Absolutely confused with the performance of my SSTO frame:

[KristopherKerblumbus]

Okay, so maybe someone can help me out here. As you can see in this screenshot: http://imgur.com/a/1DNJ4 according to the stats I normally pull from KER, I should have an abundance of all the good things I need to get me into space (TWR, Thrust, DeltaV, etc.) But when I get to about 340m/s, everything just flatlines. I think drag may have something to do with it, but I don't remember how to pull up the drag overlay or calculate that... Here's the craft link if anyone's interested: https://kerbalx.com/KrisKolumbus/Stock-SSTO-Test

Edited December 10, 2016 by KristopherKerblumbus

[Goomblah]

RAPIERs tend to struggle near the sound barrier; it's fairly common for builds using only RAPIERs to have some difficulty going supersonic. I would suggest staying subsonic and climbing to a little under 10km before going for your speed run; you'll encounter much less drag at that altitude than your 4200m screenshot. If necessary (though with 1.5TWR I don't think it should be) you can actually pitch down slightly as you begin your speed run to break the sound barrier, at which point the RAPIERs will start producing much more thrust and you can pitch up again.

You may also save a little mass and drag by removing your surface-mounted intakes and replacing the fuselage tail pieces in the wing with shock cone intakes, as well as removing the 4-way RCS ports on the top and bottom and instead using single RCS ports on the sides. You'll end up with more IntakeAir than you need, but AFAIK shock cones are still probably the best nosecone option out there.

Edited December 10, 2016 by Goomblah

[KevinW42]

44 minutes ago, Goomblah said:

You'll end up with more IntakeAir than you need, but AFAIK shock cones are still probably the best nosecone option out there.

Strangely enough, shock cones are actually one of the draggiest nosecone options in the game and you're actually better off using the circular intake.

[foamyesque]

7 minutes ago, KevinW42 said:

Strangely enough, shock cones are actually one of the draggiest nosecone options in the game and you're actually better off using the circular intake.

Not if you want air beyond Mach 1 you're not

EDIT:

 

You have a bunch of draggy radial attachments (the radiators in particular are murder) and you're climbing at a pretty sharp angle. Drop closer to horizontal and/or do some streamlining work and you should blast right through it.

Edited December 10, 2016 by foamyesque

[KristopherKerblumbus]

Okay. Well it's not the air, because I've got a wicked surplus of intake air already, as KER shows. Also, it's not the fact that the radials can't take in enough air because if I can get far enough beyond the sound barrier I can power clear up to 11-1200m/s before things start getting crazy, which you can easily demonstrate if you strip all the greebles off (RCS, parachutes, etc) and fly it "bare". *Is* there a "Drag overlay" button like there is for temp and aero?

[foamyesque]

12 minutes ago, KristopherKerblumbus said:

Okay. Well it's not the air, because I've got a wicked surplus of intake air already, as KER shows. Also, it's not the fact that the radials can't take in enough air because if I can get far enough beyond the sound barrier I can power clear up to 11-1200m/s before things start getting crazy, which you can easily demonstrate if you strip all the greebles off (RCS, parachutes, etc) and fly it "bare". *Is* there a "Drag overlay" button like there is for temp and aero?

 

The aero overlay does display drag; the giant red lines. If you want more specific numbers, hit Alt-F12 and go to the Aero tab (under Physics) to turn on the action menu and GUI tips. Handy for diagnosing problems.

In your case, though, I'm pretty sure it's just too much stuff hanging off the airframe

[GoSlash27]

If you're crapping out at Mach 1, you have too much drag. You have more than enough thrust for a ship of that size, but drag is the #1 most important thing. You're going to have to look through your design for items that are acting like brakes.

Remember that in KSP objects are just as draggy as they look and occluding them doesn't hide them from the airflow. If you have boarding ladders, lights, unconnected fuselage sections, etc. they are exposing their full blunt surface area to the airstream.

Best,
-Slashy

[swjr-swis]

7 hours ago, foamyesque said:

the radiators in particular are murder

Start with this. Radiators may look like they should be pretty aerodynamic, but they work like extended airbrakes when it comes to KSP stock drag.

[GoSlash27]

It looks like you've got a retractable boarding ladder installed. I bet if you get rid of that, it'll be fine.
 Those things are *seriously* draggy.

Best,
-Slashy

 

[KristopherKerblumbus]

Well thanks. And crap. I like having all of those "features" for sorta a roleplay style of play--being able to exit the craft, an emergency rescue/abort system, lots of RCS control...works *really* well with B9 parts, but I was specifically trying to branch out into stock KSP sets. Ah well. Thanks for the advice guys. So I'm basically trying to punch a brick through the sound barrier with all of the crap I've got hanging off my airframe. I'll revise and try again!

[fourfa]

You can often get around having ladders by retracting the landing gear, then kerbals can clamber up onto the plane and board.

Intakes are a matter of having just enough and no more - try replacing the front tapers on your engine nacelles with a pair or spike or ramscoop intakes.  Delete the radial-mount intakes.  

There is no need for radiators at all in the stock game, except for mining.  They don't solve any heating problems with SSTOs that can't be better solved with a different flight path that heats the plane less.

If you're having the cockpit explode from heating (not uncommon if you go too fast at too low altitude), try the Magic Antenna trick.  Put an unextended Communitron-16 antenna on the nose, and use the offset and rotation widgets (2 & 3) to get it exactly on the tip of the nose like a spike.  You can do the same with a Place-Anywhere RCS thruster.

