Spaceplane SSTO mass/thrust/lift/intake ratios

[Psycix]

My problem:

I've spent hours designing SSTO's, and while I am capable of building a one-mane spaceplane that can reach orbit and land, anything bigger than that tends to run into problems.

I generally keep going into circles like this:

-Spaceplane is too slow -> add engines

-Spaceplane won't lift off at 120+ m/s -> add wings

-Spaceplane doesn't have enough intakes for high altitude -> add intakes

-Spaceplane has too much drag on the front and becomes unstable -> add control surfaces

-Spaceplane has too much drag in general and is too slow -> add engines, and the cycle starts over

The question:

All tutorials for SSTO's cover balancing of CoM and CoL, but none have given me basic figures of the amount of thrust, lift and intake one needs per unit of mass.

Do any of you have any guidelines, ratios or "rules of thumb" that can help me select the right amount of lift, thrust (what about jet thrust vs rocket thrust?) and intakes for a certain size or weight aircraft.

[SnappingTurtle]

I'd recommend getting the B9 aerospace parts. http://forum.kerbalspaceprogram.com/showthread.php/25241-0-21-B9-Aerospace-Pack-R3-3-New-pods-IVAs-engines-fuselages-structures

It gives you more options for intakes, wings, engines and fuselage sections.

Spaceplanes in stock are a little under-served for now.

[RocketTurtle]

^ Ohmygosh another Turtle!!!!!

In all seriousness though.

Well for one thing, you should use about 2 intakes for each engine, an you typically only need 1 engine, 2 engines are dangerous at high altitudes, only use 2 engines if you have a heavy plane. Also the amount of wings depends on the mass of the plane. Once you get the hang of it it's not too hard. Also, if your plane isn't lifting of, make sure your rear landing gear is JUST behind the COM. Also, only use RAM intakes, the other ones are heavy and make planes unstable. Typically having too much drag is from overbuilding, KSP calculates drag not on the shape of your design but the parts on you design, so a plane with a flat nose would go faster than a plane with a nose cone, because the nose cone is another piece.

[Psycix]

I'd recommend getting the B9 aerospace parts.

I'm already using B9.

Well for one thing, you should use about 2 intakes for each engine, an you typically only need 1 engine, 2 engines are dangerous at high altitudes, only use 2 engines if you have a heavy plane. Also the amount of wings depends on the mass of the plane. Once you get the hang of it it's not too hard. Also, if your plane isn't lifting of, make sure your rear landing gear is JUST behind the COM. Also, only use RAM intakes, the other ones are heavy and make planes unstable. Typically having too much drag is from overbuilding, KSP calculates drag not on the shape of your design but the parts on you design, so a plane with a flat nose would go faster than a plane with a nose cone, because the nose cone is another piece.

Hmm well I'd like to use my SSTO's for more than crew transport, so how does this apply to heavy planes that need at least 4 jet engines?

I only use RAM or the B9 ones.

The landing gear thing seems like a good idea for tipping upward. The problem is that I always mount it as far back as possible in order to protect my engines from hitting the runway.

[SnappingTurtle]

You can add an extra wheel at the back higher up so the plane can still pivot but if you pull back too far the back end is still protected. Put two wheels in the middle and one at the front so there's more braking force coming from the back which will stop the plane tipping.

Here's some images of my spaceplane. It's a lot of payload, has space for four Kerbals but doesn't require more than two SABREs. It can reach about 25km before flameout.

http://imgur.com/a/WD0mU

[antbin]

-Spaceplane is too slow -> add engines

Recommend that you add engines in odd numbers - 1,3,5 etc., and always mount one in the back center. Helps reduce flameout risk.

-Spaceplane won't lift off at 120+ m/s -> add wings

Not necessarily. Adjusting the landing gear position to just behind the center of mass helps more. Can use struts or upside-down Mk3-Mk2 fuselages to get the gear further down so you don't rip the tail off. 80 m/s should be plenty for liftoff of any SSTO.

-Spaceplane doesn't have enough intakes for high altitude -> add intakes

Yup. 3 Ram Air intakes per engine is enough, especially if you use action groups to turn off engines 2 at a time when you're at near-flameout altitudes

-Spaceplane has too much drag on the front and becomes unstable -> add control surfaces

Other solutions: Move wings back, move fuel to middle of plane, transfer fuel around while in-flight. Be sure to rotate a wing near the front of the plane 1 small unit up (for lateral stability).

-Spaceplane has too much drag in general and is too slow -> add engines, and the cycle starts over

If you find you have to fly with the nose up in the air, you could also add wings. The less you have to raise the nose to get level flight, the less drag. And wings are light.

none have given me basic figures of the amount of thrust, lift and intake one needs per unit of mass.

