Spaceplane Woes: Can you feel the rumble of the tumble?

[BlazeFallow]

Hello All,

As is the case with so many of our Kerbal sisters and brethren, I can build rockets for days. Impressive Whackjobian behemoths that can conquer the intire Kerbol system to little probe jumpers to get to the moon and back with just enough fuel.

Spaceplanes however... haunt me...

I have reached a point where I can successfully fly a SSTO spaceplane to orbit and deorbit. That's where my trouble begins.

My problem is, and it happens without fail, that when I come back into ATMO, my plane tumbles without any ability to regain control, powered or unpowered. I've still got electricity for days, so that's not the problem of my loss of control, and I'm coming into atmo straight on in my prograde (I've also tried edging it maybe 5 or so degrees above prograde to mimic what the Shuttle entry does) however without fail my ship just starts spinning wildly on all 3 axis with no ability to regain control.

Please help give me some tips and clues on how to build a simple SSTO space plane that can re-enter atmo without crashing and burning. I've rained death and fire down upon the poor KSP airfield for too long.

Edited February 15, 2014 by BlazeFallow

[Rodyle]

So, without any further info on what they look like, this is going to be a bit hard. Could you perhaps post a few pictures of the general designs of you spaceplanes?

Also: while you're at it: go to the SPH, empty all the fuel tanks, and check where the centre of mass is compared to the centre of lift. Is it in front of it or behind it?

[capi3101]

OP: I'm right there with you. In fact, you might be able to learn a fair amount from my woes with the Auk VIII and I daresay if Claw gives you any future advice on this thread, I'd follow it without question.

Follow Rodyle's advice - and while you're at it, take the wheels off your spaceplane. Wheels have mass in the SPH but not in flight, so they throw off what the game is telling you about its location. Obviously you don't want to save after you've done this; you're just looking to see what's happening to your center of mass as the flight continues.

Positioning of your control surfaces can also be crucial. To help you with that, we'll need to see some screenies.

[ThomsenX]

If you're aiming to build a properly functioning SSTO, look into Ferram Aerospace Research (FAR). It revamps KSP's lift/drag system to make it much more realistic, taking into account everything from wing angles to lifting body principles. It also features a nice little tool for simulating lift/drag/stability with respect to angle of attack, Mach, etc.

Stock KSP does very little for spaceplane simulation, focusing mainly on rockets (which it does very well), but FAR is arguably a requirement for deploying a functioning spaceplane/SSTO program

If you want to treat yourself right, and you've got a bit of RAM to spare, add the B9 Aerospace parts pack for some awesome wing and fuselage packs.

[WhiteKnuckle]

Also: while you're at it: go to the SPH, empty all the fuel tanks, and check where the centre of mass is compared to the centre of lift. Is it in front of it or behind it?

This was my first thought as well, you're probably burning off most or all of your fuel on accent/orbit/de-orbit, and once you get back into atmo your CoL is ahead of your CoM, which will cause massive uncontrollable oscillations.

In the SPH, you can right click on any fuel tank and slide the bars to any fuel level, check your CoL/CoM with both full and empty fuel. You may want to add a "ballast tank" at the nose of your plane that is only used to keep the CoM forward during re-entry.

[Claw]

I'll echo what all three have said in the above posts.

-A picture is worth a thousand posts.

-Without any pictures or description of how your plane is going out of control, my initial guess is your CoM has shifted aft during space flight.

-If/when you try to adjust your CoL vs. CoM in the SPH, do like Capi said and take off the landing gear.

My goal is always to help you make your design fly, not how to build my designs. So if I can see yours, it'll help a lot. There are so many reasons why this might happen that we need to know more.

EDIT: If you can provide a craft file, I can usually tell a lot more from that.

Good luck

[eurybaric]

And when you reply back please tell us also if you using the FAR add on.

Cheers.

[1greywind]

Besides obvious problem with CoL/CoM placement you can try very shallow deorbit trajectory. With spaceplane you don't need to lower you periapsis below ground. Making deorbit burn so periapsis is <30 km is enough.

