[1.12.5] Silly Photon Drives

[triple cheeseburger]

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(Also on CKAN)

Although they carry no mass, photons still have momentum. We can use this principle to build drives that use light itself as an exhaust.

This is a small mod which adds three engines that use electricity to produce light – a photon engine. This is different from a solar/photon sail, which uses light from produced from an external source (whether that’s the sun or a laser beam) to push on a sail. These engines produce their own light and throw it out the back like a typical rocket to push themselves forwards.

The gimmick here is that these engines only take in electric charge as an input, they do not require any physical propellant. The downside however is that the thrust is so small it makes other electric engines look beefy in comparison.

Dependencies, recommendations, and suggestions.

Dependencies: Module Manager, B9 Part Switch, and Community Resource Pack. The mod will not work without these mods installed.

Recommendations: Waterfall, Persistent Thrust, and Near Future Electrical. These are needed to make the engines actually useful and looking their best.

Suggestions: Near Future Solar, Community Tech Tree, Far Future Technologies, Cryo Tanks. These are not strictly necessary, but can improve the experience. Far Future and Cryo Tanks are only for if you plan on using the large engine’s antimatter fuel mode since this mod does not include its own tanks for storing antimatter and LH2.

How realistic is all this?
To be honest, not very… While yes the idea of using photons as an exhaust does work and would produce thrust, the thrust produce would be much, much smaller than what it is in this mod. Like, in the range of hundreds of µN to single-digit mN.

We can calculate the thrust of an engine based on its exhaust velocity and thrust power, in the case of these photon drives the exhaust velocity is going to be the speed of light because, well, the exhaust is just photons. So the thrust depends entirely on how much power you pump into the thing. The equation is just the thrust power in watts divided by lightspeed in meters per second, this tells us that a single Newton of thrust would require nearly 300 megawatts of power! If we work with the assumption that 1 electric charge per second is 1 kilowatt, then 1 Newton of thrust would take almost 300,000 ec/s. So for the sake of gameplay, the engines produce about 200,000 times more thrust than they would realistically.

The most common way of generating photons I’ve seen in concepts for these sorts of engines involves electrically heating a block of material so that it gives off blackbody radiation, by exposing a side of this hot material to space the photons are allowed to escape and produce thrust.

The other method involves antimatter. By reacting electrons and positrons (anti-electrons) together you produce pure gamma rays. Contrary to what you may have heard, most antimatter reactions do not produce pure energy – for instance a proton-antiproton reaction spews out a myriad of particles, including pions which quickly decay into gamma rays. For simplicity sake the antimatter mode for the large engine just uses liquid hydrogen and the nondescript antimatter resource. Reflecting gamma rays produced by the annihilation poses a challenge… A gamma ray mirror, and one with a good enough efficiency to not instantly turn to plasma from the heat of the reaction, is not something we currently know how to make.

Furthermore, trying to actually get all of the positrons and electrons to react would likely be near-impossible. A good chunk would probably escape the engine without reacting or hit the walls of the engine and damage it. You could increase the odds by making the reaction zone larger, like up to hundreds of meters long. This mod just handwaves all of that though.

Before anyone mentions it, yes I know that realistically you shouldn’t be able to see the engine plume in vacuum unless it was pointed straight at you… But I like the way it looks, so it’s staying in.

So are these engines actually useful?
From the large amount of messing around testing I’ve done… not really. The premise of an engine that runs on purely electric charge sounds overpowered, but due to the amount needed and the miniscule thrust it can really limit their uses. Yay for balance I guess? With pretty much any rocket you make with these engines (barring the antimatter mode for the big engine), your TWR will probably be between 0.001 and, like, 0.02. Dealing with low TWR vessels in KSP can be a bit of a headache at times, so this can make the engines unattractive and difficult to use.

However, the biggest limit is the question of where do you get the electricity from.

