SSTO cant maintain lift at 20k altitude

[CmdrRimmer]

I've built several iterations of a 20-30 ton SSTO plane. Currently it is about 24 tones, 2 basic jets, 2 spikes and. 1 Nuclear engine.

I've got to the point I can fly it up to about 20k and 1200-1300 m/s but I then cannot maintain altitude. I end up in a very high nose up trying to keep the climb rate positive.

Is the answer more wings and less weight. Or could it simply be the layout ? I've had basically the same issue with 29 tonnes and 4 jets. I'm holding myself to 8 intakes per jet engine so far.

I'll post the vehicle file when I get back to my pc if anyone could see what I am doing wrong.

Thanks

CmdrRimmer

[antbin]

Is the answer more wings

Can't hurt. If you're using Basic Jets though, your answer's more simple: They lose power around 20km. Use Turbojets with at least 2 intakes / engine.

[Dispatcher]

Please forgive me for stating some obvious things. No matter how may intakes you have for the jets, sooner or later you simply don't have any more oxidizer for the jets. And higher up wing lift becomes less significant, and finally only dead mass. But your question rightly deals with mass or lift or efficient design. Are you maxing the throttle or holding back? Regardless, while basic jets are more fuel efficient and less massive, they don't have the thrust or iSP of the turbojet. As such they can deal with a little more payload than the basic. You might want to switch out to turbos. Besides, their iSP actually increases with altitude while there is oxidizer available; the basic starts whining down well before that. You might want to try that switch out before modifying your craft further.

Edited September 2, 2013 by Dispatcher
Grammar.

[Brevillance]

You may want to consider adding a bit more surface for lift, although at that altitude and speed you should have more then enough with the rockets to punch out into space. My Kerbal rescue plane which is only 4 jet engines can maintain altitude at 20k quite easily, make sure the fuel is distributing equally and that your center of lift compensates for a changing center of mass. Generally at 20k you should be able to pitch up quite a lot and keep going, although you'll probably want the turbo jet engines after about 14k and 600m/s.

My rescue plane is only jet engines as its inter-kerbin (not an SSTO) so i go for ISP since it doesnt need to cruse at 1k m/s or 20k+ altitude, but if your building a SSTO you should ditch the normal jet engines for turbos, they output better power and can give you more kick at higher altitudes and faster speeds. Try adding a wider wing span, and more lift (you can lay down flaps flat onto your wings/under your wings to give bonus lift, if your having problems, slap a few extra flaps near the back to give some extra lift).

Heres a pic of my Inter-kerbin design.

[Zaeo]

Not only that, but adding a tilt on your wings downwards at the tips will actually improve lift at high altitudes.

[Tiron]

Not only that, but adding a tilt on your wings downwards at the tips will actually improve lift at high altitudes.

...What exactly do you mean by that? Are you referring to Anhedral?

Or rotating the wings to change the Angle of Attack? (and if so, which way?)

[KerbMav]

Word is new to me, but the picture looks right.

Meaning: Dont attach the wings parallel to the thrustvektor of your engines, but rotate them a bit so the front of the wings is higher.

[bsalis]

Hard to say without a picture.

Possibly just need more wings.

[Stochasty]

Two turbojets (you are using turbos rather than basic jets, right?) is plenty for a craft in the 20-30 ton range, so your issue is that you need more wing surface.

[Spyritdragon]

Go back to SPH, and bring up your Centre of Mass and Centre of Lift indicators - compare the size of the arrows. My guess is that, indeed, you need more lift.

Two turbojets (you are using turbos rather than basic jets, right?) is plenty for a craft in the 20-30 ton range, so your issue is that you need more wing surface.

I use slightly more power. I like having the leftover.

[Hodo]

Two things.

Around 1200-1300m/s will force your nose down, forcing SAS/ASAS to go higher on the nose pitch. The easiest way to overcome this, is to pitch down slowly and maintain a higher speed. Once your projected flight indicator shows you are nearly flying level, you can then slowly increase your nose angle back up and you will begin to climb again.

The other thing is, you may not be getting enough airflow to the engines at this point because of altitude. I am not sure what mods you are running, but the FAR mod is by far the most useful for building a SSTO.

[Tex_NL]

Looking at the picture you're hauling way more jetfuel then you need. I guesstimate you'll need less than a quarter of what you currently have. Getting all that 'useless' weight up to orbit will cost a lot of fuel, weighing you down even more.

[Hodo]

Tex_NL what picture?

[Tex_NL]

Tex_NL what picture?

This one:

But I just realized this is not the plane mentioned by CmdrRimmer.