Henry Spencer

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.

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.

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.

NASA has never had a problem finding capable people to be astronauts. NASA's problem was, and still is, finding ways to cut the list of capable applicants down to a manageable length.

Trying to build a spaceship by making an aeroplane fly faster and higher is like trying to build an aeroplane by making locomotives faster and lighter - with a lot of effort, perhaps you could get something that more or less works, but it really isn't the right way to proceed.

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 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.

Reusable rockets promise much easier testing because you should usually get them back, and you can debug as you go rather than having to get everything perfect the first time.

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.

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.

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.

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.