Why cant other space craft go that fast?

[Naemeth]

It's important to remember G is not speed, it is a measure of acceleration compared to the gravity of the Earth. There is no physiological reason you couldn't travel at 0.2C if you accelerate slowly . No-one is suggesting accelerating at 60G, that would clearly be stupid, but if you accelerate at 1G beyond the orbit of Earth, you will accelerate at 9.8ms^-1, if you leave it long enough, you could travel at a high enough speed without causing any ill effects.

[jeremy1]

As you said G is the effect of gravity on earth. In space it is Zero G as there is no gravity as such that wants to hold you back. So why cant acceleration at silly speeds be possible in Zero G ?

[Naemeth]

As you said G is the effect of gravity on earth. In space it is Zero G as there is no gravity as such that wants to hold you back. So why cant acceleration at silly speeds be possible in Zero G ?

That's a good point - the effect of gravity would decline rapidly as you moved further away, there would be no negative acceleration trying to pull you back (g= -9.8ms²), so you shouldn't feel a 'G Force' . Once you are outside the Earth - Moon system though.

[Jessun]

So chemical rockets can now go do 90G instead of 25? The G - whatever number is not meaningless if it's only a byproduct of the calculation to reach this star in 100 years. I can't recall if the example mentioned live passangers at all.

Franklin, perhaps you could elaborate on how you measure the velocity of space vehicles in G? What are these 100G you referred to earlier? Don't be afraid to confuse things, I think I'll be able to follow. If it makes sense...

/Jesper

[Chris Rowland]

Newton's laws of motion F = Ma or a = M/F

The acceleration is the mass divided by the force so to accelerate at 1 G you need a force of 9.8 Newtons for every Kilogram of mass.

Some of the original assertions don't seem right to me. Ignoring any gravitational accelerations a 1G acceleration for six months will get you to about 0.5c. Stop accelerating, wait about 8 years, turn round and accelerate backwards for six months and you are at Alpha Centuari. (I've chosen to stop accelerating at 0.5c to avoid large relativistic effects.)

Ion drives and chemical rockets work in the same basic way, they throw stuff backwards and, because of another one of Newton's laws, the force required to throw the stuff backwards is the same as the force you get to accelerate. Chemical rockets throw their propellants back at about 4 to 6 km/sec, Ion rockets at about 30 kn/sec so a smaller amount of propellant is required.

The critical thing with rockets is what's called the specific impulse. This is measured in seconds and is essentially the time for which one unit of propellant can generate one unit of thrust. Good chemical rockets are about 450 seconds, ion rockets are about 3,000 seconds.

Getting something that can accelerate at 1G for a year will be quite a trick, especially if it has to carry it's own fuel.

Chris

[J_M_Franklin]

So chemical rockets can now go do 90G instead of 25? The G - whatever number is not meaningless if it's only a byproduct of the calculation to reach this star in 100 years. I can't recall if the example mentioned live passangers at all.

Franklin, perhaps you could elaborate on how you measure the velocity of space vehicles in G? What are these 100G you referred to earlier? Don't be afraid to confuse things, I think I'll be able to follow. If it makes sense...

/Jesper

Jesper, let me unconfuse you;

When I was referring to a vehicle (space) travelling at 90G, that means a speed that is the same as 90 x the acceleration of gravity on Earth, so that would be 882m/s^-1 and that is a speed of 3175.2km/hr. This is about the limit for current the chemical rockets we currently use, however as you correctly pointed out in an earlier post, strapping several together, so long as the thrust to weight ratio increases, will increase the forward velocity of the vehicle, so more than 3175.2km/hr is comfortably achievable. The use of a G reference has it's origins back in the early days of the space age, it is not common to refer to it outside of the aerospace industry in this way. I will endeavour to be clearer when referring to the acceleration and forward velocity of vehicles to avoid confusion.

