SSTO limitations and interplanetary spaceplanes

m4rt14n · December 11, 2014

Bah. Duna is overrated. Go Single Stage To Dres!

Hodo · December 11, 2014

Hi all
I've been trying for a while to make an interplanetary ssto space plane with no success. I'm disconsidering refueling in orbit, of course. My best designs are able to make a round trip to mun and minmus (including landing) but I never managed anything remotely close to going to another planet.
There seems to be an upper limit to spaceplanes, they simply don't scale up. When I try to make a bigger spaceplane based on a successful smaller version it takes so much fuel to get it to orbit that it gets there with the same amount of delta v as the previous version, approximately.
Anyone has any thoughts on it? ever succeeded in making an interplanetary spaceplane in one stage or maybe a non-space plane ssto? again, without refueling.

I have sent SSTO space planes to other planets, mainly Duna and Ike way back in .21. But there was a refuel in orbit of Kerbin before it left for the trip and then it used the Kethane to refuel itself for the return trip.

I have been planning on another trip out to the other planets, like hitting up Jool and Dres. But I haven't had time to do it yet.

Rune · December 11, 2014

Bah. Duna is overrated. Go Single Stage To Dres!

That is awfully ironic coming form you! Tired of the redness at home? In all seriousness, though, that should be about the same, at least in delta-v.

Pecan · December 11, 2014

I didn't say SSTOs aren't useful. Well, I say it now
The only two places where they have somewhat questionable advantages are Kerbin and Laythe. And you must carry the dead weight of jets and wings to all other places. I could probably agree that on the direct Kerbin - Laythe - Kerbin route a jet SSTO might have its uses, but this would be about all. Plus you'll need a flat 'runway' to take off and land.
Other use might be Kerbin-LKO-Kerbin but in order to lift a big payload you will need several trips thus burning more fuel in the end.

I am no great fan of spaceplanes because they're a pain, but their potential efficiency isn't in question. As you've said, a well-designed rocket can give 18-20% payload-ratio to orbit but no-one seems to have mentioned that a jet-SSTO can easily give you twice that even in stock, simply because the jets are so efficient. The heaviest spaceplane I've built was 38t and designed to ferry 40t (detachable, selectable, replaceable, modular) payloads into orbit and back - better than 50% payload-ratio with minimal fuel-costs and complete reusability. Other people can (and do) do better than that but my preference is for VTVL* SSTOs since wings are only extra mass, except for landing. I can get close enough to KSC on de-orbit that I'm recovering 98% of costs anyway and I don't find the potential extra 2% pays for itself or is worth the effort. Taking a spaceplane any further than LKO (apart from Laythe) is totally inefficient, especially without refulling.

LethalDose vehemently disagrees with me about terminology (I think an interplanetary single-stage shouldn't be called "a SSTO" amongst other issues) but even we agree that "SSTO" does not mean "Spaceplane" and that far too many people 'do' spaceplanes because they mistakenly think they are necessarily more efficient.

FWIW, here is the general "Single Stage" consensus I've gleaned from everyone's comments and shouldn't be too controversial:

A "SSTO" vehicle is one that can (at least) go from launch to orbit without jettisoning parts
...Therefore a SSTO you can't get back and reuse is almost always a mistake (I call this a disposable-SSTO whereas LethalDose doesn't consider it a "SSTO" at all)
...Rockets, VTVL jets and spaceplanes can all be SSTOs
...Staging is always more mass-efficient except when a single small engine & fuel-tank is all you need anyway (tiny probes and/or low gravity)
Since the point of a SSTO is to be recoverable/reusable fuel-cost per launch is almost all that counts
In oxygen atmosphere jets always beat rockets except where even a single jet is overpowered
...Therefore it is more efficient to SSTO with jets than rockets
...But SSTO rockets can be simpler and quicker to build and fly and should be lower part-count
Wings are excess mass
...That make landing where you want to a lot easier in atmosphere (LethalDose again makes this a requirement of the definition of "SSTO")
...And mean you can take-off with a TWR < 1 (I'm unconvinced about the value of this, but Wanderfound and others who are masters of spaceplanes seem to advocate it)
...But VTVL jets, without them, can be very efficient (Stratzenblitz's designs are my exemplar here)
Single Stage Beyond Orbit (except to Laythe) is always inefficient because at the very least you're carrying partially-empty tanks and the wrong/too many engines
...To be useful a SSTO usually has to deliver a payload and/or rendezvous/dock once in orbit (pointed out by Slashy)
...Therefore it is more efficient to launch with SSTOs, then decouple/undock any space-based vehicles/payloads
...It is most efficient to keep those space-based things in space, rather than re-landing and re-launching them
...So for ultimate efficiency only launch (with a SSTO) new vehicles, then the fuel to keep reusing them
...Alternatively, re-dock to a SSTO to land those things that need to come down again (science, crews having a rest, etc.)

