Just can't build effective heavy space planes

[TheMonkeytect]

I have an average skill on general ksp matters, and an improved, after so much failures, skill in flying, stalling and landing. Also, a decent knowledge of CoM, CoL, etc.

Besides all that, I can't build a heavy space plane that gets into orbit with two extra fuel tanks, use them, dump them, refuel a space station and come back to Kerbin, Space-Shuttle-style. This is the only way in career to refuel a space station with minimal cost, I think (apart from the SSTOs, but don't get me started on building heavy SSTO)

And when I say heavy, I don't mean the mass in tons, but I'm referring to the 3.75m diameter, the largest fuselages there are for space planes.

I've been trying for a quite long time now, and making them survive both basic orbital manuevers and a landing is simply impossible. Yes, the CoL is behind the CoM, yes, I saw where the CoM and CoL are after the fuel gets reduced to the amount it has on reentry. I did the painful calibrations just right, tested the plane, most of the times it's fine. Yet still, after orbit, the thing seems to break upon the slightest bump on the ground or spin around (I have tested the very same plane, with depleted fuel, at Kerbin and works fine). I have tried different control surfaces, larger, smaller. I have landed a spaceplane on Eve, I have managed numerous missions with ordinal "flat" space planes, but heavy space planes just don't work for some reason.

I just think the game doesn't like heavy space planes.

Has any of you tried persistently and failed to build a heavy orbiter? Is anyone avoiding the heavy, 3.75m diameter design because of its impossibility to cooperate, or am I doing something incredibly wrong? If you have any working designs or images of a 3.75 orbiter or, better, SSTO, please help me, this matter is anything I am trying to do with ksp these days.

[WhiteKnuckle]

Are you using the Mk3 spaceplane parts, or the 3.75m rocket parts?

Generally the rocket parts don't have as much impact tolerance as the spaceplane parts, and may go boom on touchdown. 

Do you really need to move that much fuel, because getting a full orange tank to orbit isn't that difficult anymore.

 

Edit: I remembered seeing this pic a few days back, one of the few 3.75m spaceplanes I've ever seen anyone make:

By @Temstar maybe you could check out his stuff to see how he makes it work.

Edited February 5, 2016 by WhiteKnuckle

[JoeNapalm]

48 minutes ago, TheMonkeytect said:

I just think the game doesn't like heavy space planes.

I don't think it's that, I'm afraid. They are different from smaller varieties, but they can work.

I've built many Mk 3 SSTOs of unnecessarily large size (sorry, don't have access to my pics right now, but it's what I do every time I start feeling bored). The thing about them is, due to the inherent bendy-ness of Kerbal construction, and the massive tonnage of such a vessel, you can't usually ram them around like a smaller spaceplane, or you'll get structural failures.

I use lots of struts, and when I touch them down, I try to land them like you're trying not to break through the crust of newly fallen snow.

I often will festoon them with radial chutes, as well, so if you're coming in heavy or fast, you can use either drogues or even full chutes to arrest your descent. (You need to balance the chutes right, or they can tear the ship apart, as well).

If you're spinning out on landing, and launching space shuttle style, my guess is your gear is misaligned, or your CoM is too far forward or back - do you wipe out on touchdown or when you brake?

 

-Jn-

[Tex_NL]

First of all I'd like to suggest two great mods: TweakScale, which lets you resize existing parts. And KerbalJointReenforcement which strengthens the connections between parts, making everything much stronger and rigid. No longer will you be flying a wet noodle or fall apart without warning.

Secondly it's all about design and how you launch it. You can build true monsters if you want to. It is heavily TweakScaled but this beast lifted 90tons of cargo safely to 100*100km orbit. To give you an idea of scale, the cockpit, tail fins and Rhino engines have not been TweakScaled. And neither have those 5 fully loaded large ore containers.

Click to see how it was launched.

