on super-heavy air-breathing (space)planes

[Cirocco]

Hello all,

so I've come back to KSP after a few months of dry spell and as I always do when I'm given new spaceplane parts, I immediately went to work on heavy-duty, interplanetary SSTO spaceplanes. A ton of people have used the Mk III parts for space shuttles (and I don't blame them, space shuttles look awesome), but I haven't seen many people using them for air-breathing SSTO spaceplanes.

And then I found out why.

The new Mk III parts are REALLY heavy when compared to the Mk II parts. That also means that in comparison, turbojet thrust is REALLY low. Seriously, even small Mk III spaceplanes immediately go over 80 tons and require 8 or more turbojets.

So I'm wondering: what have you guys built in terms of the heavy and super-heavy (90 tons and up) air-breathing spaceplanes?

Also: that bug that reverses the pitch on control surfaces placed near the CoM of a plane is really annoying. Anyone know any fix to that?

[Epthelyn]

I've yet to make a successful Mk3 aircraft. If you're using jet engines you need to have a lot of them...and then add some more. It's equivalent to trying to launch a 3.75m rocket with 1.25m engines; it's possible, but more trouble that it's worth.

Hopefully we'll get some (at least) 2.5m jet engines at some point. That'll solve this problem.

As for the bug, I wasn't aware that such a bug existed. Can't help you there

[SorryDave]

I use side-by-side stacked jet engines above and below the wing. The parts are heavy because they hold a lot of fuel - I empty what I do not need.

The parts hold enough fuel that air-breathing engines are not necessary to build a SSTO craft. Air breathers help better with landing as gliding onto the runway is tougher. But if used for this, you only need a couple because the craft is much lighter empty.

After a couple frustrating attempts, I save my craft under a different name before I add the struts that keep my huge wings from sagging on the runway. Then I revert to the original save to move things around or add more lift.

A lot of my craft have had to have pretty crazy angles of attack to fly from the runway (otherwise I fly right off the runway).

These parts make exceptional rockets.

[Cirocco]

I use side-by-side stacked jet engines above and below the wing. The parts are heavy because they hold a lot of fuel - I empty what I do not need.

The parts hold enough fuel that air-breathing engines are not necessary to build a SSTO craft. Air breathers help better with landing as gliding onto the runway is tougher. But if used for this, you only need a couple because the craft is much lighter empty.

After a couple frustrating attempts, I save my craft under a different name before I add the struts that keep my huge wings from sagging on the runway. Then I revert to the original save to move things around or add more lift.

A lot of my craft have had to have pretty crazy angles of attack to fly from the runway (otherwise I fly right off the runway).

These parts make exceptional rockets.

huh. Do you rely almost purely on rocket engines to get to orbit then? I always figured that if you're going to use rocket engines on a plane, you might as well take off the wings and convert your craft into a rocket. The Mk III parts do indeed make for some very cool looking spaceships, but I'm looking for something that can basically take off from the KSC, go interplanetary (Duna is the current goal, might hit up Laythe later), land and return to KSC, all on one tank of gas. It seems like an air-breather would be most efficient for that seeing as I'll be expending about 30-50% of the fuel supply just getting into Kerbin orbit.

[Yukon0009]

One thing: The mk3 parts carry wayyyy more fuel than you'll probably need.

[brianorca]

Why are you expecting control surfaces near the CoM to control pitch at all? Being so close, they have no control authority/leverage. If you right-click them in the hanger, you can disable the control separately for pitch, roll, and/or yaw, as needed.

In "real" planes, the control surfaces on the main wing (i.e. near the CoM) only control roll. (Fighter jets that have delta wings are far enough back to also be used for pitch.)

The reason for the reversing "feature" is so you can build a canard plane with control surfaces in the front.

[Cirocco]

Why are you expecting control surfaces near the CoM to control pitch at all? Being so close, they have no control authority/leverage. If you right-click them in the hanger, you can disable the control separately for pitch, roll, and/or yaw, as needed.

In "real" planes, the control surfaces on the main wing (i.e. near the CoM) only control roll. (Fighter jets that have delta wings are far enough back to also be used for pitch.)

The reason for the reversing "feature" is so you can build a canard plane with control surfaces in the front.

Huh, good point. Up to now, all my planes have always had canards, but i'd like to build some without, which is why I tried moving the pitch control to the wings since I can fit a lot more there. But you are correct: nearer to the CoM they'll have far less control authority. Guess I should go with either tailless planes with canards or pitch control near the tail.

Ugh. I love aerodynamics and the accompanying engineering challenges, but I swear sometimes they make me feel like such a moron >.<

[SorryDave]

I always figured that if you're going to use rocket engines on a plane, you might as well take off the wings and convert your craft into a rocket.

The rocket plane has the advantage of being re-usable. The lift of the wings allows you to launch from the runway with less than 1 TWR.

