whats a good TWR for a spaceplane

[endl]

my aerodynamics are solid but im having problems generating enough lift thrust before the runway ends

Edited August 25, 2014 by endl

[Taki117]

That may not be a TWR problem, but a landing gear placement problem. If they are too far behind the CoM then you won't be able to pull the nose up to get lift. A pic of the craft would help.

[R0cketC0der]

TWR isn't the only thing affecting how long you will need to take of. The lift and the position of the wheels are also major factors when it comes to that.

A common mistake is to place the rear landing gear too far back(and I have also done that for a long time). The rear landing gear should be a little bit in front of the middle between the COM and the rear of the plane. Placing it closer to the COM makes the plane able to lift of faster, but also increases the tailstrike risk. Placing it closer to the tail makes the plane take of slower, but lowers your tailstrike risk.

[EdFred]

I always make my planes with an angle of incidence of 5° on the wings. If I make it 0°, I need the entire runway.

[AlexanderB]

Also control surfaces, if they act on top of the landing gear, all they do is push the tires into the tarmac. Best way for me seems to put the plane about 5 degrees nose-up, it'll lift off by itsself in many cases. That, or give it canards at the front for better leverage. (I tend to design delta+canard planes, instead of airliner-shaped ones.

[endl]

That may not be a TWR problem, but a landing gear placement problem. If they are too far behind the CoM then you won't be able to pull the nose up to get lift. A pic of the craft would help.

your right it was the wheels, i had them too far back i thought it would improve my angle of attack since it placed my plane pointing up but how the hell did it stop my lift

[Alshain]

Also control surfaces, if they act on top of the landing gear, all they do is push the tires into the tarmac. Best way for me seems to put the plane about 5 degrees nose-up, it'll lift off by itsself in many cases. That, or give it canards at the front for better leverage. (I tend to design delta+canard planes, instead of airliner-shaped ones.

Or if they act on the center of mass, which not only prevents lift off, but pitch in general.

[capi3101]

Getting back to your original question, you want your turbojet TWR somewhere around 0.75 at takeoff, or roughly 1 turbojet per 15 total tonnes of craft. You also have to have enough lift-generating surfaces to actually take off; swept wings and delta wings support about three tonnes of craft per wing (i.e. six per pair), while wing connectors generally support one tonne each. Control surfaces also support about one tonne a piece. That's just some general guidelines when it comes to design.

[Alias72]

your right it was the wheels, i had them too far back i thought it would improve my angle of attack since it placed my plane pointing up but how the hell did it stop my lift

it probably wasn't enough of an AoA for takeoff. when you tipped up you likely went to 10 or 15 degrees. doesn't sound like much but has a major effect on your lift. KSP airfoils are considered symmetrical. at 0 degress they get no lift. in fact here is something useful

this graph shows an ideal lift and drag to AoA correlation. this is for a flat plate, but KSP likes this kind of oversimplification. note how steep the lift to AoA curve is.

*note I am using some assumptions about how lift works in the base game. I use far and so have access to features not in stock. I really hope this lift model applies to stock KSP or all I did was throw a picture and a poorly worded paragraph at you.

[Laie]

After all that AoA talk: in my opinion, a spaceplane should have a TWR of "one-and-a-little" on the runway: 1.1 is (barely) alright, 1.5 is plenty. If you're using mechjeb, your figure should read twice that (mechjeb doesn't account for Turbojets starting at half their nominal thrust).

[Taki117]

your right it was the wheels, i had them too far back i thought it would improve my angle of attack since it placed my plane pointing up but how the hell did it stop my lift

It didn't stop your lift, it stopped the front of your aircraft raising into the air. Because the wheels act as a lever the further they are from the CoM the harder it is to take off because you need more oomph on the nose, which most aircraft don't have. Too close to the CoM however and your run the risk of tail strikes, so it is really a balancing act.

[capi3101]

Personally, unless you're clipping the little lights at the end of the runway and tearing stuff up, I wouldn't sweat it too much if you have to use the whole runway to take off; wheels too far back eliminate the threat of tail strike as a rule. Where you're in trouble is if, after flying off the end of the runway, you start nosing down towards the drink instead of picking up altitude. If I see that, I revert to the VAB toot-sweet...

If your back wheels are right by your CoM and you still can't take off, I'd suggest throwing on a pair of canards or three (as long as it doesn't overcorrect your CoL, that is) to give yourself more torque by the nose. In my experience, craft in KSP (stock anyway) fly better with a combo of canards and elevons. If that doesn't work - check the mass of your craft, make sure you're using enough thrust and lifting surfaces, and don't try taking off until you do unless you want to send your Kerbals swimming.

[DocMoriarty]

Space plane construction is all about putting the correct number of everything together. All outlined in chapter "Numbers You Should Know Before Construction" in my documentation about space planes (link in signature). That said my simple approach to have the correct minimum of engines/TWR is the rule: 1 RAPIER per 13t takeoff weight.