Awkward thing going on with my SSTO "Space"plane

[StainX]

Hey there, lovely ksp fans

So... i kinda messing around since 1.0.1 release, and noticed something changed, a new atmosphere layer maybe?

Or twice the drag now!?

I don't know exactly whats going on, and thats why i'm here.

Pictures mean more than hundred words, so:

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Also tried other intakes, still nope...

Edit: Craft file

Edited May 2, 2015 by StainX

[Bilfr3d]

With the recent change in aerodynamics, the CoM + CoL alignment is pretty much useless to follow to begin with. A craft that has completely centered CoM and CoL has the potential to flip rapidly out of control depending on the streamlining of the craft. The engine speeds are also aligned with aircraft speed, and the with how high the craft is in the atmosphere. The higher you go, the thrust will begin to further cut off. I find that about 12-11km a sweetspot for air-breathing crafts (including semi-airbreathing SSTO's). By the time you're at about 16-20km, the thrust on the engines is down to about 30-10. In-case you didn't know about any of this, KSP has had a very large aerodynamics overhaul, which now pretty much mimics FAR (a mod that made the aerodynamics somewhat realistic), and has made aerodynamics far more realistic than pre-1.0.

[StainX]

Thanks for the quick respons ToTheMun!

It doesn't flip or anything... its just stuck on speed ~300m/s (surface) at about 15km up with TWR 1.

Its coasting at that altitude and speed until fuel runs out.

The only thing that works is to use an rocket motor to push me higher up... then the RAPIER's start to get some power again, it just looks like there is a wall of soupyness at about 10-20km

[eddiew]

My first thought is that you're climbing too aggressively for that particular airframe. It looks like you have some wing surfaces on the underside, and they're draggier than ever before in 1.0.2 (and you may not need them all). You may just need a bit more speed before you begin your ascent properly.

Best advice is a reactive flight path where you follow the mach effect visuals. This will keep your engines effectively turbocharged and delivering 2-3x as much power.

Turns out the key is to follow the Mach effects, without blowing up. Basically, the closer you are to Mach effect territory, the more air you will have for your engines and the higher their thrust will get (the effect is quite dramatic, actually). If you go too slow, they won't give you enough oomph, so pull down AoA again until you get mach effects and a decent acceleration, then go up basically as fast as you can. But, once you get close to the thermal barrier (about 1km/s at 20kms, less if lower) you have to pull up to go higher, faster and thus avoid burning up.

TLDR: stay ASL until you see white air effects around the plane. Pilot so's to stay in the white fog during ascent, without entering the red mist until 20km+. Level off if you lose the fog, pull up more if it goes orange. May or may not work, but worth a try!

[StainX]

ok eddiew, just tested it.... still at an ascend profile of constant 10° it maxes out at 300m/s

I guess its really just those circular intakes that doesn't allow more than 300m/s at altitude ASL to 20km.

If i switch to closed cycle and push me up further (speed 700m/s) and turn back to air breathing its start gaining speed again.

All in all i guess its just... those intakes don't like altitudes between 10-20km

So i replaced em with ram air intakes.... its working much better, but there is still that "airbrake" effect at about 10km up

Edit: Is it because im flying at terminal velocity!? Alt 8km; speed 300m/s; TWR 1.5; not accelerating; pitch: 10°

Edited May 2, 2015 by StainX

[FlipNascar]

So I grabbed a copy of your craft and had a quick run with it. Two ways to solve this, both include switching to ram air intakes.

1) Moar boosters! Or...

2) Change the way you're flying it. Rather than lighting the rockets earlier, you can always descend to use gravity and let it help you gain some more airspeed. It seems that this plane's problem is that it suffers massively at the transonic boundary and simply cannot push through it. So you can either flip on the rockets to push through it. Or you can dive to get gravity to help you push through it. Once you're through the sound barrier, you'll see that your TWR goes up the faster you go, until you hit Mach 3.7.

Here's an image album that might help. You'll have to find the sweet spot for your Firebird, but I let it climb to around 15km before I just told the crew to hold prograde, let it drop down a couple of thousand metres before pointing the nose back up to start getting positive rate of climb.

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[StainX]

Tanks for the help FlipNascar...

