How to handle the gravity turn on a rocket with wings at the top

[cantab]

This is my Chamberlain "space bus" on top of its launcher. It has wings so I can glide it to a controlled landing site, and it does glide fairly well at low altitudes.

Launch, though, is a problem. Straight up is fine, but attempting to make a gravity turn at 10km leaves the rocket wanting to pitch head-over-heels. For this rocket and payload, gravity turn happens while still on the first stage.

I can make a late gravity turn where the air is thinner and keep it under control that way, the launcher does have the delta-V to spare, but that still doesn't feel ideal. Are there any better solutions, besides "Take the wings off" and "Build a spaceplane"?

Edited April 9, 2014 by cantab

[FenrirWolf]

I'd try starting your turn earlier and pitching over slower. Doing a 45-degree turn at 10km is rather abrupt.

[xcorps]

RCS thrusters

High vector engines tweaked for low thrust mounted at the bottom of the last stage

[maccollo]

Add wings at the bottom to balance it out, and on the second stage if you are still in the atmosphere at that point.

[sal_vager]

I get around this by keeping my craft sideways so the wings couldn't get any lift, it works well enough

[Tex]

I abandoned the 10km gravity turn. It's a sudden pitch to 45 degrees, which, especially for real rockets, would normally completely destroy the rocket in the process. I have adopted a very gradual and hands-on turn.

Start it after your solids run out of fuel (if you have any) and begin pitching over extremely slowly, degree by degree. Use fine controls if you don't have the ability to tap the key very quickly. If you don't have solids, start the maneuver at about 2km, the same technique. After you reach about 30 or so degrees, stop the maneuver and pay attention to the map view and your apoapse for further adjustment.

[Bobnova]

Put some of the little wings with control surfaces on the tail of the rocket. That usually does it for me.

[Tank Buddy]

Just keep you wings pointing in such a way that prevents them from creating lift. (Rotating the craft 90 degrees)

[Claw]

Add wings at the bottom to balance it out, and on the second stage if you are still in the atmosphere at that point.

This is probably the most "appropriate" solution and will likely work even if the aerodynamic model suddenly changes. Just about any of the other suggestions would work as well.

As Maximus said, reality dictates you should gradually do your gravity turn. However, KSP doesn't yet force that on you. That being said, if you have a lot of wing surface up top you'll want to avoid large, sudden pitch changes. Especially if you have control surfaces (and not just wings) up top like those canards. There's a bug with the control surfaces that causes their lift to get absurdly high with excessive AOA.

So if you only put wings at the bottom, you'll still have a lift imbalance when you put AoA on the rocket. You'll want some control surfaces down low too to balance it out.

[Superfluous J]

I had this problem with v2 of my "glide an asteroid home" mission. My first mission had small plane parts, but v2 has a lot more and mounting them to the top of the rocket just proved too unstable. Even thrusting straight up out of atmo, the thing ended up wobbling so much it fell apart. Even with struts.

So I'm rebuilding the craft tonight. Instead of one rocket under the whole thing I'm launching it with twin rockets attached to the side. That way my plane can be lower and face forward (if you think your way is hard, try launching the plane BACKWARD )

[Geschosskopf]

Another option is to build a "cage" launcher. This has the airplane assembly facing down on the bottom of a central fuel stack, so you can still use the rear end of the airplane to attach the rocket to. Then you have radial tanks/engines coming off the central fuel stack so that their nozzles are down by the airplane. In this way, the airplane's wings act as the tail fins of the rocket, the rocket's fuel is all balanced as normal, and all is well. It just looks ridiculous.

VERY IMPORTANT: You need to put a probe core on top of the central fuel stack and remember to select "control from here" on it before you light the rockets. This will give your navball the correct orientation during launch.

Here's an example. This plane was going to Duna so has a totally insane amount of lift on the smallest possible mass. It is utterly incapable of being launched any other way than backwards with a cage.

[maccollo]

Another option is to build a "cage" launcher.

I just love how many different solutions there are to this problem. I find this one particularly creative!

[Geschosskopf]

I just love how many different solutions there are to this problem. I find this one particularly creative!

Thank you but the idea is not mine. Launching the airplane backwards, although with the plane on top of the rocket, I got from somebody I can't remember. Launching forward in a cage I know I got from Bothersome. So all I did was combine these elements.

I should also point out that the cage rocket pictured was an SSTO lifter, which makes things much simpler when doing cages. Just get the airplane out of the atmosphere and all worries are over. So the payload was not only the airplane but also its transfer stage.

[Kasuha]

This has the airplane assembly facing down on the bottom of a central fuel stack

Be warned: active control surfaces mounted wrong way cause all sorts of strange and unwanted effects.

Personally I would balance the action of control surfaces on the top by adding some more control surfaces at the bottom of the rocket.

[cantab]

Thanks for the advice everyone. I'll try some of them my next launch.

[Geschosskopf]

Be warned: active control surfaces mounted wrong way cause all sorts of strange and unwanted effects.

Personally I would balance the action of control surfaces on the top by adding some more control surfaces at the bottom of the rocket.

Yeah, I should probably have mentioned that. But it all depends on the situation. Each airplane is different; some are more finicky than others. You have to tune the lifter for the specific plane and it might take a few crashes to sort it out. In my example here, I put the tail feathers on the central stack from the get-go because I suspected I might have a problem with the backwards controls on the airplane, so the rocket fins would maybe cancel them out, leaving torque to steer with. I don't know if this is actually what happened or whether the airplane cooperated with the rocket, but anyway it flew just fine.