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

[Nobody_1707]

If I post my craft file would someone please tell me why my rocket can't gravity turn below~25k without starting a slow, but unstoppable, pitch down towards Kerbin?

[Taverius]

Sure. Post away

[ferram4]

@Doomydoom: The first problem is that FAR doesn't actually have the physics implemented to properly simulate a waverider, so it won't behave exactly the way you think it should. The second problem is that it looks like you'll have a very large positive zero-lift pitching moment; basically, the angled up wings near the front of the vehicle are producing enough lift to overpower anything else you do. Solutions include an up-angled wing part near the tail, more control authority, modifying the angle of the engine exhaust so you have more control, angling the front down a little bit and shifting the CoM forward.

@seagull42: Here's a not-quite exhaustive list of what things do; many of the help windows available through the GUI should answer these as well and have more information if you want it:

Static Analysis:

• Cl: Lift coefficient; the lift of a vehicle after removing the effects of wing surface area and dynamic pressure; larger means more lift.
  
• Cd: Drag coefficient; the drag of a vehicle non-dimensionalized in the same way as lift coefficient; larger means more drag.
  
• L/D: Lift to drag ratio; measures how efficient the plane lifts; larger corresponds to a more efficient vehicle.
  
• Cm: Moment coefficient; the pitching moment (torque) of the vehicle non-dimensionalized in the same way as lift coefficient and drag coefficient; positive indicates pitch-up; must have a negative slope wrt angle of attack for the vehicle to be stable; elevators increase and decrease this to control the lift of the vehicle.
  

Stability Derivatives:

These are used to describe the motion of the vehicle in response to a disturbance at any flight condition; basically, if you have a plane flying along, and then a gust hits it, these numbers will describe how it responds. The numbers themselves matter less than the sign of the numbers, which is indicated by a tooltip that appears when you mouse-over the number. You'll probably find more useful information here: http://en.wikipedia.org/wiki/Flight_dynamics_%28fixed-wing_aircraft%29

Simulation:

This takes the stability derivatives calculated in the previous tab and lets you use them to simulate the vehicle's response to a disturbance; basically, you can see what it does if it is angled 2 degrees to the left in flight. It'll let you see most of the motions described in the help section for this tab, such as:

• Longitudinal Short-Period: Fast oscillations of angle of attack and pitch angle; basically what happens when you tap and release the elevator quickly.
  
• Longitudinal Phugoid: Slow oscillations of forward velocity and pitch angle; the plane constantly exchanges altitude for velocity and vice-versa.
  
• Lateral Roll Damping: Highly-damped non-oscillatory rolling motion; it just describes how the plane responds to aileron movement.
  
• Lateral Dutch Roll: Lightly-damped oscillations in yaw and roll; see what happens if you take a plane into the air and tap and release the rudder.
  
• Lateral Spiral: Lightly-damped or slightly unstable yaw and roll combination; the plane slowly goes off-course and starts to turn into a downward spiral; note: this is not a tailspin.
  

You can find more information using the Wikipedia page on Flight Dynamics; it's fairly well written and covers most of the basics. If you want more info, feel free to ask more specific questions; they're easier to answer and the answer can convey more information.

@toadicus: Excellent! I'll make sure that works and then implement it officially. The main reason the cfg gets written to is to save the position of the GUI windows, which I suppose doesn't have to be done so much.

@Nobody_1707: Without the picture yet, I can take a stab in the dark and say you need more control authority; so more gimballing engines and fins.

[Doomydoom]

Aww, is it possible to simulate the physics that allow waveriders and compression lift in KSP?

[ferram4]

Well... maybe, but I'd have to scrap all the current wing code and switch over to a more robust, but far, far, FAR more computationally intensive CFD simulation.

If you really want to know the main problem: you're asking me to simulate physics that's currently being researched in real-time on your computer; how do you think that's gonna work?

