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

[ferram4]

Actually... that indicates that the problem is more likely in FAR's modeling. The difference in real life is due to real gas effects, which include reacting flows; FAR does not model this at all, and it shouldn't appear until around Mach 10.

I suspect that it's actually an error in how FAR accounts for lift from wings instead.

[tech_op2000]

bummer. Well Im testing re-entry with the new model soon. At least it will give you something to chew on. I still think that CoP curve for the Fuselage is pretty extreme.

[SkyHook]

And finally, for just the Wing:

*snip*

Well here are a couple of considerations to consider when reentering the atmosphere in a shuttle:

-Never reenter with empty fuel! The angle that you reenter at will almost always make you flip over when you hit the thick part of the atmosphere.

-B9 cockpits are very heavy. Seriously. They cause a lot of drag and will wreak havok on your CoM. Your CoM shifts wildly forward as your fuel empties.

-Keep your reentry angle to 30 degrees or less.

My recommendations for your specific craft:

-Halve the fuel that you take. Compensate with more fuel in the lift stage and redesign the wings to fit the new (and better, imo) CoM.

-Use pwings. They offer more flexibility in wing design (although I think you did an excellent job with the b9 wing).

Pictures of my large shuttle for design cues:

Otherwise:

Assuming that you have tested low-speed performance, how does she fly and land? I am curious. Happy flying.

Edited October 30, 2013 by SkyHook

[tech_op2000]

Otherwise:

Assuming that you have tested low-speed performance, how does she fly and land? I am curious. Happy flying.

With a quarter to half fuel left on-board it flies and lands great. on empty it is a bit nose heavy and doesn't have much up control authority, but it can still land ok. My frustration is just getting into a flight attitude on re-entry. I want to do a high alpha re-entry but the physics won't allow that.

[SkyHook]

With a quarter to half fuel left on-board it flies and lands great. on empty it is a bit nose heavy and doesn't have much up control authority, but it can still land ok. My frustration is just getting into a flight attitude on re-entry. I want to do a high alpha re-entry but the physics won't allow that.

Yea I have the same nose-forward problem at low speeds, so the canards help. The problem intrigues me. I will load up my game to see if I have the same problem with high-alpha reentry a little bit later. What are your landing speeds? Mine generally range from 120-140 m/s.

[localSol]

With a quarter to half fuel left on-board it flies and lands great. on empty it is a bit nose heavy and doesn't have much up control authority, but it can still land ok. My frustration is just getting into a flight attitude on re-entry. I want to do a high alpha re-entry but the physics won't allow that.

High alpha sounds fun. But it's not a problem that enough gyro torque can solve? Looks like your spaceplane has relatively few parts and that can be more fun because the game runs well. Something that would be extra fancy for people testing their FAR spaceplane designs is some kind of auto reetry scenario : pick your craft, pick a planet and ap/pe, or.. start at the equator and pick altitude, direction, and speed. I guess if you're familiar enough with the savegame format, it can be ok to setup something similar but it would be nice for FAR especially since reentries are extra special for each plane. I have done way more (failed with 'splosions) SSTO spaceplane orbit attempts than reentries.

[MoffKalast]

Just one question, why does my every single rocket suddently spinout and stall after it uses about 3/4 of the first stage fuel?

(they work just fine without FAR)

[NathanKell]

Question: when are you starting your gravity turn?

Answer: about 500m off the pad.

Question: what's your max angle off the surface-prograde marker?

Answer: 1-5 degrees.

If either answer is not your own....there's your problem right there.

[MoffKalast]

"Answer: about 500m off the pad."

Well i have no choice since i'm using the real life kerbin size mod where you have to start your gravity turn very early (according to Scott Manley) so you can gain the proper orbital speed (around 5km/s)

So I just have to add a ton of torgue wheels?

[taniwha]

MoffKalast: the basic problem is probably that your CoM is shifting behind the CoL as you burn fuel.

[MoffKalast]

So uhm, what do i do?

EDIT: Okay doing the gravity turn later helps alot.

Edited October 31, 2013 by MoffKalast

[Farmer Bobathan]

Add fins, kind of like an arrow, It fixed the flipping for me on real kerbin.

[Lurven]

For some wierd reason after I installed this mod, when I hit the water, my pod accelrates violently and explodes. The times this happened I was using the MK 1-2 pod (2.5m 3-crew one, think name is right). In the after death view I'm far away from where I hit the water. May be another mod I innstalled at the same time.

Can you guys please help me?

may not be the right thread to post it in?

EDIT: Can it be the Joint Reinforcement mod and kerbal engineer thats trolling me?

