How can I get this SSTO to orbit?

[Aero_T]

So, I just created a new SSTO that has about 5,000 meters per seconds of deltaV, but I can't get it into orbit. It goes well until I hit the higher atmosphere, and then it screws up. Can someone please help?

Craft Image

Edited January 29, 2019 by Aero_T
Adding an Image

[AeroGav]

8 minutes ago, Aero_T said:

So, I just created a new SSTO that has about 5,000 meters per seconds of deltaV, but I can't get it into orbit. It goes well until I hit the higher atmosphere, and then it screws up. Can someone please help?

 

 

Define "screws up".   

Goes out of control ?  Overheats ?   Accelerates but runs out of fuel before getting to orbital velocity ?   Plunges back towards the ground ?  Or simply fails to  accelerate ?

[Aero_T]

28 minutes ago, AeroGav said:

Define "screws up".   

Goes out of control ?  Overheats ?   Accelerates but runs out of fuel before getting to orbital velocity ?   Plunges back towards the ground ?  Or simply fails to  accelerate ?

The SSTO starts running out of delta-V and begins to lose altitude, even though my speed is above 1000 m/s. It also begins to become harder to control.

[AeroGav]

3 minutes ago, Aero_T said:

The SSTO starts running out of delta-V and begins to lose altitude, even though my speed is above 1000 m/s. It also begins to become harder to control.

I am assuming that you are using RAPIER engines.     1000 m/s is not fast enough for rocket mode.     Rapiers can get you to 1400-1500 in level flight in airbreathing mode.  Level off at 17-21km for your "speedrun".     Once the acceleration starts to die down,  switch to rocket mode and pitch the nose up gently so that it's pointing 5 degrees above prograde.

For the loss of control problem,   i suggest installing the mod called RCS build aid.     It shows a red ball in the SPH which is where your CoM will be when the tanks are empty.   It could be that you have more fuel at the front of your ship than at the back and your CoM is shifting rearward, causing instability , later in the flight.   Alternatively,  your engines are not in line with the airplane's centre of mass.  This can happens with engine pods mounted low under the wing or above the fuselage - and as the air gets thin,   the torque from the off axis thrust overwhelms the aerodynamics.   RCS build aid helps with this too, it gives you a torque number which you can try to minimise.

Finally,  you can make it easier to make small adjustments to your flight path (you need to be gentle at very high speed) by right clicking on control surfaces (Especially pitch / roll control ones) and using the "limit authority" slider.    When flying to orbit you never want more than 10 degrees pitch anyway.

You can also use the fine rotate tool on your canards/tailplane so that the airplane cruises with a slight nose up angle (2-5 degrees above prograde) with SAS off and no control input from the pilot.   You can do that by trial and error by making a few short flights lasting less than a minute.   Makes the plane less hard work at high speed

[Aero_T]

Great, thank you. Hopefully, I can get this thing into orbit.

[Foxster]

One word: Drag. 

It is far too busy. Loads of stacks and stuff all over the place. You need to simplify and streamline it. 

[Aegolius13]

 

6 hours ago, AeroGav said:

It could be that you have more fuel at the front of your ship than at the back and your CoM is shifting rearward, causing instability , later in the flight.

This seems like a major suspect.  You have a lot of heavy engines at the very back, and not a lot of weight up front to balance it out.  With other improvements, you can probably lose some engines.  But also consider ways to move your engines further up.  The real-life Skylon spaceplane concept, which has engines at the wingtips, is a good example.  

 

4 hours ago, Foxster said:

One word: Drag. 

It is far too busy. Loads of stacks and stuff all over the place. You need to simplify and streamline it. 

This too for sure.  It's kind of a vicious cycle.  When a plane is draggy you need to add more engines, which add extra weight, shift mass around, and generally cause as many problems as they fix.  

A few other points:

• I can't tell for sure based on the screenshot angle, but you don't have much wing at all for something that size.  And wings that angle up or down (dihedral / anhedral) have less lift still. BigS wings are really good as they are big (so decrease part count and floppiness), have high heat tolerance, and can store fuel.
• All the little protuberances (RCS ports, solar panels, antenna) add drag.  If possible, either eliminate or stow in your cargo bay. 
• On the antenna note, you can get by with the basic one if you're just in LKO.  Might not need any at all.  
• If you really need RCS (and you probably don't unless docking with a space station or whatnot), consider using  Vernors instead.  They're lower drag, more powerful, and then you can get rid of monoprop altogether.  On that note, you have a LOT of monoprop.  Not sure what your mission is, but you likely won't need all of it, so can trim some weight there.
• Those airbrakes on the front of the plane are going to kill your stability when deployed.  If you need them, move to the back.
• Echoing @AeroGav, KER is showing a lot of torque.  This is likely because your center of thrust is above or below your center of mass.  This will make your plane want to pitch up/down, which will force your control surfaces to compensate (adding drag).
• At least at the outset, I'd ditch the nukes.  They're great for long-range, low-thrust stuff, but if you're just trying to get this thing to orbit, they're not really in their wheelhouse.
• Your readout shows only 2200 ISP - it should be 3200 for Rapiers in jet mode.  Do you have something else going (rocket mode Rapiers, the nukes, etc) when you start? 
• That FAT tailfin is liable to blow up -- its heat tolerance is really poor.  You don't need much (or possibly any) yaw control, so consider replacing with a wing segment.  Also move to the back - that's what makes it work as a stabilizer (think fins on a dart or arrow).

