Official FAR Craft Repository

[capi3101]

Here is one of the variants with which I manage to reach Mach 2.7-2.9 before it becomes too hard t control it.

It is interesting that at the given speeds and altitude it shows all green(and I am using TAC Fuel Balancer in order to keep the CoM stationary during atmo flight), but in actual flight, with some

What were the conditions under which you took that simulation screenie (#14)? Speed and altitude.

In any event, here's the deal: you've got a plane that has got positive lateral static stability but negative lateral dynamic stability. You can read about this here; those "Oscillatory Dynamic Stability" graphs about a third of the way down the page is what FAR simulation study shows you. The actual values of the graphs don't matter nearly as much as the tendencies.

The closest equivalent to a "yaw dampener" in KSP? I hate to say it, but either a PID controlling mod, or (barring that) a SAS module as a last resort. The other option is to increase the vertical tail moment (put those Cessna Starship wingtips back on - completely ignore what I said before - but make them as thin as you can, and see what that does).

Edited June 30, 2015 by capi3101

[FourGreenFields]

The closest equivalent to a "yaw dampener" in KSP? I hate to say it, but either a PID controlling mod, or (barring that) a SAS module as a last resort. The other option is to increase the vertical tail moment (put those Cessna Starship wingtips back on - completely ignore what I said before - but make them as thin as you can, and see what that does).

So a yaw dampener doesnt work like setting a control surface to react to it's AoA? Mind to explain how they work (or send a link)?

EDIT: Thinking about it, I suppose they also take yaw rate and stuff like that into consideration. The AoA response still helps though.

Edited July 1, 2015 by FourGreenFields

[capi3101]

The Wikipedia article on them is pretty short though - I'll just copy and paste it in:

A yaw damper is a device used on many aircraft (usually jets and turboprops) to damp (reduce) the rolling and yawing oscillations known as the Dutch roll mode. It requires yaw rate sensors and a processor that provides a signal to an actuator connected to the rudder. The use of a yaw damper helps to provide a better ride for passengers, and on some aircraft the yaw damper is a required piece of equipment to ensure that the aircraft stability remains within certification values.

Here's an article on Dutch Roll, also from Wikipedia, and then the page I linked in my last post also talks a lot about the Dutch Roll mode in particular. That's if you're interested in more on that subject.

I guess I called it the wrong thing - its yaw damper, not yaw dampener. But it's basically a flight computer system that sends the necessary signals to the control surface to counteract unwanted oscillatory motion, which is what the SAS modules do in KSP (hence my prior recommendation). I've added SAS to my planes that have had static stability/dynamic instability, and can confirm that it does help matters (with a minimal amount of change to an overall design to boot).

[tetryds]

Yes, select rotation gizmo, click on the part you want, press F, turn on angle snap.

Then, click on any direction of the gizmo, it will snap to the nearby global snap angle.

Just move it around a bit and it will align perfectly with the SPH.

Now you can align anything, anywhere.

Good for landing gears

Fixed.

[smunisto]

I was wondering why it did not work, haha.

Anyways - I will have more screenshots for you aerodynamic-heads on friday.

With my latest design(the one I didn't post screens of), with a dihedral angle of 5 degrees and a second wing section, I manage going suborbital.

But by adding more fuel to be used for the rocket circularization burn, I lose quite a lot of speed and I am forced to start the rocket engine around Mach 2.5, at a maximum alt of 20km. This is because the B9 Turbofans start overheating by the time I reach that speed and altitude. Adding some stock radiators did not help.

Maybe I am imagining it, but giving the canards in front an inherent 5 degree dihedral and 5 degree pitch, made the craft accelerate much slower, even though the general wave drag area did not increase by more than 0.02

[capi3101]

I was wondering why it did not work, haha.

Anyways - I will have more screenshots for you aerodynamic-heads on friday.

With my latest design(the one I didn't post screens of), with a dihedral angle of 5 degrees and a second wing section, I manage going suborbital.

But by adding more fuel to be used for the rocket circularization burn, I lose quite a lot of speed and I am forced to start the rocket engine around Mach 2.5, at a maximum alt of 20km. This is because the B9 Turbofans start overheating by the time I reach that speed and altitude. Adding some stock radiators did not help.

Maybe I am imagining it, but giving the canards in front an inherent 5 degree dihedral and 5 degree pitch, made the craft accelerate much slower, even though the general wave drag area did not increase by more than 0.02

Okay......hmm. I'll admit I'm not familiar with the B9 engine parts. What can you tell me about the rocket engine you're using? Specifically -

*What's the takeoff mass of the plane?