Edited February 5, 2016 by Tex_NL

[Evanitis]

1 hour ago, TheMonkeytect said:

If you have any working designs or images of a 3.75 orbiter or, better, SSTO, please help me, this matter is anything I am trying to do with ksp these days.

Hope you mean Mk3, and not the actual 3.75 rocket parts. It wouldn't be much harder to use the latter, but such plane should be landed -really gently- as those have lower crash tolerance. Rockets were never intended to land - not that it stops anyone to do so.

I recently build an Mk3 spaceplane that's as close to being a shuttle as it can be, without being one. Stock parts only, on the bottom of this post if interested. The big blueprint picture is a download link. It intended to be a joke, but to my great surprise it wasn't just be able to do a Shuttle challenge by jettisoning the lifter parts, but it could even reach orbit in one piece (with an orange tank as payload).

Though it has some silly design choices - the shuttle part can only land on water as it has no gears (by default). But you could just add wheels.

1 hour ago, TheMonkeytect said:

I've been trying for a quite long time now, and making them survive both basic orbital manuevers and a landing is simply impossible.

Hope you mean 'atmospheric manuvers', cause there's nothing on orbit that should make a craft go boom.

One should only steer a heavy aircraft -very gently- in the air when it travels with escape velocity or on re-entry, as the aerodynamic forces can rip it apart if pitching too fast. As a rule of thumb, if the dot in the middle of the navball goes out of the circle of the prograde marker, trouble is near. But you -need- to pitch up on reentry to slow down. If it still breaks that way, you could add struts. Struts are magic.

Landing needs some practice, and on land it doesn't hurt to have a mod (KER or MJ) that displays true altitude instead of the stock sea level one. The trick is to bleed as much speed as possible without gaining altitude. Ideally, I'd aim for a horizontal flight on less than 50m. You will notice that as the plane slows down, the prograde marker crawls downwards on the navball. When that happens, slowly raise the nose, so the marker keeps sitting on the horizon line. So you'll keep the altitude while slowly decelerating. This way by the time the plane stalls it will travel less than 60, or even 40 m/s (depending on the relative wing area). That should grant a gentle touchdown.

I suggest posting a picture or two of the craft if you require further assistance, as it will greatly help to rule out some possible problems if the above method fails.

Edited February 5, 2016 by Evanitis

[Tex_NL]

11 minutes ago, Evanitis said:

...

Hope you mean 'atmospheric manuvers', cause there's nothing on orbit that should make a craft go boom. One should only steer a heavy aircraft -very gently- in the air when it travels with escape velocity or on re-entry, as the aerodynamic forces can rip it apart if pitching too fast. As a rule of thumb, if the dot in the middle of the navball goes out of the circle of the prograde marker, trouble is near. If it still breaks that way, you could add struts. Struts are magic.

...

I agree atmospheric flight is often tricky and re-entry can be dangerous but keeping the nose that close to prograde is NO GUARANTEE for a safe flight. Staying at prograde will generate the least amount of drag and won't slow you down quickly, exposing the craft to a lot of potentially deadly heat. I prefer to keep a high angle of attack with the nose sometimes 30 to 45 degrees above prograde to slow down as quickly as possible.
How to gets the best results comes down to how a craft is designed and a lot of experimentation.

Edited February 5, 2016 by Tex_NL

[Evanitis]

15 minutes ago, Tex_NL said:

I agree atmospheric flight is often tricky and re-entry can be dangerous but keeping the nose that close to prograde is NO GUARANTEE for a safe flight. Staying at prograde will generate the least amount of drag and won't slow you down quickly, exposing the craft to a lot of potentially deadly heat. I prefer to keep a high angle of attack with the nose sometimes 30 to 45 degrees above prograde to slow down as quickly as possible.
How to gets the best results comes down to how a craft is designed and a lot of experimentation.

Ahh, indeed. What I should have wrote is to 'only pitch really slowly'. Indeed, keeping the nose on prograde on reentry is a silly advice.

