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

[ferram4]

I've been looking at adding multithreading in some form, but I've been somewhat reluctant, mostly because I haven't messed with parallel processing before. Odds are some form will come eventually^TM but I want to make sure that I have a very robust single threaded version of FAR first, this way any new bugs can be relied on to be caused entirely by the multithreading implementation.

[a.g.]

Unity engine isn't multithreaded, and you probably can't do any raycasts etc from non-main threads, so they would be useful only for stuff like solving more complicated equations which don't require access to the world.

[Van Disaster]

Fair. I try for TWR-Max of 1.5-2.5 for my booster stage, so 100 works about right. I find things get a bit squirrely if I pitch over at 60, because my TWR isn't so high.

(Though--I do a very gentle pitchover, however, at the start, and try for a real gravity turn. Which you _can_ simulate in MJ2, just takes tweaking of the ascent path.)

Currently set to start turning at 200m off launch, if I'm using MJ like usual between 30 and 55% for a profile depending on TWR - that gives as gradual a turn as you're going to get, other than the pitchover it still does at 35km-ish. Would have to be a really sluggish rocket to go up to 55. TWR off the pad for heavies is usually around 1.3 at best. On the odd occasion I do it by hand I start pretty much off the pad too.

I like sitting back & watching launches a lot more than flying them, but I've also wondered how to do a proper gravity turn with the tools we have. Poking SmartASS's prograde lock and nudging the rocket over by hand at the start probably isn't going to go well

[taniwha]

Poking SmartASS's prograde lock and nudging the rocket over by hand at the start probably isn't going to go well

Yeah, particularly since MJ's SmartASS uses orbital velocity for prograde rather than surface.

[foamyesque]

Yeah, particularly since MJ's SmartASS uses orbital velocity for prograde rather than surface.

Under the SmartASS screen, hit the ADV button, tab through the options to SURF_VELOCITY, choose "Forward".

Switch to normal orbital prograde lock in the upper atmo.

(I use 60 and even 70% turns on my heavier rockets)

[foamyesque]

I've been using canards at both the extreme back and front ends for pitch control, so I assume those are just too small for anything but tiny SSTO's?

If you nail your CoL/CoM balance with FAR you can throw around even pretty huge planes around with fairly small control surfaces.

I'd prefer not to add something that has no basis in reality just because I look at what's already modeled and say, "that seems wrong," especially since this is supersonic flow, where fluids behave a lot differently than at the subsonic speeds we're used to.

Oh, for sure, no arguments here on that score. It just struck me as odd, especially since I haven't noticed it (to that extreme, anyway) with my other planes, which usually stall out at 30deg or so even at high Machs. If it's working-as-intended then that's fine.

On an unrelated note: Does anyone know of some decent cargo bays that work with FAR?

[jrandom]

On an unrelated note: Does anyone know of some decent cargo bays that work with FAR?

The B9 Aerospace ones seem to work okay.

[SyX]

I think FAR needs it's own guide or much more comprehensive FAQ now. This thread is much too large to be scanning.

Anyway, does anyone else find gravity turns impossible with FAR?

I find that due to the reduced drag of a well designed rocket, my apoapsis leaves the atmosphere before my rockets are even 20km up.

I also find that for some reason, unlike with the original aerodynamic model, if I try to gravity turn whilst the atmosphere is thick, my rockets have a tendency to become uncontrollable, as though they are stalled aircraft. How can that be when my rocket is just a rocket shape with fins at the trailing edge?

[ferram4]

@SyX: I'm thinking about making some kind of illustrated guide for the concepts, mostly because graphs and examples help people more than words. I want to make sure that FAR is very accurate before I do that, since inaccurate images would be far worse than they are now.

A picture of your rocket would help, but here are my ideas:

1. Your rocket is becoming unstable as fuel drains; this is more of a problem for squat rockets or ones in which the first stage extends to above the CoM. The solution is to make the rocket taller / use a heavier payload to shift the CoM further forward / add more fins.
   
2. Your rocket has a very high TWR and is getting up to very high speeds low in the atmosphere. The solution is to use a smaller engine, larger payload or throttle control to keep your velocity lower; as Mach number increases above 1, vehicles steadily become less stable.
   
