Cannot hit Mach 1

[CubertFarnsworth]

Any ideas on why this design cannot hit Mach 1? Even leveled out I can only get to 305 m/s or so. 

 

[Daveroski]

Afterburners on?

 

[WanderingKid]

Speed affects speed with those engines, so the faster you're going (up to Mach 2.7) gets you more TWR.  Since airflow and heavy drag don't appear to be the problem, it's TWR against Mass + Drag.  What's the weight on that puppy?

[Palaceviking]

Weight and drag.

Bit much for panthers 

Try a bit higher (about 10-16km)

Edited November 14, 2016 by Palaceviking

[AeroGav]

1 hour ago, Palaceviking said:

Weight and drag.

Bit much for panthers 

Try a bit higher (about 10-16km)

He won't be able to go higher because the wing is quite small for weight.  I can see he's at a 5 degree angle of attack just holding level flight at 5km at 0.9 mach, which is pretty much the optimum AoA already.  If he goes higher without increasing speed he'll probably have more drag as the AoA goes way up.

 

He needs to do an aerodynamic cleanup of that aircraft.  I can see a few things that will add drag

1. Fuel ducts.   They are no longer necessary since 1.2 as fuel drains from all tanks evenly.  Ever since the game came out of beta they've had horribly, bugged levels of drag - more than your cockpit.

2. Are those a four OX-Stat XL panels on the top fuselage? V draggy and way overkill.  Radially attached parts are evil.   Put the solar panels inside the cargo bay and don't deploy till orbit.   

3. While we're at it, cargo bays have been causing a lot of grief lately.  Radially attaching stuff to the walls of the cargo bay can have the game think its on the outside.   So get one of the size 0 batteries or reaction wheels, attach that to the cargo bay's node, then put your solar panels radially on the size 0 battery/rcs tank/whatever  to make absolutely sure the game realises it's on the inside.

4. As the jet engines are out on pods, I'm assuming you've got a pair of rockets on the back of the fuselage?   Be aware that rocket engines (and the RAPIER) are stackable thus they have rear attach nodes.   The trouble is, the aerodynamics model treats unused nodes on the back of engines the same way it would treat an inline cockpit with no nose cone - it slaps on a big flat plate drag penalty.  Get a 1.25m nose cone and attach it to the back of the engines.  Disable the shroud, then use the offset tool to move the cone forward into the engine so it can no longer be seen, or at least no longer looks weird.

Spoiler

One panther is good for about 20 tons if the aircraft is clean,  a terrier is ok for 15 or so.  As an aside, i think terrier is the best engine for early career spaceplane because you don't need super high TWR like you do in a rocket, it's a nice light engine and it has the highest vacuum isp of any 1.25m motor - sea level rating doesn't matter as you wont be using below 15km

Taking things further 

Spoiler

1. MK2 fuselages are very draggy, it's a bit of a game balance issue, and a shame because they look awesome.  There's two ways to reduce the penalty

a) angle the wings upwards at 5 degrees where they attach to fuselage.  That way the wings can make lift at optimum 5 degrees while the fuselage is directly aimed at prograde for minimum drag.  

b) add more wing area generally, so you fly efficiently (AoA 5 degrees or just under) a bit higher, where there is less air resistance.  You don't want to go too far with this however, as over 10km engine start to seriously lose power, which will make it harder again to pass mach 1.

Wing sizing is controversial, i do prefer more than most folks.  Generally you know you've got too much wing when you're at the altitude your engines start to loose power, and can't stop the plane climbing without pushing the nose down so AoA is less than 5.  This is inefficient and a sign you have too much wing.

You can tell that you don't have enough wing, if you hold 5 degrees AoA all the way to orbit for efficiency, but the plane doesnt make enough lift and doesn't gain altitude quickly enough as speed rises, and parts start overheating.  The higher you are, the faster you can go without melting.

 

This is the mk2 plane i built in my last career game

https://kerbalx.com/AeroGav/Bute-Ranger

It can supercruise in non-afterburner mode at high altitude.

[CubertFarnsworth]

1 hour ago, AeroGav said:

 

Just took this bad boy for a spin.....nice! Very jealous how it can hit  Ma1 without afterburners. Got it up to Ma2.8

[AeroGav]

10 minutes ago, CubertFarnsworth said:

Just took this bad boy for a spin.....nice! Very jealous how it can hit  Ma1 without afterburners. Got it up to Ma2.8

Anything over 2.5 is a bonus ! 

[bewing]

It's a hell of a lot easier to get a MK1 design with 1 panther to fly fast than it is for an MK2 design with 2 panthers.

