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NookRitzia

SpacePlane Slips out of control

Question

So when I came in for a landing My spaceplane keeps rolling out of control once it goes below 27,000 Meters. It basically doest the motion you do when you press A and D. It is uncontrollable my whole place is symmetrical, anyone knows what it could be? 

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After the fuel burns off and the cargo is released , CoM moves aft on designs with engines at the back (fuel and cargo bays forward).   If this results in more horizontal lifting surfaces being in front of CoM than behind it, pitch stability goes negative and it wants to flip.   The other, less common problem is that this rearward CG shift results in more vertical facing surface ahead of CG than behind it, leading to negative yaw stability.

Try moving the vertical stabilizer (fins) further aft, if they are not already right at the back,  then try adding more vertical stabilizer area.   Alternatively,  think about moving some or all engines off the back of the plane, so CG don't shift around so much.

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There's a number of potential reasons, depending on the shape of your craft. Pictures are by far the most helpful thing for diagnosis here (and maybe a .craft file). If you could take some pics of the plane in the hangar with CoM/CoL turned on, that would be even better. 

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Pix when the fuel tanks are empty, especially.

 

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10 hours ago, Jarin said:

There's a number of potential reasons, depending on the shape of your craft. Pictures are by far the most helpful thing for diagnosis here (and maybe a .craft file). If you could take some pics of the plane in the hangar with CoM/CoL turned on, that would be even better. 

 

Here is the picture https://postimg.org/image/sjx7sf39t/, some of my Liquid Fuel is taken out before hand due to the fact that I will be using more liquid fuel on my ascent so I were to be carrying dead weight. https://postimg.org/image/ponh59egx/ This picture is my "glider" after all fuel is out and there is no cargo.

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24 minutes ago, NookRitzia said:

Here is the picture https://postimg.org/image/sjx7sf39t/, some of my Liquid Fuel is taken out before hand due to the fact that I will be using more liquid fuel on my ascent so I were to be carrying dead weight. https://postimg.org/image/ponh59egx/ This picture is my "glider" after all fuel is out and there is no cargo.

Okay, quick observations.

1. I'm not seeing any vertical stabilizers on there (tail fins) so sideslipping is practically a guarantee. 
2. Waaaayyyy more intake than you need. Two of those shock cones could feed all your airbreathing engines on their own. The nacelles, precoolers, and inline intakes are overkill. Make those fuel tanks. 
3. Those front bays are going to be draggy and low-mass empty, leading to more instability on re-entry. Once you replace those forward nacelles with mk1 LF tanks, set those tanks to lower fuel priority so they drain last, keeping more weight forward. See how your CoM is behind the CoL in that second picture? This is how you fix that.
4. Canards make for strong pitch control, but they're also inherently unstable, especially on re-entry. If trouble continues, you might try building without them. This would also help the dry CoL issues.

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Posted (edited)
8 minutes ago, Jarin said:

Okay, quick observations.

1. I'm not seeing any vertical stabilizers on there (tail fins) so sideslipping is practically a guarantee. 
2. Waaaayyyy more intake than you need. Two of those shock cones could feed all your airbreathing engines on their own. The nacelles, precoolers, and inline intakes are overkill. Make those fuel tanks. 
3. Those front bays are going to be draggy and low-mass empty, leading to more instability on re-entry. Once you replace those forward nacelles with mk1 LF tanks, set those tanks to lower fuel priority so they drain last, keeping more weight forward. See how your CoM is behind the CoL in that second picture? This is how you fix that.
4. Canards make for strong pitch control, but they're also inherently unstable, especially on re-entry. If trouble continues, you might try building without them. This would also help the dry CoL issues.

 
 

What would be best for putting at the end of my rocket instead of the shock intake cones? And how do I make the fuel in the front be the last to be depleted, is there a way other than clicked on the green arrow?

Edited by NookRitzia

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The "full" picture looks good, but the "empty" picture is a disaster waiting to happen, it's unstable due to CoL being ahead of CoM.  The reaction wheels probably make it work anyway until you hit 27 km or so, but then you're having to fight too hard against the forces, and once it starts going it keeps going.

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7 minutes ago, NookRitzia said:

What would be best for putting at the end of my rocket instead of the shock intake cones? And how do I make the fuel in the front be the last to be depleted, is there a way other than clicked on the green arrow?

You can leave the 4 shock cones in place. It's overkill, but not really hurting anything. The rest should go, though.

You may need to turn on "show advanced tweakables" in game options, but there should be a +/- setting on each fuel tank. 

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1 hour ago, Jarin said:


4. Canards make for strong pitch control, but they're also inherently unstable, especially on re-entry. If trouble continues, you might try building without them. This would also help the dry CoL issues.

As regards to pitch, that is not true at all.    I built an aircraft, moved the elevons from the back to the front, adjusted the main wing position so they blue ball was in the sme place relative to the yellow and there was no difference in handling at all.   I actually put it on a re-entry trajectory and made a note at what angle the aero forces would push the nose down from pointing straight up - in both cases,  the wing unstalled at the same point.     

You can get into trouble if you don't move the wing, obviously.   And if you put an incidence angle on the main wing but not the canards, so the main wing stalls before the canards,  it'll go out of control at high angles of  attack.   

The only other drawback is that canards usually have a shorter tail, so the vertical stabilizer is bigger and needs more area.

On the up side, stalling speed is a bit lower, because when pitched up for max lift , the canard is helping to lift the plane, not push down on the tail to make the nose go up (negative lift).

1 hour ago, NookRitzia said:

1. I'm not seeing any vertical stabilizers on there (tail fins) so sideslipping is practically a guarantee. 

THIS.

