[WIP] R.E.L Skylon C2. Alpha Released. FAR config broken. (08 Dec 2014)

[nli2work]

I'm using FSAnimateGeneric, but the nose gear is still using stock modules, so I'll switch to FS.

Also the invisible meshes I tried to use for nodes are casting shadows, so they have to go.

KineTech clash only matters if you have another animation module on the same part (engine); gears and other things shouldn't be a problem.

How are you doing the invisible meshes? transparent texture? tried unchecking "Cast Shadow" and "Receive Shadow" in the mesh renderer component... maybe export without mesh renderer component ?

[CaptainKipard]

How are you doing the invisible meshes? transparent texture?

Yes

tried unchecking "Cast Shadow" and "Receive Shadow" in the mesh renderer component... maybe export without mesh renderer component ?

Yep, I should have noticed that.

Although since the meshes aren't working as I hoped, I'll probably just create a "Nacelle" part that's a wing and precooler in one. I'll wait and see what lo-fi is able to dig up.

Edited November 1, 2014 by Cpt. Kipard

[Guest]

I'm not too fond of that idea due to potential FAR issues, though. It'd be great if you managed to solve it.

[CaptainKipard]

I'm not too fond of that idea due to potential FAR issues, though. It'd be great if you managed to solve it.

Can you be more specific?

[Master Tao]

FAR handles wings in a completely different way than other parts, so merging the two parts will make it impossible for FAR to simulate either the wing as a wing or the nacelle as a non-wing part.

[nli2work]

I think Dragon01 is referring to the invisible meshes you use to reinforce joints, they would be perpendicular to airflow and potentially creating massive drag (similar to the folding MW array from Interstellar generated massive drag unless in a fairing). Wing/Nacelle combined part probably isn't too big a problem since you will need to specify explicit FARDragModel values for it.

[CaptainKipard]

Good news everyone!

After a coincidental change in mass of the parts I remembered (I'm not sure if correctly) that KJR also changes stiffness based on the part mass. Initial tests of the wings and precooler were a success. No wobble at all.

I can safely have separate wings and precoolers now.

[Virtualgenius]

Why are we configuring for FAR when a completely different aerodynamic model will be introduced in the Beta

[Ikaneko]

Why are we configuring for FAR when a completely different aerodynamic model will be introduced in the Beta

Because the planned stock aerodynamics will most probably not be FAR standard.

[Virtualgenius]

Thats my point many people dont use FAR or even want to, it should be configured for stock aerodynamics whatever they maybe with a MM config supplied for FAR, having a dependency on a certain aerodynamics mod is not ideal it turns a lot of people off using that particular mod.

[CaptainKipard]

I'm fine with either way. The reason FAR was mentioned though has little to do with configs, but the meshes which, as has been pointed out, will create unnecessary drag. Anyway, that problem is history now.

[CaptainKipard]

This is now on hold until someone explains to me why my stabiliser and rudder aren't working properly.

Edit

Problem solved. I'm coming back to this.

Edited November 12, 2014 by Cpt. Kipard

[CaptainKipard]

All the parts are now in the game. The next task is configuring eeeeeverything. Here are some current troubles I'm having. They are all related. I'm configuring it for stock aerodynamics first.

1. I managed to get the plane flying once, however as soon as I added a proportional amount of fuel and oxidiser (based on the values and dimensions of the largest tank in the game) The joints again started to wobble and/or slouch a bit, and the plane would not take off. The wobbling gear also makes the aircraft swerve a bit during acceleration.

2. The deflectionLiftCoeff values are all based on Spaceplane+ values, scaled up according to surface area. Not enough to lift the plane.

3. The engine thrust values are already buffed by 1.5 (from their real-world values), and I'm only reaching about 140m/s by the end of the runway in rocket mode. The actual Skylon takeoff speed is 0.5 Mach (171.5 m/s) in air-breathing mode.

