# Space Travel & G Forces

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#### bfenster

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Regarding the article on 10/26 at space.com â€œLooking to Lasers, Microwaves and Anti-Matter for Space Travelâ€. I see G force issues as a major obstacle to achieving usable speeds for distant space travel. Therefore, I have the following two fold question, the second part of which is way beyond my math capabilities:<br /><br />Questions:<br /><br />1. What G-force can a human tolerate over an extended time period? You can use a hypothetical â€œbest caseâ€ and assume it is happening in space, meaning no effects from Earth gravity, just the effect of constantly overcoming inertia.<br /><br />2. Using the best case G force established in the answer to 1 above, how long would it take to accelerate to half the speed of light? Three quarters the speed of light? And if it were possible, the speed of light itself?<br /><br />Sorry if this has been beat to death before but I didn't find it.<br /><br />Thanks, Bruce<br />

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#### usn_skwerl

##### Guest
the average person can withstand 3-5 G's without suits. a fighter pilot can handle as much as 7.<br /><br />a friend came up with a theory that if we could find the technology to accelerate at 1 G over 24 hours, we would be able to accelerate to C. doesn't sound TOO hard, right?<br /><br />Well, 1g on Earth is 32ft/s/s....<br /><br />after 1 second we climb 32 feet<br />after 2 secs, 64 ft<br />3=128<br />4=256<br />5=512<br />6=1024 (at this point we're now traveling at almost 1/4 mile per second.)<br />7=2048<br />8=4096<br />9=8192<br />10=16,384 ft/s<br /><br />After 20 secs, we're at 16,777,216 f/s or 3,177.503030 (3030 repeating) miles per second. We'll change that to 3,000 mps to allow some tolerance for fluxuations in the theoretical rocket's thrust<br /><br />At 26 seconds with an acceleration of just under 32 ft/s^2, we're traveling 192,000 miles per second. This is faster than light (186,000 mps). <br /><br />At 50 seconds, with an acceleration the same as above (slightly less than 32 ft/s^2), we're travelling at 6,442,450,944,000 (over 6.4 <b>TRILLION</b> miles per second). This is WELL over the speed of light, and we would watch Alpha Centauri rapidly get bigger in just a couple hours, then get smaller even faster. (editIf we stopped accelerating, and just continuted coasting at this rate, we would pass Alpha Centauri in 4,042.5 seconds<br /><br />Considering we're breaking all laws of physics at this point, we might as well go for broke, and see what happens when we enter a black hole. Hold onto your custom-made seat liner!!!! <br /><br />(edit time scale) <div class="Discussion_UserSignature"> </div>

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#### siarad

##### Guest
Whoa! you're doubling your speed every second, that's not 1g.<br />At 1g it would take something like a year to reach c.

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#### kelvinzero

##### Guest
This'l be pretty simplistic. Im sure someone here could do better.<br /><br />Lets think about how long it takes to get to another star with constant acceleration. I think Newtonian maths give the right answer for the traveller, but an observer would see some slight lag.<br /><br />Newton: s=ut+0.5at^2 where<br />u=initial speed<br />a=acceleration<br />t=time.<br /><br />How long to travel to alpha centauri using 1g acceleration to the midpoint, then turning around and decelerating at one g?<br /><br />To find the time to midpoint:<br />s = distance to midpoint (= half total distance = 0.5d)<br />u = 0 (disregarding relative speeds of stars)<br />t= time to midpoint<br />= /> 0.5d= 0.5at^2<br />= /> d = at^2<br />= /> t=sqt(d/a)<br />Since we want the total time T which is double t<br />T = 2*sqrt(d/a)<br />where<br />a= acceleration for 1g = 10ms^-2<br />d= 4.4ly where one ly=9.5*10^15m<br />= />d= 4.2*10^16m<br />So T(seconds) = 2*sqrt(4.2*10^16/10)<br />= about 4 years???? sorry. I think it is meant to be more like 24 years. Im a bit drunk. Can someone check my maths?<br />

