[1.3.1] Ferram Aerospace Research: v0.15.9.1 "Liepmann" 4/2/18

[ferram4]

@bs1110101: Try doing this:

Measure the chord of one of the sections for the fin (so that's the distance from the front to the back; it should be very, very small). Then measure the span of one of the sections (the side of one of those grid squares). Figure out how many sections there are, and multiply chord and span by those; use those values for MAC and B_2, respectively. Set MidChordSweep to 0. Set taperRatio to 1. Set e to 0.85. See if that sort-of works.

@bulletrhli: It'll be awhile. I think I've figured out an algorithm that can remove my dependency on attach nodes being the correct size that still works with pretty much any mod part I can think of (especially fairings), but I haven't started implementing that, so that might take a few days. It's especially critical with the new, larger stock size parts we'll be getting in 0.24; everyone speculates that they'll be some multiple of 1.25m wide, but I don't see any compelling reason for that myself; if that's the case then FAR will be completely wrong at simulating even stock part aerodynamics, and that will not do.

I've also just implemented the exact equations for the wave drag around slender bodies with conical sections, along with a little bit of a fudge to make it more accurate at hypersonic velocities. That works well, but there are quite a few parts in KSP that violate slender body theory, so I'm going to have to implement proper conical shock relations to handle that sort of thing; unfortunately, the values I need are not the type that can be calculated using an explicit formula and instead have to be either read off of a graph (which means I'll have to rig up an approximate equation or look-up table) or solved numerically (which will be fairly slow, even if I only have to do it on part loading.

There's still a lot to do.

@Dragon01: So basically, a way to set the zero-input angle of the control surfaces? I think I can do that.

@ANWRocketMan: Probably the best way to do it is a flight test, where you're going to keep track of the BC at various Mach numbers. Basically, boost the craft (unmanned, I would hope) into space on a sub-orbital trajectory and then boost it down into the atmosphere. While that's happening, take a look at the BC at Mach ~5, ~2, ~1.2, ~0.8, and ~0.5 and record them. Make sure that it has the mass that you expect it will have on atmospheric entry. From that, you can figure out terminal velocity at any gravity and air density based on this equation:

V_term = SQRT(2 * BC * g / density)

You can look up g for a planet. Density is a little more difficult, since you'll have to use the ideal gas equation to calculate it from pressure, temperature and the molar mass of the gas; I'd approximate it using atmospheric pressure in atm and assuming that's density in kg/m^3; it'll be low, but that gives you margin to work with. If you want more exact, send an atmospheric probe in to get better pressure, density and temperature data to use. BC will vary a bit with gas composition at any Mach number, but it shouldn't be by much. You'll select your BC based on what you think your Mach number will be when you have to start the burn; there's no need for Mach number data above 5, since it doesn't change too much above there.

That should give you the amount of velocity you need to get rid of, but you'll need to add some for gravity losses and for margin. Of course, adding a lot of dV to that will increase the mass of the vehicle enough to mess up the BC, so you'll want to scale BC up or down to compensate; it's just mass divided by drag area, so you just need to multiply it by the ratio of the current mass to the original tested mass to get the new BC, but then you need to run the terminal velocity numbers again.

[MAKC]

I roughly tallied the drag for each part on the plane and got ~55 kN of drag. It's the drag that's eating all the thrust! Nothing wrong with the TWR, it's almost 1. Maybe a problem with excessive PWing drag?

[ferram4]

Except the numbers I get (from using the Cd, reference area and Q values) that you're dealing with 65 kN; I think the difference is due to the intakes, which might not be set up correctly. Now it's possible that the cockpit is making too much drag due to the approximations used, but I don't think the pWings would be making too much drag.

[camlost]

Except the numbers I get (from using the Cd, reference area and Q values) that you're dealing with 65 kN; I think the difference is due to the intakes, which might not be set up correctly. Now it's possible that the cockpit is making too much drag due to the approximations used, but I don't think the pWings would be making too much drag.

Right now the intake drags are just what FAR calculates. The stock "dynamic drag" is disabled if you use AJE. I've tested all the B9 intakes and the ones with excessive drag are SABRE M and S. The one shown in the picture is not very draggy.

[MAKC]

Except the numbers I get (from using the Cd, reference area and Q values) that you're dealing with 65 kN; I think the difference is due to the intakes, which might not be set up correctly. Now it's possible that the cockpit is making too much drag due to the approximations used, but I don't think the pWings would be making too much drag.

