.05C

[dragonstar57]

according to the theory of relativity nothing can move faster than light.

so then why has a craft that moves a substantial fraction of the speed of light been so elusive (such as .05) or even .25)?

[ydoaPs]

Energy. E²=(mc²)²+(pc)². Using that equation, calculate the energy of a mere 100kg ship at rest relative to you. Then calculate the energy of that same ship at 0.25c relative to you. Is the energy difference large or small?

[dragonstar57]

so that's the reason?

so if some major breakthrough in energy occurred than .25c would be no problem?

[swansont]

To put ydoaPs's point another way, relativistic speeds are in a regime where the kinetic energy starts become comparable to the rest mass energy. A craft attaining such a speed would have to convert a noticeable fraction of its mass into kinetic energy.

[Janus]

so that's the reason?

so if some major breakthrough in energy occurred than .25c would be no problem?

 

Well, it's a bit more complex than that. The only way to propel a craft through space is by the rocket principle.

 

Rockets operate by the equation:

 

[math]\Delta V =V_{ex} \ln (MR)[/math]

 

Where [math]\Delta V[/math] is the change in velocity of the rocket

V_ex is the exhaust velocity of the rocket.

and MR is the mass ratio or the mass of the fully fueled rocket divided by the mass of the rocket after the fuel is used.

 

For example, a typical chemical rocket might have a exhaust velocity of 4500 m/s. Escape velocity from the surface of the earth is about 11000 m/s.

 

Solving the above formula for MR for a rocket leaving Earth gives a value of ~11.5 , meaning your fuel must out-mass the the rocket by about 10.5 times.

 

From this it is readily apparent that a chemical rocket has no chance of reaching 0.25c as it would need more fuel than there is in the observable universe.

 

The limiting factor is the exhaust velocity, if we can increase the exhaust velocity, we can decrease the amount of fuel we need.

 

Some ideas for fusion rockets boast possible exhaust velocities of up to 1000,000m/s. But even at this, it would take a mass equal to the Sun's worth of fuel just to get 100 kg up to 0.25c.

 

So to get up to even to 25% of c practically would not only take huge amounts of energy but also a way to generate, contain and direct that energy in such a way as to produce much higher exhaust velocities.

Edited May 18, 2011 by Janus

[DrRocket]

according to the theory of relativity nothing can move faster than light.

so then why has a craft that moves a substantial fraction of the speed of light been so elusive (such as .05) or even .25)?

 

The issue is not really relativistic effects, which are quite small until you are closer to c. Neither is it energy per se, though that can become an issue. The velocity of a rocket is a result of conservation of momentum of a body with variable mass (due to the expulsion of propulsive material.

 

[math] V_{rocket}= V_{exhaust} \times ln( \dfrac {Mass_{initial}}{Mass_{final}})[/math] where [math] V_{exhaust}[/math] is the speed of the exhaust gas relative to the rocket.

 

[math]V_{exhaust}[/math] is typically quoted in the industry, by gross abuse of units, in "seconds", the result of using English units and "cancelling" pounds-force by pounds-mass (yeah, it is ugly but traditional. Multiplying [math]I_{sp}[/math] in "seconds" by 32.2 gives you exhaust velocity in feet per second.

 

A hydrogen-oxygen rocket would have an [math]I_{sp}[/math] of about 440 seconds. A solid rocket might have an [math]I_{sp}[/math] approaching 300 seconds (large space boosters are a bit less). Mass fraction, ratio of propellant mass to total mass are usually well below 90%, and a lot less for exotic systems where [math]I_{sp}[/math] might approach 10000 seconds (say ion propulsion).

 

Do the math. 0.05 c is not in the cards. Stretching assumptions a lot you might get to 0.00075 c.

 

 

Note that this does not address thrust which is dependent on mass flow rate. Since this involves consumption of power to accelerate propellant, if thrust is appreciable power consumption can be quite high (grows like the square of exhaust velocity). Therefore energy and power considerations dictate that exotic propulsion technologies are usually very low thrust.

Edited May 18, 2011 by DrRocket

[dragonstar57]

when/if a major development in antimatter occurs would it fit the requirements of such speeds?

[CaptainPanic]

when/if a major development in antimatter occurs would it fit the requirements of such speeds?

If you add enough "when"'s and "if"'s, then yes, it's possible. But you need a helluva lot of "when" and "if".