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Its not just hard to ignite fusion of any isotope with chemical explosives - its impossible. They just can't produce the necessary pressures/temperatures high or long enough before the device is destroyed. Nowhere close to high enough for what you want.

With a "fizzle" "dud" nuke you have a similar problem, except igniting fission instead of fusion; the bomb blows up before much fission takes place, so no/little energy is released. This is what happens when nukes don't work/aren't built properly.

There are several reasons actually,

First is that such a system would be quite expensive if it were to be powerful enough to be effective, and so you would need lots of launches to pay for its construction. Nobody needs that many launches right now.

Second is the thing would not accommodate today's light-weight rockets very well, which must be launched at a high angle. Making a rail-gun long enough to be effective as well as gentle on the rocket could not be built at such a high angle cheaply enough. Building a rail gun as well as a new launch vehicle to go with it would be very very expensive.

Third is that a rail gun able to reach useful speeds as a "booster" would be huge, you need to be able to fling hundreds of tonnes up to supersonic speeds!

A direct-to-orbit would severely limit the payload mass to let you build a rail gun of practical size, plus the payload "bullet" would have to resist extreme acceleration forces. It would be difficult to build such a bullet able to stabilize its orbit after launch (rockets and guidance required). Oh, and splitting up your payload into a bunch of little bullets makes space construction a nightmare.

Another reason is that if there was a failure on a launch pad, all you lose is a concrete slab and maybe some cheap steel tower. If you have a failure of the rail gun, a multibillion dollar supermachine goes kaboom.

Its not a bad idea, but its not an idea we ought to look into right now.

One of the great lies of Greenpeace et al and one one of the things most laymen don't understand is that Uranium is not appreciably radioactive. The atoms that result when Uranium splits, thats the radioactive stuff that should scare you. However, purified Uranium splits very very slowly on its own, so effectively none of these nasty product atoms are formed until the fission reaction (in the reactor) is started.

Uranium is little more dangerous than Lead. Workers at French nuclear reactors and Canadian fuel element factories need no more shielding from fresh Uranium than plastic bags or kitchen gloves - not lead and concrete.

It is only after the reactor is started up that the fuel becomes intensely radioactive, so if a brand new reactor crashes? No problem.

Mass drivers? As in going directly to orbit? Or just as a boost for a launch vehicle?

The amount of thrust you could generate with the very low pressure Hydrogen aboard the balloon would be very small. Remember that the pressure in the balloon decreases with altitude, and so by the time you reached near-space height its pressure would be nil. Pumping the gas would itself require energy, and given the low pressure the amount of energy needed to compress the hydrogen would be prohibitive.

Drag really isn't much of a problem, and neither is altitude, speed is everything... stop trying to fight tooth and nail for a little altitude, speed is what you need.

Even at that high of an altitude there is still some drag to contend with, so a low-thrust electric engine (ion, vasimr) probably can't approach orbital velocity with such a bulky space ship.

So, a DSS station is only good for giving you altitude, which as previously established is only good for suborbital flights, not orbital.

That and the area of the shield is much smaller. And the shape is much simpler. Oh, and its not upside down like Shuttle.

As far as the SM size, it was reduced because of the higher density of hypergolic fuels hence making the SM lighter. Instead of making a "stubbier" SM of the same diameter as the CM would limit the size of big efficient spherical fuel tanks plus make it harder to stow the solar arrays/communications antenna.

So what you are saying is that your opinion is founded on baseless, uninformed arrogance instead of what the well informed professional engineers say.

You are dodging the issue, if NASA says there is no weight problem, then where did the rumor come from? And why should we believe the rumor?

Yes, the new plan is a take-off on DRM-III, redesigned for safer landing, improved provision for a future base/hopper system, and trading NTR for more Ares-V launches.

If the Aussies donated a billion to NASA annually, NASA could shave YEARS off getting to Mars.

Yeah basically,

And the fuel tanks would be huge for a nuke-powered SSTO.

With 850sec, then something like a chubby delta wing plane or maybe even a Delta Clipper style VTOL rocket.