Hypothetical future alternative to common RCS thrusters

[Rejected Spawn]

Haven't seen any mention of this yet, if I simply missed it then sorry my bad.   Was reading up on attitude control mechanisms for spacecraft and came across the mention of small ion thrusters being used in place of common verniers on some craft, now as much as that's terrific and all they are made of pure weaksauce and they do run out of propellant after some time but that's when I had this idea; since thrusters only need to provide a little puff every now and then to make minimal adjustments it wouldn't be unreasonable to run them on just about any gas you happen to have on hand at the cost of reduced fuel efficiency. Now as far as I understand specifically regular "ion" thrusters need a pure noble gas to prevent (maybe more like "slow down"?) erosion and other nasty business, but there are similar systems. I found the one called PIT, Pulsed Inductive Thruster, to be very appealing and the article even makes mention the possibility of scooping carbon dioxide from the atmosphere of Mars to use as propellant.

My proposal is that KSP2 ought to have "compressed gas" as a resource, it can very rapidly be replenished in an atmosphere or very slowly replenished using special capturing equipment in space. This random unidentified gas along with PIT nozzles would be used just like the monoprop + thrusters in KSP1 and make it possible to keep adjusting a craft slightly over very long periods of time without running out of gas if you have a collector module on the craft. Collecting "space gas" in real life would slow a craft down marginally leading to some slight orbital decay but I think the minute difference might be better omitted in a game, though that's just my personal opinion. Naturally there might very well be some regular engine in KSP2 that runs on this stuff (Time to spill some tasty beans Nate, is it in already?!) but I figured there's a pretty substantial probability that the resource and propulsion types have not yet been included as options for RCS.

[Entropian]

51 minutes ago, Rejected Spawn said:

Now as far as I understand specifically regular "ion" thrusters need a pure noble gas to prevent (maybe more like "slow down"?) erosion and other nasty business, but there are similar systems.

You're right about the noble gases; most ion propulsion systems use argon, xenon, or krypton because they are easier to ionize and they erode the thruster slower than other gases.  However, some new research is being done on Hall effect thrusters that can use air as their propellant: https://patents.google.com/patent/US6834492B2/en

SITAEL and Busek both have done research into air-fueled ion propulsion, although I can't find the links.

As far as I know, PITs are confined to the laboratory because of their shorter lifespans when compared to other ion thrusters.  NGST's PITs and FARAD both perform worse than most ion thrusters in many ways.

I think the compressed gas idea is a good one, just not with PITs, maybe some other ion propulsion system.

[Draco T stand-up guy]

We need LASERS

Couple of those would make excellent RCS thrusters.

[jimmymcgoochie]

Take the Expanse route- high pressure water/steam for RCS, possibly heated by whatever fission/fusion reactor is on board to boost thrust and/or ISP. Easily replenished unless you’re really close to the local star, you need it anyway for your crew and it can also act as radiation shielding.

Ion systems just won’t produce the thrust needed to control a huge and heavy interstellar ship. Even KSP’s massively overpowered ion thrusters aren’t up to the job; the KSP version of BepiColombo produces about 7kN from its ion thrusters whereas the real version’s produce about 0.0003kN of thrust.

[SciMan]

To be honest, I don't know why we need another low thrust attitude control system. Sure it's "realistic", but this is a game in the guise of a very simplified simulator, not a simulator. We don't need realism. What we need are solutions to problems, and currently I don't know how they're going to handle the attitude control problem on all these gigantic interstellar craft.

Currently, every kind of electric propulsion is what's known as "thrust limited" because we can't get enough power to go thru the propulsion system. That might change if we can get compact and lightweight reactors that produce 10s of kilowatts per kilogram of reactor, but that's only possible right now if you strip out the shielding from a fission reactor. Maybe fusion reactors will be different, I don't know yet because nobody's built one that can self-sustain yet.