And another option: even if your plane starts to tumble, you can wait until speed drops to resonable level (~400-500 m/s), when toggle your engines at full throttle and they can overvelm overwhelm. Check this video at 13m 06s moment:

My spaceplane starts to spin after to step reentry, but I've managed to overwhelm spinning.

[BlazeFallow]

I'll have to post some pictures once I have the opportunity, but oddly enough, one of my plane designs looks eerily similar to Greywind's. My though, i'm sure, was very similar to his. That being, put your cargo in the back so when you drop it in orbit, your COM shifts back to balanced, even with the fuel shift. My plane still tumbles horribly even so... :'(

The part that doesn't make a whole lot of sense to me is that I've got stabalization fins WAY back to the rear of the plane, but they don't seem to do anything. I'd have though they'd have more of a dart effect, but they do not...

[m4rt14n]

As with anything in life, start small. Try using the stock Aeris to go into orbit and back to land. Spaceplane is hard enough to control let alone build and troubleshoot. On rentry once you get to 15 or 10 km put the full thrust on and slowly pull up close to the horizon. Never let the prograde marker be too far away.

Also as other posters have remarked, a picture would be very helpful.

[satcharna]

Consider using an airbrake or braking parachute to increase the drag on the tail of your craft.

[capi3101]

BTW, I've heard it said that you should not attempt to fire up your jets after re-entry until a) your surface velocity is less than 400 m/s or your altitude is becoming "critical" (which I define as about 10,000 m). That said, if you're tumbling at a consistent altitude, you might try firing up the engines just before then; thrust vectoring might be sufficient to prevent loss of control.

Monoprop (RCS) makes good ballast; lemme say that. Cylinders in particular.

[Hodo]

So, without any further info on what they look like, this is going to be a bit hard. Could you perhaps post a few pictures of the general designs of you spaceplanes?

Also: while you're at it: go to the SPH, empty all the fuel tanks, and check where the centre of mass is compared to the centre of lift. Is it in front of it or behind it?

All I have to say is Rodyle is exactly right.

On top of this I also suggest getting the FAR plugin, it helps with all aspects of KSP and should honestly be in the base game.

But spaceplanes are my speciality.

You want to make sure your dry COM and wet COM are still ahead of the CoL. And that your CoT is inline with the CoM. The best thing to do is to start simple, not small, but simple. Build an aircraft that does not break the atmosphere. Test how it works in atmo, then work on improving that design to get it to space.

[BlazeFallow]

Hey Guys, here's my plane

Javascript is disabled. View full album

[satcharna]

The problem you're having is likely caused by how your centre of mass is behind your centre of lift when you reenter. This leads to overly manoeuvrable and flip-prone planes. Move your tanks and wings around so that your centre of lift is the length of the FL-T400 or so behind the CoM, and so that it's roughly the same or slightly less when you reenter.

[Hodo]

In the third picture you can clearly see the CoM moves behind the CoL, which is a BIG problem, and your CoL also becomes tilted forward which means you are getting your lift further forward of your CoL ball marker. My suggestion on fixing this is move your dry CoM further back, which will affect your loaded CoM.

The design may not work as designed right now, you may have to move your wings or reduce the size of them to move your CoL back further.

[BlazeFallow]

Hey guys, I actually developed a fix that worked flawlessly. I added two RTG's to the mix, and also added 4 SAS units. one ILRW up front, one ASAS unit in the middle at the main docking port, and two ILRW's at the tail bases. This seems to have completely fixed my problems on re-entry as the plane is completely stable and controllable now. I can even keep it at 25km with the engines going 1500m/s for the inevitable undershooting the runway (which I did on my test run)

Once the fuel runs out of her, she touches down on the runway really gentle

[Kerba Fett]

I was just about to suggest more reaction wheels. My thinking was that one of your wingtips catches drag first and the pod doesn't have enough torque to prevent it from tumbling. I built this thing for suborbital testing and it's only controls are engine vectoring and reaction wheels. It handles pretty good once you get up to 300 m/s or faster. Once one engine flames out you have a few seconds to lower the throttle before it spins. I also find stock SAS holds better than mechjebs.

Takeoff is vertical using launch clamps.