Shuttle won't fly straight.

[Frozen_Heart]

I've been trying to build a space shuttle for quite a while now with little success. Strangely I'm not having problems with the balance on the way up, but the gliding on the way back.

I know all the basics of aircraft design in KSP (aka CoM slightly in front and CoL etc) but for some reason this shuttle doesn't seem to want to follow those rules.

The CoL is a very long way behind the CoM but for some reason the shuttle will only fly backwards, spin everywhere, do everything but fly straight. On the off chance I do get it going in the right direction then it lawn darts like the CoM/L suggests it should. How is it that i'm having the problems of CoM/L too far back and too far forwards on a single aircraft? Fuel doesn't make a difference.

I'm trying to make it look like a normal shuttle so don't want canards, etc.

Edited September 19, 2015 by Frozen_Heart

[Tex_NL]

In my opinion the Big-S has never been Big-S enough. It looks to me like you need either more wing or less fuselage.

TweakScale would help in scaling up the wings.

[xrayfishx]

Well, I'd try adding air brakes to the rudder. The real Shuttle had its barn doors open during re-entry. You can't do the split control surface thing that the real Shuttle did, but you can add the normal air brakes. And yes, Big-S is not big-s enough. If you look at pictures, you'll see that the real Shuttle had proportionately larger wings. However I recall Scott Manley saying in one of his videos that such big wings weren't actually necessary and there was some other reason for adding them so that might be somewhat irrelevant. Whü knuws...

Edited September 16, 2015 by xrayfishx

[The_Rocketeer]

Move the wings forward. If you look at the NASA STS, the trailing edge of the wings is level with the fueslage and the engine cones stick out behind that. Also, the wing strakes start almost right at the join between the cabin and the cargo bay. This isn't just aesthetic, it'll bring your CoM and CoL closer together.

Also, the control surfaces are huge, so you don't want them sharing jobs too much or you'll get massive oversteer. Set the inner elevons to pitch only, and the outer ones to roll only, or if you don't have enough pitch authority anymore, to pitch and roll. Also, make sure your stabiliser is set to yaw only. If it tries to contribute to roll (which it will) it stuffs everything.

Too much separation between the wings and the CoM will always cause problems and not always ones that are symptomatic of what you've done. Keep in mind there are different forces like lift and drag, pushing your craft around with varying strength at any given moment. Once you drift off centre, things get rapidly weirder, and steering to correct it could just make it much worse.

To sum this up, take it back to basics. If your CoM, CoT or CoL look wrong, they are.

Edited September 16, 2015 by The_Rocketeer

[Val]

In my opinion the Big-S has never been Big-S enough. It looks to me like you need either more wing or less fuselage.

TweakScale would help in scaling up the wings.

Wut? A pair of Big-S can easily lift a 100 t spaceplane to orbit, and land it at very modest speeds when returning empty at around 35 t. (in the picture his craft weighs 37 t fully fueled).

100+ t take-off weight

36t when landing

I've been trying to build a space shuttle for quite a while now with little success. Strangely I'm not having problems with the balance on the way up, but the gliding on the way back.

I know all the basics of aircraft design in KSP (aka CoM slightly in front and CoL etc) but for some reason this shuttle doesn't seem to want to follow those rules.

The CoL is a very long way behind the CoM but for some reason the shuttle will only fly backwards, spin everywhere, do everything but fly straight. On the off chance I do get it going in the right direction then it lawn darts like the CoM/L suggests it should. How is it that i'm having the problems of CoM/L too far back and too far forwards on a single aircraft? Fuel doesn't make a difference.

I'm trying to make it look like a normal shuttle so don't want canards, etc.

http://i.imgur.com/FoqkCQe.png

Does your CoM move towards the front or the rear when you have no cargo and fuel is drained? Edited September 16, 2015 by Val

[Frozen_Heart]

Move the wings forward. If you look at the NASA STS, the trailing edge of the wings is level with the fueslage and the engine cones stick out behind that. Also, the wing strakes start almost right at the join between the cabin and the cargo bay. This isn't just aesthetic, it'll bring your CoM and CoL closer together.

