Power Required to Accelerate Space Craft to Near Light Speed

In summary, the conversation discusses the estimation of power required to accelerate a spacecraft from 0.9c to 0.9999c, taking into account special relativity and time dilation effects. It also mentions the use of classical and relativistic energy, and the calculation of power levels from different reference frames. The relativistic rocket equation is also mentioned as a tool for calculating the required power. The conversation ends with a discussion on the power from the point of view of accelerating observers onboard the rocket.
  • #1
edgepflow
688
1
I hope I can explain my question clearly enough for someone to answer.

Suppose we wish to estimate the power required to accelerate a spacecraft from say 0.9c to 0.9999c in a certain time. By basic definition: Power = Change in Energy / Change in Time.

Due to special relativity, there is a difference in these quantities as measured by the stationary Earth and the space craft. The classical change in energy would be m*v^2 /2, while the relativistic energy would include the term for increase in relativistic energy. The time dilation effect may also be directly calculated.

So to figure the power, which energy (classical or relativistic) and which time (earth or spaceship) should be used as seen on (a) earth, or (b) the spaceship. Are these powers equal?
 
Physics news on Phys.org
  • #2
If your spaceship is moving at relativistic speeds, you need to use the relativistic energy. As far as power levels go, it depends on what you're interested in. You can calculate either, depending on your interest, and should note which one you are calculating.

Note that rockets are not 100% efficient though. You might be interested in the relativistic rocket equation, which would give you answers to what would be required if the spaceship were a rocket.

http://math.ucr.edu/home/baez/physics/Relativity/SR/rocket.html [Broken]
 
Last edited by a moderator:
  • #3
The time as measured in the Earth frame (t) for a rocket accelerating with constant acceleration as measured in the rocket frame (a) to reach a given velocity (v) is:

[tex] t = \frac{v}{a\sqrt{1-(v/c)^2}}[/tex]

The time interval measured in the Earth frame to go from 0.9c to 0.9999c is:

[tex]\Delta t = t_2 - t_1 = \frac{0.9999c}{a\sqrt{1-(0.9999)^2}} - \frac{0.9c}{a\sqrt{1-(0.9)^2}}[/tex]

The energy of the rocket at a given instant in the Earth frame (ignoring loss of mass due to using fuel) is:

[tex] E = m c^2 \sqrt{1+(at/c)^2} [/tex]

The change in energy in the Earth frame is:

[tex]\Delta E = E_2 - E_1 = m c^2 \sqrt{1+(at_2/c)^2} - m c^2 \sqrt{1+(at_1/c)^2} [/tex]

where m is the invariant mass.

The average power going from 0.9c to 0.9999c in the Earth frame is then:

[tex] W = \Delta E / \Delta t [/tex]


Now if we switch to the point of view of an inertial observer moving at 0.9c relative to the Earth frame, the initial velocity of the rocket is zero and the final velocity using the relativistic velocity addition formula is:

[tex] v_2 = \frac {0.9999c-0.9c}{(1-0.9999*0.9)} = 0.9981c [/tex]

The equations given earlier can be reused to calculate the time interval and the power in the new frame using the new velocity values. It turns out that the change in energy and power in the new frame is less than in the Earth frame, but that makes sense because the final velocity of the rocket is lower according to the observer moving at 0.9c relative to the Earth.

The proper time that elapses according to the rocket observers as they accelerate from 0.9c to 0.9999c is given by:

[tex]\Delta T = (c/a)*atanh(0.9981) [/tex]

As for what the power is from the point of view of accelerating observers onboard the rockets, I am not sure. What would they consider their velocity and energy state to be at any given time?
 
  • #4
kev said:
As for what the power is from the point of view of accelerating observers onboard the rockets, I am not sure. What would they consider their velocity and energy state to be at any given time?

Nice exposition. As for the power seen in the ship frame, it's just the jet-power which doesn't change for a constant thrust scenario. Exhaust velocity and mass ejected would remain the same in that case at all speeds, in the ship's reference frame.
 

What is the concept of power required to accelerate a space craft to near light speed?

The power required to accelerate a space craft to near light speed refers to the amount of energy needed to increase the velocity of the spacecraft to a speed that is close to the speed of light.

How is the power required to accelerate a space craft to near light speed calculated?

The power required to accelerate a space craft to near light speed is calculated using the formula P = (0.5 x m x v2) / t, where P is power, m is mass, v is velocity, and t is time. This formula takes into account the mass of the spacecraft, the desired velocity, and the time it takes to reach that velocity.

What factors affect the power required to accelerate a space craft to near light speed?

The main factors that affect the power required to accelerate a space craft to near light speed are the mass of the spacecraft, the desired velocity, and the propulsion system used. Other factors such as air resistance, gravitational pull, and external forces may also play a role in determining the power needed.

Is it possible to calculate the exact power required to accelerate a space craft to near light speed?

No, it is not possible to calculate the exact power required to accelerate a space craft to near light speed due to the uncertainty principle in quantum mechanics. This principle states that the more accurately we know the position of a particle, the less accurately we can know its momentum, and vice versa. Therefore, the exact power needed to accelerate a space craft to near light speed cannot be determined.

What are some potential challenges in achieving the power required to accelerate a space craft to near light speed?

One of the main challenges in achieving the power required to accelerate a space craft to near light speed is the amount of energy needed. The power required is immense and current propulsion systems are not yet capable of providing such high levels of energy. Additionally, there are also technical challenges in building a spacecraft that can withstand the extreme speeds and forces involved in near light speed travel.

Similar threads

  • Special and General Relativity
2
Replies
45
Views
3K
  • Special and General Relativity
2
Replies
65
Views
4K
  • Special and General Relativity
Replies
5
Views
1K
Replies
130
Views
7K
  • Special and General Relativity
Replies
15
Views
1K
  • Special and General Relativity
3
Replies
75
Views
3K
  • Special and General Relativity
2
Replies
36
Views
3K
  • Special and General Relativity
Replies
33
Views
2K
  • Special and General Relativity
Replies
4
Views
1K
  • Special and General Relativity
Replies
5
Views
1K
Back
Top