Prograde below where I point my plane

[Numerlor]

I don't know how to call this but I'm sure you understand, It results in unwanted AoA. I want to launch single stage to Mun but I'm missing around 200m/s that could be lost during atmospheric flight because of this. I believe there is simple obvious solution to this?

[HebaruSan]

Assuming you're making a single stage spaceplane, you can tilt the wings up slightly to give more lift when locking prograde.

[Capt. Hunt]

It would probably help if you had a picture of this SSTM, but my first guess would be that the camber of the wings is too low.

Edited April 25, 2017 by Capt. Hunt

[cephalo]

I find that if you just put wings on a spaceplane using the snap your plane will usually fly like this. As was said, you can tilt the wings so that your fuselage points more prograde. It's hard to tell if this causes less drag or not.

[Numerlor]

This is also my first real Mk3 SSTO So any more advice would be appreciated too

[AeroGav]

55 minutes ago, Numerlor said:

I don't know how to call this but I'm sure you understand, It results in unwanted AoA. I want to launch single stage to Mun but I'm missing around 200m/s that could be lost during atmospheric flight because of this. I believe there is simple obvious solution to this?

That effect is called Angle of Attack,  Lift and drag are both dependent on Angle of Attack (as well as mach number and air density).

At zero AoA , you get zero lift, but you still get some drag.

As  AoA rises,  lift increases rapidly at first, increasing faster than drag, until your best lift:drag ratio AoA is reached (about 5 degrees in supersonic flight).   After that, gains in lift slow down, and are outpaced by drag buildup.    At 30 degrees the wing stalls and lift gets less with increased pitch up, but drag still continues to increase.

> yes , in critical phases of flight you want to keep AoA under 5 degrees.

In steady state flight that means adding more wing area, or flying lower for a given airspeed.   In KSP you can cheat this a bit by adding incidence angle to your wings, since fuselage parts have nearly 10x the drag of wing parts.   This allows you to keep the fuselage at 0 while the wings are making lift at 5 AoA.

But if you're gonna do that  ,  make sure you add the same amount (or more) incidence to the front wings as to the rear ones.   That way, as you pitch up, the front wings start to stall first and gently limit max AoA.   Otherwise the main wings are the first to loose lift which will cause the nose to want to rise further at high AoA.

Incidence angle - easiest way to put it on your craft is to install Editor Extensions Redux.   It allows you to create custom angle snap angles for rotate part mode (instead of just 15 degree snaps).     so you can tell it to do 5 degree angle snaps, or whatever you like.

I also strongly recommend you install CorrectCoL mod if you're going to be getting into such advanced aerodynamic techniques.   The stock blue indicator only takes  account of aerodynamic forces from parts that have a "lift rating" in the part description, and cannot be trusted, especially on a craft like yours with more fuselage in front of the wing than behind it. 

[AeroGav]

37 minutes ago, Numerlor said:

This is also my first real Mk3 SSTO So any more advice would be appreciated too

Stock aero doesn't care much about shape, but it cares a lot  about how parts are attached, so we really need you to link the craft file so we can examine it properly.

Four rapiers is a lot for a small plane like that, and that's a huge amount of  oxidizer too , so i'm worried there might be something in the way stuff is attached making excess drag.

The golden rule - every stack must start and end with something streamlined (intake, cockpit , nose cone, or jet engine*)

When two attach nodes meet, they must always be of the same diameter.

eg. sticking a 1.25m engine on the back of a mk3 fuselage (3m node) bad !

use a proper mk3 to 1.25m adapter between them.   Open nodes are bad too.    The rapiers and nukes have attach nodes on the back end which you are not using,  these cause drag because it is a 1.25m stack ending abruptly with no "cone".   Stick 1.25m cones on the back of these engines then offset them forwards to clip inside the engine for minimum drag,  this reduces the drag of engines with attach nodes like rapiers and (all rocket engines) down to that of jet engines that don't have one, eg. whiplash.

Download this subassembly for example of streamlined mk3 mount

https://kerbalx.com/AeroGav/Streamlined-6-Nuke-Mk3-engine-mount

 

[Numerlor]

@AeroGav Well, I made adjustments to that craft AoA problem is kinda solved, but I have no idea how to "close" those open nodes, I made something but I have no idea if it is just mass and drag or it fixed something (It looks funny)
Craft File

Edited April 25, 2017 by Numerlor

[AeroGav]

51 minutes ago, Numerlor said:

Well, I made adjustments to that craft AoA problem is kinda solved, but I have no idea how to "close" those open nodes, I made something but I have no idea if it is just mass and drag or it fixed something (It looks funny)

You've done it correctly,  you can offset the cones as far inside the engine as you want (so they can't be seen) or rotate them so the flat base is still visible like you have done. 

