SSTO troubles

[AFSATCOM]

Looking for a bit of advice into getting an SSTO stable on re-entry. Seems lately all of my craft tend to want to fly backwards in the upper atmosphere. Once they hit thicker atmo, they'll flip back the right way. Problem is the plane gets ripped apart when this happens. I'm using FAR and procedural wings among a number of other mods.

I've done everything I can think to get it stable but nothing seems to work. The last thing I tried was setting a few air brakes at the back of the plane and hyper editing it into a low orbit. I've checked CoM and CoL in the VAB with the fuel full, empty and everywhere in between. I've even taken off with the forward fuel tanks empty with no issues. I've actually just installed procedural wings to try to fix it with no change. Any help would be appreciated but I'm beginning to think something isn't right with FAR.

[WhiteKnuckle]

To really help you we're going to need a screenshot of your craft with the CoM and CoL shown, and a FAR stability derivatives page at the altitude and speed your craft wants to flip.

[Hodo]

Looking for a bit of advice into getting an SSTO stable on re-entry. Seems lately all of my craft tend to want to fly backwards in the upper atmosphere. Once they hit thicker atmo, they'll flip back the right way. Problem is the plane gets ripped apart when this happens. I'm using FAR and procedural wings among a number of other mods.

I've done everything I can think to get it stable but nothing seems to work. The last thing I tried was setting a few air brakes at the back of the plane and hyper editing it into a low orbit. I've checked CoM and CoL in the VAB with the fuel full, empty and everywhere in between. I've even taken off with the forward fuel tanks empty with no issues. I've actually just installed procedural wings to try to fix it with no change. Any help would be appreciated but I'm beginning to think something isn't right with FAR.

Do you have pictures of the craft?

It sounds like you are coming in at to steep of an angle and your CoM is shifting behind your CoL. At what AP are you when you start your deorbital burn? How low are you when this happens?

Generally I dont come in any higher than 100km AP, and set my PE to around 5km above the KSC for my target. I try and stay around 20-30km till I get my speed down below mach 5, then I slowly drop down below 20km. I highly suggest airbrakes on the aircraft or spoilers to help slow yourself down.

This is generally my re-entry and launch profile for getting into orbit with FAR.

This one is a good picture of one of my craft FB-1 reaching lower atmosphere. Notice the speed it is going and the angle it is traveling.

This is my F/A-106A Thundergod, on its re-entry burn. Again note its angle and its altitude. You can even see the airbrakes on the craft deployed.

I run FAR and DRE so my re-entry burn is a bit more of a challenge but I also have been building SSTO space planes since 2013, and have a great deal of experience with them.

[AFSATCOM]

What I don't get is how a plane can go from stable in one direction to stable in the complete opposite direction. The CoM isn't changing much if at all and it's pretty far in front of the CoL.

[Hodo]

What I don't get is how a plane can go from stable in one direction to stable in the complete opposite direction. The CoM isn't changing much if at all and it's pretty far in front of the CoL.

Your problems are many on that craft....

First issue I see is your CoL is WAY to far behind your CoM. You ideally want it just behind it, but not super close. Next your numbers are bad, bad, bad. To much red.

Ideally you want something closer to this.

Notice how the numbers are all green. This is good, red is bad. You can hover over them to see what they mean in game. But for a stable aircraft or SSTO you want something that can stay in the green across a wide variety of speeds and altitudes. I generally sacrifice high speed maneuverability on my larger cargo hauling SSTOs in favor of low speed stability and gentleness of flight. This makes them easy to land and fly normally, but a bit of a pig at hypersonic speeds just before it gets to orbital velocities.

Edited February 7, 2015 by Hodo

[AFSATCOM]

The main question here is how can the plane be stable in reverse with the CoL that far behind the CoM. It seems completely wrong to me. I get that the air is thinner up there but it shouldn't make the plane stable in reverse just make the surfaces have less impact on control. The derivatives are all green below 30km. By placing the CoL that far behind the CoM which was done intentionally, I've essentially made a dart. How can a dart fly stable backwards?

- - - Updated - - -

I should mention that I hit higher speeds on the way up with no issues.

[Hodo]

The main question here is how can the plane be stable in reverse with the CoL that far behind the CoM. It seems completely wrong to me. I get that the air is thinner up there but it shouldn't make the plane stable in reverse just make the surfaces have less impact on control. The derivatives are all green below 30km. By placing the CoL that far behind the CoM which was done intentionally, I've essentially made a dart. How can a dart fly stable backwards?

- - - Updated - - -

I should mention that I hit higher speeds on the way up with no issues.

The reason is pretty simple, in FAR your CoL isnt a static thing. The faster you go your CoL will shift like real life. At supersonic speeds your CoL will shift back on the wings of the craft, moving your overall CoL back on the aircraft. This becomes more exaggerated at hypersonic speeds. And if you have the wrong shape of wing it could cause the craft to stop generating lift where you expect it to altogether. If you look at most of my designs you will a common design feature, same for Wanderfound, the other SSTO "expert" here on the forums. Between the two of us we have put pretty much anything with wings into orbit and on other planets in KSP.

Another issue I just remembered is if you end up "overflying" the aircraft, or getting your AoA to far from your direction of travel you will cause the craft to generate lift in different ways. Some wings may stall and lose lift, others may start to turn into the main wings shifting your CoL drastically in another direction.

Again FAR is not easy, but it isnt that hard once you get a grasp on it. The best bit of advice I can give is start small, and work your way up from there. After all the first supersonic aircraft wasn't the Concord, it was the Bell X-1 which was tiny by comparison.