SSTO no longer able to achieve orbit?

[drtricky]

So a few months back, I built a SSTO that was (barely) capable of getting in and out of low-kerbin orbit. When I tested the SSTO now, I found that it was completely unable to get into orbit, even if I removed some wings and added more fuel. 

 

My question is: Was there an update that changed things like aerodynamics or physics? I suspect that to be the culprit, but I could be wrong.

Edited January 16, 2017 by drtricky

[AeroGav]

You'll need to share the craft file to get a proper answer.

The atmosphere is a bit more slippery than it used to be, less lift, less heat and less drag. Most of us have found it easier to ssto than at any time since the game came out of beta.

[drtricky]

1 hour ago, AeroGav said:

You'll need to share the craft file to get a proper answer.

The atmosphere is a bit more slippery than it used to be, less lift, less heat and less drag. Most of us have found it easier to ssto than at any time since the game came out of beta.

Huh, guess it must be me then.

Here's the craft in mind: https://kerbalx.com/drtricky/FAB-41-Taurus-Spaceplane

[bewing]

First try:

 

I lost the canards just as I finished my suborbital burn. I think I could have saved them if I had known they were on the edge. Not sure it could reenter/land with no canards, though. All I can say is that it definitely makes it to orbit.

There are quite a few things I'd change with this spaceplane, though, if I were going to mess with it. The disconnected rocket fuel tanks. All those unused/unnecessary precoolers. Swapping out the central rapier for a nuke, for cruising around in space. And I'd definitely swap in some higher temp canards.

Edited January 16, 2017 by bewing

[drtricky]

31 minutes ago, bewing said:

(See post above)

Thanks for testing it! Seems like I'm simply not the best at getting into space, as I've mostly worked on combat aircraft so far. As a matter of fact, that spaceplane was modified from a heavy-bomber/fighter hybrid I built previously.

Just one more question, though. Do you still remember how you got into orbit? Did you climb at a certain angle, or follow a certain flight path?

[bewing]

Certainly.

Takeoff at 140 m/s. Reduce to 1/3 throttle. Initial climb at just above 10 m/s. Slowly increase throttle to 50% by 3500m, and not touching any attitude control keys at all. Full throttle at 7500m.

Use F to lower the nose a bit at 9km. Throttle back down to 50% at 1150 m/s for overheating in the canards (adjust throttle to get slow acceleration). Use F a couple more times to lower the nose just a bit over the next 10 km of altitude.

Wait for max airbreathing speed at a little over 1300 m/s and go back to full throttle. Nose about 10 degrees above horizon, prograde about 7 degrees above horizon. Wait for automatic rapier switchover at 29.8km. Speed falls slowly to 1300 m/s while waiting.

At closed-cycle switchover, change to Map mode & watch the Ap.

Now, what I should have done at that point was to switch SAS into prograde mode as soon as the Ap got to 45km or so. I could have throttled back a bit then, too. But I didn't -- I let it rise to 58km or so. When I switched back to stage view, one canard was already gone, and the second one blew right at that moment.

But once it's in prograde mode, get the Ap to 58km or so and then kill the engines. Coast up to 50km or so and then use gentle engines to push the Ap to 71.5km. Wait until Ap and then circularize.

 

Edited January 16, 2017 by bewing

[luizopiloto]

from ver. 1.1.3 to 1.2, drag mechanics has changed, and global drag was increased from 0.6 to 0.8.
Now spaceplanes needs to be more aerodynamic to achieve sufficient speed and reach suborbital flight.
This craft can reach orbit using less fuel than before:


This one in counterpart can not reach orbit anymore:

[drtricky]

32 minutes ago, luizopiloto said:

from ver. 1.1.3 to 1.2, drag mechanics has changed, and global drag was increased from 0.6 to 0.8.

Okay, so it's a combo of me not knowing rocket science, AND a drag increase.

[AeroGav]

This is a good opportunity to delve into Drag optimisation in the stock aero model.

The most important thing these days seems to be to ensure that every stack starts and ends with something pointy,  and that when going to a part with a smaller or larger diameter, the transition must be handled by an aerodynamic adapter.

 

Before changes -  Total drag 419kn at 202 m/s,  AoA  2.156 degrees

 

So, the core of the Taurus is a mk3 size stack which starts with something pointy (the cockpit),  but ends with a flat plate.  There is a rapier engine attached to the rear node of the last mk3 fuselage section, but this barely reduces drag because the 1.25m engine only has a tiny fraction of the surface area of the a mk3 part,  so drag for this section is calculated as 90% flat plate mk3 size,  10% engine nozzle.

You've also got a lot of radially attached 1.25m pre-coolers and fuel tanks.   Some of these have a nose cone (very pointy) or intake (somewhat pointy) at the front, but you've left a lot of these open with flat plate drag at the back.  Some of this could have been avoided by merging two short radial tank stacks into one longer one.

 

 

At the rear of the ship, you've got 6 pre-coolers radially attached to the final fuselage section.  These have rapier engines mounted on the back and are clipped into the fuselage.  Needless to say, the clipping does not reduce drag.  There is nothing on the front of these pre-coolers, so they are hit with flat plate drag penalty.

To resolve this,  I put a mk3 engine mount on the back of the fuselage.  This has 3 x 1.25m attachment nodes and a single 2.5m node in the centre.

The centre 2.5m node is used for a TVR-300L, an aerodynamic adapter for splitting a 2.5m stack into 3 x 1.25m ones.  This enables us to get a total of 6 engines on the back , without resorting to radially attached nacelles.

Finally, on a mildly controversial node,  most jet engines are highly pointy ways to end a stack.  Unfortunately,  RAPIERs, in common with most rocket engines, have 1.25m attach nodes on the back so they can be used in staged rockets.   This rear attach node, when left unused, generates a "flat plate" drag penalty.   Since we don't have the option to disable the engine's attach node, I attach a 1.25m nose cone to the rear of these engines, then use the offset tool to clip it inside the engine so it doesn't block thrust.  This reduces the drag of a rapier engine to be the same as a whiplash, which lacks a rear attach node.

 

After - proper engine mounts, stacks merged, cones on back of every engine, intake or cone front of every nacelle 

197kn drag at 201.4 m/s and 2.208 AoA

In other words, less than half the drag, despite being at a higher AoA  and producing more lift (931 instead of 880)

[bewing]

1 hour ago, AeroGav said:

Some of these have a nose cone (very pointy) or intake (somewhat pointy) at the front

Other way around. The shock cone intake has significantly less drag than either nose cone -- especially at supersonic speeds, which is where it makes the most difference.