[Doomydoom]

The wiki page on WaveRiders mentions that they were using a greatly simplified 2D model of airflow around the aircraft in the 1950s. They didn't have **** for computers in the 50s, maybe ours can solve this simplified problem and come up with something that halfass works in KSP?

[Taverius]

Well... maybe, but I'd have to scrap all the current wing code and switch over to a more robust, but far, far, FAR more computationally intensive CFD simulation.

If you really want to know the main problem: you're asking me to simulate physics that's currently being researched in real-time on your computer; how do you think that's gonna work?

I cannot see the problem here ;D

Since we're asking for the moon, does FAR do any accounting for ground effect? I wanna make an Ekranoplane

[Subcidal]

Well during a tragic copy & paste incident i somehow managed to loose my original craft file that was acting odd. So i did my best to recreate it and It seems to be functioning as I would expect this time around. Though my CoM icon is in a totally different spot in a near copy of my original creation. I think I may have had a bug with the CoM indicator. Though the SR-71 is still a pain to fly, the craft I build are behaving more as I would expect them to now.

The wing placement on both my old and new recreation are damn near identical, Just my CoM icon changed and is a lot further back towards the CoL now. I'll keep messing around and see if I can't recreate the CoM being off like it was originally.

[seagull42]

@Doomydoom: The first problem is that FAR doesn't actually have the physics implemented to properly simulate a waverider, so it won't behave exactly the way you think it should. The second problem is that it looks like you'll have a very large positive zero-lift pitching moment; basically, the angled up wings near the front of the vehicle are producing enough lift to overpower anything else you do. Solutions include an up-angled wing part near the tail, more control authority, modifying the angle of the engine exhaust so you have more control, angling the front down a little bit and shifting the CoM forward.

@seagull42: Here's a not-quite exhaustive list of what things do; many of the help windows available through the GUI should answer these as well and have more information if you want it:

Static Analysis:

• Cl: Lift coefficient; the lift of a vehicle after removing the effects of wing surface area and dynamic pressure; larger means more lift.
  
• Cd: Drag coefficient; the drag of a vehicle non-dimensionalized in the same way as lift coefficient; larger means more drag.
  
• L/D: Lift to drag ratio; measures how efficient the plane lifts; larger corresponds to a more efficient vehicle.
  
• Cm: Moment coefficient; the pitching moment (torque) of the vehicle non-dimensionalized in the same way as lift coefficient and drag coefficient; positive indicates pitch-up; must have a negative slope wrt angle of attack for the vehicle to be stable; elevators increase and decrease this to control the lift of the vehicle.
  

Stability Derivatives:

These are used to describe the motion of the vehicle in response to a disturbance at any flight condition; basically, if you have a plane flying along, and then a gust hits it, these numbers will describe how it responds. The numbers themselves matter less than the sign of the numbers, which is indicated by a tooltip that appears when you mouse-over the number. You'll probably find more useful information here: http://en.wikipedia.org/wiki/Flight_dynamics_%28fixed-wing_aircraft%29

Simulation:

This takes the stability derivatives calculated in the previous tab and lets you use them to simulate the vehicle's response to a disturbance; basically, you can see what it does if it is angled 2 degrees to the left in flight. It'll let you see most of the motions described in the help section for this tab, such as:

• Longitudinal Short-Period: Fast oscillations of angle of attack and pitch angle; basically what happens when you tap and release the elevator quickly.
  
• Longitudinal Phugoid: Slow oscillations of forward velocity and pitch angle; the plane constantly exchanges altitude for velocity and vice-versa.
  
• Lateral Roll Damping: Highly-damped non-oscillatory rolling motion; it just describes how the plane responds to aileron movement.
  
• Lateral Dutch Roll: Lightly-damped oscillations in yaw and roll; see what happens if you take a plane into the air and tap and release the rudder.
  
• Lateral Spiral: Lightly-damped or slightly unstable yaw and roll combination; the plane slowly goes off-course and starts to turn into a downward spiral; note: this is not a tailspin.
  