I guess i just need to put more time into spaceplanes, with that new complex aerodynamic and heat stuff going on.

But i still don't get why this bird will not accelerate anymore at certain altitude even with a TWR of 1.5.

[eddiew]

ok eddiew, just tested it.... still at an ascend profile of constant 10° it maxes out at 300m/s

...

Edit: Is it because im flying at terminal velocity!? Alt 8km; speed 300m/s; TWR 1.5; not accelerating; pitch: 10°

Constant ascents are no good in 1.0, you need to be following the visual mach effects. The way I've been flying is a gentle 10-15 degrees off the runway until the visuals appear, then nose up to just keep on the edge of the air disturbance, usually around 30-40 degrees. Around 15km that gets a lot more shallow, but by 20km I'm doing 650-750m/s even with just turbojets. Rapiers keep pushing for at least 4 more km and should take you beyond 850.

Of course, if you're never getting mach effects, even ASL... you may have too much drag going on. Without the visuals, your engines aren't getting the full airflow, and you're bumbling along at half power or worse; which of course just makes reaching the sweet spot even harder.

What sort of kN are your engines pushing out? I'm getting ~200-250 per turbojet with the ascent profile above.

I don't think there's a terminal velocity issue going on, but the 1.0.1-2 patches did make the air thicker below 14km, so it's possible that a high drag ship might well be hitting it.

[Merandix]

Looking at your mass, you might need an extra engine.

Your ship weighs in at 52-53 tonnes. That's pretty hefty. For example, my 31 tonne space plane does a reasonable orbit with 3 RAPIERS... you're over 20 tonnes heavier, yet only have a single extra RAPIER. As a rule of thumb, I go by 1 engine per 10 tonnes of plane. Though your fuel consumption will go up too obviously.

Ideally, you want to be going 1000+ m/s when switching to closed cycle. It's more involved piloting that it was before, and thus a bit harder. My best was 1120 m/s at switch. The faster and higher you end up going while on air, the less dV needs to come from your LF+O supply. It's a delicate balance, quite fun in fact! Reminds me when I figured out spaceplanes the first time

Edited May 2, 2015 by Merandix

[eddiew]

But i still don't get why this bird will not accelerate anymore at certain altitude even with a TWR of 1.5.

I'm not convinced KER is accurately reporting the TWR of engines at this time... I've had problems getting a rocket reading 1.4 TWR off the ground at all!

- - - Updated - - -

Looking at your mass, you might need an extra engine.

Inclined to say that the engine/mass ratio is actually spot on...

and can get to orbit. I really think it's about not overdoing the wing area (which got both a drag and lift buff in 1.0.1 so you really don't want/need much) and chasing those visual effects

[Merandix]

I do believe there's a major difference in flight characteristics between RAPIERS and turboramjets. So that may make a difference, but that's speculation, as I haven't experimented enough to say anything about that yet.

Judging by how you react, I do need to step up my game, if you haul 13 tonnes per engine up there, and I only 11, and we get similar results... then I definitely need improvement

[StainX]

Those engines are capable of pushing those 55t into orbit, and not even that bad.. i was just wondering why it wouldn't accelerate with a TWR like a rocket in atmosphere...

Oh also eddiew seems right here, maybe Engineer is having some hard time calculating stuff at the moment, in orbit it told me i have 580m/s dV left, so i made a node with 580m/s and after that burn i still got 160m/s left xD

[Merandix]

Yup, this does it, I'm an official spaceplane idiot now

Thanks for showing this to me... *cracks knuckles* I have work to do

I'm using 8 engines on an 80 tonne + 10 tonne payload space plane. This tells me that I have work to do, time to rip those wings off and attempt a more minimalist approach perhaps. *goes off building space planes*

[eddiew]

Judging by how you react, I do need to step up my game, if you haul 13 tonnes per engine up there, and I only 11, and we get similar results... then I definitely need improvement

I apologise; I lied. Onager C is actually 30.7T wet and loaded

Really think there's a lot to be said for cutting down on wing area in 1.0.2... you just don't need much at all. Because nuStock is new and improved like that

I haven't made a rapier plane since 1.0.1 hit, will make that my next task. At least we've got nervas sorted out (a bit) now

[StainX]

Built something new with tweakscale, and it works sooooo much better!

