SOMETHING TO DO WITH SPACE...
It's the damn Russians, of course.
The English believe that the mysterious domes that have appeared on the moon were built by the Russians: that's why the mission is so urgent. But one of the Woomera scientists is a Russian spy. (It is rather hard to tell one scientist from another, so the spot-the-traitor whodunnit falls a bit flat.) The spy sabotages the mission; the rocket crashes on re-entry...and Chris is killed! The photos are saved (the plucky lad hugged them to his body on the way down) and it turns out that the domes are not created by the Russians after all, but possibly alien. As a result, the British and the Russians end their rivalry and pledge to work together. Our hero's self-sacrificial pluck has ended the the Cold War and secured the future of the space programme.
Ronald Reagan reportedly told Gorbachov that if the earth were invaded by aliens, then the Americans and the Russians would bury their differences and come together as fellow members of the humans race. In Clarke's disappointing sequel to 2001: A Space Odyssey, America and Russia step back from the brink because the Black Slab turns Jupiter into a second sun. And, of course, it is the ending of Watchmen: Ozymandias's faked squid incursion averts World War III at the eleventh hour. It is nice to think that a furniture salesman and part-time boffin had the same thought twenty five years earlier. It is quite possible that Alan Moore has read Blast Off at Woomera: Alan Moore has read everything.
Fortunately, Chris turns out to only have been mostly dead. Not only does he miraculously recover, but the cosmic radiation has given him a growth-spurt! So Hughes gets to have it both days: a death scene of monumental sentimentality in Chapter 20, and a happy ending in Chapter 21. In a way, it would have been a better story if he had stayed dead: but you don't kill off heroes in kids fiction, and anyway, it would have been a shame to have missed out on sequels with names like Passage to Pluto, Mission to Mars and Something to Saturn.
But there is another reason why our hero survives his near-certain death. Astute readers will have spotted it already. The book doesn't only have a plot and a sub-plot. It has a sub-text. A huge, massive, in your-face subtext that I was totally unaware of for 50 years.