High lift devices

[Tourist]

I tend to use the Lemming method of getting off the runway. In other words, I let it run off the edge of the runway and then just start praying. 

[Nich]

5 minutes ago, Tourist said:

I tend to use the Lemming method of getting off the runway. In other words, I let it run off the edge of the runway and then just start praying. 

Thats how all the best SSTO take off

[Nich]

3 hours ago, Rune said:

Actually, it works the other way around. Your Lift/Drag ratio is a function of your Angle of Attack (AoA from now on). Low angle of attack, and lift is a big percentage of the total aerodynamic forces. High angle of attack, and drag grows faster than lift, to the point that you "stall" (it's modeled derpy, but it works out surprisingly like the real thing). In fact, at really high angles of attack (>30º), lift in fact goes down, while drag keeps going up. With more wing, you can mantain a lower AoA for the same lift, meaning you see less drag at the same airspeed and altitude. High wing-loaded planes can see more drag at low airspeeds for this reason, if they have to maintain a high angle of attack to fly level. You can go the other way and have too much wing, of course, but that is mostly because wings do weight somewhat, not because of drag, or at least not aerodynamic drag. If you want to call it gravity drag...

Also, with the proper caveats about how you can't divide a given engine (so you can only go in discrete increments up or down) your size is totally irrelevant to your flight characteristics. I cannot stress that enough. Rocketry and flight are all about ratios, and given how KSP parts are scaled, everything part-relevant scales more or less linearly (twice the weight is twice everything else, like thrust in engines and lift in wings). If your small SSTOs work better than your big SSTOs, that's because you get the ratios in your big ones wrong(er). Study your small ones, copy the ratios in a bigger one (same TWR, mass ratio, payload fraction and so on), and it will have the same flight profile, with the same dV when it gets to orbit.

 

Rune. Because physics.

Well it doesn't exists in KSP but there is always reynolds number to ruin your ratios day

[JoeNapalm]

15 hours ago, Rune said:

Actually, it works the other way around. Your Lift/Drag ratio is a function of your Angle of Attack (AoA from now on). Low angle of attack, and lift is a big percentage of the total aerodynamic forces. High angle of attack, and drag grows faster than lift, to the point that you "stall" (it's modeled derpy, but it works out surprisingly like the real thing). In fact, at really high angles of attack (>30º), lift in fact goes down, while drag keeps going up. With more wing, you can mantain a lower AoA for the same lift, meaning you see less drag at the same airspeed and altitude. High wing-loaded planes can see more drag at low airspeeds for this reason, if they have to maintain a high angle of attack to fly level. You can go the other way and have too much wing, of course, but that is mostly because wings do weight somewhat, not because of drag, or at least not aerodynamic drag. If you want to call it gravity drag...

Also, with the proper caveats about how you can't divide a given engine (so you can only go in discrete increments up or down) your size is totally irrelevant to your flight characteristics. I cannot stress that enough. Rocketry and flight are all about ratios, and given how KSP parts are scaled, everything part-relevant scales more or less linearly (twice the weight is twice everything else, like thrust in engines and lift in wings). If your small SSTOs work better than your big SSTOs, that's because you get the ratios in your big ones wrong(er). Study your small ones, copy the ratios in a bigger one (same TWR, mass ratio, payload fraction and so on), and it will have the same flight profile, with the same dV when it gets to orbit.

 

Rune. Because physics.

My point was simply that you can't have lift without drag. Yeah, you can have more or less effecient examples, but adding lifting surfaces adds drag.

And size is not irrelevant to your flight characteristics. What you're saying is true on paper, but if someone is trying to build SSTOs and can't get one to work, building a smaller one is a HECK of a lot easier - in stock KSP, a big SSTO is prone to a lot more difficulties in wing geometry, structural failures, stability issues on take off and landing, fuel flow, fuel balance, and a myriad of other problems that are much MUCH easier for a newbie to handle on a small scale build. In terms of efficiency (the point I was addressing) you can generally, in KSP, build a small vessel or a large vessel to accomplish the same tasks - and yeah, if you have the same TWR and everything you'll have the same DV - but the bigger one will COST more. Have fun explaining to your bean counters that it's "just as efficient" when you spent more money to complete the same contract for the same pay.

 

-Jn-

[ment18]

Larger SSTO's will do slightly better because of drag.  They have less cross-section for their mass.  They also suffer less from putting a shielded docking port on the front.  On a 3.75m ssto the 1.25m port adds negligible drag compared to nose-cone but on a 1.25m SSTO it significantly increases drag.

[Rune]

15 hours ago, Nich said:

Well it doesn't exists in KSP but there is always reynolds number to ruin your ratios day

Good that I was talking KSP physics, then. In real life, though, I would wager the square/root law, changes things "slightly" more, tough...

3 hours ago, JoeNapalm said:

My point was simply that you can't have lift without drag. Yeah, you can have more or less effecient examples, but adding lifting surfaces adds drag.

And that is the point I attacked. If your design has very little wing, you will need a high angle of attack to fly straight, and you will produce more drag than you would with a larger wing that lets you follow the same trajectory with less AoA.

As to the size comments, I reiterate that size has nothing to do with your flight characteristics. It has to do, of course, with your payload, I never put that in doubt, but not with your payload fraction. Your more expensive flight will lift a proportionally larger payload. Maybe size does have something to do with part count on extreme examples, I can grant that, but you can build a 80 part Mk3 SSTO that lifts an orange tank, and an 80 part Mk1 SSTO that lifts a tiny sat. And as @ment18 says, size has its own benefits too. You don't need a bigger probe core or crew capacity to pilot a bigger plane...

 

Rune. However, if you change in your size comments "big" with "high part count", then I wholeheartedly agree with them.