Plane veers to the left. Stumped.

[jsisidore]

p.s. I can fit another reaction wheel if it will help.

[jsisidore]

I made it to orbit, 1670 fuel remaining. http://pastebin.com/Qx4FhMrV

Here's my ascent profile, it is not copied, I was simply adapting to the behavior of the ship. I doubt it's optimal.

1. I start with 5', at ~3000m and 240m/s the ship pitched up to 10' by itself, I stay there.
2. At 10000m I go back to 5', use nukes to go past 360m/s, turn off the nukes and watch the ship accelerating.
3. Watch the ship accelerating, going past 600m/s and slowly pitching up to 10', stay at 7-8'.
4. I'm past 1000m/s and 20000m and 8'. Go to 5' or even 1' to go past 1400m/s.
5. Back to 4. I can't go past 1150m/s without nosediving a bit. I trade some 2000m for 1400m/s.
6. Pitch up to 5' slowly gaining speed and altitude.
7. Lose speed at 24000m - switch on nukes, stay at 10'. At 30000m and 1600m/s rapiers burn out. (turns out closed cycle went on and I lost all of my oxidizer)
8. I stay at 15-20', go past 2000m/s and stop at 71000m.

edit:

I changed my ascent profile, this time I went mach 1 before 10km so at 10km I was already past 500m/s, that saved me a substantial amount of fuel, I reached orbit this time with 2043 fuel left.

Edited December 10, 2016 by jsisidore

[jsisidore]

Wow I have no idea how you've made it to Duna. With 2043 fuel I had 1265 delta-v in the orbit, lost 900 to escape it, then I needed at least another 900 to intercept Duna at a 44.38 angle.

edit:

I decided to go to Duna anyway with the infinite propellant cheat, to see if I can land. And whaddayaknow! It's impossible. Impossible to control the ship. I assume the CoL shifted back to the end of ship again. So much for even fuel flow distribution.

Edited December 10, 2016 by jsisidore

[AeroGav]

3 hours ago, jsisidore said:

Wow I have no idea how you've made it to Duna. With 2043 fuel I had 1265 delta-v in the orbit, lost 900 to escape it, then I needed at least another 900 to intercept Duna at a 44.38 angle.

edit:

https://alexmoon.github.io/ksp/

It depends on when you launch,  if your launch date is when the planets are lined up right then it takes hardly any more delta v than to escape kerbin system.

 

[AeroGav]

9 hours ago, jsisidore said:

I made it to orbit, 1670 fuel remaining. http://pastebin.com/Qx4FhMrV

Here's my ascent profile, it is not copied, I was simply adapting to the behavior of the ship. I doubt it's optimal.

1. I start with 5', at ~3000m and 240m/s the ship pitched up to 10' by itself, I stay there.
2. At 10000m I go back to 5', use nukes to go past 360m/s, turn off the nukes and watch the ship accelerating.
3. Watch the ship accelerating, going past 600m/s and slowly pitching up to 10', stay at 7-8'.
4. I'm past 1000m/s and 20000m and 8'. Go to 5' or even 1' to go past 1400m/s.
5. Back to 4. I can't go past 1150m/s without nosediving a bit. I trade some 2000m for 1400m/s.
6. Pitch up to 5' slowly gaining speed and altitude.
7. Lose speed at 24000m - switch on nukes, stay at 10'. At 30000m and 1600m/s rapiers burn out. (turns out closed cycle went on and I lost all of my oxidizer)
8. I stay at 15-20', go past 2000m/s and stop at 71000m.

edit:

I changed my ascent profile, this time I went mach 1 before 10km so at 10km I was already past 500m/s, that saved me a substantial amount of fuel, I reached orbit this time with 2043 fuel left.

Drag is lowest with the body of the plane as close to 0 AoA as possible, so i flew with Prograde Assist set whenever possible.