The 25G I referred to earlier was the actual force of gravity exerted on a rocket when launched, this is the momentary acceleration force and was actually a typo on my part as the physical limits for the rocket are actually 250G and not 25, apologies for that. Clearly no Human could withstand this level of acceleration, the limit I mentioned regarding the horizontal G force has an upper limit of 25G for a human for short periods of time, but a rocket launch actually exerts a vertical G-Force on a human body and this has a upper limit estimated to be about 12G.

[J_M_Franklin]

N

Some of the original assertions don't seem right to me. Ignoring any gravitational accelerations a 1G acceleration for six months will get you to about 0.5c. Stop accelerating, wait about 8 years, turn round and accelerate backwards for six months and you are at Alpha Centuari. (I've chosen to stop accelerating at 0.5c to avoid large relativistic effects.)

Chris, accelerating at 1G (9.8m/s^-1) would require (According to NASA) 12.7kg of fuel per day for an Ion drive, that means you would need about 4.6 metric tonne of fuel to safely get to A Cen A/B and stop...sadly they cannot build one yet that can acheive this goal so they are saying

[Jessun]

Franklin, that didin't help one bit. G is an acceleration - fixed at 9.81m/s² at 'standard gravity'. Just multiplying G does not result in a speed, unless time is factored in. 90G is therefor no example of speed. If you accelerate with 1G for a year, it seems you'll be travelling at near the speed of light. That leaves 98 years for coasting and 1 more year to decelerate (within the proposed 100 year journey time in the example by professor Kaku). By then you'd have overshot our nearest star by a loooooong way.

Here's an article - OK Wiki stuff - on interstellar spaceflight with some interesting reading:

http://en.wikipedia.org/wiki/Interstellar_travel

And here is a page from NASA JPL covering basic units of measure for space flight:

http://www2.jpl.nasa.gov/basics/units.php

I can't see G meaning speed in any shape or form. Not that I expected to.

/Jesper

[J_M_Franklin]

Jesper, firstly could I ask you to call me Jim and not Franklin, that is my surname..

Secondly.If my explanation did not help then i am at a loss to help you understand this concept.

We refer to speed in the atmosphere by Mach numbers, which is a multiple of the speed of sound at 10m above the average surface (sea) of the planet when the air is at 20°C and 1003mb, in the early days of space flight, and many in the aerospace industry who deal with spaceflight, it is common to refer to the forward velocity of a vehicle as a multiple of G, therefore a multiple of 9.8m/^-1. This is not an acceleration velocity, but a forward velocity. The two are inexorably linked, but they are different.

When you see it in technical articles they normally right the forward velocity as fG90 as an example (forward velocity = 90x9.8m/s^-1) this is simply an abbreviation in the same way as aeronautical engineers refer to M2.5 as an abbreviation for ~3067km/hr (1905mph)

When referring to the true acceleration of a vehicle at G is used, this refers to the physical force exerted on the vehicle by it's acceleration. It will feel this force whether it is inside a planetary atmosphere or in space.

For those who keep stating G only refers to earth, this is only true when referring to Earth, the G for a given astronomical body will vary with it's mass/volume, Mars is less than Earth, Jupiter vastly more so.

Linking to a wiki article and a laymen's language article on NASA simply adds to confusion.

[DaveS]

Someone needs to make a "Dark Energy" warp drive .

Maybe someone *did* try this, 6 billion years ago which is why the universe is accelerating now. I can imagine a group of alien engineers saying oops....!

And yes acceleration (Measured in g) has to be muliplied by time to give you speed. It's metres per second per second. A rifle bullet will be accelerated by something like 5000g but will still be going fairly slowly compared to the speeds we're taliking about, whereas an ion drive only has a small acceleration, but it's the seconds that add up.

[Jessun]

Jim it is then, no worries, but I can't really be expected to guess your first name?

We refer to speed in the atmosphere by Mach numbers, which is a multiple of the speed of sound at 10m above the average surface (sea) of the planet when the air is at 20°C and 1003mb,

It's perfectly safe to say that you add to confusion with a statement like that. Mach has absolutely nothing to do with 10m over the sea, 20 degrees or 1003 mBar. If you were aiming for the International standard atmosphere (ISA) you're off by a bit anyways. (15° at 1013.25mBar or 101325 Pa at precisely sealevel - just simple fixed values).