4. The engines in air-breathing mode are being starved of IntakeAir after only a second or two of functioning.

Edited November 14, 2014 by Cpt. Kipard

[White Owl]

I've avoided posting here so far, simply because I'm not a modder and don't know enough to help you solve these problems. But I just gotta say damn that thing is gorgeous! This is one of the very very few times I'm so interested in using a complete prebuilt vehicle mod instead of designing my own.

You say it won't take off at 140 m/s. Will it at least lift the nosewheel?

I wonder how long a runway the real bird is supposed to use. KSC's runway seems pretty short for something Skylon's size.

Edit: I know nothing about part modeling, or why things wobble, or why they don't. But I understand KSP-style aerodynamics and changing part configs to make it work. May I try to help?

Edit #2: The real SABRE's are going to burn liquid hydrogen, which is significantly lighter than stock liquidFuel. So your Skylon is probably way too heavy when full of fuel. I know you want it to work in stock aerodynamics... are you opposed to using a mod fuel?

Edited November 14, 2014 by White Owl

[blowfish]

There's definitely something weird about wheel friction in KSP - even many of my lift-oriented designs need TWR > 0.5 to take off. I'm using FAR too, so the soupiness of stock drag isn't an issue.

How much runway will the real-world skylon need to take off? The trajectory puts it somewhere between 2.5km and 4km, but I can't tell for sure.

[Haze-Zero]

All the parts are now in the game. The next task is configuring eeeeeverything. Here are some current troubles I'm having. They are all related. I'm configuring it for stock aerodynamics first.

1. I managed to get the plane flying once, however as soon as I added a proportional amount of fuel and oxidiser (based on the values and dimensions of the largest tank in the game) The joints again started to wobble and/or slouch a bit, and the plane would not take off. The wobbling gear also makes the aircraft swerve a bit during acceleration.

2. The deflectionLiftCoeff values are all based on Spaceplane+ values, scaled up according to surface area. Not enough to lift the plane.

3. The engine thrust values are already buffed by 1.5 (from their real-world values), and I'm only reaching about 140m/s by the end of the runway in rocket mode. The actual Skylon takeoff speed is 0.5 Mach (171.5 m/s) in air-breathing mode.

4. The engines in air-breathing mode are being starved of IntakeAir after only a second or two of functioning.

http://i.imgur.com/Xg4Idyr.png

As for joint wobble, trying adding these lines to your cfg file. They typically go beneath the 'crashTolerance = X' line. If 50 is not enough, go ahead and raise it a bit. The higher the value, the tighter the parts should be.

breakingForce = 50

breakingTorque = 50

As for landing gear, I am not sure what to tell you. A while back there was the temporary 'B9-fix' which gave the landing gear that gave something in the cfg files for them a super high stat to keep them nice and stiff until there was an official fix. Finding this old fix and seeing what stats may help your landing gear perhaps?

I suspect that the Skylon employs a lifting body design, so you may need to add lift to the body as well as the wings. I also suspect that the wings on the skylon have a high camber, meaning that they are designed to provide a lot of lift, more so than the wings that Space-plane Plus values would allow. I would also suspect that the angle of attack on those wings is quite high as well.

Regardless, I think that the flight profile in mind for the Skylon is more like rocket ascent than the normal SSTO ascents we use in KSP. This is all just speculation on my part anyways, I could very well be wrong.

Edited November 14, 2014 by Haze-Zero

[CaptainKipard]

I've avoided posting here so far, simply because I'm not a modder and don't know enough to help you solve these problems. But I just gotta say damn that thing is gorgeous! This is one of the very very few times I'm so interested in using a complete prebuilt vehicle mod instead of designing my own.

Thanks that means a lot coming from a veteran.

You say it won't take off at 140 m/s. Will it at least lift the nosewheel?

Like a blonde teenager... It can't even.

Edit: I know nothing about part modeling, or why things wobble, or why they don't. But I understand KSP-style aerodynamics and changing part configs to make it work. May I try to help?

Please. There's not even a need to ask.

