The issue of G-forces during acceleration of near light-speed spacecraft

[Fireworking]

So it occurred to me that if humans were to ever make a half-light-speed spacecraft, (a spacecraft that travels at half the speed of light) the acceleration required to get up to that speed would crush a human if it was too fast.

Alpha-Centauri, the nearest star to our sun, is 4 light-years away. A spacecraft traveling at half the speed of light would take 8 years to arrive there. This is a long time, but its short enough for a single generation of humans to get there. However, the ship couldn't accelerate instantly, for multiple reasons, so it would take longer, due to the acceleration required. But this would cause the issue of how fast the ship could accelerate without crushing the crew members. On a regular rocket, Falcon 9 for example, the G-forces are around 5-ish (i think). But this only accelerates the crew to 27,000 kph (not exact, just a ballpark range), where as half light speed is roughly 5,000,000 kph. Of course, there would be much more 'space' to accelerate, but what i don't have time to calculate is how long would it take for a spacecraft to accelerate to half the speed of light? Keeping the G-forces in a bearable range, of course

Now, if it takes too long, would there be any way to artificially negate the G-forces? It seems like this would make interstellar travel (at least in a single generation) much more difficult.

Please let me know your thoughts.

[Redclaws]

You know, the calculation really is quite simple, just division - half the speed of light is roughly (not very roughly, it’s quite close…) 150,000 kilometers/second.

1 G of acceleration is 9.8 m/s/s, call it 10 for napkin math.

So that’s 150,000,000 meters/second / 10 m/s/s = 15,000,000 seconds

Calculator for this next part…
3600 seconds per hour, 86400 seconds per day….  173 days.

At 1g of acceleration, it would take 173 days to reach half the speed of light.  The actual number is a bit higher because I rounded up 1g, but it’s close to that.

…… now find out how to maintain 1g of acceleration for 173 days and we’re in business!  Oh and do it again on the inbound leg.  :X

[Orbiter]

Using some form of ion propulsion or solar would be the best route... but man, it would take you sometime to build up speed. Even if you tried moving at 0.01g, it would still take you a better part of a century to reach the speed of light.

[MDMoery]

Using some form of ion propulsion or solar would be the best route... but man, it would take you sometime to build up speed. Even if you tried moving at 0.01g, it would still take you a better part of a century to reach the speed of light.

Well, the nice part of it taking ~173 days to get to .5C at a 1g acceleration is the fact that you have that 1G of artificial gravity for those 173 days.  And of course you have to decelerate that same .5C at the arrival point.  So maybe instead of 1g for 173 days, maybe .6 G for longer.  In other words design your flight plan to accelerate constantly until the mid-point of the flight, coast only for a few days, then turn around and start the deceleration.  Then you have some amount of artificial gravity for the whole trip.

[Nomadd]

You know, the calculation really is quite simple, just division - half the speed of light is roughly (not very roughly, it’s quite close…) 150,000 kilometers/second.

1 G of acceleration is 9.8 m/s/s, call it 10 for napkin math.

So that’s 150,000,000 meters/second / 10 m/s/s = 15,000,000 seconds

Calculator for this next part…
3600 seconds per hour, 86400 seconds per day….  173 days.

At 1g of acceleration, it would take 173 days to reach half the speed of light.  The actual number is a bit higher because I rounded up 1g, but it’s close to that.

…… now find out how to maintain 1g of acceleration for 173 days and we’re in business!  Oh and do it again on the inbound leg.  :X

The numbers are a little more complicated since Einstein rearranged the universe. At half the speed of light, you need to take relativity and reference frames and stuff into account.

[DanClemmensen]

Using some form of ion propulsion or solar would be the best route... but man, it would take you sometime to build up speed. Even if you tried moving at 0.01g, it would still take you a better part of a century to reach the speed of light.

Well, the nice part of it taking ~173 days to get to .5C at a 1g acceleration is the fact that you have that 1G of artificial gravity for those 173 days.  And of course you have to decelerate that same .5C at the arrival point.  So maybe instead of 1g for 173 days, maybe .6 G for longer.  In other words design your flight plan to accelerate constantly until the mid-point of the flight, coast only for a few days, then turn around and start the deceleration.  Then you have some amount of artificial gravity for the whole trip.