[Temstar]

Mk3 has an advantage over 3.75m because it has an insane 50m/s crash tolerance vs 6m/s for 3.75m part, Mk3 also has very high max temperature. Even so it's perfectly possible to use 3.75m tanks as structural elements as the above picture of the Aurora Winged Booster shows.

Anyway, it sounds like you trouble is mostly with the actual touch down right? We can work on this. Here's two screenshots of touchdown, pay close attention to the horizontal and vertical speed on the top centre of the screen (this is part of KER's HUD display)

40 ton SSTO touching down, Mk3 based fuselage.

 

42 ton winged rocket touching down, high wing load, mixed Mk3 and 3.75m fuselage.

Okay so the main thing you have to manage when touching down is vertical and horizontal speed. You want to hit the deck at less than 10m/s vertical speed, preferably less than 5m/s. You do this by getting your horizontal speed to around 80-100m/s just before touching down, then before you hit the deck you flare (pitch up), trading away horizontal speed for vertical speed. Here's a chain of screenshots showing the flare process for that big winged booster:

Spoiler

Approaching the runway. Note that you don't need to touch down on the very end of the runway in KSP because the brakes on your landing gears are incredibly powerful and can stop your plane in a hurry. Notice here I'm flying with a slight downward glide angle, this is to maintain forward airspeed while I descend.


Only 60m off the deck now, pitching up how to the horizontal, airspeed down to 80m/s, descent rate 11m/s.


30m off ground, starting landing flare! 5 degrees pitch up above the horizon. Descent rate down to 8m/s which is now within the safe range for touch down, airspeed down to 72m/s. At this point as you as long as you can hold the plane like this you can safely touch down.


For a more gentle touch down though, keep the flare up to keep decreasing your descent rate. Ideally you want both your descent rate and your altitude to both hit zero at the same time for a very smooth landing.


Touchdown, taxiing to a stop using wheel brakes and air brakes.

Horizonal speed is important because it governs how your craft will behave when you flare. Too slow and you will stall and fall on your ass. Too fast and you will start to gain altitude instead of land, you will also lose air speed and probably then stall. The exact value depends on how well your aircraft can glide - a glider will have very slow landing speed while a fly brick of a shuttle need very high landing speed.

Edited February 5, 2016 by Temstar

[Fearless Son]

2 hours ago, Tex_NL said:

I agree atmospheric flight is often tricky and re-entry can be dangerous but keeping the nose that close to prograde is NO GUARANTEE for a safe flight. Staying at prograde will generate the least amount of drag and won't slow you down quickly, exposing the craft to a lot of potentially deadly heat. I prefer to keep a high angle of attack with the nose sometimes 30 to 45 degrees above prograde to slow down as quickly as possible.
How to gets the best results comes down to how a craft is designed and a lot of experimentation.

Aye, that will do.  However, in the case that the craft really is so delicate and heavy that even that seems like a risk, then that is what A.I.R.B.R.A.K.E.S. are for.  Put them at the rear of the craft, turn them on during descent, and they can both keep the nose pointed prograde and help shed velocity at the same time.  

They are not required for every design, but they can certainly help some of them.  

[TheMonkeytect]

4 hours ago, WhiteKnuckle said:

Are you using the Mk3 spaceplane parts, or the 3.75m rocket parts?

Yeah, I mean the Mk3 spaceplane parts. I tried to keep the plane design cylinder-free. I just tried to go for a Space Shuttle design, just to see if it's efficient, and not only did I not see whether it is or not, but I also could not score a simple orbit-and-return.

[TheMonkeytect]

3 hours ago, Evanitis said:

Hope you mean 'atmospheric manuvers', cause there's nothing on orbit that should make a craft go boom.

No, what I mean is that, for some reason, handling the craft without exiting the atmosphere is just difficult. Like normally difficult. After a reentry, landing is impossible.

 

[TheMonkeytect]

3 hours ago, JoeNapalm said:

I use lots of struts, and when I touch them down, I try to land them like you're trying not to break through the crust of newly fallen snow.