The shuttle already flies backwards. Moving the CoL forwards will only make it worse. I did have it like that initially but moved them back to try to stop it only going backwards/sideways.

Also, the control surfaces are huge, so you don't want them sharing jobs too much or you'll get massive oversteer. Set the inner elevons to pitch only, and the outer ones to roll only, or if you don't have enough pitch authority anymore, to pitch and roll. Also, make sure your stabiliser is set to yaw only. If it tries to contribute to roll (which it will) it stuffs everything.

Already done that.

Does your CoM move towards the front or the rear when you have no cargo and fuel is drained?

Draining fuel and adding cargo both make the CoM move forwards. Even empty it still flies sideways or backwards though.

[Val]

...

Draining fuel and adding cargo both make the CoM move forwards. Even empty it still flies sideways or backwards though.

Is the shuttle sitting level in the SPH? CoL is calculated as if it was moving through the air towards the opening. If the shuttle is tilted up or down the CoL will be shown as if you were flying with high AoA. And that will move the CoL.

If you want to post a link to the craft, I'd take a look and see if I can figure out whats happening?

[Frozen_Heart]

Here is the CoM/L usually

Like this the shuttle will simply fly backwards, spin madly, fly sideways, despite the CoL/M being in the correct place. I do the glide tests without much fuel as that's how its designed to come back.

This is what its like with some cargo. I've done tens of flight tests, gradually moving the CoM forwards. At a certain point it suddenly stops flying backwards and lawn darts instead. In between these two extremes it will fly backwards until I manage to wrestle control, and then go straight to lawn darting in the same flight.

- - - Updated - - -

Found what doing it. The Mk-3 Cargo bay is creating a massive amount of lift, much more than the wings or the tail fin which is spinning the shuttle round, but when it is flying straight that part doesn't make lift which causes the lawn darting.

Don't know how to solve this apart from not using Mk-3 pieces or adding massive wings which takes away from the shuttle look. I'm seen plenty of people using identical configurations and parts so not sure how they make it work.

Edited September 16, 2015 by Frozen_Heart

[Val]

Is the cargo bay, by any chance the root of your vehicle? That has been known to cause havoc with lift and drag.

[Frozen_Heart]

Is the cargo bay, by any chance the root of your vehicle? That has been known to cause havoc with lift and drag.

The cockpit is. I'm still stuck in the old ways of only ever using the crew pod/core as the root and never remember I can use anything else.

I didn't actually look at the drag numbers though so maybe something funny is playing up. I've rebuilt this same craft from scratch about 10 times now so the chance of the same bug every time is quite slim.

[The_Rocketeer]

Is the cargo bay, by any chance the root of your vehicle? That has been known to cause havoc with lift and drag.

Hmm, fair call on the wings.

As for the cargo bay issue.... this might sound a bit nuts, but you could try using 2 of the middle length bays or 4 of the short ones instead of the single long one. This would spread that lift force throughout the trunk of the craft rather than hanging it all off one point near the middle.

Depending on your attitude to clipping, you could also add some hidden control surfaces somewhere in the nose. That would give you nose down-force without spoiling the look.

[adsii1970]

I know this may sound silly, but check your symmetry. A while back I discovered two things about symmetry:

• In the space plane hanger, there are times that the symmetry will default to radial (use [R] key to cycle through symmetry modes to make sure the setting is on MIRROR).
  
• Although it may appear that your wings were done in mirror mode, if they were put into place in radial mode, the surface area of the far side of the wing is switched to what it was on the near side of the craft. In other words, one wing will be correct and the other wing will be upside down. Since the change to 1.0.X, this has caused me a lot of pain (and reverts) because of craft that do not handle well reentering into the Kerbin atmosphere when a wing is upside down. Although you think that the wings appear to be identical, regardless to their appearance and one has been placed upside down or not, computations made by the game do see the difference.
  

This could account for the control issues you described.

[xrayfishx]

Wut? A pair of Big-S can easily lift a 100 t spaceplane to orbit, and land it at very modest speeds when returning empty at around 35 t.

