Space shuttle really unstable at landing

[Andiron]

Hi !

I'm trying to create a space shuttle (for the STS challenge) and I don't have any problem going in orbit, but the problem is the landing. It's very stable until maybe 500m/s speed where the orbiter starts to wobble unmanageably (roll axis) and finish to flip upwards (pitch axis) mostly towards retrograde. However, I can fly the orbiter from the runway and land easily when staying under 100m/s. Do you have any tips to improve or pilot my shuttle ? Thanks

Edit : The center of lift is behind the center of mass

Edited September 26, 2016 by Andiron

[Waseemq1235]

How far behind is the Center of Lift? It has to be very slightly behind the Center of Mass.

Edited September 26, 2016 by Waseemq1235

[Andiron]

10 minutes ago, Waseemq1235 said:

How far behind is the Center of Lift? It has to be very slightly behind the Center of Mass.

This is when most tanks are empty, the center of mass is closer to the center of lift with all fuel inboard

Edited September 26, 2016 by Andiron

[Gaarst]

Centre of lift /= center of pressure.

The blue vector shows your centre of lift, but it only accounts for wings (maybe body lift but I'm not sure).

Thing is, your wings are at the back and most of the shuttle's body is in front of your CoL, this doesn't cause any problem when flying with a low AoA; but as soon as airflow becomes a but rougher (transsonic phase, manoeuvring, or high AoA) this part in front will generate a lot of drag, bringing your centre of pressure (or aerodynamic centre) forward, causing instability and flipping.

If you try reentring with a high AoA (like the real thing) you will see this problem a lot sooner and I guarantee you will not be able to bring it down safely.

 

The rolling instability may be caused by too aggressive roll control surfaces: 500m/s should correspond to your reentry max-Q, the point where dynamic pressure (combination of velocity squared and air density) is the highest and therefore the point where control surfaces are the strongest. If you have a bit too much roll control, SAS will cause wobbling pretty quickly (especially in prograde hold, for some reason).

 

Combine this and the centre of pressure issue, and you have the cause of your problem.

Edited September 26, 2016 by Gaarst

[Snark]

2 hours ago, Andiron said:

Edit : The center of lift is behind the center of mass

Irrelevant.  Or, at least, marginally relevant.

40 minutes ago, Waseemq1235 said:

How far behind is the Center of Lift?

Not super important.

It's a pretty common myth in KSP that "CoL behind CoM = stable."  Yes, you want your center of lift to be behind your center of mass... but that's not even vaguely the most important thing.

This screenshot pretty much says it all.  Look at where your CoM is!  It's way the heck at the back of the craft.  In a situation like that, it doesn't matter where your CoL is... a CoM at the back of the craft, with that huge lightweight draggy fuselage sticking out in front, is going to want to fly backwards.

The center of mass of the craft wants to be in the front of the craft.  That's how we all build rockets for ascent, after all-- try to keep the CoM high, towards the front.  The same principle applies during descent.

It also doesn't help that you have basically no control authority on the craft.  All your control surfaces are located at the back of the ship, right next to the CoM, which means they have practically no lever arm to work with and therefore might as well not even be there.

Basically, what you're trying to land there is a badminton birdie that's pointing feathered-end forward.

This happens all the time with people who want to build space shuttle replicas in KSP.  It's kind of unfortunate:  the game makes it really really easy to build something that looks just like the space shuttle, because Squad has gone out of their way to make parts (the Mk3 cockpit, the Big-S tailfin, the Big-S delta wings, the shape of Mk3 fuselage parts, the Vector engine) that are designed to look just like the shuttle... except that it won't fly like the shuttle.  Because if you build something in KSP that looks just like the real life shuttle, the CoM of your replica will be nothing at all like the real-life shuttle.

There are lots of ways to make your craft stable on landing, but they're going to involve doing things that make it look less like the US space shuttle, so if you're counting on the aesthetics of looking like that, you're going to be disappointed.  On the other hand, if you're just interested in building something that works and don't care whether it looks like the shuttle, then you have options. 