The intakes had really low drag, I don't think they are the problem. Unless they're causing some behind the scenes drag?

The main wings were 10 kN each. JFYI

Edit: Velocity was topped out and the thrust was 55 kN so the drag had to be 55 kN as well since the plane was neither accelerating nor slowing down. Right?

Edited March 9, 2014 by MAKC

[Dirt_Merchant]

@ferram4: In reagrds to Dragon01's request for more control surface configuration options, I'd first like to second his request, so many new possibilities would result from this. Additionally I'd like to humbly request the ability to manually enter control surface deflection ranges, I've some interesting ideas (which I'd keep to myself for the moment) I'd like to try which require the ability to set negative deflections... The ability to configure the control surfaces on an individual basis would be rad as well. Essentially, I'd enjoy a return of the old control surface interface in addition to keeping all the fun new sliders and such. As always, your work inspires and motivates my projects in KSP, without your work, I'd not be playing!

A Few Selected FAR powered Projects/

[ferram4]

@MAKC: I've made some changes that will reduce the basic skin friction drag (it was ~50% too high) and the transonic and supersonic drag of wings. My F-15 had no problem getting up to speed using those settings, but the F100 apparently overheats at really low speeds, or it's stuck using a normal shock inlet, since getting it past Mach 1.6 quickly led to burning off. It seems about right now, though you still kind of need to go into a dive to get through Mach 1 efficiently. So wait until FAR v0.13.

@DirtMerchant: Negative deflections should also appear in the next update, but I'd prefer not to bring back the old GUI, since that GUI always kind of annoyed me. It was just a necessary evil since I didn't have something like tweakables to use.

[SSSPutnik]

Hi ferram.

I spoke with you recently about excessive drag from a piece from the LLL mod. (Where I went from a narrow section to a wider section).

A redesign recently with a much more tapered design still exhibits huge drag on the tapering part. (So much so that the craft actually flies backwards!)

Any chance I could send a craft file? (You would need LLL installed too). The way it acts, its almost at though the part is actually broken somehow and exhibits behaviour as though its just a flat surface.

If it is a part issue, I can ask Lack to look into it. If it's my design then you can smack me upside the head.

[ferram4]

If it's the part I'm thinking of, that single part tapers too quickly for its own good. Now, that said the numbers will probably drop a bit in the next FAR update, since I've got a somewhat better algorithm for that sort of thing partly-worked up. For the most part, it's a part issue, and it's an issue that's shared with almost all the tapering parts that exist in KSP; they're designed to look cool, not to be aerodynamic.

Go ahead, post the craft and a picture of the problem part so I know that I'm checking the right thing. I'll see how it plays out in the dev build and make sure nothing truly horrible happens.

[NathanKell]

@MAKC: I've made some changes that will reduce the basic skin friction drag (it was ~50% too high) and the transonic and supersonic drag of wings. My F-15 had no problem getting up to speed using those settings, but the F100 apparently overheats at really low speeds, or it's stuck using a normal shock inlet, since getting it past Mach 1.6 quickly led to burning off. It seems about right now, though you still kind of need to go into a dive to get through Mach 1 efficiently. So wait until FAR v0.13.

This is in part why I didn't redo the JRC Mk III to use AJE for its engine, and, I think, why I was getting some weirdness with the HARC. Looking forward to it.

[SSSPutnik]

Here is the craft file, though you can whip one up with LLL installed in no time.

Drag peaks near about 175kN at 20,000m (on the one offending part). (It's almost like a parachute )

I really want the rectangular section at the back, the front of the craft I'm not really fussed about the design, but it needs to hold 5 Kerbals, which this one can.

I have experimented with pure square cross section and it works great, the only issue then is the craft is too long. (It's designed to go on top of a lander).

The part is the tapering section behind the cockpit + SAS unit, (both LLL parts).

PS: when I launch the whole ship, that tapered part generates 4-5x the drag than the rectangular fuel tanks flying sideways to direction of travel!

Edited March 11, 2014 by SSSPutnik

[Boamere]

@ferram4: In reagrds to Dragon01's request for more control surface configuration options, I'd first like to second his request, so many new possibilities would result from this. Additionally I'd like to humbly request the ability to manually enter control surface deflection ranges, I've some interesting ideas (which I'd keep to myself for the moment) I'd like to try which require the ability to set negative deflections... The ability to configure the control surfaces on an individual basis would be rad as well. Essentially, I'd enjoy a return of the old control surface interface in addition to keeping all the fun new sliders and such. As always, your work inspires and motivates my projects in KSP, without your work, I'd not be playing!