Exactly. So, it's normal, I guess. I just might have to upgrade my landing and calibrating skill then.

[Evanitis]

1 hour ago, TheMonkeytect said:

No, what I mean is that, for some reason, handling the craft without exiting the atmosphere is just difficult. Like normally difficult. After a reentry, landing is impossible.

I don't really get it. If you could orbit and land a small spaceplane, the Mk 3 one should be pretty much the same.. You need to handle it a -bit- more slowly and carefully, bit the difference is -not- big. If the balance is right. But you already said you did check the full/empty CoM / CoL, so it should be good. Temstar's guide with the pics is excellent. Maybe it's really just some practice you need.

Though I'm really curious how the plane you fly looks. If you're lazy to fiddle with images or file-sharing, you could just open the craft-file with a text-editor and copy-paste the content here.

[Nich]

heavy SSTOs are much easier then asymmetric shuttle types but I do have some tips. that apply to both

1.  Attach wings to heaviest parts and move them into position for correct COL COM.  

2.  Wings should attached to central core if not they will flex too much.

3.  If using multiple wings attach them to different core parts (helps with core flexing)

4.  Use both canards and rudders (helps with core flexing)

5. Most of my heavy SSTOs need more engines to get off the runway then they need to get to space so make sure to keep your speed in check.  300 m/s below 5km 500 m/s below 7km 800 m/s below 10km.

6.  When landing empty your plane will have a TON of lift and a TON of speed I normally get below 300 m/s before I try any significant deviations from propragrade. and most of my craft manavur best around 130 m/s and land about 80 m/s when empty

[Vegetal]

You said your plane is shuttle-style?

Beware cargo bays. They generate a LOT of lift and don't register on the SPH or VAB CoL indicators. Try flying with F12 turned on, see if any big cyan lines pop up.

[quasarrgames]

Haha, my friend! I, too have fiddled with rocket SSTO spaceplanes, and i can offer two ways to maybe possibly hopefully fix your problem:

1. add more wing area. most likely you're breaking up on contact with the runway because you're going too fast. if the empty plane can glide at 50m/s, or at least flare enough to reduce its vertical speed to 0 right before touchdown, it should be fine.

2. make sure the brake torque on the back gears is up at its maximum. This will make the plane slow down as quickly as possible to reduce the dreaded wobbling/flipping upon landing.

also, no shame in deploying parachutes just above/upon touchdown. Space shuttle did it, so...

And yes, mk3 parts are notorious for disconnecting themselves from other parts even in the smallest impacts. Still, that's wierd if it behaves differently after achieving orbit. Never heard of that before.

[TheMonkeytect]

17 hours ago, Evanitis said:

Though I'm really curious how the plane you fly looks. If you're lazy to fiddle with images or file-sharing, you could just open the craft-file with a text-editor and copy-paste the content here.

Here it is:https://scontent-mxp1-1.xx.fbcdn.net/hphotos-xtf1/v/t1.0-9/12662563_10204304742452621_1540416650183001014_n.jpg?oh=48d2086acae76ebf091b87e4547ff2d8&oe=576D2C9F

I also noticed something strange: The SAS is in Stability Assist, yet the RCSs wobble it up and down slowly. I warp and then unwarp to stabilize it, and still, it starts wobbling after I unwarp.

[TheMonkeytect]

Ok, guys, I think I found the culprit. As Vegetal pointed out, I have two large Mk3 Cargo Bays, and they produce this kind of side thrust:

https://scontent-mxp1-1.xx.fbcdn.net/hphotos-xfp1/v/t1.0-9/12669418_10204304752732878_1391728401621623239_n.jpg?oh=11279d077ba1457266d88cae72e16728&oe=576A7B7E

So I guess I'll have to forget about the great feeling of an actual Space-Shuttle-like orbiter with room for letting payload into orbit?

