Noob's question on jet planes and jet plane physics.

[ArmchairPhysicist]

I'm going to start off with saying that although I've invested i large amount of time into this game, I still qualify as a noob.

Ive never legitimately performed a mission to anything farther than Minimus, and even Minimus missions are reasons to stand up and cheer if I manage to get my kerbals back safely on the same ship they left on (long story, I'll share if anyone cares).

My question is whether it really is impossible to get a spaceplane to orbital altitude before you get the super advanced jet engines? 

Yesterday I was toying around with a space plane that used panther turbofans to get to high altitude and then would attempt to boost into orbit with swivels, no luck, mostly because of the physics glitches that impede takeoff. Only one out of ten try's got off the runway. It's not due to my planes design, it's a solid ship. But for some reason when I engage my engines the ship careens to the left or right, at which time my ship's sas attempts to correct the turn, but only causes the plane to veer at exponential rates. This continues until the ship either turns off the runway and self destructs upon taking that two inch fall, or the craft completely flips and I end up with a streak of fire. 

Tl:DR

Can a ssto be built using parts up to supersonic flight?

Why does my plane come equipped with flight avoidance protocols?

 

[ArmchairPhysicist]

And to cover a few points, no my wheels are not angled.

Yes everything is perfectly symmetrical, I'm rather OCD when it comes to symmetry.

Yes, anything that could possibly bend or flex has been bolted down and strutted.

Yes I have the center of mass ahead of the center of lift. Found this requirement on the 3rd plane i built years ago because I got sick of the magical gymnastic space plane. 

Yes, control surfaces are adjusted to only control one aspect of the flight, my ailerons don't control yaw.

[Streetwind]

Welcome to the forums

Yes, you can build SSTOs easily with low tech. The earliest one takes just five science points.

You can also build spaceplanes with early tech - even Junos. But keep in mind that what you are doing at that point is basically building a rocket SSTO, bolting wings and engines to it, and then hoping that the airbreathing engines offset the fact that you bolted wings and engines to it. If you succeed, you have a vehicle that's potentially easier to recover than a straight rocket SSTO, but no more capable. In order to really unlock significant capability, you need panthers at minimum.

EDIT: To receive as much help as possible with your plane, please post screenshots. Expert players can usually spot most beginner mistakes with just a glance at a picture, but it's much more difficult for you to explain to us in text what you don't even know yourself is wrong.

Edited April 26, 2017 by Streetwind

[ArmchairPhysicist]

My plane uses panthers, four in fact (probably too many but it's a medium lift class craft). The problem I've encountered is assuming I got the damn thing to get airborne without suiciding, it can only get to about 25-30 thousand meters in the air before it can the panthers can no longer produce enough thrust to climb higher. 

This is where I would switch to rockets, but I only get a few seconds of burn, and it plummets down to air breathing altitude. No way to try again because it burned all the oxider. The space plane program is more of a fun experiment. Any success my space center has is with disposable multi stage rockets, not environmentally friendly, but cheep and effective.

I just wish I could solve the flight avoidance protocols. Having a space plane to do contracts would be nice.

[Champ]

You say you are a noob, but you can affirm the problem is not due to your plane's design ?

[bewing]

When things veer, either in the air or on the ground, it's because (as a general physics rule) there is too much drag at the front, and not enough at the back to maintain passive stability.

In the air, having CoM in front of CoL is necessary, but it is not sufficient. You also have to guess where your Center of Pressure is, and make sure that is in front of your CoL, too. CoP changes dynamically, so it's not possible to display in the editor with a ball.

On the ground, you have to pay attention to ground drag. Your wheels, as they touch the ground, generate drag. If you have too much drag in the front, and not enough in the back, your craft will veer (almost always to the left because reasons). The devs gave you access to the friction controls on the wheels for this reason. Each wheel that touches the ground forms a lever arm -- the drag on that wheel creates a torque around the CoM. So one quick and easy fix is to set the friction on the front wheel to .4 or less. Many people put their rear wheels almost at the CoM, and this reduces any effect of changing the friction of those wheels -- but you can try increasing the friction there and see if it also helps.

Beyond that, yes, all sorts of low-tech SSTO spaceplanes can be built. I built one last night that uses a pair of wheesleys, a pair of junos, and a poodle. However, as Streetwind implies, low tech spaceplanes have pathetic capabilities. The better the jet engines you use, the more they can lift.

 

 

Edited April 26, 2017 by bewing

[GluttonyReaper]

1 hour ago, ArmchairPhysicist said:

it can only get to about 25-30 thousand meters in the air before it can the panthers can no longer produce enough thrust to climb higher

This is actually pretty standard - even the highest tier jet engines can't go higher. What's more important is speed. The idea is that you cruise at around 25km until your speed stops decreasing, then fire up your rockets to get you to orbital velocity and altitude.

Not sure if you're already doing this, but for some reason I've always had better luck starting the Panthers in afterburner mode straight from takeoff. I don't know if it's efficient, but it gets you moving pretty fast.

In regards to your takeoff problems - are you sure you have enough air intakes? AFAIK, if you don't have enough KSP prioritizes some engines over others, which can cause asymmetric thrust. It's not always noticable once you're flying because you're moving faster, which feeds more air into the intakes.