*How much of that mass is dedicated to rocket fuel?

*What is the vacuum thrust of your rockets?

*What is the vacuum Isp of your rockets?

And while we're at it, what does a typical ascent profile look like for you?

[Wanderfound]

I lose quite a lot of speed and I am forced to start the rocket engine around Mach 2.5, at a maximum alt of 20km. This is because the B9 Turbofans start overheating by the time I reach that speed and altitude. Adding some stock radiators did not help.

Any reason why you aren't just using stock turbojets? They'll manage a lot better than Mach 2.5.

[smunisto]

Okay......hmm. I'll admit I'm not familiar with the B9 engine parts. What can you tell me about the rocket engine you're using? Specifically -

*What's the takeoff mass of the plane?

*How much of that mass is dedicated to rocket fuel?

*What is the vacuum thrust of your rockets?

*What is the vacuum Isp of your rockets?

And while we're at it, what does a typical ascent profile look like for you?

To start off with - I think something is wrong with the calculations and I have made so many changes to my design in the past weeks, that I don't know what to believe. After each part change I make, I am afraid the numbers FAR gives are all broken.

For example these 2 shots:

https://bg3.biz/cloud/index.php/s/l1mP6fVCNPCGC4f - after moving some parts around in the editor.

and after a scene reload:

https://bg3.biz/cloud/index.php/s/v4U8NGE3wrPJbSn

What is the safest way to calculate? Right after a scene reload or without exiting SPH? I think the CoL indicator is all wonky too.

I have no idea which screens are credible anymore, since the numbers seem to be different after each scene reload. I may have to ask Ferram to check it out.

With regard to the other questions:

*What's the takeoff mass of the plane?

50 t.

*How much of that mass is dedicated to rocket fuel?

Well, LF + O's total mass is 28 929 kg. (These can be seen in the shots)

*What is the vacuum thrust of your rockets?

Vacuum thrust of the rocket engine(Kiwi from NecroBones' SpaceY Heavy Lifter pack) - 425

*What is the vacuum Isp of your rockets?

295. The only other engines I have unlocked are the stock Swivel and Reliant ones. They don't have enough oomph for that craft.

Edited July 3, 2015 by smunisto

[kcs123]

Before/after reloading scene, have you tried to toggle gears ?

Possible issue what first pop up on my mind is that you have toggled gears trough FAR.

More info is needed from your side, as it not seen clearly from pictures if on both pictures gears are up or not, but Mach 1 cross section drag area is certainly different for some reason on first and second picture.

2.023 vs 1.68 m2

My thoughts about it is that you toggled gears trough FAR graph instead trough right click menu - FAR shows proper cross section value, but craft is not saved as in raised gear state - possible bug.

Or you have done something opposite. So, pay attention what have you did on first screenshot and what on second.

For example:

1. created craft and saved
   
2. raised landing gears - trough FAR GUI or by right click on parts
   
3. making screenshot
   
4. test flight on runway
   
5. reverting back to SPH
   
6. make screenshot - craft have different drag cross section area at mach 1
   
7. reloaded saved craft - same or different cross section value ?
   

It is all on you if you can describe reproducing steps accurate as possible.

[smunisto]

1. I close the gears - Baha's Adjustable ones btw - via the right click menu(the FAR toggle button does not work with them).

2. I check WDA.

3. Change scene - going back to Space Center, instead of launching a test flight.

4. Going back to SPH from the Space Center screen - craft loads with gear up.

5. I check WDA - it's a lower value.

[gilflo]

Mind to show the cross section area + cross section curves with raised gear? The way it looks with dropped gear, I wouldn't exactly call it a low-drag design.

Maybe be you're right, but anyway this very long range cargo is able to deliver a 6t satellite around Minmus or Mun, then land on Minmus and Mun and flyback to Kerbin, without refuelling. You will fly through Mach 1 before 10000m and switch to rockets around 28-29 km around M 4.2

[Van Disaster]

While I wait for the opportunity to do a full mod install I've been playing with stock parts & old designs; this is a conversion of a 0.90 FAR challenge craft.

So, questions:

* What wave drag area vs max cross section ratio are we targetting?

* I'm also looking for tail designs for engine cluster installations; I've tested a different design which looks better aerodynamically, but the problem is it's very long & with current parts rather hard to shorten.

