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

[Hodo]

Ferram4,

I think I am having a problem with FAR. Not sure though. It seems like lately I have had HUGE success with designs that should not work like this one below, I call it the A-1.

It isn't air hogging, it has 2 intakes per jet engine, and 2 rocket VTOL engines, which are a bit underwhelming when compared to any other rocket. Yet it climbs and flies without any problems under the jet engines, reaches a respectable mach 6.32 before I HAVE to shut down the jets at an altitude of 28km. And then I kick on the rockets for a whole 5-8 seconds and I am at a 80km orbit. All this with the smallest of wings, procedural wings are the base that are holding up the vtol engines and the rest are just B9 winglets. With a Firespitter body and cockpit. If you would like the craft file I will gladly give that to you as soon as I find a decent place to post files to share.

[ferram4]

Well, to start you have a lot of wing for the size of the vehicle there. Is it using B9 jet engines? I haven't made any changes to mod engines, so they would be absurdly overpowered compared to the stock engines. It looks like you've just combined massive engines and wings with a tiny payload and successfully proved just how significant vehicle mass is in how it performs.

[Hodo]

Well, to start you have a lot of wing for the size of the vehicle there. Is it using B9 jet engines? I haven't made any changes to mod engines, so they would be absurdly overpowered compared to the stock engines. It looks like you've just combined massive engines and wings with a tiny payload and successfully proved just how significant vehicle mass is in how it performs.

Actually using the stock turbojet engines. And the B9 VS-1 VTOL rocket engines.

And thanks for the quick response and letting me know that I didn't break something in my install, lol.

[camlost]

@Hodo,

All jet engines should have thrust curved against velocity. That is, at very high speed it will lose power, eventually. I remember that B9 VTOL jet is missing a line 'usethrustcurve=true' or something, and that might be the cause of this phemomena. Anyway

[Hodo]

@Hodo,

All jet engines should have thrust curved against velocity. That is, at very high speed it will lose power, eventually. I remember that B9 VTOL jet is missing a line 'usethrustcurve=true' or something, and that might be the cause of this phemomena. Anyway

The thing is this craft isn't using the B9 jet VTOL engine VA-1, it's using the B9 Rocket VTOL engine VS-1. And the stock Turbojet engines from KSP.

Yeah I just really looked at the thrust to weight ratio of it full and on just jets alone it has a 3.74:1 thrust to weight ratio.

EDIT-

One other thing Ferram, I noticed that the stock control surfaces suddenly lost all FAR programability, I can not program them for Pitch, Yaw or Roll anymore for some reason. It only affects the stock KSP parts with control surfaces or the stock control surfaces themselves.

[camlost]

If the turbojet lose power at mach 8 then I don't see anything wrong then. Seems your small spaceplane is a success. If there's something wrong with some engine, your can probably find the bug in its cfg.

[camlost]

Ferram,

Now that the earth-size-kerbin plugin is out, are you going to try to add some hypersonic aerodynamics into FAR?

Also that I assume in the current algorithm of FAR the speed of sound is constant, right? Do you think it's best to model temperature variance from altitude?

[Hodo]

If the turbojet lose power at mach 8 then I don't see anything wrong then. Seems your small spaceplane is a success. If there's something wrong with some engine, your can probably find the bug in its cfg.

Was this changed? Back in .21 turbojets lost power around Mach 4.5 and were useless by Mach 6.8 or so.

Otherwise this is good news, because I slapped together that little SSTO in 10 minutes just testing out a look.

[ferram4]

@Hodo: Turbojet got modified a bit; it's thrust falls off more gradually than it did before, but the other parameters should be balanced to compensate.

@camlost: FAR already has a hypersonic aerodynamics calculated in; the nonlinear supersonic equations I use tend towards the hypersonic limit for assumptions of negligible viscous effects. I can't actually implement hypersonic aerodynamics including proper shock-boundary layer interactions without causing your processor to become an experiment in electrically-powered heating and KSP to drop frames like crazy.

[a.g.]

One other thing Ferram, I noticed that the stock control surfaces suddenly lost all FAR programability, I can not program them for Pitch, Yaw or Roll anymore for some reason. It only affects the stock KSP parts with control surfaces or the stock control surfaces themselves.

That suggests broken Module Manager to me.

[Surefoot]

I can't actually implement hypersonic aerodynamics including proper shock-boundary layer interactions without causing your processor to become an experiment in electrically-powered heating and KSP to drop frames like crazy.

That leads me to thinking, are the mods like yours able to make use of multi cores / multi threads, and work asynchronously (thus giving results on a best effort basis, depending on total CPU power) ? I'm not familiar with the game engine..

