Aircraft, MkII slower than MkI?

[HardKerbin]

I'm experiencing something of a curiosity with a couple of my aircraft.

Two designs in particular, one is an older, earlier game version I made.  It's pretty simple, fairly light, designed for speed, and initially had the Wheezy engine.  I've since upgraded it to the Panther afterburner engine and I can make it do some interesting stuff that I haven't been able to do with a MKII airframe.  But before I get to the MkII, let me talk about the MkI and what it is, and what it does.

Here is a hangar shot of the MKI http://steamcommunity.com/sharedfiles/filedetails/?id=860593732.  It's a single engine design, weighs almost 8,500 kilos. If I take off in dry mode, climb to 6000m, I will reach a top speed (full fuel) around 312 m/s.  Kick it into wet mode and it'll push 900+ m/s.  Now here is the oddity, when I kick it back into dry mode, it'll maintain a speed of about 612m/s.  Freaking awesome, right? 

(MkI In flight, 612m/s maintained dry mode) http://steamcommunity.com/sharedfiles/filedetails/?id=860594477

Here is a hangar shot of the MkII http://steamcommunity.com/sharedfiles/filedetails/?id=860593672  It's a twin engine design, weighing in just over 10k kilos.  Quite a bit more thrust per kilo with this one.  However, it has a similar top speed a the same altitude in both wet and dry modes, but it wont do the trick that the MkI will do, it'll just slow all the way back down into the 320m/s range.

(MkII in flight, 316m/s maintained dry mode) http://steamcommunity.com/sharedfiles/filedetails/?id=860594319

I'm fairly sure I've controlled as many variables as possible.  Same altitude, same flying conditions, same procedures.  Is my MkI just experiencing a bug in my favor, or is there some aero wizardry I'm unaware of?  If it's the latter, can I solve it on my MkII?

[Reusables]

Imo it's simple: The problem is the sonic boom. In transonic region, the shockwave creates lots of drag. I guess it's because the plane should haul the big shock cone as well. (It gets smaller on supersonic speed)

For the less draggy mk1 plane, supersonic lift/drag ratio would be big enough to sustain its speed with the thrust. For the other one (mk2), the lift/drag ratio would be worse(2~3 times more draggy). So it can't sustain its speed and goes subsonic. (L/D is bigger for subsonic than subsonic)

I think this is one of the reason there are two kinds of airplane in the world: subsonics and supersonics.

EDIT: In some extent, bigger L/D ratio (for same AoA) makes the craft experience less drag since AoA needed is decreased. This doesn't apply when AoA is already small enough(<1deg). Mk2 might be more draggy with certain AoA, similar to the wings. In this case, more/inclined wings will improve the problem. (By decreased AoA of Mk2 hulls)

Edited February 10, 2017 by Reusables
More explanation.

[FullMetalMachinist]

Simply put, the Mk II parts give lots of lift from the central 'body' parts (meaning you can get away with less wings), but at the expense of increased drag. The Mk I parts give very little body lift, but also have much less drag. 

You can get around this a little bit by building in some angle of attack into your wings. Rotate them so that the front edge is higher than the back by a few degrees. This will let you have less drag from the Mk II parts (because they stay prograde), while the wings have some angle of attack to generate lift. This is called angle of incidence. 

[bewing]

That's right. The unbelievable drag of the MK2 fuselage parts will slow you right down, and gobbles up all your thrust and fuel. For spaceplanes on reentry, that's useful. And MK2 parts are pretty. And they have decent heat resistances. But in all other ways, MK2 parts stink.

Edited February 10, 2017 by bewing

[HardKerbin]

Ah, I see.  I thought KSP had drag over-simplified, as in it was tied into weight.  Not sure where I read that, somewhere else on the forum, or the interwebz, or youboob, who knows.  The larger profile of the MkII parts making more drag makes sense.  (I actually do understand aerodynamics fairly well, I was a jet mechanic in the Marine Corps, I built and flew model aircraft, I also just enjoy the subject, etc.)  So the obvious actually makes sense.  I was over-thinking by under-thinking.

Since I made that post, I got a contract which gave me the Whiplash RamJet engine.  Holy hell, it about burned up the MkI's intake.  Was pushing 1,300m/s and had to back off.  The MkII topped out below 1200m/s with two of them, still, pretty sick.

