I love that you point this out. People are so used to even "hard" science fiction hand-waving this away they often don't realize the magnitude of the problem.
For example, if you use a fusion drive (I_sp = .12 c) and start at 99% fuel (9:1 fuel ratio for initial acceleration and then again for deceleration at the end of the journey, so m_0/m_1 = 10 both times) then you can only travel at .27 c which means time dilation is nearly negligible, only about 4%. Similarly, if you start with 99.99% fuel, you still only achieve 0.5 c, 99.9999% (the payload is now one-millionth of the ship) gives you .68 c, one part per trillion gives you .93 c, etc. We assumed perfect engineering, used a trillion tons of fuel for one ton of payload, and we're still only at Lorentz factor of 2.7. If someone invents an antimatter rocket we can improve the .12 to 1, but that's only a one-order-of-magnitude constant difference. The tyranny of the (relativistic) rocket equation is brutal. This wikipedia article contains the equations used for the above calculations:
Now, you might not have to take all your own fuel with you: Bussard ramjet style designs gather their own fuel as they go once they get up to speed, thereby breaking free of the tyranny of the rocket equation. These are in the realm of science fiction at the moment, but at least they are within the known laws of physics.
Realistic projects like Breakthrough Starshot usually assume (a) an external power source (such as lasers pushing a mirrored object), (b) no deceleration at the end (an order of magnitude savings), (c) very small payloads, and (d) only moderate velocities.
I've been moderately critical of Starshot for other reasons (how can a useful camera or other sensor fit into such a small device? How could it have a directional antenna large enough to focus over 4 light years? How could it contain a power source large enough the send a signal back?) but in terms of just getting a payload a certain distance, it seems workable and is perhaps the best design we have.
So yes, bringing your own fuel is not a good idea if you are trying to get close to light speed!
For acceleration, the best idea I've seen is using a laser on Earth, a mirror on the ship, and another mirror on Earth. The more bounces, the higher efficiency.
I love that you point this out. People are so used to even "hard" science fiction hand-waving this away they often don't realize the magnitude of the problem.
For example, if you use a fusion drive (I_sp = .12 c) and start at 99% fuel (9:1 fuel ratio for initial acceleration and then again for deceleration at the end of the journey, so m_0/m_1 = 10 both times) then you can only travel at .27 c which means time dilation is nearly negligible, only about 4%. Similarly, if you start with 99.99% fuel, you still only achieve 0.5 c, 99.9999% (the payload is now one-millionth of the ship) gives you .68 c, one part per trillion gives you .93 c, etc. We assumed perfect engineering, used a trillion tons of fuel for one ton of payload, and we're still only at Lorentz factor of 2.7. If someone invents an antimatter rocket we can improve the .12 to 1, but that's only a one-order-of-magnitude constant difference. The tyranny of the (relativistic) rocket equation is brutal. This wikipedia article contains the equations used for the above calculations:
https://en.wikipedia.org/wiki/Relativistic_rocket#Relativist...
Now, you might not have to take all your own fuel with you: Bussard ramjet style designs gather their own fuel as they go once they get up to speed, thereby breaking free of the tyranny of the rocket equation. These are in the realm of science fiction at the moment, but at least they are within the known laws of physics.
https://en.wikipedia.org/wiki/Bussard_ramjet
Realistic projects like Breakthrough Starshot usually assume (a) an external power source (such as lasers pushing a mirrored object), (b) no deceleration at the end (an order of magnitude savings), (c) very small payloads, and (d) only moderate velocities.
https://en.wikipedia.org/wiki/Breakthrough_Starshot
I've been moderately critical of Starshot for other reasons (how can a useful camera or other sensor fit into such a small device? How could it have a directional antenna large enough to focus over 4 light years? How could it contain a power source large enough the send a signal back?) but in terms of just getting a payload a certain distance, it seems workable and is perhaps the best design we have.
So yes, bringing your own fuel is not a good idea if you are trying to get close to light speed!