Calculating the proper amount of lift?

[moleman122]

So i've been designing an SSTO spaceplane for a few hours of gametime now. I just kicked up KSP yesterday for the first time in a while, looked at all the shiny new stock parts, and decided I wanted to go to space with a plane.

The first design was able to get into a stable bare minimum (70,000m) orbit and retroburn back into the atmosphere, allowing it to land using monopropellant VTOL. Think the ranger interstellar, that was the initial inspiration for the first model:

With this Model (Hunter I) I just slapped on wings in a way that would look smooth and make the craft not take up much more space than the main body already did, and then afterwards I put on control surfaces in a way that placed the center of lift behind the center of mass.

The second design was meant to be an improvement over the Hunter I, so I took off all the wings, auxiliary functions, and started fresh with the base body to make it more effective. Eventually I came up with this thing, which was basically just a pretty rocket with RAPIER engines.

Infuriatingly enough, this thing was leagues more efficient in getting into orbit and back, despite having no lift whatsoever. But when I went to add necessary functions to it (batteries, landing gear, ladders, etc.) that minute amount of weight made it unable to orbit effectively. Adding wings only made it worse, as if the weight of the wings overshadowed the lift rating.

So, i'm curious, does anyone know what, exactly, lift rating actually does in game? It's been a bit wonky since it was first introduced, which was right around when I started playing, and the only thing it seems to do is allow you to change the direction of your craft with aiming, but it doesn't always do that.

Essentially: Are there any equations that allow you to calculate how much lift rating you need for the mass of your craft, or for the twr AND mass of your craft? Does anyone have extensive knowledge of the way lift works that could maybe help me get this thing working again?

Pre-emptive thanks to anyone who answers

Edited December 11, 2014 by moleman122

[Empiro]

Lift doesn't really reduce the amount of delta-V you need to get into orbit. What it lets you do is get away with lower TWR because you can accelerate horizontally while using the lift to keep a relatively slow climb.

Landing gears and ladders are massless (and the small batteries are as well). Small accessories add very little extra weight, so I'm thinking that your ascent profile needs to be tweaked. How fast are you going before you switch the RAPIERs to closed-cycle? In stock, I've heard of people going 2000 m/s on the air-breathing mode, which means that you almost need no LFO to make it into orbit after that.

If you're really interested in making space planes, I'd recommend FAR, though I believe RAPIERs max out around 1600 m/s there before having to switch to closed-cycle mode.

[capi3101]

Point of order there - you do have three parts on your little blaster pod thingie there that generate lift - the MK1-2 adapter up front, the inlike Mk2 cockpit and the Mk2 bicoupler are all lift generating parts - about 0.9 all told (0.3 for the adapter, 0.4 for the cockpit, 0.2 for the adapter).

I'm not sure of the specific workings behind lift - I just know the general rules:

- 10 tonnes maximum takeoff weight per Basic Jet Engine, 13 tonnes per RAPIER or 15 tonnes per Turbojet

- No less than 0.03 intake area per engine

- A total lift coefficient roughly equal to the mass of the craft.

I'ma guessing the pod design flew as well as it did due to the amount of thrust it had available to it. I'd have to try out the design myself to be sure.

[moleman122]

Lift doesn't really reduce the amount of delta-V you need to get into orbit. What it lets you do is get away with lower TWR because you can accelerate horizontally while using the lift to keep a relatively slow climb.

Landing gears and ladders are massless (and the small batteries are as well). Small accessories add very little extra weight, so I'm thinking that your ascent profile needs to be tweaked. How fast are you going before you switch the RAPIERs to closed-cycle? In stock, I've heard of people going 2000 m/s on the air-breathing mode, which means that you almost need no LFO to make it into orbit after that.

If you're really interested in making space planes, I'd recommend FAR, though I believe RAPIERs max out around 1600 m/s there before having to switch to closed-cycle mode.

Typically the I let the RAPIERs autoswitch, flying flat once I get to about .05 intake air left. At that point I just let it go as fast as it can, which usually ends up being about 14-1500 m/s. Once it's at top speed I change my angle just a little bit, slowly ascend, then they switch to rocket mode and I do standard orbital procedure from then. Maybe I just need more air intakes to allow myself for a little higher elavation and, as a result, more speed?

Also, I don't know what FAR is.

[moleman122]

Point of order there - you do have three parts on your little blaster pod thingie there that generate lift - the MK1-2 adapter up front, the inlike Mk2 cockpit and the Mk2 bicoupler are all lift generating parts - about 0.9 all told (0.3 for the adapter, 0.4 for the cockpit, 0.2 for the adapter).

I'm not sure of the specific workings behind lift - I just know the general rules:

- 10 tonnes maximum takeoff weight per Basic Jet Engine, 13 tonnes per RAPIER or 15 tonnes per Turbojet

- No less than 0.03 intake area per engine

- A total lift coefficient roughly equal to the mass of the craft.

I'ma guessing the pod design flew as well as it did due to the amount of thrust it had available to it. I'd have to try out the design myself to be sure.

Those general rules are all new to me, so thanks for that, that might help. And yeah, the way I figure it's only working so well because of the thrust. Mass is only about 8.61 tonnes, and calculated TWR at maximum engine efficiency (which only really happens higher up in the atmosphere) was somewhere around 4.0, I lost the calculation and i'm too lazy to redo it.

[Empiro]

In that case, it sounds like you've already got enough lift. More lift is only needed if you're having trouble getting off the runway or if you can't maintain level flight at ~25km while building your speed up to 1500 m/s or so. The general advice is that as long as your speed is still increasing, try keep the engines in air-breathing mode longer by throttling down. Did you try to reduce the mass further by not carrying excess oxidizer? The perfect fuel mix would be if you're left with 0 LF and 0 Oxidizer if you burn until your engine shut off.

FAR is a mod: http://forum.kerbalspaceprogram.com/threads/20451-0-25-Ferram-Aerospace-Research-v0-14-4-11-24-14 which adds more realistic aerodynamics. I generally only make spaceplanes in FAR, so I can't really give the best advice for stock aerodynamics. It does weaken air-breathing engines quite a bit though (stock jet engines are way too powerful), so a lifting body probably won't work and you'll probably need wings on your craft.

[moleman122]

Thanks! Adjusting oxidizer, adding winglets to rapidly stabilize flight, and messing around with the intakes and maximum operating height was all it needed. This will make things a lot easier for me from now on.

[Pecan]

In stock KSP wings are only worth their mass and drag on the way down (if you need them for landing).

On the way up you're better-off using a VTVL design.