How many "lift units" do you need per ton?

[Pbhead]

So trying to build space planes, and it always seems that no matter how much wing I put on, I dont put on enough wing, until I make it a biplane or triplane.

What is the angle of attack you all use for takeoff, (how do you achieve that angle of attack without messing up cog to much... whole pile of cubic struts?) how many turbojet engines/weight, and how many "lift units" per weight do your planes end up having?

(Because the space plane tutorials in the sticky are really kinda pathetic.)

[Xaver]

For me it works with 1 big jet engine per 10 tons of weight. Then your take off at the end of the runway has a speed around 110 m/s.

[falofonos]

My spaceplanes tend to crash before reaching the end of the runway but the ones that work have a strut to add height to the front wheel and the rear wheels are as far back as I can get them to prevent engines hitting the runway. Gives you an angle of attack on the ground of about 5 to 10 degrees. I even made a jet that took off without pulling back on the stick It flipped and crashed moments later because the cog was too far back... but it was a victory of sorts.

Recently I have started putting low altitude efficient jets onto drop pods on the wingtips to get extra thrust for takeoff.

Oh, I also use the stability support clamps to hold the plane until it reaches a decent level of engine thrust output.

[Pbhead]

At one big jet engine per ten tons of weight you should throw out all your lifting surfaces, and just launch vertical..

[Shania_L]

I dont exactly count how much wing I add per tonne, (Until the info tab in the 0.20 map screen I didn't even know how many tonnes I had). My early designes did suffer from a lack of wing until I reverse engineered someone elses design from the exchange to see how they did it, now I put on a lot more wing.

I'm sorry I cant specify any numbers for you, as I said I go by eye but I will link a few images of a few of my designs that work to give you an idea of what I go for. You will see I have a running theme in my design style.

As for design techniques I have a few pointers

- Build your plane around the fuel tank, keep your COG on that fuel tank, if you need multiple tanks arrange them across the plane not along so your COG doesnt move as fuel drains.

- Place your main undercarrige just behind your COG, too far back and you will have difficulty rotating, too far forward and you will sit on your tail.

- I do not angle my wings so my angle of attack is 0, other than canards to fine tune the COL/COG balence.

- Try to get your wheels so that your nose gear is "just" longer than your main gear, this will give you some takeoff AoA. By dragging your plane to the bottom of the build area you can see the difference in length by watching the wheels dissapear into the floor, try to get your main gear to touch the floor within the diameter of your nose gear.

- A lot of people go for single jets due to the high altitude flameout issues, I use two and then 1-2 ram intakes per jet. Although recently I have started trying to stack the two jets vertically so on flameout I get a pitch error not a flat spin

Hope this helps, there is a reason why spaceplanes/ssto planes are considered the pinnacle of the KSP learning cliff.

[Pbhead]

Thanks for the awesome reply Shanaia. I still want numbers, (lift/weight ratios, engine/weight ratios) but your post has given me some good ideas. I really like how you use lifting surfaces as covers over the fuel tanks and engines and such.

[Thrombo]

I love that Jeb is thrilled you're dry on fuel in the middle pic.

[numerobis]

I tend to fly on 1 turbojet per 15-20t launch mass for spaceplanes. That gives you a TWR on launch about 0.6 or so, approaching 1.0 at the top thrust (at 1km/s).

I haven't calculated the number of wings, but tilting them by one notch in the SPH helps you fly at a positive angle of attack while having the engines and intakes aligned with your flight path.

[PolecatEZ]

My general rules for aircraft and SSTO design:

Wing Count = 1 "lift unit" per 2 tons for a high altitude screamer, 4 units per ton for low altitude glider. This does not include flaps. Changing angles on the wings will throw this off, do so with caution. At around 5000m check your angle of attack (angle difference between your pro-grade and your nose) at level flight. If you're at full throttle, this should be no more than 5-10 degrees. If the gap is bigger, you need more wing.

Wing Placement = Always place so Center of Mass is overlapping Center of Lift, just forward of it (the two spheres should touch, with the CoM in front) - when you get better at design you can loosen this rule up a bit.

Landing Gear Placement = Place the rear wheel set centered on the center of mass so the back wheel is just barely behind the CoM. If the plane tips over backward on the runway, bump the rear gears backward just a hair in the SPH and try again. Shania gave another great tip about the forward gear placement. Pro-tip: These do NOT count for your weight outside of the SPH, on the runway they are weightless.

Turbojets = For SSTO you want 1 per 10 tons. For atmospheric aircraft, 1 per 15-20 tons is sufficient (thought more is always better, speed rocks!).

Rockets = For SSTO aircraft, provided you followed the other rules, you want 60-75 kN of thrust (about 1 LV-909 or NIRVA) per 10 tons. Using LV-30's or Toroidals you can use 1 per 30 tons max. Round up.

Rocket Fuel = At the time you flip on your rockets, you'll want about 1200 dV bare minimum to make orbit and subsequently de-orbit. 1600 dV is a safer amount for SSTO noobs. More dV lets you do more things.

Intakes = For a moderately challenging SSTO, 4 intakes per engine is good. Fewer make for greater challenge, more makes for it being too easy. Do not waste time with anything other than Ramjets for SSTO. For Atmospheric Fighters, use any intake you want, 1 intake of any type per engine is sufficient.

