SSTO limitations and interplanetary spaceplanes

[TokiTech]

The best efficiency interplanetary spaceplanes seem to break the definition of SSTO. For vanilla game we must have tons extra air-intake parts and maybe few addition Turbojet engines powerful enough to shoot to orbit at high velocity. But those parts are dead weights once in space and has no use anywhere else except Laythe that has oxygen. Ideally is to throw those weights off when the plane is in orbit so to saves DV to go else where, but that strips 'SSTO' title off the plane. Is the title really that important? I wondered.

[GoSlash27]

The best efficiency interplanetary spaceplanes seem to break the definition of SSTO. For vanilla game we must have tons extra air-intake parts and maybe few addition Turbojet engines powerful enough to shoot to orbit at high velocity. But those parts are dead weights once in space and has no use anywhere else except Laythe that has oxygen. Ideally is to throw those weights off when the plane is in orbit so to saves DV to go else where, but that strips 'SSTO' title off the plane. Is the title really that important? I wondered.

It's worse than you think.

I have actually built a successful Kerbin-> Laythe and back manned SSTO. It also did Duna and Eve (yeah, Eve SSTO) on the same trip. With absolutely no fuel expended at all.

Combining an infiniglider with Kraken drive completely breaks the game.

But that's not what we're talking about here.

Best,

-Slashy

[King Arthur]

Ideally is to throw those weights off when the plane is in orbit so to saves DV to go else where, but that strips 'SSTO' title off the plane. Is the title really that important? I wondered.

https://en.wikipedia.org/wiki/Single-stage-to-orbit

A single-stage-to-orbit (or SSTO) vehicle reaches orbit from the surface of a body without jettisoning hardware, expending only propellants and fluids.

Subsequent disposal of hardware once in orbit has no bearing on whether something is an SSTO or not.

[cicatrix]

(yeah, Eve SSTO)

This! How is it possible? No oxygen, 11 km/s dV?

[GoSlash27]

This! How is it possible? No oxygen, 11 km/s dV?

Abusing the loopholes in the physics engine.

Using control surfaces as wings allows you use lift as thrust and drag as lift. Then attempting to disengage parts while travelling faster than 800 m/sec will result in an imparted acceleration indefinitely.

This allows you to do all sorts of crazy stuff without fuel. It's cheaty as hell.

Best,

-Slashy

[diegzumillo]

Hello, crazy people. It's the op reporting back with great news! I learned a lot from this thread, so thanks everyone, and goslash27 specially!

Here's the application of what I learned:

Orbiting Kerbin with ~4800 dv left. Might need more than that to be really called interplanetary but I can totally see that happening now, it really doesn't have an upper limit.

At least to me the bit of information that really tipped the scale is about TWR and ascent path. For some reason I was under the impression that large spaceplanes with large TWR were impractical so they needed an ascent that relied more on rocket engines. So now I threw that assumption out the window and only used a nuke engine for rocket propulsion, and 4 jets (with 2 air intakes for each). That gave the plane a TWR larger than 3, which allows it to pretty much reach 'orbital' speed inside the atmosphere.

[GoSlash27]

Outstanding! I love seeing this sort of thing.

Now that you've got the first one under your belt, the rest will be easy.

Best,

-Slashy

[Pecan]

...Ideally is to throw those weights off when the plane is in orbit so to saves DV to go else where, but that strips 'SSTO' title off the plane...

As King Arthur said Single Stage To Orbit doesn't say anything about what happens once the thing has got to orbit, including even any subsequent ability to return or be reused. Efficiency and reusability are both best served by using a three-part mission profile - (reusable) SSTO vehicles at Kerbin and target destinations and space-resident/refuelling transfer vehicles. Jets & Wings at Kerbin and Laythe, Rockets/Ions only everywhere else (wings optional at Duna).

Unless you're abusing the system, as Slashy says - which is also fine, of course :-)

[diegzumillo]

You could have a detachable wings system to get more delta v. I don't think it would declassify it as an SSTO, seeing you should put it back on when you want to return to kerbin. So you fly to orbiy, leave your wings, jets and air intakes orbiting kering, go do whatever you want, come back, put them on and start reentry..

[Pecan]

...So you fly to orbiy, leave your wings, jets and air intakes orbiting kering, go do whatever you want, come back, put them on and start reentry..

Yep, that's the idea - or if the mission's going to take years, just de-orbit your SSTO launch vehicle to keep the place tidy and launch it/similar later to refuel or land whatever comes back. Simple, efficient and modular.

