No liftoff with SSTO

[Phony Sphinx]

Having trouble with an SSTO, which based on most everything I've read/tried, should work. What happens is whilst on the runway it will refuse to lift. however, leaving the runway by reaching the end and dropping slightly it will fly just fine, albeit with a habit of clipping the tail off on the runway.

TWR at runway with my engines set for atmo is 1.48

all gears are set perpendicular to runway

i have tried every location of center of lift-mass i could easily do, including ones so far forward for CoL that it would backflip once it got off the runway. the only thing i can't really do is move the CoG in front of the CoL easily because of the placement of the engines, but i get around it with tweaking the CoL. CoL is artificially far back in pics due to me angling the small front wings 5* down.

and here is the pics

construction details

Not to worry. I'm still flying half a ship

[Tiron]

it's your tail gear. when on the runway, the plane acts as a lever, with the fulcrum at the rearmost landing gear (that's touching the ground) and the force applied at the control surfaces. If they're too close together you get no leverage and the nose won't come up. It helps to have the rearmost landing gear near the Center of Mass.

[JiWint]

Hah,that's what happens when a newbie uses MODS. Do not use mods when you're about to begin playing a game.

Your center of thrust isn't even aligned with center of mass and center of lift is not even aligned with center of mass....

Edited October 6, 2013 by JiWint

[arq]

Hah,that's what happens when a newbie uses MODS. Do not use mods when you're about to begin playing a game.

Your center of thrust isn't even aligned with center of mass and center of lift is not even aligned with center of mass....

Your post was not constructive or helpful.

The plane apparently flies fine after dropping off the runway, so weight/balance is not the issue. If an aircraft has insufficient lift, you will not be able to get it off the runway. However, that is not the problem. The issue is the pivot point of the craft. When taking off, your plane is essentially a lever that pivots on the rearmost wheels. Your lifting surfaces must move the lifting force sufficiently forward, relative to the pivot, to make the nose rise.

There are two ways to do this. You could add more pitch-control (force) or move the pivot further forward. I'd recommend trying the second. Move the rearmost wheels forward (or in your case, with all those wheels, just remove them). The closer your rear wheels to the CoM, the easier it will be to rotate and lift off the ground.

[TMS]

Attach a series of downwards-facing separatrons to your command module.

It'll still be SSTO as they don't detach.

[Hodo]

Hah,that's what happens when a newbie uses MODS. Do not use mods when you're about to begin playing a game.

Your center of thrust isn't even aligned with center of mass and center of lift is not even aligned with center of mass....

Thank you doctor useless post.

Actually his CoM and CoT are inline, just the angle of the picture. His problem is his CoL is to far behind his CoM and he has that tail wheel that is so far back it acts as a lever keeping the nose down on the runway.

He may also suffer from a great deal of fuselage flex which is causing the CoT to shift around the CoM. I have those problems from time to time with some of my bigger SSTOs. More struts will help.

[JerryIRacer]

There is something else going on too! When you look at the side picture its easy to notice that front wing is has negative angle of attack and acts like a spoiler on the car. It keeps your nose pushed into the ground. I dont know if you need it that way for your plane to be stable in flight. It would definitely help on lift of if you have it inclined with positive angle off attack. That and the fact that your gear far behind CoM makes hard to lift up the nose to produce any lift...

[boomerdog2000]

Quick question. Where are you trying to go with this SSTO, because that thing's a monster. You can make it into orbit with just the Vonnegut if you want to. 100km circular with fuel to deorbit and land.

[Leyvin]

Hah,that's what happens when a newbie uses MODS. Do not use mods when you're about to begin playing a game.

Your center of thrust isn't even aligned with center of mass and center of lift is not even aligned with center of mass....

They don't actually need to be, nor does he realistically need a TWR above 0.6 for Take-Off (in Space is a different matter, but then Lift has absolutely no baring on that)

Still as far as Take-Off is concerned, ideally you want your Lift to be Behind and Above your Centre-of-Mass; reason for this is because while Up-Force Lift below the Mass requires the Lift to be a "Pushing Force"; which is fine for VTOL Craft (such-as Helicopters) where this is assisted with Thrust but when you're Thrust is so far back it actually begins to act like a Down-Force. Instead positioning it so that it is above the Mass will turn it into a "Pulling Force" that will aid the Engines in creating Lift.

The easiest solution to this with your current design is to provide your Wings with a Positive Angle of Attack (i.e. Pitch them so the ends are aimed upward Shift-E 2-3x should do it), doing this will not only increase the Lift Height; but it will also increase Stability at higher Speeds while also allowing for some basic "Glide" Capabilities.

The second thing you need to keep in Mind is that your Rear Landing Gear are a 'Pivot' Point for the whole Centre of Mass, so ideally you want them close to that point but behind it slightly so you don't just fall over backwards.

