I mean, sure, ultimately mass==energy. It broadens your options but doesn't change the problem that you have to take it with you, it gets used up, and the more you take the more you have to use.
The difference is quantitative, but multiple magnitudes big: The ratio of impulse to mass is given by the exhaust velocity, and velocities for massive propellants barely reach 0.01% of the speed of light. Thus the thrust can be more than 10 000 times higher for the same propellant mass.
On the flip side, the exhaust velocity also determines the ratio of (kinetic) energy to impulse, so energy beam propulsion needs over 10 000 times more stored energy.
Energy beam propulsion is only viable if you have compact high-capacity energy storage available (antimatter?).
The thing is we know how to harvest solar energy while in space. Harvesting matter has been proposed with ram scoops but contrary to solar cells, these are just theoretical propositions.
Being able to use energy directly instead of propulsion mass would make a lot of travels more sustainable and with potentially lighter vehicles.
Neat idea. How much energy do you lose (assuming a vacuum)?
You could imagine some kind of dish that catches the energy, but that's the extent of my knowledge -- I don't know how you'd convert it back to propulsion.
Not entirely the right question. Usually in spaceflight you are overwhelmingly concerned with efficiency, since you have to carry your fuel with you. But with laser-boosted light sails, you leave your engines at home. At reasonable distances, (100+ AU) most of the beam is wasted, but the spacecraft doesn't care about that.
Your main losses will be just from inverse-square law. Even if your ship is reflecting a laser off its rear end, you can only focus the beam so much; at some point the spot size will be bigger than the ship, and you'll be in the inverse-square regime (same surface area, 1/r^2 energy within it).
That doesn't seem fundamental, though. You could have a series of relays that catch the beam, convert it to energy, and re-beam it out.
But I'm sure the efficiency would be awful, and if you have a chain of N of these things, now you're dropping off exponentially with N. And N is linear in distance. Hm... this isn't sounding like such a great workaround anymore.
Not to mention that the incoming beam would be shoving your relay forward. Does the outgoing beam push it backwards? I don't know how that works. (Even if it does, you'd be shoved forward proportionally to the energy loss.)
Bleagh. You'd be better off sending out a series of energy pellets well in advance that a traveling ship would scoop up along the way. That must be what Pac-Man was all about...!
> That doesn't seem fundamental, though. You could have a series of relays that catch the beam, convert it to energy, and re-beam it out.
That's a lot of hardware to send out, though. With a dedicated relay you could afford very large collecting surfaces, to compensate a bit for the conversion/retransmission efficiency loss, but there's a bigger problem: you can't just put a chain of relays on a line inside a planetary system. You have to put them in orbit of the Sun (even if by proxy of an orbit around a planet/moon). This means your initial line of relays will quickly drift out of alignment, making the path through them much longer than beaming straight at a ship that's transferring between planets or out of the system. You'd have to put rings of relays at various heights above the sun to guarantee a reasonably short path, and that would take a lot of relays. And work only for a single plane - if you want relayed power at arbitrary plane, you'd have to build shells of relays - so the amount of satellites you need to deploy just squared.
You don't even need a dish to catch the energy; just use a mirror that reflects the laser. You will lose energy to particles in the laser beam, and possibly also due to redshift.
Wouldn't you push your satellite in the opposite direction? Not saying that it isn't a good idea. The satellite might be in orbit, so it could use gravity to negate that.
Note that in contemporary usage, the term 'mass' without qualifier is normally understood as a reference to invariant mass (aka rest mass). Photons have energy, but no mass. They can be used for propulsion due to having nonzero momentum.
While the 'working mass' in this case is massless, nevertheless the spacecraft still loses its own actual mass as it accelerates, which is something you might not expect from something labeled as "thrust without reaction mass".
True. After sending out a photon of energy E, a spaceraft of mass m will have its (invariant!) mass reduced by a factor of sqrt(1 - 2E/mc²).
In terms of energies, this is all rather trivial conceptionally: Rest energy (aka 'mass') gets converted into the kinetic energies of the spacecraft and the photon.
I mean I would just load couple of metric tons of antimatter in the ship and then just use up picked up interstellar space mass (1 atom per cubic meter is not much, but you will be moving trough shitload of them) along the way to annihilate. the faster you move the faster you will go up to a couple of limits. BUT deceleration may be a bit of a problem.
