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Docking Megaships, thoughts on how well KSP 2 will handle it?


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43 minutes ago, shdwlrd said:

dozens of maneuver drones are used to maneuver the Nauvoo/Behemoth.

That was well done 

One of those scenes that I watched and then afterwards thought 'yeah, I guess that would work!' 

... 

Elegant 

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I think the best approach to those "maneuver drones" would be for them to each only carry a small amount of fuel for their own use (say 1km/s Delta-V for use performing orbital rendezvous and docking to the very large target) but when being used to move around larger ships they act more like "detachable RCS/aux propulsion clusters" in that they're basically all thruster and next to no fuel tank by mass.
VERY HIGH TWR on their own, but also low Delta-V (similarly to a tugboat, they have HIGH THRUST but low speed, and don't need enough fuel to cross an ocean).
Additionally, they'll probably have to use that "disable the high thrust engines and RCS" trick I mentioned in my last post here, BEFORE they undock, to avoid incidents.

Now I know what you're going to say, the drones won't have enough fuel to complete turning a gigantic interstellar mothership around even a single time.
And you'd be right. But you're forgetting something that makes the whole idea work out just fine.
Docking ports in KSP allow fuel crossfeed.
Assuming the interstellar mothership uses the same propellant for its own RCS as the maneuver drones use for their (intentionally oversized) RCS and maybe the aux propulsion engines, you could simply put the majority of the propellant for all the RCS thrusters on the interstellar mothership itself, so that even without the maneuvering drones, it is still capable of a lot of turning, but with the drones, it can turn a lot but quickly as well.

After all, you're probably going to be using the translational RCS to do extremely fine translational corrections to fine-tune an interstellar encounter, unless you just wait until you're closer to do that (but that costs more fuel).

 

Then again, it sure would be nice if we could take "pretty much any engine" and slave it to the RCS controls without having to mess about with a bunch of action groups (basically turning it into a gigantic Vernor engine).
It would be doubly nice if we could do that with a selection of the high performance engines that KSP 2 is adding. The metallic hydrogen engines seem a good choice for this, but perhaps some of the "torch" type thrusters would also be useful for this (or able to be downsized for better use in such cases).

Barring that, my other previous idea of using the craft's main reactor to power Arcjet or Resistojet electro-thermal RCS thrusters of high thrust and moderate ISP (around what you can get out of an NTR, so between 900 and 1800 seconds) would likely be able to be made to work quite well enough, assuming you sized the vessel's power reactor to power such things.

And that last idea is pretty much exactly what they do in The Expanse, tho instead of electrical thrusters they're steam thrusters using water heated by the main reactor, in what's called "teakettle mode", and that's even what they use for landing the Roci on planets (can't really run a Epstein drive that close to the surface because you'll sandblast everything nearby, including the ship with said Epstein drive).

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18 hours ago, shdwlrd said:

you'll need a few more than a "couple" tugs

I guess it depends on how patient you are with long braking burns.  Small tugs can work with a lot of thumb twiddling and boredom on very large craft.  Of course sometimes patience isn't the issue, but accuracy.  A long burn centered at a hyperbolic periapsis for capture can be so long that the periapsis drops way too low during the burn; I get that

 

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3 hours ago, darthgently said:

A long burn centered at a hyperbolic periapsis for capture can be so long that the periapsis drops way too low during the burn; I get that

Well, generally no matter how large the ship is, for that specific type of maneuver you'd be using the main drive and not some small tug's aux propulsion, right?

Generally my thinking of when to use these small tugs is for exactly that, when you'd use a tugboat on something like an ocean liner.
So basically "in port" which translates to "low orbit around a planet or moon", or "in close proximity to a station".

You would not be using a tugboat to propel said ocean liner all the way across an ocean, it has its own main engines for that purpose.

That being said, there are ocean-going tugboats, but they're generally used for moving around things like oil rigs and other "constructed to near completion elsewhere but intended to be stationary" type floating installations, so I don't think they're that applicable to the case of a "auxilliary thruster cluster with guidance and a small fuel reserve built in".

