Aircraft, MkII slower than MkI?

[ajburges]

On 2/11/2017 at 6:49 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

Slash,

Can you please elaborate on how Mk2 can be aerodynamically cleaner?

Both Mk1 and Mk2 have the same minimal set of radial stacks. One fuselage, wings, gear, and control surfaces/stabilizers. However Mk2 only offers up to 2 times the Mk1 engine nodes for 3 times the frontal drag. Sure the volume inefficiency can be translated into smaller control surfaces due to the length, but that does not make up for the drag increase. Also that longer fuselage means that you may want to go up a size for landing gear to avoid tail strikes.

Honestly the only time I seriously consider Mk2 is for the cargo bay. Service bays are uncomfortably small to launch payloads and can't be extended. On a less serious note: my super-cruise Goliath concept did handle better terminating to a Mk2 cockpit and I was never able to pin-point why quickly.

[GoSlash27]

ajburges,

 If you refer to the drag cubes in the PartDatabase.Cfg, you will see that the Mk.2 cockpit has a lower drag coefficient than the Mk.1; .201 and .232 respectively. This is where the bulk of drag comes from in transsonic and supersonic flight. Moreover, Mk.2 parts have a lower laminar drag to mass than their Mk.1 counterparts. All of this conspires to make Mk.2 airframes *less* draggy than their Mk.1 equivalent, but only when aligned with the airflow.

An example of how "slick" Mk.2 can be:

16 Kerbals and a docking port to orbit using just one RAPIER.

 I think Mk.2 based designs get a bad rap because they are super- draggy when not aligned with the airflow. They are actually an excellent platform for crew shuttles and light cargo when done correctly.

 Best,
-Slashy

[ajburges]

Slash,

I don't claim to have a deep knowledge of the drag system, but I would think values via debug options would be a more reliable source than interpreting configuration files. I put together a test craft to compare two obvious nose solutions (Shock Cone + adapter and NCS + nose cone) against each other. Since they are on the same craft, they should be close enough to equivalent conditions for evaluation. Note: any craft I make in 1 min has interesting flight characteristics; I wanted to capture a speed run, but wound up on a ballistic trajectory. The speed is still over mach 5 and climbing. I also forgot to auto-strut the radial stacks together so that could also throw values slightly.

Unless I read the values wrong, the Mk1 nose configuration totals at .62 drag while the Mk2 nose configuration totals 5.20 drag! These results collaborate experiments I tried with SRBs to compare Mk2 fronts against paired Mk1 frronts.

Don't get me wrong: well thought out design can take Mk2 far, but it just does not hold up against the aerodynamic efficiency of Mk1 or the "volumetric" efficiency of Mk3.

Edited February 14, 2017 by ajburges

[GoSlash27]

ajburges,

 That result actually doesn't surprise me; Adapters in general aren't very clean. Nevertheless, you can't build a spaceplane that seats 18 and runs on a single RAPIER from Mk.1 parts.

I do agree with you on the volumetric efficiency and crazy- low laminar drag of Mk.3.

Best,
-Slashy

[AeroGav]

1 hour ago, GoSlash27 said:

ajburges,

 That result actually doesn't surprise me; Adapters in general aren't very clean. Nevertheless, you can't build a spaceplane that seats 18 and runs on a single RAPIER from Mk.1 parts.

I do agree with you on the volumetric efficiency and crazy- low laminar drag of Mk.3.

Best,
-Slashy

Sounds like  a challenge ! 

I'm pretty certain the problem would not be weight or drag,   rather one of having it not impersonate a snake.  Anyone remember Scott Manley's mecha-dragon?

[ajburges]

15 hours ago, AeroGav said:

Sounds like  a challenge ! 

I'm pretty certain the problem would not be weight or drag,   rather one of having it not impersonate a snake.  Anyone remember Scott Manley's mecha-dragon?

I agree! I assume Slashy meant 16 seats since that is what his craft shows.

The biggest difficulty flying this was dealing with pitch caused by poorly placed wings.

In case He did indeed mean 18.

This one I may refine a bit more for a crew carrier. It made orbit with more dV than my current 7 seat ferry (though it does use 33% more Ox and have less surplus LF). Given the ease it made Mach 1, I could theoretically squeeze some more efficiency out of the RAPIER.