My guess:

• 1 jet per 15 tons
  
• 3 intakes / jet
  
• 20-30 Lift Units / 15 tons.
  

Rockets enough to get a Thrust-To-Weight ratio of 1.5. Use rockets in even numbers (usually 2 is enough).

My cargo SSTO takes 40 tons into orbit, using a 90 ton aircraft. Have a look at the .craft file.

Edited August 12, 2013 by antbin

[Sauron]

My record for SSTOs stands at about 55 tons or so. Away from my PC at the moment, so no word on lift or wings, but 3 jets and 12 intakes is adequate to take off and fly at over 30km/1.5 km/s, and 2 LV-T45s are enough to finish the orbit. Avoid excessive engines. You need less than you think.

[numerobis]

What antbin said; also, tilt the wings a little bit (5-15 degrees) in the SPH so that the wings are at a positive angle of attack while your nose is level; helps takeoff and landing.

[iueras]

How do you guys actually get into orbit with one? I have a spaceplane that can get to about 1.6-1.8km/s at 28-29km altitude, but I can't quite seem to finish the get to orbit part. Have been trying to a week now. I thought I was going too slow for this type of spacecraft LOL. TWR is about 8-9 with the jets on, 7.2 on the runway before burning any fuel; this thing SCREAMS. But I can't get that last bit to orbit after cutting the jets. Sorry if I am hijacking the thread, let me know if it's an issue and I'll start a new one.

[allmhuran]

Another way to improve your drag profile is to put the intakes at the back, since intakes generate a lot of drag at high speed. It does look sorta weird, but it definitely works well.

[foamyesque]

The trick's to pitch up and punch it for orbit. That's where you want your TWR (unless you're abusing intakes). The other thing is to build such that your CoM does not shift as you burn fuel.

F'rexample:

[Anton P. Nym]

Handy trick for keeping the CoM issue under control; have a jet fuel tank well forward of the initial CoM on the craft and shut off its fuel flow before take-off. (Right click on the tank, then click the green arrow next to the fuel gauge to turn it into a red circle-and-bar.) That reserves the fuel, and its balancing mass, during ascent. Treat that as reserve fuel, and open up its flow only when the other jet tanks are empty and you need it to finish landing. This trick keeps my big Broadhead SSTOs stable upon re-entry when otherwise they'd end up in a Sabre dance on final approach.

-- Steve

[allmhuran]

How do you guys actually get into orbit with one? I have a spaceplane that can get to about 1.6-1.8km/s at 28-29km altitude, but I can't quite seem to finish the get to orbit part. Have been trying to a week now. I thought I was going too slow for this type of spacecraft LOL. TWR is about 8-9 with the jets on, 7.2 on the runway before burning any fuel; this thing SCREAMS. But I can't get that last bit to orbit after cutting the jets. Sorry if I am hijacking the thread, let me know if it's an issue and I'll start a new one.

A bit of a hijack but a useful question. If you have such a high TWR you should be able to get quite a lot faster than 1.6-1.8 at 30-odd-thousand. This suggests to me that not all of your thrust is being directed into forward velocity, or to put it another way, it sounds like you might be forced to take a high angle of attack (nose up) at high altitude so that your engines can keep you up there, meaning some of their thrust is being "wasted" as lift, which suggests you might need more lift so you can fly closer to nose-level. Add a couple of high lift surfaces, either delta wings, or the long straight ones, or even a couple of small delta winglets (0.7 lift each, and very light). This will also mean your intakes get more air, and you can fly even higher. I find that if you break 2100-2200 at 35000 getting to orbit only takes a little bit of a squirt (at 45 degrees nose up) to pop you up above the atmosphere, and usually only requires one or two quite small engines like LV909's or LVT45s.

Edited August 13, 2013 by allmhuran

[iueras]

Thank for all the answers guy, you are all awesome! I will work on it some more with everything that has been said in mind. and let you all know when I finally make it

EDIT: I redesigned some after taking into consideration what everyone said in this thread and I am happy to report I made it to orbit in my little spaceplane.

Edited August 14, 2013 by iueras

[Blaster]

...but it looks too awesome to have a pair of quadpack engines on the plane :/

[ChevronTango]

My protip is to get as much lift out of the wings as you can, by tilting them back to 25 degrees to horizontal (the optimal angle of attack in Kerbal... for now). It means the wings are more efficient, you can keep the plane pointing horizontal and not waste fuel by having the jet engines firing nicely prograde. Less wings means less drag too.