TWR Ratio and wing loading ratio

[jsisidore]

I've read that small tutorial but I still don't fully get it,

Now, the more twr ratio is the faster the speed right? The more mass I add the lower ratio I get.

Now about lift rating, if say I want to build an efficient space plane that has mass of say 8 tones, then I will need a lift rating of 10.7 tones to get a 2/3 ratio right?

edit: I think I've made the calculation wrong, it should be 12 tones and not 10.7 tones.

Edited February 22, 2015 by jsisidore
answered

[*Aqua*]

Now, the more twr ratio is the faster the speed right?

To be more specific: The more TWR the higher the acceleration.

The more mass I add the lower ratio I get.

Right.

I don't know where the lift rating in tones come from. (The unit is tones? Really? I always thought it's a made up number without a unit.)

[Taki117]

lift ratio and TWR, while related, are not mutually exclusive. the higher your TWR the faster you will accelerate. Wing loading on the other hand is the loaded weight of the aircraft divided by the area of the wing. All this means is that for a given aircraft a larger wing results in less wing loading, which means you have more lift and a corresponding slower takeoff speed.

[eddiew]

On the other hand, wing area is synonymous with drag. There's an argument to be made that the right amount of lift is just enough that you can nose up at the end of the runway, so's to ensure you have lowest possible drag and least 'dead weight' of wing. The more likely truth is that there's a good balance to be found when you can take off somewhere between the middle and the end of the runway

[jsisidore]

lift ratio and TWR, while related, are not mutually exclusive. the higher your TWR the faster you will accelerate. Wing loading on the other hand is the loaded weight of the aircraft divided by the area of the wing. All this means is that for a given aircraft a larger wing results in less wing loading, which means you have more lift and a corresponding slower takeoff speed.

These are two new concepts that baffled me, till this moment I was just playing with ksp, but now I want to understand what the hell I'm actually doing, of course I will not abandon my beloved intuition.

[jsisidore]

The more likely truth is that there's a good balance to be found when you can take off somewhere between the middle and the end of the runway

I know ksp is easy like that "just slap on stuff - give it a go and have a blast" but after a while the trial and error approach turns more into a chore than an adventure.

[GoSlash27]

jsisidore,

Unfortunately, the relationship between t/w ratio and speed isn't that simple in KSP.

As you get up around 27km altitude, the air becomes so thin that the system throttles back to avoid flameout. When this happens, adding engines doesn't add any more thrust, just mass and drag. You're limited by intake area in that critical regime a lot more than pure thrust rating.

Likewise, having too little thrust means that you're hard- pressed to overcome your drag as you attempt to slog through the 25-32km "wall". Not that big a deal so long as you have enough wing area, but because your fuel efficiency is down in that part of the flight... it will cost you fuel.

This is why we have come up with an efficient balance through empirical testing and experience that says we want one turbojet for every 10-15 tonnes of spaceplane and at least .018m^2 intake area per engine.

Lift has a similar balance problem. Too much wing means that you never get into the most efficient angle of attack during your climbout (23*) which means you're carrying more mass and drag than you could, which means more fuel wasted.

Too little wing, OTOH, means you may not be able to climb over the wall.

So we set a rule-of-thumb balance of 1.0 lift coefficient per tonne of spaceplane.

Best,

-Slashy

[capi3101]

What Slashy said. So for your eight tonne aircraft, you'll want to use one Turbojet as your main engine and a combination of wing parts that add up to a lift rating of "eight". Delta Wings and Wing Connector A/B are "two" each, so you'd want four total of those parts. Some of the wing parts actually result in better "lift per tonne" than others (the Deltas and Wing Connectors are actually really lousy in that regard); for example the old-style Swept Wings are actually really decent wing parts, they're just hard to stick control surfaces onto.