If you wish to launch a interplanetary SSTO then I agree you should use air-breathers. One craft I use has a set of jet engines above and below the wings ( 20 total) and twice as many intakes. Even still, this craft has just under 3Km/s dV when in orbit.

I do not believe an interplanetary MK3 SSTO recoverable vehicle would be cheaper than a rocket designed for the same thing, but would be more fun.

Good flying!

[Wanderfound]

m

Quite a few jets are required if you want to go big.

[numerobis]

A turbojet every 20t is enough, so you're only talking about 5 jets. Maybe add a couple basic jets to help you lift off and get to altitude quickly.

What's missing is big wing surfaces.

And yeah, the Mk3 parts have just a stupid amount of fuel in them (even Mk2 parts have more jet fuel than I typically use). To get my passenger shuttle up I had to drain most of it.

[InterCity]

I find the MK3 parts rather weak and in desperate need of a buff. My solution is called B9.

And I also think that mk2 planes look way more stylish B-)

- - - Updated - - -

I find the MK3 parts rather weak and in desperate need of a buff. My solution is called B9.

And I also think that mk2 planes look way more stylish B-)

[Cirocco]

A turbojet every 20t is enough, so you're only talking about 5 jets. Maybe add a couple basic jets to help you lift off and get to altitude quickly.

What's missing is big wing surfaces.

And yeah, the Mk3 parts have just a stupid amount of fuel in them (even Mk2 parts have more jet fuel than I typically use). To get my passenger shuttle up I had to drain most of it.

A turbojet may have a theoretical max thrust of 225, but it's a hell of a lot less at launch, so unless I want to do RATO/JATO, I'll need more.

Though the wing surface is definitely a problem. I've been racking my brain trying to come up with aesthetically pleasing ways to add more lift surfaces without clipping like crazy. Have one or two ideas I plan to test this evening.

thanks for the replies anyways guys, quite a few things here that sparked my imagination.

[viktor19]

Are FAR SSTOs relevant here?

[Cirocco]

I'm using stock aero for now

[numerobis]

A turbojet may have a theoretical max thrust of 225, but it's a hell of a lot less at launch, so unless I want to do RATO/JATO, I'll need more.

Absent ground friction (which I don't model at all), a single turbojet can push 19t of 0.2-drag parts to 56 m/s at the altitude of the runway. Using the KSP-scripts, I define a plane with a turbojet, an intake, and 19t of extra mass at 0.2 drag:

python
from planeDesigner import part, plane, jets
p = plane(part(jets.turbojet), part(jets.structuralIntake), part(None, Cd = 0.2, extraMass = 19))
p.equilibriumHorizontalSpeed(0, altitude=70)
56.346893310546875

So add in enough lift surfaces to take off at 50 m/s and you'll be fine, no rocket assist required. On liftoff you'll have a TWR about 0.57, better than a lot of airliners.

Way better, actually, according to this page.

Edited January 7, 2015 by numerobis

[Van Disaster]

You can get to space with TWR in the 0.30s with FAR, although you may need more runway...

The Mk3 parts are compatible with B9 HL parts ( as they should be, they're based on the HL cross section ), so you can take bits from there. Bac9 or DJY's procedural wings would solve any lack of large wing issues too.

[numerobis]

At the other end, in stock, the turbojet per 20t will get you to 1750 m/s surface speed, whereas you need about 2100 m/s to orbit. You can get a bit faster if you use a lot of Mk2 parts, which have lower drag (and wings have low drag too).

In FAR, drag is less of an issue, but the turbojet is nerfed, so IIRC you get a broadly similar top speed.

[GeorgeG]

I've yet to make a successful Mk3 aircraft. If you're using jet engines you need to have a lot of them...and then add some more. It's equivalent to trying to launch a 3.75m rocket with 1.25m engines; it's possible, but more trouble that it's worth.

Hopefully we'll get some (at least) 2.5m jet engines at some point. That'll solve this problem.

If you REALLY want 2.5 meter Turbojets that are simply scaled up stock turbojets, you can have those right now!

"Tweakscale"

http://forum.kerbalspaceprogram.com/threads/80234-0-90-TweakScale-Rescale-Everything!-(v1-50-2014-12-24-10-40-UTC)

It will dynamically scale existing parts, so a 200% scale-up of a 1.25 meter turbojet engine to 2.5 meter size has 6 times the thrust, and weighs nearly 6 times as much (6.788 ton vs 1.20 ton).

Unfortunately, scaled-up air intakes do not seem to behave correctly. For example a 2x shock cone intake has 8 times the intake air max capacity of a 1x. But on a flight to higher altitude, when the 1x intake is down to .30 air out of .8 (37.5%), the 2x intake is down to 1.38 out of 6.40 (21.6%).