You are into the realm of Science Fiction here, because there is no known propulsion system that can provide this acceleration for this length of time. The fundamental problem: where does the energy come from? To reach 0.5C, you need KE=0.5 MV², where V 0.5 C, so KE=.125 MC².  Even with a theoretical total mass convertor (E=MC²) you would need to convert .125 of your mass to energy, and the same to decelerate.  But because of the rocket equation, you need to carry more mass to accelerate the fuel mass.  The usual Sci Fi trick is to use a Bussard ramjet, but these have recently been shown to be theoretically unworkable. If you are trying to reach the stars using Sci Fi techniques, I suggest you opt for an FTL drive.

[whitelancer64]

Traveling 2 light years (to flip around and decelerate the other two light years), with the velocity peaking at 0.5 C, the constant acceleration required is about 0.121 g (this is less than lunar gravity).

A constant acceleration higher than that will result in reaching 0.5 C sooner than the halfway point and require some coasting time.

Handy online calculator: https://spacetravel.simhub.online/

Also, for reference, a VASIMR 40 day flight to Mars (near its closest approach to Earth) would require a constant acceleration of about 0.0041 g

[Robotbeat]

Using some form of ion propulsion or solar would be the best route... but man, it would take you sometime to build up speed. Even if you tried moving at 0.01g, it would still take you a better part of a century to reach the speed of light.

Well, the nice part of it taking ~173 days to get to .5C at a 1g acceleration is the fact that you have that 1G of artificial gravity for those 173 days.  And of course you have to decelerate that same .5C at the arrival point.  So maybe instead of 1g for 173 days, maybe .6 G for longer.  In other words design your flight plan to accelerate constantly until the mid-point of the flight, coast only for a few days, then turn around and start the deceleration.  Then you have some amount of artificial gravity for the whole trip.

You are into the realm of Science Fiction here, because there is no known propulsion system that can provide this acceleration for this length of time. The fundamental problem: where does the energy come from? To reach 0.5C, you need KE=0.5 MV², where V 0.5 C, so KE=.125 MC².  Even with a theoretical total mass convertor (E=MC²) you would need to convert .125 of your mass to energy, and the same to decelerate.  But because of the rocket equation, you need to carry more mass to accelerate the fuel mass.  The usual Sci Fi trick is to use a Bussard ramjet, but these have recently been shown to be theoretically unworkable. If you are trying to reach the stars using Sci Fi techniques, I suggest you opt for an FTL drive.

Beamed propulsion of various types. Magnetobraking. Fission fragment rockets with really high mass ratios. There are ways.

[deltaV]

The numbers are a little more complicated since Einstein rearranged the universe. At half the speed of light, you need to take relativity and reference frames and stuff into account.

The relativistic corrections aren't big enough to change the qualitative picture. See https://en.wikipedia.org/wiki/Hyperbolic_motion_(relativity) for the relevant equations. If you accelerate enough at constant gee-force (i.e. proper acceleration) so that Newton would put you at speed v_n your actual speed is v_e = v_n / sqrt(1 + (v_n / c)^2). For v_n = 0.577c this works out to v_e = 0.500c. So accelerate for 204 days instead of 173.

[Nomadd]

The numbers are a little more complicated since Einstein rearranged the universe. At half the speed of light, you need to take relativity and reference frames and stuff into account.

The relativistic corrections aren't big enough to change the qualitative picture. See https://en.wikipedia.org/wiki/Hyperbolic_motion_(relativity) for the relevant equations. If you accelerate enough at constant gee-force (i.e. proper acceleration) so that Newton would put you at speed v_n your actual speed is v_e = v_n / sqrt(1 + (v_n / c)^2). For v_n = 0.577c this works out to v_e = 0.500c. So accelerate for 204 days instead of 173.

Eventually, since the question ignores all practical considerations, someone will say "Why not just keep accelerating at 1G to the half way point, flip around and decelerate the rest of the way?" Then the relativistic effects are pretty major.

[deltaV]

The relativistic corrections aren't big enough to change the qualitative picture. See https://en.wikipedia.org/wiki/Hyperbolic_motion_(relativity) for the relevant equations. If you accelerate enough at constant gee-force (i.e. proper acceleration) so that Newton would put you at speed v_n your actual speed is v_e = v_n / sqrt(1 + (v_n / c)^2). For v_n = 0.577c this works out to v_e = 0.500c. So accelerate for 204 days instead of 173.

Oops I think I used the wrong equation. The equation should be v_e = c tanh (v_n / c). More tomorrow if I remember.

