Spaceplane issues in the transsonic region...

[MaverickSawyer]

I've been trying to build a successful SSTO spaceplane in 1.0+, and EVERY. SINGLE. DESIGN. has had issues around the Mach 0.95-1.45 region. I just seem to hit a wall, and I need to use rocket boosters to get through it. I've been trying to climb and accelerate at the same time, and I'm usually in the 3-7 km altitude region when I hit the wall.

The only thing that's come remotely close to making it was this:

With this a close second:

I've done every practical engine combination, without success, so I'm pretty certain it's an aerodynamics issue I'm not familiar with. Is wing shape a contributing factor? Or am I trying to use wings that are too large?

Edited July 28, 2015 by MaverickSawyer

[I_Killed_Jeb]

Here are two really helpful answers from my own recent thread on this

http://forum.kerbalspaceprogram.com/threads/129775-SSTO-build-flight-tips-for-0-9-vet?p=2102219&viewfull=1#post2102219

http://forum.kerbalspaceprogram.com/threads/129775-SSTO-build-flight-tips-for-0-9-vet?p=2102518&viewfull=1#post2102518

Using those tips I'm back to my old form of designing & flying SSTOs with ease

[Lighthawk]

First hunch, you're starving your engines by climbing too aggressively, so you're not moving fast enough to shove enough air into your intakes to offset the decreased pressure as you climb. Try right clicking on one of your jet engines after take off and check the thrust level. If it's steadily dropping, you're climbing too fast. Pull your nose down and get it to a point where your thrust is slowly climbing as you ascend, and keep nudging the nose down as needed to keep your thrust increasing for as long as you can.

[Feradose]

Cool Blackbird lookalike.

Try leveling out and going faster for a few secons before pulling up at 7-10kms

[satnet]

The thrust from jet engines now depends on airspeed. If you right click on your engine you'll see that as you pitch up and down the thrust will vary a little and that as you lose speed it will fall and as you speed up it increases. It also varies with altitude, but you aren't high enough yet for that to be a major factor. Both of your planes look like they use RAPIERs which have a peak thrust much higher on the velocity curve than other engines. This makes them great high altitude, high speed engines, which is why I use them in most of my post-1.0 planes, but means that if you aren't careful you can get stuck at an apparent wall when you lose too much speed and you no longer have the thrust to go faster. You need to either pick up speed before climbing or add an engine that peaks at a lower velocity to give you a boost. Don't climb too aggressively unless you have a lot of excess velocity/thrust (too aggressive depends on your plane, but usually 5-10 degrees above prograde is my rule of thumb below 10 km). If you start to lose thrust and airspeed you can point prograde and build it back up. This works even if you lose altitude as long as you don't fall too far and you pull up slowly because you'll maintain most of the speed you gained which will give you the thrust to push through. Alternatively adding a Turbo RamJet is a very easy way to get plenty of thrust in the low altitude, low speed portion of your flight, but means extra weight and air intake requirements.

Also drag and weight are extremely important factors. Yours look reasonably aerodynamic. They look like they might be a little heavy, particularly with just two RAPIERs, but I tend to try to design my SSTOs as light as possible which may give me a bias. You can probably get away with less wing area, but your designs are still fairly reasonable.

The posts pointed out by I_Killed_Jeb are good specific examples how to design planes and ascent profiles that work. SSTOs are so varied it is good to know why things like this are necessary so you can accommodate them in your design or ascent profile when they don't quite match up with specific examples.

[GoSlash27]

MaverickSawyer,

Both of those spaceplanes look underpowered to me.

You should keep your takeoff weight under 20t per turbojet and under 17t per rapier.

Your wing loading looks fine, but I'd recommend not using deltas as your primary lifting surface. their parasitic drag is higher than average. Try the swept wing type b. If you have them unlocked, the shuttle wings are excellent. Plus you can store LF in them, which kills 2 birds with 1 stone.

Best,

-Slashy

Edited July 28, 2015 by GoSlash27

[MaverickSawyer]

First hunch, you're starving your engines by climbing too aggressively, so you're not moving fast enough to shove enough air into your intakes to offset the decreased pressure as you climb. Try right clicking on one of your jet engines after take off and check the thrust level. If it's steadily dropping, you're climbing too fast. Pull your nose down and get it to a point where your thrust is slowly climbing as you ascend, and keep nudging the nose down as needed to keep your thrust increasing for as long as you can.

