SSTOs Flip over

[Sovnheim]

Hi all,

So I have decided to give a go at the SSTO construction. I am more or less able to build a plane that takes off and lands safely (by Kerbal standards).

The one I have built (unmanned) works just fine until the Toroidal rocket engines kick in. At which point, the heavy fuel burn rate empties some tanks, imbalances my plane and it flips over.

I made a small diagram explaining how physics interact with my SSTO :

(╯°□°）╯︵ >==>

I have read on the Internets that it was an issue of fuel imbalance in the craft, and all I had to do was transfer fuel to reestablish balance. Something that I have been able to do, thus stabilizing the craft and thus saving the day. Yet as soon as I start the rocket engines the fuel is used again, all hell breaks loose and I never reach space.

How can I tackle this issue ? What am I missing here ?

Thanks a lot for your help

Edited March 25, 2015 by Sovnheim

[impyre]

Using a fuel-balancing mod works very well; however, a little design consideration will ensure even fuel consumption also. Just ensure that each engine has its own fuel supply (with the same amount of fuel) and that tanks are placed in symmetry. If you must, you can use decouplers to strictly forbid fuel flow (and many parts allow you to disable crossfeed, which will accomplish the same goal). Fuel lines can help as well (to a point anyway). Uneven thrust limiter settings will cause uneven consumption as well.

[jake9039]

I would recomend pulling the fuel into a different source, than pull it to the front of craft. I do that but with radial engine pods, and they drain first, then back to front.

[capi3101]

My first thought is to (if you're not adverse to mods) get RCS Build Aid. It will show you where your plane's center of mass will be once it's out of fuel, which is information you can use to ensure that the plane will remain "balanced" as it flies. From what you're describing it's likely that the CoM is shifting aft of the CoL after you've made the switch to rockets, which is what's causing your plane to flip over. I might also recommend switching out that aerospike for something less massive; you fly a plane correctly and you won't need all that much rocket thrust to make space.

I also generally give the advice in this link to most spaceplane rookies. Keptin's guide is universally good, and DocMoriarty can help you get into space every time.

[Deutherius]

Or, you could (and should) do some SPH analytics. Mod free, good practice.

Switch on the center of mass and center of lift markers and start draining fuel tanks (right click, drag the amount of fuel to zero). I suspect that at some point your CoM gets way behind CoL, which can cause what you described. If you make it so the CoM is behind CoL at all times, no fuel drain will cause your plane to flip. (Well, unless your fuel flow is really creative and you managed to make it vertically/horizontally asymmetrical, in which case I request pictures for inspiration and lols)

Also, posting a picture of the craft and/or craft download is generally a good practice, helps us spot what's wrong instead of just guessing

[Mister Dilsby]

In addition to CoM being on the wrong axial side of CoL, you could also flip because the center of thrust of your rockets is not lined up certically with CoM. I have had this happen when putting the rockets on the wings. The ship is stable at low altitude, but then when it's high enough that atmospheric control surfaces don't work anymore it flips. I eventually debugged it by (1) using the tools in the spaceplane hangar, especially the part shifting 'gizmos' with snap ON, and (2) hyperediting to deep space and confirming that the rockets could indeed thrust in a straight line.

[Pecan]

...What am I missing here ?

I think you are missing the fact that SSTO means Single Stage To Orbit - start simple, with a rocket. Also remember that 1.0 will almost certainly make all your designs obsolete.

[Sovnheim]

Or, you could (and should) do some SPH analytics. Mod free, good practice.

Switch on the center of mass and center of lift markers and start draining fuel tanks (right click, drag the amount of fuel to zero). I suspect that at some point your CoM gets way behind CoL, which can cause what you described. If you make it so the CoM is behind CoL at all times, no fuel drain will cause your plane to flip. (Well, unless your fuel flow is really creative and you managed to make it vertically/horizontally asymmetrical, in which case I request pictures for inspiration and lols)

Also, posting a picture of the craft and/or craft download is generally a good practice, helps us spot what's wrong instead of just guessing

Thanks, that's what I did

I got bored of tweaking a broken craft so I restarted from scratch, added the parts without fuel far in front to create equilibrium, and tested it at various levels. Thanks a lot for the input - I didn't add any mods but I tested the plane's CoM with empty tanks and I made it work.

Thanks to you guys, I now have a small unmanned shuttle that can get a small payload on a 120k orbit, with adequate control during the entire operation - I wanted to send some pictures but I am in class so it's rather complicated right now.

I'm still a bit short on fuel and I frequently have to calculate additional orbits to make my rendez-vous without running out of gas, but it works.

Also regarding the upcoming 1.0 obsolescence - I don't care The challenge in itself is rewarding enough.

I've seen some inspiring designs on the forum so I look forward to new challenges - larger payloads, a SSTM or, maybe a spaceplane for interplanetar missions.

That should prove interesting.

Thanks again.

[Pecan]

...Also regarding the upcoming 1.0 obsolescence - I don't care The challenge in itself is rewarding enough...

:-) That's the attitude.

While the designs will be obsolete and some of the specific design-criteria we all (think we) learnt will be useless the approach to designing will be valuable and fun :-)

So - is your shuttle a rocket?

[Deutherius]

Thanks, that's what I did

I got bored of tweaking a broken craft so I restarted from scratch, added the parts without fuel far in front to create equilibrium, and tested it at various levels. Thanks a lot for the input - I didn't add any mods but I tested the plane's CoM with empty tanks and I made it work.

Thanks to you guys, I now have a small unmanned shuttle that can get a small payload on a 120k orbit, with adequate control during the entire operation - I wanted to send some pictures but I am in class so it's rather complicated right now.

I'm still a bit short on fuel and I frequently have to calculate additional orbits to make my rendez-vous without running out of gas, but it works.

Also regarding the upcoming 1.0 obsolescence - I don't care The challenge in itself is rewarding enough.

I've seen some inspiring designs on the forum so I look forward to new challenges - larger payloads, a SSTM or, maybe a spaceplane for interplanetar missions.

That should prove interesting.

Thanks again.

Congratulations!

I also forgot to mention, and you have probably figured it out already - engines always drain fuel from the furthest source first. That allows rockets to drain their boosters first (if they are connected to the core), and generally causes the front fuel in spaceplanes to be drained first - something to consider when fiddling with the CoM in SPH

About the fuel problems, there's (almost) always room for improvement. Flight profile, choosing the right vac. engines, maximizing turbojet time/performance, allocating correct amounts of fuel for different engines, improving orbital manuevers... If you need/want any help with the design/flying, feel free to ask