Space plane flat spins

[Cloakedwand72]

Why does my space planes always flat spin right during reentry? I've tried STS/Buran style designs also a MK3 robotic space plane with two side detachable boosters & it also flat spins. Any tips?

[Spricigo]

Without a picture, just the usual generic tip:

Move heavy things further forward and draggy things further backwards.

 

[Incarnation of Chaos]

It's either the COM or not enough authority/large enough Horizontal and Vertical stabilizers.  Also your angle of attack does definitely matter, too shallow or too steep and aero forces might very well take what looks stable into a spin abruptly as you plunge into thicker and thicker air.

This can be tricky, i won't lie.  But once you've exhausted all potential sources of error on your end, that's when I'd look at the plane itself.

[KevinW42]

Always make sure your center of lift is behind the center of mass.

Since you say it's during reentry, a common problem with planes is that your COM often moves as you burn off fuel. So when you check to make sure your COL is behind the COM in the VAB/SPH, make sure that's the case both with fuel full and with fuel empty.

 

[Snark]

48 minutes ago, KevinW42 said:

Always make sure your center of lift is behind the center of mass.

...while also bearing in mind that this is not sufficient-- it's absolutely possible to have an unstable plane whose CoL is behind the CoM.

If the CoM is near the rear of the plane, then it's almost certainly going to be unstable, regardless of where the CoL is.

@Cloakedwand72, could you please post a pic of your plane?  Ideally in the SPH with CoM displayed.  (Especially if you could do so with empty tanks, i.e. mimicking the plane's condition on reentry, so we can see where your CoM is when you're not carrying a heavy fuel load.)

[Cloakedwand72]

5 hours ago, Snark said:

...while also bearing in mind that this is not sufficient-- it's absolutely possible to have an unstable plane whose CoL is behind the CoM.

If the CoM is near the rear of the plane, then it's almost certainly going to be unstable, regardless of where the CoL is.

@Cloakedwand72, could you please post a pic of your plane?  Ideally in the SPH with CoM displayed.  (Especially if you could do so with empty tanks, i.e. mimicking the plane's condition on reentry, so we can see where your CoM is when you're not carrying a heavy fuel load.)

I’d try to show you a a image of my space plane but imgur is being finicky with the copy & past of photos so I can’t until I figure that out. Wish the forums made it easy to show photos. Btw my col/com is near the rear.

[Snark]

Just now, Cloakedwand72 said:

Wish the forums made it easy to show photos.

They do!  People do it all the time. 

You can upload your pic to imgur.com just fine, yes?  In that case, to show it here, just do this:

1. go to the page on imgur.com where the image is
2. right-click on the picture and choose "Copy Image Location"
3. paste that URL (should end in .jpg or .png) here

That's it!  When you paste the URL of the image, the forum will automagically replace it with an in-line image.  Like this:

(note, this works with any image-hosting site, not just imgur)

3 minutes ago, Cloakedwand72 said:

Btw my col/com is near the rear.

If your CoM is in the rear, then that's almost certainly your problem right there.  It's very difficult to make any craft aerodynamically stable if the CoM is near the rear; that's like trying to throw a badminton birdie backwards.  The massive end wants to be in front.  So you very probably need to design your craft with the CoM farther forward.

Will wait on seeing a picture before trying to offer any advice that's more specific, though. 

[Cloakedwand72]

On 2/1/2021 at 8:58 AM, Snark said:

...while also bearing in mind that this is not sufficient-- it's absolutely possible to have an unstable plane whose CoL is behind the CoM.

If the CoM is near the rear of the plane, then it's almost certainly going to be unstable, regardless of where the CoL is.

@Cloakedwand72, could you please post a pic of your plane?  Ideally in the SPH with CoM displayed.  (Especially if you could do so with empty tanks, i.e. mimicking the plane's condition on reentry, so we can see where your CoM is when you're not carrying a heavy fuel load.)

Hear is my Space plane COM/COL fully fueled and not. Yeah and I'm still sure I want to make space planes. Only way to send up specialty modules with being rockets look silly bye doing so.

Edited February 3, 2021 by Cloakedwand72

[Snark]

48 minutes ago, Cloakedwand72 said:

Hear is my Space plane COM/COL fully fueled and not.

Thanks, those are helpful shots!

Yeah, the biggest problem is that your CoM is near the rear.  That enormous, lightweight fuselage poking out in front of the CoM is your problem-- it's gonna be very hard to keep that pointing forward.  Anything you can do to put something heavy farther forward, to move the CoM nearer the front, would help. 

The real problem, I expect, is the engine.  You've got a big heavy engine in the rear-- and if the rest of the plane is basically a bunch of empty fuel tanks and empty cargo bays, then there's just not much that has significant weight to it.

One design strategy that some folks use to combat the "heavy engine in the rear" problem is to build a more Skylon-like design.  Instead of having a big heavy engine on the rear of the fuselage, have a pair of engines mounted amidships, on the left and right sides of the fuselage.  Since this brings the heavy engine mass forwards, it really shifts the CoM and can help with aerodynamic stability.

