Electric Propulsion: Lasers and Rail Guns

[Racescort666]

I realized that after I wrote this post, I found out that this exists already in the form of pulsed plasma thrusters. Rather than deleting the post, what does everyone think about pulsed plasma thrusters?

I was thinking about laser ablation as a means of propulsion since it's basically one of the proposals for asteroid redirect when I started to think about it for spacecraft propulsion. Laser ablation by itself probably has pretty terrible ISP so why not accelerate the gasses via electromagnetic propulsion? The propellant could be literally anything but probably an inexpensive inert metal would work the best (maybe?). 

Please excuse the crudeness of this, I literally spent 5 minutes in SolidWorks drawing it. So the 3 parts to it are the fuel which gets fed into one end of the engine, the laser that ablates the fuel, and an electromagnetic accelerator to eject the fuel at high velocity. 

Hypothetically, you'd like it to be adjustable because as you ablate it away it would need to be repositioned closer to the laser. There would probably need to be a shroud of some kind to keep the molten metal contained which I didn't draw. Then the accelerator accelerates said ablated material out the back as reaction mass. Exit of the engine is at the right of the picture.

I really have no idea if something like this would even work much less work well enough as a propulsion system so what's everyone else think? Someone may have thought of this already and there's something that I'm missing that makes this a dumb idea. I don't know, I'm not really that involved in novel spacecraft propulsion.

A drawback of this design would be the power consumption. I don't know how much power this would require to be feasible or if it would even be on par with existing ion/hall effect thrusters. 

I would think that a benefit would be having an inexpensive, easy to store fuel. You wouldn't need a high pressure gas tank on the spacecraft or really a tank of any kind. Hypothetically, it would be easy to refuel if it came back to Earth, just add a new chunk of metal to the engine. Fuel package space would be considerably less than existing ion/hall effect thrusters

Maybe I'm not thinking about this right but at a high level, it's this: ablate solid with laser, accelerate ablated products. Would this work or not? What would you have to look out for if it hypothetically worked?

[KSK]

I don't think it's a dumb idea because conceptually it should work. What you've got is basically a space-going MALDI (Matrix Assisted Laser Desorption Ionisation) injector feeding into a linear accelerator.

How well it would work and how it would compare to other ion drives though, I don't know. I suspect that for any spacecraft bigger than a satellite you'll be faced with the usual problem of finding a suitable power supply.

[shynung]

A bit of confusion in the terminology here. What you're describing as fuel is actually the propellant or reaction mass. The fuel would be what you'd chuck into the power generator to power this thing. Note that if you have solar panels, your fuel is essentially limitless

The EM accelerator on the right looks like some sort of coilgun. While a single coil would be effective, you can increase the specific impulse by using multiple stages of coils. When the plasma reaches the center of the first coil, the second coil would be activated (and the first killed), so the plasma would accelerate some more. This continues until the last coil, giving the thruster an improved exhaust velocity.

And yes, you put forward a good point - this would consume quite a bit of power. I'm guessing you can get by with solar panels for smaller applications such as satellite OMS, but larger things like manned spacecrafts would need either big panels or fuel cells.

[xehpuk]

I have read that usually ion thrusters use an inert gas like argon since it will do the least wear and tear on the engine. 

[Shikashi]

A reacton mass of this density must make up for a truly wonderful ISP if you have a compact and low density electromagnet to accelerate it

[Darnok]

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