Space plane/ shuttle flips backwards during re entry?

[Billyk88]

How do u fix this and if u have pics of ships with the weight thrust and lift shown plz upload them. Any advice is appreciated!

 

Thank you

Edited March 28, 2017 by Billyk88

[Richy teh space man]

Sounds like the centre of mass is falling behind the centre of lift. Either pump fuel forwards or redesign it so it can't happen. Empty fuel tanks in designer to see how it will work.

[LordKael]

Also, check your drag. You may have more drag on the nose than the tail, which would cause the nose to reenter more slowly than the tail, resulting in the whole vehicle flipping around to reenter backwards.

[Hodo]

First question are you running mods?

FAR or anything like that?

And the others have pretty much hit the nail on the head with the problems most common on re-entry designs.

 

A useful mod to download is RCSBuild aid.  It gives you a second indicator for dry CoM, which tells you where your CoM is when tanks are empty.

[Gargamel]

Worst case scenario, you could add a rear facing pod from which your kerbals can transfer to and control the ship from there, giving your navball the correct orientation.  You can then add a second set of landing gear, facing the other way so it rolls correctly when landing. 

But that's assuming you will be gliding into land, not under powered flight.  You may have to use some of those nifty reverse thrust engines in that case.  

Edited March 28, 2017 by gargamel

[Snark]

Moving to Gameplay Questions.

52 minutes ago, Billyk88 said:

How do u fix this and if u have pics of ships with the weight thrust and lift shown plz upload them. Any advice is appreciated!

Here's the problem:

1. Lots and lots of players want to make a Shuttle lookalike.
2. So KSP provides precisely the parts you need to slap together something that looks exactly like the shuttle.  Mk3 cockpit, Mk3 fuselage parts, Big-S delta wings and tail fin, Vector engines.
3. Except that if you build something that looks like the US space shuttle out of those parts, it's horribly unstable aerodynamically and will flip on reentry.

So everyone has this problem.  I've lost count of how many times I've seen people run up against "why does my shuttle keep flipping".  The answer is this:

• Your ship is unstable (i.e. wants to fly tail-end first) because the CoM is in the back.
• Also, your ship is unsteerable, because the CoM is sitting right next to your control surfaces.

So, fix those two things and you should be good to go.

Here's someone with essentially the exact same problem as you.  Nice discussion, with practical answers. 

 

[Billyk88]

Thank you all i will try everything except i might give up on the look a like shuttle.

[Naito]

12 hours ago, Snark said:

 

Except that if you build something that looks like the US space shuttle out of those parts, it's horribly unstable aerodynamically and will flip on reentry

 

Why do so many people think this? Look up shuttles on KerbalX like Inigmas STS, or my Shuttle Resolution. No tricks, they look pretty authentic, and they launch, orbit and even re-enter and fly well.

Its challenging, but that's the point!

Edited March 29, 2017 by Naito
Gawd I misspelled my own shuttle lol

[Alshain]

12 hours ago, Snark said:

1. Except that if you build something that looks like the US space shuttle out of those parts, it's horribly unstable aerodynamically and will flip on reentry.

I have to disagree here... mostly because I've done it.  You can build a shuttle look-alike, you just can't slap it together quickly and have it work.  Rocket launched glider planes are one of the hardest things to do in the game, they take a lot of time in the VAB and in testing.

 

That said, a lot of people think they are building a shuttle look-alike, but don't pay attention to small details (like exact wing position) which makes it not a shuttle look-alike and causes issues.  That thread you linked, he built a plane much bigger than a shuttle (it's cargo bay is huge in comparison) but kept the same sized wings.  That's why he had issues.  The small Mk3 cargo bay is about the size of the real shuttle's (in relative proportion).  If you are using the large one, your craft is a lot bigger.

Edited March 29, 2017 by Alshain

[Snark]

Certainly, it's by no means impossible.

But the "path of least resistance"-- i.e. slapping together a ship that looks just like the shuttle, in the simplest and most straightforward way-- tends not to work.  The problem is further exacerbated by the rather misleading nature of the CoM and CoL markers in the SPH. A simplistic shuttle design often puts the CoL in the back, behind the CoM... which is also in the back. That leads players astray, because they've heard the oft-repeated but misleading mantra "CoL behind CoM means stability." So they get really frustrated and confused: "Why does it keep flipping? My CoL is behind the CoM!" Add the difficulties with shifting CoM as fuel drains (another tendency of simplistic shuttle designs), and it's a recipe for failure.

Yes, you can build a shuttle that flies. But you practically have to trick KSP into making it work, and you really have to know what you're doing. Which is a pity, because the shuttle is such an iconic spacecraft that it's one of the first things that many new players want to rush out and build... and the presence of all those specialized shuttle-mimicking Mk3 parts can fool newbies into thinking it's easier than it is.

[Alshain]

I agree you have to know what you are doing, but I don't consider it tricking KSP into making it work.  I just consider it advance craft building.

As for the CoM behind CoL means stability,  I think the context of the word "stable" is missing.  Stability is the opposite of maneuverability.  Neither are good or bad, but too much of either can be very bad.  Stability does not mean your plane won't flip.  Having the CoL forward of the CoM has too much maneuverability, making the slightest tap on the controls go nuts, but having it behind the CoM doesn't guarantee it will fly well, it's just a necessity to fly well.

Edited March 29, 2017 by Alshain