In this case there's a few too many engines to start spreading in embedded tank installations on wings, I think.

[kcs123]

I don't think that ratio between Max cross section and Mach 1 cross section area matters.

Based solely on game building craft experience, I think that more important factor is critical mach number. That shows speed when you expirience largest drag before going supersonic. Tricky thing abut it is that all engines behave different on different speeds, chosen engine for your craft must have enough thrust at given speed to breach trough highest drag speed to supersonic speeds.

Once you breach speed shown in critical mach number, drag becomes lower, but lift from wings too, so you must find sweet spot to get highest lift with low drag as much as possible to get highest L/D ratio.

As nuFAR is released, lots of people are focused on new cross section area feature and forgot about old still valid points in craft building.

L/D ratio is still of high importance for given craft and payload it needs to carry. With low weight payload it is better design with low drag area as much as possible.

While you don't have much lift from wings, you are still able to take off plane from runway due to high TWR ratio from engines that alows plane to accelerate quickly.

On high speed altitude such plane does not have much lift from wing, but it have much lower drag that gives pretty much high L/D ratio.

Wanderfounds Kerbodyne craft fleet are good example for such design aproach, although those designes don't offer much room for any pilot mistakes.

With KSP 1.0.x we also got some major changes with engines, overall ISP that engines could give is much lower than it was in KSP 0.90 and older releases.

Meaning, large craft that was capable to reach orbit in 0.90 does not have enough fuel to do the same in KSP 1.0.x

It is still possible to create large spaceplane, but due to lower ISP, you need to carry more fuel to counterpart this, so more fuel weight cuts off payload weight that you were able to carry in 0.90. Assuming you are using same type and number of engines as in 0.90.

Adds area ruling on top of that and you got quite a chalange to design craft that is capable to carry as much as payload as possible, still be able to take off from runway and reach low Kerbin orbit. SSTO planes that I used to create with high wingspan are still possible, but there is need to find sweet spot between needed fuel, number of engines/air intakes, payload weight that you want to deliver in orbit and be carefull that craft is not melted down due to heat before reaching orbit.

[FourGreenFields]

I don't think that ratio between Max cross section and Mach 1 cross section area matters.

Based solely on game building craft experience, I think that more important factor is critical mach number. That shows speed when you expirience largest drag before going supersonic.

Thing is that you have to go supersonic to reach orbit. Critical Mach number is, afaIk, of little importance at Mach 3...

* What wave drag area vs max cross section ratio are we targetting?

* I'm also looking for tail designs for engine cluster installations; I've tested a different design which looks better aerodynamically, but the problem is it's very long & with current parts rather hard to shorten.

From my recent experience building high-speed planes, you won't target Mach 1 wave drag, but Mach 3-5 (depending on your design, and what it can handle) drag at 20-25 km.

If I am not mistaken, that means you'll need to shift your max cross section area tailwards, and keep wing span low (more so near nose) so your wing tip doesn't generate extra drag (might be wrong on both, still need to experiment).

Use the "scary looking numbers" to calculate Cd and Cl at 20km, Mach 3, 4 and maybe around 4.5. That is what you're headed for I'd say.

EDIT: In my previous designs, many of the engines weren't on the tail, but on the side of the fuselage, often with gimbal limit to prevent exhaust gasses from damaging stuff. Might slightly change how I build with the added experience of high-speed planes though.

EDIT2: Thinking about it, I guess drag is more importand at about 15-20 km, instead of 20-25 km, as you'll want to level out and gain speed, before pulling up, there (at least I suppose you'd want that, never did any calculations on delta-V or anything).

Edited July 5, 2015 by FourGreenFields

[Van Disaster]

From my recent experience building high-speed planes, you won't target Mach 1 wave drag, but Mach 3-5 (depending on your design, and what it can handle) drag at 20-25 km.

If I am not mistaken, that means you'll need to shift your max cross section area tailwards, and keep wing span low (more so near nose) so your wing tip doesn't generate extra drag (might be wrong on both, still need to experiment).

Theoretically that's what we've always needed to do in FAR which has had a shock cone; I've just never noticed any effect before now. That's also somewhat important for stability anyway. Pitching the plane in the hangar to whatever AoA you're predicting at speed to check where the highest drag areas are is also a good thing.

Critical mach is where wave drag starts having an effect, if you're going to Mach 5 then it's not a big deal - you don't stop having wave drag at anywhere faster anyway ( and it shows on the sweep Mach curve pretty visibly ). If you're building a subsonic aircraft then it is a very big deal.