[*VTVL = Vertical Takeoff, Vertical Landing - used as a name for jets that fly like rockets]

numerobis · December 11, 2014

Then it's the trip back, and that's like a pittance if you drop really close to Jool doing it, perhaps 1.5km/s or two

The best trip back from Laythe is to go from Laythe 55km straight to Kerbin at the right time. Then it's a bit under 1200 m/s. With jets, you can get a couple hundred of those for nearly-free: you can get your orbit to be up near the SoI boundary (but you can't quite escape Laythe on jets alone).

Dropping down to Jool is way more expensive.

Anyway, long story: it's about 3300 m/s to go from LKO to Laythe and back if you bring a turbojet. Sufficient lift surfaces to reduce your liftoff TWR to 0.5 will be lighter than doubling up on turbojets. Compare two scenarios at 1km on Laythe: two turbojets, you can just barely take off; or one turbojet plus wings, liftoff at 50 m/s.

The first one costs you 1.2t extra in turbojets.

The second one costs you 17 units of lift if you have a 20-degree angle of attack. That'll cost you 0.57t to 0.85t, depending what kind of wing you use.

So you saved at least 300 kg -- probably more since with the dual-jet version you probably would bring parachutes -- and you got the ability to more easily land on land rather than water. And you have reduced drag on the way up.

(Edit: or it'll cost you 0.12t in delta-deluxe winglets at 60-degree angle of attack, saving you 1.08t.)

Edited December 11, 2014 by numerobis

Van Disaster · December 11, 2014

With jets, you can get a couple hundred of those for nearly-free: you can get your orbit to be up near the SoI boundary (but you can't quite escape Laythe on jets alone).

I spent several hours orbiting Laythe in an aircraft one night while I was doing something else, trying to get as close to orbital as I could - if you're patient you can get an orbit with just a few of mins of a few metres inside the atmo boundary ( which would be enough for RCS at least ). But of course you'd have to spend hours messing with trace amounts of thrust

Admittedly was using FAR.

numerobis · December 11, 2014

FAR turbojets max out at about Laythe's 55km orbital speed IIRC. Stock turbojets can go far faster. You can get the apoapsis way, way high up.

Escape velocity at 55km is about 2600m/s in surface speed, which is slightly more than what stock turbojets can provide.

Rune · December 11, 2014

I am no great fan of spaceplanes because they're a pain, but their potential efficiency isn't in question. As you've said, a well-designed rocket can give 18-20% payload-ratio to orbit but no-one seems to have mentioned that a jet-SSTO can easily give you twice that even in stock, simply because the jets are so efficient. The heaviest spaceplane I've built was 38t and designed to ferry 40t (detachable, selectable, replaceable, modular) payloads into orbit and back - better than 50% payload-ratio with minimal fuel-costs and complete reusability. Other people can (and do) do better than that but my preference is for VTVL* SSTOs since wings are only extra mass, except for landing. I can get close enough to KSC on de-orbit that I'm recovering 98% of costs anyway and I don't find the potential extra 2% pays for itself or is worth the effort. Taking a spaceplane any further than LKO (apart from Laythe) is totally inefficient, especially without refulling.
LethalDose vehemently disagrees with me about terminology (I think an interplanetary single-stage shouldn't be called "a SSTO" amongst other issues) but even we agree that "SSTO" does not mean "Spaceplane" and that far too many people 'do' spaceplanes because they mistakenly think they are necessarily more efficient.
FWIW, here is the general "Single Stage" consensus I've gleaned from everyone's comments and shouldn't be too controversial:
A "SSTO" vehicle is one that can (at least) go from launch to orbit without jettisoning parts
...Therefore a SSTO you can't get back and reuse is almost always a mistake (I call this a disposable-SSTO whereas LethalDose doesn't consider it a "SSTO" at all)
...Rockets, VTVL jets and spaceplanes can all be SSTOs
...Staging is always more mass-efficient except when a single small engine & fuel-tank is all you need anyway (tiny probes and/or low gravity)
Since the point of a SSTO is to be recoverable/reusable fuel-cost per launch is almost all that counts
In oxygen atmosphere jets always beat rockets except where even a single jet is overpowered
...Therefore it is more efficient to SSTO with jets than rockets
...But SSTO rockets can be simpler and quicker to build and fly and should be lower part-count
Wings are excess mass
...That make landing where you want to a lot easier in atmosphere (LethalDose again makes this a requirement of the definition of "SSTO")
...And mean you can take-off with a TWR < 1 (I'm unconvinced about the value of this, but Wanderfound and others who are masters of spaceplanes seem to advocate it)
...But VTVL jets, without them, can be very efficient (Stratzenblitz's designs are my exemplar here)
Single Stage Beyond Orbit (except to Laythe) is always inefficient because at the very least you're carrying partially-empty tanks and the wrong/too many engines
...To be useful a SSTO usually has to deliver a payload and/or rendezvous/dock once in orbit (pointed out by Slashy)
...Therefore it is more efficient to launch with SSTOs, then decouple/undock any space-based vehicles/payloads
...It is most efficient to keep those space-based things in space, rather than re-landing and re-launching them
...So for ultimate efficiency only launch (with a SSTO) new vehicles, then the fuel to keep reusing them
...Alternatively, re-dock to a SSTO to land those things that need to come down again (science, crews having a rest, etc.)
[*VTVL = Vertical Takeoff, Vertical Landing - used as a name for jets that fly like rockets]

