Spaceplane Issues

[FREEFALL1984]

Hi all, I have been building conventional rockets for a pretty long time and decided I should try to build an SSTO spaceplane,

I have encountered a recurring issue with my designs which I cannot for the life of me overcome, After reaching about 20km and flattening out my flight path where I can gain speed quite easily with no notable stability issues, I happily reach 1300m/s. At this point my intake air is about 0.2 (not 0.02) thanks to a bit of light intake spamming (ram intakes only so no ugly 50 radial intakes attached to every possible surface) At this point my nose starts to climb and drift to the left or right and I end up in a flat spin which is often so aggressive that I drop to less than 5000m before being able to recover (if I can recover at all) Disabling all engines does nothing to help and my rockets are not yet fired.

Also my designs tend to have fuel efficiency at their heart so I use two small LV909 engines and a FL-T800 tanks meaning there is a slightly inevitable speed drop when trying to switch engines which I may switch out to aerospikes due to the ISP being the same but the huge step up in thrust.

Ultimately I would like to be able to build an SSTLAB plane (single stage to laythe and back) thanks to the newly overpowered ion engines. But to do this I need at least enough rocket fuel for 2 transfers to orbit (one on kerbin an one on laythe) and enough fuel to be able to explore laythe for a reasonable time (perhaps a half circumnavigation) and I also need enough xenon and solar panels to allow for 2 interplanetary transfers, all while carrying around 2 tons of science apparatus. If this is all too big of an ask I could send a forward command post out to laythe with fuel for the return to orbit and return to kerbin although I would rather just do it in one hop. Any advice you could give would be much appreciated.

[Rodyle]

this point my nose starts to climb

Typically a symptom of the weight balance being off. Check if the centre of mass is in behind the centre of lift when the craft is emptied of all fuel.

[bsalis]

Some pics help.

However, from what you say, it start to pull left and right as you run low on IntakeAir? First guess would be an incipient flameout. Try throttling back and see if that makes it stop. If that is the problem, you just need to adjust your ascent profile and throttle.

Second guess would be that you have spammed all your intakes at the front of the craft. These have a lot of drag, possibly moving the CoD in front of the CoM. As the craft noses a little left or right, the drag will pull it more in that direction. I have noticed this is worse for a craft that is very wide but not long (ie. like a flying wing).

Third guess is that old shifting CoM chestnut. As the fuel drains, the CoM drifts behind the CoL. This will cause the craft to become uncontrollable. It may not be noticed until you reach the thinner atmo since the control authority of the control surfaces should keep it flying where SAS wants it to in the thicker atmo.

In any case, some more tailplane might help keep it flying straight.

[FREEFALL1984]

Second guess would be that you have spammed all your intakes at the front of the craft. These have a lot of drag, possibly moving the CoD in front of the CoM. As the craft noses a little left or right, the drag will pull it more in that direction. I have noticed this is worse for a craft that is very wide but not long (ie. like a flying wing)

This sounds the most plausible since my designs tend to rely on ram intakes and engine nacelle for almost all of the intake supplies.

unfortunately I cannot post pictures just yet, I'm currently at work, most of my designs tend to be as long as 1 FL-T800, a mk2 inline cockpit an engine nacelle and a ram intake, at the longest, and are a total of 5 tanks wide, with generally 3 Delta wings on each of the the outermost tanks. but I have ram intakes on ALL tanks. Which could cause the symptoms.

[KerbMav]

Second guess would be that you have spammed all your intakes at the front of the craft. These have a lot of drag, possibly moving the CoD in front of the CoM. As the craft noses a little left or right, the drag will pull it more in that direction. I have noticed this is worse for a craft that is very wide but not long (ie. like a flying wing).

I ... this ... back to the lab ... erm, SPH!

[Claw]

bsalis brings up all great points. Also watch out for misfeeding fuel as that can cause your plane to go into a spin, but that sounds less likely if your plane spins both left and right.

If you are building a small plane, be wary of the CoM/CoL relationship. The small gear bay is massless in flight but not in the SPH. This makes the CoM marker in the SPH lie about its position, and it typically shows it too far forward. So if you have small CoM/CoL margins, the CoM might actually be behind the CoL.

If it turns out to be an intake drag problem, here is an idea on how to intake spam that isn't a complete eyesore.

[FREEFALL1984]

Thats some very pretty intake spam, I will be trying that as soon as I get home

[r4pt0r]

intake spam is so stupid

[Tidus Klein]

1 you say the nose pitches up at altitude.... this is one of two problems

one your cod as stated above is to far forward,

two your com is shifting behind your col, I have found RCS build aid to be invaluable to every thing I do as it adds a D COM or (dry COM) and (A COM) or Average COM markers.