You can power them with solar panels – though you’ll probably want a mod like Near Future Solar so that you don’t have to spam tons of stock solar panels. This can actually work decently well if you’re visiting somewhere like Moho, though very quickly the mass of solar panels needed becomes too large to be viable and kills the already low TWR of the craft. At this point you’d likely save more mass just using a typical electrical engine with some fuel and a smaller power source.

The other main option is nuclear power. Since this mod is designed to integrate well with Near Future that usually means the fission reactors from Near Future Electrical, or the fusion reactors from Far Future Technologies. However… the whole point of these engines is that they require no propellant, and nuclear reactors require enriched uranium or fusion fuel… So really you’re just back to square one, it’s just a more typical engine with an extra step (come to think of it, powering a photon drive with solar panels is also like a solar sail with extra steps…). You could calculate a kind of “effective specific impulse” based on how much nuclear fuel you get through and what your final velocity is after burning the stuff, I’ve tried this a couple of times in testing and results vary – usually between many hundreds of thousands of seconds and a couple million seconds of specific impulse.

Which still isn’t too bad, but at that point there’s fusion engines from FFT that can get you similar Isp with much higher thrust. Unfortunately I haven’t really been able to test how well they work with FFT’s fusion reactors, because fusion reactors in that mod have this feature where they auto-adjust their power output based on how much your ship consumes. Persistent thrust seems to break this entirely, so fusion reactors don’t work properly under timewarp in my experience.

Yes, if you’re feeling especially silly you can power the engines with RTGs. I imagine if you’re using a patch that gives RTGs limited lifespans then this option isn’t even worth discussing, but if not then shenanigans can ensure… Indeed, this path does offer you truly infinite delta-v with no worries of fuel or location. However, you need a huge mountain of RTGs to power these engines… or alternatively you can turn the thrust limiter on the engines way down to save cost/part count. This brings your acceleration down another octave. Your TWR will likely be floating somewhere around 0.001. Preferably you’d want to use the bigger RTGs from Near Future Electrical because they produce more power per unit of mass than the stock RTGs.

A few caveats and useful tips and tricks for getting the most out of these engines.
Unfortunately KSP is not very good at handling resource management when under timewarp. So often times what I’ve found when using the photon drives at high timewarp is that Persistent Thrust erroneously thinks they’ve ran out of fuel or electricity and kills your warp. This is very frustrating. To get around this I’d recommend simply going to the Alf+F12 menu and turning on infinite fuel and electricity. Infinite fuel doesn’t really change anything because the engines don’t use any external fuel in the first place, but it partially stops the issue mentioned above.

Using infinite electricity does mean that you can run the engines without needing a power source, which is a bit more dubious. However I’d say that as long as you remember to turn it off again once you finish your burn then it’s all fine. Reactors that consume a fuel still continue to consume fuel even when infinite fuel+electricity are enabled. As annoying as these limitations are, I don’t really know how I could go about resolving them.

Because electric charge in KSP has no mass, you can’t make a functional engine that uses only EC in its configs. The game gets very confused, I suspect the lack of mass causes a divide by zero somewhere in the calculations. To get around this the engines use EC and another resource, this custom resource is just called radiation pressure and purely exists to make the engines function properly. Radiation pressure has very low mass, and each engine stores and automatically produces its own radiation pressure for free all the time. You don’t have to pay any attention to it in-game.

Due to the nature of these engines, delta-v kinda goes out the window. The delta-v readout in the editor or in Kerbal Engineer isn’t going to be very helpful because it only considers the radiation pressure inside the engine. If you’re using solar panels to power your drive then it’s not a huge deal, as long as you’re in enough sunlight then you’ve got infinite delta-v. However it can be a bit annoying if you’re using a reactor of some sort, just like earlier I suppose you could calculate a sort of effective delta-v based on the lifespan of the reactor… But that would be pretty tedious, and accounting for the time when the engine isn’t firing would be hard.

