Why does my plane dive at high altitude

[Cerberus738]

Yes, this. For me it seems much more noticeable at 32km. At that point, wings don't matter and it's all about having a bit of thrust fed by 3 or so intakes per engine. I just completed a challenge with a wingless SSTO VTOL that circumnavigated Kerbin (below 68km) on about 72 units of fuel including a VAB landing. Most of that fuel was used up getting to altitude.

72 units of fuel seems impressively small! I think I need to try that. I have a vtol with 300 units and it can only make it half way around. Me thinks lack of intakes are to blame. It can only run engines up to 25000 but gets up to 1800m/s

[arq]

You can easily cruise above 30km with airhogging. I made a plane that could cruise above 40km. The wings were actually quite small, but I had pitched them up about 20deg from horizontal, so that I could face forward to max air intake while still producing upward lift. It had 5 intakes and 1 turbojet.

Above 40km, air is so thin (lo drag) and speed is so high (low gravity losses) that you can circumnavigate Kerbin on fumes. The 'plane' I had could fly around Kerbin close to 3 times with the relatively small reserves it had. It actually took more fuel to get to 40km than to circle the planet.

But this was mostly an experiment to screw around with airhogging. I avoid it most of the time. What I wanted to get at was you don't need massive wings, necessarily.

[MaverickSawyer]

Use at least 2 ram intakes per turbojet to get up to 35k or higher. I barely touch my OMS reserves to make orbit with a twin Ram, single Turbojet design. Wings are respectably large, but not outlandishly. It actually looks like something you would have seen on the drawing boards at the major aerospace companies back in the '60s.

[barrenwaste]

I can see a couple of things here.

At high altitudes more engines is less good than few engines. The more jets you have the more air you need and air is scarce, even when airhogging. What happens is that the jets don't share the air intake equally, that can cause your plane to dip and dive in strange and often uncontrollable ways. I would suggest pairing that thing down to a single engine for your first builds.

I would also say that you still have far to much wing and and control area. Not only does an overabundance of wing produce more drag, it makes the craft structurally unsound. While flying, crank the physical fast forward up to three or four, does the plane eat itself instantly? If so, time to go back to the drawing board because it's likely that parts you can't see are not working properly.

Finally, I understand this platform isn't designed to go to space. That's fine. But it's best to design the plane as if you were going to space with it. That's because if you build it well, it's going to blast out of the atmosphere and make a couple of orbits before being dragged back down by gravity. Make sure you have enough electrics to see you through the time you aren't powered up and enough air intakes to gag a pack of pachyderms. Ram air intakes seem to work best, but all of them can be useful. To get above 40k I tend to use 6 or more intakes per jet, and if you use enough you can keep powered flight right up to, and slightly beyond, 60k.

Finally, if you desperately need more than one jet, try dual mounting a jet engine. Take a small square strut. Mount it backwards in the center of the tail of the plane. Make sure it doesn't auto-snap into place. Now you can mount a jet engine on both sides of the small strut, allowing you to get two engines in one area without using cheats. It takes practice, and you need to zoom in real close, often looking inside existing parts of the plane, but it's easy enough once you get the hang of it. This will get you the added power you need without asymetrical flame-outs that send you into flat spins and dives.

[Cerberus738]

Hey Claw!!!

I made a fun little "plane". it has no wings.... so i dunno if its still a plane ha ha ha.

I set up a powered orbit at 53km. Just enough air to get 1kn of thrust from the engine. Made 1 full orbit and on the second orbit attempted a landing at KSC.

Totally botched the landing sequence and ended up waaaaay east of KSC.... no save to go back to... hopefully we make it to land?!

Uh... Nedming? Please sit up straight in your chair.... It's going to be a bumpy landing... Jeeez. lazy kerbals.

So Nedming just stayed relaxed in his lazy boy command chair the whole way to the ground! It was actually pretty tricky. I use the navball a lot and for this landing it was pretty much useless. you should all be proud of me!!!

[MaverickSawyer]

Now THAT is ballsy. I like it!

[Claw]

Hey Claw!!!

I made a fun little "plane". it has no wings.... so i dunno if its still a plane ha ha ha.

I set up a powered orbit at 53km. Just enough air to get 1kn of thrust from the engine. Made 1 full orbit and on the second orbit attempted a landing at KSC.

Totally botched the landing sequence and ended up waaaaay east of KSC.... no save to go back to... hopefully we make it to land?!

Uh... Nedming? Please sit up straight in your chair.... It's going to be a bumpy landing... Jeeez. lazy kerbals.

So Nedming just stayed relaxed in his lazy boy command chair the whole way to the ground! It was actually pretty tricky. I use the navball a lot and for this landing it was pretty much useless. you should all be proud of me!!!

Haha, awesome!

I find for VTOLs it helps to have a probe core on top, so you can orient the navball to point straight up for takeoff/landing. And if you're up at 50km, I find I need to cut off the engines for reentry when I'm about straight south of KSC2.

Edit: Here was mine that I was referring to.

Edited February 18, 2014 by Claw

[barrenwaste]

Nice craft there. Don't need a probe core, though, if you make your vtol with a cockpit of some kind. There are several vtol designs about the size of one of the ram air nacelles on the above ship, here is a link I to one I made.

http://forum.kerbalspaceprogram.com/threads/66123-KC-T3-Runabout-VTOL-SSTO

[Claw]

Yeah, you just need any probe core, cockpit, or seat oriented the right way. Unfortunately orienting external seats to get the straight up nav focus makes the kerbal face straight up, which looks odd.

Not that any of this is on topic.

[Cerberus738]

Ha ha.. yeah an oversight until i got to the landing part. it launched fine... ha ha ha ha. didn't even think about it until i couldn't tell where my vector was coming down. but success!

So i just built the Hamster Mk II. 4.56t at launch. 37 parts.

took some inspiration from your design barenwaste.

do i get in trouble for highjacking my own thread???

[Claw]

I'm still a big fan of small craft. I like the challenge of having to pick and packing on all the stuff I need for a trip without just adding moar.

[Dimetime35c]

Just agreeing with what everyone else is saying. As you get higher the air gets to thin to hold your plane up. Its the same thing in real life. That's why high altitude planes have REALLY long wings. It helps to reduce the amount of lift they lose at high altitude. You could also enable the RCS to try and help hold the climb.

[Cerberus738]

I never thought of using RCS. If i just put a couple of ports on the underside of the nose. i wonder if that would be enough.

[MaverickSawyer]

If you're not using Deadly Reentry, it will work just fine. DRE will cause the ports to explode on your way back down if they're under the nose.

[Claw]

Let me know how the RCS works out. My suspicion is that you would be better off adding more SAS units before RCS because of the fuel, but I've never tried it the RCS way.

[m4rt14n]

If you had to put RCS to fly usually its a sign you need a better design. I'm pretty sure this is a Moar lift issue. Try putting more wings before going RCS

[barrenwaste]

Yeah, a sas unit, especially in atmo, is most often the way to go. I find that rcs is only worth the weight for docking maneuvers. Have you tried putting a control surface at the nose? Canards can work well if you can balance the com and col. If canards are to much you can always attach one of the small control surfaces to the inside of your cockpit. It won't add as much lif, but will provide some control while remaining concealed.

[MaverickSawyer]

Also, the CoM/CoL relationship can be altered on the fly by pumping fuel around the aircraft. I use the trick all the time to trim out my SSTO during ascent and entry.