Edit #2: The real SABRE's are going to burn liquid hydrogen, which is significantly lighter than stock liquidFuel. So your Skylon is probably way too heavy when full of fuel. I know you want it to work in stock aerodynamics... are you opposed to using a mod fuel?

Aaaah! Yes! This could be it. I'll have to look into the Open Resource System. I did plan on using authentic resources, it just didn't occur to me to do it from the beginning.

I'm only configuring it for stock at first. FAR and DRE compatibility will come later for sure. I don't ever play without those two mods.

There's definitely something weird about wheel friction in KSP - even many of my lift-oriented designs need TWR > 0.5 to take off. I'm using FAR too, so the soupiness of stock drag isn't an issue.

At the moment my T/W ratios are ok for both modes. ~0.7 for jet and ~2 for rocket.

How much runway will the real-world skylon need to take off? The trajectory puts it somewhere between 2.5km and 4km, but I can't tell for sure.

I wonder how long a runway the real bird is supposed to use. KSC's runway seems pretty short for something Skylon's size.

This huge thread on Nasa Spaceflight has been my go-to place for info. A quick search gives various numbers somwhere between 3km and 5km.

As for joint wobble, trying adding these lines to your cfg file. They typically go beneath the 'crashTolerance = X' line. If 50 is not enough, go ahead and raise it a bit. The higher the value, the tighter the parts should be.

breakingForce = 50

breakingTorque = 50

I'll try raising them.

As for landing gear, I am not sure what to tell you. A while back there was the temporary 'B9-fix' which gave the landing gear that gave something in the cfg files for them a super high stat to keep them nice and stiff until there was an official fix. Finding this old fix and seeing what stats may help your landing gear perhaps?

I've never heard of it, and I don't know how to look for it.

I suspect that the Skylon employs a lifting body design, so you may need to add lift to the body as well as the wings.

I very much doubt it. The fuselage is pill-ish-shaped, with a circular cross section.

I also suspect that the wings on the skylon have a high camber, meaning that they are designed to provide a lot of lift, more so than the wings that Space-plane Plus values would allow.

Actually I tried looking for some wing cross sections when I was making my initial preparations, and came up empty, but at least this gives me an excuse to increase the number.

The images in this paper show no asymmetry at all, so I don't know

I would also suspect that the angle of attack on those wings is quite high as well.

Again I haven't been able to find official numbers. One image I found shows an angle of about 1.5 degrees, but I used a different image for my backgrounds when modelling and that resulted in an angle of only 0.5 degrees. I might have to cheat a little by rotating the part origin.

Regardless, I think that the flight profile in mind for the Skylon is more like rocket ascent than the normal SSTO ascents we use in KSP. This is all just speculation on my part anyways, I could very well be wrong.

I wrote a rough description of this here.

[White Owl]

In the real world, even a perfectly round fuselage produces some lift. It just needs to meet the air at the right angle of attack. I'd say there's no cheating at all to give your fuselage sections a lift factor. Being larger but rounder than the stock/SPP fuselage pieces, maybe it would be reasonable to start with the same numbers?

MODULE
{
    name = ModuleLiftingSurface

    deflectionLiftCoeff = 0.6
    dragAtMaxAoA = 0.3
    dragAtMinAoA = 0.1
}

Whether you go with that solution or not... not getting the nosewheel off the ground points to either the CoM too far forward of the landing gear, not enough lift from the canards, or both. Tweak the CoM by adjusting fuel and oxidizer balance since in this case we don't want to move the landing gear.

[MainSailor]

As for landing gear, I am not sure what to tell you. A while back there was the temporary 'B9-fix' which gave the landing gear that gave something in the cfg files for them a super high stat to keep them nice and stiff until there was an official fix. Finding this old fix and seeing what stats may help your landing gear perhaps?

I've never heard of it, and I don't know how to look for it.

I think he's referring to the community fix made under .23 and .24 that changed the landing gear sidewaysStiffness (actually IIRC, reduced it) that greatly improved the B9 gear.