@CubertFarnsworth: You want to try taking this one for a spin at 1km altitude and see what it does in both Wet and Dry modes. (OK, dry mode actually works better when you're above 5km.)

http://pastebin.com/raw/DDdxhbhi

MK2 planes have maybe 3 times the drag of an MK1 plane, and they weigh a lot more, too. So as pretty as they are, they do not go fast.

Edited November 15, 2016 by bewing

[WanderingKid]

4 hours ago, AeroGav said:

4. As the jet engines are out on pods, I'm assuming you've got a pair of rockets on the back of the fuselage?   Be aware that rocket engines (and the RAPIER) are stackable thus they have rear attach nodes.   The trouble is, the aerodynamics model treats unused nodes on the back of engines the same way it would treat an inline cockpit with no nose cone - it slaps on a big flat plate drag penalty.  Get a 1.25m nose cone and attach it to the back of the engines.  Disable the shroud, then use the offset tool to move the cone forward into the engine so it can no longer be seen, or at least no longer looks weird.

Can you go into "George of the Jungle" levels of detail instruction on this with pictures, either here or some other thread?  I keep imagining you're explaining to block the output nozzles on the engines with reversed nosecones and then translate them into the ship, which would block the output and thus just blow them off the ship... no?

[AeroGav]

3 hours ago, WanderingKid said:

Can you go into "George of the Jungle" levels of detail instruction on this with pictures, either here or some other thread?  I keep imagining you're explaining to block the output nozzles on the engines with reversed nosecones and then translate them into the ship, which would block the output and thus just blow them off the ship... no?

If you watch the first minute of this video, you'll see me cone a rapier and then a pair of nerv engines.   I'm not sure how far forward you'd have to slide the cone so as to avoid thrust blocking, I'm sliding mine as far forward as i do for cosmetic reasons, to avoid z-fighting etc.

 

Edited November 15, 2016 by AeroGav

[WanderingKid]

1 hour ago, AeroGav said:

If you watch the first minute of this video, you'll see me cone a rapier and then a pair of nerv engines.   I'm not sure how far forward you'd have to slide the cone so as to avoid thrust blocking, I'm sliding mine as far forward as i do for cosmetic reasons, to avoid z-fighting etc.

I... What?  Why does that remove the drag yet not block the engines.  You'd think either it's exposed, and thus both draggable and burnable, or it isn't.  Well, thank you, that's what I thought you said but I didn't think it would work.  I'm tempted to start inverting nosecones on my rockets now too... particularly my high altitude terrier burns.  You need to do this with any stackable engine to reduce the drag I take it?

Though, that's not the part that's truly got me boggled now.  Your air intake has a nosecone and a gas tank in front of it and it doesn't care.  I'm going to have to play around with that... though I assume the pre-cooler is simply a side loading air intake and not a front loading one.

[Val]

46 minutes ago, WanderingKid said:

Though, that's not the part that's truly got me boggled now.  Your air intake has a nosecone and a gas tank in front of it and it doesn't care.  I'm going to have to play around with that... though I assume the pre-cooler is simply a side loading air intake and not a front loading one.

Intakes can't get blocked. They only care about facing the correct way.

Only engine exhaust can be blocked.

Here's another Panther SSTO for you to try out.

Spoiler

Craft and flight instructions

 

9 hours ago, AeroGav said:

The trouble is, the aerodynamics model treats unused nodes on the back of engines the same way it would treat an inline cockpit with no nose cone - it slaps on a big flat plate drag penalty.  Get a 1.25m nose cone and attach it to the back of the engines.  Disable the shroud, then use the offset tool to move the cone forward into the engine so it can no longer be seen, or at least no longer looks weird.

You give really great advice. Keep it up... Except for the above quote.

Rear end drag is very small compared to front drag, whether or not it has a node. See the tests @Gaarst performed in the link below.

Difference between tail cone and none: 31 km vs 28 km, around 10% (Advanced Nose Cone)
(in the paragraph, Cones on the bottom of a single core rocket)

Certainly nowhere near, the difference between having a nose cone or not, like you describe it.

Difference between nose cone and none is big: 71 km vs. 15 km, around 470% (Advanced Nose Cone)
(in the paragraph, Cones on single core rocket)

The difference probably increased with the aero tweaks in 1.2.1, but in my experience, if you follow all the other good advice you give, and pass the transonic drag low and climb supersonic (>450 m/s), then you still end up hurting more from the mass penalty, than from fuel spent, with tail cones.

But it may very well be worth it when doing a subsonic climb, because so much more time is spent in the draggy atmosphere. I just avoid doing the subsonic climb, because it always uses more fuel and because it takes so damn long.

