SSTO essentials?

[Claw]

...

Also, remember that the plane landing gear is actually massless while flying. If they're on the plane in the editor, your CoM will be a lie.

You can get around this one by editing the part.cfg for the gear and changing to mass = 0. No more lies from the SPH/VAB.

[GoSlash27]

#1 thing I can recommend is this:

Balance the center of lift to the center of gravity for the fuel tanks first so it doesn't go all hinky as the fuel drains off. Everything else can be balanced after that.

-Slashy

[Aghanim]

First of all: Make sure that you have enough TWR for the rocket 'stage'! You are allowed to have TWR <1 for the airbreathing stage but up there, in ~25km where the air is thin and wings doesn't produce that much lift, you need TWR >1 so you could point straight up and still accelerating

[Claw]

First of all: Make sure that you have enough TWR for the rocket 'stage'! You are allowed to have TWR <1 for the airbreathing stage but up there, in ~25km where the air is thin and wings doesn't produce that much lift, you need TWR >1 so you could point straight up and still accelerating

While this is a valid way of doing things, a TWR greater than 1 for the rocket stage is absolutely not a requirement. You wouldn't quite need a TWR of 1 even if you want to point straight up because gravity is lower in LKO, but I'm not sure if that's what you meant.

Anyway, for most typical space plane designs you want the air breathing stage to do most of the pushing up to orbital speed. Then the rocket engines just do a bit of work finishing the orbit. Assuming you get the speed up where it ought to be with the jets, you won't need rockets with a TWR greater than 1 to finish the orbit.

[airelibre]

Wow, I didn't think I'd get so much good advice! Thanks guys.

[RexKramer]

Lots of good advice here.

Several posters have suggested starting small and simple, and I heartily agree. Personally I was quite frustrated with my initial attempts to get 3 Kerbals and a boatload of fuel into orbit with a winged SSTO.

After playing around with very small and simple SSTOs things began to come together.

Start small and simple..

[Hodo]

All I did for most of my 1000+ hours in KSP is build SSTO space planes. I have built them to work in FAR+DRE, and I have built them to work in RSS Kerbin (not Earth). I am now working on building them in Realism Overhaul, which is proving quite challenging with AJE but I am learning slowly but surely.

Here are some of my old favorite designs to inspire.

SVO-25 Bison II based on my first successful SSTO.

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F-145 (then XF-145) my first combat capable SSTO craft in KSP. And built with only stock parts.

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My pride and joy, SP-406 super heavy lifter that can haul 108 tons into orbit.

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And its VTOL variant

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SVO-9 that was my first SSTO to go to another planet and landed on Ike.

So you can see you can build space planes, and they can be fun, and practical even though some people will say you are wasting your time. It is ultimately your game play how you want.

[O-Doc]

You haven't mentioned whether or not you have flown other people's SSTOs. So, I'll just recommend that learning to build and learning to ascend go hand in hand. There are many different ascent styles and you should learn to do them all. Check out youtube for some basics tutorials so you know how to get into space efficiently.

I'll also recommend this shopping list to scratch build your first SSTO:

1x Mk1 Cockpit

1x R.A.P.I.E.R. Engine

1x FL-T400 Fuel Tank(half oxidizer)

1x Inline Reaction Wheel

2x XM-G50 Radial Air Intake

2x Delta Wing

2x AV-R8 Winglet

1x Delta-Deluxe Winglet

2x Standard Control Surface

3x Small Gear Bay

1x Telus Mobility Enhancer

1x PB-NUK Radioisotope Thermoelectric Generator

These parts will get you into orbit an back again in a plane-like SSTO.

[Claw]

You know, I would also argue that if you have very little experience with planes in general, that you should spend some time just making a simple plane and fly it around with (initially) no intention on going to space.

Move wings, flight controls, engines, and mass all over the place and watch what happens when you fly. Try all kinds of stuff including unstable designs. When you start to understand what kinds of response comes from different designs, it makes it easier to troubleshoot complex craft.

[gchristopher]

Lots of good advice, especially the "just fly a plane around and have fun" advice.

There're a LOT of ways that a SSTO spaceplane can be successful. Even stupid spaceplanes can work. I've built plenty. For me, there were three major areas of learning:

- Getting it off the runway. (Other posters/links have already covered the major points: structural stability, enough thrust, enough lift, relative COL/COM position, and having the COM close to the rear wheels.)

- Getting to space: I have a couple thoughts here that others haven't covered.

