Help with large spaceplanes

[Mitchz95]

So I've been playing around with SSTOs lately. I've built a rugged plane capable of carrying a Kerbal and some science to orbit and back, but that's pretty much it. Now I'm looking to build more ambitious craft, for transporting smallish payloads to orbital ships and facilities. Unfortunately, it seems no matter how many wings and control surfaces I add, I either can't get off the runway or can't keep it in the air. I'd love some help from all you spaceplane wizards out there.

Here's my first attempt.

I'm pretty sure that's how the COM and COL are supposed to be set up, and I positioned the landing gear so I can pull up easily on the runway. Unfortunately, when flying it I always have to keep the nose way up to stay in the air and SAS doesn't really help.

My second attempt (this one is exclusively for crew):

This one won't even get off the runway.

Both use the Spaceplane Plus mod, but I imagine the mechanics are more or less the same.

[LethalDose]

Just for info: Are you using FAR?

[TheSaint]

If you are using Spaceplane Plus without FAR then either make sure that you are using the latest version of Module Manager or that you delete the SP+FAR.cfg file from the Spaceplane Plus folder. If you don't, your wings will have no lift.

[Laie]

I keep my Center of Lift much closer to the Center of Mass. Usually they make contact, sometimes to the point where the blue bubble nearly vanishes inside the yellow one. When there's a visible gap between the two, it's becoming too lawn-dartish for my taste. But I'm by no means an expert.

Otherwise, canards. I find that no matter how many aft control surfaces I have, I tend to lack control at some point. A pair of canards or maybe even two, as far ahead as possible, works miracles.

[Cirocco]

Alright there are definitely a few tips I can give you regarding larger and heavier spaceplanes. This is gonna be a long post so I included a TLDR at the end

The most important thing I am not seeing on the spaceplane you show in the pictures is canards. Why do heavy planes almost always need canards and light planes don’t? Because of the way pitch control works in relation to your center of mass. Think of it this way: there’s two ways of pitching up (and down, but up is what you need to get off the runway). You either push down the back of the plane or pull up the nose. Canards will pull up the nose, ailerons will push down the back. Now also remember that a plane will ALWAYS try to rotate around its center of mass. If you look at your pictures, you can already tell that the center of mass lies more to the back of the plane. This is very typical for heavy planes and only increases the heavier the plane gets because you need more engines, which often need at least one fuel tank to attach to, which makes the plane very back-heavy (at least that’s the case if you use the standard all-engines-in-the-back triangular shape configuration).

It’s a LOT harder to pitch up when you have to push down so close to the center of mass. This is due to leverage: the longer the lever distance, the easier lifting (or lowering) something is. The shorter the lever distance, the harder it gets. By the same principle, the further back the CoM lies, the more effective canards become. So add canards to your heavy planes to help with pitch control. Note that this will shift the center of lift forward by quite a lot though. Keeping the center of mass in front of or on the center of lift is one of the biggest challenges in building heavy planes. On a side note, pitching up on the runway gets easier the closer your rear landing gear is to the CoM. But putting them too far inward means you’ll smash the engines into the runway. That’s also a balance you need to find with experience.

In short: Having pitch controlling surfaces only on the back of the wings works well for light planes because their center of mass lies pretty central of the entire plane. For heavy planes which have a center of mass that lies much more to the back, canards or other pitch control near the front of the plane are very often if not always a requirement.

Other improvements I would suggest: smaller wings. You have a huge wing there, you really don’t need THAT much wing surface, even in stock. Wings provide lift, but also drag and mass. Don’t go overboard on them. Secondly: more engines. Heavy spaceplanes need a lot more than 2 engines. My latest creation - which is an SSTDuna - has 4 rapiers, 4 turbojets and 3 nuclear engines. Obviously you don’t need nuclear engines until you want to go interplanetary, but you do need quite a bit of thrust to get to orbit efficiently in order to counteract aerodynamic and (perhaps more importantly) gravity losses. Easiest way to add engines without losing symmetry or smashing them into the runway on take-off/landing is to add a 1,25m tank on either side of the main tank in the same plane as the engines you already have (get it? Same plane? As in mathematical plane versus aircraft? … I need more friends…) and put more engines and air intakes on those.

So yeah, TLDR: add canards, add more engines, don’t go with such a long wing, keep an eye on your CoM and CoL. Keep all that in mind and you should be able to upscale quite well.

Edited June 18, 2014 by Cirocco

[Red Iron Crown]

Overfloater shared some of his large spaceplane building secrets in a great video here.

[Overfloater]

Assuming you DON'T use FAR...

For good atmospheric flight:

1. keep making wings, until the center of lift is where it needs to be*.

2. only NOW you add flaps. Add an adequate amount of flaps way behind, and way in front on the center of mass.

3. the rear flaps - tilt them up. So it will be easier to pull up then to pull down. And the plane will naturally want to pull up

A little picture to show what I'm blabbering about:

-Tilt the front flaps only when desperate.

* Slightly behind & above the mass center. Or anywhere near...

Overfloater shared some of his large spaceplane building secrets in a great video here.