In general, I agree with everything you are saying. I will comment, though, that I am in the camp of "spaceplanes should fly with low TWR ratios, and that is an efficient thing". Note I say "spaceplanes", and not "SSTOs". The whole idea is that you improve you mass ratio for the rocket part of the ascent, since the mass you need for a TWR=0.5 ascent to >200m/s, in the form in engines+wings+empty fuel tanks (to account for the extra fuel you are bound to bring up in the discussion, everyone does); is smaller than the mass of jets to make a vertical TWR>1 ascent to >2000m/s. I pick 2000m/s arbitrarily, could be any one cutoff speed, the only thing is that it should be the same to compare both philosophies of airbreathing. That used to be more true than it is now, BTW, since wings have become substantially heavier. But it's the basic principle, and it's theoretically sound: High TWR means a significant fraction of the weight is engines, and we all know how the rocket equation goes.

The best trip back from Laythe is to go from Laythe 55km straight to Kerbin at the right time. Then it's a bit under 1200 m/s. With jets, you can get a couple hundred of those for nearly-free: you can get your orbit to be up near the SoI boundary (but you can't quite escape Laythe on jets alone).
Dropping down to Jool is way more expensive.
Anyway, long story: it's about 3300 m/s to go from LKO to Laythe and back if you bring a turbojet. Sufficient lift surfaces to reduce your liftoff TWR to 0.5 will be lighter than doubling up on turbojets. Compare two scenarios at 1km on Laythe: two turbojets, you can just barely take off; or one turbojet plus wings, liftoff at 50 m/s.
The first one costs you 1.2t extra in turbojets.
The second one costs you 17 units of lift if you have a 20-degree angle of attack. That'll cost you 0.57t to 0.85t, depending what kind of wing you use.
So you saved at least 300 kg -- probably more since with the dual-jet version you probably would bring parachutes -- and you got the ability to more easily land on land rather than water. And you have reduced drag on the way up.
(Edit: or it'll cost you 0.12t in delta-deluxe winglets at 60-degree angle of attack, saving you 1.08t.)

Judging by the precision of your numbers, I'm inclined to agree with you, but... wouldn't Oberth at Laythe + Oberth at Jool (the biggest gravity well in the game you can gravity-assist with!) be the most efficient departure possible? Throw in a Tylo gravity assist if you can manage the alignment. Not that I could work out the launch window to do that, of course... Anyhow, yeah, we all agree Laythe can be cheaper than Duna on airbreathers, and it is more forgiving regarding TWR.

Rune. It can, however, be more expensive... you can always increase delta-v needed through errors, up to infinity and beyond! :sticktongue:

Landge · December 12, 2014

This is going back 2 years. My first attempts to send SSTOs to Eve and Laythe...

Click on the image above to view the mission album.

My Jool/Laythe encounter was very sloppy. I didn't know about aerobraking at Jool would save dV at that time.

Edited December 12, 2014 by Landge

numerobis · December 12, 2014

Rune: Laythe is already pretty deep in Jool's gravity well. The marginal benefit you get from dropping lower is outweighed by the cost of dropping lower. But you mentioned Tylo: indeed, if you can get line up an assist off Tylo on the way out, that might save you a bit more. It costs about 650 m/s to get there from Laythe 55km (about half of which you can do with jets); I'll bet you only need a little more to turn that into a Kerbin return.