- Landing: for me, this mostly consisted of flying slowly enough, which meant having enough wings.

Getting to Space: (These are all stock game observations. Mods such as FAR do change everything.)

Once you're in the air, there are a lot of flight patterns that will get you to space. All have some variation on using air-breathing engines until you run out of air, then use rockets. (Or switch RAPIERS) There's lots of good advice already referenced on how to switch your engines off to avoid spins, and how to manage air intakes.

Early on, it's probably easiest to just bring up some extra rocket thrust and fuel, and just switch over to a familiar rocket ascent once the air runs out. This is straightforward, and doesn't require any fancy piloting. All the jets and wings need to do is get you up and out of the first 30k of thick atmosphere.

If you start focusing on efficiency, then how you fly the plane matters more. Looking at Kasuha's flight reports taught me the most about how this works. Don't worry about things like this until WAY later, but for the sake of discussion, here's what I've learned.

Typically there's an ascent at lower altitudes (<20k) where all you're doing is trying to climb out of the thick soupy lower atmosphere.

Then you begin a careful balance of speed vs. altitude. Jet engine max thrust depends solely on speed. As you go faster, you have less thrust. This took me a while to really start understanding the implications.

If you start going too fast, too low, your jet engine thrust might not be enough to continue an ascent, and your altitude will drop until you slow down enough to gain more thrust and give the ascent another try. If you're going too slow, too high, you run out of air for your intakes and you don't make good use of your jets.

As far as I can tell, a really good jet ascent gains speed and altitude smoothly up to 25k-30k (more on that in a second) such that you can power up to as high a velocity as possible (2,300 m/s is great) during the region when you have enough atmosphere to feed your air intakes, but not so much that the drag stops your acceleration. This takes trial and error and varies a lot by plane.

This is where the number of air intakes start mattering more than any other concern. ("air hogging" is the term, mostly used derisively, because it's unrealistic, but I'm just taking the game as it is, and leaving opinion out of it.)

Once you're up high, the limiting factor is only how much air you can get to power the jets. If you have enough air intakes (up to 1 per ton!) then you can keep just enough thrust going from the jets to keep up with the tiny amount of atmospheric drag. All you're trying to do here is maintain that velocity you built up earlier, while slowly creeping up in altitude.

This is the really weird thing. At 30K altitude, moving at 2,300 m/s won't put you in orbit. But if you can eke out just enough jet thrust to maintain that speed, then by the time you're pushing 40-50k altitude, the same orbital velocity will give you a apoapsis that's ABOVE the atmosphere! So just zip along on a tiny bit of jet thrust and you'll eventually skip up and out of the atmosphere on the far side of your orbit! (Which is probably crazy and unrealistic)

This is how people get the ultra-efficient payload fractions, because this way, the jet does almost all the work. You might need only 20 m/s of rocket thrust in space to circularize your orbit in this case. Even if you can't get all the way out of the atmosphere on jet thrust alone, you still have done most of the work and you'll be going fast enough in thin enough atmosphere to be able to rely on the low-thrust, high-efficiency nuclear engines.

The best I've done is having 70% of the launch mass be payload this way.

Apologies, I suspect this is an advanced topic, but I haven't read much discussion of it. I wonder if I've summarized the idea well?

Edited June 18, 2014 by gchristopher

[Hodo]

My last bit of advice is, remember not all SSTOs are planes.... So don't be afraid to think outside the box.

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[Pecan]

...Apologies, I suspect this is an advanced topic, but I haven't read much discussion of it. I wonder if I've summarized the idea well?

There has been an awful lot of discussion on spaceplanes because they are so hard to get used to. You have summarised it all very well I think, the critical thing being to build horizontal, not vertical speed. The one thing you didn't mention though is that you can keep the jets thrusting - and the plane accelerating - when they would otherwise flameout by reducing throttle, and therefore their air requirement. That was the one thing that made spaceplanes work for me - until I nicked Claw's intake-stack clipping technique.

[gchristopher]

The one thing you didn't mention though is that you can keep the jets thrusting - and the plane accelerating - when they would otherwise flameout by reducing throttle, and therefore their air requirement.

Oh, true! In practice, the tasks of toggling air intakes, sequentially shutting down jets, and reducing throttle was tedious, annoying, and ate up all my action keys. That's once case where the Mechjeb utilities to manage intakes and clamp the throttle do a lot to increase the fun. That'd be extremely difficult to do by hand for large planes. Thanks!