You can find more information using the Wikipedia page on Flight Dynamics; it's fairly well written and covers most of the basics. If you want more info, feel free to ask more specific questions; they're easier to answer and the answer can convey more information.

Thanks, this is very helpful!

Also, I've used fairing factory (http://nathannifong.com/FairingFactory/) to cover up a landerprobe on my rocket and it didn't reduce the drag at all. Is there anything I could do so it works as it should?

[Nobody_1707]

My ship:

My Launch:

My Grav Turn:

My Unrecoverable Nosedive:

Here is the ship, and some more screen shots.

Doh, the staging is wrong. It didn't fire the main engine. Let me see if that helps... Nope, firing the LV-T45 just meant that I could do the whole process quicker.

[ferram4]

@Doomydoom: Yes, but they also knew the exact specifications of what the vehicle would be; I don't. I'd love to do an aerodynamic analysis of the entire vehicle rather than part-by-part, but that can be a lot more complicated and can end up being less accurate. Also, how do I detect when a vehicle is a waverider and when it isn't?

@Taverius: All my attempts to implement it have resulted in things either not working properly or no noticeable difference.

@seagull42: It should be working properly, unless the drag of the probe wasn't much to begin with. I can take a quick look into it.

@Nobody_1707: Your problem is that you're starting your gravity turn too soon and at too low a throttle. More throttle, start the turn when you're going 60 m/s, start the turn gently, don't use SAS, since it looks stable enough. You should be going much faster than you are in all of these shots (except the last one, which is closer to the right speed, but still low, direction aside). The gravity turn that it appears you're taking would work if you were running at a TWR > 2 the whole way up, but it would be scary and hard to control.

Only other thing I'd do is to combine the first core and second core stages into one stage so you can get further up before staging makes the rocket shorter, and thus, less stable.

[taniwha]

Nobody_1707: farram4's already given you great advice, but it looks to me like you're playing with stock aerodynamics based thinking: that just doesn't work with FAR. With that ship design, you should be able to go full throttle all the way with very little waste. The only reasons you would want to back off on the throttle are engines overheating, and going fast enough for heating to be an issue if you use deadly reentry.

[o_O]

Basically the difference between a rocket and a plastic airplane toy is stupid amounts of power. If your rocket wont stay pointed the right way it needs more power, or sometimes more gimbal/control.

[foamyesque]

Nobody: You're doing your grav turn way too hard. You should only be a couple degrees out from ninety max at that altitude, and only if you're climbing a lot faster than shown.

EDIT:

Also: I'm looking for a fairing that can enclose a hitchhiker module and attached rovers. Suggestions? KW would be my first pick, but there seems to be some confusion about a .20 compatible version.

[grom]

Doh, the staging is wrong. It didn't fire the main engine. Let me see if that helps... Nope, firing the LV-T45 just meant that I could do the whole process quicker.

Try changing deflection angle on those wing pieces to maximum of 3

[Senshi]

@Nobody: It also helps to attach your boosters parallel to your intended grav turn direction. By your pics, you want an equatorial orbit, so rotate the booster attachment 90° each. Allows for proper aerodynamics during your grav turn.

You shouldn't even need wings for this type of rocket, I deliver those kinds to orbit all the time without them. Just heed the advice of others: Getting fast very quickly after takeoff is key to getting a sweet ascent.

[White Owl]

Nobody: You're doing your grav turn way too hard. You should only be a couple degrees out from ninety max at that altitude, and only if you're climbing a lot faster than shown.

EDIT:

Also: I'm looking for a fairing that can enclose a hitchhiker module and attached rovers. Suggestions? KW would be my first pick, but there seems to be some confusion about a .20 compatible version.

KW works just fine with .20.2, and has well made payload fairings from 1.25m all the way up to 3.25m. It should work for you.

[foamyesque]

KW works just fine with .20.2, and has well made payload fairings from 1.25m all the way up to 3.25m. It should work for you.