On the subject of using reactors, I’ve found that using nuclear fuel drums as drop tanks works quite nicely. Nuclear fuel is still very heavy even when its used up, so it’s good to drop it when you can. Remember! Unless you have a high-level engineer kerbal on your ship you won’t be able to transfer nuclear fuel, though this setting can be changed or disabled in the difficulty settings for your save file.

On a similar note, unfortunately I haven’t been able to test out the viability of also using the nuclear fuel re-processor from Near Future Electrical. This part takes in spent fuel and turns it back into fuel with what seems to be 50% efficiency. I suspect that in an optimal situation this could let you double the lifespan of your reactor. Sadly though, Persistent Thrust strikes again and breaks the nuclear re-processors – for some reason they just keep churning out new nuclear fuel even if there isn’t any nuclear waste to process.

Despite everything, there is one thing I have found to be actually decently useful. The antimatter fuel mode for the large engine can work well as a means of travelling to other stars. The insane Isp makes it pretty trivial to create a ship with a couple hundred million m/s of delta-v. Antimatter tanks in FFT have horrible fuel to mass ratios, they weigh basically the same when empty (fun fact! All the antimatter tanks besides the big ring tank also cost zero funds when empty, for some reason). So using drop tanks for the antimatter tanks works well.

The large engine also does not require radiators, combined with its fairly low mass this lets it weigh a whole lot less than most other big FFT engines. The other antimatter engine, the Frisbee, weighs around 80 tons at its full length and only ~35 tons at its shortest length. The large flashlight weighs only 9 tons! (another tip, if you’re using the Frisbee for an interstellar ship you can use its shortest length and turn the thrust limiter down to like 1%, it takes hardly any radiators and weighs a lot less than the full length version. Your TWR will be small but it’s an interstellar ship so that doesn’t matter too much).

I have seen that sometimes the burn time is so long the large flashlight actually takes longer to reach a destination compared to a ship with less delta-v purely because it takes ages to reach its cruising speed.

 

This is my second ever KSP mod, and this time it’s actually being posted in a finished state… I don’t really have any other ideas for stuff to add to this mod, if you encounter any bugs or have suggestions on how to fix some of the caveats mentioned earlier then please do let me know! I hope you enjoy messing around with these engines, if you make any pretty builds with them then I’d love for you to show me! Thank you for your time.
Liscense: CC-BY-SA

Edited November 27, 2023 by triple cheeseburger

[Zarbon44]

Awesome mod and the "plume" looks amazing. Parts are very good quality and fit with restock-alike perfectly.
One thing i thing would be beneficial, is perhaps adding a optional patch making engines slightly stronger so that those who don't use mods such as Persistent Thrust could just have fun with spicy flashlights of unusual size. Keep up the amazing work!


https://imgur.com/kIsjUhJ



https://imgur.com/kIsjUhJ

[triple cheeseburger]

Thank you very much I know I already replied to the screenshots and images you made back on discord, but I'll say again that these ships are really pretty... and the the infographic you made looks extremely awesome!

[Kavaeric]

Photon drive real?! Congrats!

[Ameoba]

This is cool, thanks for early access btw!

Edited November 28, 2023 by Ameoba
not enough detail

[Isabelle.V.Fuchs]

Now I'm curious

[Poodmund]

If I install this mod and turn the vessel's cabin lights on, will it cause a torque to be induced?

Congrats on the release.

[Cyne]

This is so insanely silly but so scifi looking, I love it! I’ll have to try it back home and well done on the release 

Edited November 28, 2023 by Cyne

[idiot]

looks awesome

[Emilius73]

Looks great! Can't wait to try them.

 

 

 

Now watch as For All Mankind makes this tech a thing in 2019

[Starwaster]

Seeing some tri-hex style trusses in the first picture. Are those from this mod or another?

[triple cheeseburger]

20 minutes ago, Starwaster said:

Seeing some tri-hex style trusses in the first picture. Are those from this mod or another?

