Feasibility of Antimatter Propulsion

[SOXBLOX]

So the devs say colonies will produce "advanced fuel types". Of course that is where we'll get our mH, but what else will they produce? I'm thinking that the torchdrive will at least be antimatter catalyzed. How feasible do you think antimatter propulsion is, and do you think it will appear in KSP2? 

Edited April 28, 2020 by SOXBLOX

[Bill Phil]

Antimatter isn't particularly feasible. Possible, yes. But doing so would require some advanced technology we just don't have.

Antimatter initiated nuclear drives are promising, but it seems you can do the same without antimatter using either Z-Pinch or some other clever methods.

But even beyond that, once you have the energy to manufacture large amounts of antimatter you would have better uses for that energy. Namely beamed propulsion of some sort. The kinetic energy of an object moving at 0.866c is about equal to its rest mass energy. Manufacturing antimatter takes, at a minimum, twice its rest mass energy (because such reactions generate normal matter as well). It's much worse than that though. Even assuming that efficiency, that would take 2 kg of mass-energy to make 1 kg of antimatter. But what does that get you?

Let's say we have an antimatter beam core rocket with an exhaust velocity of 100 million meters per second.

We can use the relativistic rocket equation to solve for the mass ratio required to reach 0.866c.

https://en.wikipedia.org/wiki/Relativistic_rocket#Relativistic_rocket_equation

Solving for the mass ratio and using 100 million m/s as the exhaust velocity, I get a mass ratio of 51.84.

Keep in mind this is antimatter, rockets don't get much better than this. Photon rockets might, but they'd have even worse power requirements.

So let's be more reasonable and lower the deltav to 100 million m/s. Now our mass ratio is only 2.83. Much more reasonable.

Or is it?

You see, for a 1000 tonne ship, you'd need 914 tonnes of antimatter and another 914 tonnes of normal matter. 

That 914 tonnes of antimatter would require twice that in mass-energy to produce - at minimum. That'd be 1830 tonnes of mass-energy. Or 1.6447e+23 Joules of energy. That's 290 times the global primary energy supply in 2013. The kinetic energy of the 1000 tonnes at 100 million m/s is 5.4601e+21 Joules. So we spent 30 times as much energy just to make the antimatter. And remember, this is a best case scenario. It only gets worse when we take into account how inefficient antimatter production would be.

But do we have an alternative?

Yes.

Mass beam propulsion.

https://www.researchgate.net/publication/303682465_Mass_beam_propulsion_an_overview

Such a system would use a similar concept to photon sails. But instead of using massless photons that must travel at the speed of light you use particles that can travel at any energy. Then you "reflect" them off the vehicle, likely using something similar to a magnetic nozzle among other hardware. Such a system would be vastly more efficient - the particles can be accelerated by an accelerator and then decelerated by the vehicle to rest (relative to the accelerator). And it doesn't suffer from the rocket equation since the energy source and the propellant are external to the vehicle. Such a setup would maximize the kinetic energy delivered to the vehicle. Accelerating 1000 tonnes to 100 million m/s would then just take the kinetic energy of 1000 tonnes moving at 100 million m/s - assuming perfect efficiency like we did for antimatter. We already know that kinetic energy - 5.4601e+21 Joules. Thirty times less. But this is scalable - to go to 0.866c like we wanted to earlier we would need 51840 tonnes of mass-energy to create the necessary antimatter, but only 1000 tonnes of mass-energy if we use beamed propulsion. We can also go the other way - more sedate speeds of 1000 km/s, something more likely for interplanetary travel. At such a velocity you'd be wasting massive amounts of energy to use antimatter, you spend more than 1800 times the kinetic energy you get. Antimatter just isn't practical for propulsion. Because the energy expenditure is utterly insane. 

Of course antimatter propulsion comes in many flavors, but the amount of energy required is just too large when there are better alternatives. Sadly mass beam propulsion doesn't really fit into KSP's gameplay, but it's a much better option than antimatter for high energy trajectories and is actually quite good for high speed interplanetary travel as well. The main issue is, of course, slowing down. This isn't so much of an issue in the solar system but it is for interstellar travel. Solutions exist but they're generally suboptimal.