Mm, I've tried it subsequent to my post. KW's stuff's too small. CORE has a 5m one, which is just barely big enough, but the awkward construction precludes stacking anything above it. :/

[Senshi]

If you cannot get your stuff either inside a fairing (seriously, the 3m expanded size of KWRocketry fits pretty much ANYTHING) and your payload is too asymetrical to be launched without fairings, you simply should think about splitting the payload and assembling it in space.

[Harpoon]

The jet engines in lates release are too powerfull. In .19 my 5 turbojet SSTO managed to reach 24km at 1600-1700m/s max. Now with the same altitude it can easily exceed 2k m/s.

I noticed that in prev version you nerfed down engines but I didn't found anything about them in the latest .rar file.

Cheers

[zzz]

Getting fast very quickly after takeoff is key to getting a sweet ascent.

I have opposite experience - if my Q reach 10k it's usually deadly spin. I'm look for G-meter and push up to Q= 6-8k and try do not rise it. And my turn is usually very smooth and high.

Maybe it's uneffective and wrong but it's work for me. With small simple rockets It's usually not need to worry about it but with complicated or big need to be very gentle. With REALY anti-aerodinamic payloads I begin to turn at a height of ~17K meters.

I use KW fairings and it's very helpful, maybe it's OP in some way but if I can put thing in fairing then rocket is ascent like a charm.

Edited May 31, 2013 by zzz

[Photonically]

Rockets do seem a bit overpowered with this mod - if I'm just launching a satellite or small ship it's hard to build a rocket which isn't an SSTO. I suppose that's unavoidable when realistic physics mesh with KSP's unrealistic scale, so I wondered if nerfing the atmospheric ISPs on rocket engines would help to compensate. I don't know anything about modding - would a plugin to do this automatically be trivial to make (in which case I might ask for it in Addon Requests) or would it be easier to just go through all my engine cfg's by hand?

[foamyesque]

If you cannot get your stuff either inside a fairing (seriously, the 3m expanded size of KWRocketry fits pretty much ANYTHING) and your payload is too asymetrical to be launched without fairings, you simply should think about splitting the payload and assembling it in space.

I need the payload, most crucially, to re-enter; I'm sending it to Eve and I want it all down in the same place at the same time. This is what it looks like:

Hence my need for a big fairing.

[Senshi]

Use Payload factory to create a custom fairing that fits your payload. Or still assemble in space (sending up main hitchhiker and rovers up in separate launches). Considering your "small" rovers, I doubt the docking nodes will not survive a reentry. If that's the case, use quantum or Docking Struts to stabilize your connected payloads.

Using this approach I dropped entire space-assembled colony bases on Eve, being in the 100t+ range...

[Doomydoom]

@Doomydoom: Yes, but they also knew the exact specifications of what the vehicle would be; I don't. I'd love to do an aerodynamic analysis of the entire vehicle rather than part-by-part, but that can be a lot more complicated and can end up being less accurate. Also, how do I detect when a vehicle is a waverider and when it isn't?

@Taverius: All my attempts to implement it have resulted in things either not working properly or no noticeable difference.

@seagull42: It should be working properly, unless the drag of the probe wasn't much to begin with. I can take a quick look into it.

@Nobody_1707: Your problem is that you're starting your gravity turn too soon and at too low a throttle. More throttle, start the turn when you're going 60 m/s, start the turn gently, don't use SAS, since it looks stable enough. You should be going much faster than you are in all of these shots (except the last one, which is closer to the right speed, but still low, direction aside). The gravity turn that it appears you're taking would work if you were running at a TWR > 2 the whole way up, but it would be scary and hard to control.

Only other thing I'd do is to combine the first core and second core stages into one stage so you can get further up before staging makes the rocket shorter, and thus, less stable.

Could you cast a cone of tracers as the nose shock cone and see if there's a good wing surface to "attach" the shock wave to to provide lift?