That would be something: 400 m/s in rocket DV to return from Laythe.

GoSlash27 · December 12, 2014

We haven't heard from the OP for a while.

How's it going, diegzumillo?

diegzumillo · December 12, 2014

Wow, this thread really took off. Pun unfortunately intended. But now I'm convinced it can be done

GoSlash27 · December 12, 2014

Absolutely it can be done. Most of the folks participating in this thread have done it.

But how is your interplanetay SSTO spaceplane coming along? Anything we can help with?

Best,

-Slashy

diegzumillo · December 12, 2014

I didn't have much time to play today, but I'll report back as soon as I make some progress

Rune · December 12, 2014

Rune: Laythe is already pretty deep in Jool's gravity well. The marginal benefit you get from dropping lower is outweighed by the cost of dropping lower. But you mentioned Tylo: indeed, if you can get line up an assist off Tylo on the way out, that might save you a bit more. It costs about 650 m/s to get there from Laythe 55km (about half of which you can do with jets); I'll bet you only need a little more to turn that into a Kerbin return.
That would be something: 400 m/s in rocket DV to return from Laythe.

But if you eject at the right angle, you already come out of Laythe with a low periapsis over Jool... right? So Oberth for free, and I don't know which one would be stronger for the effect, Laythe's upper atmosphere or the periapsis around Jool you get out of a Laythe escape. In any case, the Tylo thing is totally cool (and free, since it's a true assist and not Oberth effect)... and totally outside the realm of my navigation possibilities. It would indeed be something if I ever manage it, as you say.

In any case, I think we have done a really good job of showing how a few navigation tricks can slash the delta-v budget of a mission. Imagine the kind of billiard voodoo the guys at JPL play around Saturn!

Rune. And all that is possible with KSP's "crappy" patched conics.

Rocket Farmer · December 12, 2014

I've managed it. Trip to and from Ike. I have also tried landing on Duna but found the thin atmosphere really difficult on a SSTO design. That was managed pre rapier engines. I just never got around to trying again.

Use the standard jet engines and a nuke engine. Ensure you boost with the jets into orbit the. Use the nukes to go the distance. It takes a multi jet liftoff but only a single nuke and lots of fuel.

numerobis · December 12, 2014

But if you eject at the right angle, you already come out of Laythe with a low periapsis over Jool... right?

You come out with Jool Pe at Laythe's orbit -- which is indeed pretty low.

The right angle to leave Jool if you were at 27,000 km above Jool is ... uh... where's my favourite calculator? Anyway, at the right phase angle you burn 1500 m/s prograde, and that ends up with you exiting out Jool's retrograde.

If you're orbiting Laythe, you're also orbiting Jool at 27,000km. So you want that your exit from Laythe's sphere of influence is 1500 m/s to Laythe's prograde.

Rune · December 12, 2014

You come out with Jool Pe at Laythe's orbit -- which is indeed pretty low.

Or lower. If you don't eject prograde relative to Laythe's orbit, your lateral component will translate to a more eccentric orbit. And a more eccentric orbit is easier to escape from due to Oberth effect at periapsis... because It would have a lower periapsis than Laythe's orbit.

The right angle to leave Jool if you were at 27,000 km above Jool is ... uh... where's my favourite calculator? Anyway, at the right phase angle you burn 1500 m/s prograde, and that ends up with you exiting out Jool's retrograde.
If you're orbiting Laythe, you're also orbiting Jool at 27,000km. So you want that your exit from Laythe's sphere of influence is 1500 m/s to Laythe's prograde.

That would be a direct ejection with all the burn happening in Laythe's SOI. As I said, you are limited to using the gravity wells in low Laythe orbit that way. By breaking your burn in sections in different SOIs, some of those can take advantage of the game's biggest gravity wells. And as I also said, the tiny fraction of a fraction you save is probably not worth it really... other than for bragging rights of course!

Rune. If anything, this discussion is making me realize stuff I short of knew already. I've got to try all this and see how hard it is to time things!

numerobis · December 12, 2014

If you don't eject prograde from Laythe, you'll get a lower Pe at Jool, and you'll be wasting fuel.