As to whether it still applies under .25 I have no idea. I think the behavior of landing gear under .25 hasn't been an issue.

This is the MM script I had in my backup directory:

@PART[B9_*_Landing_Gear_*]
{
  @mass = 0.5

  @MODULE[FSwheel]
  {
      @sidewaysStiffness = 0.01
  }
}

EDIT: Looking back also I think the mass was increased to help the old connection node system operate that determined node strength based on mass rather than (if I understand correctly) node size which is used now? Don't quote me on this, I'm not an expert.

[nli2work]

Looks great!

LiquidHydrogen is in CRS, ~half density of LiquidFuel. Seems like it should be lighter? Assuming Squad's LiquidFuel is kerosene. Looking at Squad's resource definitions, IntakeAir has same density as LiquidFuel, bizarre. it's not much hassle to bundle, just a single resource definition file.

the B9 landing wheel fix was to deal with Unity's wheelCollider, sideways slip is friction of the wheel when moving in a direction that's not in the same plane as the radius ring. at normal settings it's incredibly high so any deviation side to side at more the ~50m/s causes the wheel to "snag" on the ground, this was aggravated as vehicle mass increases. the B9 fix lowered it to basically nothing. the B9 "fix" is probably necessary until Unity changes their wheelCollider code to handle higher speeds better. If you use stock wheel module you have no way of modifying it through config, and have to set the values before export from Unity.

I set my breakingForce and breakingTorque between 20k to 30k. Surface attach nodes still require some struts.

CKAN is released, might consider adding integration. it should make it easier to keep everything updated.

Edited November 14, 2014 by nli2work

[Tcy16]

I do think the body has some form of lifting effect. Especially when the plane(rocket) is in "cruise mode" in the atmosfere.

I have about the same knowledge of aerodynamics and physics as you (Uni level), but it has been explained to me that even symetrical airfoils (or bodies, of course) will generate lift if you rotate them, giving them an angle of effect against the incoming wing.

So please reconsider if you cant figure it out with other aproaches.

[K.Yeon]

I had really similar problems just a while ago, i recommend test part by part, such make a fuselage using stock mk23pod and 3 fuelTankX200-32, then put skylon landing gear on sides of it, use hyperedit to give it something like 10 or 20 up velocity give it a 'drop test' see if it survives, if the stock parts just breaks then theres problem with your gears because i think gear that size should survive 35. And if you base your fuselage config from the fuelTankX200-32, there should be no problem.

If the problem is the gears, try make wheel collider larger and 'enhanced' wheel collider thinner and smaller

Hope this helps!

[White Owl]

Looking at this beast, it seems clear that keeping CoL behind CoM, but not too far, will be a pretty delicate balancing act. Obviously the CoM at takeoff needs to be just in front of the landing gear so it can rotate. And that also puts the CoM in the middle of the cargobay, so payload doesn't change the balance. But the Skylon has a whole lot of wing forward of that landing gear too, not to mention the canards waaay up in front just to make things difficult.

The official manual mentions the difficulties in balancing payload mass and aerodynamic pressure. Says it will work if payloads attach at the forward part of the cargobay, so more weight is forward. And of course all that heavy LOX is centered around there too.

I bet this bird takes off with the CoM only barely ahead of the CoL. I bet it burns fuel from the rear tanks first, to shift the CoM forward in flight. At supersonic speeds the lift will move backwards, so that'll help. Of course, not a factor in stock aerodynamics. And moving mass forward will also greatly help keep the thrust vector pointed at the CoM, so those offset engines aren't as much of an issue.

I think I'll put together a Procedural Wings and Procedural Parts mockup in stock aerodynamics. Play around with fuel balance and whatnot.

If you get the CoM farther ahead, then maybe you can make the delta wing ailerons into elevons. That might help.

[Akira_R]

Even a nice round fuselage is going to have a significant amount of lift, especially in the areas where the wings and front canards meat the fuselage, and given the way the engine nacelles are curved I would also suspect that it is expected to fly with a fairly significant AoA, which would increase the lifting effect of the body, so i would indeed suggest adding some lift to the body.