Yeah, the wings in stock do create ridiculous amounts of lift for their size. It'd make sense if the Big-S created less lift while being a bit larger. All other wings should just produce less lift. I still wish the guys at Squad would do the right thing and talk to Ferram about the aero system. Anyways, I'll stop de-railing this thread now.

Edited September 16, 2015 by xrayfishx

[GoSlash27]

FrozenHeart,

First of all, damn fine looking shuttle! I like how you've approached it.

Silly question, but where is your CoM when the payload is removed? That's what really matters.

Shuttles (at least *honest* shuttles) have crappy leverage for pitch control. You can help that by adding a body flap in the back like the real thing. Build it nose heavy (lawn darty) and rely on the body flap to help keep your nose up.

Finally, nearly everything will fly backwards if given the chance no matter where the lift is in relation to the CoM. This is due to the low drag of the engine bells.

It's important to not get it tumbling. Just because it has control surfaces doesn't necessarily mean they *should* be active. On mine, the inboard flaperons and body flap are pitch only and the rudder is locked.

Question: Where's your OMS system?

Good luck and I look forward to the flight!

-Slashy

Edited September 16, 2015 by GoSlash27

[Val]

I recreated your Shuttle (Craft file) as best I could and took it for a spin.

It was very stable.

I tried to make it spin out during descent, but it wouldn't.

(The launcher I made, on the other hand, is a horrible mess. You need to constantly adjust the Thrust limiter on the booster. Use at own risk. Or use your own if you have one.)

Javascript is disabled. View full album

Dry CoM courtesy of RCS Build Aid.

Edit: Deorbit burn was 80 m/s from AP of a 80x100 km orbit.

Edited September 16, 2015 by Val

[Frozen_Heart]

Silly question, but where is your CoM when the payload is removed? That's what really matters.

Without payload the CoM is very slightly in front of the CoL. I might try adding some body flaps and some airbrakes to see if that helps.

~Question: Where's your OMS system?

The OMS system is a pair of thuds on the top back, quite hard to see. Going to angle them as having some minor control problems in space.

~recreated your Shuttle (Craft file) as best I could and took it for a spin.

It was very stable.

I tried to make it spin out during descent, but it wouldn't.

(The launcher I made, on the other hand, is a horrible mess. You need to constantly adjust the Thrust limiter on the booster. Use at own risk. Or use your own if you have one.)

Hmmm. I've seen a lot of shuttles built in a completely identical way to mine and they function fine. Maybe its a bug with my install. I've had a few odd physics things in the last few version. (aka working k drives don't work on my copy of KSP)

On my shuttle i'm going without the engine on the booster. Done it before so i'm very stubborn that i'm gonna do it again.

Edit: Tried your craft and the mainsail on the booster overheats and explodes it in a single second....

Shuttle does the same and mine as well. Tumbles and flies backwards. It only does this if I've edited it. If I load and launch straight away works fine.

Edited September 16, 2015 by Frozen_Heart

[Val]

...

Tried your craft and the mainsail on the booster overheats and explodes it in a single second....

Shuttle does the same and mine as well. Tumbles and flies backwards. It only does this if I've edited it. If I load and launch straight away works fine.

Very, odd. I have no hypothesis to explain that.

Here's a link, if you want a copy of my Physics.cfg to try out. Download is one of the buttons at the top.

[GoSlash27]

Hmmm. I've seen a lot of shuttles built in a completely identical way to mine and they function fine. Maybe its a bug with my install. I've had a few odd physics things in the last few version. (aka working k drives don't work on my copy of KSP)

I'm inclined to agree. Especially since

Shuttle does the same and mine as well. Tumbles and flies backwards. It only does this if I've edited it. If I load and launch straight away works fine.

Best,

-Slashy

[Frozen_Heart]

Very, odd. I have no hypothesis to explain that.

Here's a link, if you want a copy of my Physics.cfg to try out. Download is one of the buttons at the top.

I should have the latest version considering I reinstalled KSP only a couple of weeks ago. Post 1.0.4. Will give it a try though.

Here is a link to my shuttle to see if it flies for you.

https://www.dropbox.com/s/t3gxtu0wz1zs1z6/Space%20Shuttle.craft?dl=0

Edited September 16, 2015 by Frozen_Heart

[Val]

I should have the latest version considering I reinstalled KSP only a couple of weeks ago. Post 1.0.4. Will give it a try though.