If you're worried about the Tail, then note there are 2 Solutions to this:

1 • On the Interconnection Joint between the end of the Tail and Body, take one of the landing gear and just slide it up the middle so it is just sticking out of the end. This will generally act like a Skid Wheel, but sometimes it can lead to hilarious torque and explosions so experiment if it's the right thing for that Aircraft.

2 • The M-1x1 Structural Panel, is actually incredibly resilient and can often be used as a Skid Plate (or even landing gear in some cases) but keep in-mind they can only really take getting hit at perhaps 60m/s (130mph)

I would say though being 180 MG, might make it a considerably Challenge to make it in to Orbit with that... ideally you want to be a light as you can possibly make it as you want your Air-breathing Engines to ideally do the Majority of the work for you before switching to the Orbital Thrusters. Still that'd just be a suggestion

[Phony Sphinx]

okay, first of all, thanks for all the help so far, it was far beyond what i thought to see in such a short period of time, and it has led to a second round of iterations now that i know what to look out for, and has helped, but it still refuses to lift off the runway.

these pictures are from the most stable iteration, but please take my word i have tested out numerous positions of moving the CoL as far forward as i can, any farther forward than is shown in the picture and my control surfaces can't seem to keep the craft from back-flipping out of control

on a side note SAS seems to not be working at all, I'd like ideas on why? i thought it might be not piloted from cockpit or maybe something involving the mini-probe i have inside, but none of these things seem to be the case.

as for people wondering why i have 300+ part 250 ton SSTO, the goal is to have it reach near any planet in the system and refuel off of a station at that point, or to launch a probe at a 90* orbit and hold while it scans the planet whilst waiting for a station to come and be assembled, more or less an advanced recon SSTO for places with virtually no infrastructure.

if anyone would care to download the design and tweak it i'd love to hand it off, I've just about done everything i can think of, though the parts count might mess with some peoples rigs.

and now for pics

in flight, probably the best and worst part of this whole thing, it launches just fine and no longer clips the tail end...just not off the runway.

i swear, if this keeps up i might rename this thing Caliburn from how firmly grounded it seems to be.

[NKL]

i swear, if this keeps up i might rename this thing Caliburn from how firmly grounded it seems to be.

Hahahaha, yeah. You apparently tweaked the rear wheel in order to get them perfectly align... but too far behind CoM as I can see.

You really should try on a simpler but long test plane how lever affect the take off with KSP stock physic.

I usually do rear gear right behind CoM, like in the CoL spot, and add a little tilt wheel (which do not touche the ground at all) in order to protect my tail in case of rough take off.

[Claw]

Hahahaha, yeah. You apparently tweaked the rear wheel in order to get them perfectly align... but too far behind CoM as I can see.

...

I usually do rear gear right behind CoM, like in the CoL spot, and add a little tilt wheel (which do not touche the ground at all) in order to protect my tail in case of rough take off.

Yes, this (your wheels). A few people have mentioned it.

Place your rear wheels slighly aft of the CoM. If you put them too close to the CoM, your plane will sit on its tail. Too far back, and it won't rotate for takeoff.

Also, your CoL should typically not be in front of your CoM.

[Taki117]

Something else you can do is use the B9 wheels, They are sturdier and will give you more ground clearance. Also you could probably save some weight and do away with the SABREs.

[capi3101]

Still the wheels even after your tweaks. Get 'em lined up right with the center of mass and you should at least be able to lift off. If you've got problems with tail strikes, keep the angle down (5-10 degrees at most) when you go to take off.

You're using mod parts, and I don't know how heavy your plane is, so I can't offer more specific advice. You do want the CoL behind the CoM though unless you want the thing to be flip happy (and getting worse as you burn fuel). Vertical alignment of the two is not terribly important; a CoL below the CoM increases maneuverability, above the CoM increases stability.

[Tarmenius]

Also notice that in the pictures during flight, the SAS is applying as much pitch control as it can to keep the plane level. In the picture where CoM is in front of CoL, the SAS looks like it's trying as hard as it can to pitch up to compensate. And where CoM is behind CoL, the SAS is trying to pitch down. That strongly suggests that the CoL and CoM need to be closer together. So if this were my design, I'd focus on fixing that and making sure that no landing gear is very far behind the CoM. Then see how it behaves and continue adjustments as needed.

[vetrox]

put the b9 wheels on the front and leave the stock ksp gear at the rear. If memory serves the b9 wheels are larger, this will make the front of your (rediculous ) craft point upwards.

Have you calculated how much dv that thing actually has once its in space? You might find you can achieve your goal with a smaller more efficient craft

[Hodo]

This thread is over 5 months old. I think either the OP has figured it out or given up by now.

[Claw]

This thread is over 5 months old. I think either the OP has figured it out or given up by now.

Good gravy. I didn't even look at the date. Just saw that it was bumped to the top...

I wish these would age out a little sooner. Haha