# Hello all, new member with a strange question

1. Oct 9, 2009

### BJBeard

At least that is what I am told when I ask it around here...

How long would it take to go get to 45 miles per second at human tolerable acceleration?

I am not a math whiz obviously, but I have some questions that get me some really strange looks, and no answers.

I know that currently the fastest man made-object, Helios-2, was estimated to have been at 42-43mps at one time during its mission.

Thanks for any help,
Brian in Florida.

(PS if this isn't the right sub-forum for this question my apologies.)

2. Oct 9, 2009

### Molydood

Wow, that's pretty fast!
The answer to the question comes down to establishing the maximum acceleration a human can sustain for extended periods of time (I *think* the maths are pretty simple as even I can do them, unless I am oversimplifying)

ALL calcs that follow are worked out in my head, so probably nowhere near accurate, and could even be wrong

Speed
45 miles a second = 1600*45 metres /s = approx 80,000 metres/s

Time
At 1g = 9.81 m/s/s, time taken would be 8000 seconds or 120 minutes or 2 hours
At 2g = 20 m/s/s, = 4 hours and so on

I reckon 1g accel would be ok for a couple of hours? (1g is the initial accel. you would 'feel' if you jumped out of an aircraft)

Those with bigger brains feel free to correct anything I have said, it's just a start, and possibly wrong too :-)

3. Oct 9, 2009

### uart

It depends on exactly what human tolerable acceleration limit you accept (which in turn depends on the seating, suiting and training of the pilot).

At 10g it would take approximately 12 minutes and 20 seconds to reach that speed.

4. Oct 9, 2009

### Molydood

beat 'ya :tongue:

5. Oct 9, 2009

### A.T.

I live at 1g for years now.
No, you would 'feel' 0g,

6. Oct 9, 2009

### fawk3s

Shouldnt it take less with bigger mass? Or Im worng.

Last edited: Oct 9, 2009
7. Oct 9, 2009

### Molydood

Agreed, this is nonsense, sorry for misleading anybody, that was a really terrible example I gave, and *completely* wrong A better analogy for getting a 'feel' for 1g or 2g is probably to imagine how it feels to weigh twice as much (by that I mean having twice as much mass, but still in earths gravitational field), seeing as we have no experience of walking on other, larger, planets and experiencing higher levels of accel. Alternatively, anybody who pilots jets or similar may have a good idea about what 2g, 5g or 10g feels like and hence be able to comment on an acceptable top limit for sustained acceleration.

I was just trying to give some real world examples to help get a feel for things, my bad for rushing my answer, and being an idiot

8. Oct 9, 2009

### Molydood

and the errors continue, I am really on form today

you are right, I am wrong

9. Oct 9, 2009

### sganesh88

As long as we're talking in terms of accelerations, mass doesn't enter the picture. V=a*t. No mass. Molydood is right there i suppose.
And i don't get the 0g part A.T mentioned. Isn't it 1 g? We don't feel any force as we're in free fall but isn't "g" quantity just a measured acceleration from an inertial frame expressed in multiples of g (9.8 m/s/s)? The wikipedia article about it, i just referred is confusing. It initially says g-force is actually acceleration but later talks about g-force acting on some body. How can 'acceleration' act?

10. Oct 9, 2009

### Molydood

I just glossed over that as I figured mass' was just a typo from fawk3, he meant 'g' or accel. I believe

Anyway, the acceleration or 'g' thing is confusing, and this could easily head into a massive discussion about semantics if we start talking about definitions, but in the context we are talking about here I beilieve things are as follows:

the difficulty humans have with acceleration is that a 'force' must be exerted on them to accelerate (or decelerate) them, usually a seat back in a car or spacecraft, or whatever, but because this force does not act on every atom within your body with equal measure, you experience some sort of (usually) temporary bodily 'deformation' in varying levels depending on the severity of the acceleration, and how the 'force' is applied

The deformation can come in many different forms, but for the example of mild accel, such as that experienced in a fast car, it would just be the skin on your back compressing, and maybe some other minor sensations, but nothing serious. Larger accelerations would see you being 'pushed' by bigger forces and in turn, more deformation (this could be too much blood moving into your head in a jet, causing redouts, or whatever) either way I think 'deformation' is a decent enough catchment term for this effect, as essentially that is what is happening (atoms which are supposed to be in one place relative to one another end up in another place relative to one another, and the body doesn't like it too much)