The problem is at low altiudes the wings make a bit too much lift even on prograde so it's constantly pulling up.   If you don't correct this, the flight path will become a series of climbs and dives,  which is ok to a point  but it does mean crossing the sound barrier repeatedly before staying supersonic for good.    As the transonic region is high drag,   i figure it;s best to only do it once and get from 250 m/s to 440 m/s  as quick as i can.

so

1. after takeoff, set prograde

2. if it tries to climb so steeply that we start loosing speed, ( climb angle over 20degrees), i lock the nose angle with stability assist mode 

3.  As we start getting above 7km and near 240 m/s,  i stop the plane from arcing over into a shallow dive when the air gets thinner by locking the nose angle with SAS again, i make it stay below 250 degrees.

4. sometime near 10km,  the plane is having to fly with a positive angle of attack  in order to get enough lift.  In other words the nose is pointing a couple of degrees above prograde.   At this point,  i set Prograde Hold again and the plane noses over into a shallow dive, goes through the sound barrier really quickly in a dive with 0 aoa,  then will start to pull out all by itself as the speed goes over 400.  

Due to generating too much lift, it can zoom over flameout altitude a couple of times before reaching max airbreathing speed.   I just leave prograde lock on and wait for it to come back down and relight.  

The best altitude for speed in horizontal flight on the Rapier is 21.5km.   Up to that point, drag falls faster than thrust, after that the engines really wimp out.  I did try locking the nose with SAS again to stop the plane cilmbing above 21.5km on the final speed run but the downside is that makes the plane fly with a nose down body angle which is bad for drag ?

Once you're on Nuke power it's really simple - just stay on prograde lock for best possible lift to drag ratio. You no longer care about staying under 22km to get enough air for the jets.  Don't worry about any shallow dives in between climbs, it will climb more than it sinks.

And after 35km, the navball will switch itself to orbit mode - i prefer to switch it back to surface until we're over 70km, because that keeps us closer to 0 body AoA in the atmosphere.

[jsisidore]

15 hours ago, AeroGav said:

Drag is lowest with the body of the plane as close to 0 AoA as possible, so i flew with Prograde Assist set whenever possible.

Ouch, never done that. To my mind the prograde was always too erratic to trust it. I need to rethink that.

15 hours ago, AeroGav said:

And after 35km, the navball will switch itself to orbit mode - i prefer to switch it back to surface until we're over 70km, because that keeps us closer to 0 body AoA in the atmosphere.

By "switch it back to surface" you mean stay below 35km and burn prograde to 70km?

[jsisidore]

How much delta-v you had? I have 13.571m/s. Maybe I should add more wing and more fuel.

Is there a way to check CoM in flight? I need to find out why what causes the ship to spin out of control in Duna's atmosphere. 

[jsisidore]

28 minutes ago, jsisidore said:

Is there a way to check CoM in flight? I need to find out why what causes the ship to spin out of control in Duna's atmosphere. 

Anyway, I checked it in the hanger bay and yea it gets very close to the CoL. If I will keep some fuel locked in the mk2-3 adapter I might just be able to even it out.

Vernor engines won't work I think, CoM is too faraway into the back, the only place I can mount them is on the fuselage next to the engines.

Parachutes. I think I won't make it without them.

[jsisidore]

Hmm I cannot fly into prograde without losing altitude. It is either CoM too far from CoL or I need to tune the elevons a bit more. I just can't balance the ship, something always goes off. Or maybe there is a certain altitude and speed I need for that?

[AeroGav]

8 hours ago, jsisidore said:

Ouch, never done that. To my mind the prograde was always too erratic to trust it. I need to rethink that.

By "switch it back to surface" you mean stay below 35km and burn prograde to 70km?

If you click on the bit where it tells you your speed in the Navball  "Surface 1150m/s,   or Orbit 2110 m/s" you can toggle between surface and orbit modes.

In surface mode,  Prograde is direction the wind is coming from.  In orbit mode,  the prograde vector also takes into account the rotation of the planet, but that means if you have Prograde hold set within the atmosphere, and the Navball is set to orbit mode, you are not directly heading into the wind , at 0 aoa.

At 35km it automatically switches to orbit mode, i click to switch it back until i'm over 70km.    And yes once the nukes are on i like to keep prograde hold set .