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10 hours ago, SciMan said:

Well, generally no matter how large the ship is, for that specific type of maneuver you'd be using the main drive and not some small tug's aux propulsion, right?

Generally my thinking of when to use these small tugs is for exactly that, when you'd use a tugboat on something like an ocean liner.
So basically "in port" which translates to "low orbit around a planet or moon", or "in close proximity to a station".

You would not be using a tugboat to propel said ocean liner all the way across an ocean, it has its own main engines for that purpose.

That being said, there are ocean-going tugboats, but they're generally used for moving around things like oil rigs and other "constructed to near completion elsewhere but intended to be stationary" type floating installations, so I don't think they're that applicable to the case of a "auxilliary thruster cluster with guidance and a small fuel reserve built in".

To be clear, the concept I was posting was using tug engines as "retrograde" engines for the main craft, not as attitude engines.  Using the tug engines would make having to reverse the attitude of the craft unnecessary for retrograde burns.  It works fairly well depending on TWR of tug engines used to brake main craft etc

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On 5/20/2022 at 10:33 PM, shdwlrd said:

Braking engines facing forward so you don't have to rotate the craft unnecessarily.

Why spend the extra weight to save on ""unnecessary"" rotation? Why have an unnecessary engine when you can rotate the ship instead?

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5 hours ago, Bej Kerman said:

Why spend the extra weight to save on ""unnecessary"" rotation? Why have an unnecessary engine when you can rotate the ship instead?

That was my thinking as well, surely you're not going to be able to get the performance out of the "auxiliary" engines that you'd be able to get out of the main drive right?

Now there might be a case where you do need those auxialliary engines, but my thinking was that the maneuver drones would mostly use those engines with the internal fuel reserves in order to move from ship to ship.

The only reason I can think of that you'd want to use the aux engines for a significant orbit change would be if the main drive is significantly radioactive (outside of whatever kind of shadow shield your craft may have on it to shield the craft's own crew and/or payload from radiation), and you need to match orbits with something.

Now I don't think they'd simulate the fact that said drive might STILL be radioactive after sitting idle for over a week after doing something like an orbital capture burn, but if they do that just lends even more validity to the need for such "non-radioactive alternative propulsion means".

NTR's may be a whole lot better than chemical rockets, but they're also fantastically harmful to anything that gets irradiated by them, potentially including the vessel's structure itself (neutron embrittlement is a nasty thing, and not easily solved unless you want to re-do the entire heat treatment on whatever alloys you made that structure out of, IIRC that's one of the reasons we like to have most operation licences for nuclear power plants require review at periodic intervals and usually require reactor pressure vessel replacement at around 50 years of operation, but I forget the specifics if I ever even looked them up in the first place). That's why most nuclear thermal rocket powered spacecraft have propellant tanks that keep the propellant and the fuel tank (and any radiators, if needed) well inside the radiation shadow shield of the NTR's reactor itself.

But aside from radiation concerns, I can't really think of any reason to prefer using retrograde-facing aux engines over simply further upsizing the RCS of the vessel (or the attached maneuvering drones). Because what can you do with an engine that you can't do with RCS? Well aside from the fuel efficiency problems that RCS usually has compared to a main engine that is...

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On 5/20/2022 at 11:45 PM, Bej Kerman said:

will also allow persistent rotation.

This was already confirmed somewhere, I don't remember in which interview or post.

No more "tap timewarp to stop rotation".

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  • 2 weeks later...
On 5/24/2022 at 2:49 AM, Master39 said:

This was already confirmed somewhere, I don't remember in which interview or post.

No more "tap timewarp to stop rotation".

Persistent Thrust was confirmed in the latest show and tell. Not sure about rotation. Though, if one is allowed, it wouldn't be too much of a stretch to assume the other is allowed as well. 