An ultimate reusable infrastructure has SSTO landers everywhere you want them (optionally with orbital 'stations' with fuel/science as well) and space-resident transfer vehicles that haul everything around. Put them into orbit with SSTOs in the first place, use the transfer vehicle(s) to deliver the landers/stations, bring the transfer vehicles back and SSTO extra fuel, replacement crews, as required. Optimised SSTOs for each body (except Eve, obviously) do the lifting, optimised space-only vehicles do the long-distance haulage. With science labs everything except fuel gets reused again and again without anything except the Kerbin SSTOs having to land back at KSC and only the transfer vehicles having to come back to Kerbin orbit :-)

[TokiTech]

As King Arthur said Single Stage To Orbit doesn't say anything about what happens once the thing has got to orbit, including even any subsequent ability to return or be reused..

Hm.. didn't cross my mind the definition is simply what it reads. I always think SSTO crafts must take off and come back in one piece and always appear as plane, but it doesn't have to be in this sense..

[Spatzimaus]

None of this counters what I've said, so either I gave a misleading impression of what I meant or you misinterpreted what I said.

Probably a bit of both. But this isn't like dealing with my insurance company after that guy T-boned me a few months ago, where assigning blame matters, so who really cares?

A vertical lift SSTO requires more engines to lift the same mass as a horizontal takeoff SSTO, so it is somewhat less efficient due to the extra mass.

True, although you do save a bit of mass in wing surfaces and such. The real advantage is that it's a lot easier to scale a vertical SSTO up to ridiculous sizes, as shown in the picture I posted; my largest vertical booster masses about 14000 tons (payload capacity ~1200 tons), and I'm going to double that in the near future to lift my REALLY big space stations (~2000 tons). My largest current cargo spaceplane is only about 400 tons, with a cargo in the 50-ton range, and it'll be really hard to scale that any further. So, when I wanted to launch my latest 600-ton rover, a horizontal launch just wasn't really an option; I ended up using a 6000-ton vertical design.

And while it is trivial to recover a vertical lift SSTO intact, it's not liable to end up on the runway at KSC at the end of the mission, whereas a (properly designed) spaceplane will do that every time, thus recovering all of your funds.

I don't have the exact numbers, but the recovery percentage for the runway isn't noticeably different than what you get for landing in the grass nearby. With a little practice, it's not hard to get a parachute landing within sight of the KSC. It just takes a bit of trial-and-error to know how much the air resistance will affect your landing site on the way down.

A spaceplane can land somewhere safely in the event of a failed launch without destroying the cargo, whereas a vertical mass lifter will doom it's cargo if something goes wrong.

On a vertical design, all you need to do is deploy the parachutes and land, same as you would once the payload is gone. Sure, a couple of the chutes might be blocked by the payload itself, but if you design right this isn't an issue. I used to have an SSTO lifter I called the Bucket, which carried its payload inside a U-shaped cavity in the middle and ejected it out the front once in orbit. Nothing blocked its chutes in case of emergencies. My cargo plane is the reverse, with a U-shaped body that ejects its cargo out the back (between the engines). The advantage of this is that the payload is sitting right on the center of mass, so the balance doesn't fundamentally change once it's gone. My more recent "Brick" rocket designs (like the one in that picture above) have the payload sitting on top, but the booster is wider than any payload so most of the chutes won't be blocked even if it's still mounted.

The bigger issue is the much higher mass (due to payload and all the fuel) than the chutes were designed for, but as long as you have any engines left you can use them to slow down the descent at the critical moment to keep this from killing you. Just make sure to disable engines that lost their counterparts...

The real problem with this whole argument, though, is that we're talking about a thin shell of metal around tons of high explosive regardless of whether it's a vertical or horizontal launch. Any "failed launch" is likely to have destroyed sizeable parts of the craft, if not the whole thing, so the ability to recover the cargo intact will depend entirely on what you're left with; a VTOL that's lost most of its engines and such can still land, since the chutes are at the other end, but a plane that's lost its left wing mid-flight isn't going anywhere but straight down. In fact, I'd give the VTOL better odds than the horizontal launch, because of WHERE any critical issues come up; on a plane, the most dangerous parts of the flight are the takeoff and landing themselves, and if you screw up there nothing at all will survive including the pilot. (See also: the threads talking about "ejection systems" for pilots.)

and on another note,

For vanilla game we must have tons extra air-intake parts and maybe few addition Turbojet engines powerful enough to shoot to orbit at high velocity. But those parts are dead weights once in space and has no use anywhere else except Laythe that has oxygen

That's kind of what RAPIERs were added for. Turbojets are dead weight in orbit, as are wings, but by using a RAPIER that turbojet also doubles as your emergency low-efficiency thrust. You'll still want something more efficient if you're doing interplanetary missions, but it's still handy to have the ability to make a short burst of high thrust if something starts to go wrong while landing on a planet with decent gravity. So, it's not quite dead weight in orbit, and the jet mode does come in handy on the landing even though the full thrust isn't often needed.