What we need is a high thrust multi-directional propulsion system. The Vernor only puts out 12kN per thruster, in a single direction. The only advantage it has is that it uses LF/OX for propellant, and that it's connected to the RCS controls instead of the main throttle. What is needed is something that has 4 or 5 directions of thrust in a single part, and 100 kN of thrust or more. More is better, because despite the ships in the KSP2 trailers already being huge, you just know that people are going to want to build bigger ones, and having to use less thrusters means less part count means a higher frame rate.

[Soda Popinski]

While I agree with SciMan this would be unnecessary, game play wise, I just wanted to note Space X uses compressed nitrogen for RCS on the Falcon.  The so-called "cold gas thrusters," which will also make it into the Tesla Roadster Space X package.   Terrible ISP, but you can compress it using electric power.

[magnemoe]

8 hours ago, Soda Popinski said:

While I agree with SciMan this would be unnecessary, game play wise, I just wanted to note Space X uses compressed nitrogen for RCS on the Falcon.  The so-called "cold gas thrusters," which will also make it into the Tesla Roadster Space X package.   Terrible ISP, but you can compress it using electric power.

Cold gas trusters are simple, easy to scale up but has low ISP. They works well for stuff like the first stage, upper stage and falcon uses conversational RCS as weight is more important here. Its an move over to less toxic monopropelant. This is mostly an safety and cost issue.
Vernor engines running on main gas is also getting focus, the centaur replacement upper stage plan on using this, same with statship, most believe this will also be used to land starship on moon. As fuel and oxidizer is gas under pressure this is much easier to start than liquid fuel ones making is suitable for RCS uses, you can always use cold gas to fine tune. 

[Rejected Spawn]

Finally I'm back to clear up the confusion I caused, this current global fiasco has kept me occupied with some other tedious stuff.

First off let me just state the obvious: I was dumb enough to pounce on a half baked thought based on 10 minutes of badly sleep deprived wikipedia browsing, that's bound to be a very incomplete level of knowledge to stand on and produce low level stepping stone thoughts at best. Second I totally failed to specify the type of craft I thought this would be a good option for, that was silly considering that some craft in KSP2 will be very large. Third I completely and utterly failed to get across the exact nature of the propulsion I had in mind. Fourth I also completely failed to bring the most spectacular point of the entire idea into focus.

Now then. Armed with some better understanding of electric propulsion in general I'll try to explain the design I'd go for. The whole point is to use electricity to accelerate any random gas to a useful speed somehow, but I should have made it far more obvious I wasn't at all thinking of the 30km/s or so that's common among regular ion thrusters. Those use insane amounts of power because they have to make the fullest use of every single atom of the gas, that's not what we need at all. What we need is an average ejection speed of more like 2km/s and the ability to fire off a comparatively astronomically much bigger quantity of gas at that speed per second. My original interest in the PIT was quite flawed on a number of levels but it at least gave me an idea, though it's embarassing I didn't even read far enough to see the FARAD variant at first. Now finally we get to the actual design; rather than a flat disc of a PIT or any other variant of magnetic field I've seen it would be better to make a rather deep "cone" pointing inward. The cone would be surrounded by a coil to induce a strong magnetic field but the inside of the cone where the gas is located would be lined with a protective layer that would also strongly reflect RF waves. Using the FARAD low energy ionization of gasses we could then inject a big puff of gas into the cone, fire a strong RF burst from the open end but trailing along the walls to ionize only the layer closest to the coil, then activate the coil to "squeeze" this "membrane" of ions around the unionized gas to make the still unionized gas interact with the magnetic field and be ejected at great speed. Any gas remaining after the membrane has exited the cone is likely to have absorbed a lot of heat and will be ejected from that instead so it's not entirely wasted either. Naturally the main process would only work if you have a high enough concentration of gas present but that's the hidden beauty of this: you can switch mode of this thruster and either use it as a regular ion drive while firing thin puffs of gas (of course much less efficient than a standard dedicated ion drive but still useful) or you can burn gas like there's no tomorrow at a massively reduced efficiency but with thrust values possibly approaching regular monopropellant systems.