[AeroGav]

Thanks for the picture.    I'm surprised it flies at all !

1.  Too many engines, too many fuel tanks,  not enough wing for the massive weight this thing must have

2.  Too much of the airplane's dry weight is at the back of the ship. You can see the yellow ball is at the back of the cargo bay.    All of the cargo and most of the fuel is in front of the yellow ball.    When these are gone the airplane will be very tail heavy.

3.  The tail fin should be as far back as possible so it has more leverage

4.  Pitch controls are very close to the centre of mass.   This might correct itself if you can move the CoM forward a bit, or you will need to add some canards.

As for drag...

Fuselage generates most of the drag in KSP.    I suspect that due to the poor flying qualities of that airplane,  it is only able to get enough lift by pitching the nose far above prograde and is only able to maintain control by operating the control surfaces at large deflection angles.   I suspect this is especially the case at high altitude.

The Space Shuttle style engine mount at the back of the main stack has three 1.25m engine attach points, which you are using, but it's also got a single 2.5m attach point in  the middle.    Unused attach nodes create a lot of drag in KSP,  it treats them like a missing nose cone (pointing forwards or backwards is the same in KSP), also all joining nodes must be of the same size,  eg. you have to attach something with a 2.5m mating surface to this unused node to avoid the drag penalty.     So you can either

a) Put a 2.5m tri coupler on the unused 2.5m mode.  and move some of your wingtip engines to the extra mounts this gives you 

eg. like on this

or 

 

b) say goodbye to the space shuttle engine mount altogether, and fit a mk3 to 2.5m adapter instead,  with a 2.5m triple mount behind that

[KerikBalm]

Quote

How can I get this SSTO to orbit?

I don't think you can... you need a massive redesign.... to the point where its not the same craft

1) You have no excess liquid fuel, and your dV readout is showing your dV from only using rapiers in LF only mode (3200 Isp) and LV-Ns (800 Isp). That dV readout is trash in this instance. Your dV will change, a lot, when the air runs out. You also have less dV than that in airbreathing mode (assuming you run the nukes at the same time) because the nukes give you less dV in the atmosphere. You will have excess oxidizer that is just useless mass

2) You probably need more wing area, and you fly it too slow. " begins to lose altitude, even though my speed is above 1000 m/s. "... air up high is very thin, and rapiers can get you much faster than 1,000 m/s... 1,000 m/s is not enough. At high altitude, you should be much closer to orbital velocity so that your wings don't need to do much

3) Your Com is going to shift way back after deploying a payload and using fuel. Move your side engines forward by a lot... along with some wings, your CoL seems pretty far back for flight on takeoff.

4) That vertical stabilizer is far too close to the CoL and Com, move it much farther back, its not doing anything where it is

Get rid of the airbrakes in front of your CoM, those are going to make it really unstable when deployed

Remove extraneous doodads... I see a Communotron 88-88 (undeployed) on your craft... what is that even for? its useless during flight (can't extend it until space/high atmosphere), that's something you'd use to connect to home from Jool, not LKO (since this seems to be an attempt at a shuttle to LKO)

[NHunter]

You also seem to have way too much oxidizer.

Based on KER readout, all of your tanks are LfOx ones. Nukes don't use oxidizer, nor do rapiers in air-breathing mode. You can safely replace half of your fuel tanks with Lf-only ones (Mk3 does have those, right?). That should give you some extra dV.

[steve_v]

23 hours ago, KerikBalm said:

I don't think you can... you need a massive redesign.... to the point where its not the same craft

This^.
If it were me, I'd start again with an airframe that looks vaguely plane-ish.

Why would anyone put airbrakes there anyway, of all places?

[Foxster]

5 hours ago, steve_v said:

Why would anyone put airbrakes there anyway, of all places?

If you wanted to de-orbit back-end first then they could be very handy there. 

[steve_v]

4 hours ago, Foxster said:

If you wanted to de-orbit back-end first

What, give up pretty much all control, and all that lovely high-AoA braking? Of course you do need reasonable wing area for that.

I suppose you could re-enter facing the wrong way, but why would you, there's no advantage.

[Foxster]

9 minutes ago, steve_v said:

What, give up pretty much all control, and all that lovely high-AoA braking? Of course you do need reasonable wing area for that.

I suppose you could re-enter facing the wrong way, but why would you, there's no advantage.

More heat-resistant parts in the direction of travel?

[Aegolius13]

10 hours ago, Foxster said:

If you wanted to de-orbit back-end first then they could be very handy there. 

I would think opening a mk 3 cargo bay would also generate quite a bit of front-side drag, if one were so inclined.

[steve_v]

9 hours ago, Foxster said:

More heat-resistant parts in the direction of travel?

Perhaps, but then a controlled lifting re-entry largely removes the need for that heat tolerance.
If you're not going to use those wings to control your descent, why not just put a 'chute on the nose and be done with it?

I don't know much about stock aero these days, but at least in FAR a craft that is stable going backwards is probably a bear to fly going forwards. I'll take stability in normal flight over a little more heat-tolerance any day of the week.