EDIT2: Tried with the mk1 pod, without the kebal engineer thing it could land normally, with it; I ended up 350m below sealevel

Edited October 31, 2013 by Lurven
Additional info

[ferram4]

Does removing KJR fix it? That mod can (apparently) cause very weird issues with very light parts.

[rpayne88]

I've been having problems with some of my aircraft. The FAR assistance systems can not be turned on, off, or changed. This is causing my aircraft to yaw violently while rolling because I can not use the yaw dampener.

[ferram4]

You tried to activate the control systems using the buttons on the bottom of the main Flight GUI and it didn't work? Did you leave SAS on? It might interfere with the FAR control systems. Are you using any other mods that might conflict with this?

[Tharios]

What is that long track in the background, and how do I get one? I must have it!

[SFJackBauer]

Its a mass driver.

I am having the same troubles converting my space shuttle designs to far - they are heavy in the back, due to the engines, and I can only design a wing that is either stable on high AOA re-entry, or stable on landing. If I get good results I will post here.

[Hodo]

Its a mass driver.

I am having the same troubles converting my space shuttle designs to far - they are heavy in the back, due to the engines, and I can only design a wing that is either stable on high AOA re-entry, or stable on landing. If I get good results I will post here.

Check in the SSTO thread in the space craft exchange forum. There are quite a few great designs to borrow ideas from. I know my SP-314 works just fine in DRE and FAR, both on leaving atmosphere and returning. If I ever to recreate my first shuttle that worked beautifully.

[SFJackBauer]

Check in the SSTO thread in the space craft exchange forum. There are quite a few great designs to borrow ideas from. I know my SP-314 works just fine in DRE and FAR, both on leaving atmosphere and returning. If I ever to recreate my first shuttle that worked beautifully.

Thanks, but... 1) I am not looking for a baked solution. I want to understand how to make my design flyable, by going through the process. 2) my design constraints were not satisfied by the crafts I looked (frankly I didn't went through the entire forum, cause of point 1).

Anyway, I didn't said my craft isn't flyable in FAR - I mentioned the high AOA regime, which generates more drag and makes the reentry quicker and more predictable. I am going by the same trouble tech_op2000 is going - on high AOA, the rear parts of the wing stall first than the front. Up to 20 AOA I can fly the re-entry just fine, it just takes longer and I have to guess better when to make the deorbit burn (hint - one of my design constraints is unpowered reentry/landing). I wonder if with procedural wings it will be different? I will give it a shot next.

On the way to understand better, I've been analyzing the FAR steady-state analysis. One of things I find odd is the fact that the way I attach the engines to the back influences the way the craft handles. It does not makes sense to me. For example, these are the curves I get with three engines surface-attached and slightly angled:

And these are the curves I get with these same three engines attached with a tri-coupler:

Perhaps its nothing, since the curves change completely with the wings attached.

[ferram4]

What's happening is that when the engines are tilted like that at zero angle of attack they are producing 1) more drag, due to their angle, 2) negative lift, due to the same thing and 3) a pitch up moment at that lower angle of attack due to the negative lift. At a higher angle of attack they result in, overall, less lift, a more positive pitch moment, but less drag due to their lower angle with respect to the flow.

[SnappingTurtle]

I suddenly recalled hearing that the ratio of a rocket's diameter to its length is important for stability. Is this an aerodynamic effect? Does it matter with FAR?

[Hodo]

Thanks, but... 1) I am not looking for a baked solution. I want to understand how to make my design flyable, by going through the process. 2) my design constraints were not satisfied by the crafts I looked (frankly I didn't went through the entire forum, cause of point 1).

Anyway, I didn't said my craft isn't flyable in FAR - I mentioned the high AOA regime, which generates more drag and makes the reentry quicker and more predictable. I am going by the same trouble tech_op2000 is going - on high AOA, the rear parts of the wing stall first than the front. Up to 20 AOA I can fly the re-entry just fine, it just takes longer and I have to guess better when to make the deorbit burn (hint - one of my design constraints is unpowered reentry/landing). I wonder if with procedural wings it will be different? I will give it a shot next.

.

I can understand point 1 quite well, but there are some really good designs in that thread, in stock and modded forms. Sometimes the answer to the problem you are experiencing has been solved by someone else, and it is most likely posted somewhere.

Ferram4, pretty much summed up your problem with the engine angle. I myself quit placing engines at angles due to the difficulty in controlling the craft when under thrust. But later I found out that it also increased drag.

Just wait till you get it flying correctly then you put a cargo in the bay and it throws the whole thing out of whack again! I think you are far better than I and will stick with it and find a solution... I myself said forget shuttles I am building space plane SSTOs! Now I can put 72 tons in orbit with a SSTO plane, or go to Duna and land on Ike with a VTOL SSTO plane.

