Inhabitants of another planet (their biology is yet undefined, and if helpful to solving the problem, suggestions can be included in the answer), have a spaceship in which they plan to accelerate to 0.2c over a two earth year period. Technology level is more or less undefined as it's an alien species who can have whatever tech is required.

Of course, F = ma means that the force of accelerating at 0.1c per year is still a big number, even for an entity that weighs something similar to ourselves. 0.2c is important as anything above this tends to incur significant time dilation, and any slower will mean too much time elapsed to reach their destination.

So my question is, How can I prevent my space explorers from being crushed by the acceleration? The parameters of time and final velocity are set and critical to my story, so can't be altered. I'd prefer a feasible but not necessarily completely hard science answer. If there really isn't any way to do this, then a plausible hand-wavy answer would suffice.

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    $\begingroup$ @RonJohn Because I didn't do the calculation beforehand :) I expected it to be a much bigger number, but clearly I was wrong :) $\endgroup$ – Jane S Sep 17 '18 at 22:07
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    $\begingroup$ Jane, that's a textbook example of the downvote reason "This question does not show any research effort". $\endgroup$ – RonJohn Sep 17 '18 at 22:34
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    $\begingroup$ @RonJohn I will wear that deserved downvote then. I guess I didn't trust myself to do the math properly and made an incorrect assumption. Lesson learned! $\endgroup$ – Jane S Sep 17 '18 at 22:42
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    $\begingroup$ Kind of seems a little harsh, I mean it was a moderately popular question. $\endgroup$ – Mark Rogers Sep 18 '18 at 1:01
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    $\begingroup$ @imallett Fortunately as an alien ship, I don't have to get too specific as to the mechanism of the drive and propellant, but I want to make sure that what I write is scientifically plausible and not in direct contravention with what we know breaks the laws of physics as we know them :) $\endgroup$ – Jane S Sep 18 '18 at 8:20

They won't be crushed. In fact, they will feel less than Earth gravity, so they will need exercise to preserve their bones and muscles' integrity.

0.2c equals 59,958,491.6 meters per second. Two years have exactly 63,115,200 seconds. What this means is that if they were to accelerate at 1m/s2, which is pretty close to 0.1g, they would be going faster than 0.2c relative to us in two years while feeling a gravity weaker than the Moon's.

You can dial those thrusters to 11 without fear. We were made for that.

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    $\begingroup$ @GretchenV I think Renan means 11 as in turn it up to 11, not 11g :) $\endgroup$ – Jane S Sep 17 '18 at 9:06
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    $\begingroup$ am I getting this right that that means I can have a spaceship accelerate at about 10m/s^2 and have artificial gravity similar to earth on the spaceship (rear wall will be the "bottom")? $\endgroup$ – RancidCrab Sep 17 '18 at 12:11
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    $\begingroup$ @RancidCrab yes :) $\endgroup$ – Renan Sep 17 '18 at 12:16
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    $\begingroup$ @RancidCrab: Remember => The Gate Is Down. $\endgroup$ – Matthieu M. Sep 17 '18 at 14:39
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    $\begingroup$ An helpful unit conversion is 1G≈1 lightyear / year / year. $\endgroup$ – Jetpack Sep 17 '18 at 17:52

They probably don't slam the accelerator to the floor.

Chemical rockets (like we use here on earth) are great for one thing; getting out of the Gravity Well. Why? Because they burn all their fuel really quickly, generating a MASSIVE amount of thrust all at once. They're so good at this, that we build our rockets in stages, allowing the expended fuel and engines drop to lighten what gets taken further by the next stage of the rocket.

If you're already out of the gravity well though, and you're considering interstellar travel in particular, you don't want to use rocket thinking. It's a long distance after all, and you're FAR better off using constant thrust the whole way. One reason is for exactly what you describe; accelerating constantly at 1G worth of force for half the distance (and then reversing attitude and thrusting at -1G for the second half of the trip) is going to be more comfortable on the crew, AND it's going to get you there a lot faster because your velocity is more of a ballistic curve.

Time Dilation isn't so much of an issue for you in this instance, even if at the halfway point you're travelling much faster than 0.2C. This is because you're not travelling at that speed for the entire journey, just for the peak of your velocity curve, right in the middle. At the beginning and end of your journey, you're travelling much slower in fact. I haven't run the numbers, but I suspect that you don't have that much of a difference overall. That said, your crew probably wouldn't mind a little time dilation if it appears to get them where they want to go faster.

Put simply, think less chemical rockets, more ion or plasma engines. Look these up as they're under active development by NASA for interplanetary and (potentially) interstellar probe engines of the future. In every other respect, it's like your car. It might be able to accelerate from 0-100Kph in 3 seconds, but it doesn't necessarily make it a good idea to do that.

