minimum/optimum lift in stock

[Thrain]

Is there any way to determine what the minimum and optimum amount of lift is for a give spaceplane mass in stock? Currently, when I try and design a new plane and take it for a test flight, I have no idea if the poor performance is due to insufficient lift, too much lift/drag, or not enough thrust/power (moar boosters!). I am sure this all depends on each individual craft design, but is there any general rule of thumb people use?

Edit: changed to answered. Thanks to all who responded.

Edited February 1, 2015 by Thrain

[AlexinTokyo]

I tend to follow Claw's Rules of Thumb for Spaceplanes

- 1 TurboJet per 12 tons, (no more than 15 tons)

- 3 Ram intakes per TurboJet (no less than 1 per TurboJet)

- 150 liquid fuel per TurboJet (no less than 100)

- 1.0 lift rating per ton (No more than 1.0, no less than 0.5 per ton)

These criteria are interchangeable. So if you have a craft that's lighter (i.e. 1 TurboJet for less than 12 tons), then you can ease up on the fuel requirements, or intakes, or lift rating. I can give you more reasoning behind why I chose these, but it was a bit more in depth than "because it feels right." These rules of thumb also utilize a climb profile that is very close to terminal velocity on the way up.

Also, if you're interested in reducing flameout chances, then you want to place your intakes/engines in an alternating pattern. Split up your intakes equally between your engines, then place "Intakes", "Engines", "Intakes", "Engines", and so on.

These have always proved to be a good starting point for my designs (not that I'm a great plane designer at all).

[Superfluous J]

I watch a youtuber (DFWanderingKid) who always makes sure his planes have roughly a 1:1 ratio of "lift units" to tonnes of mass. So if his plane weighs 6 tonnes, it must have 6 "lift units"

You can find the "lift units" in the VAB by right clicking the wing segments in the part selector. Add up the "lift rating" of all your wing portions and you have the "lift units" of the plane. That should be just about the tons of the plane's weight.

I have no idea if this is a good rule of thumb, what they actually intended when they came up with this "lift rating" number, if any of this will matter in 1.0, or any of that. I actually asked him about it last night and got what I told you above and I've not even tested it

EDIT: Ninja'd with better info. Le sigh

Edited February 1, 2015 by 5thHorseman

[GoSlash27]

^ I'll second that, though I use more jet fuel and less rocket. I also don't use nearly that much intake area.

The key to a successful spaceplane is climbing through the 25-32km altitude region with a pitch of 23* at terminal velocity.

If you can't climb and you're at or above terminal velocity, add wings.

If you can't climb and you're below terminal velocity, add engines or lose weight.

If you can't climb and you've had to reduce throttle to keep the engines lit, add intake area.

I use 1.0 Cl per tonne for wings, 1 turbojet per 15 tonnes, and .018 m^2 intake area per turbojet.

Best,

-Slashy

[Pecan]

...The key to a successful spaceplane is climbing through the 25-32km altitude region with a pitch of 23* at terminal velocity...

23 degrees seems like a lot of pitch to me, not that I claim to be good at (or particularly interested in) spaceplanes.

What rate of climb are you getting at that pitch and what Pe do you get from the jet(s)?

Using much shallower pitch (+/- 5 degrees) at that altitude - and intake-spamming 8 per jet - I usually have a Pe of 50-60km.

[Thrain]

thanks so much, that helps explain some of my issues then. My latest behemoth was at max pitch just to maintain altitude at ~21km and seemed stuck at < 1300m/s. Now I know why.

Mass: 65t

Lift: 93

6 turbojets with 1.25 ram intakes each and 300 liquid fuel for each turbojet.

Time to redesign this with less lift and cut some mass too. Thanks again for the answers.

[GoSlash27]

23 degrees seems like a lot of pitch to me, not that I claim to be good at (or particularly interested in) spaceplanes.

What rate of climb are you getting at that pitch and what Pe do you get from the jet(s)?

Using much shallower pitch (+/- 5 degrees) at that altitude - and intake-spamming 8 per jet - I usually have a Pe of 50-60km.

Pecan,

23* is the maximum lift/ drag possible for KSP wings. If it is necessary to hit 23* to continue the climb at around 30 km up, then you're at the smallest wing area possible for a successful spaceplane. Any less AoA than that and you're carrying more wing mass than you need, which costs DV. Any more than that, and you're probably not going to space today

My wings pretty much crap out at 32 km, and it sits there for a while building speed until the centrifugal force is sufficient to get me climbing again.

This is generally good to get me between 65-80 km pe before I finally have to kick in the rocket.

Best,

-Slashy

[Pecan]

Pecan, 23* is the maximum lift/ drag possible for KSP wings. If it is necessary to hit 23* to continue the climb at around 30 km up...

Ahh, right - with the intake-spam I'm relying on speed by the time I get to 30km and wings are pretty much irrelevant except for initial rotate and final landing. Since my planes are over-fed they are giving more thrust -> acceleration -> speed. No surprise that I follow the same ascent with VTVL jet 'tail sitters' that don't have any wings at all (which can't quite fit "more wing mass than you need", but the difference is in the spamming, so I know what you mean).