In the MkI, its actually reasonably fuel efficient at 23,000 meters, but there is virtually no control at that speed, that high up, so it's fairly useless as it turns out....Or, I just need to get my "foot out of it".

Still, its good fun

 

Maybe I should add a canard...

Edited February 10, 2017 by HardKerbin
Added a thought.

[HardKerbin]

I played around with the MkI, found some more heat resistant parts, made the wing smaller, added a canard, and now I'm cruising at 23,000 meters at 1342m/s while consuming 0.08 units (litres?) of fuel per second.

Doesn't get much better than that! http://steamcommunity.com/sharedfiles/filedetails/?id=860720121

[Foxster]

There are big steps in the inefficiency of the different body size parts. 

This is as important for efficient rockets as it is for planes. 

Never go up a size unless you really have to or you will face a steep increase in dV losses and/or drag. 

[FullMetalMachinist]

9 hours ago, HardKerbin said:

Ah, I see.  I thought KSP had drag over-simplified, as in it was tied into weight. 

FYI, this used to be the case, but ever since version 1.0 came out KSP has used drag cubes. Basically each part has an imaginary cube built around it, and each face of the cube has a certain drag value assigned to it. Then it figures out how much of each face is exposed to the airflow, and which direction (is it prograde and head-on to the air, or is it the trailing part at the back). Then it applies that amount of drag to each part of the ship/plane/craft. 

This has the benefit of making stuff that looks streamlined and 'fast' actually behave that way. 

[Sharpy]

Also note: your Mk2 cockpit will be mildly warm long after your Mk1 cockpit evaporated.

[AeroGav]

Mark 2 parts are in need of a serious re-balance.    The mk2 fuel tanks are scarcely any heavier than a mk1 of the same capacity , despite appearances.  In that respect the extra heat and impact tolerance come for free.  But they have about twice as much drag.  Comparing a mk2 cockpit with a mk1 is not really a like-for-like comparison since the mk2 holds twice as many Kerbals.  

As such, the best use for mk2 might be in low speed utility aircraft that are expected to lug science juniors around , and  make rough field landings to collect samples and sometimes crash a bit as a result.    

For high speed work, leave the SR71 Blackbird lookalike at home and bust out the F-86 Sabre 

Also on the subject of heat, your mk1 plane will resist heat better than your mk2, because the game now takes better account of a part's position when calculating aerodynamic heating.   Your mk2 has the cockpit right at the very front of the ship, whereas the mk1 has an intake ahead of it.

To really make the mk1 go fast , swap that circular intake for a mk1 diverterless supersonic ramp intake fuselage section, then put an NCS adapter ahead of that,  then a small nose cone at the very front.   With the cockpit the 4th part in the stack,  it won't get much in the way of heat 

To minimise the drag penalty of that mk2 fuselage, you need to do two things 

• Fly high.   Where the air is thinner, drag will be less.  Note that you will need larger wings to reach these altitudes without resorting to large angles of attack, which are draggy.   Up to 14km,  drag falls faster than thrust on the Panther.   The critical altitudes for the Whiplash and Rapier are 17 and 22km respectively.

• Minimise body AoA.    As @FullMetalMachinist said , you can build incidence into your wings so you can fly the craft directly on prograde with zero body AoA.      Even then, there is still a lot of drag, so you need to fly high as well, which requires a lot of wing area.       Sizing the wing for flying at 14km on Prograde means it'll have too much lift low down, and will want to keep flying loops if you gun the throttle at lower altitude.

 

My Bute Ranger is a mk2 career mode utility freighter.   It uses both these tricks to supercruise.   Flys really high,  and the wings have built in incidence, so it cruises on Prograde hold.   Very squirrelly at low altitude however,  don't go too fast or it will start trying to "loop the loop". It's very fuel efficient in supercruise and i think it could circumnavigate.

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

 

14 hours ago, HardKerbin said:

If I take off in dry mode, climb to 6000m, I will reach a top speed (full fuel) around 312 m/s.  Kick it into wet mode and it'll push 900+ m/s.  Now here is the oddity, when I kick it back into dry mode, it'll maintain a speed of about 612m/s.  Freaking awesome, right? 

