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There's a lot of chat about SSTOs for KSP. I recently managed to design, launch into orbit, and recover an SSTO.

However I have to say that having achieved this, I don't really see what the appeal is. The remaining d/v is fairly bad, managing the different engines and fuels is cumbersome, and it's trickier to launch and land than a regular rocket. Furthermore, since you would be keeping the wings and structure for landing, you can't even toss them off once you're in space to lose mass.

Furthermore, it seems to me like there's always an awkward gap between actually being in orbit and when the airbreathing engines run out, when you still need a moderately high TWR to finish getting into orbit. I've looked at things like the Rapier but it seems to me like their vac performance is pretty bad TWR and mediocre ISP at best.

So in short, what actually is the point of making and using SSTOs or even just spaceplanes in general?

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The primary purpose of spaceplanes is fun and challenge.

While you can theoretically operate them on cost of fuel alone (providing you don't crash), if you take profits "per hour of gameplay" from completing contracts with unrecoverable or partially recoverable launchers to profits you can achieve in the same (real) time with spaceplanes, the balance goes way towards rockets - you can finish three contracts earning 70% value from each in the time it takes you to complete one contract earning 98% of its value with a spaceplane.

This goes even worse for the two sandbox modes where money is non-issue.

The value of spaceplanes lies in the difficulty curve of building and flying them. Rockets are easy. After a while you'll want to install MechJeb simply to avoid the tedium of launch process. Meanwhile, spaceplanes are challenging.

Sure, the "official" goal of them is completing contracts. Tourists, satellites, maybe site surveys after a suborbital fight. But it takes skill and patience to build a spaceplane that handles well on climb, reaches orbit reliably and in non-excessive time, can carry a non-token payload, handles well on reentry and landing, looks nice and maybe even doesn't cost a fortune.

[I've purposely listed handling on climb, reentry and landing separately... I might add early and late climb. These are all flight phases where different traits of the plane are important - e.g. on late climb you can't count on air to stabilize you if your thrust is off-center; on reentry your Center of Mass will have shifted after burning the rocket fuel. On landing you must fly slowly and control your descent rate actively...]

In short, spaceplanes only marginally fit into the "big picture". You can visit all the planets, complete the science tree and win many challenges without ever visiting the SPH or upgrading it. But they are a game of their own, a separate challenge, something fun to do next to all that standard work.

Plus there's few things in the game looking more impressive than a mighty, big spaceplane docked to a space station.

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  • Let me add using launch-stage SSTO rockets is the best of both worlds. They are easier to launch and operate than staged launchers, they take a fairly short time to land, and you recover great most of their value. At expense of maybe 3-4 minutes of your time you can easily and reliably recover most of your costs. And if you don't want to bother with the recovery, just dump them 30m/s before reaching the orbit and let them burn, they cost only marginally more than staged launchers. – SF. Sep 22 '15 at 14:37
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The point of a single-stage-to-orbit vehicle is that it can return back to the space center in one piece, where you can recover it for 100% of the initial cost (Maybe a better acronym would be SSTOAB - single-stage-to-orbit-and-back). That means getting the payload into orbit costs you nothing except fuel.

A common use-case for SSTOs is as delivery vehicle for satellites. When you have a "satellite in Kerbin orbit" contract, add a cargo bay to your spaceplane and place a satellite in it which has a small fuel tank and thruster so it can maneuver itself into place. Use your spaceplane to get it into orbit, release the satellite, maneuver the satellite into the desired orbit and deorbit the spaceplane so it returns to the space center. Fulfilling the mission then costs you nothing except the price of the satellite itself and a bit of spare-change for fuel.

When you unlocked the Mk3 spaceplane parts, you can even transport rockomax-scale parts into orbit in cargo bays, so manned missions which get into low-kerbin orbit through SSTO become an option. A more ambitioned mission might require to dock several sections in orbit, which you can all launch per SSTO.

Regarding the rapier: While it looks quite mediocre on paper in both of its roles as a jet engine or a rocket engine, the ability to fulfill both roles in one part can make more than up for it. Dedicated engines for one flight phase are just dead weight in the other. That means when you compare the TWR of rapiers with that of a combination of jet- and rocket engines, you need to add the mass of the rocket engines to that of the jet engines and vice versa. Also, having more engines means you will have to design a craft with a larger aerodynamic cross-section.

  • SSTO/Spaceplanes are completely nerfed, IMHO. Their part requirements (and the science and funds to unlock them) totally diminishes their cost savings. The Rapier is usually the last part you unlock and by then (end game) funds aren't at a premium so you might as well use a rocket. Also, getting rockomax-scale parts into orbit is child's play with rockets. – Coomie Sep 7 '15 at 7:54
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The answers above are great: SF is correct that SSTOs are largely for fun and challenge, and Philipp points out that it can deliver small payloads to orbit. I'd like to a points.

First, SSTOs can also deliver 'non-structural' payloads (resource in tanks, science, and Kerbals) to and from orbit. I've used SSTOs to get Kerbals and supplies (TAC-LS) up to an LKO station, and bring Kerbals and science back down.

Second, while the term "SSTO" is typically applied to hybrid open-/closed-cycle spaceplanes that can can make the surface-orbit round trip at Kerbin, you can apply it to any lander that can make a in one-stage round trip (down and back). So, SSTOs on other bodies (Minmus, Mun, Laythe, etc) can be useful for moving all kinds of stuff from surface outposts to orbital stations repeatedly if you make the SSTO reusable, and if it's not reusable, what's the point?

The third point is really a sub-point of the second. If you can build an SSTO spaceplane that works on Kerbin, it should work on Laythe, too. Again, this is a very useful method for going to and from Laythe's surface for a minuscule amount of fuel, which is ALWAYS useful far from home.

Hope that helps.

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