I've been running a solid 10-15 degrees pitch hold the entire way. I guess I need to hold the nose down a little more.

Cool Blackbird lookalike.

Try leveling out and going faster for a few secons before pulling up at 7-10kms

Thanks. It's using parts from SuicidalInsanity's Mk2 expansion pack.

The thrust from jet engines now depends on airspeed. If you right click on your engine you'll see that as you pitch up and down the thrust will vary a little and that as you lose speed it will fall and as you speed up it increases. It also varies with altitude, but you aren't high enough yet for that to be a major factor. Both of your planes look like they use RAPIERs which have a peak thrust much higher on the velocity curve than other engines. This makes them great high altitude, high speed engines, which is why I use them in most of my post-1.0 planes, but means that if you aren't careful you can get stuck at an apparent wall when you lose too much speed and you no longer have the thrust to go faster. You need to either pick up speed before climbing or add an engine that peaks at a lower velocity to give you a boost. Don't climb too aggressively unless you have a lot of excess velocity/thrust (too aggressive depends on your plane, but usually 5-10 degrees above prograde is my rule of thumb below 10 km). If you start to lose thrust and airspeed you can point prograde and build it back up. This works even if you lose altitude as long as you don't fall too far and you pull up slowly because you'll maintain most of the speed you gained which will give you the thrust to push through. Alternatively adding a Turbo RamJet is a very easy way to get plenty of thrust in the low altitude, low speed portion of your flight, but means extra weight and air intake requirements.

I knew about the airspeed and altitude causing thrust changes, but the bit about the thrust curve was a new one for me. And the "wall" description is spot on. Once I clear Mach 1.45ish, I would shut off whatever booster I was using and would be able to accelerate on jets alone.

They look like they might be a little heavy, particularly with just two RAPIERs, but I tend to try to design my SSTOs as light as possible which may give me a bias.

MaverickSawyer,

Both of those spaceplanes look underpowered to me.

You should keep your takeoff weight under 20t per turbojet and under 17t per rapier.

Guess I should have added a second pic of the big Mk 4 bird when viewed from in front... It's got a matching set of engines below the wing for added thrust and to minimize the offset between the center of thrust and the center of mass.

Your wing loading looks fine, but I'd recommend not using deltas as your primary lifting surface. their parasitic drag is higher than average. Try the swept wing type b. If you have them unlocked, the shuttle wings are excellent. Plus you can store LF in them, which kills 2 birds with 1 stone.

Best,

-Slashy

Those are the B9 procedural wings, actually. and they can be made wet wings, as those ones are. I tend to use the LF/O ones, though, to provide more rocket burn time once it's been refueled on-orbit.

As for the deltas... didn't know about that. That would certainly explain why the upper craft was having so much trouble. Looks like I'm going to have to go the route of the lower one, then.

Thank you for the tips and ideas, folks. I'm going to go back to the Hangar and see what I can shake out with the new information.

- - - Updated - - -

Okay, going to mark this one as "ANSWERED" now... Turns out I was underpowered on all my designs. Woefully underpowered. I switched to some MUCH bigger engines and she was supersonic in no time at all.

[gilflo]

To get more thrust to pass over M 1.45 just add afterburner on your jet engines by using the Kerbokatz small utilities: http://forum.kerbalspaceprogram.com/threads/116034-1-0-X-KerboKatz-SmallUtilities-V1-2-2-07-08-2015-Now-with-AutoBalancingLandingLeg!

If your engines are not in the config file, look at the file, it's very easy to add an engine.

[Sharpy]

Yep, way underpowered for their bulk.

Also, Rapiers have a poor performance curve early on. You might add a pair of turbojets - or just push through transsonic to some 400m/s using rockets. Still, I don't see your Speedbird Heavy reaching orbit with less than 6 engines, more likely 8. Speedbird 1 might do it in 4, but to me it seems it has too much wing area anyway - with so much lifting MK2 surface it really needs token amount of wings - ones so big introduce some serious drag.