I don't even want to think about the real amounts of energy required to make a kilogram of antimatter... 

[SOXBLOX]

Someone's been on Atomic Rockets! Great post, man! I certainly wasn't considering a pure matter-antimatter drive concept for those reasons and more. A much better use would of course be AM catalyzed fusion, which is what I believe we'll see in KSP2, if we do get antimatter.

[kerbiloid]

There is antimatter in KSPI-E, why can't in KSP2.

***

There's still some problem with the particle beams. They should be ionized to accelerate, but neutral to stay beams.
And even then they will be blurring with distance due to the diffraction. Probably, the Compton wavelength is enough good to estimate this.

Actually and unexpectedly, "ionized" not to H⁺, but to H^-.

***

6 hours ago, Bill Phil said:

I don't even want to think about the real amounts of energy required to make a kilogram of antimatter... 

If the civilisation is enough hi-tech, they can have little blackholes and just mine virtual antimatter particles and store them.

But it anyway should be a really developed hi-tech, not Kerbal one. Because the blackhole farm creation requires a lot of energy. Much more that it can actually produce as antimatter.

So, probably it's a "terraform the planets of the white dwarf" level.

Edited April 28, 2020 by kerbiloid

[jimmymcgoochie]

Fusion? Yes. Antimatter? No.

Antimatter needs to be created using huge quantities of energy and will produce matter in equal quantities; fusile materials like tritium or helium-3 can be made pretty easily or found naturally in gas giants.

Antimatter storage requires magnetic fields to stop the antimatter meeting, and annihilating, regular matter i.e. the whole rest of the spaceship, so the slightest leak will be potentially catastrophic; fusion fuels can be stored pretty safely in liquid or possibly solid form and the technologies already exist (tritium is just hydrogen and liquid hydrogen has been used in rockets for decades).

Antimatter will react with any matter it touches, wherever that may be, requiring lots of effort to keep it under control and most likely using power to do so; fusion requires a lot of effort to actually get it started so there's almost no chance of spontaneous reactions.

And so on and so forth, but the simple version is this: antimatter requires so much energy to produce in the quantities required for space travel that you're better off using a fusion reactor to power the spaceship than to make antimatter with. Using a tiny pellet of antimatter to start a fusion reactor still carries the risks of premature ignition/annihilation and the problems of storage, whereas using some massive lasers (as in the Daedalus-type engine in the trailer) is a lot safer and easier to do, if a bit more power-hungry for the spaceship itself.

 

[mcwaffles2003]

On 4/27/2020 at 10:50 PM, Bill Phil said:

Antimatter initiated nuclear drives are promising, but it seems you can do the same without antimatter using either Z-Pinch or some other clever methods.

Fusion does nowhere close to the same amount of matter-> energy conversion as matter-antimatter annihilation

For example, deuterium-deuterium fusion yields a mass difference of 0.025602‬ Au (2.014102*2 - 4.002602)[2 deuterium - 1 helium] where as antimatter annihilation would yeild a mass difference of 4.028204 Au ( 1 deuterium annihilating with 1 anti deuterium). That is over 150x the energy density per unit mass

On 4/27/2020 at 10:50 PM, Bill Phil said:

But do we have an alternative?

Yes.

Mass beam propulsion.

https://www.researchgate.net/publication/303682465_Mass_beam_propulsion_an_overview

Such a system would use a similar concept to photon sails. But instead of using massless photons that must travel at the speed of light you use particles that can travel at any energy. Then you "reflect" them off the vehicle, likely using something similar to a magnetic nozzle among other hardware. Such a system would be vastly more efficient - the particles can be accelerated by an accelerator and then decelerated by the vehicle to rest (relative to the accelerator). And it doesn't suffer from the rocket equation since the energy source and the propellant are external to the vehicle. Such a setup would maximize the kinetic energy delivered to the vehicle. Accelerating 1000 tonnes to 100 million m/s would then just take the kinetic energy of 1000 tonnes moving at 100 million m/s - assuming perfect efficiency like we did for antimatter. We already know that kinetic energy - 5.4601e+21 Joules. Thirty times less. But this is scalable - to go to 0.866c like we wanted to earlier we would need 51840 tonnes of mass-energy to create the necessary antimatter, but only 1000 tonnes of mass-energy if we use beamed propulsion. We can also go the other way - more sedate speeds of 1000 km/s, something more likely for interplanetary travel. At such a velocity you'd be wasting massive amounts of energy to use antimatter, you spend more than 1800 times the kinetic energy you get. Antimatter just isn't practical for propulsion. Because the energy expenditure is utterly insane. 