The red and white truss in the first screenshot is just made out of Procedural Parts structure pieces that have been recoloured using SimpleRepaint

Edited November 28, 2023 by triple cheeseburger

[Starwaster]

4 minutes ago, triple cheeseburger said:

The red and white truss in the first screenshot is just made out of Procedural Parts structure pieces that have been recoloured using SimpleRepaint

Including the end cap at the front?

[triple cheeseburger]

2 minutes ago, Starwaster said:

Including the end cap at the front?

The end cap at the front is just some of the structural pieces from Near Future Construction and a few of the stock structural panels. This mod doesn't add any other parts besides the three engines shown in the infographics.

[610yesnolovely]

This is very cool, plus the tip about using procedural parts is nice.

Also I looked up the infographic address. Hah! Very Kerbal. Congratulations on an excellent mod release and supporting ships (which are amazing also).

[Vesuvius]

For some reason i can't find the 1.25 and 5m drives, anyone else have this problem?

[Alpha_star]

Wow man, great mod! I have been searching mods this type for ages, and it seems the search has ended, really cool concept and hopefully it works well in-game.

[TheSpaceWarlock]

Quite a cool mod. Even though photon rockets aren't very realistic, they are quite a fun idea. Excited to cook up some funny ships with this!

[triple cheeseburger]

5 hours ago, Vesuvius said:

For some reason i can't find the 1.25 and 5m drives, anyone else have this problem?

Are you sure you installed the mod properly? If you installed it manually then I'd recommend using CKAN instead to prevent any issues happening. You could check inside GameData -> SillyPhotonDrives -> Parts -> Engine, and see if the files for "photon_5" and "photon_125" exist or not, if not then I'd recommend reinstalling the mod.

[TheSpaceWarlock]

Such a pretty engine!

Simple Scout ship using the lepton antimatter mode. TWR of  around 0.05 with persistent thrust isn't so bad considering the deltav.

 

Edited December 1, 2023 by TheSpaceWarlock
fixed typos

[Vesuvius]

On 11/29/2023 at 3:18 AM, triple cheeseburger said:

Are you sure you installed the mod properly? If you installed it manually then I'd recommend using CKAN instead to prevent any issues happening. You could check inside GameData -> SillyPhotonDrives -> Parts -> Engine, and see if the files for "photon_5" and "photon_125" exist or not, if not then I'd recommend reinstalling the mod.

the photon_5 and photon_125 files do exist, yet don't appear in the parts list, i've tried redownloading manually and through CKAN, any other recommendations?

[triple cheeseburger]

3 minutes ago, Vesuvius said:

the photon_5 and photon_125 files do exist, yet don't appear in the parts list, i've tried redownloading manually and through CKAN, any other recommendations?

Hmm... Are you sure you've downloaded all the dependancies for the mod? The 1.25m and 5m engines are the only ones with B9 configs so you might be missing B9 Part Switch, or maybe you could check if it's up-to-date enough for the mod.

[Vesuvius]

5 hours ago, triple cheeseburger said:

Hmm... Are you sure you've downloaded all the dependancies for the mod? The 1.25m and 5m engines are the only ones with B9 configs so you might be missing B9 Part Switch, or maybe you could check if it's up-to-date enough for the mod.

I've got all dependencies downloaded and B9PartSwitch on its most recent version (i hope), im gonna try redownloading B9, but if that doesn't work, any other recommendations?

[JadeOfMaar]

@Vesuvius Most likely case is you have another mod for photon drives installed and its parts have the exact names as this one. If that's the case, there is a conflict and the affected parts become invalid. I've informed this mod owner on an action to take which will prevent this occuring later on but will break existing craft files. If you have another mod for photon drives installed do let us know.

[triple cheeseburger]

Alright I've updated the mod to change the internal names of the engines, so this should prevent any conflicts with other mods. Unfortunately this will probably break any ships you have that currently use engines from this mod, I suppose you could remove the engines in the editor before updating and re-add it once you've got the new version of the mod.