3. You are staging while going at high speed low in the atmosphere and your upper stage / core rocket without boosters is not as stable as the complete configuration was. The solution is to push more of the rocket into the first stage or to independently design the upper stage / core to be aerodynamically stable.
   

Posting pictures of the rocket would be helpful, since then we can give you more specific advice, unless there's a bug in the code, in which case I find out about it and can try to get a fix into the next release.

[Van Disaster]

SyX: also, "gravity turn" isn't "violent pitchover to 45 degrees at 10km" like stock aero needs, you really want to be turning gradually the entire flight - the last few posts above yours were a discussion about launch profiles and if it's possible to do a real gravity turn. Any violent turns with FAR are going to present the side of your rocket to the airstream, which is really bad news for various reasons.

[SyX]

@ferram: Base on your advice, I'm almost certain that the problem is my first stage extending above my CoM. Can someone explain why this causes instability though?

edit: I mean, my first stage is usually large due to a long fuel tank, not a chain of fuel tanks, so as long as the CoM is still above the center of the main tank, shouldn't the CoM gradualy move upwards with fuel burn as expected?

Edited August 11, 2013 by SyX

[jrandom]

Anyway, does anyone else find gravity turns impossible with FAR?

Like the other's have said, you need to turn more gradually, and starting at a lower altitude.

This is a horrible experimental jet-assisted rocket, but it's highly unstable and I was still able to get a good gravity turn out of it:

[ferram4]

@SyX: If the center of another fuel tank object isn't above the CoM, then the CoM should gradually move upwards and my first suggestion doesn't actually get at the problem. You should post pictures so that we have something more to go off of than guesses.

[TheTriniFlyer]

EDIT: Don't Bother any longer, with a little ship redesign, i have mostly remedied the issue.

Edited August 12, 2013 by TheTriniFlyer

[Torminator]

I apologize if this has been covered- I did try to search the thread -but I periodically get a strange behavior from the CoL indicator.

Plane is built perfectly symmetrically, as far as I am aware. I figured it was just a graphical error, but while flying, it would in fact roll very strongly, even at rather low speeds. Nothing's in the hold.

Also, is there a guide, or helpful post in this thread, for those of us who don't have a clue about aerodynamics? Things like "Your center of lift shifts back past the sound barrier" that I only found out randomly clicking pages in the thread. At this point, it's quite too large to manually search, and I don't know what keywords to use with the thread search tool. An idiot's guide to the data window would be especially helpful.

My planes are often quite difficult to control, even though I have a fairly close CoL/CoM. Then again, I'm often trying to fly at trans-sonic speeds, so now that I know how that affects things it might help.

[ferram4]

The first thing I'd check is that you have struts between the wings and the body; wings do not necessarily bend the same amount and that could cause your rolling tendency. The second thing I'd check is right-clicking on each of the wing parts once the flight scene has loaded to make sure that the cargo bays are not accidentally "shielding" them from the airflow; this has been a problem in the past, and though I believe I've solved it, the bug may still exist.

Besides that, I'd suggest placing your engines underneath the wings rather than above them (you have to fight the pitch-down moment from the engines on top of the wings currently, which will require more pitch control surface deflection, creating more drag). I'd also tilt the vertical stabilizers less, since they're not being as effective as they could be (in the configuration they're in they also make the plane less stable in roll; if they were angled outward they would make it more stable in roll).

A guide is planned, but first a question to FAR users: is a .docx file good? I'd like to include graphs and illustrations with a guide and that filetype is my go-to for that kind of situation.

[MAKC]

FAR seems to be quite buggy atm. I'm having issues with the CoL not displaying properly, the lift indicator pointing in strange directions and changing drastically when I'm just moving non-aerodynamic parts like batteries. I'm also getting weird behavior with using the B9 Aerospace S2 Tail as a nose for my partly-reusable unmanned resupply craft. It's as if it wants to fly backwards despite CoL being behind CoM.

Also, stabilators seem to do whatever they want regardless of what you configured them to do. If I have a pair of them set up as ruddervators (45 degree tilt) they still react to roll commands even though I have separate control surfaces for that.

Edited August 12, 2013 by MAKC

[ferram4]

First, pictures please, since those can help explain several issues.

