If you are a fan of science fiction, one of the standard scenarios is the crushing weight that astronauts face on launch: the terrible pressure of terrific g-forces, suddenly slamming the helpless astronauts into their couches and the terrible effort to move that this produces.
This is part of the legend of space flight. In fact, reality is quite different. Let’s have a look at what the mighty Saturn V did, in reality, to its occupants. To help us, we will compare the Saturn V rocket with a legend of track athletics: the great Usain Bolt.
If you have seen a Saturn V launch, you will have noticed just how slowly it moves initially. This was a problem faced previously by the designers by the V2 rocket: the V2 was moving so slowly at launch that any breeze would push it off course and strong wind could even topple it and so cause a crash. The engineers at Peenemunde resolved this by using steering vanes in the jet of exhaust to correct these small movements. In contrast, the Saturn V could move its engines themselves to vector its thrust: that is, adjust the direction of thrust to compensate for the wind. One of the things that caused great nervousness in the astronauts was just how close to the launch tower the rocket was for the first few seconds of flight and the threat that there could be a collision as the rocket lifted agonisingly slowly off Pad 39A; the vectored thrust of the giant, F-1 engines ensured that a collision with the launch tower could not happen.
That launch tower was 106 metres high. The Saturn V took a fraction over 9 seconds to clear it. This was an important moment in the launch because it marked the moment that Cape Canaveral handed over control of the flight to Mission Control at the Manned Spaceflight Center in Houston.
In fact, the big surprise is that the Saturn V’s initial acceleration was not actually anything special, as the time taken to clear the tower indicates. It was so loaded with fuel that it took time to get moving properly. So much so, that the acceleration in the first seconds after launch was very low indeed. Probably you have felt the force pushing you back into your seat when an airliner starts its take-off run: initially the g-force that the Saturn V produced was no greater than that.
If you are a fan of Usain Bolt – my daughter is a huge fan (the picture below was our view of him from our seats at his penultimate race before retirement and, that evening, we had a more distant view of his last race) – you know that he is quick, but did you know that he is faster than a Saturn V rocket from a standing start?
Usain Bolt holds the World Record for the 100 metres and 200 metres. His time for 100 metres – 9.58 seconds – will, like Bob Beamon’s legendary long jump of 8.90 metres at the Mexico Olympics, undoubtedly last for years and, initially, only be beaten by a fraction.
The Saturn V is – and will remain, until at least the end of the decade – the most powerful rocket ever built. Just 13 were launched and only nine of them went to the Moon but, for many of my generation, the Saturn V symbolises space flight.
Imagine, we place them in the starting blocks for the 100 metres together – or, at least, Bolt on his blocks, the Saturn V on Pad 39A at the Cape. The gun goes!
Usain Bolt was famous for being a rather slow starter. In the only major 100 metre race that he lost, in the 2017 London World Championships, had he reacted to the starter as fast as the winner, he would have won Gold, instead of Bronze. Bolt’s average reaction time was 0.158s in major championships, but, that night in London, his reaction time was a much slower 0.18s: one of the slowest reaction times registered in the entire 100 metres competition.
However, the Saturn V was slower still. The engines fired up at T-8.7 seconds. First motion of the rocket was at T-0.4 seconds. The Saturn V then took 0.4 seconds from first motion to lift-off physically from the launch pad. By 0.4 seconds, even slow-reacting Usain Bolt is out of his blocks and heading down the track at increasing speed.
Bolt pulls away
- Three seconds into the race and Usain Bolt is building a substantial lead: he is 22m down the track, already about 13 metres ahead of the Saturn V, but his speed has only increased a little in the last 20 metres.
- Five seconds in and Bolt is pulling away still further: his lead is now 17 metres, but the Saturn V is starting to react.
- At 50 metres, Bolt is still hanging-on to that 17 metre lead, but it is no longer increasing.
The Saturn V starts to come back
- At 60 metres, physics is starting to work against Bolt. His speed has topped-out at 12.2 metres/s (44 km/h), but the Saturn V continues to accelerate, as it gulps down 10530 kilogrammes of kerosene and liquid oxygen each second. Bolt is through 60 metres in 6.32 seconds, but his lead is down to 15 metres.
- At 70 metres the Saturn V is clawing-back its disadvantage. Usain Bolt’s lead is now down to 12 metres.
- At 80 metres, the Saturn V is beginning to cut the gap rapidly: Bolt’s lead is just 7 metres and reducing quickly.
The lunge for the finishing line
- At 90 metres, Bolt is still just ahead, but there is now only half a metre in it.
- With the finishing line in sight, the Saturn V powers ahead and starts to increase its advantage.
- Victory! The Saturn V completes the 100 metres in 9.26 seconds, with Bolt trailing by 4.4 metres.
Of course, we have placed the Saturn V under a handicap: Usain Bolt was running on the flat, but we have the Saturn V climbing vertically, or almost vertically but, still, it makes an interesting comparison.
Science v Science Fiction
The point that this makes is that, at launch, the acceleration of a Saturn V was modest and, contrary to what you might believe, the g-forces on the astronauts, quite small. For the initial seconds of flight, the astronauts were only pulling about 1.3g: in other words, feeling only about 30% more than their normal weight.
To put it another way: a sports car will probably get from 0-100 km/h in about 2.5 seconds; the Saturn V had 0-100 km/h in 12 seconds!
What is the trick here? It is, of course, that the acceleration of the Saturn V increased rapidly as the fuel load decreased whilst, like Usain Bolt, your sports car will reach a maximum speed and not be able to accelerate any more, although the Saturn V will continue to accelerate as long as it has fuel to burn.
The rocket reached Mach 1 – 1235km/h – in 60 seconds. By this time the astronauts were feeling a little under 2g (the average over those first 60 seconds was 1.6g).
As the immense tanks on the Saturn Ic emptied, the acceleration increased rapidly. The maximum g-force permitted on that 1st Stage was 3.98g: to stop that being exceeded, the centre engine cut-out, 125.6 seconds after first motion, 25.4 seconds before the outboard engines, at which the acceleration dropped significantly, before building up again as the fuel tanks emptied.
So, it is true that the astronauts felt considerable g-forces during launch, but these only became important from about 100 seconds after launch. The increase was slow and progressive for that first minute and a half or so of flight, allowing them to become accustomed to it.
In contrast to the 1st Stage, which reached peaks of 3.75g before centre engine cut-off, 3g after and 3.98g at staging, the 2nd and 3rd Stages were very gentle. The Saturn II peaked at about 1.9g and the Saturn IVb, at under 1g during the short burn to Earth orbit. Even the much bigger burn for Trans-Lunar Injection subjected the astronauts to just under 1.6g.
So, the Saturn V, at least, was really relatively mild in its acceleration – a true, Gentle Giant – and nothing like science fiction stories would have us believe.