2 the nose starts to drift from side to side.... well again this can be one or both of two or three problems

1 you are about to flame out,... left click on an engine and then ALT left click the other and look at there thrust if its not the same the craft will pull in the direction of less thrust.

2 you dont have enough tail plane to compensate you coarse corrections, I have found that trusting SAS to fly a plane is pretty much worse then giving jeb an all expenses payed trip to the rocket factory.

and then that brings me to 3, you may be over correcting, I know everyone loves there SAS but there is a time when SAS is just not the answer, especially in the higher atmosphere. so ether turn off SAS and use fine control (PRESS the caps lock button) or keep SAS on and Fine control on in order to keep SAS in check.

Hope all this helps

[FREEFALL1984]

I'm pretty sure im not overcorrecting, my sub orbital flights seem fine when below 1200m/s and between 20km and 30km, and my aircraft are stable enough to generally hold a heading without pitching up or down by any margin. but as soon as I hit that magic 1300m/s all hell breaks loose. Tonight Imma try using an independently fueled engine design with much more drag behind the center of mass, making it hopefully fly like a dart.

[KerbMav]

Any experience reports with the FAR flight control helper options?

The do not seem to do much whenever I try them.

[Claw]

I'm pretty sure im not overcorrecting, my sub orbital flights seem fine when below 1200m/s and between 20km and 30km, and my aircraft are stable enough to generally hold a heading without pitching up or down by any margin. but as soon as I hit that magic 1300m/s all hell breaks loose. Tonight Imma try using an independently fueled engine design with much more drag behind the center of mass, making it hopefully fly like a dart.

Pictures are good (which I know you said you'd get to), but if you want you can post the .craft. I find it easier to give specific advice with unusual problems like this.

[J.Random]

My guess: it's starting with loss of thrust on one of engines (check it with Alt-RMB next time) and then asymmetric drag kicks in. Try putting intakes closer to the rear end of your plane.

[Pecan]

...If it turns out to be an intake drag problem, here is an idea on how to intake spam that isn't a complete eyesore.

Curse you foul fiend, I am corrupted and undone by your evil clipping.

*cough* Sorry about that. Finally decided that that method is just so effective and good-looking that I had to use it on my 'infrastructure' crew shuttle. Went a bit mad too, so I have 10 or so on an I-beam.

[Claw]

Curse you foul fiend, I am corrupted and undone by your evil clipping.

*cough* Sorry about that. Finally decided that that method is just so effective and good-looking that I had to use it on my 'infrastructure' crew shuttle. Went a bit mad too, so I have 10 or so on an I-beam.

haha, sorry. At least you can do it with normal editor clipping, tends to cause less physics attacks. If you use ALT+F12, be warned.

I also find that for most applications, I really only need 3 intakes (6 at the most) per engine. Obviously you can get more air with more intakes, but it gets to where you have to double the number to get much of an improvement.

Edited April 22, 2014 by Claw

[FREEFALL1984]

Moving my main area of intakes behind the center of mass seems to work a treat, now my spaceplanes fly straight and true regardless of my speed and altitude, the only minor issue now is that I cant get a correct "looking" approach, I like the idea of lifting the nose and flaring through the upper atmosphere, but now my spaceplanes are so stable I cant seem to get this effect and they always seem to go pointy end first when travelling at re-entry speeds. Not that it matters in kerbins thick soupy atmosphere and I dont use DRE so not an issue Thanks for the tips guys

[undercoveryankee]

Moving my main area of intakes behind the center of mass seems to work a treat, now my spaceplanes fly straight and true regardless of my speed and altitude, the only minor issue now is that I cant get a correct "looking" approach, I like the idea of lifting the nose and flaring through the upper atmosphere, but now my spaceplanes are so stable I cant seem to get this effect and they always seem to go pointy end first when travelling at re-entry speeds. Not that it matters in kerbins thick soupy atmosphere and I dont use DRE so not an issue Thanks for the tips guys

If your aircraft is overstable, reduce the distance between the center of lift and the center of mass. Since the CoL varies with Mach number (at least in FAR; I haven't actually learned stock aerodynamics) and the CoM varies with fuel use, you can be as close as you can get in the hangar and still overstable in reentry condition. Try pumping fuel toward the rear of the aircraft.

More pitch authority can also help to push a stable plane into an attitude it would rather not stay in.