Yes, it can be awkward to design around depending on how the rest of the craft is built-- not saying you have to do that, just mentioning that it's one technique that works well for a lot of folks.

Some other suggestions/observations:

• Canards.  You may wish to consider a pair of canards, mounted way up at the front of the ship, as an assist for maintaining pitch.  The effectiveness of a control surface depends on its lever arm, and with your CoM way in the back of the craft, it means that any control surfaces back there are fairly close to the CoM and therefore can't exert much control authority.  Some little canards way up at the front, on the other hand, are really far from the CoM and therefore have a lot of leverage to work with.  So they may come in handy.  If you add them, I'd suggest adjusting their actuators so that they're pitch-only and won't attempt to help you with roll or yaw.
• Vertical stabilizer.  As far as I can tell, you don't really have any vertical stabilizer at all-- meaning you don't have any control surfaces to provide yaw stability.  It's therefore not surprising that you're encountering a flat spin.  I'd strongly suggest adding a vertical stabilizer.  Make it as far back as possible, and also try not to stick up above the central axis of the ship too much.  When you add it, make sure to adjust the actuators so that it assists with yaw only.  In particular, make sure that it has roll authority turned off  (it's on by default, so you need to manually disable that in the editor).  Reason:  if you leave roll turned on, it'll try to activate for roll assistance and will end up fighting your ailerons for control and make the craft less stable.

 

[FleshJeb]

@Cloakedwand72 The tank that's sitting at the CoL can be swapped out. Use one half the size, and then put another half tank in front of the cargo bay.  You may also want to set the priority on the front adapter tank so it drains last.

Cargo bays can also be very draggy, and the CoL doesn't show it. Make sure you close the internal nodes to get the minimum drag. I use 2.5m docking ports or 2.5m batteries.

That's a very elegant craft; I like it a lot.

[Cloakedwand72]

1 hour ago, Snark said:

Thanks, those are helpful shots!

Yeah, the biggest problem is that your CoM is near the rear.  That enormous, lightweight fuselage poking out in front of the CoM is your problem-- it's gonna be very hard to keep that pointing forward.  Anything you can do to put something heavy farther forward, to move the CoM nearer the front, would help. 

The real problem, I expect, is the engine.  You've got a big heavy engine in the rear-- and if the rest of the plane is basically a bunch of empty fuel tanks and empty cargo bays, then there's just not much that has significant weight to it.

One design strategy that some folks use to combat the "heavy engine in the rear" problem is to build a more Skylon-like design.  Instead of having a big heavy engine on the rear of the fuselage, have a pair of engines mounted amidships, on the left and right sides of the fuselage.  Since this brings the heavy engine mass forwards, it really shifts the CoM and can help with aerodynamic stability.

Yes, it can be awkward to design around depending on how the rest of the craft is built-- not saying you have to do that, just mentioning that it's one technique that works well for a lot of folks.

Some other suggestions/observations:

• Canards.  You may wish to consider a pair of canards, mounted way up at the front of the ship, as an assist for maintaining pitch.  The effectiveness of a control surface depends on its lever arm, and with your CoM way in the back of the craft, it means that any control surfaces back there are fairly close to the CoM and therefore can't exert much control authority.  Some little canards way up at the front, on the other hand, are really far from the CoM and therefore have a lot of leverage to work with.  So they may come in handy.  If you add them, I'd suggest adjusting their actuators so that they're pitch-only and won't attempt to help you with roll or yaw.
• Vertical stabilizer.  As far as I can tell, you don't really have any vertical stabilizer at all-- meaning you don't have any control surfaces to provide yaw stability.  It's therefore not surprising that you're encountering a flat spin.  I'd strongly suggest adding a vertical stabilizer.  Make it as far back as possible, and also try not to stick up above the central axis of the ship too much.  When you add it, make sure to adjust the actuators so that it assists with yaw only.  In particular, make sure that it has roll authority turned off  (it's on by default, so you need to manually disable that in the editor).  Reason:  if you leave roll turned on, it'll try to activate for roll assistance and will end up fighting your ailerons for control and make the craft less stable.

 

Yeah no problem it took me a while to figure out how to send pics. And I noticed how light the MK3 fuselage & how heavy my engine is. Should I switch to a vector engine or something similar? Also I can add some ore tanks in the front to balance this. And wondering if y’all have similar designs like this that I can barrow. And I’m pretty adamant when it comes to making stuff like this with cargo bays it just makes it realistic sending gravity rings or unity node modules or fuel tanks with something like this or traditional shuttles. Btw what should my tail section look like?

[Spricigo]

2 hours ago, Cloakedwand72 said:

Only way to send up specialty modules with being rockets look silly bye doing so.