10 hours ago, quasarrgames said:

Haha, my friend! I, too have fiddled with rocket SSTO spaceplanes, and i can offer two ways to maybe possibly hopefully fix your problem:

1. add more wing area. most likely you're breaking up on contact with the runway because you're going too fast. if the empty plane can glide at 50m/s, or at least flare enough to reduce its vertical speed to 0 right before touchdown, it should be fine.

2. make sure the brake torque on the back gears is up at its maximum. This will make the plane slow down as quickly as possible to reduce the dreaded wobbling/flipping upon landing.

also, no shame in deploying parachutes just above/upon touchdown. Space shuttle did it, so...

And yes, mk3 parts are notorious for disconnecting themselves from other parts even in the smallest impacts. Still, that's wierd if it behaves differently after achieving orbit. Never heard of that before.

It's not a SSTO plane, but these comments have been really helpful 

Edited February 6, 2016 by TheMonkeytect

[TheMonkeytect]

Ok, now I am sure the reason were the cargo bays. They throw the plane out of control, and the reason this happens after orbit seems to be the speed alone. The plane cannot slow down with the normal nose-slightly-over-prograde way. It's just impossible.

And the Cargo Bays do not, of course, show any effect on the CoL indicator.

Does this mean that Cargo Bays cannot be put in orbit on an orbiter?

[Nich]

No just means you need to move the CoL further back than you think.  btw just about every plane I fly if I see cyan at all I am very close to crashing.  The fact that you can get a what appears to be a 45 degree AOA means it is a very well designed plane.

Most of your breaking should occur above 35km.  by 25km I am down to 1700m/s and I glide there until I hit 500 m/s

[Tex_NL]

42 minutes ago, TheMonkeytect said:

Ok, now I am sure the reason were the cargo bays. They throw the plane out of control, and the reason this happens after orbit seems to be the speed alone. The plane cannot slow down with the normal nose-slightly-over-prograde way. It's just impossible.

And the Cargo Bays do not, of course, show any effect on the CoL indicator.

Does this mean that Cargo Bays cannot be put in orbit on an orbiter?

Indeed. Cargo bays do not affect the CoL in the VAB/SPH but they do generate body lift.

Of course you can get craft with cargo bays to orbit. The forum is overflowing with examples. This very thread shows several examples.

4 minutes ago, Nich said:

No just means you need to move the CoL further back than you think.  btw just about every plane I fly if I see cyan at all I am very close to crashing.  The fact that you can get a what appears to be a 45 degree AOA means it is a very well designed plane.

Most of your breaking should occur above 35km.  by 25km I am down to 1700m/s and I glide there until I hit 500 m/s

I prefer to BRAKE instead of BREAK at 35km. .

[TheMonkeytect]

3 minutes ago, Tex_NL said:

Of course you can get craft with cargo bays to orbit. The forum is overflowing with examples. This very thread shows several examples.

Ok, so now my only concern is how to add more wing area without making my plane unsufferably ugly. This thread has many good looking planes, but mine just looks like a wooden ship, with the struts and the pieces of metal 'n' all.

Do you know whether TweakScale affects wing area, or just the geometry? Most possibly it does

[luizopiloto]

all the MK3 tanks creates a lot of body lift... and this can cause some problems during reentry...

 

Edited February 6, 2016 by luizopiloto

[Nich]

Oh WOW so I just built an MK3 SSTO and there is definitely something wrong with the MK3 cargo bay.  At one point braking it had more lift then my wings :0  In addition the lift does not scale the same way as wings so at low AOAs it provides no lift but at large AOAs Tons of lift.

I think the only way around this is to put cargo bays as far back as possible and tanks up front

[Vegetal]

Yeaaaaaah those cargo bays always get people who are not used to making space planes.

The trick is to put them as close as possible to your CoL, that way they act at the same point as your wings, thus not messing up your stability.

One thing to note though: A lot of people try to copy the space shuttle design, thinking it looks cool or it should be a good design, if you don't know better. Actually, it got to space holding on to a big rocket and cam back gliding. It didn't have to be very good at flying actually. And it wasn't as far as I know....