4 hours ago, jsisidore said:

Hmm I cannot fly into prograde without losing altitude. It is either CoM too far from CoL or I need to tune the elevons a bit more. I just can't balance the ship, something always goes off. Or maybe there is a certain altitude and speed I need for that?

I was flying your dragon ship and i had a hard time stopping it from climbing.

Can you turn on Aero Data GUI again, what is your angle of attack (AoA) when using Prograde hold on that screen?

It should be close to zero , maybe half a degree out,  postive is better than negative.

I suspect you have very strong nose down tendency and the ship flies at like -5aoa without any input,  and with Prograde hold set it still flies a couple of degrees nose down.

If you look back on this thread you will see i linked the version of the dragon craft i flew, where i angled the strakes and elevons to make it fly straight , about 0.2 degrees AoA when prograde set.   

8 hours ago, jsisidore said:

 

Vernor engines won't work I think, CoM is too faraway into the back, the only place I can mount them is on the fuselage next to the engines.

 

I've said this at least twice already, but  you can mount them to the bottom of your cargo bay over the CG.

If you offset them upwards slightly - clip them inside a little bit - you can have the vernors without the drag, because the game will think they are inside the cargo bay.   This has the downside that you will have to open the cargo bay roof to use the vernors, but when landing you won't care about drag .

[jsisidore]

1 hour ago, AeroGav said:

I've said this at least twice already, but  you can mount them to the bottom of your cargo bay over the CG.

If you offset them upwards slightly - clip them inside a little bit - you can have the vernors without the drag, because the game will think they are inside the cargo bay.   This has the downside that you will have to open the cargo bay roof to use the vernors, but when landing you won't care about drag .

I've done that. How will I use them on Duna if CoM is way behind? Maybe I was too high up but I couldn't use them without spinning out of control. I need to think it out a bit more.

1 hour ago, AeroGav said:

I was flying your dragon ship and i had a hard time stopping it from climbing.

Can you turn on Aero Data GUI again, what is your angle of attack (AoA) when using Prograde hold on that screen?

It should be close to zero , maybe half a degree out,  postive is better than negative.

I suspect you have very strong nose down tendency and the ship flies at like -5aoa without any input,  and with Prograde hold set it still flies a couple of degrees nose down.

If you look back on this thread you will see i linked the version of the dragon craft i flew, where i angled the strakes and elevons to make it fly straight , about 0.2 degrees AoA when prograde set.

I'll try to match this number. Can I fly prograde just after taking off?

[jsisidore]

On 12/8/2016 at 9:05 PM, AeroGav said:

 

BTW I used the mod CorrectCoL to work the above out.   You can see in that graph, the line needs to slope downhill from left to right.  If it does not , you need to move your CoL back some more.

The point where the line crosses the x axis (horizontal axis) is the AoA your airplane will try to find if it's got no control input from you or SAS.   I adjusted the canards/elevons until the plane was trimmed to fly at a small positive AoA when left with no input.

The horizontal axis is basically AoA and the vertical axis is the pitching moment the aerodynamics generate.  Positive values on the vertical axis mean the nose is trying to go up, negative values mean it's pushing down.   If that line slopes "downhill" then that means it tries to pitch itself up when the nose is down  , so the plane is stable.

It's hard to see in that picture, but it also generates a thin vertical blue line.  That is the AoA your plane needs for level flight at the conditions you've specified.   So if you want to fly at 21.5km and 1350 m/s, you can put those numbers in and see how much AoA it will need in those conditions. Helps you work out if you need more or less wing.

According to this graph the CoL has to be miles behind CoM for the dry craft pitch to be partially stable, which I don't think is right. This is as far as I could shift it, the graph is either wrong or I need to completely rethink my design, which is crazy since it did look like a design you would see in a real life. Honestly I'm about to give up on KSP, it is only good for flying boring rockets.