If your control surfaces aren't responding properly to control inputs make sure that the control surfaces are set properly and that the craft is saved, and then attempt to fly it. See if the control surfaces deflect instantly in flight or if they take time to fully deflect; this will indicate whether the appropriate part module has been applied to the part. If it hasn't then there is likely a problem with ModuleManager; post a copy of your output_log so I can see if any errors got thrown in it (only posting a snippet doesn't necessarily help me, since that snippet might not include all the relevant data).

[jrandom]

A guide is planned, but first a question to FAR users: is a .docx file good? I'd like to include graphs and illustrations with a guide and that filetype is my go-to for that kind of situation.

A PDF would be more platform-independent and all readers will see the exact same formatting, fonts, images, etc.

[ferram4]

Well, then a PDF; that good for everyone? It may take some time to get the file together, but I'll distribute it with the next update (hopefully).

[foamyesque]

Anyway, does anyone else find gravity turns impossible with FAR?

I find that due to the reduced drag of a well designed rocket, my apoapsis leaves the atmosphere before my rockets are even 20km up.

I also find that for some reason, unlike with the original aerodynamic model, if I try to gravity turn whilst the atmosphere is thick, my rockets have a tendency to become uncontrollable, as though they are stalled aircraft. How can that be when my rocket is just a rocket shape with fins at the trailing edge?

It sounds very much like you're turning too late and/or too sharp (and possibly have too high a TWR). Can you provide pictures of the rocket and the ascent you're having problems with?

Ferram: My own suggestion for fixing stability issues is generally "add another stage". Guarantees you have a nose heavy rocket without costing you dV. Evens out the TWR curve, too. Lets you drive around fin-less rockets unless you're launching a spaceplane or similar.

[Torminator]

The first thing I'd check is that you have struts between the wings and the body; wings do not necessarily bend the same amount and that could cause your rolling tendency. The second thing I'd check is right-clicking on each of the wing parts once the flight scene has loaded to make sure that the cargo bays are not accidentally "shielding" them from the airflow; this has been a problem in the past, and though I believe I've solved it, the bug may still exist.

Besides that, I'd suggest placing your engines underneath the wings rather than above them (you have to fight the pitch-down moment from the engines on top of the wings currently, which will require more pitch control surface deflection, creating more drag). I'd also tilt the vertical stabilizers less, since they're not being as effective as they could be (in the configuration they're in they also make the plane less stable in roll; if they were angled outward they would make it more stable in roll).

A guide is planned, but first a question to FAR users: is a .docx file good? I'd like to include graphs and illustrations with a guide and that filetype is my go-to for that kind of situation.

If I'm using a wing placed at the side of the fuselage, I usually do engines above and below the wing- you just can't see the ones on the underside. I'll try the struts method to see if it fixes the wing flex, but I've flown some real noodles and they didn't have that odd roll tendency. I mean I was using Damned Robotics hinges, which... don't work for wings.

As to the stabs: Yeah, my sense of aesthetics is constantly fighting my sense of good airframe design. Inward rudders are cool. I didn't even think about the difference in roll stability, though. That's quite helpful. (Also, on that plane the stabilizers were right in line with the CoM, so they probably weren't doing much. I'm learning. Slowly.)

[google123]

What is Flap Key? I know Air-Brake is 'B', but how do you deploy flap?

Also,

How do I change Flap and Air-Brake keys?

Is there any other keys I should know about?

[birrhan]

One more problem ferram. Rockets. Spinning out of control. I have tried the following (individually and in succession):

1) fins, of all sizes, with and without ailerons

2) locking gimbal

3) disabling ASAS

4) inline SAS

Suggestions? Am I missing something? What are the causes of this? I have had this issue before, and some combination of the above fixed it before.

[jrandom]

One more problem ferram. Rockets. Spinning out of control. I have tried the following (individually and in succession):

1) fins, of all sizes, with and without ailerons

2) locking gimbal

3) disabling ASAS

4) inline SAS

Suggestions? Am I missing something? What are the causes of this? I have had this issue before, and some combination of the above fixed it before.

Do you have a screenshot of your rocket? Where is it's center of mass? How fast are you attempting your gravity turn, and at what speed/altitude?