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    $\begingroup$ @JaneS yeah, you really wouldn't want to do that. Just put the slow burn on to the half way point, then turn around and use the other half of your fuel slowing down. You'll get there a LOT faster. $\endgroup$ – Tim B II Sep 17 '18 at 3:53
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    $\begingroup$ It's kinda important to my story to do the "quick" burn at the beginning and the end of the journey, so while it's more efficient the other way, it breaks some of my other logic that I'd prefer to keep :) $\endgroup$ – Jane S Sep 17 '18 at 3:55
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    $\begingroup$ @TimBII If your slow burn still only peaks at 0.2c before the turnaround point, your trip will be slower (though probably more efficient) with constant acceleration. A quick burn with an extended cruise results in a higher average velocity over the whole trip, which results in a shorter trip time. $\endgroup$ – chepner Sep 17 '18 at 11:34
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    $\begingroup$ If you need a reason for 2 years acceleration - 20 years cruise and 2 years deceleration, you can just use engineering/repair. The plasma drive needs a lot of maintanance after 2 years of constant use, it may take several years of repairs and reconstruction to make it safely usable again. $\endgroup$ – Falco Sep 17 '18 at 12:00
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    $\begingroup$ @TimBII If you have a constant acceleration which would obtain 0.2c over 2 years and you don't care about the amount of fuel/reaction mass used or the safety of your maximum speed, then, yes, you would get there faster by accelerating 50% of the way, then decelerate for the rest. However, there are many reasons why you might choose to limit your maximum speed (e.g. your shielding for impacts in the interstellar medium is only good up to X% of c), or limit the fuel/reaction mass used (you can only carry so much of either). Thus, having a coast phase is reasonable, and may result in least-time. $\endgroup$ – Makyen Sep 17 '18 at 21:09

The other answers have shown that you can accelerate much faster than you originally planned. This means time saved getting to your destination. If you really want to get to .2c quickly, sci/fi gives a few possibilities. My favorite comes from the book Death's End. Interstellar explorers use "deep sea acceleration fluid." You can see the original description here, but the gist is that specially designed passenger compartments are filled with oxygen-rich fluid that helps prevent the human body from being crushed under the intense pressure of acceleration.

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    $\begingroup$ Thanks for your answer! I haven't as yet defined the anatomy of these particular space voyagers, but I was keeping in mind the use of "breathing fluid" under high pressure from The Abyss and thought to adapt the idea if the crush of acceleration was high. As it turned out, that wasn't an issue, but I will definitely keep the idea in mind! $\endgroup$ – Jane S Sep 18 '18 at 1:17

As others have said you will be accelerating at less than 1G for 2 years so you may want to look at rotating habitats, unless you want to have problems with muscle and bone loss (and to talk about it or the medication to prevent it).

As your final speed is related to how fast the rocket exhaust is going out the back of the craft it is possible to hand wave it to say the ship could go faster but it is not economical to do it (as you have to burn a lot more fuel for the same change in velocity).

Also of note is that your acceleration (and deceleration) are a product of your mass so as you burn off your fuel it is easier to change your velocity. This could mean a 3 year acceleration and then a 1 year deceleration if you wanted (depends on fuel mass vs rest of ship mass) that would still keep the journey time the same.

Hope that gives you some options and food for thought.

  • $\begingroup$ Thanks for your answer! I haven't really given too much thought to crew habitat yet, but I appreciate your ideas, they will be of use! :) $\endgroup$ – Jane S Sep 17 '18 at 22:23

As others have said, that's not actually a huge amount of acceleration. However, it's worth noting that if you did need very high acceleration that humans could survive, you have an option - don't just accelerate the frame of the ship, directly accelerate the ship and its contents.

Gravitational slingshots do just that. By aiming just right, you can whip yourself around a moving object and gain tremendous speed. Within the craft, you wouldn't feel a thing any more than you would on the ISS - you can't feel gravity when you're falling. There are some limitations to how well this can work. You need everything to be nicely arranged, so .2c is a bit of a pipe dream. But gravity is a good lead

My suggestion: attach some rockets to a colossal sphere of something heavy. Depleted uranium might work. You accelerate that at the acceleration you need. Your ship floats behind it, using small manoeuvring thrusters to keep the correct distance, so that it also accelerates at the correct speed.

The problem here is that if you want significant acceleration, you'll need significant mass. Planet scale. Depleted uranium is about 4 times as dense as planet earth, so an earth-sized ball of uranium would max out at 4g. Try to accelerate the ball faster than that and gravity won't be enough for your passengers to keep up with it. The amount of fuel that you would need to accelerate that much mass is literally astronomical.

The fuel problem has an entertaining solution, though: instead of depleted uranium, use weapons grade uranium. Now it's a rocket!

TLDR: For a wildly impractical and unnecessary solution, build a planet sized nuke, have a controlled detonation propel it through space, drag yourself behind in its gravitational field.


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