[AeroGav]

2 hours ago, jsisidore said:

According to this graph the CoL has to be miles behind CoM for the dry craft pitch to be partially stable, which I don't think is right. This is as far as I could shift it, the graph is either wrong or I need to completely rethink my design, which is crazy since it did look like a design you would see in a real life. Honestly I'm about to give up on KSP, it is only good for flying boring rockets.

The graph appears to be saying that when fuelled (green line) you are stable because it is sloping downhill.    When empty your craft is neutrally stable.  Also, you're crossing the horizontal line (x axis) at -2 degrees angle of attack, so that is what your plane will go to when SAS is off and you're not making any manual inputs.   Ideally, you'd use fine rotation mode to make a really small adjustment to the canards (angle them up) and/or the elevons (angle them down) so it crosses the axis at 0 to 5 degrees positive AoA.

The main problem seems to be that your dry CoM is too far aft.    You have more fuel at the front than the back, when it burns off your CoM will move rearward.   You seem to have moved the engines towards the back of the ship again,  compared with the ship you showed us last week  - that's going in the wrong direction if you ask me.

If you want, share the craft file again and i'll have another go at fixing it.   Actually i could just develop the "Dragon 001" from last week further,  it had interesting styling and wasn't far away from being a viable Duna transport.

[Boris-Barboris]

1). Use larger aoa range, 30+ will show the whole picture better.

2).

Simple rule - center of mass of your fuel should be as close as possible to CoM of the craft completely dry. Only then you'll be able to forget about CoM shift due to fuel burnout (only to discover that you need to balance cargo load too). There is a mod somewhere that pumps fuel in realtime to keep CoM at the same place it was in SPH, google should help you, but it's the way of casuals. Besides, it only delays the problem, it does not solve it.

Typical plane in KSP - engines are far in the back, fuel forward - nowhere near the way real planes are built. That's a rocket's composition, and rockets are unstable.
Good way to balance fuel-chassis pair is to have some fuel blocks on your craft that you can easily move forward-backwards while looking at the graph (see pic below). When green and yellow line merge, you hit the right spot. After that you move wings again to balance the craft and get stability, then fuel blocks again. After a couple of iterations you'll reach good balance.

Edited December 16, 2016 by Boris-Barboris

[jsisidore]

Thanks Boris, that's all very interesting. I'll look into it more.

@AG, here's the 002 version: http://pastebin.com/RnTKvJi6

[foamyesque]

13 hours ago, jsisidore said:

 Honestly I'm about to give up on KSP, it is only good for flying boring rockets.

Nah, you can make some really cool planes, but there's some rules.

 

1. If you're building something to launch a deployable payload, that payload's CoM should always be on your plane's dry CoM. That way your CoM doesn't shift if the payload is removed or exchanged with something of a different mass.

2. Fuel mass should be as centered as possible around your dry CoM. This means that your CoM will not shift as fuel is burned off, giving you relatively consistent aerodynamics. It will generally handle better as it gets lighter, in fact.

3. On most planes, and especially on large ones like yours (and double especially ones using nuclear engines, which are very heavy), the biggest determination of your dry CoM position is where you put your engines. That means that the only ones you can afford to put at the back are ones that are counterweighted by some other fixed mass, such as cockpits, passenger modules, reaction wheels, and so on. Fuel tanks cannot be used counterbalance engines.

The upshot of this that basically for a large plane, you need fuel tanks behind your engines. I believe @Val has a layout with engines fore-and-aft, but I can't stomach the exhaust problems inherent to it; in general, I find the solution is to put my engines on midbody pods. It has issues of its own, structurally, but it's my solution to the aerodynamic woes. The nuclear engines, particularly, as they are both the heaviest pieces and have no gimbals, should be moved. Given what appears on the nose of the craft, you might be able to keep a pair of RAPIERs at the back to balance the cockpit and passenger module and continue to provide you with TVC.

Edited December 17, 2016 by foamyesque

[AeroGav]

3 hours ago, jsisidore said:

Thanks Boris, that's all very interesting. I'll look into it more.