Of course, any type of single-stage design has this issue. What use are landing legs and parachutes in orbit? Sure, you'll need them at the end of the trip, but it'd be more efficient to just put some small docking ports on your design and have it meet up with a skyhook-style landing assist module on the way back. And obviously, your efficiency would improve if you could dump all of those now-empty fuel tanks you're carrying along... which is basically what multiple-stage designs do anyway.

As for terminology, I'll agree with the others: having a BOOSTER that's single stage to orbit, carrying a payload exclusively designed for interplanetary flights, doesn't disqualify you from using an "SSTO" label, at least as applied to the booster itself. That big stack of rockets I use as a booster obviously isn't going anywhere but orbit, but it's still an SSTO by any reasonable definition. I've used it to launch space stations, giant rovers, tiny ion probes, you name it, and none of those are aerodynamic. Likewise, my heavy-lift spaceplane is used to loft things like 30-ton rovers with engines that fly themselves to whatever planet/moon needs one; the plane is clearly an SSTO, but the payload is what goes anywhere. (With that plane I don't usually aim for the runway on landing, but on my last trip I coasted on the grass until the plane was BETWEEN the buildings at KSC.)

That being said, this thread's title also includes the phrase "interplanetary spaceplanes". If the "interplanetary" part is completely separate from the "spaceplane" part, it's not quite the same thing.

------

Part of the problem IMNSHO is that there are only three destinations with atmosphere, and only one has oxygen, so all those plane bits seem unnecessary once you're in orbit. If more bodies had even a trace of atmosphere, that could change. Imagine if Tylo had a thin Duna-type atmosphere; wings would be a lot more useful there for descent, although the ascent would be even tougher than it is now. (There's at least one mod that does this, that I know of.) Or take Dres or Eeloo and give a thin (Duna-level, i.e. 20% of Kerbin) oxygen atmosphere; what I really want is a moon with low gravity and a decent oxygen atmosphere, as a perfect place to play around with efficient spaceplanes. (Physics-wise that'd be completely unrealistic, though.) Back when there was talk about adding a second gas giant, I was really hoping one of its moons would be like that. (Or imagine if the gas giant itself had enough oxygen in it to allow intakes to function... now THAT would be a fun place for planes.)

Of course, this also depends on what mods you use. Eve gets a lot easier if you're using blimps, electric propellers, or kethane-burning turbojets. With those sorts of things, it's theoretically possible to make a design that can go to Eve's surface and back in a single stage...

Edited December 13, 2014 by Spatzimaus

[GoSlash27]

Spatzimaus,

Any "failed launch" is likely to have destroyed sizeable parts of the craft, if not the whole thing, so the ability to recover the cargo intact will depend entirely on what you're left with; a VTOL that's lost most of its engines and such can still land, since the chutes are at the other end, but a plane that's lost its left wing mid-flight isn't going anywhere but straight down. In fact, I'd give the VTOL better odds than the horizontal launch, because of WHERE any critical issues come up; on a plane, the most dangerous parts of the flight are the takeoff and landing themselves, and if you screw up there nothing at all will survive including the pilot. (See also: the threads talking about "ejection systems" for pilots.)

In this case, I'm talking about KSP failure modes, not real- world.

A KSP launch is most likely to fail because either a) control is lost or the vehicle ran out of fuel before establishing orbit. At least, that's how it works at my house.

KSP launch failures generally don't happen where large chunks of the vehicle have decided to take a separate vacation.

So option A: vertical lifter with payload

Hanging enough parachutes on it to save the payload severely reduces the efficiency, which is why I'm using an SSTO lifter in the first place.

If I'm clearly not going to orbit, I have no choice but to jettison the payload and pop the chutes. The payload is dead.

Option B: spaceplane with payload

I don't need chutes because I can fly.

If I'm clearly not going to orbit, I can simply fly to the nearest dry land, set 'er down, and hail a cab.

Option B is clearly the better option when Kerbin lives are in the balance. And since wings weigh less than engines, my cost efficiency for my spaceplanes is actually *better* than with my vertical lifter.

So from a safety standpoint, my Kerbals are forbidden to hitch a lift on a vertical lifter. Strictly cargo only.

From a cost- effectiveness standpoint, the spaceplane is still more economical than the vertical lifter.

So "if it fits, it ships", as the old commercial goes.

For everything else, it's the chunker.

Now... that's not to say that a chunker *couldn't* be made safe. Just a matter of adding an escape tower at the very least.

Sorry for the rambling!

-Slashy