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" 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. "

Henry Spencer

Wonder

Burn

Space

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" If your goal is to change the world, you can't start by doing things the same old way because it sells better. "

" Spaceflight, especially in the Mercury spacecraft, clearly wasn't going to be much like flying an airplane. "

Henry Spencer

Flying

Much

" 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. "

" Claiming that solid rockets are necessary for a heavy-lift launcher is obvious nonsense. "

" Rocket engines generally are simpler than jet engines, not more complicated. "

" 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. "

" Sometimes a malfunctioning test setup actually gives the tested system a chance to show what it can do in an unrehearsed emergency. During a test of an Apollo escape system in the 1960s, the escape system successfully got the capsule clear of a malfunctioning test rocket. "

" Solid-fuel rockets can't easily be shut down on command. "

" 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. "

" 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. "

" As plans for the first lunar landing started to be made, nobody had really thought about who would be out first. "

" The demise of Constellation is not the death of a dream. It's just the end of an illusion. "

" Developing expendable rockets is always going to be painful and expensive. Throwing the whole rocket away on each attempt not only costs a lot, it also hampers figuring out just what went wrong because you don't get the rocket back for inspection. "

" Foul-ups in testing are not uncommon, especially when the test setup is being tried for the first time. "

" Not until the space shuttle started flying did NASA concede that some astronauts didn't have to be fast-jet pilots. And at that point, sure enough, women started becoming astronauts. "

" It's true that Apollo 10's lander was overweight. Late in the craft's development, it became clear that its ballooning weight was endangering the whole mission. "

" 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. "

" On the technical side, Apollo 8 was mainly a test flight for the Saturn V and the Apollo spacecraft. The main spacecraft system that needed testing on a real lunar flight was the onboard navigation system. "

" 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. "

" One of the headaches of high-tech test programmes is having to debug the test arrangements before you can start debugging the things you're trying to test. "

" Liquid oxygen is one of the cheapest manufactured substances on Earth. In large quantities, it costs pennies per kilogram - cheaper than milk or beer. "

" 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. "

Henry Spencer

Rocket

Space

" An experienced designer with more freedom to act might have realised that there was just too much optimism in the Ares I concept: that a shuttle SRB was simply too small as a first stage for a rocket carrying the relatively heavy Orion spacecraft. "

" Is manned space exploration important? Yes - not least because it simply works much better than sending robots. "

" The Apollo programme of the 1960s had some weight problems, too; in particular, the lunar lander needed some fairly drastic weight-reduction work. "

" 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. "

" SpaceX does seem to have had a run of bad luck, with its first three launches all failing. "

" Large solid rockets have never been a very good way to build launchers that might have crews on top, especially because of the problems in getting the crew away from a failing launcher. "

" 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. "

Henry Spencer

People

Earth

" Testing a parachute drop of a heavy object is not simple. "