SSTO Space Planes - going bigger

[mellojoe]

I can't seem to make SSTO space planes with any size. They are all typically super small crafts. Any larger, and I find that they die out at around 38km, and then use all the fuel burning rockets to get to orbit. Is there some trick to going bigger? More wings = more lift? Or more wings = more weight? Which is better?

Here are some failed attempts to go big:

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This one worked perfectly. So good, in fact, that I only had to use the rockets to circularize, as I got an AP of 70k on jet alone. But its so tiny.

Whats the trick to getting SSTO's bigger? More wings? Less wings? More power?

((edit))

You can clearly see in the 2nd picture that I'm not scared to try airhogging either. I put 4 intakes per intake.

[Jouni]

I'm not sure about KSP aerodynamics, but if you remove wings, SSTOs scale up quite well. If you want to make the ship ten times bigger, you just add ten times more engines, intakes, and fuel. At some point, the designs start looking ridiculous, because there are no size-2 jet engines.

It's probably enough to add ten times more wings and control surfaces as well, assuming that you can still make the plane balanced and reasonably rigid. Alternatively, add even more engines, intakes, and fuel to compensate for the lack of lift.

[Monger]

One of the problems with big SSTOs is that you need increasingly bigger wings, and therefore a bigger body to attach them. Making the aircraft a lot longer or wider makes it increasingly more difficult to fly. Also, wing parts have pretty weak junctions, i.e. you can't stack a lot of them without breaking them.

At some point, you need some kind of sturdy frame. Just using the fuel tanks as skeleton won't suffice.

By the way, this is a real world problem as well. Bigger planes are usually much more cost-efficient than small ones, and still they didn't grow very much in the last 50 years. The Airbus A380 is the biggest civilian plane ever built, and it "just" has a length of 79m. The Saturn V measures 110m.

[TheOfficialStorm]

I would recommend going less intricate than slowly going bigger. That way you will know the requirements to get one off the ground.

[Pecan]

Getting a high apoapsis isn't the problem, it's building up enough speed to get your periapsis up that's where the efficiency of turbojets pays. To get a high apoapsis you just need to go up as fast as possible but that'll leave you with a big circularisation burn on rockets. 38km is definitely high for jets but - somewhat lower than that, before you need to cut the throttle right back - you should be flying more or less level, which means your apoapsis might only be a kilometre or two higher than you, As your speed increases in the thin air there though your periapsis will rise then, eventually, you nose-up and add rockets to raise apoapsis, coast there and do a small circularisation burn.

[m4rt14n]

You need multiple wings. Tri wing design works best. I'll post some pics whn I get to my comp

[Claw]

I missed this thread last night. When you say "go big," how heavy are you talking about?

If you're looking for space planes the size of what is in your pictures, they look reasonable from a design standpoint. It leads me to think your flight profile might need some help. Can you talk us through how you're flying it up to 38km?

I'll try and recreate some of your ships later (or if you want to provide a .craft for us) and try to get a sense of what you're flying. They look like they have plenty of thrust (maybe too much) and enough wing area though.

[capi3101]

General guidelines for spaceplanes:

A)1 Turbojet engine for every 9-12 tonnes of payload (everything but the wings, jets, jet fuel tanks and intakes; the ratio of payload to engine is highly dependent on number of intakes).

B)200-250 units of liquid fuel per jet engine.

C)One pair of swept wings or delta wings per six total tonnes of aircraft (that includes your jets and their fuel tanks).

D)No fewer than three Ram Air Intakes per jet. With 3:1, use 9 tonnes per engine and add half a tonne for each whole step increase in the ratio, to a max of 12 (4:1 = 9.5, 5:1 = 10, 6:1=10.5, 7:1=11, 8:1=11.5, 9+:1 = 12)

4:1 is not really scary airhogging; its not unusual for me to go 8:1 with my designs, though there are exceptions:

(The Auk 8 runs out of jet fuel all the ruttin' time...)

I think in your case it's a combination of not enough lift (moar wings) and not enough thrust. Difficult to tell without taking the time to calculate how much mass you're talking about.

Any case, check your designs against the guidelines I've listed above. If you're meeting them all and you're still having trouble, let us know.