As for scale I see no immediate reason this can't come in more than one size and no reason you can't use several at once with minimal performance difference due to the greatly improved specs of KSP2. I also see no reason other RCS thrusters can't have vastly different sizes since they simply operate like little rockets, I don't understand the fear that KSP2 wouldn't have far bigger and more powerful ones if the game literally encourages ships that are many times bigger than anything possible to fly in KSP1?

Lastly the biggest selling point of the OP that seems to have been lost on most of you; the random gas used as propellant. Not only does it self replenish over time so you can RCS all you want (if you have a few months on your hands to wait for the tanks to refill) but this self refilling property means you can literally send a ship with NO PROPELLANT at all in the gas tanks, how much more weight reduction can you possibly ask for? Alternatively you can of course have a main rocket that also runs on compressed gas and also has unlimited dV at the expense of likely quite poor acceleration. Originally I had meant for this entire system to be used to dock things like 2-man landers and orbiters, you know the regular stuff that regular players build in KSP1. Giving less perfect players another chance to redock their vessels if they fail the first try by a narrow margin would most likely be greatly appreciated. The whole system in itself is meant to be your one safeguard against those "my Kerbal is stranded alone on the other side of the solar system because I missed 1m/s dV" moments, you get this tiny bit of wriggle room that will always be there for you as a shining little beacon of hope, "the seat belt that keeps on giving" or something like that.

Edited July 26, 2020 by Rejected Spawn
Spellgoof.

[Starhelperdude]

orion rcs...

[SciMan]

Well if you're going to go with the "any random gas" idea, and you still want electric propulsion, there's not much better than some kind of electro-thermal thruster.

It's very much a thermal propulsion method, as you're heating propellant to produce thrust, but the heat comes from electric energy. There are several types of electro-thermal thruster, with most of the difference in the types being in exactly how the electric energy is transferred to the propellant.

On paper a resistojet is about the simplest and therefore most reliable electrothermal thruster you can get. Just stick a heating element in a thrust chamber, and feed it with enough electric power to heat the incoming propellant hot enough to meet your specifications.

Arcjets are a little more complicated as the "heating element" is replaced with an electric arc, but because you can't melt an electric arc you can heat the propellant to a hotter temperature, resulting in increased performance. Lower thrust than a resistojet, but higher specific impulse. One of the downsides to this type of electrothermal thruster is that you have to worry about electrode erosion, just like with a gridded ion thruster.

OK so you want arcjet performance, but you don't like the problem of electrode erosion. Well, then the Microwave Electrothermal Thruster is the thing for you!. The hint's in the name, these use microwaves to heat the propellant. I'm pretty sure that IRL there is a company developing these to use on small satellites, running on Water of all things. If you're wanting a green propellant that is easy to get via ISRU, I can't think of a better one than water. Plus, the exhaust from a microwave electrothermal thruster using water is a nice shade of purple thanks to the ionization that happens.
Thermal ionization, not electrical. So no, it's still not quite an ion engine but yes a magnetic nozzle would help things. It's in a weird spot, kind of like where a fusion rocket is. Obviously thermal, but there's a lot of ionization happening simply because things are too hot to NOT be ionized.

And IMO the microwave electrothermal thruster comes the closest to being the "teakettle" thrusters in The Expanse. High system efficiency since I assume they get most of their power thru direct energy conversion of charged particles inside the fusion reactor into electric current output (otherwise they'd need obvious radiators which are absent), and the thrust can be as high as you need provided you have the power available.

[Guest]

Alternatively, go for a high thrust plasma engine, such as MPDT. Those can have high thrust and can use almost any propellant, though they also need a lot of power. PITs would actually work, too, just scale them up. These, like MPDs, are not thrust limited, but require a lot of reactor power.

We do need realism, because that's where the solution to the problem is. There are plenty of high thrust electric propulsion technologies. If you want unrealistic, just pile on reaction wheels like in KSP1, and forget about RCS altogether (I hope KSP2 improves on this, RCS really should be needed).