[edzieba]

Since we're in Torchship Country, a read through of the torchship page on the excellent Atomic Rockets would be a good idea. The equation section of that page is classical, but there is also the relativistic rocket equation section for the self-flagellation enthusiasts.

[KelvinZero]

Looks like it is answered: you can get up to half lightspeed with just 1g acceleration in some moderately small fraction of your total trip time.

If you are just interested in dealing with high-g, there probably are some interesting solutions to discuss that might be required for other problems.. for example magnetic launch systems, or drives like in The Expanse tv show where military objectives may require outflying an enemy, and thus pushing the human body to it's limits.

[Robotbeat]

Looks like it is answered: you can get up to half lightspeed with just 1g acceleration in some moderately small fraction of your total trip time.

If you are just interested in dealing with high-g, there probably are some interesting solutions to discuss that might be required for other problems.. for example magnetic launch systems, or drives like in The Expanse tv show where military objectives may require outflying an enemy, and thus pushing the human body to it's limits.

Beamed propulsion has range limits, which is one reason why you might have g-load limitations.

[qraal]

So it occurred to me that if humans were to ever make a half-light-speed spacecraft, (a spacecraft that travels at half the speed of light) the acceleration required to get up to that speed would crush a human if it was too fast.

Alpha-Centauri, the nearest star to our sun, is 4 light-years away. A spacecraft traveling at half the speed of light would take 8 years to arrive there. This is a long time, but its short enough for a single generation of humans to get there. However, the ship couldn't accelerate instantly, for multiple reasons, so it would take longer, due to the acceleration required. But this would cause the issue of how fast the ship could accelerate without crushing the crew members. On a regular rocket, Falcon 9 for example, the G-forces are around 5-ish (i think). But this only accelerates the crew to 27,000 kph (not exact, just a ballpark range), where as half light speed is roughly 5,000,000 kph.

Lightspeed is 1,080,000,000 km/h, so 0.5 c is 540,000,000 km/h.

Of course, there would be much more 'space' to accelerate, but what i don't have time to calculate is how long would it take for a spacecraft to accelerate to half the speed of light? Keeping the G-forces in a bearable range, of course

Now, if it takes too long, would there be any way to artificially negate the G-forces? It seems like this would make interstellar travel (at least in a single generation) much more difficult.

Please let me know your thoughts.

Gerald Nordley's Beam-Riders accelerate at 5 gee to 0.866 c. The crew float in baths for most of the trip and use powered wheelchairs if they need to get around. But your ship isn't cruising so fast. Accelerating at 5 gee to 0.5 c (measured by external observers at the local standard of rest) takes just 39 days. Ship-board the integrating accelerometer reads 0.55 c since acceleration began.

[CameronD]

Of course, there would be much more 'space' to accelerate, but what i don't have time to calculate is how long would it take for a spacecraft to accelerate to half the speed of light? Keeping the G-forces in a bearable range, of course

Now, if it takes too long, would there be any way to artificially negate the G-forces? It seems like this would make interstellar travel (at least in a single generation) much more difficult.

Please let me know your thoughts.

Gerald Nordley's Beam-Riders accelerate at 5 gee to 0.866 c. The crew float in baths for most of the trip and use powered wheelchairs if they need to get around. But your ship isn't cruising so fast. Accelerating at 5 gee to 0.5 c (measured by external observers at the local standard of rest) takes just 39 days. Ship-board the integrating accelerometer reads 0.55 c since acceleration began.

Actually, of even more interest might be what happens if you hit something at 0.55c.  I suspect there isn't a material known to man that could withstand even tiny impacts at that speed over even a mere 39 days..

[Lampyridae]

Looks like it is answered: you can get up to half lightspeed with just 1g acceleration in some moderately small fraction of your total trip time.

If you are just interested in dealing with high-g, there probably are some interesting solutions to discuss that might be required for other problems.. for example magnetic launch systems, or drives like in The Expanse tv show where military objectives may require outflying an enemy, and thus pushing the human body to it's limits.

Beamed propulsion has range limits, which is one reason why you might have g-load limitations.

You can use beamed propulsion to send mass pellets to a spacecraft to detonate against a pusher plate / lasers & magnetic nozzle. As range increases, the acceleration imparted from the pellets would drop off since each beamed launch would consume more energy to send it to the target ship's velocity.