Mk2 & Mk3 Feasibility?

[Right]

So I am having a hard time seeing the point to Mk2 & Mk3 parts.

Heres a breakdown. Mk2 parts compared to size 1 AKA 1.25m parts, Mk3 parts compared to size 3 AKA 3.75m parts.

Mk2 Disadvantages:

• The lift from Mk2 body parts is offset by mass proportional to any fixed wing - not a disadvantage technically, just a wash.
  
• The Z+/Z- area is twice as bad as a size 1 equivalent part. To make matters worse, its Z+/Z- drag coefficient is horrible. This causes much more parasitic drag (not the good kind) when pitching. One test I did showed twice the drag at only 5 degrees AoA!
  
• The Y+/Y- area is twice as bad, and is harder to fully occlude with low mass, low drag-coefficient alternatives.
  

Mk2 Advantages:

• The X+/X- area is a little worse, but its drag coefficient is pretty decent so makes up for it. But personally, my spaceplanes rarely need to yaw, so this advantage is lost on me.
  
• These parts have a higher crash tolerance, overall more sturdy.
  
• 25% better max temperature.
  

Mk3 Disadvantages:

• Both the Z+/Z- and the X+/X- area is almost twice as bad (for an equivalent mass of fuel), On top of this, the drag coefficients are worse. Both pitching and yawing cause considerably more drag than a size 3.
  
• The Y+/Y- area is, again, harder to fully occlude with low mass, low drag-coefficient alternatives.
  
• Dry mass per fuel point is 14% higher.
  

Mk3 Advantages:

• Y+/Y- area is like 8% smaller, if the stack remains Mk3 throughout the stack (which I don't often see).
  
• Higher crash tolerance
  
• 35% better max temperature
  

So, considering how much time spaceplanes spend in the atmosphere, it seems like the aerodynamic penalties are pretty damning. I can't help but feel like its backwards... rockets blunder straight up through the atmosphere with high TWR while spaceplanes carefully navigate through it, shouldn't spaceplane parts be more aerodynamic and rocket parts be sturdier?

[TheXRuler]

Well try building a spaceplane out of regular tanks, without massive clipping that will just look weird apart from that mhhhh, there are no other 1.25m crew tanks and no proper cargo bays. Also the parts work well for spaceplanes since as you said they are in the atmosphere longer (subjecting them to more heating). Apart from all that they just work, the ISP of the airbreathing engines is just so much higher but I guess if you were really minmaxing things the regular tanks might make a small difference. Assuming all your data is correct

- - - Updated - - -

Also if I am not mistaken (and I may well be) rockets are actually more flimsy than aircraft IRL as well since they only have to cope with forces along one axis whereas the plane has lift and drag and it has to deal with both

[Right]

Not sure I follow. I make them all the time, they look fine, no clipping.

There are twice as many 1.25 crewed pods than Mk2.

You have fairings that can often do the job cargo bays can.

Heating isn't a matter of length of time in atmo (E.g.; doesn't add up past 1-2 seconds), but speed and air density. Besides, only the leading part is exposed to significant heat, which is typically a high max temp nose cone or intake. Even if not, I've used the FAT wings on spaceplanes (40% less max temp than a regular tank) without too much trouble.

Engines have nothing to do with it, there are no mk2/3 configured engines (which is actually yet another disadvantage).

In terms of aero losses, its more than a small difference.

[BudgetHedgehog]

Sure, if you look at just numbers, those seem at a net disadvantage. But look at the big picture - show me a Service Bay that can hold a Rockomax tank, show me a stackable size 1 cargo bay, show me a size 3 part that can hold 16 kerbals.. The Mk2 and Mk3 parts have advantages that no other size 2 or 3 parts have. Yeah, you could put 16 lander cans on a craft, but then you'd be using 16 parts instead of 1 (by the way, the Mk2 crew compartment has the lowest weight to kerbal capacity ratio, beating the much larger Hitchhiker). If you need to carry a big cargo to orbit in a spaceplane, the Mk3 cargo bay is ideal, you're never going to fit it into anything smaller (yes, you could put it in a fairing on a 2.5m stack, that's why I said "in a spaceplane". I'll assume you want your spaceplane to remain plane shaped after deploying the cargo so fairings on a plane are not feasible).

Also, IIRC, the Mk3 fuel tanks have the best dry mass ratio of any other tanks.

Edited November 4, 2015 by ObsessedWithKSP

[GoSlash27]

Right,

The Cd and area nose-on is the bulk of the drag in spaceplanes. The Mk.2 has an advantage over Mk.1 in that respect due to the low Cd of the Mk.2 cockpit.

As for the Mk.3, it's side drag doesn't increase with longer sections.

I'm a big fan of Mk.2 for personnel transports and Mk.3 for tankers. I suppose it depends on what you're trying to do...

Best,

-Slashy

[Right]

@Obsessed

As you probably already guessed, I'm interested in the most efficient and most effective designs.

Good points, I'll grant you kerbal storage, and cargo bays for looking cool. But theres nothing about fairings that necessarily hurt your spaceplane's post payload functionality. You just put your payload at the center of the craft with docking ports, and re-dock your plane for re-entry. Not only do you retain you're spaceplane's maneuverability, but you've reduced its drag profile, which can't happen with cargobays.

Mk3 fuel tanks did have the best ratio once upon a time, but they are among the worst now.