Mach is unit less, due the division in the formula, and is simply the fraction of the speed of sound you travel at. (M=v/v sound). The 'v sound' varies with temperature. Mach is positively never, never, ever directly translated to speed in aviation - not even by approximation. I know this from flying fighter aircraft for 10 years, and commercial aircraft for 15. Mach differs too much with altitude - as a function of temperature only - for any approximation to be of sensible use. Especially if you fly at around M1 vertically which was always fun .

As for velocity expressed in for example fG90, I can understand your latest explanation, but not the limits you appear to put to speed in general in previous posts.

I have, personally, never come across this unit of measure for velocity in literature of any kind, and I don't see its immediate use - given of course that I don't read all. Fractions of speed of light seem way more relevant for serious interstellar travel. And I have no doubt in my mind that very, very high speeds indeed can be reached in space - even with current technology. Nuclear power - by means of exploding small charges behind the ship - was only dismissed after a treaty was signed to ban such overground tests necessary to develop this propulsion type. It's current though, and very efficient indeed. As for chemical rockets, I stand by prof Kaku any day.

I'll leave this discussion now. Let me just extend an apology to the OP for unwillingly taking the thread in surprisingly odd directions.

/Jesper

EDIT, some spelling. As always...

[J_M_Franklin]

Jesper, you are taking this too literally FFS and I simply used mach as an example of how one term is used to describe something. As for how Mach is calculated, the calculations I have always used and been instructed to use are as I stated.

Mach does not change as much as some people seem to think. At 0m altitude the mach No for 340m/s is 0.998, at 1000m it is 1.01 (the speed of sound dropping to 336m/s) and at 10,000m it is 1.134 (with the speed of sound dropping to 299m/s

So Mach has no relevance in many ways to ground speed, my comment in my last post was based on the fact that Mach 2.5 at sea level is approximately 3067km/hr (1905mph). However as a pilot you should know that this ground speed figure would vary as altitude, air pressure and air temperature vary, as these all impact the speed of sound in the given air mass. And to be totally pedantic, I thought all commercial pilot spoke about their speed in knots??

I am pleased you finally got what I was saying.

[gkec]

Jesper, firstly could I ask you to call me Jim and not Franklin, that is my surname..

Secondly.If my explanation did not help then i am at a loss to help you understand this concept.

We refer to speed in the atmosphere by Mach numbers, which is a multiple of the speed of sound at 10m above the average surface (sea) of the planet when the air is at 20°C and 1003mb, in the early days of space flight, and many in the aerospace industry who deal with spaceflight, it is common to refer to the forward velocity of a vehicle as a multiple of G, therefore a multiple of 9.8m/^-1. This is not an acceleration velocity, but a forward velocity. The two are inexorably linked, but they are different.

When you see it in technical articles they normally right the forward velocity as fG90 as an example (forward velocity = 90x9.8m/s^-1) this is simply an abbreviation in the same way as aeronautical engineers refer to M2.5 as an abbreviation for ~3067km/hr (1905mph)

When referring to the true acceleration of a vehicle at G is used, this refers to the physical force exerted on the vehicle by it's acceleration. It will feel this force whether it is inside a planetary atmosphere or in space.

For those who keep stating G only refers to earth, this is only true when referring to Earth, the G for a given astronomical body will vary with it's mass/volume, Mars is less than Earth, Jupiter vastly more so.

Linking to a wiki article and a laymen's language article on NASA simply adds to confusion.

G is the gravitational constant. g is the ACCELERATION due to gravity. Which (for Earth) is 9.81,/s² .

So for every second of g acceleration the velocity of the object is 9.81 metres per second faster than it was.

g at the surface of a planet is determined by the formula g = Gm/r² where m and r are is the mass and radius of the planet.

There is no confusion.