EDIT: In my previous designs, many of the engines weren't on the tail, but on the side of the fuselage, often with gimbal limit to prevent exhaust gasses from damaging stuff. Might slightly change how I build with the added experience of high-speed planes though.

Ditto, although previously I've almost exclusively used B9 engines - the larger SABREs were the equivalent of 3-4 rapiers, so packaging was much easier. Trying to find a slick packaging solution with stock parts that lets me pull up off the runway is being awkward.

EDIT2: Thinking about it, I guess drag is more importand at about 15-20 km, instead of 20-25 km, as you'll want to level out and gain speed, before pulling up, there (at least I suppose you'd want that, never did any calculations on delta-V or anything).

Rapiers degrade badly above 20km, but you can't really burn too fast below that because of heating issues anyway, so I have a narrow window at present of about 19-21km where I pick up most of my speed - which frankly is not far off my old 21-25km profile anyway, just a little more compressed. Mostly having enough wing not to have to pull high AoA at altitude is still one major factor in design, there's just this new wrinkle to consider and I've no idea of the point to stop bothering reducing it.

[Wanderfound]

Critical mach isn't when you stop experiencing transonic drag; it's when it begins. Airflow speed varies around the craft, with blunt surfaces forcing air to accelerate into the high-drag transonic range before the ship itself gets there. Critical mach is when this first happens anywhere on the airframe. As the ship continues to accelerate, different areas of the airframe will reach and then exceed transonic airflow.

Drag usually continues to build up until Mach 1.

Where it comes into play is with long-range or heavy lift SSTOs. Both of those are generally running with the bare minimum of jets necessary to get up to speed. Low-altitude speedsters also benefit from reduced drag.

For engine clusters, underwing works for me:

A quad-adaptor on the back works well if you don't mind losing some ground clearance. Offsetting wing position vertically helps in controlling thrust torque. A bit of pitch-up from turbojets is no bad thing; it takes the load off your elevators.

I've yet to see anything that needs more than eight jets to fly, and if you're carrying nukes, two is plenty.

Edited July 5, 2015 by Wanderfound

[kcs123]

That was exactly what I was try to say in previous post, critical mach is much more important for planes designed for atmosperic fligths with minimum amount of craft parts/engines and weight, not for SSTO planes with speeds of 3 mach or more.

I agree, for SSTO planes critical speeds are near mach 3 at 19-23 km, and you should aim for craft design that is stable to fly at 5-10 AoA near 20 km altitude at 3 mach.

Depending on craft weight you need enough wingspan to have stability at those speeds/altitudes and low enough to minimize drag.

How much of lifting area you need strongly depends on craft weight or payload that you try to put in orbit.

Point is, wave drag area ruling is important, it helps you to minimize overall drag of your plane, it helps you to find flaws in your design, so you can improve it.

But it is not of ultimate importance to have certain overall cross section area vs mach one wave drag cross section ratio, or have certain critical mach number.

For space planes more important is L/D ratio and stability for 3 mach at 19-23 km and 4 mach or more at 30 km or above.

When you fly over 30 km aerodynamics becomes less important and aligment of CoM with engine thrust and gimbal capabilites becomes more important to overcome backfliping and similar design issues.

On some design, if you have enough engine thrust and enough fuel you can overcome those issues more easy. Creating craft with underpowered engines with minimum amount of fuel and stable on critical flight areas: take off/landing, stability on high altitude/low speed, decent L/D ratio to maximize efficieny that is quite a chalange that gives lot of satisfaction when you create it.

What I wanted to say, people are focused on area ruling too much, forgeting other things that was important in previous KSP/FAR releases and are still valid today.

[Van Disaster]

A quad-adaptor on the back works well if you don't mind losing some ground clearance. Offsetting wing position vertically helps in controlling thrust torque. A bit of pitch-up from turbojets is no bad thing; it takes the load off your elevators.

I've yet to see anything that needs more than eight jets to fly, and if you're carrying nukes, two is plenty.

This rebuild/adaptation I'm working on carried four nukes just for some decent TWR - given how heavy it is in it's previous life the rather small percentage of extra mass more than made up for the rather large increase in dV cost for long burns. Engines have changed a lot since then, haven't got round to seeing what's best there yet. This actually seems a reasonably slick shape but it definitely seems to like all 7 rapiers, so I don't know if that's the shape - currently at 20km/1200m/s, L/D of 2.5/absolute drag about 400kN - or just how it is in the current state of things.