[AeroGav]

They can be done but career mode with standard difficulty settings doesn't really reward re-use - it's mostly a grind to get the cash to max upgrade the science building.        I've done a few airplane only career modes as a self imposed challenge, and i've also done custom difficulty where the "Funds Penalties" (ie. building upgrade cost) and "Funds Rewards" (cash payouts for contract completion) were both reduced by 1/3.      This meant cost for vessel parts was actually a bigger expense than building upgrades, which made re-usable craft actually worth bothering with.

My best Panther SSTO is this one , which can do return trips to the Mun, and could give you a return trip from the surface of Duna with a bit of in-orbit refuelling -

https://kerbalx.com/AeroGav/Stretch-Ray

However, it also has a NERV engine, which is hardly a low tech part.

If you've only got Panthers and Terriers,  I'd recommend something like this to deliver a Kerbin SOI exploration vehicle to orbit.

https://kerbalx.com/AeroGav/Aero-Tug-1

(here it is hauling a science lab, but obviously you'd just replace that with an exploration vessel for payload) 

Once the exploration vessel is in orbit ,  dock panther/terrier spaceplanes with it to exchange crew/transfer science and refuel the thing for another trip to Mun and Minmus.   You'll soon have enough science for NERV, and then you can really start going places.

Share your craft file on here we can help diagnose it.

[Jarin]

4 hours ago, ArmchairPhysicist said:

My plane uses panthers, four in fact (probably too many but it's a medium lift class craft). The problem I've encountered is assuming I got the damn thing to get airborne without suiciding, it can only get to about 25-30 thousand meters in the air before it can the panthers can no longer produce enough thrust to climb higher. 

This is where I would switch to rockets, but I only get a few seconds of burn, and it plummets down to air breathing altitude. No way to try again because it burned all the oxider. The space plane program is more of a fun experiment. Any success my space center has is with disposable multi stage rockets, not environmentally friendly, but cheep and effective.

I just wish I could solve the flight avoidance protocols. Having a space plane to do contracts would be nice.

Part of the problem is that there's just legitimate physics issues on takeoff, but there's some design tricks to help deal with them. Pending pics and .craft file there. 

Past that though, I can tell you right now that you're building too much "plane" and not enough "rocket". Even the best high-tech jet SSTO is going to be getting a large portion of its orbital velocity from LFO mode. With Panthers, you're topping out your air-breathing speed around 1000m/s at 20km so you still need 1500+ dV of rocket thrust to get to orbit. Remember, speed is life, altitude gets you jack without speed.

Edited April 26, 2017 by Jarin

[Spricigo]

5 hours ago, bewing said:

In the air, having CoM in front of CoL is necessary, but it is not sufficient. You also have to guess where your Center of Pressure is, and make sure that is in front of your CoL, too. CoP changes dynamically, so it's not possible to display in the editor with a ball.

Not quite accurate. Actually CoL also change dynamically and you can fly pretty well with CoL in front of CoM if other variable are correct.

The point is that CoL its neither CoDrag or CoPressure while if affects both. Also important that the editor's CoL indicator is itself a approximation that only take in account parts that have a Lift Value listed in description. From the thread above:

[AeroGav]

21 minutes ago, Spricigo said:

The point is that CoL its neither CoDrag or CoPressure while if affects both. Also important that the editor's CoL indicator is itself a approximation that only take in account parts that have a Lift Value listed in description. From the thread above:

That's a nice GIF.       The blue indicator is sorta reliable if all surfaces are at the same AoA (except for the fact it doesn't take account aero forces acting on non wing parts, which is why you need CorrectCoL)  but you can get into trouble when adding incidence to wings.

What you're  effectively doing there is adding some nose up trim to the elevators.  At zero AoA, CoL is ahead of CoM,  and the plane will pitch up, but because the main wing is already starting out from a higher AoA than the elevators, it  gains lift more slowly as  the AoA increases.  This causes CoL to move aft as AoA increases and the plane will tend to settle at its trimmed AoA.

Again , CorrectCoL makes this easier to visualize, you can bring up a graph of stability vs AoA,  you want stability to increase as AoA rises.

These days I build my planes to fly "hands off" at a small positive AoA with no control input, so it doesn't just dive into the ground if you let go of the stick.

ps.   "Centre of PRessure" , "Centre of Lift" etc.  people get tied in knots over these terms.    We talk of "Lift" and "Drag" but isn't that a false dichotomy?  In reality aerodynamic forces act on the structure, and we call the component acting in opposition to the direction of travel "Drag" and the forces that are perpendicular to the direction of travel "Lift", but really it's the same thing.  Especially with respect to the way the physics engine simulates this stuff.

[Spricigo]

29 minutes ago, AeroGav said:

 really it's the same thing.  Especially with respect to the way the physics engine simulates this stuff.

Agreed. My point was more "CoL indicator don't show the whole picture" than the concern  about the terms itself.

[Urses]

As discripted the problem with the wearing on runway may be to weak undercarriage. For pure Stock a good indicator for this is the moment after physics kick in. If the prograde indicator jumps in this moment your undercariage is to weak and needs some support. You go for a higher tier or double the actual wheels. Like beewing mentioned above your wheels generate drag and if they are to weak to hold the mass you have a drunken balerina on the runway.

Funny Kabooms 

Urses 

PS: for airborn the Chummers above are the guys with the knowledge

[EpicSpaceTravel]

Simple answer is yes.