[seppel]

1kg matter at 0.1c has a kinetic energy of 4.5x10^14 J

It is certainly very expensive to spend this amount of energy. But is it also extremely challenging to even extract this out of an engine that has to accelerate itself.

1kg matter at 0.995c has a kinetic energy of 8x10^17 J

This is the amount of enery you get if you convert 9kg in pure enery. That seems impossible to reach, even with an anti matter drive.

[selestify]

A 9:1 ratio of fuel to mass doesn't sound particularly bad. Are there known theoretical limits to extracting the full E=mc^2 worth of energy from an object?

[squirrelicus]

No, you can do this with an antimatter rocket.

Good luck capturing and containing antimatter and feeding/containing the explosion though. It's going to be practically impossible to get a 90% fuel ratio from an antimatter reactor, since literally 45% of your ship will be antimatter, by mass.

[adrianN]

You can also do this by feeding a black hole with ordinary matter and using Hawking radiation for propulsion. That is probably more practical than carrying around large quantities of antimatter.

https://en.wikipedia.org/wiki/Black_hole_starship

[m_mueller]

If you watch PBS Spacetime on the 'Kugelblitz' idea, it seems that one way to go about it (over the course of a couple hundred years) would be to convert Mercury into a solar energy collecting swarm by use of Von Neumann probes and essentially harvest much of what the sun has to give. That kind of energy apparently would suffice to create tiny black holes (simply by redirecting enough energy to a focused point), which is I think the theoretically most efficient battery (or star system destroying bomb, depending on how you go about it...).

[squirrelicus]

I... can't.... Half of me recoils in horror at the impracticality, but the other half of me has a good enough armchair sense for the practicality of a high fuel ratio antimatter rocket, so I can't say you're wrong :P

[seppel]

You need to accelerate the fuel as well. And for a reaction engine, you need to carry reaction mass that you accelerate. And at some point you want to decelerate again.

Would be interesting to see whether this is at least theoretically feasible.