So, there are two things you can do:

• Move the CoM far forward.  Way far forward.  As far as you can manage it.
  • This can be tricky, given that your main ballast (fuel) is all gone, and your heaviest components by far (the engines) are all clustered at the back of the ship.
  • One design that can work well is to go with more of a Skylon-like design:  put your engines close to the middle of the ship, mounted on the sides, rather than having them way back in the back of the ship.
  • There are other things you can do, too (KSP's great at giving design options!) ... but given how heavy the engines are, moving them forwards is probably the easiest thing to do.
• Make sure you have control surfaces that are far away from the CoM.
  • Ideally, this would mean move your CoM way far forward, and keep your control surfaces at the back.  The fact that you've moved the CoM forward will give your surfaces the lever arm they crave.
  • However, if you're having trouble moving the CoM forward... put some canards way out on the front, as far forward as possible.  They'll help maintain control if you can stick closely to prograde (i.e. you have SAS to help with that).  A CoM-at-the-back ship is still going to be aerodynamically unstable, which means if you ever waver even slightly off prograde, it's going to flip backwards and stay that way.  But if you can manage to ride the knife-edge of stability, poised precisely at prograde, then this can work.  It's just a bit of a nail-biter, is all.
• Make sure you've turned off everything except yaw authority on your tail fin.
  • The only reason to have that big ol' vertical stabilizer is to give you yaw stability.  It shouldn't try to compensate for anything other than yaw.
  • However, by default it'll try to help you with roll, too... and the result is a complex interaction that causes it to fight itself (trying to yaw induces roll that it tries to compensate for by rolling the other way which requires deflecting in the opposite direction and cancelling out the yaw...).  Long story short:  you'll find better yaw stability if you turn off everything else on your vertical stabilizer.  You'll also be more stable if you can move your tail fin down some (i.e. in the ventral direction); right now it's sticking up so high that it induces a lot of roll whenever it operates.  Use the offset tool to move the fin a bit closer in to the central axis of the ship.

[Andiron]

Thanks a lot for your help ! That's really useful

I will try to make a better one with COM really in front of the vessel. In fact, trying to make a STS copy wasn't a great idea, so my next craft will be really different.

[Gaarst]

2 minutes ago, Andiron said:

In fact, trying to making a STS copy wasn't a great idea.

Fixed it for you  

[Andiron]

It went better than I expected. Little turn around to test it.

[Capt. Hunt]

1 hour ago, Gaarst said:

 

1 hour ago, Andiron said:

In fact, trying to make a STS copy wasn't a great idea, so my next craft will be really different.

Fixed it for you  

To be fair, there's a lot of negative bias directed against the STS on this forum.

However, they called it a flying brick for a reason.  Try adding ballast forward of the CoM.  If there's any fuel left on reentry, pump it to a forward tank.  Maybe you could replace that nose docking port with one of the nose cones that has fuel tanks in it.

Edited September 26, 2016 by Capt. Hunt

[Andiron]

24 minutes ago, Capt. Hunt said:

To be fair, there's a lot of negative bias directed against the STS on this forum.

I like it a lot, it was really a technological marvel. But we can't forget that 14 astronauts died because of it.

[ZooNamedGames]

59 minutes ago, Andiron said:

I like it a lot, it was really a technological marvel. But we can't forget that 14 astronauts died because of it.

We lost several space goers in Soyuz' capsules. No vehicle is perfect. But let's no debate it here.

[michal.don]

Yeah, my shuttle has similar issues. It is nicely stable in high atmosphere in high speeds, almost too stable (like a brick) in low atmosphere, but has a tendency to misbehave around 15 kilometres. Like the people before me said, this is an inevitable problem with all the STS replicas. But two things that already had been said help reduce it greatly:

- disabling roll/pitch od the tail fin (if not disabled, it tries to "help" you in roll maneuvers, and sends your mission to hell doing so)

- adding a fuel tank to the front part of the vehicle - you rarely land completely out of fuel, at least in the STS challenge missions. You can than move your CoM quite significantly, and use it to stabilise your shuttle in the most critical part of the approach. Even a bit of fuel helps a lot.

This is my shuttle, Kolumbia, that I used in all the mission (with slight modifications). It's not an exact replica, but looks similar to the original shuttle. And after I got used to it, it flies really great.

