Help with spaceplane.

[magnemoe]

I'm making an space plane for use on Laythe. 

Parachutes is an optional landing method on Laythe as the base might end up an uneven place, don't laugh it works. 
I took plane to orbit around kerbin with fuel to save. Added more features like a bit more in the cargo bay and the parachutes. 
Rear landing gear is for landing and taxing. hotkey to retract for takeoff. 

However my problem is that plane is now speed caped to around 350 m/s both at sea level an 10Km, first I though it was drag because of the parachutes or stuff in cargo bay so i removed it all. 

Aerodynamic overlay. Wings might be a bit large but it worked the first time. 

[bewing]

The problem is your MK2 fuselage. As soon as your AoA goes a few degrees above prograde, the drag on an MK2 fuselage goes up exponentially.

I understand that you said you got it to orbit before you added features. The features you added changed your CoL/CoM a little, so now it needs more AoA to fly.

I assume you want to keep the cargo bay, so you want to keep the MK2 fuselage. In that case, you need to adjust the AoA on the wings. Thank you for the pics. According to them, it looks like you need another 3 or 4 degrees of AoA incidence on the wings. As soon as you do that, your nose will drop to approximately 0 degrees AoA, and the drag on your spaceplane will drop tremendously. Then you will be able to get to orbit again. Doing that to your wings will completely change your CoL, of course, and would force you to change your design a bit.

If you don't want to change your design, then try slowly flying up to 10km altitude, going to full throttle, and holding down F (to temporarily cancel SAS). If your plane can make it to 400 m/s and level out before you hit the ground, then you can probably make it to orbit.

Edited November 13, 2018 by bewing

[Rayder]

17 minutes ago, bewing said:

As soon as your AoA goes a few degrees above prograde, the drag on an MK2 fuselage goes up exponentially

It's been a long time since I've used Rapier engines but is this because there's limited thrust to overcome the drag? Since only increasing the drag slightly the craft loses too much speed?

OP, do you still have the original version of the craft that made it to space? If so, try reverting back and do another test run. Then start adding parts one at a time until you start having the speed limiting issue again.

Edited November 13, 2018 by Rayder

[bewing]

17 minutes ago, Rayder said:

It's been a long time since I've used Rapier engines but is this because there's limited thrust to overcome the drag? Since only increasing the drag slightly the craft loses too much speed?

Yes, rapiers (and really all jet engines) have a thrust minimum at mach 1. So if the drag on your plane at mach 1 is just a little above that thrust minimum, then you'll either never break the mach barrier -- or you will need to use some extreme method such as going into a shallow dive, lighting an extra rocket engine, or lighting any afterburner you have. Getting to a 10km altitude and locking your SAS on "prograde" will sometimes get you through if you are on the edge. (Alternately, holding F to temporarily turn off SAS works well on stable planes.) A rapier's thrust ramps up extremely fast after mach 1, so if you make it through and can pull out of your dive, then you're good to go.

Edited November 13, 2018 by bewing

[Rayder]

9 minutes ago, bewing said:

- snip -

Agreed, however he claims he's only had this speed issue since slightly modifying the craft. To me, it seems there's a part he added that's adding excessive drag when it shouldn't be.

I know this occasionally happens with some modded parts - Mk IV spaceplanes had serious drag problems with the crew cabin. I've also seen anecdotally that some cargo bays have this problem as well, even if they're occluded. Whether this is the problem OP is facing though, I have no idea. OP also stated he removed the parachutes but didn't say if that resolved the problem.

Edited November 13, 2018 by Rayder

[bewing]

Nah. I guarantee it's the MK2 fuselage itself and that 4 degree AoA he has. That's just instant death for spaceplane performance.

Edited November 13, 2018 by bewing

[AeroGav]

Try to reduce the mk2 parts to a minimum - if you really need a multi purpose cargo bay,  have that part only be mk2, and immediately transition down to mk1 size with an adapter before and aft.  Use an inline mk1 cockpit and crew parts.  Any rocket fuel you need above what is stored in the mk1 to mk2 adapter short x 2,   put in mk1 tanks.

 

You could try combining rapiers and panthers for better low speed punch to get over mach 1.   I  can't see how you're ending the main fuselage stack from either picture shared,  that could also be a drag issue.

 

I presume you have a lot of liquid fuel storage for laythe transfer ?   Keep your liquid fuel in big S wings and strakes.  They have lowest drag per unit of volume of any tank.  Strakes orientated vertically make good tail fins/vertical stabilizers.

 

Finally, it appears you not only have drag from angle of attack,  but also from sideslip,  because you are using SAS stability hold mode.  Every 2 degrees off prograde doubles drag.      Better way to control the airplane is to use fine adjust tool on the elevons so it naturally gives the right angle of attack to make lift with low drag when on prograde hold mode or with sas off altogether.  This will reduce drag from sideslip.

[magnemoe]

1 hour ago, bewing said:

Nah. I guarantee it's the MK2 fuselage itself and that 4 degree AoA he has. That's just instant death for spaceplane performance.