[KerbMav]

@Ratzap: TL;DR: Your vehicle is not stable for landing on an atmospheric body; the CoM is too close to the top and the CoL is too close to the back.

The main problem is that empty fuel tank on the bottom, which makes quite a bit of drag, particularly in that orientation. A cylinder one meter in length in cross flow (which is what you have going on there) has a drag coefficient of 1.2 based on its cross-sectional area.

Basically, the long story short is that those cylinders all make more drag and lift than you think, your CoM is higher than you think, and the pod torque is weaker than you think; your vehicle is in its proper stable orientation and it is not properly designed for landing on an atmospheric world. Add more weight at the bottom, lighten up the top, or put the parachute on a tower or something silly like that.

I have the same problem, but I just let the whole thing drop and slow down, open a single chute do increase drag and slow down more as soon as it is slow enough to not rip anything off.

Yesterday I could even delay my impact by pressing controls really hard and keep the top of the 3-man-pod above the horizon - 23km up, 2,3km/s fast, coming in from Mun to a PE of 34km initially.

So, it really is true, wings are not the only things creating lift!

Going to try to place some small control surfaces and see if that keeps me pointing the way I want to - could end badly in thinner atmospheres if I could not burn to slow down.

[Surefoot]

A reaction wheel is enough to control a lander. Just remember to turn SAS off !

[tech_op2000]

Well, this thread is getting long! Alright, Lets see if you can help me out. I am building a space shuttle right now and have hit a barrier. The normal flight characteristics are great... until it approaches stall. Which is a problem for high AOA re-entry! My best guess is that the COP(center of pressure) is moving forward to the extreme. I know that it moves forward in reality, but it seems to be moving forward an unreasonable amount. So, Here is the current shuttle:

The Static COP is perfect and in fast flight it is quite stable and maneuverable.

Unfortunately I can't show images of the COP moving( A Data line in "Flight Data" for COP would be fantastic! BTW), so here is the flight data as AOA increases.

From Minor stall onward it is impossible to nose forward. A classic symptom of aft CG or in this case forward COP

From this point on it actually starts flying backwards, and does so quite well unfortunately.

Here is some info that might help:

I am guessing that the fuselage is not stalling as deeply as the wing, which in turn gives the fuselage(which sticks forward much more than back) much more lift comparatively. It then has a very far forward CoP. In reality shouldn't it pitch forward from a 90 deg AoA since there is more surface area aft of the CG? Side Note: yes I can fix this issue by moving the wings back, but the normal flight characteristics suffer to significantly. I have done tests with canards and varying aspect ratio wings. the equation however still seems to be factoring the fuselage for aspect ratio so it still stays near 1.

[Tsuki]

From this point on it actually starts flying backwards, and does so quite well unfortunately.

A bit noncontributing post, but still: I can still very well remember how one of my first SSTOs could only fly backwards at reentry, and at that time I decided not to fix the problem, but to improve the flight characteristics in backward, resulting in perfect landings on the KSC landing strip... in reverse. And by landing in reverse I accidentally solved one of my main issue with SSTO re-usability... i would not need to turn it around for takeoff

[Hodo]

Tech_op, I am no expert but from my POV it looks like your flight path indicator is trying to compensate for the drop in air speed to maintain the current flight path.

In other words the slower you go the higher your nose has to be to stay level.

Try maintaining a certain airspeed while descending. Or the perfect/near perfect glide path.

[localSol]

[...] So, it really is true, wings are not the only things creating lift!

Going to try to place some small control surfaces and see if that keeps me pointing the way I want to

I tend to forget this! Good information.

@Hodo , I think something has maybe gone wrong with your install. My stock type control surfaces are all ok. KSPModAdmin can really help to prevent problems with 'what files did I install and overwrite for mod #87?...' , and installing mods with it is easier and faster.

So let's do some math here. [...]

Drag Force = Dynamic Pressure * Drag Coefficient * Reference Area

Dynamic Pressure = 0.5 * Air Density * Velocity2

Dynamic Pressure = 0.5 * 0.8211 * 602

Dynamic Pressure = 1478 Pa

Your reference area is the cross-section of that cylinder, which is:

Area = Pi * Diameter2 / 4;

Area = 1.227 m2

Drag coefficient of 1.2 for a 1 m cylinder, so extend that to a 2.5m cylinder, and we get:

Drag Force = 1478 Pa * 1.227 m2 * 1.2 * 2.5;

Drag Force = 5440 N

Good stuff! Can you add it or something like it to the Readme too for reminders? I was thinking it would be cool to have the latest readme in the ingame GUIs too, maybe even while in flight, just for fancy convenience. For those long flights where you can't time-accelerate. Some light(heavy) reading!