Higher and faster (15 km and 351 m/s), the 1x intake is at .10 out of .8 (12.5%), and the 2x intake is at .47 out of 6.40 (7.3%).

At 26 km, the 1x intake is at .02 out of .8 (2.5%) while the 2x intake is at .10 out of 6.4 (1.56%).

So, for best performance, six or more 1 meter intakes would be better than one 2x scaled up intake that is supposed to have 8x the airflow.

Fortunately, with part clipping on, the shock cone intake can be stacked onto itself multiple times, to achieve a better looking plane that doesn't have ridiculous amounts of intakes stuck everywhere.

BTW - BIG planes also tend to need decent landing gear. And frankly the stock landing gear is too ancient (unchanged since what, 0.15?), rickety, finicky, and does not look so great.

So another mod I have started using recently is "Adjustable Landing Gear".

http://forum.kerbalspaceprogram.com/threads/99660-0-25-Adjustable-Landing-Gear-v1-0-4(doors-fixed)-Nov-14

Here is a link to a 49 second video showing what it is like and how it works.

It is fantastic! THIS is what KSP's "stock" landing gear ought to be these days.

Adjustable Landing Gear, plus "Tweakscale" produces landing gear suitable for just about any size of space plane that you can get to fly.

Below are two images of a SSTO shuttle I made, which uses a 2.5 meter turbojet plus six 1.25 meter turbojets, and stacked intakes (also a lot of the skinny long Structural Intakes along the bottom of the wing). Action groups shut down the six 1.25 meter turbojets as the thin air risks flameouts of random engines that could cause it out go out of control. So then it keeps accelerating on the single 2.5m Turbojet. Then when it flames out, it does not make it go out of control, and it's time to fire up the two aerospikes and climb steeper.

This also uses Adjustable Landing Gear, and the main wing used tweakscale to be 200% larger.

- GeorgeG

Edited January 7, 2015 by GeorgeG

[Wjolcz]

Not much, really. Although I menaged to built a semi-working VTOL Skylon and a possible SSTO, that might work but I havent tested it fully yet.

Here it is:

Javascript is disabled. View full album

I was able to take off from the runway with it, but I was too lazy to do that over and over during testing.

A cargo, air-only one:

And this poor little thing

Edited January 7, 2015 by Veeltch

[Van Disaster]

At the other end, in stock, the turbojet per 20t will get you to 1750 m/s surface speed, whereas you need about 2100 m/s to orbit. You can get a bit faster if you use a lot of Mk2 parts, which have lower drag (and wings have low drag too).

In FAR, drag is less of an issue, but the turbojet is nerfed, so IIRC you get a broadly similar top speed.

FAR's interpretation of the stock jets will run out of puff at ~1300m/s or so no matter what you do or how many you have, they'll just stop making thrust. You can cruise on up to orbit with very little thrust from 25km up in FAR though, not much air up there to cause drag.

This is B9 HL - exactly same fuselage format as Mk3, so eminently something this size is buildable using mk3. Also using pWings to make the entire wings in one piece, so perhaps anyone at least a little open to mods might want to look at pWings until there's large stock wings available.

Can't do much about the engines though...

[Cirocco]

hmmm, from most posters here it seems the problem with the new parts is not so much the fact that they're heavy but the fact that we're lacking large wings. I did some testing yesterday evening and it turns out that while you do need several turbojets for a spaceplane, it's actually far less than I initially thought. Insufficient wing surface seems to be a bigger problem.

I'll see if I can figure out a way to aesthetically add more surfaces and larger wings. Thanks again guys

[KotDemopan]

hmmm, from most posters here it seems the problem with the new parts is not so much the fact that they're heavy but the fact that we're lacking large wings. I did some testing yesterday evening and it turns out that while you do need several turbojets for a spaceplane, it's actually far less than I initially thought. Insufficient wing surface seems to be a bigger problem.

I'll see if I can figure out a way to aesthetically add more surfaces and larger wings. Thanks again guys

How did landing go for ya? Did the plane break into pieces?

[Cirocco]

I only tested one configuration for landing. I'm focusing more on take-off at the moment.

as for the one "landing" I did do... well let's just say that at least it was quick and accompanied by very pretty explosions.

[DunaRocketeer]

I've found that landings usually have to be extremely benign events with the mk3 parts. Soon I'm going to experiment with a series of probes that I'll fly to locations due-west of the runway that can be targeted by returning spaceplanes in order to give myself super-accurate alignment to the KSC runway, then all I have to worry about is descent rate and keeping the wings level.

As for lift, if you make your wings double thickness, you'll get acceptable lift and the wings themselves will look more realistic due to looking thicker.

[r4pt0r]

As for lift, if you make your wings double thickness, you'll get acceptable lift and the wings themselves will look more realistic due to looking thicker.

does this work with far?