Again this is a case of the Mk2 parts have no advantage in gaining kinetic energy over Mk1. The Cabins have the same seats per mass as Mk1 but Mk1 wins on drag without a thought. Mk2 wins on part count especially with the included mono-prop and reaction wheel options, but I have yet to hear of single engine craft needing part count reductions. My best outcomes with Mk2 always leverage either their cargo bays or utilize their crazy high drag and improved heat tolerance for better, more aggressive aerobraking.

17 hours ago, GoSlash27 said:

ajburges,

 That result actually doesn't surprise me; Adapters in general aren't very clean. Nevertheless, you can't build a spaceplane that seats 18 and runs on a single RAPIER from Mk.1 parts.

I do agree with you on the volumetric efficiency and crazy- low laminar drag of Mk.3.

Best,
-Slashy

Ah, That explains why a Mk3 Cockpit + Aerodynamic Nose Cone has similar drag than a series of adapters from Mk3 to a nose cone. Before comparison I thought the adapter series would win on account of being more "pointy."

Edited February 15, 2017 by ajburges

[AeroGav]

2 hours ago, ajburges said:

 

 

Ah, That explains why a Mk3 Cockpit + Aerodynamic Nose Cone has similar drag than a series of adapters from Mk3 to a nose cone. Before comparison I thought the adapter series would win on account of being more "pointy."

With most stack sizes, there are two adapter options - aero, non aero. The non aero is usually a tiny bit lighter and shorter.  Often there is a third category - aero + fuel.

The aero options all give roughly similar drag when using similar stack diameter. Eg, Mk3 cockpit vs adapters from mk3 to mk1.     If the cockpit wins, bear in mind the series of adapters also store fuel, you'd probably find that mk3 cockpit + short mk3 lfo fuselage is equivalent to  the "series of adapters " option.

Main drawback of pointy, stack fronting cockpits is that they suffer the worst of heat and also any crash impacts.  OTOH, the string of adapters give a load of LFO capacity you may not actually have use for, and being right at the front can cause weight and balance issues.

[GoSlash27]

6 hours ago, ajburges said:

The biggest difficulty flying this was dealing with pitch caused by poorly placed wings.

Wow, I stand corrected! Plus that plane looks pretty

Kudos,
-Slashy

[Blaarkies]

I found that the MK3 tail section creates an excellent nosecone for mk3 size parts...yes, the cargo bay tail ramp part

[HardKerbin]

On ‎2‎/‎10‎/‎2017 at 1:42 PM, AeroGav said:

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.

 

Hey AeroGav, what's the reason for the reversed aerodynamic nosecones placed behind the Engines and moved forward?

[bewing]

3 hours ago, HardKerbin said:

Hey AeroGav, what's the reason for the reversed aerodynamic nosecones placed behind the Engines and moved forward?

It's a slightly soul-destroying trick that reduces the in-game drag of the engine somewhat, without decreasing the engine's performance.

 

[fourfa]

I can answer that.  In KSP's model of aero drag, an open rear attachment node signifies an un-smooth, non-aerodynamic trailing edge.  In reality, flat trailing edges can dominate total drag (blunt noses and tapered tails are usually more aerodynamic than pointy noses and blunt tails).  So when KSP sees an open rear node - like the back of the RAPIER - it assigns lots of drag to it.  Same goes with almost all the other rocket engines, which need rear nodes to make staging possible.  (Though I've not often seen any designs that have RAPIERs staged in the middle of a ship, as they have nearly the worst rocket ISP in the game).

Traditional rockets, with their typically compact frontal area and vertical launches that quickly get out of the draggy thick atmosphere, don't really suffer much from that open node drag (it's certainly measureable, but it's relatively small).  Planes on the other hand, spend a lot of time in atmosphere and top speed on air-breathers suffers terribly from extra drag, so open node drag can be a real problem.

The funny thing is that the other jet engines don't have rear nodes at all, and don't get this extra drag applied to them.  The Whiplash and Panther etc make for very aerodynamic tail cones.  But stick an extra nose cone on the back of a RAPIER, all of a sudden its drag is very much in line with the other powerful jet engines.  Obviously it'll block the engine's thrust and probably heat up and explode.  But if you offset it forward, it no longer blocks thrust, and still counteracts that extra drag, and they fall right in line with the other jets in this regard.

Of course some people find the RAPIER to be inherently overpowered, and the open node drag to be a fair nerf.  Your game, your rules.  I'm a big fan of circular intakes placed over aerospikes and Terriers, and offset forward until matching the lines at the front edge.  Very fashionable.  I usually put the plain 0.03t nose cones on the back of RAPIERs.  Lightweight, cheap, and the difference vs the other options is very small.