As you can see, there is a LF/Ox tank in front of and behind the cargo bay and lacks the monoprop OMS. other than that, looks, shuttle-ish enough.

 

Of course, you can design your own shuttle, that likes nothing like the original (in fact, most of the people in the STS challenge do), but I like the additional challenge. It's worth the extra effort.

Never give up

Michal.don

 

 

 

[adsii1970]

1 hour ago, Andiron said:

I like it a lot, it was really a technological marvel. But we can't forget that 14 astronauts died because of it.

More people die in one Airbus or Boeing crash than did in both orbiters put together. Every astronaut/cosmonaut knows and accepts the risk.

And with that said, the shuttle was an outstanding demonstrator of what space flight could be. A lot of unrealized potential squandered by pencil-necked politicians.

It did its job in spite of national politics.

[AeroGav]

On 26/09/2016 at 4:47 PM, Snark said:

 

This happens all the time with people who want to build space shuttle replicas in KSP.  It's kind of unfortunate:  the game makes it really really easy to build something that looks just like the space shuttle, because Squad has gone out of their way to make parts (the Mk3 cockpit, the Big-S tailfin, the Big-S delta wings, the shape of Mk3 fuselage parts, the Vector engine) that are designed to look just like the shuttle... except that it won't fly like the shuttle.  Because if you build something in KSP that looks just like the real life shuttle, the CoM of your replica will be nothing at all like the real-life shuttle.

The biggest culprit I'd say are those Vector engines.   4 Tons each, that's 12 tons of empty mass at the back, the mk3 cockpit weighs just 4 tons?

KSP makes rocket engines far heavier than they should be,  but offset this by making orbital velocities much lower than IRL.

I can't help but feel 3 Vectors is actually overkill on something the size of a Kerbal space shuttle replica.   The real shuttle was supersonic and above airliner cruising altitude at booster separation,  so if your replica is at 12km+ and mach 1.5+ by the time the boosters pop off, you really don't need that much power.

My attempt at an STS was a horrid boondoggle, but it did get to orbit with a Poodle and two Puff engines,  a much lighter setup.

You could make my design much simpler by 

1) forgetting about that ridiculous Flea booster-booster.  Took more designing than the rest of the ship.  

2) mounting three SRB around the fuselage instead of two

3) waiting for the new 1.2 patch, which will make fuel drain evenly from every tank by default even with rocket engines - would make my 3rd party fuel managers redundant.   Though you'd still need a mod to get the wings to hold monoprop for the Puff motors.

4) tweakscale wont be needed in 1.2 either - the Puffs are tiny things on the current version of the game,  but they appear to have grown to the size of the real shuttle's OMS engines in the beta !

 

 

[KerbalTween]

On 9/26/2016 at 11:47 AM, Snark said:

Irrelevant.  Or, at least, marginally relevant.

Not super important.

It's a pretty common myth in KSP that "CoL behind CoM = stable."  Yes, you want your center of lift to be behind your center of mass... but that's not even vaguely the most important thing.

This screenshot pretty much says it all.  Look at where your CoM is!  It's way the heck at the back of the craft.  In a situation like that, it doesn't matter where your CoL is... a CoM at the back of the craft, with that huge lightweight draggy fuselage sticking out in front, is going to want to fly backwards.

The center of mass of the craft wants to be in the front of the craft.  That's how we all build rockets for ascent, after all-- try to keep the CoM high, towards the front.  The same principle applies during descent.

It also doesn't help that you have basically no control authority on the craft.  All your control surfaces are located at the back of the ship, right next to the CoM, which means they have practically no lever arm to work with and therefore might as well not even be there.

Basically, what you're trying to land there is a badminton birdie that's pointing feathered-end forward.

This happens all the time with people who want to build space shuttle replicas in KSP.  It's kind of unfortunate:  the game makes it really really easy to build something that looks just like the space shuttle, because Squad has gone out of their way to make parts (the Mk3 cockpit, the Big-S tailfin, the Big-S delta wings, the shape of Mk3 fuselage parts, the Vector engine) that are designed to look just like the shuttle... except that it won't fly like the shuttle.  Because if you build something in KSP that looks just like the real life shuttle, the CoM of your replica will be nothing at all like the real-life shuttle.

