Nose diving problem with SSTO & planes

[ImpulseSpaceCenter]

Hi,

After I have managed to put my first simple SSTO into orbit, I am now trying to get a more usefull one (with a docking port) into orbit.

I have a problem that when my plane reach 15km high (some of the design it did happen at 20km), my plane just nose dive and loose all of it's speed!

I don't think it's my CoM as it should move backward. I have tried to add more control surface without success. more wings for lift without success. avionotic or ASAS fail!

It's not the speed as it was doing it at 1500+ m/s with some design and just about 500m/s on this design!

Here is a screen of the plane in the hangar

Also this may help,

Usually i have about half of the intake off, and when I reach 10-12km, I open them (as I reach low intake) and my plane just have a small nose dive (that I can get back on control).

ANSWER: CoM a bit more forward for more stability and more importantly, CoT align with CoM!

Edited May 28, 2013 by ImpulseSpaceCenter
question answered

[MarkusA380]

I think that is because you have too low atm, your plane can't be controlled with rudders, of course. I would add RCS so you can control it in low atm or vacuum too.

[daveboy2000]

Might be your center of mass shifting behind your center of lift as fuel gets used up, move some weight to the front and put the wings further back.

[RoboRay]

Move your CoM forward, away from the CoL. CoM is going to shift backwards as you burn off jet fuel, and you're already critically close to neutral stability. Burning off the rocket fuel will probably bring it back forward a little, but not much, and you're not making it far enough for that to help, anyway.

And, just so you know, that docking port will probably rip off when docking. I've never gotten the shielded ports to work well when radially attached.

An Avionics Package instead of the ASAS might also be more useful.

[ImpulseSpaceCenter]

Move your CoM forward, away from the CoL. CoM is going to shift backwards as you burn off jet fuel, and you're already critically close to neutral stability. Burning off the rocket fuel will probably bring it back forward a little, but not much, and you're not making it far enough for that to help, anyway.

And, just so you know, that docking port will probably rip off when docking. I've never gotten the shielded ports to work well when radially attached.

An Avionics Package instead of the ASAS might also be more useful.

Never tried the shielded ports! and I thought a space plane are the best reasons to try them! I'll still be able to switch to a normal one if needed!

I will try for the CoM, if it move backward, behind the CoL in my current case, shouldn't the noze go up instead of down?

[RoboRay]

I will try for the CoM, if it move backward, behind the CoL in my current case, shouldn't the noze go up instead of down?

No, not unless it's extreme.

You can tilt your elevator surfaces on the back of the wings upward one notch to give the nose a natural upward deflection, and Avionics or ASAS (or manual trim) will hold it there nicely.

The reason CoL must stay behind the CoM is because lift is also drag. If you have more drag behind your CoM than in front of it, your craft will naturally "weather-vane" and keep the pointy end out front. This is positive stability.

If the majority of your drag is in front of the CoM, the craft will actively try to spin around backwards. This is negative stability, and is pretty much never desirable.

If the drag is perfectly centered on the CoM, your craft has no preferred orientation in respect to the airflow. A neutrally stable craft is extremely maneuverable and even harder to keep pointed in the direction you want to go, which is great for fighters and aerobatics, but not so good if you are trying to go space today.

On the other hand, if you put the CoM too far out in front of the center of drag, the craft can become so positively stable that it becomes difficult to make it turn. I find that putting the CoL and CoM markers just barely touching each other works well for my designs.

Never tried the shielded ports! and I thought a space plane are the best reasons to try them! I'll still be able to switch to a normal one if needed!

It may work better now... that was back on 0.18 when I tried putting them on square trusses for radial mounting. As soon as I'd dock, they'd bend and tear off. Go ahead and try it, but if it doesn't work you'll know how to fix it.

Edited May 28, 2013 by RoboRay

[ImpulseSpaceCenter]

Thanks RoboRay for the explaination, I would really like a place where I can learn all that! the wiki barely explain anything! And I don't know much about aerodynamic (except for kiting, pull the bar, more drag-> more lift -> jump higher).

I will see what I can do to move that CoM, may be switching some tanks around!

Would it be a difference between shifting the CoM foreward or shifting the CoL backward?

Aviononic-> I would love one that is not nose cone! where could I put it?

ASAS -> That's ok but I would like to be in the plane as it move so fast when I try to bring it up or down

manual trim -> I have played a bit with it but I feel like it need too much trim at the moment!

[pinolallo]

Thrust my friend. Your nose are pitching down and your ship seems balanced? your thrust vector is above the complessive mass center.

Do not try to cover the unbalance using a sas/asas, before install than modules you must find the right balance of your ship at full throttle: same weight an position up as below your thrust vector.

[ImpulseSpaceCenter]

Thrust my friend. Your nose are pitching down and your ship seems balanced? your thrust vector is above the complessive mass center.

Do not try to cover the unbalance using a sas/asas, before install than modules you must find the right balance of your ship at full throttle: same weight an position up as below your thrust vector.

I need to try but I went for a quick check in game and I guess we have a winner!

Last time I had that problem (when trying to make a VTOL), it did happen at take off not that far long after!

I'll try to fix that and try again!

also Is there a way to geet better intake without having to stack the ram or put lof of ineficient radial?, I don't really want my ship to be full of 1m tube for air intake!

[ImpulseSpaceCenter]

ok I tried and it works now!

I do start with the mose pointing down a bit (the trim is 1-2 bar [not sure how many degree is a bar, seem small for 5°]) and once I arrive a some point the noise is pointing up a bit (same 1-2 bar of trim).

I guess now it's a matter of center of mass moving.

I do need however to get the aerospike again and remove the 909 as I do loose my intake at 28km high. (I would love to have a bit more so I can reach those 1500m/s)

[RoboRay]

Thanks RoboRay for the explaination, I would really like a place where I can learn all that! the wiki barely explain anything!

My flight school was good for something.

I will see what I can do to move that CoM, may be switching some tanks around!

Would it be a difference between shifting the CoM foreward or shifting the CoL backward?

Either would work. Keep in mind that the effectiveness of your flight control surfaces increases as they get further away from the CoM also (more leverage around the fulcrum). So, shifting the CoM forward would make your rudder and elevator surfaces work better, too.

Aviononic-> I would love one that is not nose cone! where could I put it?

You could put it point backwards behind a fuel tank. Better yet, stick it anywhere if you use the little square truss to make an attachment point and jam the avionics cone on top of it.

It's a variant of the ASAS, so it won't do anything if the regular ASAS is still installed, though.

Edited May 29, 2013 by RoboRay

[ImpulseSpaceCenter]

My flight school was good for something.

You just have to put it in practice and make a KSP flight university

My flight school was good for something.

Either would work. Keep in mind that the effectiveness of your flight control surfaces increases as they get further away from the CoM also (more leverage around the fulcrum). So, shifting the CoM forward would make your rudder and elevator surfaces work better, too.

It's good to know and it make sense.

I like how you put the avionic! it doesn't look too cheaty with the clipping and it does have a nice appeal.