I do love your shuttle though. I hope you get it working perfectly because it is a work of art.

[SFJackBauer]

I can understand point 1 quite well, but there are some really good designs in that thread, in stock and modded forms. Sometimes the answer to the problem you are experiencing has been solved by someone else, and it is most likely posted somewhere.

No worries mate, I don't doubt about the quality of the designs, including quite a bit yours that I have seen. However being a "craft exchange" thread I couldn't find/don't belong there the precise answer to what I am looking for - how to design a wing in FAR for high-drag, high AOA reentry that also is maneuverable for landing.

Ferram4, pretty much summed up your problem with the engine angle. I myself quit placing engines at angles due to the difficulty in controlling the craft when under thrust. But later I found out that it also increased drag.

Just wait till you get it flying correctly then you put a cargo in the bay and it throws the whole thing out of whack again! I think you are far better than I and will stick with it and find a solution... I myself said forget shuttles I am building space plane SSTOs! Now I can put 72 tons in orbit with a SSTO plane, or go to Duna and land on Ike with a VTOL SSTO plane.

Regarding the engine angling - the key is giving the engines a large gimbal range (around 10 degrees, instead of most stock engines at 1). SAS will take care of adjusting the gimbal through the center of mass. However the static angling is still necessary to give it some headroom. This way, its perfectly possible to launch a piggyback shuttle with varying payloads that is stable at ascent.

Now, SSTOs have advantages and disadvantages... well lets not bring this discussion to this thread.

I do love your shuttle though. I hope you get it working perfectly because it is a work of art.

Appreciated! Although credits should be given also to Maxime Faget

Now back to the problem, slightly good news! I have come to terms with a reentry profile, after switching to Procedural Wings it gave me more flexibility, less weight and the stall problem is reduced, although still looking to improve it. Here are the "FAR lines" at Mach 6:

Notice the center of lift has been moved further back, so the pitching moment (yellow line) has been reduced drastically in relation to the B9 wing. (Note - the engines placement don't have much effect in the graph - the wings overpower their drag and their aero effects, so I don't worry about it anymore).

So now my reentry profile is this:

- In a 80km orbit, plan a 100m/s retro burn so that MJ reports landing at 130km west of KSC

- Mach 7.5 to 5.0 - 35º pitch on navball (corresponds to roughly 40º AOA) In this phase the wing is between 30/50% stalled.

- Mach 4.9 to 2.5 - reduce to 25º pitch / 25º AOA

- Below Mach 2.5 - just fly and land. If overshooting, apply airbrakes.

G loading is low, and thermal load is low since most of the braking is done at upper atmosphere.

Now since the center of lift is further back, the low speed handling suffered - I countered adding more control surfaces to the back (there are 5 elevators and 2 elevons in the pic above). And these add drag and reduce lift when operated, so landings have to be at higher speeds. I still need to tweak this. The truth is I have managed now the full cycle with FAR - ascent, orbit, reentry and landing - twice. So I'm happy

Edited November 2, 2013 by SFJackBauer

[Hodo]

Now, SSTOs have advantages and disadvantages... well lets not bring this discussion to this thread.

Appreciated! Although credits should be given also to Maxime Faget

Now back to the problem, slightly good news! I have come to terms with a reentry profile, after switching to Procedural Wings it gave me more flexibility, less weight and the stall problem is reduced, although still looking to improve it. Here are the "FAR lines" at Mach 6:

Notice the center of lift has been moved further back, so the pitching moment (yellow line) has been reduced drastically in relation to the B9 wing. (Note - the engines placement don't have much effect in the graph - the wings overpower their drag and their aero effects, so I don't worry about it anymore).

So now my reentry profile is this:

- Mach 7.5 to 5.0 - 35º pitch on navball (corresponds to roughly 40º AOA) In this phase the wing is between 30/50% stalled.

- Mach 4.9 to 2.5 - reduce to 25º pitch / 25º AOA

- Below Mach 2.5 - just fly and land. If overshooting, apply airbrakes.

Now since the center of lift is further back, the low speed handling suffered - I countered adding more control surfaces to the back (there are 5 elevators and 2 elevons in the pic above). And these add drag and reduce lift when operated, so landings have to be at higher speeds. I still need to tweak this. The truth is I have managed now the full cycle with FAR - ascent, orbit, reentry and landing - twice. So I'm happy

I love Procedural Wings, one of those must haves if you have FAR. I know the real life shuttle has a really high landing speed, 214-226mph (94-101m/s) according to the NASA site. In comparison a F-16 lands around 138-195mph. I am glad that you have it working, from the graph it is far more controllable than one of my less stable SSTOs. That thing is a nightmare when it drops from supersonic speeds.