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On 4/18/2022 at 8:49 PM, Pthigrivi said:

The last dev video showed some pretty enormous thruster blocks, so in theory its all the same principles as docking other large vessels, just with bigger parts and more mass. There are also options for hall thrusters that run on xenon that might be a better option than monoprop later in the tech tree. Nate mentioned in an older video that a lot of the late-game engines have big radiation cones that would effect both departure and rendezvous. This might mean using secondary hydrogen engines on approach, or if you were leaving or arriving somewhere with an existing station you could use tugs to close that safe-distance. 

If your ship is carrying a massive amount of unshielded radioactive fuel, then it shouldn’t even be possible to turn the radiation off! So it may end up being a case of just approaching and docking these kinds of ships “tip to tip” over vast distances  to keep the business ends of your reactors pointed away from each other at all times.

Edited by joratto
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Agreed about docking "tip to tip", that seems to be the agreed-on method in most of the hard sci-fi I have read or heard about (Hard sci-fi is sci-fi that only breaks "maybe one" law of physics, in one specific and well detailed way, and it always breaks in that specific way (no exceptions), that's assuming it even breaks any known laws of physics in the first place). The Expanse is hard sci-fi (the Epstein drive being more efficient than it should be is the one violation of the laws of physics, at least before the alien stuff gets involved, but even the alien stuff doesn't truly break the laws of physics), Star Trek is not.

Point being, you should "eventually" be able to approach a nuclear-powered spacecraft from any angle, but you'll have to wait for the really nasty fission products to decay a few half-lives before you can do that (IIRC nobody was considering getting near the STS NTR Space Tug engine module (which was going to be part of the Shuttle program before it all got axed because "no Bucks, no Buck Rogers") before waiting something like 90 days of having that reactor SCRAMmed at minimum, to make the radiation tolerable according to NASA standards).

It's a lot easier with fusion-driven spacecraft, especially if they use the D-He3 fusion reaction instead of the more traditional (and easier to get going) D-T fusion reaction, because of the rather low amount of energetic neutrons given off by the D-He3 fusion reaction (it's the easiest of the so-called "aneutronic" fusion reactions to ignite, but it's not strictly aneutronic because there is "some" D-D fusion happening as side reactions, and that generates neutrons aplenty).
With that kind of power source for the engine or reactor, you'd probably only have to keep the reactor shut down for a week or so before radiation dropped to levels that people could approach it relatively safely from most any angle.

Antimatter is even better, the radiation has exactly zero neutrons in it (it's pretty much exclusively gamma rays), so the main concern becomes "how do we stop the blasted thing from melting" rather than having to wait any significant time for the radiation to die down. Of course, that also requires careful selection of materials used in said reactor or engine, to prevent the possibility of transmutation or activation by high-energy gamma rays (more likely nucleic spallation (I think that's the term, but if it's not please correct me) where the incoming gamma ray is of sufficient energy to knock loose a proton, neutron, or even an entire helium-4 nuclei aka alpha particle from the nucleus of whatever atom was intended to be there, so pro tip don't make your reactor out of any kind of Lithium or REALLY INTERESTING things are about to start happening).

Edited by SciMan
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I think the question of megaships will depend (at least partially) on the way orbital construction works. We know the Interstellar ships are too big to be built on the ground, even in sections. We know that you can 'design' whole ships in completed form, even if you don't launch it as one complete piece.

Maybe mega-sized ships can be designed as single entities, and then the game remembers that enough that they can be rigid on assembly.

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  • 2 weeks later...
On 6/3/2022 at 7:31 PM, SciMan said:

Of course, that also requires careful selection of materials used in said reactor or engine, to prevent the possibility of transmutation or activation by high-energy gamma rays (more likely nucleic spallation (I think that's the term, but if it's not please correct me) where the incoming gamma ray is of sufficient energy to knock loose a proton, neutron, or even an entire helium-4 nuclei aka alpha particle from the nucleus of whatever atom was intended to be there, 

I've never heard of gamma  rays being involved in spallation, but I don't see why it couldn't happen :huh:.

5 minutes later, and TIL about photofission.

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Yep, science actually experiences a negative trend in "questions we have" versus "questions we've answered". Every time science answers a question, it brings to light at least two more questions worth asking.

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