[AeroGav]

1 hour ago, WanderingKid said:

I... What?  Why does that remove the drag yet not block the engines.  You'd think either it's exposed, and thus both draggable and burnable, or it isn't.  Well, thank you, that's what I thought you said but I didn't think it would work.  I'm tempted to start inverting nosecones on my rockets now too... particularly my high altitude terrier burns.  You need to do this with any stackable engine to reduce the drag I take it?

Yeah it doesnt make any sense, except that all this is doing is reducing the drag of stackable engines like the RAPIER down to what you get from a non stackable jet engine like the panther or whiplash.   The game really needs to distinguish between this unused attach node and genuine flat plate drag.

In  a rocket upper stage you probably can't do this because the bottom node of the terrier is needed to attach it to the lower stage.   Rockets also have huge amounts of thrust and are probably much higher in the atmosphere than a spaceplane would be at the same speed, so it would be less important.

All work the same way,  acting as intakes in the middle of a stack.   It's pretty easy to attach a pre-cooler facing backwards as it's not so obvious what the front end is unlike the others - in this way it works fine at standstill but takes in less and less air as you get faster, the opposite of normal behaviour.

[AeroGav]

16 minutes ago, Val said:

 

..pass the transonic drag low and climb supersonic (>450 m/s), then you still end up hurting more from the mass penalty, than from fuel spent, with tail cones.

But it may very well be worth it when doing a subsonic climb, because so much more time is spent in the draggy atmosphere. I just avoid doing the subsonic climb, because it always uses more fuel and because it takes so damn long.

There's a new video on drag  in the tutorials section btw, can't remember the conclusion on this particular matter, but it's very nicely done.

The thing with cones vs no cones, is that if it makes the difference between being able to get supersonic or not, then they are worth every kg.

As regards going sonic at sea level vs higher altitude,  i still think it depends on the design of your ship, esp. wing loading.    I have some low wing loading airplanes  that are only just able to go supersonic at sea level,  and this does use more fuel than climbing a bit on those designs.    The acceleration slows drastically at 280m/s, despite the rapiers getting a ram air boost that doubles their power (and fuel consumption),  then take a further minute and a half to get to 440 m/s.  Ultimately though your fuel consumption getting to 20km/hypersonic is pretty immaterial anyway,  just do whatever flies easiest on your ship.

If you climb a bit before going sonic, you're not wasting fuel provided thrust stays well ahead of drag and you're getting a good lift/drag ratio.   Once neither of these is true, you should go supersonic if you can.

Edited November 15, 2016 by AeroGav

[AeroGav]

On 14/11/2016 at 8:12 PM, CubertFarnsworth said:

Any ideas on why this design cannot hit Mach 1? Even leveled out I can only get to 305 m/s or so. 

 

 

 

I don't know if Cubert has been on KSP much this week but I kept working on his ship.

Short version -

He had a few drag problems due to wrong size cones on the back of the engines,  and because the the docking adapter was attached to the rocket fuel fuselage behind it, not the cargo bay node.  This meant it wasn't shielded, so it and whatever cargo hangs off it would be experiencing full drag.

However, this was really only the "straw that broke the camel's back", in terms of stopping him getting past mach 1.  After correcting these things it went sonic, thanks to the power boost these engines get at supersonic speed, but the lift drag ratio in the aero data GUI was horrific.  No better than 1.2 to 1 at any combo of speed or altitude in the supersonic regime.     Terriers aren't strong enough to accelerate in the face of such drag, but swivels and reliants will suck the tanks dry before you get anywhere near orbit.

Yes, he's another victim of mk2 fuselage drag.   The only way to solve that was to angle the wings and fly the whole way on prograde, with the body at zero AoA.   This brought the L/D up to 3.5 to 1 or so.    

Actually, you can mitigate fuselage drag by simply spamming wings, so that the airplane flies at a lower aoa/higher altitude for a given airspeed.  It was the default approach with my own designs. and would give you a 2.5 to 1 lift drag ratio.   But that's in a sandbox aircraft with Big S wings and strakes, giving us extra LF capacity to compensate for the extra mass of all those wings , as well as NERVs to make use of it.

I also had to deal with an overheating problem by swapping to an inline cockpit - not entirely resolved as the antenna usually melts off on ascent so not 100% re-usable SSTO !  And then I had to rework things substantially to get back to the correct ratio of engines and fuel for this class of spaceplane - two terriers for every panther, 1200 LF/O for each terrier.   This in turn required a lot of experimenting to get both the wet and dry CoM back to the middle of the cargo bay again.

As a result, the finished product no longer looks much like the original.  You could probably make it more so,  but I'm exhausted.   Declare victory and move on !

https://kerbalx.com/AeroGav/Coopie-GAVMOD

Edited November 21, 2016 by AeroGav