Of course antimatter propulsion comes in many flavors, but the amount of energy required is just too large when there are better alternatives. Sadly mass beam propulsion doesn't really fit into KSP's gameplay, but it's a much better option than antimatter for high energy trajectories and is actually quite good for high speed interplanetary travel as well. The main issue is, of course, slowing down. This isn't so much of an issue in the solar system but it is for interstellar travel. Solutions exist but they're generally suboptimal.

I don't even want to think about the real amounts of energy required to make a kilogram of antimatter... 

The problem with using beam propulsion is that any beam, matter or photon, will spread out. There is no such thing as a perfectly columnated beam, all beams disperse over distance and this isn't a problem with current technologies but fundamental physics, specifically diffraction:

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

and the formula:

Quote

Far from the aperture, the angle at which the first minimum occurs, measured from the direction of incoming light, is given by the approximate formula:

This means for any ship traveling significantly far from the beam source there will be an exponential drop off in efficiency unless were supposing an infinitely large collector. In short your beam propulsion system would work for travel near already developed areas of space but not sufficiently far away where collection drop off becomes significant. This also wouldn't work in a colonized area if the target destination doesn't have a beam propulsion system ready as now the craft has no way to decelerate.

As for efficiency for antimatter generation, any society capable of doing such a thing, I would imagine, has an abundance of excess energy seeing as simply collecting 10% of all the solar energy hitting earth currently (10% of about 170 petawatts) would surpass our current energy consumption globally by around 1000x. Not considering that though, I feel this argument fails since energy requirements to generate fuel shouldn't need to be considered if the ship itself isn't generating the fuel. The point of fueling a ship is just to pack it with as much energy as possible with as little mass as possible and a 3% efficiency seems worth it for getting 170x the energy density per unit mass of perfectly efficient fusion, especially if that mass will be brought along up to relativistic speeds.

To me though this all seems  like type 1 civilization requirements to do and Kerbals seem to be, like us, a type 0 civ. If the base lore though allowed us to progress through the arc of type 0 to type 1 and perhaps beyond I think it would be allowable. All depends on the scale the developers set for us in the game.

[Bill Phil]

8 hours ago, mcwaffles2003 said:

Fusion does nowhere close to the same amount of matter-> energy conversion as matter-antimatter annihilation

For example, deuterium-deuterium fusion yields a mass difference of 0.025602‬ Au (2.014102*2 - 4.002602)[2 deuterium - 1 helium] where as antimatter annihilation would yeild a mass difference of 4.028204 Au ( 1 deuterium annihilating with 1 anti deuterium). That is over 150x the energy density per unit mass

I was referring to antimatter initiated nuclear drives. The advantage of antimatter is that you can make arbitrarily small explosions with fusion or fission. But this is already possible with Z pinch for fission and other methods for fusion - antimatter is too much hassle to use for nuclear drives that have other methods of initiating their reactions.

A fusion drive has no hope of reaching antimatter energy densities. That isn't what I was referring to though.

Quote

The problem with using beam propulsion is that any beam, matter or photon, will spread out. There is no such thing as a perfectly columnated beam, all beams disperse over distance and this isn't a problem with current technologies but fundamental physics, specifically diffraction: 

I'm aware of the engineering difficulties associated with particle beam based propulsion. Perfectly collimated beams do not exist. But there are ways around it:

https://ntrs.nasa.gov/search.jsp?R=20190014041

The reason particle accelerators are considered is because they offer advantages in technological readiness - we understand them, have built a lot of them, and know the engineering of them. But there are other options. Instead of a beam of particles we can send a stream of discrete objects - a mass stream. This has a few advantages. No beam collimation, for one. But it can also be guided while en route to the vehicle. The issue becomes one of how to accelerate the propellant but there are solutions to that - Sailbeam is one such solution, and many others exist.