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#### usn_skwerl

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You're considerably closer to the correct answer than I am. I just realized all the effort I made was for nought. <br /><br />Why to I have the sudden overwhelming feeling that Newton would want to push me off a cliff? <img src="/images/icons/blush.gif" /> <div class="Discussion_UserSignature"> </div>

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#### SpeedFreek

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I can't do the maths myself, so I cheat and use a calculator!<br /><br />From the Relativistic Starship calculator and checked with Space Math , using relativistic calculations:<br /><br />If our target is at 4.4ly and we are using a constant 1g acceleration until mid-point, and then turn around and use a constant 1g acceleration in the opposite direction of travel until we reach our destination -<br /><br />With a constant 1g acceleration, an observer would see the ship take 1335 days (3.65 years) to reach the halfway point (2.2ly) but the travel time on the ship would only be 985 days (2.69 years) due to time dilation.<br /><br />So after doubling that, the observer would see the journey take 2670 days (7.3 years), whereas only 1970 days (5.39 years) would pass for the passengers of that ship!<br /><br />(Note: the time the observer takes to see the ship make the journey is calculated <i>after</i> light-travel time is subtracted - the difference is not due to the time the observer takes to see the light from the ship, the difference is what is left <i>after</i> the observer has calculated all that out! If the ship returned, the passengers would have aged around 4 years less than the observers back home!)<br /><br />When the ship reaches the halfway point, it is travelling at over 88% of the speed of light, so Newtonian maths will be way out. <div class="Discussion_UserSignature"> <p><font color="#ff0000">_______________________________________________<br /></font><font size="2"><em>SpeedFreek</em></font> </p> </div>

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#### lukman

##### Guest
I take it from an advertisement, a jet fighter plane pilot will experience 5-6 G for over 3 minutes, a F1 driver will experience 5-6 G for over 1.5 hour. David Purley, Silverstone 1977 (highest G-force ever recorded, 180 G, the car decelerating from approx. 110 mph to zero in half a meter. The crash was extremely violent, Purley VERY lucky to survive. Many frontal crash from 30-80G are common in F1. <div class="Discussion_UserSignature"> </div>

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#### SpeedFreek

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2. Let's then assume that a human could stand a constant acceleration of 5g.<br /><br />At 5g it would take 77 days on board the ship, to reach half the speed of light, but an observer back home would calculate that it took 81 days.<br /><br />At 5g it would take 137 days on board the ship, to reach three quarters the speed of light, but the observer back home would calculate that it took 160 days.<br /><br />At 5g it would take an infinite amount of time to reach the speed of light. You can never quite get to the speed of light, so we cannot assign a time for it take, based on <i>any</i> acceleration.<br /><br />But we have a problem in that we need more and more energy to maintain a constant acceleration, as the velocity of the ship increases, due to the relativistic mass increasing.<br /><br />This link - The relativistic rocket explains it very well. <div class="Discussion_UserSignature"> <p><font color="#ff0000">_______________________________________________<br /></font><font size="2"><em>SpeedFreek</em></font> </p> </div>

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#### kelvinzero

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<font color="yellow">When the ship reaches the halfway point, it is travelling at over 88% of the speed of light, so Newtonian maths will be way out. </font><br /><br />Oops. I always thought the newtonian formula still worked for the traveller though they saw the universe as contracting instead of exceeding light speed, and observers saw a different value. If Im reading that link right it is something much more complicated. Oh well. <br /><br />(edit) .. er if Im reading it right those sites are applying the double calculation (acceleration + decelleration) and are actually giving faster times for the trip from the travellers point of view than the newtonian formula

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#### CalliArcale

##### Guest
<blockquote><font class="small">In reply to:</font><hr /><p>the average person can withstand 3-5 G's without suits. a fighter pilot can handle as much as 7. <p><hr /></p></p></blockquote><br /><br />The average person can withstand as much as the fighter pilot can, except that they'll probably pass out and very likely will not enjoy the experience at all. <img src="/images/icons/tongue.gif" /><br /><br />Fighter pilots will go up to 9 Gs in tight turns, actually. And Soyuz crews unfortunate enough to experience a ballistic descent will get to enjoy 9 Gs. One Soyuz crew (I forget which at the moment) survived a whopping 20 Gs during a harrowing emergency abort during second stage ascent. Injuries were sustained, mostly from the vehicle rolling violently down the side of a hill after touchdown. <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>

