What's Wrong With My SSTO?

[Harvoification]

I recently constructed my first advanced spaceplane. All of them before this one have been tiny and unable to deliver any payloads into space, so it feels good to finally have created one with actual function. However, it's plagued by a pretty serious problem. At first when it takes off, it flies pretty smooth. However, I notice later in the flight that it becomes unstable when activating the rocket engines. More importantly, this only happens after a while of flying on jet engines. If I burn with the rocket engines at take off, it still seems to fly pretty smooth.

I know it's not the aerodynamics of the craft, because it happens in space too. I can actually get it into space, but it's a constant battle. Leave the engines burning for too long, the nose rises until the spacecraft goes into a complete spin. My first thought was the fuel was becoming unbalanced later on in the flight, and indeed this may still be the case. However I tried flying it with TAC Fuel Balancer active and it still does the same thing, so I'm really stumped.

It's so frustrating because without this problem, the thing would be pretty efficient and my first truly successful advanced SSTO. If anyone here can shed some light on what's happening here I'd be really grateful.

Craft File (Spaceplane Plus, KW Rocketry): https://www.dropbox.com/s/9zga07uux2njxr1/Skylon%20%28FIX%20ME%29.craft

A few pictures to help everyone out:

And yes, I know it looks nothing like the real Skylon design!

Edited August 15, 2014 by Harvoification

[capi3101]

What you're describing sounds like a classic case of shifting CoM during flight. Do this - while still in the VAB, turn on the CoM and CoL meatballs and temporarily drain the fuel tanks in the order they would drain in flight. If the CoL goes ahead of the CoM at any point, you're going to lost stability and start nosing up pretty much constantly.

Edited August 14, 2014 by capi3101

[Harvoification]

What you're describing sounds like a classic case of shifting CoM during flight.

How would I combat this?

[capi3101]

Been a while since I've looked up this stuff...lemme see if I can find it real quick.

Here we go: http://forum.kerbalspaceprogram.com/threads/52080-Basic-Aircraft-Design-Explained-Simply-With-Pictures

Basically, you need to move your center of lift further back and/or move your center of mass further forward. That might affect the smoothness of the flight in the early going, but unless you're going for a one way trip up, it's an adjustment you'll have to make. Figuring out how to get the right balance between the two throughout an entire flight is what makes or breaks a spaceplane design; I've been there myself a number of times.

I will say that TAC Fuel Balancer does quite help a bit, if you're not adverse to utilizing mods. One of the other things I tried in some of my designs was using monoprop as ballast tanks - stick an empty set towards the nose and a full set towards the tail; once in orbit, shift it to the nose - this moves the center of mass forward. I had some success with that.

The rest of y'all can jump in any time now...

Edited August 14, 2014 by capi3101

[Harvoification]

Been a while since I've looked up this stuff...lemme see if I can find it real quick.

Here we go: http://forum.kerbalspaceprogram.com/threads/52080-Basic-Aircraft-Design-Explained-Simply-With-Pictures

Basically, you need to move your center of lift further back and/or move your center of mass further forward. That might affect the smoothness of the flight in the early going, but unless you're going for a one way trip up, it's an adjustment you'll have to make. Figuring out how to get the right balance between the two throughout an entire flight is what makes or breaks a spaceplane design; I've been there myself a number of times.

I will say that TAC Fuel Balancer does quite help a bit, if you're not adverse to utilizing mods. One of the other things I tried in some of my designs was using monoprop as ballast tanks - stick an empty set towards the nose and a full set towards the tail; once in orbit, shift it to the nose - this moves the center of mass forward. I had some success with that.

Thanks! Guess it's back to the drawing board for a while!

[Tweety]

Without seeing the COM and COL indicators I reckon you should be able to shift the COL back simple by taking the canards up front off all together. JUST a guess but I would imagine with the control surfaces you have you could still make that plane work without ripping it apart and starting over.

PM me if you have any more questions or pics to hand. I'm not as good as some of the guys here but have played about with spaceplanes of about this size loads.

Tweety

[Wanderfound]

How would I combat this?

The RCS Build aid mod also gives you CoM with both full and empty tanks (without having to faff about with tweakables) and measures the distance between the two. If it's more than 1m on a spaceplane, you should think about rearranging your tanks. Spread your fuel load laterally rather than longitudinally and use fuel lines to control the order in which they drain.