@GoSlash

The Mk.2 cockpit has a great Cd, but it comes at a huge cost. First and foremost, it has twice the area, which causes it to fall behind the Mk1 even if we're talking 0 AoA (0.344 vs 0.506 thanks to a handy PM!). As soon as you start pitching up or down, Mk2 starts to really get bad. Second, it comes at a high mass expenditure.

Longer sections don't increase its drag? In other words, a single Mk3 Long Fuselage has the same X & Z drag as a stack of 2 of them do?

Edit: Oh are you talking about sideflow drag?

Edited November 4, 2015 by Right

[BudgetHedgehog]

You just put your payload at the center of the craft with docking ports, and re-dock your plane for re-entry. Not only do you retain you're spaceplane's maneuverability, but you've reduced its drag profile, which can't happen with cargobays.

It's.. not quite as simple as that. Besides adding RCS and docking ports to your craft that you wouldn't normally add (and increasing the part count and overall weight), a plane designed with a centre fuselage of X length will not perform the same way if X was changed to Y. The CoL and CoM will move, possibly making it unstable, it's extra hassle as well.. if you can show me an experiment where it's easier and more efficient to use that method vs simply opening a cargo bay, I will agree with this. Until then, I will continue to take those orange-sides to orbit with a spaceplane in a Mk3 cargo bay because it's less parts, less work and less bendiness.

(well, I won't because I don't, but if I did, I would. I'm not a spaceplane kinda guy)

EDIT: I do use FAR which probably changes things somewhat, more in the favour of cargo bays than planes in two parts.

[Right]

More efficient is rarely easier, so I'm happy to concede that it may not be easier. Believe it or not, you wouldn't need any RCS, and the mass of the docking ports/fairing is about equal to the cargobay they replace.

Yeah I'm all for it! I'd need a baseline. If anyone has a favorite mk3 payloader, I'll try to rebuilt it more efficiently without mk3 parts. Otherwise, I'll do my own later on.

[juanml82]

I think heat and crash tolerance shouldn't be underestimated for interplanetary missions. That extra heat tolerance can be the difference between aerobreaking or exploding, and you also have to deal with reentry heating while reentry.

As for crash tolerance, you can land on an MK fuel tank. If you have one of those at the bottom, you don't need landing legs.

[GoSlash27]

@GoSlash

The Mk.2 cockpit has a great Cd, but it comes at a huge cost. First and foremost, it has twice the area, which causes it to fall behind the Mk1 even if we're talking 0 AoA (0.344 vs 0.506 thanks to a handy PM!). As soon as you start pitching up or down, Mk2 starts to really get bad. Second, it comes at a high mass expenditure.

Right,

The mass is a pretty minor concern. Spaceplanes aren't particularly sensitive to that. As for the area, again... it depends on what you're doing with it. If it's a small crew taxi (which is what I use Mk2 for) you're going to be hard- pressed to build a Mk. 1 that will do the same job as cleanly. *Edit* Yeah, the drag gets bad when you pitch it. That's why I don't consider body lift to be an advantage and I design my spaceplanes to fly with zero incidence from Mach .9 on.

Longer sections don't increase its drag? In other words, a single Mk3 Long Fuselage has the same X & Z drag as a stack of 2 of them do?

Edit: Oh are you talking about sideflow drag?

Aye, I'm talking about the side flow numbers. A long section has the same side flow as a short section.

I'll kick you a copy of my "Brawndo" Mk.3 tanker to play with.

*edit*

http://wikisend.com/download/714318/BrawndoIV.craft

Best,

-Slashy

Edited November 5, 2015 by GoSlash27

[Right]

Actually, I have to retract my concession about mk2/3 mass efficiency for crew taxiing. Its not very stylish, but its practical.

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[Red Dwarf]

I think that we're forgetting one very important factor here: they look cool. A lot of spaceplanes I see are built for looks rather than pure efficiency (rune's work notwithstanding), and they look awesome. For me, that's justification enough

[Jovus]

I don't fly stock, so I can't speak to the stock aerodynamic characteristics. But there is one important area of comparison I haven't seen mentioned: wet/dry ratio

Standard wet/dry for rockets in KSP is 9. (This sucks, by the way, but whatcha gonna do?)

For Mk1 parts, the wet/dry ratio is also 9.

For Mk2 parts, the wet/dry ratio is 8 or lower. Ouch.

For Mk3 parts, the wet/dry ratio is 8 as well.

So yeah, if you care anything about efficiency in getting to orbit, eschew funny-looking tanks.

[SpaceplaneAddict]

Do never insult the Mark2 parts.

[Kagame]

Eh, I personally find the parts far more forgiving of stupid crap than normal rocket parts. For example, I just made an SSTO using SRBs (non-discarding), a single nuke, and turbojets. Rockets would balk at having to carry up their SRBs... of course, then so would any sane engineer. Sometimes I wonder just what I'm doing. Also, they look cool. And the high crash tolerance has saved the lives of many a Kerbal...

[Right]

@RedDwarf I think thats true for most people, and thats the bummer I think. Many suckered into the cool, probably never realize the performance loss.

@Jovus Yeah, I mentioned it in the OP. In stock, Mk1 and Mk2 are both 9, and Mk3 is 14% worse.

@Kagame True that, can't write off sturdy