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#### usn_skwerl

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A fighter pilot knows what muscles to constrict in a G maneuver, and how much. The average person does not. Being awake for a reentry or maneuver is kind of important, depending on the majority of air/spacecraft. I know i wouldn't want to be pass the eff out if I'm headed towards terra firma at "ludicrous speed".<br /><br />Fighter pilots can handle 9 sustained G's with a G- suit on.......unless of course, its a Blue Angel pilot; they don't wear G-suits. <img src="/images/icons/wink.gif" /> <div class="Discussion_UserSignature"> </div>

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#### CalliArcale

##### Guest
Yep -- that's why I said the regular person probably wouldn't be awake for it.<br /><br />You'd be surprised, though, at what you can pass out during and live to tell about it. One of the better points of the Soyuz is that it is designed to take care of the entire process by itself. This was lamentably proven true when Soyuz 11 returned the crew of Salyut 1, who had died during the deorbit process due to a loss of cabin pressure. Until they opened the capsule, the only clue ground controllers had that anything was wrong was the lack of any communications from the crew. <img src="/images/icons/frown.gif" /> <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>

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#### lukman

##### Guest
The only apparent weakness for an ordinary person compare to a fighter pilot and F1 driver is their neck and shoulder muscle. At sudden direction change, there is enough G to break an ordinary person neck. <br /><br />F1 driver have more muscle at their right side of neck and shoulder because they have more clock wise track race. At anti clockwise track, a driver will suffer. <br /><br />In this case I think a human can handle few tens of G, but only for a very short moment, maybe a few millisec or nanosec. Under 10G, that depends on your fitness and neck muscle. <div class="Discussion_UserSignature"> </div>

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#### usn_skwerl

##### Guest
Pilots tighten up their leg and torso muscles to keep blood in the upper body. <div class="Discussion_UserSignature"> </div>

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#### CalliArcale

##### Guest
<blockquote><font class="small">In reply to:</font><hr /><p>In this case I think a human can handle few tens of G, but only for a very short moment, maybe a few millisec or nanosec. Under 10G, that depends on your fitness and neck muscle.<p><hr /></p></p></blockquote><br /><br />Yes. Prolonged G loads can be fatal, even if one could survive the short loads.<br /><br />One thing done in spacecraft to help with this problem is the design of the seat cushions. They're not real strong on the Shuttle, but Soyuz seatliners are custom-fitted to the specific crewman for a perfect fit. This greatly reduces the risk of injury, should the vehicle have to make a ballistic reentry. (That's the 9-G flight profile. The most recent Soyuz landing was a ballistic one, and one of the crew was actually a space tourist. He was fine.)<br /><br />I'm sure you've seen the ads for the "Swedish sleep system". (Can't think of the name at the moment, though I've got one of their beds.) They tout the fact that the material was developed with NASA funding -- and it was! If I'm not mistaken, the foam was designed originally for better cushioning in spaceship seats, which would help reduce injury even further. It's not needed on the Space Shuttle, since it flies such a benign profile (and when it doesn't, it's not survivable anyway), but I would expect the new Orion vehicle to need that sort of thing for when they attempt the double-skip reentry. <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>

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#### jasonpply

##### Guest
what do you mean by a double skip re-entry?

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#### usn_skwerl

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similar to aerobraking, but faster. The object barely dips into the atmosphere, slows down enough to get below escape velocity, skipping back into space, and after a brief period in orbit, it falls back in. <div class="Discussion_UserSignature"> </div>

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#### dragon04

##### Guest
<b>If</b> you could accelerate to the speed of light at 1g acceleration, it would take about one year to do it relative to ship time.<br /><br />Actually, it would be like .98<i>c[/c].<br /><br /><br /><br /></i> <div class="Discussion_UserSignature"> <em>"2012.. Year of the Dragon!! Get on the Dragon Wagon!".</em> </div>

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