1 hour ago, NookRitzia said:

3. Those front bays are going to be draggy and low-mass empty, leading to more instability on re-entry. Once you replace those forward nacelles with mk1 LF tanks, set those tanks to lower fuel priority so they drain last, keeping more weight forward. See how your CoM is behind the CoL in that second picture? This is how you fix that.

CorrectCoL mod improves the accuracy of the blue indicator.   In SPH it does not allow for lift made by parts not on the aerodynamics tab, such as fuselages.   He has a lot of fuselage.

RCS build aid shows a red ball which is where your Centre of Mass will be with no fuel.

 

TBH the OP may be on a hiding to nothing with a design like that - all the engines at the back. It will either be excessively nose heavy with fuel and cargo, or unstable when empty.      Try to get some engines mounted forward.

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5 hours ago, AeroGav said:

As regards to pitch, that is not true at all.    I built an aircraft, moved the elevons from the back to the front, adjusted the main wing position so they blue ball was in the sme place relative to the yellow and there was no difference in handling at all.   I actually put it on a re-entry trajectory and made a note at what angle the aero forces would push the nose down from pointing straight up - in both cases,  the wing unstalled at the same point.     

What we're talking about is length of lever arm. If your CoM is dead center in the middle of your craft, then it doesn't matter whether pitch control is forward or aft. But the vast majority of "sleek" design spaceplanes (like this one) end up with CoM pushed towards the back, giving canards outsized control authority for their size. The big problem is that on planes that are delicately balanced to start with (CoL close to CoM), canards can flip the plane all by themselves by pulling the CoL forwards when pitching up (have seen this in practice countless times). If your pitch control is in the back, it should always contribute to overall stability. 

Obviously, the ideal is to keep the CoM close to the physical center of the craft, but I was trying for advice that didn't need a major rebuild of the craft.

5 hours ago, AeroGav said:

On the up side, stalling speed is a bit lower, because when pitched up for max lift , the canard is helping to lift the plane, not push down on the tail to make the nose go up (negative lift).

You've brought this up before, but I'm still not convinced it's a major concern outside of some odd edge cases like aerobatics aircraft.

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1 hour ago, Jarin said:
7 hours ago, AeroGav said:

On the up side, stalling speed is a bit lower, because when pitched up for max lift , the canard is helping to lift the plane, not push down on the tail to make the nose go up (negative lift).

You've brought this up before, but I'm still not convinced it's a major concern outside of some odd edge cases like aerobatics aircraft.

Takeoff and landing are odd edge cases? Huh. 

Snarkiness aside, those are two very important (and common) scenarios that would be helped by a slower stall speed. 

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To be honest, this is drastically over built.  Unless you are planning on a long journey.  But either way, the shock cones are the only intakes you should be using here.  Radial intakes are only hurting you, drag is paramount in space planes.  That eliminates much of the craft or gives you more space for fuel.

 

Just for comparison, the Arrow has almost the same cargo capacity (2 full size bays) in a much smaller package:

https://kerbalx.com/ForScience6686/Arrow-Network-package

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First off, I've never been happy with flight stability with any plane, no matter how well designed. I use AtmosphereAutopilot to provide superior handling. It's a low-level fly-by-wire system that helps you better control your craft.

Second, I use Correct CoL to help make sure that the center of lift is accurate. Since I started using this, I've had little to no issues with designing flyable craft.

Finally, I use RCS Build Aid to visualize the shift in CoM as fuel burns off. Useful for more than just spaceplanes, too.

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2 hours ago, ForScience6686 said:

To be honest, this is drastically over built.  Unless you are planning on a long journey.  But either way, the shock cones are the only intakes you should be using here.  Radial intakes are only hurting you, drag is paramount in space planes.  That eliminates much of the craft or gives you more space for fuel.

 

Just for comparison, the Arrow has almost the same cargo capacity (2 full size bays) in a much smaller package:

https://kerbalx.com/ForScience6686/Arrow-Network-package

I'm counting 16 intakes for 6 jet engines, which is too much.  That said, intake drag (I think it got balanced a few updates ago) is nothing compared to what the mk2 fuselage (which is very long in this design) will cause, and he's probably not got cones on the back of his rapiers which probably adds more drag than all the intakes combined.

You can check the numbers yourself - press ALT F12 for the debug menu, Physics Tab, Aero subheading, check the box that says "display aero data in action menus" and go for a flight with right click menus pinned to the screen.   Suspect it's a classic case of excess drag, need more engines to go supersonic, high dry mass from all the engines brings down delta V, add more fuel etc.

For a space plane with RAPIERs,  there's only two intakes in the running - the Pre-Cooler and the Shock Cone.   All the others - start to choke past Mach 2.5 and cause engine air starvation at high speed no matter how many are spammed.    Pre-cooler can feed one rapier each.  Shock cone can do multiple rapiers, but it's low speed performance is not as good as pre-cooler,  you sometimes have to open the throttle gradually on the runway to avoid engine surges if you're using much less than one shock cone per engine, or you have some engines with stronger low speed performance eg, whiplash/panther being used alongside the rapiers.

*though I suppose you could combine one shock cone with one subsonic intake to cover multiple rapiers, so both ends of the speed regime are covered - don't know how efficient this would be

One other thing - with 6 engines and a huge number of fuel tanks it looks pretty heavy, the wings are quite small for lifting such a weight.    That may amplify a draggy fuselage, because you 've either got to go faster lower in the atmosphere to get sufficient lift or fly with a bigger nose up angle, which increases drag on everything.   That said, nobody's forced to fit larger wings if they don't want to.  You can just brute force it with more engines, in the craft sharing thread somebody made a horizontal takeoff SSTO with only a pair of advanced canards for wings.   The more wing you have the less TWR you tend to need, and vice versa.

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