I changed the small landing gear to medium for safer handling in rough terrain because of problems with takeoff, might well be that who did it. 
Don't think I went subsonic on Laythe but I also used the small landing gear. 
First thought was parts in cargo bay who caused drag, its very packed in it
 

1 hour ago, bewing said:

Nah. I guarantee it's the MK2 fuselage itself and that 4 degree AoA he has. That's just instant death for spaceplane performance.

That should be easy, can probably also reduce wing size. 
I made an previous plane with 3 MK2 fuselages and 4 engines and just some tiny wings but that probably brute forced it

Could probably also make it with 1.25 meter parts. requirement is 4+ kerbals and science equipment and be an bush plane. 
Was not aware of any issues with MK2. 

Edited November 13, 2018 by magnemoe

[magnemoe]

1 hour ago, AeroGav said:

Try to reduce the mk2 parts to a minimum - if you really need a multi purpose cargo bay,  have that part only be mk2, and immediately transition down to mk1 size with an adapter before and aft.  Use an inline mk1 cockpit and crew parts.  Any rocket fuel you need above what is stored in the mk1 to mk2 adapter short x 2,   put in mk1 tanks.

You could try combining rapiers and panthers for better low speed punch to get over mach 1.   I  can't see how you're ending the main fuselage stack from either picture shared,  that could also be a drag issue.

I presume you have a lot of liquid fuel storage for laythe transfer ?   Keep your liquid fuel in big S wings and strakes.  They have lowest drag per unit of volume of any tank.  Strakes orientated vertically make good tail fins/vertical stabilizers.

Finally, it appears you not only have drag from angle of attack,  but also from sideslip,  because you are using SAS stability hold mode.  Every 2 degrees off prograde doubles drag.      Better way to control the airplane is to use fine adjust tool on the elevons so it naturally gives the right angle of attack to make lift with low drag when on prograde hold mode or with sas off altogether.  This will reduce drag from sideslip.

Was not aware of any issues with MK2, can use 1.25 m, don't really need cargo, just passangers and science equipment. 

The back is an MK2 to 1.25 part with an docking port, that would be critical for getting to Jool. 
I could use an rapier and an decopler however. Need to dock it as i'm doing an 3 km/s Jool burn with an 2 Km/s Tylo brake. 

No idea why the plane sideslips, has an ladder on the wing for crew access, 

[magnemoe]

Ok, tested with the original design and tilted wings, now I could send it to the Mun after reaching orbit. With medium landing gear. 
 

[magnemoe]

Also an MK1 version who looks like works just as well or better, easier to fly and land in any case. 

Gets to LEO just fine and will probalby go for the design. Testing ladders and science now. 

[bewing]

That particular MK1 cockpit has heating issues sometimes. If you have heating problems with that plane on reentry let us know -- there are very easy fixes to make it work better for heating.

Edited November 14, 2018 by bewing

[magnemoe]

25 minutes ago, bewing said:

That particular MK1 cockpit has heating issues sometimes. If you have heating problems with that plane on reentry let us know -- there are very easy fixes to make it work better for heating.

Saw a little but Kerbin reentry was no issues and Laythe is gentler. The reentry just heated up an ladder I had forgot and showed that an top mounted ladder has an second benefit of giving access to the roof for the science equipment. Thanks for the help I ended up with an much better plane, the parachutes are mostly for safety now this thing is easy to fly. 

[AeroGav]

The inline cockpits are better for heat.    Inline mk1 is better than pointy mk2 , because it is protected by whatever nose cone, intake, service bay etc. goes  in front of it.

 

My "Sparrow" speedbuild SSTO uses liquid fuel only,  no oxidizer.   Drag is so low that a single Whiplash provides all the air breathing thrust you need,  the twin nervs  provide ample power to continue cruising upwards when the jets quit.

https://kerbalx.com/AeroGav/Sparrow

 

If you follow the KerbalX link there's  a video of the build .

3 seats,  1800 dv in orbit, inline clamp o tron.

I'd recommend two mods at this point.

CorrectCoL  

Shows a nice "Static Analysis" graph in the SPH,    with aerodynamic torque vs angle of attack.    On a stable airplane, the more pitched up you are the stronger the nose down torque gets.   But you can also see at which angle of attack the aero torque drops to zero,  which is the angle of attack the plane will go to with no control input from you or from SAS. 

 So you take a basic, stable airplane, then tweak the elevons slightly with fine rotate tool to a slight  nose up trim,  until static analysis indicates the line crossing the horizontal axis at an AoA of 3 or 4 degrees or so.    Do a quick test flight (no need to get more than a few hundred metres up) to verify this is indeed the case.   You'll find you get a natural AoA of 3 or 4 degrees with SAS off,   and about 1.4 degrees with SAS set to Prograde hold .  If you want to foce the plane to level off completely for the speedrun or to accelerate through mach 1,   use prograde hold with Navball in Orbit mode, which makes the nose pitch to the horizon.      