I'd not worry that much, how they did in real life is already quite silly. *cough!*Polyus ...*cough!* 

Anyways: while the Center of Lift is behind the CoM, the Center of Drag is  not.  At least not if the craft have any deviation from surface prograde (the nose of the craft is quite a bit of extra exposed surface to the airflow ahead of CoM)

 

So the solution is the usual:

• More heavy things in the front  (Fuel a ballast is a popular choice, if not the most effective use of fuel)
• More draggy things in the rear (I'd consider adding more vertical wing parts, given that the craft don't have that much to begin with.

Well, you can try the other way,  removing weight/drag from where there is too much. However I suppose you put those parts there for a reason.

 

[Spricigo]

6 minutes ago, Cloakedwand72 said:

And wondering if y’all have similar designs like this that I can barrow

You may probably find something youlike at kerbalx and the spacecraft exchange sub-forum. But give us some details about what tech is available and the performance required (in particular, payload and orbital height) and we may come up with something.

 

 

[Guest]

Wow that's a lot of plane.

Point one: any spaceplane that big is hard. I've certainly made hundreds of planes at this point, and it would likely take me the better part of a day to make something that big work, unless I'm going with a design I already know works from having done it before. So I would recommend starting less ambitious: first figure out how to make a plane ol' plane based on Mk 1 parts, with one or two engines, then figure out how to give it enough power to get to orbit, then make a bigger one, and then work your way up from there. As a rule of thumb, if it's < 20 tons take-off weight it's pretty easy, < 40 tons is still pretty easy, < 80 tons takes a quite a bit of work, > 120 tons is a beast.

Second, as to your design specifically -- there's a reason planes have vertical stabilisers and rudders. Stick a nice big one on way back and see if it helps. Those turned-up wingtips aren't doing anything to stabilise it because they're so close to the centre of lift, the rudder needs to be well aft of it.

Edited February 3, 2021 by Guest

[lemon cup]

5 hours ago, Spricigo said:

More draggy things in the rear (I'd consider adding more vertical wing parts, given that the craft don't have that much to begin with.

 

3 hours ago, Brikoleur said:

there's a reason planes have vertical stabilisers and rudders.

BIG factor here. Take this advice especially. Your craft is most likely suffering from poor yaw stability. Your center of mass being so far to the rear means that you need a large vertical stabilizer, as far back as possible.

The real-world Space Shuttle is a good example of this. Its center of mass was roughly in the same spot as yours, and it needed a big vertical stabilizer.

Keep in mind it was a spacecraft and so it would not have carried such a big heavy component with it unless it absolutely needed every square inch of that surface area.

[swjr-swis]

19 hours ago, Cloakedwand72 said:

I noticed how light the MK3 fuselage & how heavy my engine is. Should I switch to a vector engine or something similar? Also I can add some ore tanks in the front to balance this. And wondering if y’all have similar designs like this that I can barrow. And I’m pretty adamant when it comes to making stuff like this with cargo bays it just makes it realistic sending gravity rings or unity node modules or fuel tanks with something like this or traditional shuttles. Btw what should my tail section look like?

A spaceplane to carry an Mk3 cargo bay of payload to LKO doesn't need that much engine, so you could definitely benefit from switching to smaller ones. Part of your fuel/thrust is being wasted just to lift all that engine mass up to orbit.

But who says we have to be efficient? KSP offers us the fun opportunity to be playful and extravagant, and try out all the what ifs we can come up with. I really like the looks of your concept, so I gave it a test run to see how to make this a viable -and safely returnable- cargo vehicle.

I couldn't make out from the screenshots how you constructed the boosters, so I used a simple twin-boar stack for that. The flat spin was solved quite easily just by adding the vertical stabilizer with an elevon 3 for rudder. To get the CoM more forward without having to ditch the Rhino, I replaced the nosecone by an Mk1-2 command pod, and split the Mk3 LFO tank moving half of it in front of the cargo bay. This allows manned missions as well, and the docking port on the nose means docking to stations becomes an option.

With the CoM forward, the CoL is left too far back - while this helps some with the stability on ascent, it makes it near impossible to land safely because it just wants to nose-dive. So I doubled up the front delta wing sections, and added an extra swept wing section in front of that, with a 5 degree AoI. Better, but it was still lacking on pitch authority. I chose to double up the elevon 2 and 3 at the back, which can then also be deployed as split airbrakes. Canards are an alternative, but they also add drag to the front with would make it prone to destabilize again.

Assemble it all up with the boosters, and this bird can comfortably put a 37t full length module into orbits from 70-300km equatorial, or up to 225km polar. It needs very little stick during re-entry, maintains heading even when on probe control only, and is quite easy to land safely on or off the runway and on water.

I think I managed to keep very close to your original concept, but that is of course up to you.

 

 

 

Full album: https://imgur.com/a/8FIuKEm

Craft files: Just the space plane alone (SPH) fueled without cargo, or fully assembled (VAB) with a 37t orange tank module. Note that they're both created in KSP 1.3.1 - if you load them in later versions, you may need to check some of the parts due to changes, but it should perform nearly the same. Needless to say - they're yours to use/edit/publish any way you see fit.