I've tried Mk3->Mk2 converter and adding extra engines which gave the best aero result so far, but that has major clearance issues. The quad converter didn't really seem to add anything over just sticking engines to the bulkhead of a Mk3 piece...

[Wanderfound]

Speaking of overcoming drag without area ruling:

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Craft file at https://www.dropbox.com/s/xnz42bj8w03q3vk/Kerbodyne%20Scihop.craft?dl=0

[bartekkru99]

Provide some more info about your craft in Official FAR craft repository thread. Screenshots of your plane in SPH with FAR graphs can be of great help to figure out flaws of your design. That thread is created to help with designes and show off of sucessfull crafts. That way in this thread is more easy to track down possible bugs/solutions.

And there it is...

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[Wanderfound]

I've tried Mk3->Mk2 converter and adding extra engines which gave the best aero result so far, but that has major clearance issues. The quad converter didn't really seem to add anything over just sticking engines to the bulkhead of a Mk3 piece...

Considered doing it as a three-wide fuselage job? Less gooseneck, more of a blended-body delta design. Move four of the cabins from the neck to where the engines are now, put engines on the back and intakes on the front.

[kcs123]

And there it is...

http://imgur.com/a/sZ1jx

Check out AoA sweep graph again, but this time put "1" for pitch setting value.

Based on provided AoA sweep graph, you plane will going to stall at 15 degree of AoA. You will most probably need to adjust control surfaces i a way that yelow line does not cross x axis at higher AoA angle than 15 degree.

It does not have always be 15 degree, it depends from one craft design to other, but point is that you most probably want to set controls in a way that when you picth with 100% of authority, point where yelow line cross x axis should not be higher than point where lift/drag lines starts to split.

Your craft will be less maneuverable in that way, but much more stable and easier to control, because you will less likely fall in stall.

Other way to overcome this problem is to be careful in piloting, so you do not pull controls too hard when you fly at low speeds, only to 70% of maximum available pitching.

It is hard to do it with keyboard control, so i more often use limiting pitching control in my craft designs to overcome this issue.

Another thing that may help, is dynamic deflection mod. I just didn't have time to fully test it and if it is helpfull at all, I just come across with it. But I'm looking for something like this for quite a while.

[tetryds]

Check out AoA sweep graph again, but this time put "1" for pitch setting value.

Based on provided AoA sweep graph, you plane will going to stall at 15 degree of AoA. You will most probably need to adjust control surfaces i a way that yelow line does not cross x axis at higher AoA angle than 15 degree.

This is false, at that speed his craft will stall at around 27 degrees of AoA, from the graph you can also see that the chines stall first, generating extra drag on the front which decreases stability a bit.

@bartekkru99: That front stalling is a bad behavior, you don't want to stall non-active control surfaces at high AoA and low speeds at the front, you may want to re-position those chines and maybe add deflecting leading edges to them.

What is being experienced with instability may also be related to uneven flexing, FAR does not touch joints and the stock joints are a bit borked.

With that many wing parts, I recommend adding struts to the critical points as well as small wing parts further away from the fuselage, which you did right, just try to hide them as they are accounted for aerodynamics.

You can do that by simply moving them around with the gizmos, they continue to be connected to the part you clicked, just show up on a different place.

So, fuselage flexing is usually not a very big deal when it's a single core, but yours is not, you should be careful.

Even though they won't really flex much, the wings connected to them get an amplified effect from that, you will want to sew the two engine fuselages together, a few struts connecting them will be enough.

For wave drag area, your only issues are the rear and intakes, that could be easily fixed with anti-shock bodies and re-positioning of the horizontal stabilizers, move them forward so they end exactly when the engine ends.

Edited July 5, 2015 by tetryds

[kcs123]

This is false, at that speed his craft will stall at around 27 degrees of AoA, from the graph you can also see that the chines stall first, generating extra drag on the front which decreases stability a bit.

OK, I might be wrong, I have always think that safe AoA before stall is before FAR graph lines is start to split.

On his graph they started to split at 17 degree and "safe" AoA should be below that, around 15 degree.

I'm not saying that you are wrong, but how you come up to conclusion that stall AoA is exactly 27 degree ?

[FourGreenFields]

Pretty sure that oldFAR's explanation of the graphs said that when you see the graphs seperate there is a stall involved somewhere. Can't check though, sadly.