Need some help with atmospere planes flight-carrying a booster to orbit stock career

[xcorps]

I just started messing with atmos flying a few days ago, and I've pretty much got the hang of basic flight and design. I can build flight capable planes and fly them reasonably well (with SAS). I've done nap of kerbin flying, landing on roughish terrain, gotten a turbojet equipped plane up to 1400 m/s horizontal, taken a basic jet engine to 65k, etc.

What I'm trying to do is build a "piggyback" style launch system that will put a probe out of the atmosphere before it fires a rocket. I built a turbojet equipped plane that flies perfectly well and has plenty of power and air. It's capable of getting to 70k apoapsis.

I added a single BACC SRB with a battery, wings and a probe to hydraulic manifold attached to a hardpoint on top of the plane as a test. I made enough adjustments to wing position to get COL perfectly aligned with the new COM.

It won't fly straight. The bow flips over topside and continues to loop around. I've made quite a few adjustments, moving COM up and down, forward and back. Moved the wings around to adjust COL above and below COM.

How should I go about doing this?

Edited May 14, 2014 by xcorps

[Claw]

Are you saying the nose comes up and it flips over backward?

It could be a couple things.

1) It might be that there's enough drag up front and on top now that it's forcing the nose to flip over. Especially if it's happening after you add the booster, and the fact that you have that big ram scoop on the nose.

2) You don't have enough control authority. It's hard to tell because of the angle of your picture, but you might need more control surfaces further aft on the fuselage. If they are too close to inline with your CoM, they don't help much.

3) Also, turn on your Center of Thrust (CoT) pink marker and see where that lines up. It's probably a bit low after you load on the booster, causing your craft to pitch up.

Here's something you can check, to see how borderline your base craft is. Try flying it again without the booster, but watch and see how much the SAS is working to keep the nose level. Do a couple of aggressive nose up/down maneuvers and see if it goes out of control.

Also, be aware that if you move your CoL up too much to meet up with the loaded CoM, you might have problems later if your ship doesn't have enough control authority to deal with the shift after release.

Here's an example of a piggy back space plane carrier I made a while ago. It's not nearly as big as what you're working on, but sort of demonstrates what I'm talking about. In this case, the CoL shifts up with the CoM because the space plane also has wings. However, I designed the carrier to be able to take cargo (satellites) into space on its back, but I don't have any pictures of that uploaded. Notice how the CoM is a bit below the CoL. That was so that when I load cargo up top, the CoM shifts about that much above the CoL and the carrier is still controllable. Also note the CoT relationship.

Once such way to deal with all of that is to add a torque unit to your craft (can't see any in your picture but I'm sure you have at least one). That can help you from having to redesign the wings and control surfaces for this particular design.

(Here's the rest of the album, but the other pictures probably aren't useful for design. http://imgur.com/a/mpA63)

Edited May 14, 2014 by Claw

[xcorps]

Wow, great post, thanks for taking the time.

Do a couple of aggressive nose up/down maneuvers and see if it goes out of control.

That is true of almost all of my designs, even the smallish ones. (well, the ones with any wing area) I always assumed it was stall.

Try flying it again without the booster, but watch and see how much the SAS is working to keep the nose level

How do you check, electrical use?

If they are too close to inline with your CoM, they don't help much.

Might that be the cause of some of my designs simply not willing to pitch up/down past a certain point?

Never occurred to me to mount an engine fuel tank under the wing. Derp.

[Claw]

Wow, great post, thanks for taking the time.

No sweat, hopefully it will be of some use.

That is true of almost all of my designs, even the smallish ones. (well, the ones with any wing area) I always assumed it was stall.

Well, it is stall. At least in the sense of how KSP can simulate it. If you can force it out of control, then it could be that your design is unstable or it could be that you have enough control authority. (Or both...) Generally speaking, I find that I want to be able to just barely be able to force a carrier plane out of control. Then it should have enough authority for payload carrying (assuming the payload isn't assisting). (EDIT: I should probably qualify that this also depends a lot on the payload.)

How do you check, electrical use?

Ah, sorry. I should have said. Look in the lower left corner at the trim indicators. You can watch the SAS do its inputs there. If it's all over the place, then your craft is a bit unstable. If it's way out at the ends, then your craft has just barely enough control to keep it in line. If you start to stress it, then you'll have problems.

Might that be the cause of some of my designs simply not willing to pitch up/down past a certain point?

Never occurred to me to mount an engine fuel tank under the wing. Derp.

Yes, having them too close to the CoM can cause your plane to be unresponsive. It isn't the only thing, but it can be part of it. Being able to pitch only so far up or down isn't always a bad thing, since that can help keep your plane from going out of control.

The fuel tanks are loaded under the wings to pull the CoM down. Conveniently with this setup, the CoM moves up as the tanks burn thereby making the plane easier to fly after cargo release. (I routed the fuel lines so the liquid fuel drains from the wings first.)

Edited May 14, 2014 by Claw

[xcorps]

If you can force it out of control, then it could be that your design is unstable

Generally, I can pitch straight up/down and fly when I have a plane that WILL pitch straight up and down with one turn command. But when I build the really fast planes, pitching straight up with any real speed will usually send me into a combined uncontrolled pitch and roll. I'm thinking it's stability, I never really checked SAS controls.

Now I know how to tweak it. Much appreciated.

________________________________________

Someone once posted a link to a poster style tutorial that included basic information but also some advanced stuff like canards and wing shapes. Anyone recall that?

[Claw]

This one?

http://forum.kerbalspaceprogram.com/threads/52080-Basic-Aircraft-Design-Explained-Simply-With-Pictures

Or was it in-game?

[xcorps]

That's the one!

Could you possibly Rock less? I'm thinking not.

[O-Doc]

Also, if you have enough pitch authority you can trim your aircraft on your ascent to counter-act the increasing drag to lift co-efficient once you start getting higher and faster. To trim down press alt-uparrow. For piggy backing heavy loads on an aircraft's shoulder I always hang fuel drop pods under the nose like this:

Every SSTO I build these days has this shoulder/nose docking port arrangement. You only need 50% weight on the nose compared to the load you are carrying up top. In this image the nose tanks are only carrying jet fuel. Because the top cargo s over the center of mass and the nose pod is right up the front, you get a nice fulcrum effect.

If you use counter-balancing on an orbital release craft you will end up having to move fuel around to keep the balance right. Remember that fuel goes from a small tank to a large tank faster than the other way around so, try to use one large centrally located tank in the lifer's main body to move your nose pod fuel into at or after the moment of release. Though, I'm guessing what happens to the lifter is not really important once your rocket starts it's monologue.

[Yasmy]

Is it possible that the large number of air intakes up front are adding to your instability? I think they have a higher drag than most parts, so naturally want to twist around to the rear. Someone please correct me if I'm wrong.

How do ships orient themselves, for example, on atmospheric entry? I understood it to be through higher drag forces on higher drag parts.

Edit: Now I see Claw mentioned the ram scoop on the front...

Edited May 15, 2014 by Yasmy

[O-Doc]

How do ships orient themselves, for example, on atmospheric entry? I understood it to be through higher drag forces on higher drag parts.

An unpowered entry is like a lawn dart, the heavy end falls first. If you're empty, your engines can and often do cause the craft to flip or spin. On my heavy lift SSTOs I power slightly on re-entry to keep the front facing forwards. Good SSTO designs will be much longer than a traditional planes to help prevent flipping and flat spins.

I've found lift is the initiator of aircraft tumbling on re-entry and it's not really something you can avoid with an SSTO. You just have to keep your nose in the direction of the prograde marker to avoiding/reduce those lifting moments.