issues with a mk3 Ssto to a minmus station a mun station and back

[Vortexian2]

I am having trouble with an ssto of mine. This is only my 4th ssto and 1st mk3 ssto. It is supposed to go to a mun station, pick up 4 kerbals, then go to a minmus station, and pick up 4 more and then come back to kerbin. When I was testing it for landing on the runway I took of the runway (near the end) and then aimed for the 10 degree marker on the nav ball, physical time warped till I was 1400 meters high and then I was going to turn around and land on the runway but this thing is very sensitive when turning and flips out and stalls a lot and I don't know why. I know that at like over 300 m/s planes typically do this but with must you can at least turn with some effort but this one is way to unstable for me to fly, but I m a beginner for sstos and planes in general and I am not a good pilot either (I can only land on the runway about a third of the time). I really want this to land on the runway and use mk 3 parts to be space shuttle-ish even though it doesn't have the orange fuel tank and/or the srbs. Here are some pictures to help you (I am new to the forum and don't know how to size to pictures or anything, and this is also my first time using imuger to post pictures too.). Also, please excuse any spelling mishaps I have.

Any tips?

Edited November 13, 2019 by Vortexian2

[Snark]

Moving to Gameplay Questions.

 

Regarding the plane: Your CoM is way far towards the back of the plane. This makes it aerodynamically unstable, plus your control surfaces are too close to the CoM so they can't help much.

Try moving the CoM forward, plus I'd suggest bigger stronger canards up front. Make sure you've disabled roll authority on the tail fin.

[Guest]

Yeah big SSTOs tend to get tail happy because the most obvious place for the engines is the tail. Even if it's stable on take-off it almost certainly won't be when you return as all the fuel will be gone. When I make something this size I usually put the engines in nacelles on the wings, that way they're closer to the CoM. 

Also, suggestion. You're trying to make a heavy long-range SSTO. This is a really ambitious design combining two difficult design challenges -- heavy SSTO, and long-range SSTO. I've made hundreds of planes and SSTOs for several years and it would take me hours to get something like this functional. 

Therefore I would recommend that you scale back your ambition level a bit. Either try to make a heavy SSTO that only makes LKO, or a light long range SSTO, something on the scale of two RAPIERs and one NERV. Once you're comfortable making those, try making this one.

[mk1980]

in my experience large planes are much easier to balance if you move the engines (and main wings) from the back to the center of the plane.

basically like the concept of the real life "Skylon" SSTO project.

you'll have your cockpit and passenger cabin in the front and the fuel tanks along the fuselage, with engines and wings roughly in the center. so the CoM will gradually shift a bit forward as the tanks run dry. which makes it easier to keep the plane flying straight.

[Fraktal]

6 hours ago, Snark said:

your control surfaces are too close to the CoM so they can't help much

Does it actually matter how close or far away they are? I've heard both that it does because of torque and that it doesn't because KSP applies the torque without taking that into account. Which?

[Signo]

1 hour ago, Fraktal said:

Does it actually matter how close or far away they are? I've heard both that it does because of torque and that it doesn't because KSP applies the torque without taking that into account. Which?

KSP loves "leverage": you need to apply forces in line with the CoM depending on the command you want to input and obviously the longer your "arm" the lesser the force you need to apply.

[Starhawk]

2 hours ago, Fraktal said:

Does it actually matter how close or far away they are? I've heard both that it does because of torque and that it doesn't because KSP applies the torque without taking that into account. Which?

Yes.  It absolutely matters where your control surfaces are.

Control surfaces near the CoM are good for roll authority, but should have pitch and yaw disabled.
Control surfaces near the front or back are good for pitch authority, but should have roll and yaw disabled.

Incidentally, since there is no wind in KSP, there is no need for yaw authority in atmospheric flight.

The distance from the CoM of reaction wheels does not matter in KSP, but that is another topic.

Happy landings!

[Snark]

4 hours ago, Fraktal said:

Does it actually matter how close or far away they are?

Yes, of course, it absolutely matters where control surfaces are.  That's the most important thing about them.

Remember, control surfaces don't generate "torque" inherently, at all  They generate force.  They're used to make torque, by virtue of applying that force at right angles to the lever arm between themselves and the CoM.

And torque equals force times distance.  Double the distance, double the torque.  If the distance is zero, the torque is zero.  The farther from the CoM you place the control surfaces, the more their control authority for pitching / rolling / yawing the plane increases-- in direct proportion.

4 hours ago, Fraktal said:

I've heard both that it does because of torque

Yes, exactly.  That's exactly it, that's how it works.  You've heard this all over the place because that's how reality works and that's how KSP works. 

4 hours ago, Fraktal said:

it doesn't because KSP applies the torque without taking that into account.

Really?  I've never heard anyone try to claim that, at all.  I mean, I've seen plenty of folks who didn't realize how the placement matters, simply because they weren't familiar with the physics involved, but to actually claim that it doesn't matter?  Not the case.

(In sharp contradistinction to, say, reaction wheels, where the placement doesn't matter because they directly generate torque, unlike control surfaces which only generate force.  But that's not what we're talking about here.)

1 hour ago, Starhawk said:

Incidentally, since there is no wind in KSP, there is no need for yaw authority in atmospheric flight.

