How fast a rocket can humans safely travel in?

"How fast a rocket can humans safely travel in?" Space science journalist Richard Hollingham answered this question for us...

Richard Hollingham - Okay. That's the wrong question! I'll come on to why it's the wrong question a second. We're actually travelling very fast right now; so I had to write these down, because I had to work some of these out as well so they might not be wholly accurate. So the Earth rotates at 600 miles an hour. So we're currently on the earth going 600 miles an hour, because we're on the earth we're also going 600 miles an hour. The Earth is speeding around the sun at 70,000 miles per hour. So we are also speeding around the sun at 70,000 miles an hour. The sun is speeding through the galaxy at 450,000 miles an hour, which means we're also spinning at 450,000 miles an hour. So, actually, we can go very very fast! The proper question is what acceleration can a human stand?

That's when you come into g-forces, and the idea of the amount times gravity that you can you can withstand. So, typically, a rocket, something like the Soyuz rocket, which is what astronauts use to get to the International Space Station, doesn't actually accelerate that much it's about 3 or 4g - if that - maximum going up to the space station.  Coming back, it's more unpleasant: it's kind of nearer 5g, but not for a long period of time, so that's fine. We know from experiments done in the 1950s by  Colonel John Stapp - a military doctor, who did experiments on himself, as all good doctors should do, he attached himself to a rocket-powered sled. And sped along this track and then braked very sharply to see what g-forces pilots could endure. The most he went up to was - and bursting the blood vessels in his eyes in the process but actually suffering no serious injury other than that - was 20g.

Chris Smith - Did he decelerate with his head towards the direction of travel, or away? Because this makes a difference because it makes a difference whether the blood goes away from your brain and you get 'blackout', or towards your brain and you get so-called 'red-out'?

Richard Hollingham - Interesting. He was on a chair on a sled. I guess, the acceleration would be going through his chest - through his body - the same way as astronauts. It's the reason they lie down on couches so that the acceleration - so that the forces - are going through your body rather than straight down through your head, which is something you want to avoid. So yes, you get this idea of blackouts and astronauts also wear to avoid that - and pilots do too - they have a far greater g-forces so they wear pressure suits and they will push the blood towards the head so you don't blackout. You don't want to be unconscious when you are flying a jet fighter!

Chris Smith - So returning to the question, which was what's the best sort of rocket to go in terms of speed, actually, that doesn't matter: it's the acceleration that matters and the acceleration is what determines the g you're going to feel. So what would be a comfortable g for a human to feel? I mean, when I'm taking off on an aeoroplane, what sort of g-forces are they?

Richard Hollingham - Nothing!

Chris Smith - So that's what I want. So basically yeah. 

Richard Hollingham - Actually, the best spacecraft for comfort was absolutely the space shuttle. Not too bad on the way up, and coming down it sort of circled: it looped and looped, to lose energy as it came down to Earth, and as it did that it was quite a gradual deceleration; whereas the 'Soyuz' again g-forces going up not too bad, coming back is pretty horrendous. I've heard it described as like going over Niagara Falls in a barrel - but a barrel that's on fire!