@AG, here's the 002 version: http://pastebin.com/RnTKvJi6

Last week's airplane only needed some minor fixes to make it fly, this one was harder because you'd pushed the engines right to the back, then added a very long neck to try and get the crew cabin far enough forward to balance those.    You are somewhat correct about CorrectCoL,   When the x axis crossing point was adjusted to be at one and a half degrees on the graph, it actually wanted to fly nearly 10 degrees nose up in reality.  So , lots of trial and error till i got it to fly only a couple of degrees nose up with no control inputs.    The graph was showing a downhill sloping line at this point but was indcating  an x axis crossover at a significant negative aoa, which is wrong.   It also showed a blue marker in front of the CoM in the SPH,  when the stability analysis graph of CorrectCoL shows positive stability from -90 to +90 AoA, borne out in flight testing (it won't fly much more than 15 degrees nose up no matter what).

http://pastebin.com/8xg9nUz0

Still "to do"  

- test the rcs translation system - does the nose pitch up or down when fired?

- the outer wing sections break off when i landed it on kerbin next to the runway. i might try some struts (real ones) to see if i can beef up the structure.

it does reach orbit with plenty of fuel though, and apart from the part shedding on landing is quite easy to fly.

 

[jsisidore]

good god that is one ugly plane. reminds me of a bird with a swollen head. not that i've seen one. thanks I will study your mods but I doubt I will come up with anything more visually palatable while having the same amount of fuel left for the trip. I just hate restrictions.

I officially make this design to be open to the public, if someone can make it fulfill its function that would be great, as for me i know when i'm beaten.

Edited December 17, 2016 by jsisidore

[foamyesque]

6 hours ago, jsisidore said:

good god that is one ugly plane. reminds me of a bird with a swollen head. not that i've seen one. thanks I will study your mods but I doubt I will come up with anything more visually palatable while having the same amount of fuel left for the trip. I just hate restrictions.

I officially make this design to be open to the public, if someone can make it fulfill its function that would be great, as for me i know when i'm beaten.

 

What exactly is it's function, anyway? Can you throw it in the OP? I like a challenge.

[AeroGav]

 

4 hours ago, foamyesque said:

 

What exactly is it's function, anyway? Can you throw it in the OP? I like a challenge.

14 hours ago, jsisidore said:

Thanks Boris, that's all very interesting. I'll look into it more.

@AG, here's the 002 version: http://pastebin.com/RnTKvJi6

His aim was to fly a rover (its already in the cargo bay) to surface of Duna, land there and have it drive out the ramp.   A tank truck from his duna colony drives up the ramp, connects to the docking node, rinse and repeat till fuelled for home journey.    It's very challenging because such a ship needs to excel in multiple areas simultaneously, but i have managed this with a clean sheet design.

https://kerbalx.com/AeroGav/C7-Galaxian

Not yet succeeded in getting his Dragon to do all the things he wants from it .   

[foamyesque]

I assume he's checked that the docking port is aligned for the tanker truck dock on Duna?

[AeroGav]

19 minutes ago, foamyesque said:

I assume he's checked that the docking port is aligned for the tanker truck dock on Duna?

Well, if he flies the tanker truck out to Duna as his first cargo mission, it should shouldn't it?

Ah - i see the problem.   It was attached in the VAB in zero g conditions,  so it might be floating with it's wheels off the floor and hence never be able to re-dock with the attach node in a gravity field.  Or, it will sink down on it's springs when full of fuel, and not line up.

I guess you can test this on kerbin's runway though, just use hack gravity and set to 30% of normal to simulate Duna conditions.   Ground docking is hard, but there's not much alternative, stock.

[jsisidore]

Nnnnnnope. This was a space tourism mission, "tank truck" has no fuel tanks on it, it is a sight seeing bus. The ship stays on Dune until another ship arrives but with a mining setup in its cargo bay, the ship basically becomes a refueling station for other ships with a capacity to go back to Kerbin

[jsisidore]

[foamyesque]

Honestly if I was going to try to refuel this thing, I'd just put a claw on the tanker truck and clamp on with that. Means you're not constrained with trying to fit a truck into the cargo bay.