This eliminates the horrible jerkiness and sideslip you get from flying with SAS in default "stability assist" mode, which just holds the nose at a fixed angle relative to the sun,  not taking account the curvature of the planet.    If you are a couple degrees off course , rather than roll the wings and actually turn the plane to 90 degrees magnetic, it just uses rudder to sideslip the nose and keep it pointing at the same angle relative to the sun, without changing the flight path (sideslip)

Also ,  i'd like to point you to Kerbal Wind Tunnel, which shows a flight envelope plot of "excess thrust" vs speed and altitude

https://forum.kerbalspaceprogram.com/index.php?/topic/177302-1415-kerbal-wind-tunnel-120/&

Above is 3 plots from one of my dual stage panther/nerv spaceplanes.   Left plot is panther only,  middle one is panthers and nervs running,  right plot is nervs only after jettisoning the panther nacelles.     Work out the best engine combo to fill out any holes in the ascent  path without even leaving the SPH!

[bewing]

4 hours ago, magnemoe said:

Saw a little but Kerbin reentry was no issues and Laythe is gentler.

Well, Laythe is gentler unless you are an idiot like me.  On Laythe, when I reenter, my temptation is always to fly along and along until I get to a nice island. It's really easy to just go into a nice hypersonic glide for hours and hours while you wait for the perfect island to show up. And then BOOM! your nose blows up from overheating. 

[magnemoe]

14 hours ago, AeroGav said:

The inline cockpits are better for heat.    Inline mk1 is better than pointy mk2 , because it is protected by whatever nose cone, intake, service bay etc. goes  in front of it.

 

My "Sparrow" speedbuild SSTO uses liquid fuel only,  no oxidizer.   Drag is so low that a single Whiplash provides all the air breathing thrust you need,  the twin nervs  provide ample power to continue cruising upwards when the jets quit.

https://kerbalx.com/AeroGav/Sparrow

 

If you follow the KerbalX link there's  a video of the build .

3 seats,  1800 dv in orbit, inline clamp o tron.

I'd recommend two mods at this point.

CorrectCoL  

Shows a nice "Static Analysis" graph in the SPH,    with aerodynamic torque vs angle of attack.    On a stable airplane, the more pitched up you are the stronger the nose down torque gets.   But you can also see at which angle of attack the aero torque drops to zero,  which is the angle of attack the plane will go to with no control input from you or from SAS. 

 So you take a basic, stable airplane, then tweak the elevons slightly with fine rotate tool to a slight  nose up trim,  until static analysis indicates the line crossing the horizontal axis at an AoA of 3 or 4 degrees or so.    Do a quick test flight (no need to get more than a few hundred metres up) to verify this is indeed the case.   You'll find you get a natural AoA of 3 or 4 degrees with SAS off,   and about 1.4 degrees with SAS set to Prograde hold .  If you want to foce the plane to level off completely for the speedrun or to accelerate through mach 1,   use prograde hold with Navball in Orbit mode, which makes the nose pitch to the horizon.      

This eliminates the horrible jerkiness and sideslip you get from flying with SAS in default "stability assist" mode, which just holds the nose at a fixed angle relative to the sun,  not taking account the curvature of the planet.    If you are a couple degrees off course , rather than roll the wings and actually turn the plane to 90 degrees magnetic, it just uses rudder to sideslip the nose and keep it pointing at the same angle relative to the sun, without changing the flight path (sideslip)

Also ,  i'd like to point you to Kerbal Wind Tunnel, which shows a flight envelope plot of "excess thrust" vs speed and altitude

https://forum.kerbalspaceprogram.com/index.php?/topic/177302-1415-kerbal-wind-tunnel-120/&

Above is 3 plots from one of my dual stage panther/nerv spaceplanes.   Left plot is panther only,  middle one is panthers and nervs running,  right plot is nervs only after jettisoning the panther nacelles.     Work out the best engine combo to fill out any holes in the ascent  path without even leaving the SPH!

Nice, I had to see the video to find out there you stored the fuel

[farmerben]

It is also possible to get better performance from the rapiers from a special ascent path.  

Even at low speed the rapier can usually hold ~150kN of thrust.  But you have to get airspeed above Mach 2 to see thrust go >300 kN.

Try this:

1. Climb to about 14000 m at low speed.  Pitch up at >5 degrees until you can't climb anymore.

2.  Pitch the nose down just below the horizon ( -1 or -2 degrees) and dive to 10,000 m.

3. When the thrust exceeds 250 kN level out and watch the thrust keep going up. It can go as high as 460 kN but it probably won't because of drag and reduced air intake.  Thrust in the mid 300 kN is good enough.

4.  Gradually pitch up, just a degree at a time then pause, then another degree then a pause... until pitch is >15 degrees.  

 

This should allow the plane to climb on only air breathing until the air runs out, without losing significant amounts of speed.  It should be possible to reach 30,000m and 1000 m/s before using oxidizer.