The quad-RCS thrusters are very draggy in open air - see if you can replace them with only Place-Anywhere thrusters which are much more slippery.  If you can reduce the number you need - just a single ring of four outward-facing Place Anywhere at CoM, plus one on the nose (see above) and one on the center of the tail - you'll drop a ton of drag.  Lately my favorite trick is to have a cargo bay at the center of mass, using RCS Build Aid to better visualize how the CoM changes as fuel burns off, then hide all the RCS (and other odds and ends - solar, batteries, reaction wheels, science containers, etc) from drag in there.  RCS placement doesn't need to be completely perfect either, if you have enough reaction wheel torque to fight the offset RCS torque.  I can go into more detail if that doesn't make sense.

[GoSlash27]

^ I agree with what fourfa says here. Building a successful SSTO space plane isn't just about having enough t/w and fuel. The #1 most important consideration is drag. Not just drag in the real world, but drag as KSP processes it. Even at extremely low t/w and fuel fractions, you can still build a successful SSTO space plane. It just needs to be aerodynamically clean.

So in addition to what's been said above, I want to add "static wing incidence" to the recipe. The single largest contributor to drag at Mach 1 is a fuselage that's not aligned with the air flow. If you incline the wings a little, they still make enough lift to keep you flying while your nose is perfectly aligned with your prograde vector. This allows you to slip through the sound barrier with minimal resistance instead of trying to bulldoze it. My rule of thumb is that if it can clear Mach 1, it will easily go hypersonic.

Best,
-Slashy

[theend3r]

1. Wait for FAR or get a dev build
2. Go to space

[KristopherKerblumbus]

Just now, theend3r said:

1. Wait for FAR or get a dev build
2. Go to space

I won't use FAR on personal principle.

[GoSlash27]

4 hours ago, KristopherKerblumbus said:

I won't use FAR on personal principle.

A man after my own heart

-Slashy

[MaxL_1023]

Drag coefficients in general almost always increase when you go trans-sonic - once you clear Mach 1.25 or so you often end up in better shape. My best luck with SSTOs has been to get in level flight quite high up, then pick up speed. Not so high as to run out of air, but 12000-15000 meters offers significantly less drag and lets you get away with more design-wise. 

[foamyesque]

I, too, like to include ladders, and even RCS quads, but you need to be aware you're paying a drag price for them and plan accordingly. This is particularly true if they're mounted on the nose of things; orientation can matter a lot for things like Vernor RCS engines or ladders.

 

[Sharpy]

10 hours ago, KristopherKerblumbus said:

Well thanks. And crap. I like having all of those "features" for sorta a roleplay style of play--being able to exit the craft, an emergency rescue/abort system, lots of RCS control...works *really* well with B9 parts, but I was specifically trying to branch out into stock KSP sets. Ah well. Thanks for the advice guys. So I'm basically trying to punch a brick through the sound barrier with all of the crap I've got hanging off my airframe. I'll revise and try again!

Add an MK2 cargo bay and stow that all inside. You may keep it open at all times while in orbit and your profile won't matter there. You may even stow some extendable radiators. They look silly when extended, but work fine. As for RCS, Vernors are pretty streamlined.  Also, if you're not averse to mods, MK2 stockalike expansion has some sweet RCS for spaceplanes. But in essence, stow everything you don't need out in the open into a cargo bay (or service bay from the mod), then climb to 10km and try again

Or... Moar Boosters. Give up on 'pure SSTO', attach, say, four Hammers, as soon as you breach 500m/s VTEC KICKS IN YO, and all you need to worry about is heat and fuel.

Edited December 11, 2016 by Sharpy

[KristopherKerblumbus]

1 minute ago, Sharpy said:

Add an MK2 cargo bay and stow that all inside. You may keep it open at all times while in orbit and your profile won't matter there. You may even stow some extendable radiators. They look silly when extended, but work fine. As for RCS, Vernors are pretty streamlined.  Also, if you're not averse to mods, MK2 stockalike expansion has some sweet RCS for spaceplanes. But in essence, stow everything you don't need out in the open into a cargo bay (or service bay from the mod), then climb to 10km and try again

I think I've got a solution. We'll see--running through testing right now. Yeah, I know that B9/Mk2 stockalike--hell, just about any spaceplane mod can do RCS better than stock. But I wanted an "almost-full-stock" airplane (I love Procedural Wings) as much as I could--sorta a self-restriction. I have Mk2 expansion, Mk3 expansion, B9, OPT, and Mark IV spaceplanes. Plus about 100 other mods I've got no shortage of tools in my toolbox and I love it!

[RocketBlam]

Tried to load your file, it says it has B9 parts in it (procedural wing, and something else).

Anyway, unless things have changed, those Adjustable Ramp Intakes just don't cut it for anything high-performance. I was never able to create an SSTO that relied solely on them. But I have to say the aerodynamic/air intake rules have changed, and I don't understand most of it any more. However, I'd be surprised if those intakes were sufficient.

The ascent profile in SSTO's is usually very narrow in my experience. You have to be traveling at nearly max speed at any given altitude to keep the Rapiers at max power. So if I see my m/s going up rapidly, I know I can nose up some more. If it's barely going up (or going down), I have to nose down. If you can't keep accelerating while still ascending at least a little, then your craft is underpowered.

The speed where it is narrowest is about where you're having trouble, 350-400 m/s. So either fiddle with your ascent profile (nose down if you can do so while still ascending) or you don't have enough power - although from the looks of your ship, it doesn't look underpowered with 3 Rapiers. My guess is the intakes.

Edited December 11, 2016 by RocketBlam