Here's some calculations:


import planet
def viajool(pe):
     laytheSOI = abs(planet.jool.hohmann(27.1e6, pe)[0])
     laytheDV = planet.laythe.soiBurn(55000, laytheSOI)
     joolSOI = abs(planet.kerbol.hohmann(69e9, 13.6e9)[0])
     joolCircDV = planet.jool.soiBurn(pe, joolSOI)
     joolDV = joolCircDV - abs(planet.jool.hohmann(27.1e6, pe)[1])
     return joolDV + laytheDV

What I'm calculating first is: for any given joolian periapsis pe (in meters), how would I need to get from Laythe's orbit down to there?

Second: ok, but I'm actually in a 55km circular orbit around Laythe, so the previous question answered how fast I should be going when I exit Laythe's SOI due retrograde. From circular orbit, how big a burn must I do (in m/s) so that at the edge of Laythe's SOI I'm going the required speed?

Third: unrelated question: if I'm in a circular orbit with no inclination that is roughly coorbital with Jool, how big a burn would I need to do to get my perihelion down to Kerbin's orbit? (The real solution is a bit different, but not hugely so.)

Fourth: OK, but now let's say I'm in a joolian orbit circular at pe m altitude. Then the third question just tells us how fast to be going at the SOI boundary of Jool, due retrograde. How big a burn do I need to do to get there?

Fifth: OK, but actually I'm not in a circular orbit, I've got a bunch of excess speed from coming down from Laythe's orbit. That's speed I get for free (or rather, already paid for), so what's left for me to burn?

Sixth: We have two actual burns -- the second and fifth lines. Add them up, that's how much it takes to escape Jool from Laythe 55km via a lower joolian periapsis.

Next up: loop over all possible solutions, since it's not a linear relation (or at least, loop over every km):


best = viajool(0)
bestPe = 0
for peKm in xrange(27000):
     pe = peKm * 1000
     dv = viajool(pe)
     if dv < best: best = dv ; bestPe = pe

And that shows that the drop-down approach works best if you drop down to a joolian periapsis of 3839 km -- and that will cost you a mere 1934 m/s.

By contrast, straight from Laythe is:


joolSOI = abs(planet.kerbol.hohmann(69e9, 13.6e9)[0])
laytheSOI = planet.jool.soiBurn(27.1e6, joolSOI)
laytheDV = planet.laythe.soiBurn(55000, laytheSOI)

1060 m/s.

For the "planet.kerbol.hohmann" bit I'm assuming fallaciously that Jool is in a circular orbit with zero inclination.

numerobis · December 12, 2014

Overall: my point is: if you can get a spaceplane to Mun and back with a bit of jet fuel left upon arrival home, then that same spaceplane can get to Laythe and back.

cantab · December 12, 2014

These are my go-to delta-V charts for Joolian stuff (and anything vaguely complex really).

http://www.reddit.com/r/KerbalAcademy/comments/1qu5jv/deltav_charts/

Using the Jool set of charts, the Jool map is for dropping your periapsis low over Jool and burning there. To get to the Jool-Kerbin transfer orbit takes 1960 m/s.

The Laythe map is for burning directly from Laythe. To get to the Laythe-Kerbin transfer orbit takes 1060 m/s. Vastly less.

This tallies with my experience, in that 1120 m/s was sufficient to get one of my ships back from Laythe to Kerbin by the direct burn method. Though I didn't investigate the periapsis drop method.

Edit: And it looks like you beat me to the figures numerobis. Your numbers and the chart's agree, which is a good sign.

As for Tylo assists, they can work but they're tricky to set up. Firstly bear in mind you need to eject from Laythe with a bit more than the 640 m/s for a Tylo transfer, since you need to cross Tylo's orbit to get the gravity assist not merely touch it. Then the Laplace resonance becomes a nuisance, if you want to encounter Tylo on your way out you can only do that in three positions round its orbit, and they might not be where you really want. If you encounter it on your way back in after apoapsis that gives you more freedom but you then become very sensitive to the delta-V used for Laythe ejection. Finally if you have an inclined Laythe orbit the needed plane change can undo the gains from the Tylo assist.

Edited December 12, 2014 by cantab

bitslizer · December 12, 2014

If on stock... Why bother with Wings?

Did you check out Cupcake's thread?

http://forum.kerbalspaceprogram.com/threads/27424-Cupcake-s-Dropship-Dealership

http://forum.kerbalspaceprogram.com/threads/102023-Building-Dropships-A-VTOL-Tutorial

http://forum.kerbalspaceprogram.com/threads/93802-How-To-Dropship-%28A-VTOL-Tutorial%29

based on his style/inspriation I was able build a SSTO (w/o wings... using 4 turbojets and 2 nukes) to land on both Duna and Ike and return to Kerbin with 2 Kerbals and a small duna rover. Its got over 6k DV after achieving LKO.