Two reasons for this -

If you can make it past the 0.86-1.3 mach region, you're golden.

This is how my career mode space plane gets through the rough patch -

https://kerbalx.com/AeroGav/Basic-Tourist-SSTO

Edited February 10, 2017 by AeroGav

[HardKerbin]

Yeah, I have a TON of AoA to fly my MkI at 23k M with the whiplash.  And it still wants to burn up the cockpit, I had to slow down to just below 1,200m/s, which is still twice as fast as I was cruising with the panther, and with better fuel efficiency (0.08).  I'll keep toying with this plane.

[HardKerbin]

I managed to get a MkI SSTO in orbit...barely. 

[HardKerbin]

And a simpler, more efficient MkI with the Rapier engine I just unlocked.  Hooray for science labs!

[AeroGav]

2 minutes ago, HardKerbin said:

And a simpler, more efficient MkI with the Rapier engine I just unlocked.  Hooray for science labs!

Nice.   Note that you want Val's cockpit as far back as possible to avoid the heat, I'd put the service bay & reaction wheel , inline clamp o tron (if fitted) in front of it.

I'd also put an engine pre-cooler in front of the cockpit as well, and get rid of those adjustable ramp intakes.     The amount of air they gather peaks at about mach 3  then rapidly falls off,  the Rapier power (and air consumption) doesn't peak till mach 3.75 and stays pretty large till well past mach 5.    If you go fast enough those radial intakes will choke the engine.   The pre-cooler has a better top end, and is good enough to supply one rapier at any speed.    Also, it keeps your cockpit further back !

The final measure to reduce heat is to fit larger wings,  because you rise faster, but that's not really needed on a pure chemical SSTO that gets the rocket mode part of the climb over and done very fast.

My nuke SSTO spends a bit longer in the upper atmo, so it's more of an issue. OTOH it gets to space without any proper fuel tanks, so there are upsides.

 

[HardKerbin]

I don't understand where you're getting the fuel to run the Rapier that long.

My ship blew up on re-entry, the interior exceeded the heat tolerance.  I did he same thing with a MkII.  Probably coming in too steep, I've managed reentry before just fine, maybe I'm getting greedy.

As for getting to space, I just took off, set the pitch to 35 degrees and let er fly.  Got up there and I had 500+ deltaV in reserve.

I'll play around with it like you said though.  I didn't even know about the pre-cooler, thanks for the tips.

[Wanderfound]

 

17 hours ago, HardKerbin said:

I played around with the MkI, found some more heat resistant parts, made the wing smaller, added a canard, and now I'm cruising at 23,000 meters at 1342m/s while consuming 0.08 units (litres?) of fuel per second.

Doesn't get much better than that! http://steamcommunity.com/sharedfiles/filedetails/?id=860720121

 

You've got a bit more speed to find yet...

 

Note the 0° AoA, possible due to the slight wing incidence.

Edited February 10, 2017 by Wanderfound

[AeroGav]

27 minutes ago, HardKerbin said:

I don't understand where you're getting the fuel to run the Rapier that long.

My ship blew up on re-entry, the interior exceeded the heat tolerance.  I did he same thing with a MkII.  Probably coming in too steep, I've managed reentry before just fine, maybe I'm getting greedy.

As for getting to space, I just took off, set the pitch to 35 degrees and let er fly.  Got up there and I had 500+ deltaV in reserve.

I'll play around with it like you said though.  I didn't even know about the pre-cooler, thanks for the tips.

https://kerbalx.com/AeroGav/freedom-fighter

I've uploaded the craft from the video in case you want to play with it.   Very bare bones (no solar panels).

You can see that after takeoff, i control pitch mostly using pitch trim (alt S and alt W on keyboard) to maintain fine control of angle of  attack in the steady climb.   Occasionally i resort to prograde hold if i need to level off in a hurry.  It never runs the rapier in closed cycle so fuel lasts ages.   Also,  at high altitude, thrust is weedy, but remember jet engines have constant ISP so when thrust is weak fuel consumption is also low.

It re-enters just fine too (cockpit far back, large wing area).

 

[HardKerbin]

1 hour ago, Wanderfound said:

 

You've got a bit more speed to find yet...

 

Note the 0° AoA, possible due to the slight wing incidence.