[HardKerbin]

I have another question, getting off topic, but it's my thread so...

Is there some way to activate navigation for the space center?  Sure would be handy to be able to fly home more accurately.

[fourfa]

Launch a bare capsule to the runway.  EVA, walk to the base of the runway, plant a flag.  Now you can use the flag for navigation.  I like one at each end of the runway to help line up straight on approach.

[bewing]

And make sure the flags are a couple meters off the edge of the runway -- or they will be counted as "debris on the runway" and you won't be allowed to launch planes until you get rid of them again.

[Hesp]

19 hours ago, HardKerbin said:

I have another question, getting off topic, but it's my thread so...

Is there some way to activate navigation for the space center?  Sure would be handy to be able to fly home more accurately.

Near the end of the discussion there is a link with a 1.2 compatible version

[HardKerbin]

So I've built myself an all purpose SSTO work in progress, but I really like it.  It has room for a couple passengers, so I can do Kerbal rescue missions.  Once in orbit, it has about 1700+ dV (on the solo nuke), so I can do a fly by of the Mun or Minmus if I so desire.  Parachutes so I can land anywhere on Kerbin. It has a service bay with some science gear, batteries, antenna, and monopropellant tanks.  Overall, it's a versatile SSTO MKI spaceplane that flies nicely, and gets work done.

 

This is my atmospheric workhorse.  Fly it up to 20,000m and it's fuel efficient, and cruises at 1/3 throttle at about 1350m/s.  I can make multiple flights, landings from one launch. I use my Engineer to repack the chutes for ground missions, so I can to a Short take off and get going again.  The wide stance of the landing gear makes it ideal for rover-like surface work, but makes takeoffs a bit sketchy.  Has all science needed for mission work in the service bay.  I've used several variations on this theme since the early game and it's always worked well, they just get faster as I science-up.

Edited February 20, 2017 by HardKerbin
More Info

[AeroGav]

On 11/02/2017 at 3:51 AM, 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.

Edited Saturday at 03:52 AM by HardKerbin
Added a thought.

Enough for a landing on Minmus actually.   Coming back you can aerobrake with multiple passes.    Or just put a fuelling base on the surface.

[AeroGav]

On 15/02/2017 at 5:30 AM, ajburges said:

I agree! I assume Slashy meant 16 seats since that is what his craft shows.

The biggest difficulty flying this was dealing with pitch caused by poorly placed wings.

In case He did indeed mean 18.

This one I may refine a bit more for a crew carrier. It made orbit with more dV than my current 7 seat ferry (though it does use 33% more Ox and have less surplus LF). Given the ease it made Mach 1, I could theoretically squeeze some more efficiency out of the RAPIER.

Again this is a case of the Mk2 parts have no advantage in gaining kinetic energy over Mk1. The Cabins have the same seats per mass as Mk1 but Mk1 wins on drag without a thought. Mk2 wins on part count especially with the included mono-prop and reaction wheel options, but I have yet to hear of single engine craft needing part count reductions. My best outcomes with Mk2 always leverage either their cargo bays or utilize their crazy high drag and improved heat tolerance for better, more aggressive aerobraking.

Well , this is my no-oxidizer, nuke version. 9 mk1 passenger cabins plus the cockpit.   Those strakes hold quite a bit of fuel,  ended up with too much fuel mass at the rear, so added a mk1 fuel tank up front and a pair on the nuke nacelles to balance it all out.   This results in a takeoff weight over 50 tons, half of which is fuel.   I needed the largest size landing gear to not tail strike on liftoff.

I got it to orbit on first try, but it took it's sweet time getting there.    At about 6km and 200m/s i noticed my thrust was only 25% higher than my drag, so i toggled the nukes on for a bit.   The RAPIER has four times the ISP , but it was only making 90kn at that point and drag was like 68kn or so.      Climbed at about 4 degrees AoA to just under 12km then arced over to a dive, which got me supersonic.   At 1.4 mach i cancelled the nukes, the rapiers were on song now.

Levelling off for our speedrun at 22km, we got a bit over mach 4.4, before drag rose to within 75% of our thrust, and i left the nukes on again for good.      I should probably have pitched up more agressively to 5 degree AoA at that point but i didn't , heat soaking the cockpit and frontmost cabin.     Anyway, here it is in orbit. 

Despite the mistakes, over 2600 dV, so you could take this to the surface of Minmus , or who knows even further afield?