CoL needs to be behind CoM at all stages of the flight. Only a smidge, though; the indicators should overlap. The fact that it's happening in vacuum suggests that it's probably a CoM/CoT misalignment rather than anything to do with the aero, though. Unbalanced RCS might also be a problem.

At a first guess, I'd say that your aerospikes are aimed slightly below your CoM. You can get away with this at low altitude, because the aero surfaces will counter it. But in the stratosphere or space, there isn't enough counterforce to compensate.

As mentioned above: give us screenshots in the SPH, from above and the side, with CoM/CoL/CoT indicators on, with tanks both empty and full. The spaceplane crew can diagnose most issues, but we need some info to work with.

We'll also need to know if you're using stock aero, NEAR or FAR. You've got a huge amount of wing surface on that thing; that'll work in stock, but it's counterproductive with realistic aero.

Edited August 14, 2014 by Wanderfound

[Harvoification]

Well, I've played about with it a bit more and I just cant get it right. Take the canards at the front off, and it won't have enough lift to keep the nose pointing upward. Move the engines forward helped a little bit but it's still uncontrollable. I'll post you guys some pictures in the SPH with CoM/CoT/CoL.

Oh, and I'm in stock.

There you go. Sorry about the debug menu, it often comes up when I take screenshots.

Edited August 14, 2014 by Harvoification

[capi3101]

Hmm...if you need the canards up front, you might try moving them further back along the cockpit; say as close to the intakes as you can manage sans clipping. It would have the same general effect - it'd put the center of lift further back at takeoff. Still don't know what you're going to do later in the flight.

Of course, as the canards move, so would the CoM...so maybe never mind on this one. I'll give the matter some thought and see if I can come up with something more practical.

[Harvoification]

Thanks. Honestly not sure whether to start from scratch again, but this is as close as I've got to a functional SSTO so it makes me wanna try to fix it.

[Moar Boosters]

It's hard to tell from the pictures, but it looks like the center of mass is slightly higher than the center of thrust. Fix that and it should fly straight.

[Tweety]

I see you have Spaceplane+ mod in there, if that's the only mod you're running I'd be happy to check out the craft file. I normally only run stock but I don't mind adding something I have used before.

Tweety

[pa1983]

How would I combat this?

Simplest fix for this is to start out with the Fuel tanks. Mount them in parallel instead thats the easiest way to maintain CoM. Second option is to use a forward tank and a rear tank and use pipes to make sure they drain equally. A center tank can be used to in this configuration with proper piping.

But the parallel method is the easiest to master. Once you have your tanks placed add engines and cockpit and make sure CoM is in the cetnter of the tanks then add wings in such a way that the craft maintains CoL appropriately. Lot of people wants CoL rear of CoM but thats not necessarily better in KSP from my experience. Many of my best crafts have had CoL forward of CoM.

Also a good idea to rotate the craft 90 degrees on it side and check CoL. It can shift significantly forward or rearward that can cause the craft to spin out of control during roles and turns. Add rudders etc to compensate for this if needed.

[cantab]

It's hard to tell from the pictures, but it looks like the center of mass is slightly higher than the center of thrust. Fix that and it should fly straight.

I agree with this, it seems the problem is off-centre thrust. That would account for the issue occuring in space where the wings are irrelevant.

Possible solutions:

Shift the engines up a little with the aid of cubic octagonal struts.

Change the aerospikes to a gimballed engine like the LV-T45 or the RAPIER. Since the plane flies OK on the jets, this might be sufficient. Of course you'll lose a bit of delta-V.

Add some radial engines on the top and tweak the thrust so the overall thrust is on-axis.

Angle the engines so the thrust points through the CoM. I suggest sticking an RGU in front of the engines if you do this, so you can control from it to line up the navball with the thrust axis.

Add more reaction wheels.

Add more RCS (not a GOOD solution since you'll guzzle monoprop).

[Harvoification]

Wow, I forgot how helpful the KSP community is! I won't have time to play any more tonight, but I'll keep these solutions in mind.

Oh, and Tweety, unfortunately Spaceplane + isn't the only mod being used for it. I believe there are Lack Luster Lab parts as well as Kerbal Engineer. If I had time to play, I'd go look for these parts for you but alas it is not to be. I also have no idea how to upload a craft file... :rolleyes:I could do it tomorrow but I'm not sure on the etiquette of resurrecting threads is for round here, assuming of course this gets buried. If it's still here after I get back from work in the evening, I'll upload the craft file to this thread using only the spaceplane plus parts, if you want?