If you follow these principles, you cannot make a bad airplane, ever. It will take off and fly every time. Nearly every common problem can be traced to one of the items above.

Check out my cataloge below for some examples.

Edited May 31, 2013 by PolecatEZ

[numerobis]

On the surface, you need at least 1 turbojet per 20t or so or you can't build up speed. At low altitude, 1 intake can supply 2-4 turbojets, so more intakes doesn't help (but you can close the excess ones).

At high altitude, more intakes means more thrust, because you're limited by your air supply.

PolecatEZ's numbers are more conservative than what I fly with, but I always push the limits. I typically circularize off ion engines or a pair of 24-77s, or a single LV-N if I'm sending for a large spacecraft to Jool -- the trick is getting just barely enough engines to lift off and get to about 1500 m/s at 20km, then enough intakes to push up my altitude and allow me to make about 2100 m/s surface speed.

My KSP-scripts claim to have a function to calculate all that in the jets module, but apparently it's busted (I just tried it). The basic calculations have served me well so far.

[bsalis]

I still want numbers, (lift/weight ratios, engine/weight ratios) but your post has given me some good ideas. I really like how you use lifting surfaces as covers over the fuel tanks and engines and such.

I wouldn't lock yourself into thinking theres one best way. It depends what you want out of the craft. In the end, if it works and does what you want it to, then it's a good design.

[superm18]

Canards in the front is the way to get you off the ground

[triffid_hunter]

- A lot of people go for single jets due to the high altitude flameout issues, I use two and then 1-2 ram intakes per jet. Although recently I have started trying to stack the two jets vertically so on flameout I get a pitch error not a flat spin

if you edit your GameData/Squad/Resources/ResourcesGeneric.cfg and change IntakeAir to STACK_PRIORITY_SEARCH then edit your spaceplanes as appropriate you will get symmetrical flameout.

Using that alteration, you can make a plane with 3 engines that automatically drops to one at high altitudes without risking a flat spin.

[Shania_L]

if you edit your GameData/Squad/Resources/ResourcesGeneric.cfg and change IntakeAir to STACK_PRIORITY_SEARCH then edit your spaceplanes as appropriate you will get symmetrical flameout.

Using that alteration, you can make a plane with 3 engines that automatically drops to one at high altitudes without risking a flat spin.

I'm not entirely sure what you mean by this beyond some kind of client mod that will flameout two engines rather than one. What do you mean by "edit my spaceplane as appropriate?"

[triffid_hunter]

I'm not entirely sure what you mean by this beyond some kind of client mod that will flameout two engines rather than one. What do you mean by "edit my spaceplane as appropriate?"

At the moment, the reason you get asymmetrical flameout is that each air intake feeds all the engines because it's distribution is ALL_VESSEL like ElectricCharge and MonoPropellant. When you're close to flameout, one engine will consume the last of the IntakeAir, then the next engine that wants some air will find none and flame out, which is deadly as it puts you in a flat spin.

If you change IntakeAir to STACK_PRIORITY_SEARCH, you can set your plane up so that each air intake only feeds a specific engine. Coupled with standard aeroplane symmetry and the fact that all the intakes are going at the same speed in the same altitude, each engine will use the last of the available air at the same time- giving symmetrical flameout.

This change can be done simply by editing the aforementioned file in a normal text editor such as notepad.

A game patch that would make more sense is that if 3 engines all request IntakeAir from one intake, it gives 1/3 of its IntakeAir to each one, and engines keep tagging intakes for consumption until there's no more or they have enough. That however means that resource trees need to be walked twice rather than just once, not sure what impact that would have on game speed but I suspect it would be fairly small.

This could also ameliorate the momentary flameouts that happen with rocket engines when changing from the last fuel in an empty tank to a new full one.

[Syhrus]

I've been doing a bit of experimenting on spaceplanes (you can checkout my videos here:

but I haven't done any major tests on there about lift yet). The main thing I can say about lift is that it is a very small part of actually getting your plane to fly, but it is vital in the very first stage: takeoff.

I might do a bit of research into how lift units work, but for now this is what I know: My 26 ton spaceplane has 2 turbojets and a total of 13.4 lift rating from all surfaces (this number is probably a bit higher than the truth, as some of the canards are on an angle) and it manages to takeoff by running off the end of the runway and uses the lift to remain at a stable altitude until I maneuver to go higher. At this point, it is doing 108m/s.

So at 108m/s, 13.4 units of lift can keep 26tons at a stable altitude AT SEA LEVEL. As your altitude increases you need more lift to achieve the same thing as the air is thinner.

All this said, you can't rely on lift to get you into orbit anyway. My advice is to get as enough lift to get off the runway, and then just focus on keeping the weight down and the centre of lift back so you can control the thing.

~Sy

[PolecatEZ]

Your lift also effects your angle of attack, not just takeoff, so it is important all the way into the start of the upper atmosphere. Your angle of attack then plays into a few things, such as fuel economy and climb speed. Having plenty of lift makes the difference between an SSTO getting to 30km in 3-4 minutes vs. 20 minutes.