Now, when it's mentioned about 1g being exerted on you, in this context I believe it refers to times such as when you are sitting or standing and thus preventing yourself from 'accelerating', in which case you will feel the 'force' of 1g through your feet or back or whatever. Now if you jump out of an aircraft, that 1g would still be acting on you and accelerating you (ie changing your speed), but you wouldn’t feel the 'force' of it initially, not until you reached terminal velocity anyway (and had some wind resistance to push against the outside of your body)

The 0g mentioned earlier is potentially confusing, but if you think about it, your body is not used to 'weghtlessness' hence that weird feeling in your stomach when you are freefalling, because normally your body would have your internal organs, lungs etc 'resting' on other parts of your body in the downward direction, whereas in free fall it all sort of floats about

That is my take on it anyway, it's all about context, and in this case I got the context wrong by talking about 1g when jumping out of an aircraft (even though technically you are accelerating at 9.81 m/s/s (initially) which is '1g' )

Writing this all in a rush so hopefully I haven't got it wrong again. Have a good weekend

11. Oct 9, 2009

### fawk3s

Sorry, the mass was indeed a typo. I meant bigger g.
This g part confused me and I automatically thought of gramms when seeing g, so there you go.

12. Oct 9, 2009

### Integral

Staff Emeritus

This is just wrong. It will take LESS time at a higher acceleration rate not longer.

Molhdood, if you are not going to think about what you are posting please back off.

13. Oct 9, 2009

### fawk3s

This was cleared a couple of posts ago.

14. Oct 9, 2009

### Archosaur

The "Top Thrill Dragster", a roller coaster at Cedar Point in Ohio, goes from 0 to 120 miles per hour in 3.8 seconds which is about 1.5 Gs. That may not be the most a human can handle, but I promise it's the most I can handle. Anyway, that rate would get you to 45 miles per second in 5130 seconds, or about 1.425 hours, though I really don't recommend it. (pukes just thinking about it)

15. Oct 9, 2009

### sganesh88

@Molydood
Delightful post. :) But still i am not entirely clarified. For example, when we are floating in water, we'd still be 'at' 1g but we can tolerate a lot more force from the water against gravity. We'd love to, infact. Its all in the stress value and the 'g' quantity merely confuses people i think. Just another popular science term like relativity and Black hole that no one understands but just 'knows'.

16. Oct 10, 2009

### BJBeard

I was thinking 2g acceleration would be fine, I think most average health folks could do that. Now the even bigger question...

How many RS-68's will it take to do this? The ship's mass is about 350,000 lbs, NOT including fuel.

17. Oct 10, 2009

### Molydood

Cheers :-)

I have to admit I have not really given much of this stuff a second thought until now, hence my original out of context comment, which I hope didn't upset too many people.

With water I see it as just a very efficient way of supporting your body weight in that all of the upward thrust supporting your body is acting evenly over the largest possible area (unlike when stood up for example). I think that is similar to what you were saying? So although 'g' is the same in water as when sat in a chair, in a way it's less 'stressfull' (to use your term) because there is lower pressure on the outside of your body (same force lower pressure). When we get to higher (and artificial) 'g' levels however (eg sat in the cockpit of the space shuttle during lift off) I would say that the problem shifts away from external stress (how the acceleration is applied to your body) and moves towards being about internal stress (how it effects your insides)

18. Oct 12, 2009

### BJBeard

So no one wants to help out the the delta V? How about dropping me a pre-algebraic equation to solve it, or point me to a book of orbital mechanics for morons!

19. Oct 12, 2009

### Archosaur

The "stress" you guys are talking about is nothing but distribution of force. If you weigh 180 pounds, that means that the Earth is pulling on you with 180 pounds of force. In order to not fall through the Earth, something else has to push up on you with the same force (we call that force the 'normal' or 'contact' force.)

You feel "stress" when that force is concentrated in a small area (stepping on a nail).
You feel less "stress" when the force is spread out (floating on water).

20. Oct 13, 2009

### Molydood

I thought that had been established already. I used the term 'pressure' in my previous post, and also previously the analogy of standing up (ie high pressure through feet) versus floating in water (ie low pressure distributed over surface area of body). I was using the term 'stress' for continuity reasons, but I did make it clear that this is actually the effects of 'pressure'.