Progress requires setbacks; the only sure way to avoid failure is not to try.
Testing a parachute drop of a heavy object is not simple.
Supplying fuel for a Mars expedition from the lunar surface is often suggested, but it's hard to make it pay off - Moon bases are expensive, and just buying more rockets to launch fuel from Earth is relatively cheap.
SpaceX does seem to have had a run of bad luck, with its first three launches all failing.
The Orion capsule uses an escape system quite like that of the Apollo spacecraft in the 1960s and 70s: an 'escape tower' containing a solid-fuel rocket that will pull it up and away from Ares I in a pinch.
Technically and financially, it might still make sense to give up on Ares I and simply write off the money spent on it, but politically, that's probably impossible.
Spaceflight, especially in the Mercury spacecraft, clearly wasn't going to be much like flying an airplane.
The Apollo programme of the 1960s had some weight problems, too; in particular, the lunar lander needed some fairly drastic weight-reduction work.
In the first few years, it was at least plausible to come in in the morning and read all the Usenet traffic that had come in, and 15 minutes later be off doing something useful.
The key virtue of orbital assembly is that it eliminates the tight connection between the size of the expedition and the size of the rockets used to launch it.
Sure, there were hopes that Constellation's systems could later be adapted to support more ambitious goals. But Apollo had those hopes, too. It didn't work in 1970, and it wasn't going to work in 2020.
Past experience, on the shuttle and the Titan rockets, suggests that large multi-segment solid rockets have a probability of failure of 0.5 to 1 per cent.
Sometimes people wonder why aeroplanes are so cheap and rockets are so expensive. Even the most superficial comparison shows one obvious difference: aeroplane engines use outside air to burn their fuel, while rockets have to carry their own oxidisers along.
Rocket engines generally are simpler than jet engines, not more complicated.
In the long run, it's impossible to make progress without sometimes having setbacks, although people who get lucky on their first attempt sometimes forget this.
Is manned space exploration important? Yes - not least because it simply works much better than sending robots.
The demise of Constellation is not the death of a dream. It's just the end of an illusion.
My one concern is that when money gets tight, it's easy to cut R&D funding that isn't tied to a specific project - look at what's happened to NASA's aviation research.
The communications delays between Earth and Mars can be half an hour or more, so the people on the ground can't participate minute by minute in Mars surface activities.
The Moon may not be quite as appealing as Mars, but it's still a complex and poorly understood world, with many questions still unanswered.