The option space for mass beam propulsion is quite large, from particle beams to any other method of accelerating small objects to large energies.

Quote

This means for any ship traveling significantly far from the beam source there will be an exponential drop off in efficiency unless were supposing an infinitely large collector. In short your beam propulsion system would work for travel near already developed areas of space but not sufficiently far away where collection drop off becomes significant. This also wouldn't work in a colonized area if the target destination doesn't have a beam propulsion system ready as now the craft has no way to decelerate.

As mentioned above there are methods we already know of that would deal with the problem of hitting the spacecraft. Future engineers will have even better tools and methods available to them than us. 

If the colonized area in question is a solar system then decelerating isn't too much of an issue - you can just accelerate the deceleration path before you launch the vehicle. It takes a while to set up but after a few flights you can establish a proper decelerator at the destination. This would primarily be an infrastructure investment allowing for less expensive interplanetary travel - in that specific context.

For other star systems you could use a magsail to decelerate. But decelerating is an issue for every interstellar vehicle, not just ones propelled by mass beams.

Quote

As for efficiency for antimatter generation, any society capable of doing such a thing, I would imagine, has an abundance of excess energy seeing as simply collecting 10% of all the solar energy hitting earth currently (10% of about 170 petawatts) would surpass our current energy consumption globally by around 1000x. Not considering that though, I feel this argument fails since energy requirements to generate fuel shouldn't need to be considered if the ship itself isn't generating the fuel. The point of fueling a ship is just to pack it with as much energy as possible with as little mass as possible and a 3% efficiency seems worth it for getting 170x the energy density per unit mass of perfectly efficient fusion, especially if that mass will be brought along up to relativistic speeds.

Energy requirements to generate fuel are important - this is called "Energy Returned for Energy Invested", and in the case of rockets it's a function of how much energy the fuel (and the rocket) costs to produce and how much energy you give the payload, among other things. If it takes too much energy then there are likely better uses for that energy. If a ship costs too much energy just to provide its fuel then it's not worth fueling the ship, not when there are better uses for that energy or better alternatives to get the same job done.

Our current energy consumption is small indeed, but so is 170 petawatts. That's only 62 tonnes of mass-energy per year. You'd need much, much more than that to produce antimatter since most of that energy will be used for other things. Energy abundance of the type necessary to produce antimatter isn't going to happen on Earth - the waste heat alone would turn the planet into an oven.

The problem is that the 3% efficiency figure is a best case scenario (provided the exhaust velocity is 100 million m/s). A best case scenario. What you need to understand here is that antimatter production will be vastly less efficient - that efficiency number is probably going to be in the thousandths after considering real difficulties in producing antimatter. 

Antimatter is not a useful substance. It's too much hassle and takes too much energy to use properly. Let's put it this way. If the best case scenario is 30 times as much energy than vehicle kinetic energy, then that also means we can send 30 times more vehicles using mass beam propulsion than with antimatter. Antimatter's energy costs are too large, and remember this is best case. It's far more likely to be 300 times or even 3000, perhaps more. This means that a civilization that uses antimatter for propulsion could have sent hundreds or thousands of times more vehicles using mass beam propulsion (or some other alternative). In fact, a fusion drive to the same speed (100 million m/s) would be more economical - you'd need vastly more fuel to do it but you can (at least theoretically) use the most abundant element in the universe as your fuel. You'd have to use staging and lord knows what else, but the fuel already exists, unlike antimatter. Now I don't think we should do that either, but it would be more economical than antimatter. In fact if we wanted to go slower - about 10% of the speed of light, we'd need nearly 70 times as much energy just to produce the antimatter than vehicle kinetic energy. A fusion engine would have a much larger mass ratio - but that mass would be a material that actually appears naturally, so it'd be much easier to acquire.

Antimatter gives you a lot of bang for the kilogram, but much less bang for the "buck". Or perhaps, much less bang for the Joule. And that's before getting into the difficulties creating and storing antimatter. It's possible, but it will be difficult. Let alone engineering a beam core antimatter drive...