A Few Selected FAR powered Projects/

I see you have alternis kerbol, I've always wanted to download this, but I have some questions: Does it take up lots of space? (i've heard it takes up less) Does it cause more lag? and does it delete saves?

Thanks in advance

[Kenken244]

I have been having a consistent problem with my plane designs since using this mod. All of my planes have, after flying perfectly for a while, gaining altitude and velocity, suddenly decide to rapidly pitch up and flip out. I cannot seem to determine the cause of this problem. I have tried adjusting the center of lift versus center of mass, adjusting control surfaces, tried different ascent paths, but nothing seems to avoid this problem. Furthermore, there seems to be no definite cause of the issue as far as I can see. This will happen at wildly varying altitudes and speeds, across a variety of plane designs. is there some common mistake that I might be making? What generally is the cause of such losses of control, and how can I avoid them? I really seem to be at a loss here.

[ferram4]

@SSPutnik: I can look into it, but if the drag on the part is that bad then I wonder if there's something wacky going on with the model.

@Kenken244: Without more information on each case there's no way to diagnose what's going on. The most accurate thing I can say is that something is causing your plane to become unstable, but if there's no consistent thread through all of it then it's probably completely different things happening.

One possibility is that you're causing the plane to stall, and it's not capable of coming out of a stall. Stalls happen above ~15 degrees angle of attack (for each wing), so if you're trying to fly the plane like in stock you might cause that to happen.

Another possibility is that the plane is particularly susceptible to Mach tuck and it becomes highly unstable near Mach 1. If that's the case you might have to move your wings up or down on the plane in order to deal with the drag and possibly shift the wings further back and compensate with more control surfaces.

It could be that your planes just have some stupidly un-aerodynamic thing stuffed on the front that makes it unstable, in which case that's the source of your problem.

You might just be dealing with a shifting CoM as fuel drains and you're not used to the way the fact that drag isn't dependent on mass with FAR, in which case you need to redesign your vehicles to account for the fuel drain.

Posting pictures will make it a lot easier to diagnose what's going on with each case, but for until you do that I can't tell you what you're doing wrong.

[MAKC]

I have been having a consistent problem with my plane designs since using this mod. All of my planes have, after flying perfectly for a while, gaining altitude and velocity, suddenly decide to rapidly pitch up and flip out. I cannot seem to determine the cause of this problem. I have tried adjusting the center of lift versus center of mass, adjusting control surfaces, tried different ascent paths, but nothing seems to avoid this problem. Furthermore, there seems to be no definite cause of the issue as far as I can see. This will happen at wildly varying altitudes and speeds, across a variety of plane designs. is there some common mistake that I might be making? What generally is the cause of such losses of control, and how can I avoid them? I really seem to be at a loss here.

This sounds like a case of changing CoM due to fuel consumption.

1. Get RCS Build Aid if you don't have it
   
2. Enable empty CoM along with normal CoM and aerodynamic center
   
3. Take a screenshot from dead above so we can see your control configuration as well
   
4. 
   

Edited March 13, 2014 by MAKC

[Guiltyspark]

Is there a reliable way to predict orbit after aerobraking with FAR installed? (I'm not sure if someone has asked this before but reading a 337 page thread is painful.) MechJeb (even with the FAR compatibility installed) can't give anything nearly accurate and this online calculator does not adjust for FAR either.

[ferram4]

Unfortunately, the most reliable way of "predicting" is F5, aerobrake, F9 if unsatisfied. Whenever MJ can actually manage simulating FAR stuff even that won't be perfectly accurate because it will heavily vary with how the vehicle is oriented during aerobraking. Aerobraking is fuzzy and not perfect with FAR, just as it should be, unlike in the stock game where you can reliably predict exactly what will happen down to the meter.

FWIW, aerocapture with FAR generally ends badly due to heating or aerodynamics tearing the ship apart.

[illectro]

I'm experiencing an interesting form of RUD and before I go talk about it on video I'd really like to clarify what's going on.