A fun flight with FAR, B9, Interstellar's nuclear thermal rockets/turbojets. I spent something like 15 hours building, testing and flying it. Camera booms using HullCamVDS mod, what a treat to land with the 'gear cam' though also with FAR :

Front gear cam : http://cloud-2.steampowered.com/ugc/597018368812443752/305DAA78E92E96191102EEB389EDB732FBDBE86D/

Rear gear cam (could use a little lowering next time, maybe with infernal robotics telescoping booms) : http://cloud.steampowered.com/ugc/597018368823143607/03ECD126F1786680E13562148479E8F81C34D5E3/

Plenty of GUI : http://cloud-3.steampowered.com/ugc/597018368812445361/85E89969367DD62D4028C2478966F7877B0EB33B/

Not enough to escape laythe though (TWR ~0.85(??), 1065 m/s) : http://cloud-2.steampowered.com/ugc/597018368812448687/976185A4121438EAE57880C8CBC1B8B933EFC2ED/

Edited October 29, 2013 by localSol

[tech_op2000]

Tech_op, I am no expert but from my POV it looks like your flight path indicator is trying to compensate for the drop in air speed to maintain the current flight path.

In other words the slower you go the higher your nose has to be to stay level.

Try maintaining a certain airspeed while descending. Or the perfect/near perfect glide path.

You are right about it compensating. This is normal for aircraft. The important part however is that it flies great in normal attitudes. The point of the exercise was to show stall characteristics. If it were performing normally then the attitude would continue to increase until it stalled, then would drop because of assistance from aerodynamic forces( CoP behind CG) but that is not the case for this strange flyer.

[tech_op2000]

Here is some extra info that might help illuminate the problem.

Check out the CM for the Fuselage only! That is a nasty curve!

[tech_op2000]

And finally, for just the Wing:

[localSol]

And finally, for just the Wing: [picture...]

Wow, I never once thought of analyzing them separately like that. I think I'll try it next time, it may help me get a better intuition for this stuff. But at the same time, Kerbal Joint Reinforcement 1.2 is out! Time to put some crazy wings on tall rockets?!

[ferram4]

@Surefoot: There's nothing stopping a mod from making use of multithreading, but the problem is moving the data from the mod thread to the main Unity thread for physics calculations. There's also the issue that none of the Unity methods are threadsafe, and thus unusable outside of the main thread.

@tech_op2000: Ultimately the problem is that your shuttle is stalling at the back before the front, and there are three things I can think of trying to fix that:

1. Shift the wing forward a little bit and add a separate, highly swept tail at a slightly negative angle of incidence; this will cause the tail to stall much later than the other surfaces and to be less severely stalled at any angle of attack.
   
2. Shift the majority of the wing forward and add a swept root extension just behind it, sort of an inverse of what you have going on at the front. This will have a similar effect as option 1.
   
3. Put a heavy tank / counterweight near the front too shift the CoM further forward and compensate with more pitch control surfaces.

A thing to keep in mind is that your tests were all done at Mach 0.5; this corresponds to compressible subsonic flow, which will behave much, much differently than the Mach 5 high supersonic flows you will encounter during reentry. In considering the high-AoA stall behavior at low Mach numbers you and trying to use that to judge supersonic performance you are doing the aerodynamic equivalent of designing an apple corer for the problem of peeling an orange.

Your analysis of the wing alone also includes some messy data from the structural pieces down the center, which are affecting the numbers quite a bit, though it's difficult to say how much.

[tech_op2000]

Tests at Mach 5 yield very similar results. My question is this, Why does it have such a severe pitching moment for the fuselage. And for that matter for the whole craft? I want it to resemble the Space shuttle, so shouldn't using a similar wing design work?

[ferram4]

It's possible that FAR isn't properly simulating aerodynamics for that shape; I'm going to look into the lift and drag of wings in large-scale stall and make sure that it is modeled correctly.

It's also possible that the CoM on the Space Shuttle is further forward than yours is, and that's why it works. The fuselage has such a large pitching moment because most of the weight of the fuselage is at the back while most of the surface area is near the front, which means that the slight amount of lift created by the forward fuselage is more than enough to force it to pitch upwards. It's really not too far off reality, since even a uniformly dense fuselage tends to be unstable, especially at higher angles of attack.

[tech_op2000]

Well, I think I found the problem courtesy of NASA. The answer is that the actual shuttle has a similar problem .

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19940006365_1994006365.pdf

Refer to page 5 on Center of Pressure. They used a body flap that deflected downward at the back. I am testing this design now. And the most humorous part of the whole thing... they had problems too!