I'm going to run that over to Laythe next

Edited December 12, 2014 by bitslizer

GoSlash27 · December 12, 2014

If on stock... Why bother with Wings?
Did you check out Cupcake's thread?
http://forum.kerbalspaceprogram.com/threads/27424-Cupcake-s-Dropship-Dealership
http://forum.kerbalspaceprogram.com/threads/102023-Building-Dropships-A-VTOL-Tutorial
http://forum.kerbalspaceprogram.com/threads/93802-How-To-Dropship-%28A-VTOL-Tutorial%29
based on his style/inspriation I was able build a SSTO (w/o wings... using 4 turbojets and 2 nukes) to land on both Duna and Ike and return to Kerbin with 2 Kerbals and a small duna rover. Its got over 6k DV after achieving LKO.
I'm going to run that over to Laythe next

Well,

he *did* mention non-spaceplane SSTOs. I have a crazy- efficient vertical lift SSTO that I use for lifting large, bulky assemblies (station sections, ship subassemblies and the like). Never used it for interplanetary travel, but no reason why it *couldn't* be used for that.

Only downside to a vertical lifter is that it needs more engines to do it's work and doesn't get you 100% recovery.

Best,

-Slashy

Spatzimaus · December 13, 2014

Only downside to a vertical lifter is that it needs more engines to do it's work and doesn't get you 100% recovery.

I beg to differ.

It's not hard to stick a bunch of parachutes onto an SSTO rocket stack for recovery; like any other fully recoverable SSTO all you'll lose is the fuel. Yes, using rockets means the net fuel costs will be considerably higher than you'd get from jets for the same payload, but all of the expensive bits will survive the landing. And if you make one using jets+nukes (or, say, a RAPIER) you can even avoid most of that expense.

I'd also argue the statement

If on stock... Why bother with Wings?

Four reasons come to mind:

1> Lift. Seriously, not that complicated of a concept; having your lift offset part of your weight allows you to ascend much more easily. As a side effect, it also means your intakes will be moving more directly into the airstream instead of being strongly up-angled, so that'll help as well.

2> Precision landings, for those of us who really want to be able to land right on the runway (or at least ridiculously close to the KSC). I've got a 400-ton SSTO plane (lifts 50-ton payloads with ease) that I can land right back at KSC because of this.

3> Softer landings. By making minor tweaks to your angle of attack as you approach the ground, you can have good control over your vertical speed without any parachutes or VTOL fuel expenditures. This is really important if you're trying to do a landing somewhere like Duna as you can do a landing without any kind of thrust whatsoever.

4> Horizontal spacing. A narrow body reduces your ability to place landing gear in a way that'd prevent rolling crashes, for instance, and putting your RCS jets further out gives higher torque than keeping them close in. Again, good for landing on places like Duna or in any other rough terrain.

And really, they add very little mass to begin with. My own long-range spaceplane (posted earlier in this thread) might look like a normal plane in-atmosphere, but in space it's basically just a rocket with a tiny bit of extra mass. It lands on its tail on airless bodies, with landing legs and ladders. So even outside of the atmosphere, the wings have almost no drawbacks, they allow me to do a diamond-shaped 4-point landing gear setup (MUCH more stable than the usual 3-point triangle), and they make convenient outboard mounts for the ion engines and RCS (neither of which need fuel lines).

GoSlash27 · December 13, 2014

I beg to differ.
http://i.imgur.com/RPjQfab.png
It's not hard to stick a bunch of parachutes onto an SSTO rocket stack for recovery; like any other fully recoverable SSTO all you'll lose is the fuel. Yes, using rockets means the net fuel costs will be considerably higher than you'd get from jets for the same payload, but all of the expensive bits will survive the landing. And if you make one using jets+nukes (or, say, a RAPIER) you can even avoid most of that expense.

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.

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.

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.

Another difference I didn't mention is that a vertical lift SSTO has no abort options in the event of failure, so there's a safety risk to the payload that's not a problem for spaceplanes. 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.

I use both types, and specialize them to their ideal payloads. Kerbals and supplies go in spaceplanes, while structures go aboard vertical lift SSTOs.

Best,

-Slashy

Edited December 13, 2014 by GoSlash27

SSTO limitations and interplanetary spaceplanes

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