I'm trying to launch the largest microwave transmitter from KSP interstellar, now this thing is huge but folds up enough to fit inside a 3.75m faring. Outside of the fairing it appears to make drag calculations based on its deployed state which is so huge you can actually use it as a parachute, so you need to stick it inside the an aerodynamic structure to make it move, but quite often during launches it appears that FAR decides that it's no longer protected by the shroud, whenever this happens the sudden addition of drag will tear the rocket apart. So I'm wondering how FAR is making this decision, is it just that the payload is wobbling too far laterally and pokes just a little bit of the model through the shroud and suddenly making it calculate drag for the whole object or, is the faring perhaps splitting apart at the nose a little and drag being applied that way, or is this another hilarious bug.

It's pretty spectacular when it happens, and it'll make for good video, but I'm really curious for the mechanics.

[Motokid600]

I see you have alternis kerbol, I've always wanted to download this, but I have some questions: Does it take up lots of space? (i've heard it takes up less) Does it cause more lag? and does it delete saves?

Thanks in advance

Well this is my personal experience. Alternis broke FAR to hell and back. I am not sure why... Or if was another mod maybe, buy I noticed with Alternis that EVERY craft I made acted like it was dragging a parachute. No TWR in the world could get my craft past 250m/s. So i deleted it and never looked back. ( I only wanted Alternis for the lighting ) But by all means try it and see how it works for you.

Yes you'll want a fresh save and yes it takes up just a bit of memory, but nothing over the top like planet factory. It just reorganizes the existing planets.

[Naten]

It really doesn't matter to me I have rockets powerful enough to get to space with their first stage (depending on what they're carrying) so I don't mind what the ascent is like

(but when I do I just use SAS and turn when it "feels right")

And yeah rockets do seem to "Fly" through the air, very smooth, nice feeling

.-.

boamere, you're everywhere.

Anyways, I've been playing with FAR and ProcFairings and it's the best combo. EVAR.

I love the whole idea of "Wow, this would work in real life! :D" instead of "Well, it's a plane... Maybe if it had no wings, then it would work like real life..."

Thanks Ferram!

[DaMichel]

"Wow, this would work in real life! :D" Thanks Ferram!

Indeed. I had a lot of fun recently with lightweight prop planes.

^ can pull 15g's for rapid self disassembly as well as fly super slowly

TBH, probably i wouldn't have noticed that stock aerodynamics are wrong if i hadn't read about it. However if at all possible i perfer physics to be like in real life. And it is great to be able to basically play a flight sim with DIY planes knowing that it isn't too far off from RL

[Boamere]

.-.

boamere, you're everywhere.

Indeedy I am Naten

[ferram4]

@illectro: Yeah, unfortunately FAR's drag calculation seems to grab the deployed state of those transmitters, I'm not sure exactly why though.

As for the fairing thing, what FAR does ever time the vessel part list is updated is it goes through all of the fairing parts, calculates their bounds, and then goes through the list of every part in the vessel to see if that part's origin is inside the fairing (for wings it also checks the wingtip for thoroughness, or it's supposed to; that doesn't seem to work either for some reason). So the only way the issue you're seeing should happen is if the transmitter wobbles outside the fairing at the same time as you're ditching a stage. FAR isn't calculating if something is inside the fairing every single frame, that would add lots of vector math, comparisons and iterating through part lists that are simply unnecessary. Try right-clicking on the transmitter and making sure that it stays "isShielded = true" the whole way, since that needs to be false in order for FAR to apply drag parameters; it acts the same as FAR detecting zero density and it simply stops running the rest of the code at that point.

If the payload is wobbling too much, I'd make sure you're running KJR v2.0 rather than an earlier version if you don't want to wait until the official joint fixes. Things have gotten a lot better since the earlier versions of KJR.

But anyway, the video will be good; video documentation of bugs is great, because then I can actually see everything that went wrong.

[camlost]

@MAKC: I've made some changes that will reduce the basic skin friction drag (it was ~50% too high) and the transonic and supersonic drag of wings. My F-15 had no problem getting up to speed using those settings, but the F100 apparently overheats at really low speeds, or it's stuck using a normal shock inlet, since getting it past Mach 1.6 quickly led to burning off. It seems about right now, though you still kind of need to go into a dive to get through Mach 1 efficiently. So wait until FAR v0.13.

If v0.13 is going to be long then could you release a preview version? 50% reduction in skin friction drag seems a lot and I really need this to test AJE.

[ferram4]

Alright, there's an experimental that's out that gets a lot of the features out there. Still haven't adjusted the attach node drag system or all of the control surface stuff, but there's enough there to play with.