The important thing is that the node is closed , and that the collider that checks to see if the engine exhaust is occluded doesn't run into any obstructions.   Because your cone ends before the exhaust stream starts, you still get thrust.   I did some tests and found that pointy end facing forwards or backwards makes no difference,  but sideways would be bad.  So cone facing with 0 degrees aoa with respect to pro or retro.

You can now maybe think about reducing the amount of engines.   They are very heavy (2 tons) and reduce your delta v by increasing dry mass.    Also,  when i flew your ship,  i got my AP out of the atmosphere with 4000 oxidizer still left over.   Might want to reduce the oxidizer tankage a bit for some LF you can use for the interplanetary mission...

one prob though, CorrectCoL shows your craft to be unstable when fuel tanks are empty.   Too much engine mass at the back !

Edited April 25, 2017 by AeroGav
and another thing...

[Numerlor]

13 minutes ago, AeroGav said:

BTW though , according to CorrectCoL installed on my game , your craft will be unstable when empty - 

I tried to make it stable but I clicked on few things in RCS build aid and have noidea how to turn it back and I was lazy to empty tanks

And I wanted to keep that oxidizer for Mun landing

Edited April 25, 2017 by Numerlor

[AeroGav]

21 minutes ago, Numerlor said:

I tried to make it stable but I clicked on few things in RCS build aid and have noidea how to turn it back and I was lazy to empty tanks

This is what i did 

https://www.dropbox.com/s/cees80iu2dhi3dt/SSTO 5.craft?dl=0

Got rid of the four radial rapiers.   The diamond shaped wing allows me to put the heavy nukes on the wing tips, a bit further forward.

= less dry mass at the back

I changed the forward rocket fuel tanks for a small liquid fuel one, and built a tail fin out of strakes that hold more liquid fuel

= more of our fuel mass at the rear

 

There is much less difference between empty and full.   Of course, it is now much less powerful.        The old one, with your cones on,  was very simple to fly - take off , AoA 5 degrees, try not to melt, space !       With cargo it might really test your patience.

 

Rear engine designs look much cooler imo, but only work on a mining/passenger ship where you got some heavy stuff you can put at the front that will still be there on landing.    Really hard with cargo ships that have to fly back empty.

 

[AeroGav]

43 minutes ago, AeroGav said:

This is what i did 

 

Hmm well for some reason this one turned out to be a bit unstable still,  fully loaded.

I wanted to try another engine config as the previous was sluggish accelerating

That's the mk3 engine mount with 3 1.25m nodes, and one centrally placed 2.5m node.  I use a 2.5m to 1.25m adapter on the middle node to give me four engine attach points,  but i attach the engines one at a time and use RCS build aid and the offset tool to make them symmetrical

It does accelerate well,  due to the good low speed power of the panther.   It also gets our CG even further forward, because we don't have the mk3 to 2.5m adapter pushing our engines back.  But, the CoL is now too far aft with the CoM so far fwd, you can't pull  more than 5 degrees of AoA  it's so nose heavy.

Also, loosing that fuel tankage at the back of the ship means we need to cut some from the front or CG shifts yet again as fuel burns off.

[EpicSpaceTroll139]

While unrelated to the problem at hand, I can't help but suggest that you move your rear gear forward so that they are only a short distance behind the center of mass. Having your gear at the very back like that can prevent you from taking off until the end of the runway, and sometimes cause problems during landing.

[AeroGav]

1 hour ago, Numerlor said:

And I wanted to keep that oxidizer for Mun landing

With Mun gravity, it has over 1:1 TWR  on just two nukes even when full of fuel.   Obviously by the time you reach the Mun most of the fuel will be gone, also if you are able to reduce the number of rapiers (from 7 to ?????) when you will be lighter still. 

[Numerlor]

@AeroGav With your advice and still keeping most of my design I managed to get it to Mun but with too much oxidizer like you suggested as seen here 

 

[AeroGav]

17 hours ago, EpicSpaceTroll139 said:

While unrelated to the problem at hand, I can't help but suggest that you move your rear gear forward so that they are only a short distance behind the center of mass. Having your gear at the very back like that can prevent you from taking off until the end of the runway, and sometimes cause problems during landing.

He's got huge canards on (i was going to name that vessel Dumbo because it looks like a flying elephant) and this is less of a problem for takeoff - they takeoff by lifting the nose up, so the fact gears are well aft of cg is not a big deal.   Landing on rough terrain though it can pitch you forward when the main gears hit, on Duna that can cause a fatal tumble.

If your pitch controls are via tail only, having gears aft of CG  can prevent takeoff.   They raise the nose by pushing the tail down, and if your main gears are underneath the elevons rather than at the CG,  the elevons would have to push the gear below ground level to make the nose go up.   Probably, you won't succeed at that.   If you do, then you might find yourself having to pay for a new runway.