 

CHALLENGE ACCEPTED!!!

Now to make it not explode when I kick on Rocketman mode.

G

[HardKerbin]

I like the minimalistic route really, that four engine thing I posted above is not really my style.

 

[HardKerbin]

Rocketman mode...and then it blew up...lulz.

I just realized after looking at this that I'm hauling around monopropellant for no reason.

Edited February 11, 2017 by HardKerbin
Added a thought.

[GoSlash27]

IRT the OP,

Those intakes on your Mk2 design are horribly draggy. They're probably accounting for more drag than the Mk2 parts themselves, even with the body lift and drag.

If done correctly, Mk2 designs can actually be aerodynamically cleaner than their Mk1 counterparts, especially when built to haul similar payloads.

Best,
-Slashy

[AeroGav]

11 hours ago, HardKerbin said:

Rocketman mode...and then it blew up...lulz.

I just realized after looking at this that I'm hauling around monopropellant for no reason.

I take it you're going for a speed record/testing thermal limits rather than trying to get in orbit?

Once you're on closed cycle power, there is no point trying to stay low to get enough air for the jets (as you saw from the graph i posted, their thrust falls to zero by mach 6 anyway, no matter your altitude).  You want to pitch up to about 5 degrees where lift:drag ratio is best,   which will take you out of the lower atmosphere fairly smartly - especially if the craft has larger wings than the one posted above.

[HardKerbin]

On ‎2‎/‎11‎/‎2017 at 5:49 AM, GoSlash27 said:

IRT the OP,

Those intakes on your Mk2 design are horribly draggy. They're probably accounting for more drag than the Mk2 parts themselves, even with the body lift and drag.

If done correctly, Mk2 designs can actually be aerodynamically cleaner than their Mk1 counterparts, especially when built to haul similar payloads.

Best,
-Slashy

Thanks for the tip, is there any way to know how much drag any given part imparts?

On ‎2‎/‎11‎/‎2017 at 7:34 AM, AeroGav said:

I take it you're going for a speed record/testing thermal limits rather than trying to get in orbit?

Once you're on closed cycle power, there is no point trying to stay low to get enough air for the jets (as you saw from the graph i posted, their thrust falls to zero by mach 6 anyway, no matter your altitude).  You want to pitch up to about 5 degrees where lift:drag ratio is best,   which will take you out of the lower atmosphere fairly smartly - especially if the craft has larger wings than the one posted above.

Yeah, at that point I'm just fooling around.  No purpose for that particular plane but speed.  I was trying to see how fast I could go without going suborbital, and staying low-ish.  No reason why, just because I was having fun.

I've already got a MkI design that makes orbit easily.  It doesn't do anything once it's there, but then again, I don't really need an orbiter.  I already have a geosynchronous com-sat network, a mobile space station (as in, it can go between Kerbin, the Mun, Minmus and back, but it's mostly a floating gas station and science lab so I can make repeated mun and minmus landings to keep the science lab full.  I have a probe on the way to Duna, and Dres is next in the transfer window, so I'll be sinding a probe there as well.  The science lab is making my progress on the tech tree move along pretty quickly, so there's lots of new toys to mess around with.

[GoSlash27]

1 minute ago, HardKerbin said:

Thanks for the tip, is there any way to know how much drag any given part imparts?

HardKerbin,

 There's a cheat in the Alt+F12 menu that enables you to show drag values when you right- click on a part.

You go to Physics/ Aero and enable "Display aero data in action menus".

Best,
-Slashy

 

 

[HardKerbin]

1 hour ago, GoSlash27 said:

HardKerbin,

 There's a cheat in the Alt+F12 menu that enables you to show drag values when you right- click on a part.

You go to Physics/ Aero and enable "Display aero data in action menus".

Best,
-Slashy

 

 

Dernit, something isn't working, or I'm doing it wrong.  I've done exactly as you said and I'm getting no new information.  Right clicking on parts in the spaceplane hangar/editor, correct?   On the craft, in the menu, seems to make no difference.  I even tried to restart the game, that didn't help, it just reset the cheat anyway.  Could mods be interfering?

Oh, nevermind, you have to click on it in flight.

Edited February 12, 2017 by HardKerbin
Derp