Here is a link to my shuttle to see if it flies for you.

https://www.dropbox.com/s/t3gxtu0wz1zs1z6/Space%20Shuttle.craft?dl=0

Unedited, it was completely unstable.

But if I pulled the rear fuel tank off, reattached it and launched then it was very flyable once it was empty of fuel.

Wet CoM was behind CoL. But that won't matter for a re-entry when it's nearly empty.

- - - Updated - - -

...

The OMS system is a pair of thuds on the top back, quite hard to see. Going to angle them as having some minor control problems in space.

...

BTW, I strongly recommend you mount the OMS pointing in the same direction as the command pod. Otherwise you won't be trusting in the direction the navball indicates. Thrusting through CoM is important in vacuum, but it is just as important to be thrusting in the right direction.

Move the OMS down instead of angling them.

Edit: Also attach your landing gear to the fuselage. It's easier to be sure they are correctly aligned and they will buckle less. Then use the offset gizmo to make them look like they are attached to the wings.

Edited September 16, 2015 by Val

[Frozen_Heart]

Unedited, it was completely unstable.

But if I pulled the rear fuel tank off, reattached it and launched then it was very flyable once it was empty of fuel.

Wet CoM was behind CoL. But that won't matter for a re-entry when it's nearly empty.

Exactly what's happening to me, but in reverse. That is suggesting something is wrong with my KSP copy.

Yeah its designed to come back low on fuel though so I test it empty.

~BTW, I strongly recommend you mount the OMS pointing in the same direction as the command pod. Otherwise you won't be trusting in the direction the navball indicates. Thrusting through CoM is important in vacuum, but it is just as important to be thrusting in the right direction.

Move the OMS down instead of angling them

They are angled down by 10 degrees so I simply put the nose up by that amount.

[Val]

Exactly what's happening to me, but in reverse. That is suggesting something is wrong with my KSP copy.

Yeah its designed to come back low on fuel though so I test it empty.

Good, in the sense that we're closer to finding the problem.

Did you have a chance to try the physics.cfg?

They are angled down by 10 degrees so I simply put the nose up by that amount.

That works, too

[GoSlash27]

Frozen_Heart,

I had to realign the engines a bit to get it to work with my stack for launch. On reentry it wanted to fly anywhere but straight ahead.

It had a lot of fuel left over (my stack is set up assuming the orbiter has no fuel except for the OMS), so that may have been the cause of it...

*edit* yeah, the CoL was ahead of the CoM on my try. It doesn't need to be that close. If you have the lift further back with more pitch authority it should work out...

Best,

-Slashy

Edited September 16, 2015 by GoSlash27

[Frozen_Heart]

Good, in the sense that we're closer to finding the problem.

Did you have a chance to try the physics.cfg?

That works, too

Using your config didn't help. Even empty still going everywhere.

Frozen_Heart,

I had to realign the engines a bit to get it to work with my stack for launch. On reentry it wanted to fly anywhere but straight ahead.

It had a lot of fuel left over (my stack is set up assuming the orbiter has no fuel except for the OMS), so that may have been the cause of it...

*edit* yeah, the CoL was ahead of the CoM on my try. It doesn't need to be that close. If you have the lift further back with more pitch authority it should work out...

Best,

-Slashy

I've managed to get the launchers sorted out, just needs a bit of tweaking but can get to orbit fine now. Done It before so that bit isn't too difficult. Just the re-entry that's a challenge. I have action groups set up now to turn off the main engines and turn on the OMS.

I'll try moving the wings back again, though I had them further back in the first pic and was still having the problems.

Here's some pics of the launcher. SRB boosters and engines only on the shuttle. Looking forward to those proper shuttle engines we're getting though. Just need proper sized SRBs now.

[GoSlash27]

FrozenHeart,

Minor detail, but I was able to launch the stack with just the 2 SRBs. I hear that if you clip the 14 boosters together like in your setup and turn the centers way down you can get the cool STS delayed seperation, but you don't actually need bigger SRBs to make it work.

Best,

-Slashy