Well, it's helpful for doing yaw maneuvers, and also extremely helpful for maintaining yaw stability.  Not sure how the presence or absence of "wind" enters into it?

I mean, unlike pitch and roll, it's not an absolute requirement, but it's certainly helpful in a variety of circumstances-- I wouldn't go so far as to say "no need".

[Guest]

1 hour ago, Snark said:

Not sure how the presence or absence of "wind" enters into it?

Maybe he's referring to crabbing in crosswind landings and take-offs? That would be no fun at all without yaw authority, and with no wind it's not an issue in KSP.

I agree though that it's far from the only thing yaw authority is good for.

[Snark]

1 minute ago, Brikoleur said:

Maybe he's referring to crabbing in crosswind landings and take-offs? That would be no fun at all without yaw authority, and with no wind it's not an issue in KSP.

Yah, that makes sense.  Though I've found that, conversely, there's a different reason why having yaw authority in KSP is helpful for landings.  In spoiler, since we're starting to get off-topic from the original person's question.

Spoiler

One type of "engineering problem" that's really easy to run into, when building a plane in KSP, is to make something that takes off fine and flies fine, but has a tendency to "spin out" when landing, i.e. to fishtail around.  It's a hard thing to eyeball, and (at least for me) really hard to tell for sure whether it'll be a problem with a particular design simply by eyeballing it in the SPH.  For me, the only way to really know is to go out and try to land the thing with brakes on and see what happens.

(Yes, I'm familiar with the reasons why fishtailing happens.  it comes down to the placement of landing gear, and also the strength of the various brakes.  But I've found that in practice, it's so finicky and touchy that it's really hard for me to tell for a given design "will it spin out upon landing" or not just by looking at it.)

And when a craft does have a tendency to spin out on landing, it can be really bad.  Usually the plane's still moving pretty quick when it touches down, and if it fishtails and starts to skid sideways, there's a strong chance that it will then roll and go kablooie.

So I've found that having a fair amount of aerodynamic yaw authority can be really handy to help with such situations.  If there's a good, strong yaw control available during the crucial part of landing (i.e. after touching down, but while still going pretty fast), then that allows SAS to help keep the nose pointed forward, and makes it easier to brake to a halt without fishtailing too much.

 

[Starhawk]

4 hours ago, Brikoleur said:

Maybe he's referring to crabbing in crosswind landings and take-offs?

That's exactly what I was talking about.  I haven't put a rudder on any spaceplane I've built for years, and I've built quite a number of them.  I won't take it further to avoid derailing the thread any more.

As to the topic at hand, I concur with what Snark said above.  The CoM so close to the rear makes the original poster's craft inherently very unstable.

Happy landings!

[Signo]

Well, to be back on topic, it is better to disable gimbal on that rapiers to avoid them "pushing around" instead of pushing through CoM.

[Vortexian2]

Thank You for all the information. I know the control surfaces generated torque but I did not know the location of the CoM mattered. I guess I was just stupid then. But thanks to your help it turns a lot better now. Also sorry about how long it took me to reply I was sick yesterday and couldn't do much. Still sick but better today. 

 

Also, one more thing, any tips for lining up the runway or just landing in general? I'm a bad pilot.

 

Thank You

Here is my Ssto now:

[AHHans]

11 hours ago, Vortexian2 said:

Also, one more thing, any tips for lining up the runway or just landing in general? I'm a bad pilot.

One neat trick that I got from a Scott Manley video is to put flags at the ends (just outside of) of the runway. That done, you can set the nearer flag as the target and have a much more precise marker for the direction in which to fly to get to the runway - both in azimuth and elevation(!). Then you should position your craft so that this marker is exactly at the 90 deg or 270 deg line of your navball.

And finally: training, training, and more training. I, too, used to suck at landing in KSP. But after doing it often enough I now manage to hit the runway (and not crash) more often than not.

[Guest]

11 hours ago, Vortexian2 said:

Also, one more thing, any tips for lining up the runway or just landing in general? I'm a bad pilot.

Yeah, practice. 

I would also suggest you practice with a plane that's easier to fly. Real pilots start training on trainers for that reason. If I can bang my own drum, here's a little plane that fits that bill nicely: https://kerbalx.com/Brikoleur/DHV-X1

[TheFlyingKerman]

15 hours ago, Vortexian2 said:

Also, one more thing, any tips for lining up the runway or just landing in general? I'm a bad pilot.

There is a tutorial for that

 

[Signo]

@Brikoleur: that Vampire is really cool and I second you suggestion regarding practice.

@AHHans: Manley's markers are the best suggestion after eyeball practice.

[Vortexian2]

Thant you for the information and the tutorial. Though now when ever I try to land I lose control of the plane around 100 m/s and can't pick my nose up to land.

I tried making the wing bigger (which helped with control but I had to remove the canards and now can't get off the runway) and using the fat elevons (don't know if there better or not since there big but only goes up 15 degrees not 30 degrees like others) and yes I have landed a plane before without parachutes I just have them just in case.

Sorry for all of my issues, I thought I just needed a lot of practice or something for landing larger planes, but after 2 days of trying to land it I need help.

[Vortexian2]

Never mind just started using the big-s elevons I was stupid not to use them before. Landed fine on the runway first try actually.

Sorry for all my dumbness.