even at 30km, air resistence would create too much resistive heating for a mass driver to work.  Air resistance would melt the barrel of the electromagnetic launcher before the projectile even left the launcher.

The only way around that would be to use a camera-shutter technique.  The mass driver would be sealed and evacuated with vacuum pumps and a shutter would open microseconds before the projectile reached the end of it.