There are lots of ways to make your craft stable on landing, but they're going to involve doing things that make it look less like the US space shuttle, so if you're counting on the aesthetics of looking like that, you're going to be disappointed.  On the other hand, if you're just interested in building something that works and don't care whether it looks like the shuttle, then you have options. 

So, there are two things you can do:

• Move the CoM far forward.  Way far forward.  As far as you can manage it.
  • This can be tricky, given that your main ballast (fuel) is all gone, and your heaviest components by far (the engines) are all clustered at the back of the ship.
  • One design that can work well is to go with more of a Skylon-like design:  put your engines close to the middle of the ship, mounted on the sides, rather than having them way back in the back of the ship.
  • There are other things you can do, too (KSP's great at giving design options!) ... but given how heavy the engines are, moving them forwards is probably the easiest thing to do.
• Make sure you have control surfaces that are far away from the CoM.
  • Ideally, this would mean move your CoM way far forward, and keep your control surfaces at the back.  The fact that you've moved the CoM forward will give your surfaces the lever arm they crave.
  • However, if you're having trouble moving the CoM forward... put some canards way out on the front, as far forward as possible.  They'll help maintain control if you can stick closely to prograde (i.e. you have SAS to help with that).  A CoM-at-the-back ship is still going to be aerodynamically unstable, which means if you ever waver even slightly off prograde, it's going to flip backwards and stay that way.  But if you can manage to ride the knife-edge of stability, poised precisely at prograde, then this can work.  It's just a bit of a nail-biter, is all.
• Make sure you've turned off everything except yaw authority on your tail fin.
  • The only reason to have that big ol' vertical stabilizer is to give you yaw stability.  It shouldn't try to compensate for anything other than yaw.
  • However, by default it'll try to help you with roll, too... and the result is a complex interaction that causes it to fight itself (trying to yaw induces roll that it tries to compensate for by rolling the other way which requires deflecting in the opposite direction and cancelling out the yaw...).  Long story short:  you'll find better yaw stability if you turn off everything else on your vertical stabilizer.  You'll also be more stable if you can move your tail fin down some (i.e. in the ventral direction); right now it's sticking up so high that it induces a lot of roll whenever it operates.  Use the offset tool to move the fin a bit closer in to the central axis of the ship.

Um There Is another One Called Clipping A Lots of fuel tanks to the front Try to make clean Thats How I Get My Center of mass for My New space shuttle columbia Center of mass Near to the middile if am correct And You Get More fuel

Edited October 16, 2017 by KerbalTween

[Spricigo]

5 hours ago, Kerbal Tween said:

Um There Is another One Called Clipping A Lots of fuel tanks to the front Try to make clean Thats How I Get My Center of mass for My New space shuttle columbia Center of mass Near to the middile if am correct And You Get More fuel

I just hope the OP had solved his issue months ago or that you have a time machine.

[KerikBalm]

I wish we had a center of drag display. You really need the center of all aerodynamic forces behind the CoM, but you don't see that display (and L vs D changes at different numbers)

You can probably get it to work by simply moving the wings back more - this gives you more of a lever arm, in addition to moving some lift and drag farther aft.

My designs were stable... granted that may have been from a time when the Rhino was 6.5 tons, not 9... its still less massive than 3x vectors (12 tons combined)

v I think this one was my "mk3" version, the above pics were mk4

[Spricigo]

4 hours ago, KerikBalm said:

I wish we had a center of drag display. You really need the center of all aerodynamic forces behind the CoM, but you don't see that display (and L vs D changes at different numbers)

I wish that and a windtunnel tool where the player could adjust windspeed and direction of airflow. Unfortunately I don't forsee it hapening anytime soon.

In any case my experience is easier to balance CoD/CoM when CoM is closer to midle of the craft. 

[nOtelite]

Short answer: Your CoM can be in front of the CoL. But there is a problem:

If your center of mass is on the back of the orbitter like this, your front body acts like a wing and since the front body is too far from center of mass, it is being too unstable.

So try to put some fuel tanks on front so your CoM is closer to middle of the orbitter. 

 

 

[Richmountain112]

Shuttles are actually pretty easy to fly for me. It may be because I added a body fin exclusively for Pitch in the back.