I'm being advertised! And yes, that video demonstrates what I said.

[Hodo]

First problem I see is you dont have enough struts, your wings are probably flexing and loosing lift keeping you on the runway.

Next problem I am seeing is your engines are attached by docking ports... bad idea. They are not strong enough to keep the engines from flexing around on the back of the craft.

Unlike others suggested, you don't need canards at the front of your craft to get the nose to go up. Here is a picture of one of my old SSTOs that I made many months ago that has a very close wing design.

Note the dry CoM and wet CoM in relation to the CoL.

As for cargo hauler SSTOs I HIGHLY suggest you move your cargo bay over the CoM of the vessel. This way no matter what you put in it your CoM will not move forward or back any large amount throwing off the flight characteristics of the aircraft.

Like in this picture.

[Mitchz95]

Wow, lots of great advice here. Thanks, everyone.

I am not using FAR, and updating Module Manager (and deleting that FAR.cfg for Spaceplane Plus) did indeed help me get off the runway more easily. Maneuvering is still somewhat tricky, and tailstrikes are very common, but I think I'm slowly getting the hang of it. I also shrank the wings and strutted them more. Both planes do have forward canards, though they may not be easily visible in the pics. And the reason I have my RAPIERs mounted on docking ports is so I can swap them out for NERVAs once in orbit. Perhaps I'll add some KAS struts to make them more sturdy.

I already built a prototype based on that Overfloater video (which is awesome, by the way!). It couldn't make it to orbit, so I converted it to an atmospheric cargo plane.

Again, thanks for all the advice. I'll be sure to come back if I have more questions.

[O-Doc]

Basically, what Overfloater said. You need to set aerodynamics so that the plane will have a tendency to pull up. How much it will have to pull up depends upon how many pitching control surfaces you put(or hide) on the front of the airframe. The more control you put on the front the less it has to pitch up. A good heavy lifter will take off the runway without SAS on by just tapping back on the stick once or twice.

Your tail assembly is important for this. The way you visualise this is to think of a see-saw. the CoT is the fulcrum and the tail assembly control surfaces are lifting the CoM. The further away the tail is from the CoT(and the nearer you CoM is to the CoT) the more leverage you get to create a pitching moment on the aircraft.

EDIT: TL;DR: your engines are behind your control surfaces! Move your CoT forward, a lot.

Edited June 19, 2014 by O-Doc

[Mitchz95]

Well, that was a disaster. I built a new spaceplane from scratch, taking into account everything I've read here. Gave it a small LFO tank as a test payload, and it handled pretty well. Fuel seemed to be draining contrary to the placement of my fuel lines, but I manually transferred it around and it seemed okay. I got it to around 20km, at which point I switched to rocket mode. Then everything went to hell.

The plane began twisting to starboard, and nothing I did -- cutting the engines, activating torque, dumping the payload -- made any difference. I went into an all-out stall, and had absolutely no control over the plane. And yes, I had lots of electricity.

Thankfully, I managed to get the nose pointed up and fired the engines, slowing the plane enough to turn the crash into a manageable one. Bob survived, but I don't think he'll be flying again anytime soon.

It turned out the fuel was draining from the side tanks asymmetrically, despite everything being completely symmetrical during construction. I can only assume it's more wonkiness with the fuel lines, and I've made some adjustments that may help with future flights. I'm taking a break for now, but I'll try to make another attempt before turning in.

O-Doc: Thanks for the tip. I never really put a lot of focus on the tail, but I suppose it would make in interesting thing to tweak. I'll play around with it.

[Hodo]

Tac fuel balancer or PWB fuel pump will fix this problem with fuel drain, and HIGHLY suggested for any serious SSTO work.

[Claw]

You also need to be careful shunting around fuel with fuel lines. Some of the fuel rules are tricky, even though the connections may make logical sense to you.

You can find the fuel flow rules here.

http://forum.kerbalspaceprogram.com/threads/64362-Fuel-Flow-Rules-%280-23-5%29

[Mitchz95]

You also need to be careful shunting around fuel with fuel lines. Some of the fuel rules are tricky, even though the connections may make logical sense to you.

You can find the fuel flow rules here.

http://forum.kerbalspaceprogram.com/threads/64362-Fuel-Flow-Rules-%280-23-5%29

Ah, that might be it. Thanks for the link.

[Mitchz95]

Well, quite an interesting day I had.

I built another plane, complete with test payload, and launched it. To my surprise and joy, it actually made it to orbit. Unfortunately, I didn't have enough delta-v left to go anywhere and had to do my re-entry burn right then and there. And of course it stalled and crashed on the way down, perma-killing Jeb and Bill.

Modified the design and launched it again, this one with some extra seats instead of a payload to bring home three Kerbals who were stranded in orbit. In hindsight, I probably should have tested it unmanned first: it, too, stalled during landing and killed all but one Kerbal. So five of my green friends died today.

I do think I'm getting the hang of things, though. In fact, I think that last crash was just pilot error: specifically, I forgot to pump the remaining fuel forward to keep my COM ahead. But I think I'm going to stop angling my elevators up; it makes things harder to control when the COM and COL are close together.