[Tweety]

Yeah that would be brilliant if you could. I would go and install all the mods but I'm not keep on adding lots of parts packs since I get restless waiting for KSP to load. Regardless of this I am going to reinstall Spaceplane + since I haven't messed with it on my 0.24.2 saves.

Resurrecting threads would apply to completely dead threads from months ago. You could also just PM the craft file. You only need a real simple hosting site, I use dropbox for example, mainly cause there is an app for it on my phone so I have my crafts everywhere I go.

Edit: I just noticed that the air intakes are asymmetric....your COM may be higher than your COT because you have more air intakes higher fuel tanks. That may cause the problems in space, in the atmo they would create extra drag and pull the craft up.

Edited August 14, 2014 by Tweety

[Harvoification]

Oh my, you might be right. Not sure on their mass but it could be heavy enough to make a real difference! No idea how I'd even get into space without them though

I'll play around with it tomorrow before taking the modded parts off and see if it helps. Hopefully there's somewhere decent to put them!

[Wanderfound]

Thanks. Honestly not sure whether to start from scratch again, but this is as close as I've got to a functional SSTO so it makes me wanna try to fix it.

You CoL is way too far back; you want the spherical part of the CoL/CoM indicators to be touching each other. If you do that, you'll find it much easier to keep the nose up. Instead of adding more surfaces to the front, remove some of the decorative panels from the rear fuselage.

The fact that it's happening in vacuum strongly suggests that it's nothing to do with CoL, though. Does it have any RCS or ASAS units? Is the RCS on when it's happening?

[Nurph]

As others have mentioned it sounds like your CoM is higher than your CoT. Remember that landing gear are mass-less in flight, so make sure you balance your craft before adding the gears!

[Tweety]

It should still space without the extra intakes, you just have to milk the jets some more at altitude. If I remember correctly one of those big intakes are equivalent to about 4 ram intakes. That would be plenty for this design I should imagine. BTW I started working on a stock version of your plane last night (without the cargo bay) to see if there's just a fundamental flaw in the design. I will continue it tonight when I get the boring stuff out of the way (damn you work for keeping from KSP)!

[Wanderfound]

It should still space without the extra intakes, you just have to milk the jets some more at altitude. If I remember correctly one of those big intakes are equivalent to about 4 ram intakes.

More like one and a bit. He could certainly afford to lose a few of the structural intakes, though.

[Mesons]

Oh my, you might be right. Not sure on their mass but it could be heavy enough to make a real difference! No idea how I'd even get into space without them though

I'll play around with it tomorrow before taking the modded parts off and see if it helps. Hopefully there's somewhere decent to put them!

If your evident drive to make your craft beautiful allows, you could place the intakes in equal numbers above and below the CoM. I usually place them in pairs atop and beneath wing surfaces. That might not work in NEAR or FAR but it works great in stock.

[Harvoification]

Won't be able to play KSP tonight as well, so no craft file yet I'm afraid.

...

I'm getting withdrawel symptoms...

EDIT: Never mind, I got a few minutes! I removed the Kerbal Engineer and LLL mod parts, but unfortunately some of the liquid tanks are from KW Rocketry and I can't remove them without drastically changing the craft. I'd really appreciate it if someone could take a look at the problem child in-game and try to discern what's wrong or even fiddle about with it a bit!

Here's the craft file:

https://www.dropbox.com/s/9zga07uux2njxr1/Skylon%20%28FIX%20ME%29.craft

Thanks again for all your help. If there are indeed more mod parts attached (I think I got them all other than KW Rocketry) just let me know and I'll have another look.

One more thing, I did actually quickly try removing most of the structural intakes and making the rest symmetrical on the side of the craft, but it didn't seem to help.

Edited August 15, 2014 by Harvoification

[DaMichel]

Hm .. if i was in your place i would get RCSBuildAid http://forum.kerbalspaceprogram.com/threads/35996

This little tool can show you the torque that is generate when the thrust is not aligned with the CoM.

Needless to say, i also think it is an issue with thrust-CoM alignment. It is a particular problem with the aerospikes since they don't gimbal at all.