The way the engines are angled down slightly plus the steep upward rake of the tail section that starts immediately after the landing gear leads me to believe that for take off it should rotate around the rear gear, resulting in a fairly large angle of attack which should add quite a bit of lift and would make the engines point almost directly down also helping to lift it off the runway, so I think getting the center of mass almost directly over the rear gear to be fairly important.

Based off the large canards on the front, the lack of any horizontal stabilizers on the rear and the angle of the engines I also suspect that on takeoff and through the early stages of flight the center of lift would sit in front of the center of mass, this would help to lift the nose up at take off, the angle of the engines and their location slightly behind the suspected CoM would help to counter act the instability this would cause. The engines would have to have a fair amount of gimbal in order to counteract the rotation they would cause in rocket mode once in the upper atmosphere. As it burns fuel I would suspect the CoM would then move farther forward bringing it inline with or just in front of the CoL as it would need to be the most aerodynamically stable during reentry. Is it intended to make a powered landing or glide in? If it is intended to make a powered landing then this CoM shift could cause problems due to the angled center of thrust, of course this could be solved with engine gimbaling or one technique they may employ in the real world is fuel transfer, pumping fuel around to make the desired changes in the CoM, if that is the case that can't really be solved through part cfgs. If it is intended to glide in then the forward shift in CoM would be desirable because of increased aerodynamic stability, although I kind of doubt this is the case as the shape doesn't look like a very effective glider plus given my earlier postulations of getting a significant amount of lift from the body and using a high AoA this would not be good for a glider as a high AoA would also mean a high coefficient of drag.

Of course I am pulling 90% of this out of my rear, I really haven't done any reading on the actual specifics and flight characteristics of the actual Skylon, and while I study aerodynamics and aerospace engineering in my spare time I am by no means super knowledgeable in these things, I simply have a basic working knowledge of these and am pretty confident in my grasp of physics, but given the specific design choices I highlighted (curved engine nacelles, upward slope of tail section, canards on the nose and lack of horizontal stabilizers in the rear) and the assumption those design choices were made for a specific reason, I feel as though I'm not too far off base. Of course i could be off in left field somewhere banging my head on a wall.

You also have to understand that given the manner in which KSP simulates aerodynamic flight you may not be able to make this thing fly like the real one would. For example if you were to simulate an F22 in KSP it would be horrendously unstable and pretty much completely un-flyable, and in the real world it is a fairly unstable aircraft the only reason it is flyable is due to the significant amount of computer assistance the pilot get that keeps the plane flying, the same thing may be true in this case.

[Akira_R]

Looking at this beast, it seems clear that keeping CoL behind CoM, but not too far, will be a pretty delicate balancing act. Obviously the CoM at takeoff needs to be just in front of the landing gear so it can rotate. And that also puts the CoM in the middle of the cargobay, so payload doesn't change the balance. But the Skylon has a whole lot of wing forward of that landing gear too, not to mention the canards waaay up in front just to make things difficult.

The official manual mentions the difficulties in balancing payload mass and aerodynamic pressure. Says it will work if payloads attach at the forward part of the cargobay, so more weight is forward. And of course all that heavy LOX is centered around there too.

I bet this bird takes off with the CoM only barely ahead of the CoL. I bet it burns fuel from the rear tanks first, to shift the CoM forward in flight. At supersonic speeds the lift will move backwards, so that'll help. Of course, not a factor in stock aerodynamics. And moving mass forward will also greatly help keep the thrust vector pointed at the CoM, so those offset engines aren't as much of an issue.

I think I'll put together a Procedural Wings and Procedural Parts mockup in stock aerodynamics. Play around with fuel balance and whatnot.

If you get the CoM farther ahead, then maybe you can make the delta wing ailerons into elevons. That might help.

Looks like we had some similar ideas on the flight dynamics of this thing lol

Edited November 14, 2014 by Akira_R