How to launch people into space...

...if you want them to survive the journey

Secure remote control for conventional and virtual desktops

It's Not Rocket Science Last time, I explored how difficult it is for computer hardware to operate in the noisy - from an electromagnetic radiation perspective - environment of outer space. Today’s topic is people. Don’t worry, I won’t be boring you with life support systems. That’s the least of a spacer’s worries. Before you need to worry about keeping your astronaut breathing, you’ve got to get them into orbit first, in a condition where the oxygen will still be required.

The US military has a long history of spinning people in circles for fun to find out just how tolerant they are to G-forces. There have also been some rather more gruesome tests performed by certain less-than-ethical nations in the earlier half of the 20th Century. The result of all this is a good understanding of exactly when you need to worry about your lungs collapsing or needing to press your eyeballs back into their sockets.

G-forces are simply acceleration induced by the Earth’s gravity under freefall, and there are three types that need to be dealt with:

  • Linear acceleration Moving in a straight line - you knew this one already
  • Radial acceleration Traveling around the perimeter of a circle
  • Angular acceleration Rotating around an axis passing through the body

Although all three of these can be encountered in a space mission, radial and angular can be controlled under normal operating conditions and are only experienced in anomalous events. Linear G, however, is unavoidable if you want to get off the ground at all.

Nasa's g-force centrifuge

Nasa's g-force centrifuge: survive a spin in this and you can go into space... maybe
Source: NASA

As you know, linear accelerations are encountered every day, but only rarely will you experience greater than 1g for any non-impulsive event. In a spacecraft, however, linear accelerations greater than 1g can be experienced for extended durations during launch, abort, re-entry and landing. Landing can involved coming home or arriving somewhere more interesting, but I’ll deal with settling down on planets with a gravity greater than 1g another time.

Unlike a cargo payload, the human body doesn’t react uniformly to G-loading, so an XYZ coordinate system must be defined. The Gz axis runs vertically through the upright body, with +Gz acting downwards to compress the spine. Gx acts perpendicular to the chest with +Gx compressing the lungs. Gy acts sideways into the limbs.

The effects of G-loading are primarily caused by cardiovascular pressure changes, these are most dramatic when accelerating the body in the Z axis. Under positive Gz the subject will experience difficulty moving against 2g. At 3g lifting arms and legs is almost impossible and vision begins to blur. At 4g you can’t move at all. Beyond 5g, you’ll black out after five seconds, and longer exposure results in what space scientists call "G-LOC": G-induced loss of consciousness. The astronaut’s blood pressure is insufficient to combat the acceleration and reach the brain, thus starving the higher centres of oxygen.

Under -Gz, the opposite occurs and by 2g subjects report throbbing headaches due to excessive blood pressure in the skull and redout at 3g as the conjunctiva is forced over the eyeball. Testing beyond 4g is almost intolerable beyond six seconds and G-LOC occurs rapidly.

Gx is a much more pleasant sensation than Gz, but that’s not to say it’s all fun and rollercoasters. That being said, if you’ve been on Stealth at Thorpe Park, then you’ll have an idea of what +4.5Gx feels like - at least momentarily.

In a more extreme case, humans can tolerate +2Gx for 24 hours with slight abdominal pressure. When you get up to 3g though, there is measurable compression of the chest with breathing and speaking difficulties. Around 6g heartbeats start to become irregular and at 8g limbs can’t be lifted. Above 12g breathing is nigh on impossible and is accompanied by extreme chest pains and loss of vision. However, G-LOC is uncommon.

-Gx results in similar effects, but without chest compression or breathing difficulties.

Little research has been done on ±Gy, but it is known that around ±5g hemorrhages occur in the arms and legs.

Nasa's Apollo g-force launch profile

Saw-tooth graph for a sore arse
Source: Nasa

While a cargo payload - satellite, space probe, Mars rover, nuclear hovertank with laser canon - may be able to withstand 30g or more during a launch, it’s clear that people can’t - at least if you want them to do science or zap aliens when they’re up there.

As a result, rockets carrying humans tend to have relatively low thrust-to-weight ratios to ensure minimal g-loads. For example, the Mercury, Atlas and Gemini missions put the likes of Shepard, Glen and Grissom through g-loads 6.4g for 54 seconds and 6g for 35 seconds, while folks going up on Apollo and Skylab trips experienced only around 4g for similar time periods.

To end up on a lunar bound coasting trajectory, Apollo astronauts were subjected to three different levels of acceleration, one for each stage of the Saturn V. The first of these accelerated from 1g to 4.5g over the course of 150 seconds. Next, the second stage shunt was kept at to 1.8g for 300 seconds to limit maximum dynamic pressure. The third stage has a bit more of a kick and shoved along at between 1.2g and 2.6g for the last 130 seconds.

Despite the minimised acceleration, this still isn’t tolerable to humans sitting vertically, so every single human space launch that has ever occurred has had astronauts lounging back at a leisurely angle of about 12 degrees from horizontal.

Nasa's astronaut angle chart

Sit back, you'll be more comfortable...
Source: NASA

By ensuring that their seatbacks are reclined for take off, it’s possible for trained astronauts to tolerate forces in excess of 10g for a few minutes.

So sit back and relax.... it’s the only way to stay alive if you want to get up there.

Getting back, though - that’s a whole other debacle... ®

Further Reading

Want to know more? Check out these two items:

  • Kumar, K. Norfleet, W (1992) Issues on Human Acceleration Tolerance After Long-Duration Space Flights. Available from NASA
  • Rollercoaster Database (2012) Stealth - Thorpe Park. Available online

Next gen security for virtualised datacentres

More from The Register

next story
Vulture 2 takes a battering in 100km/h test run
Still in one piece, but we're going to need MORE POWER
TRIANGULAR orbits will help Rosetta to get up close with Comet 67P
Probe will be just 10km from Space Duck in October
Boffins ID freakish spine-smothered prehistoric critter: The CLAW gave it away
Bizarre-looking creature actually related to velvet worms
CRR-CRRRK, beep, beep: Mars space truck backs out of slippery sand trap
Curiosity finds new drilling target after course correction
China to test recoverable moon orbiter
I'll have some rocks and a moon cheese pizza please, home delivery
What does a flashmob of 1,024 robots look like? Just like this
Sorry, Harvard, did you say kilobots or KILLER BOTS?
No sign of Ziggy Stardust and his band
Why your mum was WRONG about whiffy tattooed people
They're a future source of RENEWABLE ENERGY
Vulture 2 spaceplane autopilot brain surgery a total success
LOHAN slips into some sexy bespoke mission parameters
prev story


5 things you didn’t know about cloud backup
IT departments are embracing cloud backup, but there’s a lot you need to know before choosing a service provider. Learn all the critical things you need to know.
Implementing global e-invoicing with guaranteed legal certainty
Explaining the role local tax compliance plays in successful supply chain management and e-business and how leading global brands are addressing this.
Build a business case: developing custom apps
Learn how to maximize the value of custom applications by accelerating and simplifying their development.
Rethinking backup and recovery in the modern data center
Combining intelligence, operational analytics, and automation to enable efficient, data-driven IT organizations using the HP ABR approach.
Next gen security for virtualised datacentres
Legacy security solutions are inefficient due to the architectural differences between physical and virtual environments.