So you guys like pics:

[eddiew]

Lol, is that a downscaled MK3 hull, or an upscaled rapier? xD

[StainX]

Lol, is that a downscaled MK3 hull, or an upscaled rapier? xD

Upscaled rapier ~2000 kN thrust each

[eddiew]

Upscaled rapier ~2000 kN thrust each

That's just... I'm not sure if that's cheating or not tbh Doubling the size, in my head, should quadruple the power (4x the exhaust and air intake cross sections) but it sounds like it's gone on part volume and given an 8x increase...

It's nice that it works, but I think I'll wait on B9 for some 2.5m sabre engines and see how they balance those

[StainX]

This is thing is wet at 161.5t (without payload), so this thrust is needed

[eddiew]

Wow... I just tried out a plane I made for 1.0 on tuesday that has rapiers and a nerva... and rapiers really suck in 1.0.2. They just don't do anything Your earlier problem of the 300m/s threshold is very reproducible with this bird.

- now crippled as of 1.0.2.

Not sure whether it's the shock cones, or the wing area; I shall have to take this one back to the drawing board completely. Still, should be an interesting process of working how how to fix it again!

- - - Updated - - -

This is thing is wet at 161.5t (without payload), so this thrust is needed

Tbh, that's why I like B9. It comes with nice big engines for SSTOs and keeps your part count down ^^

[StainX]

So it has something to do with 1.0.2 and not with Kerbal Engineer!? A very confusing update it is

[eddiew]

So it has something to do with 1.0.2 and not with Kerbal Engineer!? A very confusing update it is

There was a little 1-liner in the 1.0.1 notes that said "aerodynamics". What actually happened was an increase in wing lift, and an increase in drag all around - it was meant to enable capsules to return to ground sensibly, without having to deploy parachutes (which now burn in 1.0.2 if you deploy them while in the fire). The aero change is apparently much heavier below 14km - although like you, I've taken a rapier based plane to above that and so far it's still not willing to go at any decent speed.

Upshot of this is you don't want any more wing than you absolutely have to have, but it's ok because falling speed is now much slower anyway. We're almost back to the soup-o-sphere days :/

On the up side, nervas don't overheat anymore, so if you can ever get them out of the atmosphere they're good again.

WRT my rebuild... this plane looks nothing like it used to for 1.0, and it still doesn't work right. I'm finding that a combination of rapiers and turbojets can be good though; the turbo produces more thrust at low altitudes and speeds, and can kick the rapiers into the sort of speed that they need to become effective. I'm fairly sure that the devs have balanced it so that rapiers alone aren't enough and you have to support them with other engines unless your plane is very tiny...

[Wanderfound]

Interestingly, I just took a smallish four-RAPIER spaceplane up for a spin. Take off, set it into a 45 degree climb and leave it there; by the time the RAPIERs switched modes, the apoapsis was already well over 100km. The new aero appears to favour ridiculously overpowered ships, so long as you manage to avoid melting your wings off during the ascent.

[eddiew]

Interestingly, I just took a smallish four-RAPIER spaceplane up for a spin. Take off, set it into a 45 degree climb and leave it there; by the time the RAPIERs switched modes, the apoapsis was already well over 100km. The new aero appears to favour ridiculously overpowered ships, so long as you manage to avoid melting your wings off during the ascent.

Nice... best AP I've had on air was 36, using 1 turbo and 2 rapiers. Wasn't carrying enough fuel to circularize though I'm starting to feel that small and large spaceplanes are viable, but medium are in some strange netherworld...

*edit* I'm starting to suspect that rapiers don't hit their power band until about 420m/s... once I can get a plane beyond that, rapier power shoots up past 200kN, but below that speed I'm lucky to get 100 out of them. Tempted to put a disposable SRB on there for some kick off the line

Edited May 2, 2015 by eddiew

[eddiew]

Claymore D, three rapiers required to get a 30T plane to orbit with 270m/s left in the tanks. Which makes them hardly better than turbojets in my book, since two of those and a pair of LV-45s can easily get the same weight to LKO :/

And the wings are ridiculous. What wings? The canards turn out to be optional for flight, but the liftoff speed becomes outright dangerous (180m/s).

Looking forward to nuFAR and some predictable behaviour...