Control during re-entry?

[Olleus]

What's the best way of controlling the attitude (ie, which way a craft is pointing) of a large craft during re-entry?

My ye-usable rockets have a tendency to go point-first when slowing down in the atmosphere on re-entry, which is not ideal. RCS works to some xtent, but this can require a lot of fuel and very large RCS exhausts to really be able to turn a rocket going at 1-2km/s at ~15km ASL. So much so that it's not feasible for very large things.

Going for absolute symmetry to do reentry engine first and trying to maintain that unstable orientation works up to a point. Above a certain mass the engine simply can't take the re-rentry heating, even with maximum heat shields.[ Btw, is there a mod that models the engine nozzle / heatshield / aerospike that Stoke Space are doing? Would be fun to play around with something like that if the physics were well represented.]

I've tried putting control surfaces on the rocket so that I can do a Starship-ish bellyflop while going hypersonic, and then go back to a vertical orientation to do a controlled landing. However, I can't get enough control authority to do this properly, and my fins invariably burn up. Any advice on what the optimal place to put them (and rough ideas of necessary size) in order to get this to work? Or is there anything like grid fins (in the game or in mods) that provide high-altitude, hypersonic control authority but not so much drag that they burn up?

[toinkertoy]

So this is a rocket stage (the booster/first stage segment) we're talking about here?

Usually the best way to orientate those is with use of it's engine gimbeling to maintain an attitude (this could also be used to slow it down a bit so the heating isn't as extreme). If you don't want that option then you might just have too put on a ablative coating (which can be added to most parts not just heat shields) onto the parts that are receiving the bulk of the heat damage.

If you're looking for strictly attitude control parts (RCS, Fins, ect.) I don't think there are really any good options in-game right now that would work at the velocities you're experiencing. You might also experiment with using a zero mass gyroscope (using tinker panel) to at least get the desired effect you want. Or try checking out some other craft players have made using reusable boosters to see of they've come up with any solutions.

[Olleus]

This is, essentially, a single stage to orbit vertical takeoff/landing vehicle (only asparagus-staged ancillary fuel tanks get staged). The 200t version works well as RCS allows me to keep the engine pointing prograde throughout the atmospheric descend, and I bleed speed to air resistance progressively enough that ablative coating can cope. I'm then in the perfect orientation to turn the engine back on for a suicide burn and control the last part of the descent.

But when I scale it up to 1500t, I simply punch through the upper atmosphere without losing any real speed (square-cube-law in action) so that the reentry heating in the lower atmosphere simply burns through any amount of coating. The obvious solution is to show more cross sectional area in the upper atmosphere to slow earlier and spread the load, and then re-orient myself for that final burn. The problem is that I simply can't control the attitude at all. I can just about keep engine-forward using a ton of RCS. But if I try any other orientation at all, I end up going nose first and cannot recover from that.

Zero-mass gyros feel like cheating in a game where gyros are already over powered. I have looked at other crafts, and fins seem to be an option. I just can't get it to work, but I have little-to-no aerodynamic intuition, either the real physics of it or how it works in game. That's what I was hoping for some help on.

[toinkertoy]

Originally posted by Olleus:

but I have little-to-no aerodynamic intuition, either the real physics of it or how it works in game. That's what I was hoping for some help on.

Try inquiring on the Juno site, more of the experienced players frequent that forum.

https://www.simplerockets.com/Forums

[Olleus]

I tried, still no reply.

Honestly, I think this game has no community. Not any more at least. The official forums get a handful of posts per day max, and most of those are rambling stream-of-consciousness junk which I can barely get any meaning out of. There only seems to be a couple of mods in development, and they're all purely cosmetic stuff. The discord does seem to be a little more active, but it looks like a few guys hanging around in a chatroom than much discussion of the game itself. In all of this, the little community that seems to remain is in using the game as CAD software, with very little interest in actual rocketry...

It's very revealing that when googling stuff, even using the new name for the game, the vast majority of mods / posts / videos that turn up are many years old. Considering how many updates and changes the game has had, it's hard to tell if any of those are still accurate.


****

I found a work around to solving my problem, posting this here in case anyone else finds it helpful in the future.

I think for large ships you simply have to use your engine for some slowing down mid-atmosphere, you can't just dip your periapsis into the atmosphere and let drag do all the work for you. I keep an eye on the temperature of my most vulnerable parts and when they hit 1000C I burn retrograde at around 1-2g. At the start the temperature keeps increasing but eventually it starts dropping again. When it does is when I turn the engines back off to save fuel. Your experiences will vary between crafts, but this seems to be a fuel efficient way to prevent taking damage but still use the ablative heat shields to best effect. For a 300t re-entry vehicle, I used about 800m/s dV for this mid-atmosphere breaking at around 35km AGL, and then another 600m/s for a suicide burn a few km before landing.

For attitude control to keep retrograde during all this, I found that adding a sort of aircraft tail to the rocket helped. Because I'm flying backwards when landing, this "tail" goes close to the nosecone of the rocket and is made up of short, stubby wings (with the biggest control surfaces possible) installed "backwards". Getting Juno to register controls properly for a rocket flying like a backwards plane took a lot of trial and error, but I get there eventually. The value of the PIDs is also very important here, it can easily make the difference between smoothly matching retrograde as I can go down a parabolic trajectory, and getting into a complete spin.

[mreed2]

Originally posted by Olleus:

Honestly, I think this game has no community. Not any more at least. The official forums get a handful of posts per day max, and most of those are rambling stream-of-consciousness junk which I can barely get any meaning out of. There only seems to be a couple of mods in development, and they're all purely cosmetic stuff. The discord does seem to be a little more active, but it looks like a few guys hanging around in a chatroom than much discussion of the game itself. In all of this, the little community that seems to remain is in using the game as CAD software, with very little interest in actual rocketry...

I agree, especially with the last point (people using this as CAD software), and add:

1. The community focus seems to be on planes, rather than rockets.
   
2. The developer focus seems to be primarily on visual improvements.

My conclusion is that if you aren't interested in writing autonomous rockets (e.g. Vizzy) and aren't interested in playing on mobile, then KSP 1 is nearly as good (without mods) and clearly superior (with mods).

[TomKerbal]

Well, I don't think so. There are not many, but very experienced users on https://www.simplerockets.com/Forums, and I didn't read your question there, @Olleus
Especially with the Parallax mod it is now further improved. And yes, I am using KSP1 with RP-1 as well, you can use both. I don't understand why so many collegues think, that there is a rule for using only one of them...

[Olleus]

Originally posted by TomKerbal:

Well, I don't think so. There are not many, but very experienced users on https://www.simplerockets.com/Forums, and I didn't read your question there, @Olleus
Especially with the Parallax mod it is now further improved. And yes, I am using KSP1 with RP-1 as well, you can use both. I don't understand why so many collegues think, that there is a rule for using only one of them...

Well the question is there, and someone even "answered" it by, essentially, saying what I was trying to do was impossible. After I'd solved the problem perfectly well.

As for parallax mod, that's purely cosmetic right? Which just reinforces my point and what mreed2 was saying. The focus of the devs and most of the community is limited to visuals. The latest update that allows extra customisability in exhaust plumes is basically a joke. That this took priority over something fundamental like, say, having the delta-v calculator in the builder actually work is a clear sign that this is moving in a direction I couldn't care less about.

As for the discord, it's more active than I thought. Far more than either the steam boards or the forum, and a substantially higher quality of chat. It's still mostly pretty pictures of planes, but there is a solid minority of chat about functional rocketry stuff.